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Evaluating the Use of Lower VMA Requirements for Superpave Mixtures

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PAGE 1

EVALUATING THE USE OF LOWER VM A REQUIREMENTS FOR SUPERPAVE MIXTURES By GREGORY A. SHOLAR A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ENGINEERING UNIVERSITY OF FLORIDA 2004

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Copyright 2004 by Gregory A. Sholar

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iii ACKNOWLEDGMENTS I would like to acknowledge my graduate advisor, Dr. Reynaldo Roque, for his patient guidance and wisdom thr oughout this long process. I would also like to thank Dr. Bjorn Birgisson and Dr. Mang Tia for their ad vice and assistance in performing the work and analysis of this thesis. I offer the most sincere appreciation to Mr. Howie Moseley of the Florida Department of Transportation. Without Mr. Moseley’s support, help and advice, this thesis would have not been finished. I would like to thank my supervisors, Mr Jim Musselman and Mr. Gale Page, for allowing me the time and for offering the mo ral support to pursue my master’s degree while working fulltime. I would like to thank Ms. Susan Andrews, Mrs. Shanna Johnson, Mr. Joshua Whitaker and Mr. Stephen Browning of the Fl orida Department of Transportation for their assistance in performing laboratory testing and the closeness that we share, not only as workers, but as friends. I would like to thank Dr. Christos Dra kos and Ms. Tanya Riedhammer for their assistance in performing testing and analysis. Their expertise far surpassed mine and I would still be struggling with an alysis if not for their help. I also thank them for their friendship. I would also like to thank Mr. George L opp for general technical advice and most importantly for his friendship and mora l support over the last five years.

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iv And finally, I would like to thank Ms. Betsy Pepine for her encouragement, support and love over the last two and half years.

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v TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iii LIST OF TABLES............................................................................................................vii LIST OF FIGURES.........................................................................................................viii ABSTRACT....................................................................................................................... xi CHAPTER 1 INTRODUCTION........................................................................................................1 1.1 Problem Statement..................................................................................................1 1.2 Objectives...............................................................................................................2 1.3 Scope of Work........................................................................................................3 1.4 Research Plan..........................................................................................................3 2 BACKGROUND..........................................................................................................5 2.1 Definition of VMA.................................................................................................5 2.2 Coarse and Fine Gradations....................................................................................6 2.3 Nominal Maximum Aggregate Size.......................................................................7 2.4 Literature Review...................................................................................................8 2.4.1 Historical Perspective...................................................................................8 2.4.2 Recent Research...........................................................................................9 3 MATERIALS AND TESTING METHODS..............................................................14 3.1 Introduction...........................................................................................................14 3.2 Materials...............................................................................................................14 3.2.1 Asphalt Binder............................................................................................14 3.2.2 Aggregates..................................................................................................14 3.2.2.1 Alabama limestone...........................................................................15 3.2.2.2 Brooksville limestone.......................................................................16 3.2.2.3 Nova Scotia granite..........................................................................17 3.2.2.4 Miami limestone (Tarmac mine)......................................................19 3.3 Testing Methods...................................................................................................20 3.3.1 Mix Design Testing....................................................................................21

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vi 3.3.2 Moisture Sensitivity Testing.......................................................................23 3.3.3 Permeability Testing...................................................................................24 3.3.4 Asphalt Pavement Analyzer Testing..........................................................26 3.3.5 Servopac Gyratory Compactor Testing......................................................31 3.3.6 Superpave Indirect Tension Testing...........................................................33 4 TEST RESULTS AND ANALYSIS..........................................................................39 4.1 Introduction...........................................................................................................39 4.2 Mix Design...........................................................................................................39 4.2.1 Mix Design Test Results............................................................................39 4.2.2 Mix Design Summary.................................................................................43 4.3 Rutting..................................................................................................................44 4.3.1 APA Test Results.......................................................................................44 4.3.2 APA Summary............................................................................................51 4.3.3 Servopac Test Results.................................................................................52 4.3.3.1 Gyratory shear slope.........................................................................53 4.3.3.2 Vertical strain...................................................................................55 4.3.3.3 Maximum shear stress......................................................................58 4.3.4 Servopac Summary.....................................................................................61 4.4 Cracking................................................................................................................62 4.4.1 Energy Ratio...............................................................................................62 4.4.2 Dissipated Creep Strain Energy (DCSE)....................................................64 4.4.3 Fracture Energy (FE)..................................................................................66 4.4.4 Cracking Summary.....................................................................................68 4.5 Moisture Damage..................................................................................................69 4.5.1 Conventional FM 1-T 283 Test Results.....................................................69 4.5.2 Superpave IDT Test Results (Energy Ratio)..............................................69 4.5.3 Moisture Damage Summary.......................................................................70 4.6 Permeability..........................................................................................................70 5 CONCLUSIONS AND RECOMMENDATIONS.....................................................71 5.1 Conclusions...........................................................................................................71 5.2 Recommendations.................................................................................................72 LIST OF REFERENCES...................................................................................................73 BIOGRAPHICAL SKETCH.............................................................................................75

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vii LIST OF TABLES Table page 3-1 Gradations for Alabama limestone mixtures............................................................16 3-2 Gradations for Brooksvi lle limestone mixtures.......................................................17 3-3 Gradations for Nova Scotia granite mixtures...........................................................18 3-4 Gradations for Tarm ac limestone mixtures..............................................................20 4-1 Volumetric mix design data for Alabama limestone mixtures.................................39 4-2 Volumetric mix design data fo r Brooksville limestone mixtures............................40 4-3 Volumetric mix design data fo r Nova Scotia granite mixtures................................41 4-4 Volumetric mix design data for Tarmac limestone mixtures...................................42 4-5 VMA difference between rounds one and two.........................................................44 4-6 APA test results........................................................................................................45 4-7 Servopac test results.................................................................................................53 4-8 Energy ratio values for the unconditioned and LTOA specimen.............................62 4-9 Moisture damage test results....................................................................................69 4-10 Permeability test data...............................................................................................70

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viii LIST OF FIGURES Figure page 2-1 Volumetric diagram....................................................................................................5 2-2 Coarse and fine gradations.........................................................................................6 3-1 Gradation plots for Alabama limestone mixtures....................................................16 3-2 Gradation plots for Brooksville limestone mixtures................................................18 3-3 Gradation plots for Nova Scotia granite mixtures....................................................19 3-4 Gradation plots for Tarmac limestone mixtures.......................................................20 3-5 Permeability test apparatus.......................................................................................25 3-6 Asphalt Pavement Analyzer.....................................................................................27 3-7 Asphalt Pavement Analyzer loading apparatus........................................................28 3-8 Measuring plate and contour gage for modified measuring technique....................29 3-9 Holder, contour gage and rut profile trace...............................................................29 3-10 Illustration of absolute rut depth and differential rut depth.....................................30 3-11 Gyratory shear slope.................................................................................................32 3-12 Vertical failure strain................................................................................................33 3-13 Framework for evaluating mixtures.........................................................................34 3-14 Superpave indirect tension test.................................................................................35 3-15 Dissipated creep strain energy..................................................................................36 4-1 VMA plots for Alabama limestone mixtures...........................................................40 4-2 VMA plots for Brooksvi lle limestone mixtures.......................................................41 4-3 VMA plots for Nova Scotia granite mixtures..........................................................42

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ix 4-4 MA plots for Tarm ac limestone mixtures................................................................43 4-5 Comparison of APA measurement met hods for 7% air voids, 75 mm tall specimens.................................................................................................................46 4-6 APA absolute rut depth using conventiona l measuring device for 7% air voids, 75 mm tall specimens...............................................................................................46 4-7 APA absolute rut depth using profile m easuring device for 7% air voids, 75 mm tall specimens...........................................................................................................47 4-8 APA differential rut depth using profil e measuring device for 7% air voids, 75 mm tall specimens...............................................................................................47 4-9 APA percent area change using profil e measuring device for 7% air voids, 75 mm tall specimens...............................................................................................48 4-10 APA absolute rut depth using conventiona l measuring device for 4% air voids, 115 mm tall specimens.............................................................................................49 4-11 APA rut depth versus VMA using conve ntional measuring device for 7% air voids, 75 mm tall specimens....................................................................................50 4-12 APA rut depth versus dust to effectiv e binder ratio using conventional measuring device for 7% air voids, 75 mm tall specimens........................................................51 4-13 Gyratory shear slope for Alabama limestone round one mixture............................54 4-14 Gyratory shear slope for Florida Br ooksville limestone round one mixture............54 4-15 Vertical strain for Alabam a limestone round two mixture.......................................56 4-16 Vertical strain for Florida Br ooksville limestone round one mixture......................57 4-17 Vertical strain for Nova Sc otia granite round one mixtures.....................................57 4-18 Gyratory shear versus percent air voids for Alabama limestone mixtures...............59 4-19 Gyratory shear versus percent air voids for Florida Brooksville limestone mixtures....................................................................................................................59 4-20 Gyratory shear versus percent air vo ids for Nova Scotia granite mixtures..............60 4-21 Gyratory shear versus percent air vo ids for Florida Tarmac limestone mixtures....60 4-22 Gyratory shear stress versus APA rut depth.............................................................61 4-23 Energy ratios for unconditioned specimens.............................................................63

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x 4-24 Energy ratios for long-term oven aged specimens...................................................63 4-25 Dissipated creep strain en ergy for unconditioned specimens..................................65 4-26 Dissipated creep strain energy for long-term oven aged specimens........................65 4-27 Fracture energy for unconditioned specimens.........................................................67 4-28 Fracture energy for long-term oven aged specimens...............................................67

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xi Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Engineering EVALUATING THE USE OF LOWER VM A REQUIREMENTS FOR SUPERPAVE MIXTURES By Gregory A. Sholar December 2004 Chair: Reynaldo Roque Cochair: Bjorn Birgisson Major Department: Civil and Coastal Engineering The Florida Department of Transportation sp ecifies coarse graded asphalt mixtures for high traffic roadways with the rationale th at coarse graded mixtures will offer better rutting performance compared to fine graded mixtures. Contractors struggle to meet minimum voids in the mineral aggregate (V MA) specification requirements, especially when using aggregates native to Florida. Contractors often gap-grade asphalt mixture gradations to obtain enough void space to m eet VMA requirements. It is generally believed that gap-grading an as phalt mixture will be detrimen tal to the mixture’s rutting performance. This study examines the e ffects on laboratory measured rutting, cracking, moisture sensitivity and permeability of asphalt mixtures that have been designed with gap-graded and continuous gradations with the thought that should the continuous gradation provide better performance, then perhaps the VMA specification requirements should be lowered to allow for this type of gradation.

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xii The Asphalt Pavement Analyzer and Ser vopac gyratory compactor were used to determine the mixtures’ rutting performance. The Superpave indirect tensile tests (IDT) and calculated parameters (energy ratio, di ssipated creep strain energy and fracture energy) were used to determine the mixtures’ cracking and moisture sensitivity performance. Additional standard laboratory tests were used to evaluate permeability and moisture sensitivity. Test results indicate that the addition of coarse aggregate on the 12.5 and 9.5 mm sieves of 12.5 mm coarse graded mixtures improved the rutting performance of the mixtures. However, cracking performance was adversely affected by the addition of coarse aggregate. Moisture sensitivity resu lts varied depending on the test method used. Permeability results were unaffected by the gradation change. Since cracking is the predominant form of distress for Florida pavements, it is recommended that no change be made to the Department’s specifications at this time. Performance test results indi cate that not all mixtures pe rform at their optimum when designed volumetrically. The Department s hould continue to conduct research and move towards implementation of one or more pe rformance tests to augment or replace volumetric mix design.

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1 CHAPTER 1 INTRODUCTION 1.1 Problem Statement The Florida Department of Transportation, herein referred to as the Department, adopted the Superpave mix design system in 1996 as a replacement for the Marshall mix design system, which the Department had used since the 1970’s. One major difference between the two mix design methodologies is the recommendation in the Superpave system to use coarse graded mixtures for pa vements subject to high traffic levels. The Department defines a high traffic level as any pavement that will be subjected to ten million equivalent single axle loads (ESALs ) or greater over the pavement’s 20-year design period. This is in accordance with the American Association of State Highway and Transportation Officials (AASHTO) Sta ndard Practice for Superpave Volumetric Design of Hot-Mix Asphalt PP 28-03. The rationale for using coarse graded mixt ures on high traffic pavements is for the prevention of rutting. Coarse graded mixtures are typically those in which the gradation curve initially starts above the maximum density line for the larger sieve sizes and then curves below the maximum density line for th e smaller sieve sizes. This results in a mixture with more coarse aggregate and more stone-on-stone contact. A coarse aggregate skeleton is created in which the voids are filled with fine aggregate and asphalt binder. Because of this, coarse graded mi xtures are thought to provide equal or better resistance to rutting than fine graded mixt ures, which have a gradation curve entirely above the maximum density line.

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2 The Superpave mix design system also sets minimum requirements for the mixture property voids in the mineral aggregate (VMA ). The VMA is the percent by volume of the air voids plus asphalt bi nder that has not been absorbed into the aggregate. Commonly, to meet the minimum VMA require ments for coarse graded mixtures, mix designers have to gap grade the mixture by removing a portion of the coarse aggregate from the mix design. This problem is exacerba ted for limestone aggregates from Florida, which are less angular, softer, and brea kdown more easily than imported granite aggregates or limestones from other states. In general, for a given gradation, an angular aggregate will result in a higher VMA than a le ss angular aggregate. Additionally, as the aggregate breaks down during the production pr ocess, it becomes more rounded and less angular, which results in a re duction in VMA. Since the Department has implemented the Superpave mix design system, asphalt cont ractors have struggled to meet minimum VMA requirements at the mix design stage and more so during production. The gap grading of the aggregate gradation is necessa ry to meet the minimum VMA requirements. However, the removal of a portion of the coarse aggregate from the mix design may nullify the benefits of the strong rut resistant coarse aggregate skeleton. 1.2 Objectives The objectives of the study are as follows: Determine the effects on laboratory performance of adding additional coarse aggregate to a mixture’s gradation resu lting in a reduction in VMA, which may violate Superpave specifications. Based on the results, make recommendati ons regarding specification changes, further research, or no changes to the current specifications.

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3 1.3 Scope of Work This research focuses on identifying the laboratory performan ce difference between mixtures which have been designed to m eet Superpave specifications and then subsequently modified by adding more coarse aggregate to the mixtures gradations. The scope of work is as follows: Construct four Superpave mix designs usi ng aggregates from different geological sources that are commonly used in Flor ida. The mixtures will all be 12.5 mm coarse graded mixtures since this is th e most common coarse mixture type used by Contractors performing work for the Departme nt. The mixtures will be gap graded to match common practice by mix designers. Determine the laboratory performance of the four mixtures by using tests that give an indication of a mixtures resistance to rutting, cracking, moistu re sensitivity and permeability. Modify the gradations of the four mixt ures evaluated in the first objective to provide more coarse aggregate on the 12.5 mm and 9.5 mm sieve sizes. This will result in a reduction of VMA, which may be less than the minimum specified value. These four mixtures will then be evaluated using the same laboratory performance tests used to evaluate the unmodified mixtures. Compare the performance between the unm odified and modified mixtures to ascertain the effects of the addition of coarse aggregate on a mixture’s performance. Evaluate the results a nd make recommendations. 1.4 Research Plan The following items constitute the research plan for this study: A literature review was conducted. Four aggregate types were selected for study: Alabama limestone, Florida limestone from the Brooksville area, Nova Scotia granite, and Florida limestone from the Miami area (Tarmac mine). Thes e aggregate types are commonly used in Florida and represent a wide ra nge of softness and angularity. Four mixtures were designed to meet S uperpave mix design criteria. All of the mixtures were 12.5 mm coarse graded mi xtures and were gap graded. Each mixture contained only one aggregate type.

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4 The Brooksville limestone mixture met all Superpave mix design criteria except for the minimum VMA requirement. Brooksville limestone is a soft Florida limestone that cannot be used solely to construc t a 12.5 mm coarse graded Superpave mixture and meet minimum VMA requirements. This aggregate type was chosen intentionally so that a mixture not able to meet VMA criteria could be evaluated in terms of performance. The following laboratory tests were used to ascertain rutting performance: the asphalt pavement analyzer with the conve ntional and modified analysis approach and the ServoPac gyratory compactor to m easure shear stress, gyratory shear slope and strain. The following laboratory test was used to ascertain cracking performance: the Superpave indirect tension test. The following laboratory tests were used to ascertain moisture sensitivity performance: tensile strengt h, the Superpave indirect tens ion test and a falling head permeability test. Each of the four mixtures was then modified by adding mo re 12.5 mm and 9.5 mm coarse aggregate. The resulting gradations were more continuously graded and less gap graded than the unmodified mixtures. A reduction in VMA occurred for each mixture. The modified mixtures were then evaluate d with the same laboratory tests used to evaluate the unmodified mixtures. The data was analyzed and conclusions and recommendations were made.

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5 CHAPTER 2 BACKGROUND 2.1 Definition of VMA Voids in the mineral aggregate (VMA) is a volumetric property and is the sum of the air voids in the mixture plus the amount of asphalt binder that ha s not been absorbed into the aggregates. This unabsorbed binder is termed the “effective binder.” The concept of VMA is illustrated in Figure 2-1. Figure 2-1. Volumetric diagram

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6 2.2 Coarse and Fine Gradations The Superpave mixture design system designate s mixtures as either coarse or fine. As mentioned previously, this study focuses only on coarse graded mixtures, which are thought to have equal or better rutting resistan ce compared to fine graded mixtures. Coarse graded mixtures have gradation curves that start above the maximum density line and curve downward below the restricted z one, whereas fine graded mixtures have gradation curves which lie solely above the maximum density line. The maximum density line represents the gradation that w ould result in the denses t possible arrangement of the aggregate particles. Superpave define s the restricted zone as an area where the gradation should not pass through. Gradati ons that pass through this zone have the potential to contain natura l rounded sands which may i nhibit good rutting performance (Asphalt Institute 1996). An example of a coar se and fine gradation is shown in Figure 2-2. 0 10 20 30 40 50 60 70 80 90 100 Sieve Size (mm)Percent Passing Coarse Fine.0751.18 .600 .300 .1502.36 4.75 9.5012.50 19.00 Maximum Density Line Restricted Zone Figure 2-2. Coarse and fine gradations

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7 The aggregate gradation curve and its distance away from the maximum density line are related to the VMA of a mixture. More area between the gr adation curve and the maximum density line increases the VMA potential of the mixture. 2.3 Nominal Maximum Aggregate Size The Superpave mix design system designa tes a mixture by its nominal maximum aggregate size (NMAS). The NMAS is defined to be the sieve which is one sieve size larger than the first sieve to retain more than ten percent of the aggregate by weight. All of the mixtures used in this study are 12.5 mm mixtures, which means that more than ten percent of the aggregate is re tained on the 9.5 mm sieve. Minimum VMA requirements are based on th e NMAS of the mixture. A smaller NMAS mixture, for example a 9.5 mm mixtur e, has a higher VMA requirement than a larger NMAS mixture, such as a 19.0 mm mixt ure. This is because the total surface area of the aggregates is greater for the smalle r NMAS mixture as compared to the larger NMAS mixture. More aggregate surface area requires more asphalt binder to coat the aggregates and hence the speci fied minimum VMA is greater. However, Superpave does not differentiate between coarse and fi ne gradations with respect to the VMA requirement. Both type s of mixtures have the same minimum VMA requirement for a given NMAS. Coarse graded mixtures have more coarse aggregate in proportion to fine aggregate than fine graded mixtures. Therefore, there is less aggregate surface area in a coarse graded mixture as comp ared to a fine graded mixture for a given NMAS. Given the same VMA requirement, mix designers are then forced to gap grade the mixture to provide ample volume between the aggregate partic les to contain the required four percent air voids and effective asphalt binder needed to meet the minimum VMA requirement.

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8 2.4 Literature Review 2.4.1 Historical Perspective Modern mix design methods can generally be dated back to the 1940’s with the most predominant method being the Marshall mix design method. Marshall had different views regarding VMA than other asphalt tec hnologists at the time. Marshall believed that VMA should be reduced to the lowest possible level and did not believe in establishing specification limits for VMA (Leahy and McGennis 1999). In 1957, Norman McLeod presented a paper to the Highway Research Board emphasizing the importance of using the aggreg ate bulk specific gravity in the calculation of VMA instead of the effective specific gravity, which was common at the time (Leahy and McGennis 1999). McLeod also believed that VMA should be specified as a minimum value of 15 percent with design air voids at five percent using the 75-blow Marshall method. No performance data was used by McLeod to determine this VMA limit. McLeod proposed VMA requirements ba sed on nominal maximum aggregate size, which were adopted by the Asphalt Institute in 1964. The current Superpave mix design system specifies VMA based on McLeod’s recommendations but has adjusted them lower by one percent realizing that McLeod designed asphalt mi xtures at five percent air voids and the Superpave system require s four percent air voids (Kandhal and Chakraborty 1996). Coree and Hislop (1999) conducted a thorough review of literatu re regarding VMA and found that there is little historical basis, if any, to support the VMA values currently specified. Minimum VMA requirements that are the same for all gradations of a particular NMAS can cause well performing mixt ures to be rejecte d. They suggest the possible use of a minimum asphalt film thic kness as a replacement for VMA. The

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9 researchers also recommend that VMA re quirements or asphalt film thicknesses be validated against field performance and that enforcement of any VMA specification not be rigidly enforced due to the im precision in current test methods. 2.4.2 Recent Research Researchers have come to recognize that VMA criteria based on NMAS alone is not adequate and that an approach based on as phalt film thickness is more rational. Work by Kandhal and Chakraborty (1996) examined film thicknesses ranging from 4 to 13 microns for one 12.5 mm coarse graded mixtur e containing limestone. Mixtures were compacted to eight percent air voids and short and long term aged. Mixture tests included resilient modulus a nd tensile strength and binde r tests included viscosity, penetration and complex modulus. The resear chers’ conclusion was that a minimum film thickness of 9 to 10 microns is desi rable to minimize accelerated aging. Work also conducted by Kandhal et al. (1998) emphasized that coarse graded mixtures are penalized by current Superpav e requirements because the VMA requirement is the same for coarse and fine graded mixtur es. This results in thicker than necessary film thicknesses for coarse graded mixtures As mentioned previously, work done by Kandhal and Chakraborty (1996) indicated an optimum film thickness of 9 to 10 microns at eight percent air voids. This study recommended a minimum asphalt film thickness of eight microns for mixtures compacted between four and five percent air voids, which would better represent the in-place density ac hieved in the roadway. The researchers’ reasoning for the lower film thickness is that at four to five percent air voids there would be less aging of the binder. Based on a f ilm thickness of eight microns, coarse graded mixtures had VMA values up to two percent lo wer than fine graded mixtures using the same aggregate type.

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10 Kandhal and Mallick (2001) investigated the effect of aggr egate gradation and aggregate type on the rutting pot ential of asphalt mixtures. Tests conducted in the asphalt pavement analyzer (APA) indicated that fo r the limestone and granite mixtures, an increase in VMA resulted in an increase in rut depth. The trend was opposite for the river gravel mixture studied. The same trends we re observed when comparing voids filled with asphalt (VFA) to the APA rut depths. Hand et al. (2001) conducted a study measuring the rut resi stance of 21 granite and limestone mixtures of varying gradations us ing the PURWheel laboratory rut tester and triaxial shear strength. Th e researchers concluded that maximum rut resistance as determined by these two tests was achieved at an asphalt binder content 0.5 percent below the value determined in the Superpave mix design process. The additional 0.5 percent asphalt binder can be attributed to minimum Superpave VMA requirements. Sholar et al. (2001) conducted a stud y measuring the effects of aggregate degradation throughout the produc tion process on the volum etric properties of asphalt mixtures. Three aggregate types commonly us ed in Florida (Georgia granite, southeast Florida limestone, and west-central Florid a limestone) were evaluated representing a range of hard to soft aggregates respectivel y. Aggregate gradations were examined at five points in the production process. Be lt cut samples were obt ained, asphalt mixture was obtained from the truck bed, asphalt mixt ure from the same truck was obtained from behind the paver but prior to compaction, asphalt mixture was obtained after roller compaction, and gradations were determined fr om gyratory compacted samples. Some of the conclusions from the research were: Aggregate breakdown was directly related to Los Angeles Abrasion values. The two limestone mixtures degraded signifi cantly more than the granite mixture.

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11 An average reduction of 0.5 percent VMA w ould be expected to occur for every one percent of dust (materia l passing the 0.075 mm sieve) that was generated due to breakdown. Coree and Hislop (2001) conducted additional research to determine the aggregate factors related to the critical VMA for a mixt ure. They determined the critical VMA by using the Nottingham Asphalt Tester, which is a repeated load triaxial test. The researchers determined the critical point by examining strain data at multiple asphalt contents and selecting the asphalt content a nd corresponding VMA wher e strain started to increase. They identified this point to be where the mixture would go from sound to unsound behavior in terms of permanent defo rmation. Only three out of 28 mixtures were correctly identified based on VMA design criteria alone. It was determined that the volume of effective binder is more reliable (ten out of 28 mixtures) than VMA alone. Aggregate factors that correlated well with the critical VMA were fineness modulus, the percent of crushed coarse aggregate and the percent of crushed fine aggregate. Anderson (2001) conducted a study compari ng the performance of 12.5 mm coarse and fine graded mixtures composed of Illin ois Dolomite with each mixture designed with 13 and 15 percent VMA. Anderson had the following conclusions: Using the shear frequency sweep test (for rutting characterization) and the shear fatigue test, the high temper ature stiffness and critical temperature and the shear fatigue characteristics of the coarse mixt ure decreased substantially as the VMA increased. These tests suggest that the co arse mixture with 15 percent VMA would be more susceptible to rutting and fatigue cracking than the coarse mixture with 13 percent VMA. Repeated shear testing (for rutting characterization) and flexural beam fatigue testing (for fatigue charac terization) indicated that a reduction of VMA from 15 percent to 13 percent should not affect the performance ch aracteristics of the coarse mixture. An increase in VMA from 13 percent to 15 percent for the fine graded mixture improved the shear fatigue characterist ics by 50 percent while only reducing the high temperature stiffness and rutting char acteristics by no more than 30 percent.

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12 The coarse mixture appeared much more sensitive to VMA changes than the fine mixture. Ruth and Birgisson (1999) identified severa l factors of high quality mixtures that would make them relatively insensitive to changes during production. They emphasized the importance of a continuously graded mixture that did not have an excess or deficiency on any one sieve size. They also believed th at the gradation should generally not be gap graded. Ruth et al. (2002) used tensile strength, fracture energy and failure strain from the Superpave indirect tension test to evaluate mixtures with a variety of gradations and determined that continuously graded mixt ures outperformed mixtures that were gap graded or had an excess or deficiency on any one sieve size, confirming the research performed by Ruth and Birgisson (1999). Nukunya et al. (2002) performed a co mprehensive study regarding VMA and presented the following findings: Mixture performance must be evaluate d through the use of physical tests and gradation analysis in addition to volumetric analysis. Current methods of calculating VMA and asphalt film thickness are ineffective across all cases. A new approach calcul ating effective VMA and effective film thickness based on only the portion of th e mixture passing the 2.36 mm sieve was presented. The percent of fine aggregate, not coarse aggregate, in a mixture appears to control binder age hardening. Coarse graded mixtures develop pockets of fine aggregate and asphalt binder, which make current methods for calculati ng film thickness and VMA irrelevant for coarse graded mixtures but rele vant for fine graded mixtures. Low effective film thickness and low effective VMA have a more pronounced effect on fine graded mixtures than coar se graded mixtures. The fine graded mixtures with low effective film thickness and VMA lose their flexibility and become more brittle during aging.

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13 The minimum VMA requirements for coarse graded mixtures may result in excessive asphalt leading to higher rutti ng based on high creep values and low shear resistance. The current Superpave criteria for a mi nimum VMA for coarse graded mixtures could be discontinued as long as other a ggregate controls were instituted to limit mix designers from using in ferior (soft) aggregates.

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14 CHAPTER 3 MATERIALS AND TESTING METHODS 3.1 Introduction This chapter provides information on the mate rials and test procedures used in this research project. It includ es properties of the materials, how the materials were combined, the test procedures performed on th e materials, and the analysis methods used. 3.2 Materials 3.2.1 Asphalt Binder A Superpave performance graded binder, PG 67-22, from El Paso Merchant Energy Petroleum (formerly known as Coastal Fuels) in Jacksonville, FL was used for this research project. This grade of binder is the standard unmodified binder used for Department projects. The binder contained no anti-stripping agent. The asphalt binder specific gravity was 1.03. The binder was sampled into ten 5-gallon buckets. 3.2.2 Aggregates Four types of aggregate were used for this study: Alabama limestone, limestone from the Brooksville, FL area, granite from Nova Scotia, and limestone from the Miami, FL area (Tarmac mine). Each aggregate ty pe was the basis for each asphalt mix design studied. All aggregates used for this study were 100 percent crushed aggregates, which is very common for Department work. All mix designs, except the Brooksville limestone mix design, were based on actual mix designs submitted for approval to the Department. Contractors do not submit 100 percent Brooksv ille aggregate mix designs because it is not possible to meet Superpave VMA criteri a as discussed in Ch apter 1, Section 1.4.

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15 Aggregate types were not intermingled and no reclaimed asphalt pavement was used. All aggregate components for each mix design we re oven dried and fractionated into individual sieve sizes from the 19.0 mm siev e to the 0.075 mm sieve prior to batching. Fractionating into all sieve sizes provided optimal control of achieved gradations and assured consistency between batches. It s hould be noted that material below the 2.36 mm sieve was typically not present for coarse aggregate components. The convention used throughout this paper will be that “Round 1” refers to the gap graded mixture which conforms to Superpave criteria. “Round 2” refe rs to the modified gradation that contains more coarse aggregate and is more conti nuously graded, yet reduces the VMA of the mixture. Each aggregate type will be discussed below. 3.2.2.1 Alabama limestone The Alabama limestone asphalt mixtur e was composed of three aggregate components: Number 7 stone from Southern Read y Mix, FDOT code 44, pit number AL-485. S-1-B stone from Southern Ready Mi x, FDOT code 51, pit number AL-526. Screenings from Vulcan Materials Co rporation, FDOT code 22, pit number AL149. The aggregate components were proportione d to give the following gradations for rounds 1 and 2 and are shown in Table 3-1 and Figure 3-1.

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16 Table 3-1 Gradations for Alabama limestone mixtures Percent Passing Round 1Round 2 19.0 100100 12.5 10092 9.5 8982 4.75 5454 2.36 3535 1.18 2222 0.600 1616 0.300 88 0.150 55 0.075 3.43.4 Sieve Size (mm) 0 10 20 30 40 50 60 70 80 90 100 Sieve Size (mm)Percent Passing Round 1 Round 2 0.075 0.15 0.30 0.60 1.182.364.759.512.519.0 Figure 3-1 Gradation plots for Alabama limestone mixtures 3.2.2.2 Brooksville limestone The Brooksville limestone asphalt mixtur e was composed of three aggregate components:

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17 S-1-A stone from Florid a Crushed Stone, FDOT code 46, pit number 08-012. S-1-B stone from Florid a Crushed Stone, FDOT code 52, pit number 08-012. Screenings (130A) from Florida Crushe d Stone, FDOT code 24, pit number 08012. The aggregate components were proportione d to give the following gradations for rounds 1 and 2 and are shown in Table 3-2 a nd Figure 3-2. These gradations are very similar to the gradations for the other three aggregate types. Table 3-2 Gradations for Br ooksville limestone mixtures Percent Passing Round 1Round 2 19.0 100100 12.5 9892 9.5 8982 4.75 5555 2.36 3232 1.18 2222 0.600 1414 0.300 99 0.150 77 0.075 5.35.3 Sieve Size (mm) 3.2.2.3 Nova Scotia granite The Nova Scotia granite asphalt mixture was composed of three aggregate components: Number 7 stone from Martin Marie tta, FDOT code 44, pit number NS-315, terminal TM-322. Number 89 stone from Martin Marie tta, FDOT code 54, pit number NS-315, terminal TM-322. Screenings from Martin Marietta, FDOT code 22, pit number NS-315, terminal TM-322. The aggregate components were proportione d to give the following gradations for rounds 1 and 2 and are shown in Table 3-3 and Figure 3-3.

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18 0 10 20 30 40 50 60 70 80 90 100 Sieve Size (mm)Percent Passing Round 1 Round 2 0.075 0.15 0.30 0.60 1.182.364.759.512.519.0 Figure 3-2 Gradation plots for Brooksville limestone mixtures Table 3-3 Gradations for Nova Scotia granite mixtures Percent Passing Round 1Round 2 19.0 100100 12.5 9892 9.5 8982 4.75 5858 2.36 3838 1.18 2424 0.600 1616 0.300 1010 0.150 77 0.075 5.35.3 Sieve Size (mm)

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19 0 10 20 30 40 50 60 70 80 90 100 Sieve Size (mm)Percent Passing Round 1 Round 2 0.075 0.15 0.30 0.60 1.182.364.759.512.519.0 Figure 3-3 Gradation plots for N ova Scotia granite mixtures 3.2.2.4 Miami limestone (Tarmac mine) The Tarmac limestone asphalt mixture was composed of four aggregate components: S-1-A stone from Tarmac Ameri ca, FDOT code 42, pit number 87-145. S-1-B stone from Tarmac Ameri ca, FDOT code 51, pit number 87-145. 5/16 inch stone from Tarmac America, FDOT code 56, pit number 87-145. Screenings from Tarmac America, FDOT code 22, pit number 87-145. The aggregate components were proportione d to give the following gradations for rounds 1 and 2 and are shown in Table 3-4 and Figure 3-4.

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20 Table 3-4 Gradations for Tarmac limestone mixtures Percent Passing Round 1Round 2 19.0 100100 12.5 9892 9.5 8982 4.75 5555 2.36 3232 1.18 2525 0.600 1818 0.300 1313 0.150 77 0.075 5.35.3 Sieve Size (mm) 0 10 20 30 40 50 60 70 80 90 100 Sieve Size (mm)Percent Passing Round 1 Round 2 0.075 0.15 0.30 0.60 1.182.364.759.512.519.0 Figure 3-4 Gradation plots fo r Tarmac limestone mixtures 3.3 Testing Methods Testing methods for this study can be categ orized into six classifications: mix design, moisture sensitivity testing, permeability testing, Asphalt Pavement Analyzer

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21 testing, Servopac gyratory compactor testing, and Superpave indirect tension testing. Each classification will be discussed below with respect to the test procedures used and the techniques used to analyze the data. 3.3.1 Mix Design Testing The design of the mixtures followed st andard Superpave practice, which is governed by four American Association of State Highway and Tran sportation Officials (AASHTO) standards: Superpave Volumetric Design for HotMix Asphalt (HMA), AASHTO designation PP 28-03. This practice outlines the overa ll design procedure from materials selection, designing the aggreg ate structure, selecting the design binder content, and evaluating the mixture for moisture sensitivity. Superpave Volumetric Mix Design, AASHTO designation MP 2-03. This specification gives detailed requirements fo r binder selection, aggregate gradation criteria, aggregate consensus property requi rements, and mixture property criteria based on traffic level. Preparing and Determining the Density of Hot-Mix Asphalt (HMA) Specimens by Means of the Superpave Gyratory Compactor, AASHTO designation T 312-03. This standard method of test discusse s specific requirements of the gyratory compactor, the compaction procedure, and density determination. For this study, all mix design specimens were gyrated in a Pine AFGC125X gyratory compactor. Mixture Conditioning of Hot-Mix Asphalt (HMA), AASHTO designation R 30-02. This practice outlines mixture conditioning for volumetric mix design and short and long-term conditioning for mechanical property testing. Specific Department test procedures n eeded during the mix design process were used to determine aggregate and mixtur e properties and are discussed below: Sieve Analysis of Coarse and Fine Aggreg ate, Florida Method of Test FM 1-T 027. This test method describes the procedure for performing a sieve analysis on coarse or fine aggregate to determine a gradation. Specific Gravity and Absorption of Fine A ggregate, Florida Method of Test FM 1T 084. This test method describes the pro cedure for determining the bulk specific gravity and absorption of fine aggregates.

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22 Specific Gravity and Absorption of Coarse Aggregate, Florida Method of Test FM 1-T 085. This test method describes the procedure for determining the bulk specific gravity and absorption of coarse aggregates. Bulk Specific Gravity of Compacted Bitumi nous Mixtures, Florida Method of Test FM 1-T 166. This test method describes the procedure for determining the bulk specific gravity of compacted asphalt mixtures, such as gyratory specimens. Maximum Specific Gravity of Asphalt Pavi ng Mixtures, Florida Method of Test FM 1-T 209. This test method describe s the procedure for determining the maximum specific gravity of uncompacted asphalt mixtures. As mentioned previously, i ndividual aggregate compone nts were fractionated to every sieve size to provide better accuracy and consistency in batching. Two fine and two coarse aggregate specific gravity tests were conducted for each aggregate component and the individual values combined mathematica lly to obtain bulk specific gravity values for the composite gradation, otherwise known as the job mix formula (JMF). Following standard Superpave guidelines, mixtures were designed with four percent air voids at the design number of gyr ations while also meeting specification requirements for VMA, VFA, and dust/effectiv e binder ratio. The design number of gyrations for all mixtures was 100. The sp ecified minimum VMA re quirement was 14.0. The VFA requirement was the range of 65 to 75 percent. The specified dust to effective binder content ratio was the ra nge 0.8 to 1.6. Once the design binder content had been selected, then additional specimens were prep ared with binder contents modified by the following amounts: +1.0, +0.5, -0.5 and -1.0 percent binder. Three asphalt specimens were made at each binder content. Having vol umetric design data at five asphalt binder contents provided enough information to construct an adequate VMA curve for each mixture.

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23 3.3.2 Moisture Sensitivity Testing Moisture sensitivity testing is a routine function in the Superpave mix design procedure and was performed for all mixtur es in this study. The main reason for performing this test was to obtain a relative measurement of the mixture’s resistance to moisture damage between rounds one and tw o of a particular aggregate type, not necessarily between mixtures of different aggr egate types. The addition of more coarse aggregate, resulting in a more continuous gr adation closer to the maximum density line, was thought to perhaps reduce the permeability of the mixture and reduce the susceptibility to moisture damage. The test method used to determine the mo isture susceptibility of a mixture was Resistance of Compacted Bituminous Mixtur e to Moisture-Indu ced Damage, Florida Method of Test FM 1-T 283. The basic test procedure is performed as follows: Samples are batched in the laboratory to a predetermined weight that will result in compacted specimens of 7.0 +/1.0 percent air voids. A minimum of six 100 mm diameter specimens are gyrated to a height of approximately 65 mm. Three specimens are broken in the unconditioned state at 25 C in the indirect tensile mode at a rate of 50 mm per minut e. The Pine breaking apparatus typically used to determine stability and flow values for Marshall mix design was used for this test. Three different specimens are conditi oned by vacuum saturating the specimens underwater to a condition of 70 to 80 percent saturation. These three specimens are then frozen at -18 C for a minimum of 16 hours and then placed in a water bath at 60 C for 24 hours. The specimens are then placed in a chamber at 25 C for two hours. These three specimens are then broken in th e indirect tensile m ode at a rate of 50 mm per minute. Peak loads obtained from the indirect tens ion testing are used to calculate diametral tensile strength.

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24 A tensile strength ratio is obtained by di viding the average tens ile strength in the conditioned state by the average tensile strength in the unconditioned state. In addition to the approach mentioned above the data from the Superpave indirect tension test was also used to evaluate mois ture sensitivity. This will be discussed in a subsequent section. 3.3.3 Permeability Testing Like the moisture sensitivity testing described above, permeability testing was performed to obtain a relative measuremen t of the mixture’s resistance to water permeability between rounds one and two of a pa rticular aggregate type, not necessarily between mixtures of different aggregate types. The addition of more coarse aggregate, resulting in a more continuous gradation clos er to the maximum density line, was thought to perhaps reduce the permeability of the mixture. The test method used to determine the pe rmeability of a mixture was Measurement of Water Permeability of Compacted Asphalt Paving Mixtures, Florida Method of Test FM 5-565, with the addition of a vacuum sa turation. The basic test procedure is performed as follows: Samples are batched in the laboratory to a predetermined weight that will result in compacted specimens of 7.0 +/0.5 percent air voids when compacted to a height of approximately 115 mm. Specimen diam eter is 150 mm. Three specimens are used for permeability testing. The top 50 mm of each gyratory specimen is then removed from the remaining portion of the specimen by saw cutting using a diamond tipped blade which is cooled and lubricated with a stream of wa ter. This prevents smearing of the asphalt binder during the cut, which would clog the permeable pores. The samples are then vacuum saturated under water for five minutes at a vacuum of 380 mm of mercury. The samples are then placed in the falling head permeability apparatus shown in Figure 3-5.

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25 Figure 3-5 Permeability test apparatus

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26 The time is recorded to flow 500 milli liters of water through the specimen. Additionally, the water temperature is reco rded so that a temperature correction factor can be applied to correct the perm eability readings to a standard reference temperature of 20 C. The permeability value for the three speci mens is then averaged to obtain an average permeability value for the mix design. 3.3.4 Asphalt Pavement Analyzer Testing The Asphalt Pavement Analyzer (APA) was one of two devices used to assess the rutting performance of the asphalt mixtures. The other device was the Servopac gyratory compactor, which will be discussed in a subs equent section. The APA is essentially a wheel tracking device that appl ies a repeating load to a cylindrical asphalt specimen and the rut depth is determined after 8,000 cycl es, or 16,000 passes (Figure 3-6). The test procedure followed is Determining Rutting Susceptibility of Asphalt Paving Mixtures Using the Asphalt Pavement Analyzer (APA), AASHTO designation TP 63-03. The highlights of the test procedure are given below. Samples are batched in the laboratory to a predetermined weight that will result in compacted specimens of 7.0 +/1.0 percent air voids when compacted to a height of 75 mm. Specimen diameter is 150 mm Four specimens are used for APA testing. As a comparison, additional samples were batched in the laboratory to a predetermined weight that would resu lt in compacted specimens of 4.0 +/1.0 percent air voids when compacted to a height of 115 mm at 100 gyrations. Specimen diameter is 150 mm. Four specimens were used for APA testing. Specimens are placed in the APA molds (t wo per mold) with the top side of the specimens facing up. The top side of the sp ecimen is the side that was in contact with the ram head of the gyratory compact or. Specimens (in the molds) are then heated to 64 C for approximately 16 hours. The specimens are then placed in the 64 C heated APA testing chamber where a seating load of 25 cycles is applied to the specimens. The load is comprised of a 445 N load applied on top of a 19.0 mm diameter hose inflated to 700 kPa (Figure 3-7).

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27 Figure 3-6 Asphalt Pavement Analyzer A measuring template is then placed on th e top of the mold and an initial depth reading is obtained using a digital measur ing device. The template contains four measuring slots, two per specimen. The specimens are then placed in the 64 C heated testing chamber and 8000 additional load cycles are applie d, as described in step four. A final depth reading is then obtained at each of the four measuring slots. The rut depth is taken as the difference betw een the initial and final readings. In addition to the method described above for measuring the rut depths, a recently developed method for measuring the rut prof ile (Drakos 2003) was used for the seven percent air void specimens. Instead of using the conventio nal digital measuring device with a small roller on the end to measure a single point maximum rut depth, the new

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28 Figure 3-7 Asphalt Pavement An alyzer loading apparatus method uses a modified measuring plate and c ontour gage that measures the entire rut profile. The profile is measured at thr ee longitudinal locations for each cylindrical specimen (Figure 3-8). The contour gage is then placed in a specia lly made holder and the contour is traced onto a paper card. The specially made holder establishes a consistent orientation and reference system for each rut prof ile that is traced (Figure 3-9). The line trace on the card is then electroni cally scanned and a best fit line is fitted to the electronic trace using computer softwa re. Through integration of the equation of the line, the area between the line and the xaxis is determined. This procedure is conducted for the initial trace after 25 rut cycl es and the final trace after 8000 additional

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29 Figure 3-8 Measuring plate and contour gage for modi fied measuring technique Figure 3-9 Holder, contour ga ge and rut profile trace cycles. The initial area is then subtracted fr om the final area and a percent area change is determined. If the percent area change is positive, then Drakos (2003) concluded that

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30 instability rutting has occurred and if the percent area change is negative, then consolidation rutting has occurred. In addition to the area change, the maximu m single point absolute rut depth (ARD) and the maximum single point differential rut depth (DRD) can be de termined from the profile traces. This is illustrated in Figure 3-10. 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 0123456 Lateral Location, X (in)Deformation Depth, Y (in) Initial Profile Profile after 8000 cycles ARD DRD Figure 3-10 Illustration of absolute rut depth and differential rut depth The absolute rut depth determined from the profile traces is the same form of rut depth measured using the conventional meas uring device described previously. The differential rut depth measurement includes th e absolute rut depth plus the shoving or heaving that occurs with mixtures that experien ce instability rutting.

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31 3.3.5 Servopac Gyratory Compactor Testing The second testing device used to examin e the mixtures’ rutting potential was the Servopac gyratory compactor located at the University of Florida Civil Engineering asphalt laboratory. This de vice performs the same func tions as the Pine gyratory compactor mentioned previously. In additi on, it has the ability to measure the force required to maintain the angle of compacti on and this force is then converted to a “gyratory shear stress.” The Servopac compactor generates an output file displaying the angle of gyration, the gyratory shear stress, the internal angle and the sample height. Another useful feature of the Servopac comp actor is the ability to quickly change the angle of gyration by simply inputting th e desired angle into the computer input screen. The standard angle of compaction per AASHTO standards is 1.25 degrees. For this study, mixtures were compacted at 1.25 and 2.50 degrees per the procedure described below. Roque et al. (2004a) developed a new pro cedure using the Servopac compactor for evaluating the rutting potential of mixtures. The procedure results in two parameters: the gyratory shear slope and the vertical failure strain. Two asphalt mixture specimens are compact ed at an angle of 1.25 degrees to the maximum design number of gyrations (Nmax). Nmax for this study was 160 gyrations. The bulk specific gravity of each specimen is determined and air voids are calculated based on the maximum specific gravity of the mixture. Based on the height measurements reco rded during compaction, the percent air voids at each gyration le vel is backcalculated. A graph is created plotting the measured gyratory shear versus the natural log of the number of gyratory revolutions. The slope of the graph is obtained in th e range corresponding to seven to four percent air voids or to the maximum gyratory shear if this is reached prior to four

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32 percent air voids. This value is design ated the “gyratory shear slope” and is an indicator of the mixture’s resistan ce to deformation (Figure 3-11). y = 25.13x + 625.75 R2 = 0.98 720 725 730 735 740 745 3.03.54.04.55.0 Natural Log RevolutionsGyratory Shear (kPa) 7% to 4% air voids Gyratory shear slope = 25.13 Figure 3-11 Gyratory shear slope Two additional asphalt specimens are prepared and compacted at an angle of 1.25 degrees until the gyration corresponding to seven percent air voids is reached. At this point the machine is stopped for appr oximately fifteen seconds while the angle of gyration is changed to 2.5 degrees. Th en the sample is gyrated for another 100 gyrations. Changing the angle of compac tion causes an unstable condition in the mixture resulting in a shear fa ilure. The behavior of th e mixture during this period provides a further indication of rutting potential and nature of the mixture. The gyratory shear versus the number of revolutions is plotted. The “vertical failure strain” is then calculated from the point of angle change to the local minimum in gyratory shear strength (Figur e 3-12). This strain measurement is during the point of aggregate rearrang ement caused by changing the angle of compaction and is an indicator of the stabil ity characteristics of the mixture. The strain value is calculated by taking the change in gyrator y pill height divided by the initial pill height at the point of angle ch ange. The magnitude of the strain is an indicator of whether the mi xture is brittle, plastic or somewhere in between. A

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33 framework for evaluating mixtures based on the work of Roque et al. (2004a) is shown in Figure 3-13. 600 650 700 750 800 850 900 1163146617691106121136 Number of Gyratory RevolutionsGyratory Shear (kPa) Average Specimen #1 Specimen #2 Change in angle from 1.25 to 2.50 Vertical failure strain region Gyratory shear local minimum Figure 3-12 Vertical failure strain 3.3.6 Superpave Indirect Tension Testing The evaluation of the mixtures’ resist ance to top-down cracking was evaluated using the Superpave indirect tension test (IDT) and the procedure deve loped at the University of Florida. Top-down cracking is the primary mode of pavement distress in Florida. Approximately 80 percent of the Stat e’s deficient highways are deficient due to top-down cracking. The research conducted at the University of Florida has been on going for many years and many papers have been published. Roque et al. (2004b) summarized the work to date and presented their framework for energy based criteria

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34 Gyratory Shear Slope Low High Vertical Failure Strain LowHigh Brittle Mixtures Optimal Mixtures Plastic Mixtures Mixtures with Low Shear Resistance Figure 3-13 Framework for evaluating mixtures related to top-down cracking in asphalt mixtures. The highlights of the procedure and analysis technique will be discussed below: 150 mm diameter gyratory compacted speci mens of approximately 115 mm tall at an air void content of 7 +/ 1 percent air voids are prepared. From these specimens, the top and bottom are trimmed off using a wet saw and then the remainder of the specimen is cut in half resulting in tw o specimens approximately 50 mm thick. The specimens are dried and gage points ar e applied to both faces. The specimens are then further dried in a dehumidif ying chamber and brought to a testing temperature of 10 C. Three different tests are performed on each of three specimens in sequential order. The final results are therefore based on the average of three specimens. A MTS closed loop servo hydraulic system was used for all Superpave IDT testing. The resilient modulus and Poisson’s rati o are determined by applying a haversine wave load for 0.1 seconds followed by a rest period of 0.9 seconds. A creep test is performed in which a cons tant load is applied for 1000 seconds. Several parameters are determined from this test including the creep compliance, creep rate and m-value, which is an indicat ion of the mixture’s resistance to creep. An indirect tensile strength test is perfo rmed at a rate of 50 mm/min. The tensile strength is determined at the point wher e the plot of the vertical deformations

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35 minus the horizontal deformations versus time reaches a peak. Figure 3-14 shows a test specimen. Figure 3-14 Superpave indirect tension test The key parameter calculated is the energy ratio and is defined as the dissipated creep strain energy threshol d of a material divided by the minimum dissipated creep strain energy needed. The dissipated creep st rain energy (DCSE) of a material is defined as the fracture energy (FE) minus the elastic energy (EE) and is shown in Figure 3-15. The minimum dissipated creep stain ener gy required is a function of material properties and the pavement structure. The relationship is described as: DCSEmin = m2.98*D1/A where, m and D1 are parameters derived from the creep test.

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36 x Stress, Strain, St (Strength) f(Fracture) DCSE MRFracture EnergyMREE x Stress, Strain, x Stress, Strain, Stress, Strain, St (Strength) f(Fracture) St (Strength) f(Fracture) DCSE MRFracture EnergyMREE Figure 3-15 Dissipated creep strain energy The term “A” accounts for the tensile stre sses induced in the pavement by vehicle loads and the tensile st rength of the material. “A” is defined as: A = 0.0299* -3.10(6.36-St)+2.46*10-8 where, is the tensile stress indu ced in the pavement and St is the tensile strength of the material. For this st udy, a standard value of 100 lb/in2 was used for Roque et al. (2004b) developed the follo wing criteria for acceptable cracking performance. The DCSE of the mate rial should be gr eater than 0.75 kJ/m3 and the energy ratio should be greater than or equa l to one. The researchers propose higher energy ratio values for higher traffic levels. The aforementioned research was conduc ted on specimens that had only been short-term conditioned in accordance with the AASHTO procedure “Mixture

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37 Conditioning of Hot-Mix Asphalt (HMA), AA SHTO designation R 30-02.” Short-term conditioning is supposed to represent the agi ng that plant produced mix will experience during the mixing and compaction process. Testing identical to that mentioned previously was conducted on specimens that had been long-term oven aged (LTOA) in accordance with AASHTO R 30-02. LTOA aging is intended to represent the aging that the mixture will experience after seven to ten years of service. The procedure for longterm aging is to place samples that have alr eady been short-term oven aged into an oven at 85 C for 120 hours. This testing was conduc ted to see if the asphalt mixtures performed in a similar manner or not compared to the short-term aged samples. In addition to the short-term and long-term oven aged samples, an additional set of samples were prepared that were moisture conditioned. Birgisson et al. (2003) found that the Superpave IDT tests and data analysis t echniques used to characterize a mixture’s resistance to cracking is also successful at identifying a mixture’s susceptibility to moisture damage. The moisture conditioning and testing procedure consists of: Uncut gyratory compacted specimens are va cuum saturated to a saturation level between 65 and 80 percent. The specimens are then placed in a 60 C water batch for 24 hours. After removal from the water bath the sp ecimens are allowed to dry at ambient room conditions for twelve hours, after whic h they are cut to a thickness of 50 mm. The suite of Superpave IDT tests is then performed on the specimens as described previously. The Superpave IDT testing was performed on moisture conditioned specimens to provide an additional means of assessing the mi xtures’ moisture sensitivity in addition to the moisture testing conducted per FM 1-T 283 described previously. The emphasis was

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38 to examine the effects on moisture sensitivity between rounds one and two for each mixture type, not necessarily between mi xtures of different aggregate types.

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39 CHAPTER 4 TEST RESULTS AND ANALYSIS 4.1 Introduction The test results and analysis will be presen ted categorically in the following order: mix design, rutting (APA and Servopac), cr acking, moisture damage and permeability. 4.2 Mix Design 4.2.1 Mix Design Test Results Following are tables of volumetric mix desi gn data and VMA plots for each of the four mixture types. Each table and plot contains data for rounds one and two. Table 4-1 Volumetric mix design data for Alabama limestone mixtures Round #1 Gap Graded Mixture Dust Ratio 3.82.5812.7440.54%3.281.02.4096.714.354 4.32.5662.7500.63%3.700.92.4235.614.361 4.82.5462.7500.62%4.210.82.4454.013.972 5.32.5272.7510.63%4.700.72.4682.313.683 5.82.5102.7540.67%5.170.72.4711.513.989 Round #2 Continuous Graded Mixture Dust Ratio 3.62.5962.7520.53%3.091.12.4176.914.251 4.12.5742.7500.50%3.620.92.4385.313.962 4.62.5562.7530.53%4.090.82.4554.013.771 5.12.5392.7560.58%4.550.72.4752.513.581 5.62.5172.7530.53%5.100.72.4861.313.591 PbeGmb Percent AC GmmGsePba Percent AC GmmGsePbaPbeGmb VMAVFA Air VoidsVMAVFA Air Voids

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40 13.0 13.5 14.0 14.5 3.54.04.55.05.56.0 Asphalt Binder (percent)VMA (percent) Round 1 Round 2 Figure 4-1 VMA plots for Al abama limestone mixtures Table 4-2 Volumetric mix design data for Brooksville limestone mixtures Round #1 Gap Graded Mixture Dust Ratio 6.52.3192.5404.53%2.272.32.1766.110.944 7.02.3052.5424.55%2.761.92.1895.010.954 7.52.2952.5494.67%3.181.72.2093.810.664 7.92.2742.5374.47%3.781.42.2053.011.173 8.42.2612.5394.51%4.261.22.2112.211.481 Round #2 Continuous Graded Mixture Dust Ratio 6.02.3352.5404.55%1.723.12.1746.910.535 6.52.3192.5404.54%2.252.42.1935.410.247 7.02.3032.5394.53%2.781.92.2143.99.961 7.52.2912.5434.60%3.241.62.2163.310.268 8.02.2752.5424.58%3.791.42.2371.79.983 VMAVFA Air VoidsVMAVFA Air Voids GsePbaPbeGmb GsePba Percent AC Gmm PbeGmb Percent AC Gmm

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41 9.5 10.0 10.5 11.0 11.5 6.06.57.07.58.08.5 Asphalt Binder (percent)VMA (percent) Round 1 Round 2 Figure 4-2 VMA plots for Brooks ville limestone mixtures Table 4-3 Volumetric mix design data for Nova Scotia granite mixtures Round #1 Gap Graded Mixture Dust Ratio 4.82.4712.6590.39%4.431.22.3046.816.759 5.32.4562.6620.45%4.881.12.3285.216.268 5.82.4352.6580.39%5.441.02.3364.116.475 6.32.4202.6610.43%5.890.92.3592.516.084 6.82.4022.6610.42%6.410.82.3671.516.291 Round #2 Continuous Graded Mixture Dust Ratio 4.62.4732.6520.30%4.321.22.3007.016.658 5.12.4552.6520.30%4.821.12.3225.416.367 5.62.4392.6540.33%5.291.02.3463.815.876 6.12.4212.6540.32%5.800.92.3582.615.984 6.62.4042.6540.33%6.290.82.3711.415.991 PbeGmb Percent AC GmmGsePba Percent AC GmmGsePbaPbeGmb VMAVFA Air VoidsVMAVFA Air Voids

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42 15.5 16.0 16.5 17.0 4.55.05.56.06.57.0 Asphalt Binder (percent)VMA (percent) Round 1 Round 2 Figure 4-3 VMA plots for Nova Scotia granite mixtures Table 4-4 Volumetric mix design data for Tarmac limestone mixtures Round #1 Gap Graded Mixture Dust Ratio 6.32.3142.5262.48%3.981.32.1726.114.558 6.82.3022.5302.55%4.431.22.1944.714.167 7.32.2912.5352.64%4.861.12.2043.814.273 7.82.2732.5312.57%5.431.02.2242.213.984 8.32.2562.5282.52%5.990.92.2321.114.192 Round #2 Continuous Graded Mixture Dust Ratio 5.62.3352.5252.46%3.271.62.1866.413.352 6.12.3202.5252.47%3.781.42.1975.313.360 6.62.3052.5262.48%4.281.22.2143.913.170 7.22.2832.5212.40%4.971.12.2192.813.579 7.62.2752.5262.49%5.301.02.2441.412.990 PbeGmb VMAVFA Air VoidsVMAVFA Air Voids Percent AC GmmGsePba PbeGmb Percent AC GmmGsePba

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43 12.5 13.0 13.5 14.0 14.5 15.0 5.56.06.57.07.58.08.5 Asphalt Binder (percent)VMA (percent) Round 1 Round 2 Figure 4-4 VMA plots for Tarmac limestone mixtures 4.2.2 Mix Design Summary Examination of the VMA curves generally shows the expected concave shaped curve for each mixture. Theoretically, it is desirable to have a design asphalt binder content that is either at the minimum point on the VMA curve or to the left of the minimum point. In this range of binder cont ents, the slight addition of additional binder, which could occur during production, will not increase the VMA. An increase in VMA (i.e. to the right side of the minimum point ) is thought to push the aggregate skeleton apart and reduce shear resistance, which is related to rutting. It appears that for all of the mixtures designed for this study that the optim um binder content is either at the minimum of the curve or to the le ft side of the minimum. It should be noted that the amount of coar se aggregate added to each mix design for round 2 was almost identical for all of the aggregate types, however, the reduction in

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44 VMA between rounds one and two was significantly different for each aggregate type. The reduction in VMA for each aggregate type is shown in Table 4-5. Table 4-5 VMA difference between rounds one and two VMA Round 1Round 2Difference Alabama limestone13.913.70.2 FL Brooksville limestone10.69.90.7 Nova Scotia granite16.415.80.6 Fl Tarmac limestone14.213.11.0 Aggregate Type 4.3 Rutting 4.3.1 APA Test Results There were five different parameters dete rmined with the APA considering various methods of sample preparation and data in terpretation. The five parameters are: Absolute rut depth using the conventiona l one-point measuring device testing 75 mm tall gyratory compacted specimens compact ed to an air void content of 7.0 +/1.0 percent. Absolute rut depth using the conventiona l one-point measuring device testing 115 mm tall gyratory compacted specimens compact ed to an air void content of 4.0 +/0.5 percent. Absolute rut depth using the complete pr ofile measuring device testing 75 mm tall gyratory compacted specimens compacted to an air void content of 7.0 +/1.0 percent. Differential rut depth using the complete profile measuring device testing 75 mm tall gyratory compacted specimens compacted to an air void c ontent of 7.0 +/1.0 percent. Percent area change using the complete pr ofile measuring device testing 75 mm tall gyratory compacted specimens compacted to an air void content of 7.0 +/1.0 percent. The results for each of the five parameters for all of the mixture types are presented in Table 4-6. Each value in Table 4-6 represents the average of four specimens.

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45 Table 4-6 APA test results Round 1Round 2Round 1Round 2Round 1Round 2Round 1Round 2 5.54.21.51.35.94.52.51.5 3.54.41.11.23.74.32.61.9 5.13.71.40.55.93.11.80.9 10.08.33.63.111.78.05.43.8 0.880.82-1.00-0.271.821.69-0.37-0.10 Profile measuring device 7% Va, 75 mm tall Differential Rut Depth (mm) Percent Area Change Single point measuring device 7% Va, 75 mm tall Single point measuring device 4% Va, 115 mm tall Profile measuring device 7% Va, 75 mm tall Profile measuring device 7% Va, 75 mm tall FL Tarmac limestone Absolute Rut Depth (mm) Absolute Rut Depth (mm) Absolute Rut Depth (mm) APA Parameter Alabama limestone FL Brooksville limestone Nova Scotia granite Figure 4-5 displays the absolute rut depths measured by the conventional measuring device and profile measuring device and the differential rut depths measured by the profile measuring device. In theor y, the absolute rut depths measured by the conventional measuring device and the profile measuring device should be approximately the same, and this is displayed in Figure 4-5. The differential rut depths follow the same trend as the absolute rut depths but with greater magnitude, as expected. Figure 4-6 displays the absolute rut depths measured by the conventional measuring device for the specimens compacted to seven percent air voids and a height of 75 mm. Figure 4-7 displays the absolute ru t depths measured by the profile measuring device and Figure 4-8 displays the differen tial rut depths meas ured by the profile measuring device for the same specimens. In a ll cases, there is a significant decrease in rut depth from round one to round two implyi ng that adding more coarse aggregate is beneficial in reducing the rutti ng susceptibility of the mixtures.

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46 0 2 4 6 8 10 12 14 AL-1AL-2BV-1BV-2NS-1NS-2TM-1TM-2 Aggregate Type and Round NumberRut Depth (mm) Absolute Rut Depth Conventional Measuring Device Absolute Rut Depth Profile Measuring Device Differential Rut Depth Profile Measuring Device Figure 4-5 Comparison of APA measurement methods for 7% air voids, 75 mm tall specimens 0 1 2 3 4 5 6 7 Alabama LimestoneFL Brooksville Limestone Nova Scotia GraniteFL Tarmac Limestone Aggregate TypeAbsolute Rut Depth (mm) Round 1 Round 2 Figure 4-6 APA absolute rut depth using conven tional measuring device for 7% air voids, 75 mm tall specimens

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47 0 1 2 3 4 5 6 7 Alabama LimestoneFL Brooksville Limestone Nova Scotia GraniteFL Tarmac Limestone Aggregate TypeAbsolute Rut Depth (mm) Round 1 Round 2 Figure 4-7 APA absolute rut depth using prof ile measuring device fo r 7% air voids, 75 mm tall specimens 0 2 4 6 8 10 12 14 Alabama LimestoneFL Brooksville Limestone Nova Scotia GraniteFL Tarmac Limestone Aggregate TypeDifferential Rut Depth (mm) Round 1 Round 2 Figure 4-8 APA differential rut depth using prof ile measuring device for 7% air voids, 75 mm tall specimens

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48 Figure 4-9 displays the percent area cha nge for each aggregate type and round. A positive area change indicates instability rutting manifest ed by shoving and heaving of the mixture on each side of the rut. A nega tive percent area change indicates that the majority of the rutting was due to consolid ation. The percent area change decreased for every aggregate type from round one to round two, a further indicati on that the addition of coarse aggregate was benefi cial in reducing the rutting su sceptibility of the mixtures. The two aggregate types with the lowest amount of rutting (Brooksville and Tarmac Florida limestones) had negative percent area ch anges indicating that the small amount of rutting those mixtures expe rienced was primarily due to consolidation. The two aggregate types with the larg est amount of rutting (Alabama limestone and Nova Scotia granite) had positive percent area changes indicating that the rutting those mixtures experienced was primarily due to instability under a load. -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 Alabama LimestoneFL Brooksville Limestone Nova Scotia GraniteFL Tarmac Limestone Aggregate TypeAPA Percent Area Change Round 1 Round 2 Figure 4-9 APA percent area change using pr ofile measuring device for 7% air voids, 75 mm tall specimens

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49 Figure 4-10 displays the absolute rut depths measured by the conventional measuring device for the specimens compacted to four percent air voids and a height of 115 mm. In contrast to the specimens compact ed to seven percent air voids and a height of 75 mm, the rut depths increased from round one to round two for the Alabama limestone and the Nova Scotia granite mixtur es. The rut depth was essentially the same between rounds one and two for the Florid a Brooksville limestone. The rut depth decreased for the Tarmac Florida limest one between rounds one and two. It is undetermined why there is a difference in tr ends between rounds one and two for the different sample types. Howe ver, it is noted that the two mixtures (Alabama limestone and Nova Scotia granite) that showed an increase in rut depth between rounds one and two for the specimens compacted to four perc ent air voids and a he ight of 115 mm were the same mixtures that expe rienced instability rutting. 0 1 2 3 4 5 6 7 Alabama LimestoneFL Brooksville Limestone Nova Scotia GraniteFL Tarmac Limestone Aggregate TypeAbsolute Rut Depth (mm ) Round 1 Round 2 Figure 4-10 APA absolute rut depth using c onventional measuring device for 4% air voids, 115 mm tall specimens

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50 Figure 4-11 displays the APA rut depth ve rsus the VMA for the eight mixtures examined in this study. The APA rut de pth is measured us ing the conventional measuring device for the specimens compacted to seven percent air voids and a height of 75 mm. The plot of the data shows a st rong correlation between rut depth and VMA (R2 = 0.70). For these eight mixtures, as the VM A increased, the rut depth increased. This effect is reasonable since highe r VMA at a fixed air void content means that there is more effective binder in the mix, which means th ere is more void space between the aggregate particles, less stone on stone contact and a poten tially less stable aggregate structure. A similar correlation existed between APA rut depth and VFA. AL-1 AL-2 BV-1 BV-2 NS-1 NS-2 TM-1 TM-2y = 0.0014x2.9439R2 = 0.700 1 2 3 4 5 6 7 91011121314151617 VMA (%)Absolute Rut Depth (mm ) Figure 4-11 APA rut depth versus VMA using conventional measuring device for 7% air voids, 75 mm tall specimens Figure 4-12 displays the APA rut depth ve rsus the dust to effective binder ratio (commonly called the dust ratio ) for the eight mixtures examined in this study. The APA rut depth is measured using the conven tional measuring device for the specimens

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51 compacted to seven percent air voids and a heig ht of 75 mm. The plot of the data shows a strong correlation between rut depth and dust ratio (R2 = 0.79). For these eight mixtures, as the dust ratio increased, the rut de pth decreased. This is reasonable since the dust mixes with and stiffens the binder, hen ce increasing the rutting resistance of the mixture. AL-1 AL-2 BV-1 BV-2 NS-1 NS-2 TM-1 TM-2 y = 3.6193x-1.7889R2 = 0.79 0 1 2 3 4 5 6 7 0.51.01.52.0 Dust to Effective Binder RatioAbsolute Rut Depth (mm) Figure 4-12 APA rut depth versus dust to effective binder ratio using conventional measuring device for 7% air vo ids, 75 mm tall specimens 4.3.2 APA Summary Two measurement techniques were used for obtaining APA rut depths; the conventional single point absolute rut dept h using a digital micrometer and the new profile measuring device which provided the full rut profile of the mixture and provided for the determination of the absolute and differential rut depths and the percent area change of the rut profile. The absolute rut dept hs measured by the conventional and new measuring techniques compared very well.

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52 APA results for the specimens compacted to seven percent air voids and a 75 mm height revealed that the addition of more coarse a ggregate to the 12.5 mm and 9.5 mm sieves, resulting in a more c ontinuous graded mixture, impr oved the rutting performance with respect to absolute and differential rut depths and percent area change regardless of aggregate type. Examining all eight mixtures as a group re vealed a strong correl ation showing that increasing VMA resulted in an increase in rut depth. An even stronger correlation showed that increasing the dust to effective bi nder ratio resulted in a decrease in rut depth. 4.3.3 Servopac Test Results There were three different parameters determined with the Servopac gyratory compactor. The three parameters are Gyratory shear slope: A graph is create d plotting the gyratory shear measured by the Servopac compactor versus the natural log of the number of gyratory revolutions. The air voids at each gyratory revolution are computed. The gyratory shear slope is the slope of the graph in th e range of compaction from seven to four percent air voids or to the maximum gyratory shear if this value is reached prior to four percent air voids. This value describes the rate at which the mixture develops shear resistance and is an indication of the mixture’s resistance to deformation. Vertical failure strain: Specimens are compacted at an angle of 1.25 degrees until the specimens reach seven percent air voids The angle of compaction is changed to 2.50 degrees and the sample is gyrated for another 100 gyrations. The gyratory shear versus the number of revolutions is plo tted. The vertical failure strain is then calculated from the point of angle change to the local minimum in gyratory shear strength. This strain measurement is an indicator of the stability characteristics of the mixture. The magnitude of the strain is an indicator of wh ether the mixture is brittle, plastic or somewhere in between. Maximum gyratory shear strength: This is the maximum shear strength achieved during the compaction process.

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53 The values for gyratory shear slope, verti cal failure strain and maximum gyratory shear strength for all of the mixture types are presented in Table 4-7. Each value in Table 4-7 represents the average of two specimens. Table 4-7 Servopac test results Round 1Round 2Round 1Round 2Round 1Round 2Round 1Round 2 251123716133235 7.0 to 4.07.0 to 4.27.0 to 6.27.0 to 6.77.0 to 6.77.0 to 5.77.0 to 4.07.0 to 4.5 1.831.67n/an/an/a1.972.131.66 744726772775679689750751 3.14.26.26.76.75.73.64.5 FL Tarmac limestone Gyratory Shear Slope Percent Vertical Strain Servopac Parameter Alabama limestone FL Brooksville limestone Nova Scotia granite Percent Air Void Range Percent Air Voids Maximum Shear Stress (kPa) 4.3.3.1 Gyratory shear slope An example of a gyratory shear slope graph is shown in Figure 4-13 for the Alabama limestone round one mixture. Six of the eight mixtures had a peak gyratory shear strength prior to reaching a compacti on level of four per cent air voids. The gyratory shear slope is then de fined as the slope of the graph from the seven percent air void level to the air void leve l at the point of maximum gyratory shear strength. An example of this is shown in Figure 4-14 fo r the Florida Brooksville limestone round one mixture. The gyratory shear slope decreased for th e Alabama limestone, Florida Brooksville limestone and Nova Scotia granite mixtures from round one to r ound two indicating that the round two mixtures did not develop shear resistance as rapidly as the round one mixtures. The Florida Tarmac mixture had a slight increase in gyr atory shear slope from round one to round two. Roque et al. (2004a) indicated that mixtures with a gyratory shear slope of less than 14 were undesirable with respect to ru tting. All of the round one

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54 y = 25.13x + 625.75 R2 = 0.98 720 725 730 735 740 745 3.03.54.04.55.0 Natural Log RevolutionsGyratory Shear (kPa) 7% to 4% air voids Gyratory shear slope = 25.13 Figure 4-13 Gyratory shear slope for Alabama limestone round one mixture y = 22.96x + 671.52 R2 = 0.83 y = 50.03x + 560.35 R2 = 0.99 y = -54.44x + 1012.75 R2 = 0.93 730 735 740 745 750 755 760 765 770 775 780 3.03.23.43.63.84.04.24.44.64.85.0 Natural Log RevolutionsGyratory Shear (kPa) 7% to 6.2% air voids 10% to 7% air voids 6.2% to 4% air voids Figure 4-14 Gyratory shear slope for Florid a Brooksville limestone round one mixture

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55 mixtures had a gyratory shear sl ope greater than 14. Only one of the round two mixtures (Florida Tarmac limestone mixture) had a gyratory shear slope greater than 14. 4.3.3.2 Vertical strain Roque et al. (2004a) indicated that mixtures with a vertical strain in the range of 1.4 to 2.0 percent were desirable, whereas mixtures with a vertical strain less than 1.4 percent would be considered “brittle” and mixtures wi th a vertical strain greater than 2.0 percent would be considered “plastic.” Examinati on of the data reveal s that there was an improvement from round one to round two fo r three of the four mixtures (Alabama limestone, Nova Scotia granite, and Florida Tarmac limestone). Th e round two vertical strain values for these mixtures were in the desirable range. It a ppears that the addition of coarse aggregate to the r ound two mixtures improved the ve rtical strain values. An example of a plot of the gyratory shear vers us the number of revolutions is shown in Figure 4-15 for the Alabama limestone round two mixture. The gyratory shear peaks initially after the ch ange in compaction angle to 2.50 degrees at a compaction level of seven percent air voids and then drops to a local minimum as the particles rearrange themselves. Shear strength then builds slow ly and reaches a final peak before dropping off. Rounds one and two of the Florida Brooksvi lle limestone mixtur e and round one of the Nova Scotia granite mixture never re ached a local minimum in gyratory shear strength after the compaction angle was changed to 2.50 degrees and hence had no vertical strain value to report. This is indicated as an “n/a” in Table 4-7. It appears that these mixtures were never able to recover st rength after the angle change in compaction at the seven percent air void level. An example of the gyr atory shear versus the number

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56 200 400 600 800 1000 1163146617691106121136 # RevolutionsGyratory Shear Change in angle from 1.25 to 2.50 Vertical failure strain region Gyratory shear local minimum Figure 4-15 Vertical strain for Al abama limestone round two mixture of revolutions for this c ondition is shown in Figure 416 for the Florida Brooksville limestone round one mixture. To determine if a mixture would recover shear strength when the angle of compaction was changed at a different air void content other than seven percent it was decided to make two mo re specimens of the Nova Scotia round one mixture and change the angle of compaction at an air void content of nine percent instead of seven percent. Figure 4-17 shows the gyr atory shear versus the number of revolutions for both of these conditions. It can be seen that the shear strength did recover slightly when the angle of compaction was changed at nine percent air voids. This demonstrates that some mixtures gain strength rapidly a nd peak at higher air void contents than other mixtures.

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57 0 200 400 600 800 1000 1200 1163146617691106121136151 # RevolutionsGyratory Shear (kpa) N o local minimum Change in angle from 1.25 to 2.50 Figure 4-16 Vertical strain for Florid a Brooksville limestone round one mixture 0 200 400 600 800 1000 1163146617691106121136151 # RevolutionsGyratory Shear (kPa) Angle change at 9% air voids Angle change at 7% air voids Figure 4-17 Vertical strain for Nova Scotia granite round one mixtures

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58 4.3.3.3 Maximum shear stress The gyratory shear strength versus the percent air voids during compaction is shown for rounds one and two of each aggr egate type in Figures 4-18 through 4-21. Examination of the data in Table 4-7 and Figures 4-18 through 4-21 reveals that the maximum shear strength peaks at higher air void contents for the round two mixtures of the Alabama limestone, Florida Brooksvill e limestone and Florida Tarmac limestone mixtures. A possible explanation of this is that these round two mixtures have a more continuous gradation and less VMA than thei r round one counterparts resulting in more aggregate interlock at higher air voids and le ss asphalt binder to act as lubrication in the compaction process. Additionally, the Alabama limestone a nd Florida Tarmac limestone mixtures tended to peak at lower air vo id contents (3.1 to 4.2 percent range) and then start to lose strength. The Florida Brooksvill e limestone and Nova Scotia granite mixtures peaked at higher air void contents (5.7 to 6.7 pe rcent range) and then lost strength. The maximum gyratory shear stress for each aggregate type correlated with the APA rut depth for the seven pe rcent air void specimens. Higher gyratory shear stress values in the Servopac compactor were equivale nt to lower rutting values in the APA (see Figure 4-22).

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59 0 100 200 300 400 500 600 700 800 0 5 10 15 20 25 Percent Air VoidsGyratory Shear (kPa) Round 1 Round 2 Figure 4-18 Gyratory shear versus percent air voids for Alabama limestone mixtures 0 100 200 300 400 500 600 700 800 900 0 5 10 15 20 25 30 Percent Air VoidsGyratory Shear (kPa) Round 1 Round 2 Figure 4-19 Gyratory shear versus percent air voids for Florida Brooksville limestone mixtures

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60 0 100 200 300 400 500 600 700 800 0 5 10 15 20 25 Percent Air VoidsGyratory Shear (kPa) Round 1 Round 2 Figure 4-20 Gyratory shear versus percent ai r voids for Nova Scotia granite mixtures 0 100 200 300 400 500 600 700 800 0 5 10 15 20 25 Percent Air VoidsGyratory Shear (kPa) Round 1 Round 2 Figure 4-21 Gyratory shear versus percen t air voids for Florida Tarmac limestone mixtures

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61 AL-1 AL-2 BV-1 BV-2 NS-1 NS-2 TM-1 TM-2 y = 3.6193x-1.7889R2 = 0.79 0 1 2 3 4 5 6 7 660680700720740760780 Servopac Gyratory Shear Strength (kPa)APA Absolute Rut Depth (mm) Figure 4-22 Gyratory shear stress versus APA rut depth 4.3.4 Servopac Summary The Servopac test results fo r vertical strain and maxi mum shear strength correlated well with the APA test results. With respect to vertical strain, the mixtures showed an improvement from round one to round two a nd the results stayed within the desirable range of 1.4 to 2.0 percent. The ranking of the mixtures with resp ect to the APA test results matched the rankings per the maximum gyratory shear test results. There was a decrease in gyratory shear slope from round one to round two for three of the four mixtures, indicating that the round one mixtures develop shear resistance at a faster rate than the round two mixtures, though they do not necessarily achieve a greater maximum shear strength. Some mixtures tend to reach maximum shear strength at much higher air void contents than other mixtures. For the eight mi xtures examined in this study, the air void

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62 content at which maximum shear strength was achieved differed by up to 3.6 percent. Additionally, three of the f our round two mixtures with lower VMA reached maximum shear strength at a higher air void content that their counter part round one mixtures. This reveals one of the main problems with nor mal volumetric mix design procedures, where all mixtures are designed at four percent air voi ds. Some mixtures may be optimal at this air void content and others may not be. This is further justification fo r the need of one or more performance tests for mix design purposes. 4.4 Cracking The Superpave indirect tensi on (IDT) test results can be best described by three parameters; energy ratio, dissipated creep strain energy and fracture energy. Test results are shown in Table 4-8 for the unconditioned and long-term oven aged (LTOA) specimens. Each parameter will be discussed separately below. Table 4-8 Energy ratio values for the unconditioned and LTOA specimen Round 1Round 2Round 1Round 2Round 1Round 2Round 1Round 2 Unconditioned3.363.182.082.533.691.312.641.62 LTOA1.201.935.935.334.043.444.658.58 Unconditioned4.24.11.51.35.74.22.61.6 LTOA1.22.01.40.86.45.01.72.4 Unconditioned4.44.31.71.55.94.42.81.7 LTOA1.42.11.61.16.65.21.92.6 Energy Ratio @ 100 psi Dissipated Creep Strain Energy (kJ/ m 3) Fracture Energy (kJ/m3) FL Tarmac limestone Alabama limestone FL Brooksville limestone Nova Scotia granite Superpave IDT Test Parameter Test Condition 4.4.1 Energy Ratio As described in Chapter 3, the energy ratio is defined as the dissipated creep strain energy threshold of a material divided by the minimum dissipated creep strain energy needed. Roque et al. (2004b) have found this parameter effective in characterizing the cracking performance of asphalt mixtures. Figure 4-23 shows the unconditioned energy ratio and Figure 4-24 shows the LTOA energy ratio for rounds one and two for each mixture type.

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63 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Alabama LimestoneFL Brooksville Limestone Nova Scotia GraniteFL Tarmac Limestone Aggregate TypeEnergy Ratio Round 1 Round 2 Figure 4-23 Energy ratios for unconditioned specimens 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 Alabama LimestoneFL Brooksville Limestone Nova Scotia GraniteFL Tarmac Limestone Aggregate TypeEnergy Ratio Round 1 Round 2 Figure 4-24 Energy ratios for l ong-term oven aged specimens

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64 Figure 4-23 shows that the energy ratio decreased for three of the four unconditioned mixtures (Alabama limestone, Nova Scotia granite and Florida Tarmac limestone). The decrease was significant for the Nova Scotia granite and Florida Tarmac limestone mixtures. It should be noted that these two mixtures ha d significant decreases in VMA from rounds one to two. The energy ratio increased a moderate amount for the Florida Brooksville limestone mixture. The implication is that the addition of coarse aggregate at round two had an overall negative effect on the cracking performance of the mixtures examined in this study. With respect to the LTOA specimens, Figur e 4-24 shows that the results were mixed between rounds one and two. Energy ra tios decreased for the Florida Brooksville limestone mixture and Nova Scotia granit e mixture and increased for the Alabama limestone and Florida Tarmac limestone mixture. Overall, only the energy ratio for the Florida Tarmac limestone mixture changed sign ificantly. The reason for this is unknown. Comparing the unconditioned energy ratios to the LTOA energy ratios reveal a significant increase in energy ratio after ag ing for the Florida Brooksville and Tarmac limestone mixtures. These mixtures contai n aggregates that are highly absorptive compared to the Alabama limestone and Nova Scotia granite mixtures. 4.4.2 Dissipated Creep Strain Energy (DCSE) The DCSE of a mixture describes the amount of energy that a mi xture can dissipate through repeated loading before fracturing. Though the DCSE by itself cannot describe completely the cracking performance of a mixtur e, as a rule of thumb, if other factors are held constant, then a mixture with a greater DCSE will perform better than a mixture with a lower DCSE. Figure 4-25 shows the unc onditioned DCSE and Figure 4-26 shows the LTOA DCSE for rounds one and two for each mixture type.

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65 0.0 1.0 2.0 3.0 4.0 5.0 6.0 Alabama LimestoneFL Brooksville Limestone Nova Scotia GraniteFL Tarmac Limestone Aggregate TypeDissipated Creep Strain Energy (kJ/m3) Round 1 Round 2 Figure 4-25 Dissipated creep strain energy for unconditioned specimens 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Alabama LimestoneFL Brooksville Limestone Nova Scotia GraniteFL Tarmac Limestone Aggregate TypeDissipated Creep Strain Energy (kJ/m3) Round 1 Round 2 Figure 4-26 Dissipated creep strain ener gy for long-term oven aged specimens

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66 Figure 4-25 shows that the DCSE decreased significantly for the Nova Scotia granite and Florida Tarmac limestone unconditi oned mixtures from round one to round two. It should be noted that these two mi xtures had significant d ecreases in VMA from rounds one to two. There was a slight, if not insignificant, decrease in DCSE for the Alabama limestone and Florida Brooksville limestone unconditioned mixtures. The implication is that the addition of coarse aggregate at round two ha d a negative effect on the DCSE of the mixtures examined in this study. With respect to the LTOA specimens, Figur e 4-26 shows that the results were mixed between rounds one and two. DCSE decreased for the Florida Brooksville limestone mixture and Nova Scotia granit e mixture and increased for the Alabama limestone and Florida Tarmac limestone mixture. This is the same trend as occurred for the energy ratios of the LTOA specimens. 4.4.3 Fracture Energy (FE) The FE of a mixture descri bes the total amount of en ergy (elastic energy plus dissipated energy) that a mi xture can withstand before fracturing. Though the FE by itself cannot describe complete ly the cracking performance of a mixture, as a rule of thumb, if other factors are held constant, th en a mixture with a greater FE will perform better than a mixture with a lower FE. Figure 4-27 shows the unconditioned FE and Figure 4-28 shows the LTOA FE for rounds one and two for each mixture type.

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67 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Alabama LimestoneFL Brooksville Limestone Nova Scotia GraniteFL Tarmac Limestone Aggregate TypeFracture Energy (kJ/m3) Round 1 Round 2 Figure 4-27 Fracture energy for unconditioned specimens 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Alabama LimestoneFL Brooksville Limestone Nova Scotia GraniteFL Tarmac Limestone Aggregate TypeFracture Energy (kJ/m3) Round 1 Round 2 Figure 4-28 Fracture energy for long-term oven aged specimens

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68 Figure 4-27 shows that the FE decreased si gnificantly for the Nova Scotia granite and Florida Tarmac limestone unconditioned mi xtures from round one to round two. It should be noted that these two mixtures ha d significant decreases in VMA from rounds one to two. There was a sli ght, if not insignificant, decr ease in FE for the Alabama limestone and Florida Brooksville limestone unconditioned mixtures. The implication is that the addition of coarse aggregate at r ound two had a negative effect on the FE of the mixtures examined in this study. With respect to the LTOA specimens, Figur e 4-28 shows that the results were mixed between rounds one and two. FE decreased for the Florida Brooksville limestone mixture and Nova Scotia granite mixture and increased for the Alabama limestone and Florida Tarmac limestone mixture. This is the same trend as occurred for the energy ratios and DCSE of the LTOA specimens. 4.4.4 Cracking Summary The energy ratio, dissipated creep strain energy and fracture energy test results from the Superpave IDT test indicate that the addition of coarse aggr egate, resulting in a more continuous gradation and reduction in VMA, had an overall negative effect on the cracking performance when examining the unconditioned specimens. Only the energy ratio for the Florida Brooksville limestone mi xture showed an increase from round one to round two. Results were mixed and not conclusive for the LTOA specimens. However, the highly absorptive Florida limestone mixtures showed significant increases in energy ratio for the LTOA specimens compared to the unconditioned specimens. This trend was not evident for the Alabama limestone an d Nova Scotia granite mixtures.

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69 4.5 Moisture Damage Moisture damage was evaluated using the standard Department test procedure (FM 1-T 283) and by using the Superpave IDT te sts on moisture conditioned specimens. From the suite of Superpave IDT tests, th e energy ratio was calculated for both the unconditioned and conditioned specimens. Test results are shown in Table 4-9. Each parameter will be discussed separately below. Table 4-9 Moisture damage test results Round 1Round 2Round 1Round 2Round 1Round 2Round 1Round 2 Unconditioned989941102612087858438551025 Moisture Conditioned764840596698647697699791 Ratio7789585883838277 Unconditioned3.363.182.082.533.691.312.641.62 Moisture Conditioned2.872.661.870.970.441.082.611.50 Ratio8584903812829993 FL Tarmac limestone FM 1-T 283 Tensile Strength (kPa) Energy Ratio Test Method and Condition Alabama limestone FL Brooksville limestone Nova Scotia granite 4.5.1 Conventional FM 1-T 283 Test Results Examination of the data in Table 4-9 does not indicate any trends with respect to tensile strength ratio (TSR). The Alabam a limestone mixture had a twelve percent increase in TSR. The Florida Brooksville li mestone and Nova Scotia granite mixtures showed no change in TSR and the Florida Tarm ac limestone mixture showed a mild five percent reduction in TSR. Ho wever, with respect to the te nsile strengths, every mixture had an increase in unconditioned and conditioned tensile strengths from round one to round two except for the Alabama limestone unconditioned mixture, which had a mild reduction in unconditioned te nsile strength (7 psi) from round one to round two. 4.5.2 Superpave IDT Test Results (Energy Ratio) Examination of the data in Table 4-9 reveals a different outcome than the FM 1-T 283 test results. Only the Nova Scotia gran ite conditioned results showed an increase in

PAGE 82

70 energy ratio from round one to round two. The other three comparisons showed a decrease in energy ratio fr om round one to round two. 4.5.3 Moisture Damage Summary Moisture damage test results were de pendent on the test method used. The standard Department test method, FM 1-T 283, revealed that tensile strengths increased for unconditioned and conditioned specimens from round one to round two. Superpave IDT test results showed that for three of four comparisons, the energy ratio decreased from round one to round two. 4.6 Permeability The permeability values for rounds one and two of each mixture type are presented in Table 4-10. Permeability values were essentially the same between rounds one and two. The addition of coarse aggregate in r ound two did not affect the permeability of the mixture. Perhaps there was an offsetting e ffect between adding more coarse aggregate, which would increase the permeability of the mixture, and the more continuous gradation which being closer to the maximum density line, would tend to decrease permeability. Table 4-10 Permeability test data Round 1Round 2Round 1Round 2Round 1Round 2Round 1Round 2 4115131561439121 24916179136416 134928271396924 n/a31n/an/an/an/a37n/a 2626192010125254 Specimen 2 Specimen 4 Specimen 3 Permeability (x 10-5 cm/s) Specimen Number FL Tarmac limestone Specimen 1 Alabama limestone FL Brooksville limestone Nova Scotia granite Average

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71 CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS 5.1 Conclusions Rutting potential, as measured by the Asphalt Pavement Analyzer (APA), showed an improvement in rut performance by the addition of more coarse aggregate on the 12.5 mm and 9.5 mm sieves, resulting in a more continuous gradation. This improvement was evident with absolute rut depth, differential rut depth and percent area change of the rut profile, when te sting 75 mm tall specimens compacted to seven percent air voids and tested at 64 C. APA test results also showed a strong correlation (R2 = 0.70) that increasing VMA resulted in an increase in rut de pth. An even stronger correlation (R2 = 0.79) showed that increasing the dust to effective binder ratio resulted in a decrease in rut depth. Rutting potential, as measured with the Servopac gyratory compactor, was evaluated with the following parameters: gy ratory shear slope, vertical strain and maximum gyratory shear stress. Vertical strain and maximum gyratory shear stress test results correlated well with the APA test results. With respect to vertical strain, the mixtures showed an improvement from round one to round two and the results stayed within the desirable range of 1.4 to 2.0 percent. The ranking of the mixtures with respect to the APA test result s matched the rankings per the maximum gyratory shear test results. There was a decrease in gyratory shear slope from round one to round two for three of the four mixtures, indicating that the round one mixtures develop shear resistance at a fa ster rate than the round two mixtures. However, the round one mixtures did not necessarily achieve a greater maximum shear strength than the round two mixtures. Servopac test results show that mixtur es achieve their maximum gyratory shear strength over a wide range of air voids compared to each other. Designing all mixtures at four percent air voids may not result in the optimum mixture design for all mixtures with respect to rut resistance. The energy ratio, dissipated creep strain energy and fracture energy test results from the Superpave IDT test indicate th at the addition of coarse aggregate, resulting in a more continuous gradation and reduction in VMA, had an overall negative effect on the cracking performance. Only the energy ratio for the Florida Brooksville limestone mixture showed an increase from round one to round two.

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72 Conclusions with respect to moisture da mage were dependent on the test method used. The standard Department test method, FM 1-T 283, revealed that the addition of coarse aggregate in round two resulted in incr eased tensile strengths for unconditioned and conditioned specimens. However, Superpave IDT test results showed that for three of four comparis ons, the energy ratio decreased with the addition of coarse ag gregate in round two. Permeability characteristics of the mixtures were not affected by the addition of coarse aggregate. Most likely there wa s an offsetting effect between adding more coarse aggregate, which woul d tend to increase the permeability of the mixture, and the more continuous gradation, which bei ng closer to the maximum density line, would tend to decrease permeability. 5.2 Recommendations Cracking is the predominant mode of dist ress (approximately 80%) for the asphalt roads in Florida. For the mixtures eval uated in this study, the addition of coarse aggregate on the 12.5 mm and 9.5 mm sieves indicated an overall reduction in cracking performance. Therefore, it is not recommended at this time to lower the VMA specification requirement for coarse graded mixtures. For situations where rutting performance is a high priority, the addition of coarse aggregate, with the potential for a lo wer than specified VMA, should be considered. The Department should continue work towards the implementation of one or more performance tests at the mix design stage. Possible candidate test methods include the APA, Servopac and Superpave indirect tension test. As a first step, the Department could specify minimum perfor mance values that mixtures would be required to meet at the mix design stage. Testing in this study and othe rs has revealed that not all mixtures have optimal performance when volumetrically designed according to current Superpave mixture design requirements. Research explor ing new mix design methodologies, which optimize a mixture’s performance based on laboratory performance test(s), should be explored. Gradations and asphalt contents would be selected to optimize performance, not to meet certain volumetric criteria.

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73 LIST OF REFERENCES Anderson, R.M., R.A. Bentsen. Influence of Voids in the Mineral Aggregate (VMA) on the Mechanical Properties of Coarse and Fine Asphalt Mixtures. Journal of the Association of Asphalt Paving Technologi sts, St. Paul, MN, Volume 70, 2001, pp. 1-37. Asphalt Institute. Superpave Mix Design, S uperpave Series No. 2, SP-2, Lexington, KY, 1996. Birgisson, B., R. Roque, G.C. Page. Ev aluation of Water Damage Using Hot Mix Asphalt Fracture Mechanics. Journal of the Association of Asphalt Paving Technologists, St. Paul, MN, Volume 72, 2003, pp. 424-462. Coree, B.J., W.P. Hislop. Difficult Nature of Minimum Voids in the Mineral Aggregate. Transportation Research Record 1681, Tr ansportation Research Board, National Research Council, Washington, DC, 1999, pp. 148-156. Coree, B.J., W.P. Hislop. A Laboratory Inve stigation into the E ffects of AggregateRelated Factors on Critical VMA in As phalt Paving Mixtures. Journal of the Association of Asphalt Paving Technologi sts, St. Paul, MN, Volume 70, 2001, pp. 70-131. Drakos, C.A. Identification of a Physical Model to Evaluate Rutting Performance of Asphalt Mixtures. Ph.D. dissertation, Univ ersity of Florida, Gainesville, FL, 2003. Hand, A.J., J.L. Stiady, T.D. White, A.S. Nour eldin, K. Galal. Gradation Effects on HotMix Asphalt Performance. Transportati on Research Record 1767, Transportation Research Board, National Research Council, Washington, DC, 2001, pp. 152-157. Kandhal, P.S., S. Chakraborty. Evaluation of Voids in the Mineral Aggregate for HMA Paving Mixtures. National Center for Asphalt Technology Report No. 96-4, Auburn, AL, 1996. Kandhal, P.S., K.Y. Foo, R.B. Mallick. A Critical Review of VMA Requirements in Superpave. National Center for Asph alt Technology Report No. 98-1, Auburn, AL, 1998. Kandhal, P.S., R.B. Mallick. Effect of Mi x Gradation on Rutting Potential of DenseGraded Asphalt Mixtures. Transportati on Research Record 1767, Transportation Research Board, National Research Council, Washington, DC, 2001, pp. 146-151.

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74 Leahy, R.B., R.B. McGennis. Asphalt Mixes: Materials, Design a nd Characterization. Journal of the Association of Asphalt Paving Technologist s, St. Paul, MN, Volume 68A, 1999, pp. 70-127. Nukunya, B., R. Roque, B. Birgisson, M. Ti a. Evaluation of Superpave Criteria for VMA and Fine Aggregate Angularity, Vo lume 1. Report for FDOT contract BB498, University of Florida, Gainesville, FL, 2002. Ruth, B.E., B. Birgisson. An Overview of Operational Aspects Relating to Volumetric Design and Construction of Asphalt Paving Mixtures. Canadian Technical Asphalt Association, Calgary, Canada, 1999, pp. 345-368. Ruth, B.E., R. Roque, B. Nukunya. Aggregat e Gradation Characte rization Factors and Their Relationship to Fracture Energy a nd Failure Strain of Asphalt Mixtures. Journal of the Association of Asphalt Paving Technologist s, St. Paul, MN, Volume 71, 2002, pp. 310-344. Roque, R., B. Birgisson, D. Darku, C.A. Dr akos. Evaluation of Laboratory Testing Systems for Asphalt Mixture Design and Ev aluation, Volume 2. Report for FDOT contract BB-888, University of Florida, Gainesville, FL, 2004. Roque, R., B. Birgisson, C.A. Drakos, B. Dietrich. Develo pment and Field Evaluation of Energy-Based Criteria for Top-down Cracki ng Performance of Hot Mix Asphalt. Paper presented at annual meeting of A ssociation of Asphalt Paving Technologists, Baton Rouge, LA, 2004. Sholar, G.A., G.C. Page, J.A. Musselman, P. B. Upshaw, H. Moseley. Examination of Aggregate Degradation and Effect on Volu metric Properties. FDOT research report 00-444, Florida Department of Transportation, State Materials Office, Gainesville, FL, 2001.

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75 BIOGRAPHICAL SKETCH Gregory Allen Sholar was born in Holly wood, Florida, in 1966 to Thomas and Barbara Sholar. Greg graduated from Pine Crest Preparatory School in Ft. Lauderdale in 1984 and then achieved a bachelo r’s degree in building constr uction from the University of Florida in 1988. Not satisfied with a car eer in the building c onstruction industry and having the thirst for more knowledge, Greg went back to the Univer sity of Florida and obtained a bachelor’s degree in civil engineering in 1996. While in college, Greg worked part time at the State Materi als Office of the Florida Depa rtment of Transportation and thoroughly enjoyed it. Fortunate ly, Greg was able to obtain a fulltime position there after graduation and has enjoyed working in bitumi nous research. In 1999, Greg enrolled at the University of Florida part time to obt ain a master’s degree in civil engineering materials. Greg plans to continue to work in his current capacity upon graduation.


Permanent Link: http://ufdc.ufl.edu/UFE0008942/00001

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Title: Evaluating the Use of Lower VMA Requirements for Superpave Mixtures
Physical Description: Mixed Material
Copyright Date: 2008

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Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
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Permanent Link: http://ufdc.ufl.edu/UFE0008942/00001

Material Information

Title: Evaluating the Use of Lower VMA Requirements for Superpave Mixtures
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
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EVALUATING THE USE OF LOWER VMA REQUIREMENTS FOR SUPERPAVE
MIXTURES















By

GREGORY A. SHOLAR


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF ENGINEERING

UNIVERSITY OF FLORIDA


2004

































Copyright 2004

by

Gregory A. Sholar















ACKNOWLEDGMENTS

I would like to acknowledge my graduate advisor, Dr. Reynaldo Roque, for his

patient guidance and wisdom throughout this long process. I would also like to thank Dr.

Bjorn Birgisson and Dr. Mang Tia for their advice and assistance in performing the work

and analysis of this thesis.

I offer the most sincere appreciation to Mr. Howie Moseley of the Florida

Department of Transportation. Without Mr. Moseley's support, help and advice, this

thesis would have not been finished.

I would like to thank my supervisors, Mr. Jim Musselman and Mr. Gale Page, for

allowing me the time and for offering the moral support to pursue my master's degree

while working fulltime.

I would like to thank Ms. Susan Andrews, Mrs. Shanna Johnson, Mr. Joshua

Whitaker and Mr. Stephen Browning of the Florida Department of Transportation for

their assistance in performing laboratory testing and the closeness that we share, not only

as workers, but as friends.

I would like to thank Dr. Christos Drakos and Ms. Tanya Riedhammer for their

assistance in performing testing and analysis. Their expertise far surpassed mine and I

would still be struggling with analysis if not for their help. I also thank them for their

friendship.

I would also like to thank Mr. George Lopp for general technical advice and most

importantly for his friendship and moral support over the last five years.









And finally, I would like to thank Ms. Betsy Pepine for her encouragement, support

and love over the last two and half years.
















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ................................................................................................. iii

LIST OF TA BLE S ....................................................... .. .......... ............ .. vii

L IST O F FIG U R E S .............. ............................ ............. ........... ... ........ viii

ABSTRACT ........ .............. ............. ...... .......... .......... xi

CHAPTER

1 IN TR OD U CTION ............................................... .. ......................... ..

1.1 Problem Statem ent ............................................ .......... .. ............ 1
1.2 O bj ectives ..................................... .......................... ....... .. ...... 2
1.3 Scope of W ork ....................................................................... ... ......... 3
1.4 R research Plan............................................................ 3

2 BA CK GROUND ........................................................ .......... ............. ....

2 .1 D efin ition of V M A ............................................................................... ...... ....5
2.2 Coarse and Fine G radations........................................................... ............... 6
2.3 N om final M axim um Aggregate Size .................................................................. 7
2.4 Literature Review ........................... ................. .................8
2.4.1 H historical Perspective ......................................................... ............... 8
2.4.2 R recent R research ....................... ............................ .......... ............ .... .

3 MATERIALS AND TESTING METHODS.......................................................14

3 .1 In tro d u ctio n ..................................................................................................... 14
3 .2 M a te ria ls ............................................................................................................... 14
3.2.1 A sphalt Binder .................. ........................... .... .. .............. ... 14
3 .2 .2 A ggregates ................................................................................. 14
3.2.2.1 A labam a lim estone .................................... ..................................... 15
3.2.2.2 B rooksville lim estone.................................... ........ ............... 16
3.2 .2 .3 N ova Scotia granite .................................. ...................................... 17
3.2.2.4 Miami limestone (Tarmac mine).................... ..................19
3.3 Testing M methods .................................. .. .. ... ... ... ...............20
3.3.1 M ix D esign Testing .................... ............................... ...............21


v









3.3.2 M moisture Sensitivity Testing.................................... ........ ............... 23
3.3.3 P erm ability T testing ................................................ ............... ...24
3.3.4 Asphalt Pavem ent Analyzer Testing .............................................. 26
3.3.5 Servopac Gyratory Compactor Testing.................................................31
3.3.6 Superpave Indirect Tension Testing.........................................................33

4 TEST RESULTS AND ANALYSIS........................................ ...................... 39

4.1 Introduction ..................................................................... 39
4.2 M ix D esign ...................................................... 39
4.2.1 M ix D esign Test R results ........................................ ........................ 39
4.2.2 M ix D esign Sum m ary........................................... .......................... 43
4 .3 R u ttin g ............................................................................ 4 4
4.3.1 APA Test Results ....................................... ............................ 44
4 .3 .2 A P A Su m m ary ........... ..... ..................................................... .... .... .. ....5 1
4.3.3 Servopac T est R esults........................................... .......................... 52
4.3.3.1 G yratory shear slope................................... ......... ............... 53
4.3.3.2 V ertical strain .......................... .. .. ......... ........ ........ ......55
4.3.3.3 Maximum shear stress ............................................. ...............58
4 .3.4 Servopac Sum m ary ......................................................................... .. .... 6 1
4 .4 C rack in g ............................. ......................................................... ............... 62
4 .4 .1 E energy R atio ....................... ............ ..................... 62
4.4.2 Dissipated Creep Strain Energy (DCSE)..............................................64
4.4.3 Fracture Energy (FE) .............. .... .......................... ... .... ........... 66
4.4.4 C racking Sum m ary ....................... ... .......... .................... ..... .......... 68
4.5 M moisture D am age............. ........ ..... ................................................ .... .. ...... .... 69
4.5.1 Conventional FM 1-T 283 Test Results ............................................. 69
4.5.2 Superpave IDT Test Results (Energy Ratio)................. ............... 69
4.5.3 Moisture Damage Summary............................ .......................... 70
4.6 Perm ability .................................... ................................ ......... 70

5 CONCLUSIONS AND RECOMMENDATIONS............................................... 71

5 .1 C o n clu sio n s................................................. .................. 7 1
5.2 Recommendations................ ......... .........................72

L IST O F R E F E R E N C E S ....................................................................... ... ................... 73

B IO G R A PH IC A L SK E TCH ..................................................................... ..................75
















LIST OF TABLES


Table page

3-1 Gradations for Alabama limestone mixtures.............................. ...... ......... 16

3-2 Gradations for Brooksville limestone mixtures ....................................... .......... 17

3-3 Gradations for Nova Scotia granite mixtures ................ .................................18

3-4 Gradations for Tarmac limestone mixtures........ .................... ..................20

4-1 Volumetric mix design data for Alabama limestone mixtures............................ 39

4-2 Volumetric mix design data for Brooksville limestone mixtures ..........................40

4-3 Volumetric mix design data for Nova Scotia granite mixtures.............. ...............41

4-4 Volumetric mix design data for Tarmac limestone mixtures............... ...............42

4-5 VMA difference between rounds one and two............................... ...............44

4-6 A PA test results .................. ................................ ........ .. ............ 45

4-7 Servopac test results ............................................... ........ ................. 53

4-8 Energy ratio values for the unconditioned and LTOA specimen...........................62

4-9 M oisture dam age test results ........................................................................ .. .... 69

4-10 P erm ability test data ...................................................................... ...................70
















LIST OF FIGURES

Figure page

2-1 V olum etric diagram ............. ............................................................ ........ ..... ....

2-2 C oarse and fine gradations ........................................ ................................. 6

3-1 Gradation plots for Alabama limestone mixtures ......................................... 16

3-2 Gradation plots for Brooksville limestone mixtures .............................................18

3-3 Gradation plots for Nova Scotia granite mixtures..................................................19

3-4 Gradation plots for Tarmac limestone mixtures.....................................................20

3-5 P erm ability test apparatu s............................................................ .....................25

3-6 A sphalt Pavem ent A nalyzer......................................................... ............... 27

3-7 Asphalt Pavement Analyzer loading apparatus......................................................28

3-8 Measuring plate and contour gage for modified measuring technique ..................29

3-9 Holder, contour gage and rut profile trace .................................... ............... 29

3-10 Illustration of absolute rut depth and differential rut depth ...................................30

3-11 Gyratory shear slope............... ... .. ............. ...... .........32

3-12 V ertical failu re strain ........................................................................ ..................33

3-13 Fram ew ork for evaluating m ixtures ........................................ ...... ............... 34

3-14 Superpave indirect tension test........................................... .......................... 35

3-15 D issipated creep strain energy........................................... ........................... 36

4-1 VMA plots for Alabama limestone mixtures ................................ .....................40

4-2 VMA plots for Brooksville limestone mixtures.....................................................41

4-3 VMA plots for Nova Scotia granite mixtures .................................. ...............42









4-4 M A plots for Tarm ac limestone mixtures ..................................... .................43

4-5 Comparison of APA measurement methods for 7% air voids, 75 mm tall
sp e c im e n s ......................................................................... 4 6

4-6 APA absolute rut depth using conventional measuring device for 7% air voids,
75 m m tall specim ens ................................................. ...... .. ............ 46

4-7 APA absolute rut depth using profile measuring device for 7% air voids, 75 mm
tall sp ecim en s ...................................................... ................ 4 7

4-8 APA differential rut depth using profile measuring device for 7% air voids,
75 m m tall specim ens ................................................. ...... .. ............ 47

4-9 APA percent area change using profile measuring device for 7% air voids,
75 m m tall specim ens ............................................... .. ...... .. ............ 48

4-10 APA absolute rut depth using conventional measuring device for 4% air voids,
115 m m tall specim ens ............................................... .. ...... .. ...... .... 49

4-11 APA rut depth versus VMA using conventional measuring device for 7% air
voids, 75 m m tall specim ens ............................................................................. 50

4-12 APA rut depth versus dust to effective binder ratio using conventional measuring
device for 7% air voids, 75 mm tall specim ens............................................ 51

4-13 Gyratory shear slope for Alabama limestone round one mixture ..........................54

4-14 Gyratory shear slope for Florida Brooksville limestone round one mixture............54

4-15 Vertical strain for Alabama limestone round two mixture.............................. 56

4-16 Vertical strain for Florida Brooksville limestone round one mixture ....................57

4-17 Vertical strain for Nova Scotia granite round one mixtures............................. 57

4-18 Gyratory shear versus percent air voids for Alabama limestone mixtures .............59

4-19 Gyratory shear versus percent air voids for Florida Brooksville limestone
m fixtures ........... ......... .................................... ............ ......... 59

4-20 Gyratory shear versus percent air voids for Nova Scotia granite mixtures.............60

4-21 Gyratory shear versus percent air voids for Florida Tarmac limestone mixtures ... 60

4-22 Gyratory shear stress versus APA rut depth.................................. ............... 61

4-23 Energy ratios for unconditioned specimens .................................. ............... 63









4-24 Energy ratios for long-term oven aged specimens................................................63

4-25 Dissipated creep strain energy for unconditioned specimens ................................65

4-26 Dissipated creep strain energy for long-term oven aged specimens ......................65

4-27 Fracture energy for unconditioned specimens ............................... ................67

4-28 Fracture energy for long-term oven aged specimens ............................................67















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Engineering

EVALUATING THE USE OF LOWER VMA REQUIREMENTS FOR SUPERPAVE
MIXTURES

By

Gregory A. Sholar

December 2004

Chair: Reynaldo Roque
Cochair: Bjorn Birgisson
Major Department: Civil and Coastal Engineering

The Florida Department of Transportation specifies coarse graded asphalt mixtures

for high traffic roadways with the rationale that coarse graded mixtures will offer better

rutting performance compared to fine graded mixtures. Contractors struggle to meet

minimum voids in the mineral aggregate (VMA) specification requirements, especially

when using aggregates native to Florida. Contractors often gap-grade asphalt mixture

gradations to obtain enough void space to meet VMA requirements. It is generally

believed that gap-grading an asphalt mixture will be detrimental to the mixture's rutting

performance. This study examines the effects on laboratory measured rutting, cracking,

moisture sensitivity and permeability of asphalt mixtures that have been designed with

gap-graded and continuous gradations with the thought that should the continuous

gradation provide better performance, then perhaps the VMA specification requirements

should be lowered to allow for this type of gradation.









The Asphalt Pavement Analyzer and Servopac gyratory compactor were used to

determine the mixtures' rutting performance. The Superpave indirect tensile tests (IDT)

and calculated parameters (energy ratio, dissipated creep strain energy and fracture

energy) were used to determine the mixtures' cracking and moisture sensitivity

performance. Additional standard laboratory tests were used to evaluate permeability and

moisture sensitivity.

Test results indicate that the addition of coarse aggregate on the 12.5 and 9.5 mm

sieves of 12.5 mm coarse graded mixtures improved the rutting performance of the

mixtures. However, cracking performance was adversely affected by the addition of

coarse aggregate. Moisture sensitivity results varied depending on the test method used.

Permeability results were unaffected by the gradation change.

Since cracking is the predominant form of distress for Florida pavements, it is

recommended that no change be made to the Department's specifications at this time.

Performance test results indicate that not all mixtures perform at their optimum when

designed volumetrically. The Department should continue to conduct research and move

towards implementation of one or more performance tests to augment or replace

volumetric mix design.














CHAPTER 1
INTRODUCTION

1.1 Problem Statement

The Florida Department of Transportation, herein referred to as the Department,

adopted the Superpave mix design system in 1996 as a replacement for the Marshall mix

design system, which the Department had used since the 1970's. One major difference

between the two mix design methodologies is the recommendation in the Superpave

system to use coarse graded mixtures for pavements subject to high traffic levels. The

Department defines a high traffic level as any pavement that will be subjected to ten

million equivalent single axle loads (ESALs) or greater over the pavement's 20-year

design period. This is in accordance with the American Association of State Highway

and Transportation Officials (AASHTO) Standard Practice for Superpave Volumetric

Design of Hot-Mix Asphalt PP 28-03.

The rationale for using coarse graded mixtures on high traffic pavements is for the

prevention of rutting. Coarse graded mixtures are typically those in which the gradation

curve initially starts above the maximum density line for the larger sieve sizes and then

curves below the maximum density line for the smaller sieve sizes. This results in a

mixture with more coarse aggregate and more stone-on-stone contact. A coarse

aggregate skeleton is created in which the voids are filled with fine aggregate and asphalt

binder. Because of this, coarse graded mixtures are thought to provide equal or better

resistance to rutting than fine graded mixtures, which have a gradation curve entirely

above the maximum density line.









The Superpave mix design system also sets minimum requirements for the mixture

property voids in the mineral aggregate (VMA). The VMA is the percent by volume of

the air voids plus asphalt binder that has not been absorbed into the aggregate.

Commonly, to meet the minimum VMA requirements for coarse graded mixtures, mix

designers have to gap grade the mixture by removing a portion of the coarse aggregate

from the mix design. This problem is exacerbated for limestone aggregates from Florida,

which are less angular, softer, and breakdown more easily than imported granite

aggregates or limestones from other states. In general, for a given gradation, an angular

aggregate will result in a higher VMA than a less angular aggregate. Additionally, as the

aggregate breaks down during the production process, it becomes more rounded and less

angular, which results in a reduction in VMA. Since the Department has implemented

the Superpave mix design system, asphalt contractors have struggled to meet minimum

VMA requirements at the mix design stage and more so during production. The gap

grading of the aggregate gradation is necessary to meet the minimum VMA requirements.

However, the removal of a portion of the coarse aggregate from the mix design may

nullify the benefits of the strong rut resistant coarse aggregate skeleton.

1.2 Objectives

The objectives of the study are as follows:

* Determine the effects on laboratory performance of adding additional coarse
aggregate to a mixture's gradation resulting in a reduction in VMA, which may
violate Superpave specifications.

* Based on the results, make recommendations regarding specification changes,
further research, or no changes to the current specifications.









1.3 Scope of Work


This research focuses on identifying the laboratory performance difference between

mixtures which have been designed to meet Superpave specifications and then

subsequently modified by adding more coarse aggregate to the mixtures gradations. The

scope of work is as follows:

* Construct four Superpave mix designs using aggregates from different geological
sources that are commonly used in Florida. The mixtures will all be 12.5 mm
coarse graded mixtures since this is the most common coarse mixture type used by
Contractors performing work for the Department. The mixtures will be gap graded
to match common practice by mix designers.

* Determine the laboratory performance of the four mixtures by using tests that give
an indication of a mixtures resistance to rutting, cracking, moisture sensitivity and
permeability.

* Modify the gradations of the four mixtures evaluated in the first objective to
provide more coarse aggregate on the 12.5 mm and 9.5 mm sieve sizes. This will
result in a reduction of VMA, which may be less than the minimum specified
value. These four mixtures will then be evaluated using the same laboratory
performance tests used to evaluate the unmodified mixtures.

* Compare the performance between the unmodified and modified mixtures to
ascertain the effects of the addition of coarse aggregate on a mixture's
performance.

* Evaluate the results and make recommendations.

1.4 Research Plan

The following items constitute the research plan for this study:

* A literature review was conducted.

* Four aggregate types were selected for study: Alabama limestone, Florida
limestone from the Brooksville area, Nova Scotia granite, and Florida limestone
from the Miami area (Tarmac mine). These aggregate types are commonly used in
Florida and represent a wide range of softness and angularity.

* Four mixtures were designed to meet Superpave mix design criteria. All of the
mixtures were 12.5 mm coarse graded mixtures and were gap graded. Each
mixture contained only one aggregate type.









* The Brooksville limestone mixture met all Superpave mix design criteria except for
the minimum VMA requirement. Brooksville limestone is a soft Florida limestone
that cannot be used solely to construct a 12.5 mm coarse graded Superpave mixture
and meet minimum VMA requirements. This aggregate type was chosen
intentionally so that a mixture not able to meet VMA criteria could be evaluated in
terms of performance.

* The following laboratory tests were used to ascertain rutting performance: the
asphalt pavement analyzer with the conventional and modified analysis approach
and the ServoPac gyratory compactor to measure shear stress, gyratory shear slope
and strain.

* The following laboratory test was used to ascertain cracking performance: the
Superpave indirect tension test.

* The following laboratory tests were used to ascertain moisture sensitivity
performance: tensile strength, the Superpave indirect tension test and a falling head
permeability test.

* Each of the four mixtures was then modified by adding more 12.5 mm and 9.5 mm
coarse aggregate. The resulting gradations were more continuously graded and less
gap graded than the unmodified mixtures. A reduction in VMA occurred for each
mixture.

* The modified mixtures were then evaluated with the same laboratory tests used to
evaluate the unmodified mixtures.

* The data was analyzed and conclusions and recommendations were made.

















CHAPTER 2
BACKGROUND

2.1 Definition of VMA

Voids in the mineral aggregate (VMA) is a volumetric property and is the sum of

the air voids in the mixture plus the amount of asphalt binder that has not been absorbed

into the aggregates. This unabsorbed binder is termed the "effective binder." The

concept of VMA is illustrated in Figure 2-1.



air Va


ashl

abore asphal


VMVmb


aggregate vsb vse




Vma = Volume of voids in mineral aggregate
Vmb = Bulk volume of compacted mix
Vmm = Voidless volume of paving mix
Vfa = Volume of voids filled with asphalt
..,. Va = Volume of air voids
Vb = Volume of asphalt
Vba = Volume of absorbed asphalt
o* Vsb = Volume of mineral aggregate
(by bulk specific gravity)
Vse = Volume of mineral aggregate
(by effective specific gravity)


Figure 2-1. Volumetric diagram










2.2 Coarse and Fine Gradations

The Superpave mixture design system designates mixtures as either coarse or fine.

As mentioned previously, this study focuses only on coarse graded mixtures, which are

thought to have equal or better rutting resistance compared to fine graded mixtures.

Coarse graded mixtures have gradation curves that start above the maximum density line

and curve downward below the restricted zone, whereas fine graded mixtures have

gradation curves which lie solely above the maximum density line. The maximum

density line represents the gradation that would result in the densest possible arrangement

of the aggregate particles. Superpave defines the restricted zone as an area where the

gradation should not pass through. Gradations that pass through this zone have the

potential to contain natural rounded sands which may inhibit good rutting performance

(Asphalt Institute 1996). An example of a coarse and fine gradation is shown in Figure

2-2.



100
90 .
80 -
a 70 -
"-
I 60
50 Maximum Density Line


30 0
30 Coarse

/'^^ -Fine
10 'Restricted Zone Fn
0
K 1.18 2.36 4.75 9.50 12.50 19.00


Sieve Size (mm)


Figure 2-2. Coarse and fine gradations









The aggregate gradation curve and its distance away from the maximum density

line are related to the VMA of a mixture. More area between the gradation curve and the

maximum density line increases the VMA potential of the mixture.

2.3 Nominal Maximum Aggregate Size

The Superpave mix design system designates a mixture by its nominal maximum

aggregate size (NMAS). The NMAS is defined to be the sieve which is one sieve size

larger than the first sieve to retain more than ten percent of the aggregate by weight. All

of the mixtures used in this study are 12.5 mm mixtures, which means that more than ten

percent of the aggregate is retained on the 9.5 mm sieve.

Minimum VMA requirements are based on the NMAS of the mixture. A smaller

NMAS mixture, for example a 9.5 mm mixture, has a higher VMA requirement than a

larger NMAS mixture, such as a 19.0 mm mixture. This is because the total surface area

of the aggregates is greater for the smaller NMAS mixture as compared to the larger

NMAS mixture. More aggregate surface area requires more asphalt binder to coat the

aggregates and hence the specified minimum VMA is greater.

However, Superpave does not differentiate between coarse and fine gradations with

respect to the VMA requirement. Both types of mixtures have the same minimum VMA

requirement for a given NMAS. Coarse graded mixtures have more coarse aggregate in

proportion to fine aggregate than fine graded mixtures. Therefore, there is less aggregate

surface area in a coarse graded mixture as compared to a fine graded mixture for a given

NMAS. Given the same VMA requirement, mix designers are then forced to gap grade

the mixture to provide ample volume between the aggregate particles to contain the

required four percent air voids and effective asphalt binder needed to meet the minimum

VMA requirement.









2.4 Literature Review

2.4.1 Historical Perspective

Modem mix design methods can generally be dated back to the 1940's with the

most predominant method being the Marshall mix design method. Marshall had different

views regarding VMA than other asphalt technologists at the time. Marshall believed

that VMA should be reduced to the lowest possible level and did not believe in

establishing specification limits for VMA (Leahy and McGennis 1999).

In 1957, Norman McLeod presented a paper to the Highway Research Board

emphasizing the importance of using the aggregate bulk specific gravity in the calculation

of VMA instead of the effective specific gravity, which was common at the time (Leahy

and McGennis 1999). McLeod also believed that VMA should be specified as a

minimum value of 15 percent with design air voids at five percent using the 75-blow

Marshall method. No performance data was used by McLeod to determine this VMA

limit. McLeod proposed VMA requirements based on nominal maximum aggregate size,

which were adopted by the Asphalt Institute in 1964. The current Superpave mix design

system specifies VMA based on McLeod's recommendations but has adjusted them

lower by one percent realizing that McLeod designed asphalt mixtures at five percent air

voids and the Superpave system requires four percent air voids (Kandhal and

Chakraborty 1996).

Coree and Hislop (1999) conducted a thorough review of literature regarding VMA

and found that there is little historical basis, if any, to support the VMA values currently

specified. Minimum VMA requirements that are the same for all gradations of a

particular NMAS can cause well performing mixtures to be rejected. They suggest the

possible use of a minimum asphalt film thickness as a replacement for VMA. The









researchers also recommend that VMA requirements or asphalt film thicknesses be

validated against field performance and that enforcement of any VMA specification not

be rigidly enforced due to the imprecision in current test methods.

2.4.2 Recent Research

Researchers have come to recognize that VMA criteria based on NMAS alone is

not adequate and that an approach based on asphalt film thickness is more rational. Work

by Kandhal and Chakraborty (1996) examined film thicknesses ranging from 4 to 13

microns for one 12.5 mm coarse graded mixture containing limestone. Mixtures were

compacted to eight percent air voids and short and long term aged. Mixture tests

included resilient modulus and tensile strength and binder tests included viscosity,

penetration and complex modulus. The researchers' conclusion was that a minimum film

thickness of 9 to 10 microns is desirable to minimize accelerated aging.

Work also conducted by Kandhal et al. (1998) emphasized that coarse graded

mixtures are penalized by current Superpave requirements because the VMA requirement

is the same for coarse and fine graded mixtures. This results in thicker than necessary

film thicknesses for coarse graded mixtures. As mentioned previously, work done by

Kandhal and Chakraborty (1996) indicated an optimum film thickness of 9 to 10 microns

at eight percent air voids. This study recommended a minimum asphalt film thickness of

eight microns for mixtures compacted between four and five percent air voids, which

would better represent the in-place density achieved in the roadway. The researchers'

reasoning for the lower film thickness is that at four to five percent air voids there would

be less aging of the binder. Based on a film thickness of eight microns, coarse graded

mixtures had VMA values up to two percent lower than fine graded mixtures using the

same aggregate type.









Kandhal and Mallick (2001) investigated the effect of aggregate gradation and

aggregate type on the rutting potential of asphalt mixtures. Tests conducted in the asphalt

pavement analyzer (APA) indicated that for the limestone and granite mixtures, an

increase in VMA resulted in an increase in rut depth. The trend was opposite for the river

gravel mixture studied. The same trends were observed when comparing voids filled

with asphalt (VFA) to the APA rut depths.

Hand et al. (2001) conducted a study measuring the rut resistance of 21 granite and

limestone mixtures of varying gradations using the PURWheel laboratory rut tester and

triaxial shear strength. The researchers concluded that maximum rut resistance as

determined by these two tests was achieved at an asphalt binder content 0.5 percent

below the value determined in the Superpave mix design process. The additional 0.5

percent asphalt binder can be attributed to minimum Superpave VMA requirements.

Sholar et al. (2001) conducted a study measuring the effects of aggregate

degradation throughout the production process on the volumetric properties of asphalt

mixtures. Three aggregate types commonly used in Florida (Georgia granite, southeast

Florida limestone, and west-central Florida limestone) were evaluated representing a

range of hard to soft aggregates respectively. Aggregate gradations were examined at

five points in the production process. Belt cut samples were obtained, asphalt mixture

was obtained from the truck bed, asphalt mixture from the same truck was obtained from

behind the paver but prior to compaction, asphalt mixture was obtained after roller

compaction, and gradations were determined from gyratory compacted samples. Some of

the conclusions from the research were:

S Aggregate breakdown was directly related to Los Angeles Abrasion values. The
two limestone mixtures degraded significantly more than the granite mixture.









* An average reduction of 0.5 percent VMA would be expected to occur for every
one percent of dust (material passing the 0.075 mm sieve) that was generated due to
breakdown.

Coree and Hislop (2001) conducted additional research to determine the aggregate

factors related to the critical VMA for a mixture. They determined the critical VMA by

using the Nottingham Asphalt Tester, which is a repeated load triaxial test. The

researchers determined the critical point by examining strain data at multiple asphalt

contents and selecting the asphalt content and corresponding VMA where strain started to

increase. They identified this point to be where the mixture would go from sound to

unsound behavior in terms of permanent deformation. Only three out of 28 mixtures

were correctly identified based on VMA design criteria alone. It was determined that the

volume of effective binder is more reliable (ten out of 28 mixtures) than VMA alone.

Aggregate factors that correlated well with the critical VMA were fineness modulus, the

percent of crushed coarse aggregate and the percent of crushed fine aggregate.

Anderson (2001) conducted a study comparing the performance of 12.5 mm coarse

and fine graded mixtures composed of Illinois Dolomite with each mixture designed with

13 and 15 percent VMA. Anderson had the following conclusions:

* Using the shear frequency sweep test (for rutting characterization) and the shear
fatigue test, the high temperature stiffness and critical temperature and the shear
fatigue characteristics of the coarse mixture decreased substantially as the VMA
increased. These tests suggest that the coarse mixture with 15 percent VMA would
be more susceptible to rutting and fatigue cracking than the coarse mixture with 13
percent VMA.

* Repeated shear testing (for rutting characterization) and flexural beam fatigue
testing (for fatigue characterization) indicated that a reduction of VMA from 15
percent to 13 percent should not affect the performance characteristics of the coarse
mixture.

* An increase in VMA from 13 percent to 15 percent for the fine graded mixture
improved the shear fatigue characteristics by 50 percent while only reducing the
high temperature stiffness and rutting characteristics by no more than 30 percent.









* The coarse mixture appeared much more sensitive to VMA changes than the fine
mixture.

Ruth and Birgisson (1999) identified several factors of high quality mixtures that

would make them relatively insensitive to changes during production. They emphasized

the importance of a continuously graded mixture that did not have an excess or deficiency

on any one sieve size. They also believed that the gradation should generally not be gap

graded.

Ruth et al. (2002) used tensile strength, fracture energy and failure strain from the

Superpave indirect tension test to evaluate mixtures with a variety of gradations and

determined that continuously graded mixtures outperformed mixtures that were gap

graded or had an excess or deficiency on any one sieve size, confirming the research

performed by Ruth and Birgisson (1999).

Nukunya et al. (2002) performed a comprehensive study regarding VMA and

presented the following findings:

* Mixture performance must be evaluated through the use of physical tests and
gradation analysis in addition to volumetric analysis.

* Current methods of calculating VMA and asphalt film thickness are ineffective
across all cases. A new approach calculating effective VMA and effective film
thickness based on only the portion of the mixture passing the 2.36 mm sieve was
presented.

* The percent of fine aggregate, not coarse aggregate, in a mixture appears to control
binder age hardening.

* Coarse graded mixtures develop pockets of fine aggregate and asphalt binder,
which make current methods for calculating film thickness and VMA irrelevant for
coarse graded mixtures but relevant for fine graded mixtures.

* Low effective film thickness and low effective VMA have a more pronounced
effect on fine graded mixtures than coarse graded mixtures. The fine graded
mixtures with low effective film thickness and VMA lose their flexibility and
become more brittle during aging.






13


* The minimum VMA requirements for coarse graded mixtures may result in
excessive asphalt leading to higher rutting based on high creep values and low
shear resistance.

* The current Superpave criteria for a minimum VMA for coarse graded mixtures
could be discontinued as long as other aggregate controls were instituted to limit
mix designers from using inferior (soft) aggregates.














CHAPTER 3
MATERIALS AND TESTING METHODS

3.1 Introduction

This chapter provides information on the materials and test procedures used in this

research project. It includes properties of the materials, how the materials were

combined, the test procedures performed on the materials, and the analysis methods used.

3.2 Materials

3.2.1 Asphalt Binder

A Superpave performance graded binder, PG 67-22, from El Paso Merchant Energy

Petroleum (formerly known as Coastal Fuels) in Jacksonville, FL was used for this

research project. This grade of binder is the standard unmodified binder used for

Department projects. The binder contained no anti-stripping agent. The asphalt binder

specific gravity was 1.03. The binder was sampled into ten 5-gallon buckets.

3.2.2 Aggregates

Four types of aggregate were used for this study: Alabama limestone, limestone

from the Brooksville, FL area, granite from Nova Scotia, and limestone from the Miami,

FL area (Tarmac mine). Each aggregate type was the basis for each asphalt mix design

studied. All aggregates used for this study were 100 percent crushed aggregates, which is

very common for Department work. All mix designs, except the Brooksville limestone

mix design, were based on actual mix designs submitted for approval to the Department.

Contractors do not submit 100 percent Brooksville aggregate mix designs because it is

not possible to meet Superpave VMA criteria as discussed in Chapter 1, Section 1.4.









Aggregate types were not intermingled and no reclaimed asphalt pavement was used. All

aggregate components for each mix design were oven dried and fractionated into

individual sieve sizes from the 19.0 mm sieve to the 0.075 mm sieve prior to watching.

Fractionating into all sieve sizes provided optimal control of achieved gradations and

assured consistency between batches. It should be noted that material below the 2.36 mm

sieve was typically not present for coarse aggregate components. The convention used

throughout this paper will be that "Round 1" refers to the gap graded mixture which

conforms to Superpave criteria. "Round 2" refers to the modified gradation that contains

more coarse aggregate and is more continuously graded, yet reduces the VMA of the

mixture. Each aggregate type will be discussed below.

3.2.2.1 Alabama limestone

The Alabama limestone asphalt mixture was composed of three aggregate

components:

* Number 7 stone from Southern Ready Mix, FDOT code 44, pit number AL-485.

* S-1-B stone from Southern Ready Mix, FDOT code 51, pit number AL-526.

* Screenings from Vulcan Materials Corporation, FDOT code 22, pit number AL-
149.

The aggregate components were proportioned to give the following gradations for

rounds 1 and 2 and are shown in Table 3-1 and Figure 3-1.






16


Table 3-1 Gradations for Alabama limestone mixtures
Percent Passing
Sieve Size (mm)
Round 1 Round 2
19.0 100 100
12.5 100 92
9.5 89 82
4.75 54 54
2.36 35 35
1.18 22 22
0.600 16 16
0.300 8 8
0.150 5 5
0.075 3.4 3.4


0.075 0.30 1.18 2.36 4.75 9.5 12.5 19.0
0.15 0.60 Sieve Size (mm)


Figure 3-1 Gradation plots for Alabama limestone mixtures

3.2.2.2 Brooksville limestone

The Brooksville limestone asphalt mixture was composed of three aggregate


components:









* S-1-A stone from Florida Crushed Stone, FDOT code 46, pit number 08-012.

* S-1-B stone from Florida Crushed Stone, FDOT code 52, pit number 08-012.

* Screenings (130A) from Florida Crushed Stone, FDOT code 24, pit number 08-
012.

The aggregate components were proportioned to give the following gradations for

rounds 1 and 2 and are shown in Table 3-2 and Figure 3-2. These gradations are very

similar to the gradations for the other three aggregate types.

Table 3-2 Gradations for Brooksville limestone mixtures
Percent Passing
Sieve Size (mm)
Round 1 Round 2
19.0 100 100
12.5 98 92
9.5 89 82
4.75 55 55
2.36 32 32
1.18 22 22
0.600 14 14
0.300 9 9
0.150 7 7
0.075 5.3 5.3

3.2.2.3 Nova Scotia granite

The Nova Scotia granite asphalt mixture was composed of three aggregate

components:

* Number 7 stone from Martin Marietta, FDOT code 44, pit number NS-315,
terminal TM-322.

* Number 89 stone from Martin Marietta, FDOT code 54, pit number NS-315,
terminal TM-322.

* Screenings from Martin Marietta, FDOT code 22, pit number NS-315, terminal
TM-322.

The aggregate components were proportioned to give the following gradations for

rounds 1 and 2 and are shown in Table 3-3 and Figure 3-3.


































0.075 0.30 1.18 2.36 4.75 9.5 12.5 19.0
0.15 0.60 Sieve Size (mm)


Figure 3-2 Gradation plots for Brooksville limestone mixtures

Table 3-3 Gradations for Nova Scotia granite mixtures
Percent Passing
Sieve Size (mm)
Round 1 Round 2
19.0 100 100
12.5 98 92
9.5 89 82
4.75 58 58
2.36 38 38
1.18 24 24
0.600 16 16
0.300 10 10
0.150 7 7
0.075 5.3 5.3











100

90 -

80 / -

70 -//

60

S__-_Round 1
50
f Round 2
40

30

20

10


0.075 0.30 1.18 2.36 4.75 9.5 12.5 19.0
0.15 0.60 Sieve Size (mm)


Figure 3-3 Gradation plots for Nova Scotia granite mixtures

3.2.2.4 Miami limestone (Tarmac mine)

The Tarmac limestone asphalt mixture was composed of four aggregate

components:

* S-1-A stone from Tarmac America, FDOT code 42, pit number 87-145.

* S-1-B stone from Tarmac America, FDOT code 51, pit number 87-145.

* 5/16 inch stone from Tarmac America, FDOT code 56, pit number 87-145.

* Screenings from Tarmac America, FDOT code 22, pit number 87-145.

The aggregate components were proportioned to give the following gradations for

rounds 1 and 2 and are shown in Table 3-4 and Figure 3-4.









Table 3-4 Gradations for Tarmac limestone mixtures
Percent Passing
Sieve Size (mm)
Round 1 Round 2
19.0 100 100
12.5 98 92
9.5 89 82
4.75 55 55
2.36 32 32
1.18 25 25
0.600 18 18
0.300 13 13
0.150 7 7
0.075 5.3 5.3


0.075 0.30 1.18 2.36 4.75 9.5 12.5 19.0
0.15 0.60 Sieve Size (mm)


Figure 3-4 Gradation plots for Tarmac limestone mixtures

3.3 Testing Methods

Testing methods for this study can be categorized into six classifications: mix

design, moisture sensitivity testing, permeability testing, Asphalt Pavement Analyzer









testing, Servopac gyratory compactor testing, and Superpave indirect tension testing.

Each classification will be discussed below with respect to the test procedures used and

the techniques used to analyze the data.

3.3.1 Mix Design Testing

The design of the mixtures followed standard Superpave practice, which is

governed by four American Association of State Highway and Transportation Officials

(AASHTO) standards:

* Superpave Volumetric Design for Hot-Mix Asphalt (HMA), AASHTO designation
PP 28-03. This practice outlines the overall design procedure from materials
selection, designing the aggregate structure, selecting the design binder content,
and evaluating the mixture for moisture sensitivity.

* Superpave Volumetric Mix Design, AASHTO designation MP 2-03. This
specification gives detailed requirements for binder selection, aggregate gradation
criteria, aggregate consensus property requirements, and mixture property criteria
based on traffic level.

* Preparing and Determining the Density of Hot-Mix Asphalt (HMA) Specimens by
Means of the Superpave Gyratory Compactor, AASHTO designation T 312-03.
This standard method of test discusses specific requirements of the gyratory
compactor, the compaction procedure, and density determination. For this study,
all mix design specimens were gyrated in a Pine AFGC125X gyratory compactor.

* Mixture Conditioning of Hot-Mix Asphalt (HMA), AASHTO designation R 30-02.
This practice outlines mixture conditioning for volumetric mix design and short and
long-term conditioning for mechanical property testing.

Specific Department test procedures needed during the mix design process were

used to determine aggregate and mixture properties and are discussed below:

* Sieve Analysis of Coarse and Fine Aggregate, Florida Method of Test FM 1-T 027.
This test method describes the procedure for performing a sieve analysis on coarse
or fine aggregate to determine a gradation.

* Specific Gravity and Absorption of Fine Aggregate, Florida Method of Test FM 1-
T 084. This test method describes the procedure for determining the bulk specific
gravity and absorption of fine aggregates.









* Specific Gravity and Absorption of Coarse Aggregate, Florida Method of Test FM
1-T 085. This test method describes the procedure for determining the bulk
specific gravity and absorption of coarse aggregates.

* Bulk Specific Gravity of Compacted Bituminous Mixtures, Florida Method of Test
FM 1-T 166. This test method describes the procedure for determining the bulk
specific gravity of compacted asphalt mixtures, such as gyratory specimens.

* Maximum Specific Gravity of Asphalt Paving Mixtures, Florida Method of Test
FM 1-T 209. This test method describes the procedure for determining the
maximum specific gravity of uncompacted asphalt mixtures.

As mentioned previously, individual aggregate components were fractionated to

every sieve size to provide better accuracy and consistency in watching. Two fine and

two coarse aggregate specific gravity tests were conducted for each aggregate component

and the individual values combined mathematically to obtain bulk specific gravity values

for the composite gradation, otherwise known as the job mix formula (JMF).

Following standard Superpave guidelines, mixtures were designed with four

percent air voids at the design number of gyrations while also meeting specification

requirements for VMA, VFA, and dust/effective binder ratio. The design number of

gyrations for all mixtures was 100. The specified minimum VMA requirement was 14.0.

The VFA requirement was the range of 65 to 75 percent. The specified dust to effective

binder content ratio was the range 0.8 to 1.6. Once the design binder content had been

selected, then additional specimens were prepared with binder contents modified by the

following amounts: +1.0, +0.5, -0.5 and -1.0 percent binder. Three asphalt specimens

were made at each binder content. Having volumetric design data at five asphalt binder

contents provided enough information to construct an adequate VMA curve for each

mixture.









3.3.2 Moisture Sensitivity Testing

Moisture sensitivity testing is a routine function in the Superpave mix design

procedure and was performed for all mixtures in this study. The main reason for

performing this test was to obtain a relative measurement of the mixture's resistance to

moisture damage between rounds one and two of a particular aggregate type, not

necessarily between mixtures of different aggregate types. The addition of more coarse

aggregate, resulting in a more continuous gradation closer to the maximum density line,

was thought to perhaps reduce the permeability of the mixture and reduce the

susceptibility to moisture damage.

The test method used to determine the moisture susceptibility of a mixture was

Resistance of Compacted Bituminous Mixture to Moisture-Induced Damage, Florida

Method of Test FM 1-T 283. The basic test procedure is performed as follows:

* Samples are batched in the laboratory to a predetermined weight that will result in
compacted specimens of 7.0 +/- 1.0 percent air voids. A minimum of six 100 mm
diameter specimens are gyrated to a height of approximately 65 mm.

* Three specimens are broken in the unconditioned state at 25 C in the indirect
tensile mode at a rate of 50 mm per minute. The Pine breaking apparatus typically
used to determine stability and flow values for Marshall mix design was used for
this test.

* Three different specimens are conditioned by vacuum saturating the specimens
underwater to a condition of 70 to 80 percent saturation.

* These three specimens are then frozen at -18 OC for a minimum of 16 hours and
then placed in a water bath at 60 OC for 24 hours. The specimens are then placed in
a chamber at 25 C for two hours.

* These three specimens are then broken in the indirect tensile mode at a rate of 50
mm per minute.

* Peak loads obtained from the indirect tension testing are used to calculate diametral
tensile strength.









* A tensile strength ratio is obtained by dividing the average tensile strength in the
conditioned state by the average tensile strength in the unconditioned state.

In addition to the approach mentioned above, the data from the Superpave indirect

tension test was also used to evaluate moisture sensitivity. This will be discussed in a

subsequent section.

3.3.3 Permeability Testing

Like the moisture sensitivity testing described above, permeability testing was

performed to obtain a relative measurement of the mixture's resistance to water

permeability between rounds one and two of a particular aggregate type, not necessarily

between mixtures of different aggregate types. The addition of more coarse aggregate,

resulting in a more continuous gradation closer to the maximum density line, was thought

to perhaps reduce the permeability of the mixture.

The test method used to determine the permeability of a mixture was Measurement

of Water Permeability of Compacted Asphalt Paving Mixtures, Florida Method of Test

FM 5-565, with the addition of a vacuum saturation. The basic test procedure is

performed as follows:

* Samples are batched in the laboratory to a predetermined weight that will result in
compacted specimens of 7.0 +/- 0.5 percent air voids when compacted to a height
of approximately 115 mm. Specimen diameter is 150 mm. Three specimens are
used for permeability testing.

* The top 50 mm of each gyratory specimen is then removed from the remaining
portion of the specimen by saw cutting using a diamond tipped blade which is
cooled and lubricated with a stream of water. This prevents smearing of the asphalt
binder during the cut, which would clog the permeable pores.

* The samples are then vacuum saturated under water for five minutes at a vacuum of
380 mm of mercury.

* The samples are then placed in the falling head permeability apparatus shown in
Figure 3-5.











mark 500 ml


LPressure gauge
Pressure / vacuum pump


Graduated cylinder
I.D. = 31.75 nun (1.25 in.)
+/- 0.5 nmn (0.02 in.)


Cap
Assembly

- Cap sealing
o-ring

Hose barb
SI'iliiig






I Pressiure
Wle


Pedestal
sealing
o-rino


Quick
connect


Figure 3-5 Permeability test apparatus


r mark 0


Clamp
issembl:

Hose
clamp


(2.0 in.)


pipe









* The time is recorded to flow 500 milliliters of water through the specimen.
Additionally, the water temperature is recorded so that a temperature correction
factor can be applied to correct the permeability readings to a standard reference
temperature of 20 C.

* The permeability value for the three specimens is then averaged to obtain an
average permeability value for the mix design.

3.3.4 Asphalt Pavement Analyzer Testing

The Asphalt Pavement Analyzer (APA) was one of two devices used to assess the

rutting performance of the asphalt mixtures. The other device was the Servopac gyratory

compactor, which will be discussed in a subsequent section. The APA is essentially a

wheel tracking device that applies a repeating load to a cylindrical asphalt specimen and

the rut depth is determined after 8,000 cycles, or 16,000 passes (Figure 3-6). The test

procedure followed is Determining Rutting Susceptibility of Asphalt Paving Mixtures

Using the Asphalt Pavement Analyzer (APA), AASHTO designation TP 63-03.

The highlights of the test procedure are given below.

* Samples are batched in the laboratory to a predetermined weight that will result in
compacted specimens of 7.0 +/- 1.0 percent air voids when compacted to a height
of 75 mm. Specimen diameter is 150 mm. Four specimens are used for APA
testing.

* As a comparison, additional samples were batched in the laboratory to a
predetermined weight that would result in compacted specimens of 4.0 +/- 1.0
percent air voids when compacted to a height of 115 mm at 100 gyrations.
Specimen diameter is 150 mm. Four specimens were used for APA testing.

* Specimens are placed in the APA molds (two per mold) with the top side of the
specimens facing up. The top side of the specimen is the side that was in contact
with the ram head of the gyratory compactor. Specimens (in the molds) are then
heated to 64 C for approximately 16 hours.

* The specimens are then placed in the 64 C heated APA testing chamber where a
seating load of 25 cycles is applied to the specimens. The load is comprised of a
445 N load applied on top of a 19.0 mm diameter hose inflated to 700 kPa (Figure
3-7).














I


j1


po


Figure 3-6 Asphalt Pavement Analyzer
* A measuring template is then placed on the top of the mold and an initial depth
reading is obtained using a digital measuring device. The template contains four
measuring slots, two per specimen.
* The specimens are then placed in the 64 C heated testing chamber and 8000
additional load cycles are applied, as described in step four.
* A final depth reading is then obtained at each of the four measuring slots. The rut
depth is taken as the difference between the initial and final readings.
In addition to the method described above for measuring the rut depths, a recently
developed method for measuring the rut profile (Drakos 2003) was used for the seven
percent air void specimens. Instead of using the conventional digital measuring device
with a small roller on the end to measure a single point maximum rut depth, the new


0000




































Figure 3-7 Asphalt Pavement Analyzer loading apparatus

method uses a modified measuring plate and contour gage that measures the entire rut

profile. The profile is measured at three longitudinal locations for each cylindrical

specimen (Figure 3-8).

The contour gage is then placed in a specially made holder and the contour is traced

onto a paper card. The specially made holder establishes a consistent orientation and

reference system for each rut profile that is traced (Figure 3-9).

The line trace on the card is then electronically scanned and a best fit line is fitted

to the electronic trace using computer software. Through integration of the equation of

the line, the area between the line and the x-axis is determined. This procedure is

conducted for the initial trace after 25 rut cycles and the final trace after 8000 additional
































Figure 3-8 Measuring plate and contour gage for modified measuring technique


Figure 3-9 Holder, contour gage and rut profile trace

cycles. The initial area is then subtracted from the final area and a percent area change is

determined. If the percent area change is positive, then Drakos (2003) concluded that










instability rutting has occurred and if the percent area change is negative, then

consolidation rutting has occurred.

In addition to the area change, the maximum single point absolute rut depth (ARD)

and the maximum single point differential rut depth (DRD) can be determined from the

profile traces. This is illustrated in Figure 3-10.


Lateral Location, X (in)
3


U. /


10.
0.9





I ARD
1.2
V--DR



1.3
Initial Profile
1.4
Profile after 8000 cycles
1.5


Figure 3-10 Illustration of absolute rut depth and differential rut depth

The absolute rut depth determined from the profile traces is the same form of rut

depth measured using the conventional measuring device described previously. The

differential rut depth measurement includes the absolute rut depth plus the shoving or

heaving that occurs with mixtures that experience instability rutting.









3.3.5 Servopac Gyratory Compactor Testing

The second testing device used to examine the mixtures' rutting potential was the

Servopac gyratory compactor located at the University of Florida Civil Engineering

asphalt laboratory. This device performs the same functions as the Pine gyratory

compactor mentioned previously. In addition, it has the ability to measure the force

required to maintain the angle of compaction and this force is then converted to a

gyratoryy shear stress." The Servopac compactor generates an output file displaying the

angle of gyration, the gyratory shear stress, the internal angle and the sample height.

Another useful feature of the Servopac compactor is the ability to quickly change

the angle of gyration by simply inputting the desired angle into the computer input

screen. The standard angle of compaction per AASHTO standards is 1.25 degrees. For

this study, mixtures were compacted at 1.25 and 2.50 degrees per the procedure described

below.

Roque et al. (2004a) developed a new procedure using the Servopac compactor for

evaluating the rutting potential of mixtures. The procedure results in two parameters: the

gyratory shear slope and the vertical failure strain.

* Two asphalt mixture specimens are compacted at an angle of 1.25 degrees to the
maximum design number of gyrations (Nmax). Nmax for this study was 160
gyrations.

* The bulk specific gravity of each specimen is determined and air voids are
calculated based on the maximum specific gravity of the mixture.

* Based on the height measurements recorded during compaction, the percent air
voids at each gyration level is backcalculated.

* A graph is created plotting the measured gyratory shear versus the natural log of
the number of gyratory revolutions.

* The slope of the graph is obtained in the range corresponding to seven to four
percent air voids or to the maximum gyratory shear if this is reached prior to four










percent air voids. This value is designated the gyratoryy shear slope" and is an
indicator of the mixture's resistance to deformation (Figure 3-11).


745



740 --
y = 25.13x + 625.75
R2 = 0.98

4-735



730

75 -* 7% to 4% air voids

725



720
3.0 3.5 4.0 4.5 5.0
Natural Log Revolutions



Figure 3-11 Gyratory shear slope

* Two additional asphalt specimens are prepared and compacted at an angle of 1.25
degrees until the gyration corresponding to seven percent air voids is reached. At
this point the machine is stopped for approximately fifteen seconds while the angle
of gyration is changed to 2.5 degrees. Then the sample is gyrated for another 100
gyrations. Changing the angle of compaction causes an unstable condition in the
mixture resulting in a shear failure. The behavior of the mixture during this period
provides a further indication of rutting potential and nature of the mixture.

* The gyratory shear versus the number of revolutions is plotted. The "vertical
failure strain" is then calculated from the point of angle change to the local
minimum in gyratory shear strength (Figure 3-12). This strain measurement is
during the point of aggregate rearrangement caused by changing the angle of
compaction and is an indicator of the stability characteristics of the mixture. The
strain value is calculated by taking the change in gyratory pill height divided by the
initial pill height at the point of angle change. The magnitude of the strain is an
indicator of whether the mixture is brittle, plastic or somewhere in between. A










framework for evaluating mixtures based on the work of Roque et al. (2004a) is
shown in Figure 3-13.


900

Vertical
850 failure strain
region Gyratory shear local minimum

g 800
Change in
angle from
5 750-- 1.25 to 2.50

18 -Average
S700 Specimen #1

Specimen #2
650


600
1 16 31 46 61 76 91 106 121 136
Number of Gyratory Revolutions



Figure 3-12 Vertical failure strain

3.3.6 Superpave Indirect Tension Testing

The evaluation of the mixtures' resistance to top-down cracking was evaluated

using the Superpave indirect tension test (IDT) and the procedure developed at the

University of Florida. Top-down cracking is the primary mode of pavement distress in

Florida. Approximately 80 percent of the State's deficient highways are deficient due to

top-down cracking. The research conducted at the University of Florida has been on

going for many years and many papers have been published. Roque et al. (2004b)

summarized the work to date and presented their framework for energy based criteria
























Low


High


Vertical Failure Strain


Figure 3-13 Framework for evaluating mixtures

related to top-down cracking in asphalt mixtures. The highlights of the procedure and

analysis technique will be discussed below:

* 150 mm diameter gyratory compacted specimens of approximately 115 mm tall at
an air void content of 7 +/- 1 percent air voids are prepared. From these specimens,
the top and bottom are trimmed off using a wet saw and then the remainder of the
specimen is cut in half resulting in two specimens approximately 50 mm thick.

* The specimens are dried and gage points are applied to both faces. The specimens
are then further dried in a dehumidifying chamber and brought to a testing
temperature of 10 C.

* Three different tests are performed on each of three specimens in sequential order.
The final results are therefore based on the average of three specimens. A MTS
closed loop servo hydraulic system was used for all Superpave IDT testing.

* The resilient modulus and Poisson's ratio are determined by applying a haversine
wave load for 0.1 seconds followed by a rest period of 0.9 seconds.

* A creep test is performed in which a constant load is applied for 1000 seconds.
Several parameters are determined from this test including the creep compliance,
creep rate and m-value, which is an indication of the mixture's resistance to creep.

* An indirect tensile strength test is performed at a rate of 50 mm/min. The tensile
strength is determined at the point where the plot of the vertical deformations


Brittle Optimal Plastic
Mixtures Mixtures Mixtures





Mixtures with Low Shear Resistance









minus the horizontal deformations versus time reaches a peak. Figure 3-14 shows a
test specimen.


Figure 3-14 Superpave indirect tension test

The key parameter calculated is the energy ratio and is defined as the dissipated

creep strain energy threshold of a material divided by the minimum dissipated creep

strain energy needed. The dissipated creep strain energy (DCSE) of a material is defined

as the fracture energy (FE) minus the elastic energy (EE) and is shown in Figure 3-15.

The minimum dissipated creep stain energy required is a function of material

properties and the pavement structure. The relationship is described as:

DCSEmin = m298*D/A

where, m and D1 are parameters derived from the creep test.










St
(Strength)



e 1MR








Strain, C Ef
(Fracture)

Figure 3-15 Dissipated creep strain energy

The term "A" accounts for the tensile stresses induced in the pavement by vehicle

loads and the tensile strength of the material. "A" is defined as:

A = 0.0299*"-3.10(6.36-St)+2.46*10-

where, c is the tensile stress induced in the pavement and St is the tensile strength

of the material. For this study, a standard value of 100 lb/in2 was used for C.

Roque et al. (2004b) developed the following criteria for acceptable cracking

performance. The DCSE of the material should be greater than 0.75 kJ/m3 and the

energy ratio should be greater than or equal to one. The researchers propose higher

energy ratio values for higher traffic levels.

The aforementioned research was conducted on specimens that had only been

short-term conditioned in accordance with the AASHTO procedure "Mixture









Conditioning of Hot-Mix Asphalt (HMA), AASHTO designation R 30-02." Short-term

conditioning is supposed to represent the aging that plant produced mix will experience

during the mixing and compaction process. Testing identical to that mentioned

previously was conducted on specimens that had been long-term oven aged (LTOA) in

accordance with AASHTO R 30-02. LTOA aging is intended to represent the aging that

the mixture will experience after seven to ten years of service. The procedure for long-

term aging is to place samples that have already been short-term oven aged into an oven

at 85 C for 120 hours. This testing was conducted to see if the asphalt mixtures

performed in a similar manner or not compared to the short-term aged samples.

In addition to the short-term and long-term oven aged samples, an additional set of

samples were prepared that were moisture conditioned. Birgisson et al. (2003) found that

the Superpave IDT tests and data analysis techniques used to characterize a mixture's

resistance to cracking is also successful at identifying a mixture's susceptibility to

moisture damage. The moisture conditioning and testing procedure consists of:

* Uncut gyratory compacted specimens are vacuum saturated to a saturation level
between 65 and 80 percent.

* The specimens are then placed in a 60 C water batch for 24 hours.

* After removal from the water bath the specimens are allowed to dry at ambient
room conditions for twelve hours, after which they are cut to a thickness of 50 mm.

* The suite of Superpave IDT tests is then performed on the specimens as described
previously.

The Superpave IDT testing was performed on moisture conditioned specimens to

provide an additional means of assessing the mixtures' moisture sensitivity in addition to

the moisture testing conducted per FM 1-T 283 described previously. The emphasis was






38


to examine the effects on moisture sensitivity between rounds one and two for each

mixture type, not necessarily between mixtures of different aggregate types.
















CHAPTER 4
TEST RESULTS AND ANALYSIS

4.1 Introduction

The test results and analysis will be presented categorically in the following order:

mix design, rutting (APA and Servopac), cracking, moisture damage and permeability.

4.2 Mix Design

4.2.1 Mix Design Test Results

Following are tables of volumetric mix design data and VMA plots for each of the

four mixture types. Each table and plot contains data for rounds one and two.

Table 4-1 Volumetric mix design data for Alabama limestone mixtures
Round #1 Gap Graded Mixture
Percent Dust
Percent m Gse Pba Pbe Dust Gmb Air Voids VMA VFA
AC Ratio
3.8 2.581 2.744 0.54% 3.28 1.0 2.409 6.7 14.3 54
4.3 2.566 2.750 0.63% 3.70 0.9 2.423 5.6 14.3 61
4.8 2.546 2.750 0.62% 4.21 0.8 2.445 4.0 13.9 72
5.3 2.527 2.751 0.63% 4.70 0.7 2.468 2.3 13.6 83
5.8 2.510 2.754 0.67% 5.17 0.7 2.471 1.5 13.9 89
Round #2 Continuous Graded Mixture
Percent Dust
Percent m Gse Pba Pbe Dust Gmb Air Voids VMA VFA
AC Ratio
3.6 2.596 2.752 0.53% 3.09 1.1 2.417 6.9 14.2 51
4.1 2.574 2.750 0.50% 3.62 0.9 2.438 5.3 13.9 62
4.6 2.556 2.753 0.53% 4.09 0.8 2.455 4.0 13.7 71
5.1 2.539 2.756 0.58% 4.55 0.7 2.475 2.5 13.5 81
5.6 2.517 2.753 0.53% 5.10 0.7 2.486 1.3 13.5 91



































Asphalt Binder (percent)


Figure 4-1 VMA plots for Alabama limestone mixtures

Table 4-2 Volumetric mix design data for Brooksville limestone mixtures
Round #1 Gap Graded Mixture
Percent Dust
Percent m Gse Pba Pbe Dust Gmb Air Voids VMA VFA
AC Ratio
6.5 2.319 2.540 4.53% 2.27 2.3 2.176 6.1 10.9 44
7.0 2.305 2.542 4.55% 2.76 1.9 2.189 5.0 10.9 54
7.5 2.295 2.549 4.67% 3.18 1.7 2.209 3.8 10.6 64
7.9 2.274 2.537 4.47% 3.78 1.4 2.205 3.0 11.1 73
8.4 2.261 2.539 4.51% 4.26 1.2 2.211 2.2 11.4 81
Round #2 Continuous Graded Mixture
Percent Dust
Percent m Gse Pba Pbe Dust Gmb Air Voids VMA VFA
AC Ratio
6.0 2.335 2.540 4.55% 1.72 3.1 2.174 6.9 10.5 35
6.5 2.319 2.540 4.54% 2.25 2.4 2.193 5.4 10.2 47
7.0 2.303 2.539 4.53% 2.78 1.9 2.214 3.9 9.9 61
7.5 2.291 2.543 4.60% 3.24 1.6 2.216 3.3 10.2 68
8.0 2.275 2.542 4.58% 3.79 1.4 2.237 1.7 9.9 83


~Round i
~ Round 2












11.5




11.0




10.5




10.0




9.5


6.5 7.0 7.5 8.0
Asphalt Binder (percent)


Figure 4-2 VMA plots for Brooksville limestone mixtures

Table 4-3 Volumetric mix design data for Nova Scotia granite mixtures
Round #1 Gap Graded Mixture
Percent Dust
Percent m Gse Pba Pbe Dust Gmb Air Voids VMA VFA
AC Ratio
4.8 2.471 2.659 0.39% 4.43 1.2 2.304 6.8 16.7 59
5.3 2.456 2.662 0.45% 4.88 1.1 2.328 5.2 16.2 68
5.8 2.435 2.658 0.39% 5.44 1.0 2.336 4.1 16.4 75
6.3 2.420 2.661 0.43% 5.89 0.9 2.359 2.5 16.0 84
6.8 2.402 2.661 0.42% 6.41 0.8 2.367 1.5 16.2 91
Round #2 Continuous Graded Mixture
Percent Dust
Percent m Gse Pba Pbe Dust Gmb Air Voids VMA VFA
AC Ratio
4.6 2.473 2.652 0.30% 4.32 1.2 2.300 7.0 16.6 58
5.1 2.455 2.652 0.30% 4.82 1.1 2.322 5.4 16.3 67
5.6 2.439 2.654 0.33% 5.29 1.0 2.346 3.8 15.8 76
6.1 2.421 2.654 0.32% 5.80 0.9 2.358 2.6 15.9 84
6.6 2.404 2.654 0.33% 6.29 0.8 2.371 1.4 15.9 91














R-ound 1
-- Round 2







I II


Asphalt Binder (percent)


Figure 4-3 VMA plots for Nova Scotia granite mixtures

Table 4-4 Volumetric mix design data for Tarmac limestone mixtures
Round #1 Gap Graded Mixture
Percent Dust
Percent m Gse Pba Pbe Dust Gmb Air Voids VMA VFA
AC Ratio
6.3 2.314 2.526 2.48% 3.98 1.3 2.172 6.1 14.5 58
6.8 2.302 2.530 2.55% 4.43 1.2 2.194 4.7 14.1 67
7.3 2.291 2.535 2.64% 4.86 1.1 2.204 3.8 14.2 73
7.8 2.273 2.531 2.57% 5.43 1.0 2.224 2.2 13.9 84
8.3 2.256 2.528 2.52% 5.99 0.9 2.232 1.1 14.1 92
Round #2 Continuous Graded Mixture
Percent Dust
Percent m Gse Pba Pbe Dust Gmb Air Voids VMA VFA
AC Ratio
5.6 2.335 2.525 2.46% 3.27 1.6 2.186 6.4 13.3 52
6.1 2.320 2.525 2.47% 3.78 1.4 2.197 5.3 13.3 60
6.6 2.305 2.526 2.48% 4.28 1.2 2.214 3.9 13.1 70
7.2 2.283 2.521 2.40% 4.97 1.1 2.219 2.8 13.5 79
7.6 2.275 2.526 2.49% 5.30 1.0 2.244 1.4 12.9 90











15.0
-Round 1
Round 2
14.5



S14.0



13.5



13.0



12.5
5.5 6.0 6.5 7.0 7.5 8.0 8.5
Asphalt Binder (percent)


Figure 4-4 VMA plots for Tarmac limestone mixtures

4.2.2 Mix Design Summary

Examination of the VMA curves generally shows the expected concave shaped

curve for each mixture. Theoretically, it is desirable to have a design asphalt binder

content that is either at the minimum point on the VMA curve or to the left of the

minimum point. In this range of binder contents, the slight addition of additional binder,

which could occur during production, will not increase the VMA. An increase in VMA

(i.e. to the right side of the minimum point) is thought to push the aggregate skeleton

apart and reduce shear resistance, which is related to rutting. It appears that for all of the

mixtures designed for this study that the optimum binder content is either at the minimum

of the curve or to the left side of the minimum.

It should be noted that the amount of coarse aggregate added to each mix design for

round 2 was almost identical for all of the aggregate types, however, the reduction in









VMA between rounds one and two was significantly different for each aggregate type.

The reduction in VMA for each aggregate type is shown in Table 4-5.


Table 4-5 VMA difference between rounds one and two
VMA
Aggregate Type V
Round 1 Round 2 Difference
Alabama limestone 13.9 13.7 0.2
FL Brooksville limestone 10.6 9.9 0.7
Nova Scotia granite 16.4 15.8 0.6
Fl Tarmac limestone 14.2 13.1 1.0


4.3 Rutting

4.3.1 APA Test Results

There were five different parameters determined with the APA considering various

methods of sample preparation and data interpretation. The five parameters are:

* Absolute rut depth using the conventional one-point measuring device testing 75
mm tall gyratory compacted specimens compacted to an air void content of 7.0 +/-
1.0 percent.

* Absolute rut depth using the conventional one-point measuring device testing 115
mm tall gyratory compacted specimens compacted to an air void content of 4.0 +/-
0.5 percent.

* Absolute rut depth using the complete profile measuring device testing 75 mm tall
gyratory compacted specimens compacted to an air void content of 7.0 +/- 1.0
percent.

* Differential rut depth using the complete profile measuring device testing 75 mm
tall gyratory compacted specimens compacted to an air void content of 7.0 +/- 1.0
percent.

* Percent area change using the complete profile measuring device testing 75 mm tall
gyratory compacted specimens compacted to an air void content of 7.0 +/- 1.0
percent.

The results for each of the five parameters for all of the mixture types are presented

in Table 4-6. Each value in Table 4-6 represents the average of four specimens.










Table 4-6 APA test results
FL Brooksville FL Tarmac
APA P r Alabama limestone im e Nova Scotia granite Lmes
APA Parameter limestone limestone
Round 1 Round 2 Round 1 Round 2 Round 1 Round 2 Round 1 Round 2
Absolute Rut Depth (mm)
Single point measuring device 5.5 4.2 1.5 1.3 5.9 4.5 2.5 1.5
7% Va, 75 mm tall

Absolute Rut Depth (mm)
Single point measuring device 3.5 4.4 1.1 1.2 3.7 4.3 2.6 1.9
4% Va, 115 mm tall

Absolute Rut Depth (mm)
Profile measuring device 5.1 3.7 1.4 0.5 5.9 3.1 1.8 0.9
7% Va, 75 mm tall

Differential Rut Depth (mm)
Profile measuring device 10.0 8.3 3.6 3.1 11.7 8.0 5.4 3.8
7% Va, 75 mm tall

Percent Area Change
Profile measuring device 0.88 0.82 -1.00 -0.27 1.82 1.69 -0.37 -0.10
7% Va, 75 mm tall

Figure 4-5 displays the absolute rut depths measured by the conventional

measuring device and profile measuring device and the differential rut depths measured

by the profile measuring device. In theory, the absolute rut depths measured by the

conventional measuring device and the profile measuring device should be approximately

the same, and this is displayed in Figure 4-5. The differential rut depths follow the same

trend as the absolute rut depths but with greater magnitude, as expected.

Figure 4-6 displays the absolute rut depths measured by the conventional

measuring device for the specimens compacted to seven percent air voids and a height of

75 mm. Figure 4-7 displays the absolute rut depths measured by the profile measuring

device and Figure 4-8 displays the differential rut depths measured by the profile

measuring device for the same specimens. In all cases, there is a significant decrease in

rut depth from round one to round two implying that adding more coarse aggregate is

beneficial in reducing the rutting susceptibility of the mixtures.







46




14
*Absolute Rut Depth Conventional Measuring Device
Absolute Rut Depth Profile Measuring Device
12 A Differential Rut Depth Profile Measuring Device
A














12

10


AL-1 AL-2 BV-1 BV-2 NS-1 NS-2 TM-1 TM-2
Aggregate Type and Round Number



Figure 4-5 Comparison of APA measurement methods for 7% air voids, 75 mm tall
specimens


7-

Round 1
6
Round 2

E 5


4


3 3


2





0
Alabama Limestone FL Brooksville Nova Scotia Granite FL Tarmac
Limestone Limestone
Aggregate Type



Figure 4-6 APA absolute rut depth using conventional measuring device for 7% air voids,
75 mm tall specimens







47




7 -

Round 1
6
ED Round 2

5m








2





0
Alabama Limestone FL Brooksville Nova Scotia Granite FL Tarmac
Limestone Limestone
Aggregate Type



Figure 4-7 APA absolute rut depth using profile measuring device for 7% air voids, 75
mm tall specimens


14

Round 1
12
E Round 2

o10





6





2


0
Alabama Limestone FL Brooksville Nova Scotia Granite FL Tarmac
Limestone Limestone
Aggregate Type



Figure 4-8 APA differential rut depth using profile measuring device for 7% air voids, 75
mm tall specimens










Figure 4-9 displays the percent area change for each aggregate type and round. A

positive area change indicates instability rutting manifested by shoving and heaving of

the mixture on each side of the rut. A negative percent area change indicates that the

majority of the rutting was due to consolidation. The percent area change decreased for

every aggregate type from round one to round two, a further indication that the addition

of coarse aggregate was beneficial in reducing the rutting susceptibility of the mixtures.

The two aggregate types with the lowest amount of rutting (Brooksville and Tarmac

Florida limestones) had negative percent area changes indicating that the small amount of

rutting those mixtures experienced was primarily due to consolidation. The two

aggregate types with the largest amount of rutting (Alabama limestone and Nova Scotia

granite) had positive percent area changes indicating that the rutting those mixtures

experienced was primarily due to instability under a load.


2.0
Round 1
1.5
O Round 2

S1.0

Q 0.5

S0.0

S-0.5

-1.0


-1.5
Alabama Limestone FL Brooksville Nova Scotia Granite FL Tarmac
Limestone Limestone
Aggregate Type



Figure 4-9 APA percent area change using profile measuring device for 7% air voids, 75
mm tall specimens







49


Figure 4-10 displays the absolute rut depths measured by the conventional

measuring device for the specimens compacted to four percent air voids and a height of

115 mm. In contrast to the specimens compacted to seven percent air voids and a height

of 75 mm, the rut depths increased from round one to round two for the Alabama

limestone and the Nova Scotia granite mixtures. The rut depth was essentially the same

between rounds one and two for the Florida Brooksville limestone. The rut depth

decreased for the Tarmac Florida limestone between rounds one and two. It is

undetermined why there is a difference in trends between rounds one and two for the

different sample types. However, it is noted that the two mixtures (Alabama limestone

and Nova Scotia granite) that showed an increase in rut depth between rounds one and

two for the specimens compacted to four percent air voids and a height of 115 mm were

the same mixtures that experienced instability rutting.


7 -
Round 1
6
l Round 2







1-
2




0
Alabama Limestone FL Brooksville Nova Scotia Granite FL Tarmac
Limestone Limestone
Aggregate Type


Figure 4-10 APA absolute rut depth using conventional measuring device for 4% air
voids, 115 mm tall specimens










Figure 4-11 displays the APA rut depth versus the VMA for the eight mixtures

examined in this study. The APA rut depth is measured using the conventional

measuring device for the specimens compacted to seven percent air voids and a height of

75 mm. The plot of the data shows a strong correlation between rut depth and VMA (R2

= 0.70). For these eight mixtures, as the VMA increased, the rut depth increased. This

effect is reasonable since higher VMA at a fixed air void content means that there is more

effective binder in the mix, which means there is more void space between the aggregate

particles, less stone on stone contact and a potentially less stable aggregate structure. A

similar correlation existed between APA rut depth and VFA.




NS-1

SAL-1


2 NS-2
& 4 re AL-2
Y= 0.0014x29439
R2 = 0.70

| ^^ TM-1


TM-2
1 \ BV-1
1 -- B V -2--------------------------



9 10 11 12 13 14 15 16 17
VMA (%)



Figure 4-11 APA rut depth versus VMA using conventional measuring device for 7% air
voids, 75 mm tall specimens

Figure 4-12 displays the APA rut depth versus the dust to effective binder ratio

(commonly called the dust ratio) for the eight mixtures examined in this study. The APA

rut depth is measured using the conventional measuring device for the specimens










compacted to seven percent air voids and a height of 75 mm. The plot of the data shows

a strong correlation between rut depth and dust ratio (R2 = 0.79). For these eight

mixtures, as the dust ratio increased, the rut depth decreased. This is reasonable since the

dust mixes with and stiffens the binder, hence increasing the rutting resistance of the

mixture.


7 -

NS-1
6 -
SAL-1


SNS-2
4
AL-2 y= 3.6193x-17889
R2 = 0.79

| TM-1
<2
BV-2
STM-2
I BV-1


0
0.5 1.0 1.5 2.0
Dust to Effective Binder Ratio



Figure 4-12 APA rut depth versus dust to effective binder ratio using conventional
measuring device for 7% air voids, 75 mm tall specimens

4.3.2 APA Summary

Two measurement techniques were used for obtaining APA rut depths; the

conventional single point absolute rut depth using a digital micrometer and the new

profile measuring device which provided the full rut profile of the mixture and provided

for the determination of the absolute and differential rut depths and the percent area

change of the rut profile. The absolute rut depths measured by the conventional and new

measuring techniques compared very well.









APA results for the specimens compacted to seven percent air voids and a 75 mm

height revealed that the addition of more coarse aggregate to the 12.5 mm and 9.5 mm

sieves, resulting in a more continuous graded mixture, improved the rutting performance

with respect to absolute and differential rut depths and percent area change regardless of

aggregate type.

Examining all eight mixtures as a group revealed a strong correlation showing that

increasing VMA resulted in an increase in rut depth. An even stronger correlation

showed that increasing the dust to effective binder ratio resulted in a decrease in rut

depth.

4.3.3 Servopac Test Results

There were three different parameters determined with the Servopac gyratory

compactor. The three parameters are

* Gyratory shear slope: A graph is created plotting the gyratory shear measured by
the Servopac compactor versus the natural log of the number of gyratory
revolutions. The air voids at each gyratory revolution are computed. The gyratory
shear slope is the slope of the graph in the range of compaction from seven to four
percent air voids or to the maximum gyratory shear if this value is reached prior to
four percent air voids. This value describes the rate at which the mixture develops
shear resistance and is an indication of the mixture's resistance to deformation.

* Vertical failure strain: Specimens are compacted at an angle of 1.25 degrees until
the specimens reach seven percent air voids. The angle of compaction is changed
to 2.50 degrees and the sample is gyrated for another 100 gyrations. The gyratory
shear versus the number of revolutions is plotted. The vertical failure strain is then
calculated from the point of angle change to the local minimum in gyratory shear
strength. This strain measurement is an indicator of the stability characteristics of
the mixture. The magnitude of the strain is an indicator of whether the mixture is
brittle, plastic or somewhere in between.

* Maximum gyratory shear strength: This is the maximum shear strength achieved
during the compaction process.










The values for gyratory shear slope, vertical failure strain and maximum gyratory

shear strength for all of the mixture types are presented in Table 4-7. Each value in Table

4-7 represents the average of two specimens.

Table 4-7 Servopac test results
FL Brooksville FL Tarmac
Alabama limestone Nova Scotia granite
Servopac Parameter limestone limestone
Round 1 Round 2 Round 1 Round 2 Round 1 Round 2 Round 1 Round 2
Gyratory Shear Slope 25 11 23 7 16 13 32 35
Percent Air Void Range 7.0 to 4.0 7.0 to 4.2 7.0 to 6.2 7.0 to 6.7 7.0 to 6.7 7.0 to 5.7 7.0 to 4.0 7.0 to 4.5

Percent Vertical Strain 1.83 1.67 n/a n/a n/a 1.97 2.13 1.66

Maximum Shear Stress (kPa) 744 726 772 775 679 689 750 751
Percent Air Voids 3.1 4.2 6.2 6.7 6.7 5.7 3.6 4.5

4.3.3.1 Gyratory shear slope

An example of a gyratory shear slope graph is shown in Figure 4-13 for the

Alabama limestone round one mixture. Six of the eight mixtures had a peak gyratory

shear strength prior to reaching a compaction level of four percent air voids. The

gyratory shear slope is then defined as the slope of the graph from the seven percent air

void level to the air void level at the point of maximum gyratory shear strength. An

example of this is shown in Figure 4-14 for the Florida Brooksville limestone round one

mixture.

The gyratory shear slope decreased for the Alabama limestone, Florida Brooksville

limestone and Nova Scotia granite mixtures from round one to round two indicating that

the round two mixtures did not develop shear resistance as rapidly as the round one

mixtures. The Florida Tarmac mixture had a slight increase in gyratory shear slope from

round one to round two. Roque et al. (2004a) indicated that mixtures with a gyratory

shear slope of less than 14 were undesirable with respect to rutting. All of the round one








54




745



740
y = 25.13x + 625.75
R2 = 0.98

4 735 A

SGyratory shear slope = 25.13

I 730

72 7% to 4% air voids

725
*


720
3.0 3.5 4.0 4.5 5.0
Natural Log Revolutions



Figure 4-13 Gyratory shear slope for Alabama limestone round one mixture



780

775 7% to 6.2% air voids y = 22.96x + 671.52
775 --
10% to 7% air voids R2 = 0.83
770 6.2% to 4% air voids ~ A"-

765

S760

y = -54.44x + 1012.75
S755
R2 = 0.93
Sy = 50.03x + 560.35
750 R 0
R2 0.99--
745

740

735-

730
3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0
Natural Log Revolutions



Figure 4-14 Gyratory shear slope for Florida Brooksville limestone round one mixture









mixtures had a gyratory shear slope greater than 14. Only one of the round two mixtures

(Florida Tarmac limestone mixture) had a gyratory shear slope greater than 14.

4.3.3.2 Vertical strain

Roque et al. (2004a) indicated that mixtures with a vertical strain in the range of 1.4

to 2.0 percent were desirable, whereas mixtures with a vertical strain less than 1.4 percent

would be considered "brittle" and mixtures with a vertical strain greater than 2.0 percent

would be considered "plastic." Examination of the data reveals that there was an

improvement from round one to round two for three of the four mixtures (Alabama

limestone, Nova Scotia granite, and Florida Tarmac limestone). The round two vertical

strain values for these mixtures were in the desirable range. It appears that the addition

of coarse aggregate to the round two mixtures improved the vertical strain values. An

example of a plot of the gyratory shear versus the number of revolutions is shown in

Figure 4-15 for the Alabama limestone round two mixture. The gyratory shear peaks

initially after the change in compaction angle to 2.50 degrees at a compaction level of

seven percent air voids and then drops to a local minimum as the particles rearrange

themselves. Shear strength then builds slowly and reaches a final peak before dropping

off.

Rounds one and two of the Florida Brooksville limestone mixture and round one of

the Nova Scotia granite mixture never reached a local minimum in gyratory shear

strength after the compaction angle was changed to 2.50 degrees and hence had no

vertical strain value to report. This is indicated as an "n/a" in Table 4-7. It appears that

these mixtures were never able to recover strength after the angle change in compaction

at the seven percent air void level. An example of the gyratory shear versus the number











1000

Vertical Gyratory shear local minimum
failure.
800 strain
region
Change in
angle from
S1.25 to 2.50
600




400




200
1 16 31 46 61 76 91 106 121 136
# Revolutions



Figure 4-15 Vertical strain for Alabama limestone round two mixture

of revolutions for this condition is shown in Figure 4-16 for the Florida Brooksville

limestone round one mixture. To determine if a mixture would recover shear strength

when the angle of compaction was changed at a different air void content other than

seven percent it was decided to make two more specimens of the Nova Scotia round one

mixture and change the angle of compaction at an air void content of nine percent instead

of seven percent. Figure 4-17 shows the gyratory shear versus the number of revolutions

for both of these conditions. It can be seen that the shear strength did recover slightly

when the angle of compaction was changed at nine percent air voids. This demonstrates

that some mixtures gain strength rapidly and peak at higher air void contents than other

mixtures.
























600


Chane m 1 No local minimum

1 16 31 46 61 76 91.25 to 2.50121 136 151
















1 16 31 46 61 76 91 106 121 136 151


# Revolutions



Figure 4-16 Vertical strain for Florida Brooksville limestone round one mixture



1000



800



600
2 IAngle change at 9% air voids
-- Angle change at 7% air voids

I 400



200



0
1 16 31 46 61 76 91 106 121 136 151
# Revolutions



Figure 4-17 Vertical strain for Nova Scotia granite round one mixtures









4.3.3.3 Maximum shear stress

The gyratory shear strength versus the percent air voids during compaction is

shown for rounds one and two of each aggregate type in Figures 4-18 through 4-21.

Examination of the data in Table 4-7 and Figures 4-18 through 4-21 reveals that the

maximum shear strength peaks at higher air void contents for the round two mixtures of

the Alabama limestone, Florida Brooksville limestone and Florida Tarmac limestone

mixtures. A possible explanation of this is that these round two mixtures have a more

continuous gradation and less VMA than their round one counterparts resulting in more

aggregate interlock at higher air voids and less asphalt binder to act as lubrication in the

compaction process.

Additionally, the Alabama limestone and Florida Tarmac limestone mixtures

tended to peak at lower air void contents (3.1 to 4.2 percent range) and then start to lose

strength. The Florida Brooksville limestone and Nova Scotia granite mixtures peaked at

higher air void contents (5.7 to 6.7 percent range) and then lost strength.

The maximum gyratory shear stress for each aggregate type correlated with the

APA rut depth for the seven percent air void specimens. Higher gyratory shear stress

values in the Servopac compactor were equivalent to lower rutting values in the APA (see

Figure 4-22).













800


700


600


500


400 !


300 2


200


100


0


25 20 15 10 5 0

Percent Air Voids


Figure 4-18 Gyratory shear versus percent air voids for Alabama limestone mixtures


900


800

700

Round 1
600
S- Round 2
5o500


400


300


200


100


0
30 25 20 15 10 5 0
Percent Air Voids



Figure 4-19 Gyratory shear versus percent air voids for Florida Brooksville limestone
mixtures



































25 20 15 10 5 0
Percent Air Voids


Figure 4-20 Gyratory shear versus percent air voids for Nova Scotia granite mixtures


15 10
Percent Air Voids


Figure 4-21 Gyratory shear versus percent air voids for Florida Tarmac limestone
mixtures


--- Round 2











7 -


6
NS-1 AL-1
5
I NS-2
4 4AL-2
4

y y 3.6193x-17889
3 R2=0.79 -TM-1

2
H TM-2 H

1 BV-2

0
0 ------------------------------
660 680 700 720 740 760 780
Servopac Gyratory Shear Strength (kPa)



Figure 4-22 Gyratory shear stress versus APA rut depth

4.3.4 Servopac Summary

The Servopac test results for vertical strain and maximum shear strength correlated

well with the APA test results. With respect to vertical strain, the mixtures showed an

improvement from round one to round two and the results stayed within the desirable

range of 1.4 to 2.0 percent. The ranking of the mixtures with respect to the APA test

results matched the rankings per the maximum gyratory shear test results. There was a

decrease in gyratory shear slope from round one to round two for three of the four

mixtures, indicating that the round one mixtures develop shear resistance at a faster rate

than the round two mixtures, though they do not necessarily achieve a greater maximum

shear strength.

Some mixtures tend to reach maximum shear strength at much higher air void

contents than other mixtures. For the eight mixtures examined in this study, the air void










content at which maximum shear strength was achieved differed by up to 3.6 percent.

Additionally, three of the four round two mixtures with lower VMA reached maximum

shear strength at a higher air void content that their counterpart round one mixtures. This

reveals one of the main problems with normal volumetric mix design procedures, where

all mixtures are designed at four percent air voids. Some mixtures may be optimal at this

air void content and others may not be. This is further justification for the need of one or

more performance tests for mix design purposes.

4.4 Cracking

The Superpave indirect tension (IDT) test results can be best described by three

parameters; energy ratio, dissipated creep strain energy and fracture energy. Test results

are shown in Table 4-8 for the unconditioned and long-term oven aged (LTOA)

specimens. Each parameter will be discussed separately below.

Table 4-8 Energy ratio values for the unconditioned and LTOA specimen
FL Brooksville FL Tarmac
Superpave IDT Test t C Alabama limestone litoe Nova Scotia granite limestone
m Test Condition limestone limestone
Parameter
Round 1 Round 2 Round 1 Round 2 Round 1 Round 2 Round 1 Round 2
Unconditioned 3.36 3.18 2.08 2.53 3.69 1.31 2.64 1.62
Energy Ratio d 100 psi
LTOA 1.20 1.93 5.93 5.33 4.04 3.44 4.65 8.58

Dissipated Creep Strain Unconditioned 4.2 4.1 1.5 1.3 5.7 4.2 2.6 1.6
Energy (kJ/m3) LTOA 1.2 2.0 1.4 0.8 6.4 5.0 1.7 2.4

S3 Unconditioned 4.4 4.3 1.7 1.5 5.9 4.4 2.8 1.7
Fracture Energy (kJ/m ) -- ^--- --- --------- -- ---
LTOA 1.4 2.1 1.6 1.1 6.6 5.2 1.9 2.6


4.4.1 Energy Ratio

As described in Chapter 3, the energy ratio is defined as the dissipated creep strain

energy threshold of a material divided by the minimum dissipated creep strain energy

needed. Roque et al. (2004b) have found this parameter effective in characterizing the

cracking performance of asphalt mixtures. Figure 4-23 shows the unconditioned energy

ratio and Figure 4-24 shows the LTOA energy ratio for rounds one and two for each

mixture type.















* Round 1

* Round 2


3.0


S2.5

2.0


1.5


1.0


0.5


0.0
Alabama Limestone FL Brooksville Nova Scotia Granite
Limestone

Aggregate Type



Figure 4-23 Energy ratios for unconditioned specimens


FL Tarmac
Limestone


* Round I

O Round 2


u.u

5.0

Z 4.0

3.0

2.0

1.0

0.0 ---
Alabama Limestone FL Brooksville Nova Scotia Granite FL Tarmac
Limestone Limestone

Aggregate Type



Figure 4-24 Energy ratios for long-term oven aged specimens


I









Figure 4-23 shows that the energy ratio decreased for three of the four

unconditioned mixtures (Alabama limestone, Nova Scotia granite and Florida Tarmac

limestone). The decrease was significant for the Nova Scotia granite and Florida Tarmac

limestone mixtures. It should be noted that these two mixtures had significant decreases

in VMA from rounds one to two. The energy ratio increased a moderate amount for the

Florida Brooksville limestone mixture. The implication is that the addition of coarse

aggregate at round two had an overall negative effect on the cracking performance of the

mixtures examined in this study.

With respect to the LTOA specimens, Figure 4-24 shows that the results were

mixed between rounds one and two. Energy ratios decreased for the Florida Brooksville

limestone mixture and Nova Scotia granite mixture and increased for the Alabama

limestone and Florida Tarmac limestone mixture. Overall, only the energy ratio for the

Florida Tarmac limestone mixture changed significantly. The reason for this is unknown.

Comparing the unconditioned energy ratios to the LTOA energy ratios reveal a

significant increase in energy ratio after aging for the Florida Brooksville and Tarmac

limestone mixtures. These mixtures contain aggregates that are highly absorptive

compared to the Alabama limestone and Nova Scotia granite mixtures.

4.4.2 Dissipated Creep Strain Energy (DCSE)

The DCSE of a mixture describes the amount of energy that a mixture can dissipate

through repeated loading before fracturing. Though the DCSE by itself cannot describe

completely the cracking performance of a mixture, as a rule of thumb, if other factors are

held constant, then a mixture with a greater DCSE will perform better than a mixture with

a lower DCSE. Figure 4-25 shows the unconditioned DCSE and Figure 4-26 shows the

LTOA DCSE for rounds one and two for each mixture type.













6.0

F Round 1
5.0
E3 Round 2


4.0


3.0


2.0



1.0


0.0
Alabama Limestone FL Brooksville Nova Scotia Granite FL Tarmac
Limestone Limestone

Aggregate Type



Figure 4-25 Dissipated creep strain energy for unconditioned specimens


7.0

0 Round 1
6.0
E Round 2

S5.0


S4.0


5 3.0


2.0


1.0


0.0
Alabama Limestone FL Brooksville Nova Scotia Granite FL Tarmac
Limestone Limestone
Aggregate Type



Figure 4-26 Dissipated creep strain energy for long-term oven aged specimens









Figure 4-25 shows that the DCSE decreased significantly for the Nova Scotia

granite and Florida Tarmac limestone unconditioned mixtures from round one to round

two. It should be noted that these two mixtures had significant decreases in VMA from

rounds one to two. There was a slight, if not insignificant, decrease in DCSE for the

Alabama limestone and Florida Brooksville limestone unconditioned mixtures. The

implication is that the addition of coarse aggregate at round two had a negative effect on

the DCSE of the mixtures examined in this study.

With respect to the LTOA specimens, Figure 4-26 shows that the results were

mixed between rounds one and two. DCSE decreased for the Florida Brooksville

limestone mixture and Nova Scotia granite mixture and increased for the Alabama

limestone and Florida Tarmac limestone mixture. This is the same trend as occurred for

the energy ratios of the LTOA specimens.

4.4.3 Fracture Energy (FE)

The FE of a mixture describes the total amount of energy (elastic energy plus

dissipated energy) that a mixture can withstand before fracturing. Though the FE by

itself cannot describe completely the cracking performance of a mixture, as a rule of

thumb, if other factors are held constant, then a mixture with a greater FE will perform

better than a mixture with a lower FE. Figure 4-27 shows the unconditioned FE and

Figure 4-28 shows the LTOA FE for rounds one and two for each mixture type.





































Alabama Limestone FL Brooksville Nova Scotia Granite
Limestone

Aggregate Type


FL Tarmac
Limestone


Figure 4-27 Fracture energy for unconditioned specimens



7.0

0 Round 1
6.0
F Round 2

5.0


4.0


3.0


S2.0


1.0


0.0
Alabama Limestone FL Brooksville Nova Scotia Granite FL Tarmac
Limestone Limestone

Aggregate Type



Figure 4-28 Fracture energy for long-term oven aged specimens









Figure 4-27 shows that the FE decreased significantly for the Nova Scotia granite

and Florida Tarmac limestone unconditioned mixtures from round one to round two. It

should be noted that these two mixtures had significant decreases in VMA from rounds

one to two. There was a slight, if not insignificant, decrease in FE for the Alabama

limestone and Florida Brooksville limestone unconditioned mixtures. The implication is

that the addition of coarse aggregate at round two had a negative effect on the FE of the

mixtures examined in this study.

With respect to the LTOA specimens, Figure 4-28 shows that the results were

mixed between rounds one and two. FE decreased for the Florida Brooksville limestone

mixture and Nova Scotia granite mixture and increased for the Alabama limestone and

Florida Tarmac limestone mixture. This is the same trend as occurred for the energy

ratios and DCSE of the LTOA specimens.

4.4.4 Cracking Summary

The energy ratio, dissipated creep strain energy and fracture energy test results

from the Superpave IDT test indicate that the addition of coarse aggregate, resulting in a

more continuous gradation and reduction in VMA, had an overall negative effect on the

cracking performance when examining the unconditioned specimens. Only the energy

ratio for the Florida Brooksville limestone mixture showed an increase from round one to

round two.

Results were mixed and not conclusive for the LTOA specimens. However, the

highly absorptive Florida limestone mixtures showed significant increases in energy ratio

for the LTOA specimens compared to the unconditioned specimens. This trend was not

evident for the Alabama limestone and Nova Scotia granite mixtures.










4.5 Moisture Damage

Moisture damage was evaluated using the standard Department test procedure (FM

1-T 283) and by using the Superpave IDT tests on moisture conditioned specimens.

From the suite of Superpave IDT tests, the energy ratio was calculated for both the

unconditioned and conditioned specimens. Test results are shown in Table 4-9. Each

parameter will be discussed separately below.

Table 4-9 Moisture damage test results
FL Brooksville FL Tarmac
Alabama limestone FL Nova Scotia granite
Test Method and Condition Alabama limestone limestone Nova Scotia grate limestone
Round 1 Round2 Round 1 Round2 Round 1 Round2 Round 1 Round2
S Unconditioned 989 941 1026 1208 785 843 855 1025
FM 1-T 283 Tensile
S engt a Moisture Conditioned 764 840 596 698 647 697 699 791
Ratio 77 89 58 58 83 83 82 77

Unconditioned 3.36 3.18 2.08 2.53 3.69 1.31 2.64 1.62
Energy Ratio Moisture Conditioned 2.87 2.66 1.87 0.97 0.44 1.08 2.61 1.50
Ratio 85 84 90 38 12 82 99 93

4.5.1 Conventional FM 1-T 283 Test Results

Examination of the data in Table 4-9 does not indicate any trends with respect to

tensile strength ratio (TSR). The Alabama limestone mixture had a twelve percent

increase in TSR. The Florida Brooksville limestone and Nova Scotia granite mixtures

showed no change in TSR and the Florida Tarmac limestone mixture showed a mild five

percent reduction in TSR. However, with respect to the tensile strengths, every mixture

had an increase in unconditioned and conditioned tensile strengths from round one to

round two except for the Alabama limestone unconditioned mixture, which had a mild

reduction in unconditioned tensile strength (7 psi) from round one to round two.

4.5.2 Superpave IDT Test Results (Energy Ratio)

Examination of the data in Table 4-9 reveals a different outcome than the FM 1-T

283 test results. Only the Nova Scotia granite conditioned results showed an increase in










energy ratio from round one to round two. The other three comparisons showed a

decrease in energy ratio from round one to round two.

4.5.3 Moisture Damage Summary

Moisture damage test results were dependent on the test method used. The

standard Department test method, FM 1-T 283, revealed that tensile strengths increased

for unconditioned and conditioned specimens from round one to round two. Superpave

IDT test results showed that for three of four comparisons, the energy ratio decreased

from round one to round two.

4.6 Permeability

The permeability values for rounds one and two of each mixture type are presented

in Table 4-10. Permeability values were essentially the same between rounds one and

two. The addition of coarse aggregate in round two did not affect the permeability of the

mixture. Perhaps there was an offsetting effect between adding more coarse aggregate,

which would increase the permeability of the mixture, and the more continuous gradation

which being closer to the maximum density line, would tend to decrease permeability.

Table 4-10 Permeability test data
Permeability (x 10-5 cm/s)
FL Brooksville FL Tarmac
Specimen Number Alabama limestone Nova Scotia granite
limestone limestone
Round 1 Round 2 Round 1 Round 2 Round 1 Round 2 Round 1 Round 2
Specimen 1 41 15 13 15 6 14 39 121
Specimen 2 24 9 16 17 9 13 64 16
Specimen 3 13 49 28 27 13 9 69 24
Specimen 4 n/a 31 n/a n/a n/a n/a 37 n/a
Average 26 26 19 20 10 12 52 54














CHAPTER 5
CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusions

* Rutting potential, as measured by the Asphalt Pavement Analyzer (APA), showed
an improvement in rut performance by the addition of more coarse aggregate on the
12.5 mm and 9.5 mm sieves, resulting in a more continuous gradation. This
improvement was evident with absolute rut depth, differential rut depth and percent
area change of the rut profile, when testing 75 mm tall specimens compacted to
seven percent air voids and tested at 64 C.

* APA test results also showed a strong correlation (R2 = 0.70) that increasing VMA
resulted in an increase in rut depth. An even stronger correlation (R2 = 0.79)
showed that increasing the dust to effective binder ratio resulted in a decrease in rut
depth.

* Rutting potential, as measured with the Servopac gyratory compactor, was
evaluated with the following parameters: gyratory shear slope, vertical strain and
maximum gyratory shear stress. Vertical strain and maximum gyratory shear stress
test results correlated well with the APA test results. With respect to vertical strain,
the mixtures showed an improvement from round one to round two and the results
stayed within the desirable range of 1.4 to 2.0 percent. The ranking of the mixtures
with respect to the APA test results matched the rankings per the maximum
gyratory shear test results. There was a decrease in gyratory shear slope from
round one to round two for three of the four mixtures, indicating that the round one
mixtures develop shear resistance at a faster rate than the round two mixtures.
However, the round one mixtures did not necessarily achieve a greater maximum
shear strength than the round two mixtures.

* Servopac test results show that mixtures achieve their maximum gyratory shear
strength over a wide range of air voids compared to each other. Designing all
mixtures at four percent air voids may not result in the optimum mixture design for
all mixtures with respect to rut resistance.

* The energy ratio, dissipated creep strain energy and fracture energy test results
from the Superpave IDT test indicate that the addition of coarse aggregate,
resulting in a more continuous gradation and reduction in VMA, had an overall
negative effect on the cracking performance. Only the energy ratio for the Florida
Brooksville limestone mixture showed an increase from round one to round two.









* Conclusions with respect to moisture damage were dependent on the test method
used. The standard Department test method, FM 1-T 283, revealed that the
addition of coarse aggregate in round two resulted in increased tensile strengths for
unconditioned and conditioned specimens. However, Superpave IDT test results
showed that for three of four comparisons, the energy ratio decreased with the
addition of coarse aggregate in round two.

* Permeability characteristics of the mixtures were not affected by the addition of
coarse aggregate. Most likely there was an offsetting effect between adding more
coarse aggregate, which would tend to increase the permeability of the mixture, and
the more continuous gradation, which being closer to the maximum density line,
would tend to decrease permeability.

5.2 Recommendations

* Cracking is the predominant mode of distress (approximately 80%) for the asphalt
roads in Florida. For the mixtures evaluated in this study, the addition of coarse
aggregate on the 12.5 mm and 9.5 mm sieves indicated an overall reduction in
cracking performance. Therefore, it is not recommended at this time to lower the
VMA specification requirement for coarse graded mixtures.

* For situations where rutting performance is a high priority, the addition of coarse
aggregate, with the potential for a lower than specified VMA, should be
considered.

* The Department should continue work towards the implementation of one or more
performance tests at the mix design stage. Possible candidate test methods include
the APA, Servopac and Superpave indirect tension test. As a first step, the
Department could specify minimum performance values that mixtures would be
required to meet at the mix design stage.

* Testing in this study and others has revealed that not all mixtures have optimal
performance when volumetrically designed according to current Superpave mixture
design requirements. Research exploring new mix design methodologies, which
optimize a mixture's performance based on laboratory performance test(s), should
be explored. Gradations and asphalt contents would be selected to optimize
performance, not to meet certain volumetric criteria.
















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BIOGRAPHICAL SKETCH

Gregory Allen Sholar was born in Hollywood, Florida, in 1966 to Thomas and

Barbara Sholar. Greg graduated from Pine Crest Preparatory School in Ft. Lauderdale in

1984 and then achieved a bachelor's degree in building construction from the University

of Florida in 1988. Not satisfied with a career in the building construction industry and

having the thirst for more knowledge, Greg went back to the University of Florida and

obtained a bachelor's degree in civil engineering in 1996. While in college, Greg worked

part time at the State Materials Office of the Florida Department of Transportation and

thoroughly enjoyed it. Fortunately, Greg was able to obtain a fulltime position there after

graduation and has enjoyed working in bituminous research. In 1999, Greg enrolled at

the University of Florida part time to obtain a master's degree in civil engineering

materials. Greg plans to continue to work in his current capacity upon graduation.