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Analysis and verification of stresses and strains and their relationship to failure in concrete pavements under heavy ve...

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Permanent Link: http://ufdc.ufl.edu/UFE0010099/00001

Material Information

Title: Analysis and verification of stresses and strains and their relationship to failure in concrete pavements under heavy vehicle simulator loading
Physical Description: Mixed Material
Language: English
Creator: Kumara, Mampe Arachchige Wasantha ( Dissertant )
Tia, Mang ( Thesis advisor )
Ruth, Byron E. ( Reviewer )
Roque, Reynaldo ( Reviewer )
Birgisson, Bjorn ( Reviewer )
Yang, Mark ( Reviewer )
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2005
Copyright Date: 2005

Subjects

Subjects / Keywords: Civil and Coastal Engineering thesis, Ph.D
Dissertations, Academic -- UF -- Civil and Coastal Engineering

Notes

Abstract: Research was performed to evaluate the behavior and performance of concrete pavement slabs at an early age under heavy vehicle simulator (HVS) loading. A concrete pavement test track was constructed at the accelerated pavement testing facility of the Florida Department of Transportation (FDOT). The test sections were instrumented with strain gauges and thermocouples to collect strain and temperature data. The finite element model FEACONS IV was used to analyze pavement behavior. Model parameters were determined by matching the deflection basins caused by the Falling Weight Deflectometer (FWD) load and the computed deflection basin, using FEACONS IV the finite element model. The measured maximum strains caused by a moving HVS wheel load were found to match fairly well with the measured maximum strains caused by a static wheel load of the same magnitude. The difference between static and dynamic strains for the same magnitude load was small and fluctuated between positive and negative values. The FEACONS program was used to calculate the maximum stresses in each test slab due to the HVS loads at various times in this study. The applicable pavement parameters (effective modulus of subgrade reaction, joint stiffness, and edge stiffness), concrete elastic modulus, HVS load, and temperature differential in the concrete slab for each particular condition were used in each analysis. The computed stress-to-strength ratio can be used to explain the observed performance of the test slabs used in the slab- replacement study. The properties needed to ensure adequate performance of concrete pavement at early age were determined. Impact echo tests were used successfully in this study to detect cracks in a concrete slab. This was manifested by a sudden drop in the apparent measured speed of P waves across the location of cracks. Cracks in the concrete slab were also successfully detected from observed changes in the measured strains from strain gauges that had been installed in the concrete.
Subject: concrete, cracks, failure, heavy, HVS, pavement, replacement, rigid, simulator, slab, strain, strength, stress, vehicle
General Note: Title from title page of source document.
General Note: Document formatted into pages; contains 163 pages.
General Note: Includes vita.
Thesis: Thesis (Ph.D.)--University of Florida, 2005.
Bibliography: Includes bibliographical references.
General Note: Text (Electronic thesis) in PDF format.

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 003322402
System ID: UFE0010099:00001

Permanent Link: http://ufdc.ufl.edu/UFE0010099/00001

Material Information

Title: Analysis and verification of stresses and strains and their relationship to failure in concrete pavements under heavy vehicle simulator loading
Physical Description: Mixed Material
Language: English
Creator: Kumara, Mampe Arachchige Wasantha ( Dissertant )
Tia, Mang ( Thesis advisor )
Ruth, Byron E. ( Reviewer )
Roque, Reynaldo ( Reviewer )
Birgisson, Bjorn ( Reviewer )
Yang, Mark ( Reviewer )
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2005
Copyright Date: 2005

Subjects

Subjects / Keywords: Civil and Coastal Engineering thesis, Ph.D
Dissertations, Academic -- UF -- Civil and Coastal Engineering

Notes

Abstract: Research was performed to evaluate the behavior and performance of concrete pavement slabs at an early age under heavy vehicle simulator (HVS) loading. A concrete pavement test track was constructed at the accelerated pavement testing facility of the Florida Department of Transportation (FDOT). The test sections were instrumented with strain gauges and thermocouples to collect strain and temperature data. The finite element model FEACONS IV was used to analyze pavement behavior. Model parameters were determined by matching the deflection basins caused by the Falling Weight Deflectometer (FWD) load and the computed deflection basin, using FEACONS IV the finite element model. The measured maximum strains caused by a moving HVS wheel load were found to match fairly well with the measured maximum strains caused by a static wheel load of the same magnitude. The difference between static and dynamic strains for the same magnitude load was small and fluctuated between positive and negative values. The FEACONS program was used to calculate the maximum stresses in each test slab due to the HVS loads at various times in this study. The applicable pavement parameters (effective modulus of subgrade reaction, joint stiffness, and edge stiffness), concrete elastic modulus, HVS load, and temperature differential in the concrete slab for each particular condition were used in each analysis. The computed stress-to-strength ratio can be used to explain the observed performance of the test slabs used in the slab- replacement study. The properties needed to ensure adequate performance of concrete pavement at early age were determined. Impact echo tests were used successfully in this study to detect cracks in a concrete slab. This was manifested by a sudden drop in the apparent measured speed of P waves across the location of cracks. Cracks in the concrete slab were also successfully detected from observed changes in the measured strains from strain gauges that had been installed in the concrete.
Subject: concrete, cracks, failure, heavy, HVS, pavement, replacement, rigid, simulator, slab, strain, strength, stress, vehicle
General Note: Title from title page of source document.
General Note: Document formatted into pages; contains 163 pages.
General Note: Includes vita.
Thesis: Thesis (Ph.D.)--University of Florida, 2005.
Bibliography: Includes bibliographical references.
General Note: Text (Electronic thesis) in PDF format.

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 003322402
System ID: UFE0010099:00001


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ANALYSIS AND VERIFICATION OF STRE SSES AND STRAINS AND THEIR RELATIONSHIP TO FAILURE IN CONCRETE PAVEMENTS UNDER HEAVY VEHICLE SI MULATOR LOADING By MAMPE ARACHCHIGE WASANTHA KUMARA A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2005

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Copyright 2005 by Mampe Arachchige Wasantha Kumara

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To my parents and to Lakmini, and Sahanya.

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iv ACKNOWLEDGMENTS I wish to express my deepest gratitude to my supervisory committee chair (Dr. Mang Tia) for continuously guidi ng and assisting me throughout my graduate studies at the University of Florida (UF). Appreciation is also extended to the other members of my committee (Drs. Byron E. Ruth, Reynaldo Roque, Bjorn Birgisson, and Mark Yang) whose opinions and guidance have been i nvaluable in the co mpletion of my study. I wish to thank the Florida Department of Transportation (FDOT) for sponsoring of the research that made this dissertation possi ble. I also give tha nks to FDOT Material Office personnel (particularly Dr. Bouzid Choubane, Dr. Alexander Appea, and Messrs. Michael Bergin, Tom Byron, Steve Ross, Aa ron Philpott, Charles Ishee, Richard DeLorenzo, Salil Gokhale, Abdenour Nazef, Jerr y Moxley and Vidal Francis). I also extend my thanks to Dr. Chung-lung Wu, and Dr J. M. Armaghani for their suggestions and guidance at project meetings. Thanks ar e also extended for contributions made by personnel from Dynatest and Florida Rock Industries. Special gratitude is also e xpressed to the staff of th e Department of Civil and Coastal Engineering (particularly George Lopp, Doretha Ray, Sonja Lee and Carol Hipsley) for providing necessary support for my research a nd academic work during my study at UF. Thanks are also extended to coll eagues in the pavement and infrastructure materials groups for helping me in different wa ys. I wish to express my sincere thanks to my former professor (Dr. Manjriker Gunaratne) at the University of South Florida for his guidance and help on my studies in the Unite d States (US). I also wish to thank the

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v University of Moratuwa, Sri Lanka for conti nuous assistance for my higher studies in the US. Finally, the greatest thanks go to my parents; to my wife, Lakmini Wadanami; and to my daughter, Sahanya, for their patience and sacrifices throughout my studies.

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vi TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES.............................................................................................................ix LIST OF FIGURES.............................................................................................................x ABSTRACT....................................................................................................................... xv CHAPTER 1 INTRODUCTION...................................................................................................1 1.1 Background..................................................................................................1 1.2 Problem Statement.......................................................................................2 1.3 Research Hypothesis....................................................................................3 1.4 Objectives....................................................................................................3 2 LITERATURE REVIEW........................................................................................4 2.1 Structural Analysis of Concrete Pavements.................................................4 2.1.1 Foundation Models..........................................................................4 2.1.1.1 Dense-liquid foundation model............................................4 2.1.1.2 Elastic-solid foundation.......................................................5 2.1.1.3 Improved models using a modi fied Winkler foundation.....6 2.1.1.4 Improved models by using a modified elastic-solid foundation............................................................................8 2.1.2 Analytical Solutions for Concrete Pavement Response to Traffic Loading............................................................................................9 2.1.3 Numerical Solutions for Concrete Pavement Response to Traffic Loading..............................................................................13 2.1.3.1 Discrete element method (DEM).......................................14 2.1.3.2 Finite element method........................................................14 2.1.3.3 Finite difference method (FDM)........................................19 2.2 Review of Concrete Pavement Failures in Slab Replacement...................19 2.3 Accelerated Pavement Testing...................................................................22

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vii 3 STRESS ANALYSIS CONVENTIONAL CONCRETE PAVEMENT SLABS..27 3.1 Method of Analysis....................................................................................27 3.2 Results of Analysis....................................................................................29 3.3 Results of Previous Parametric Studies of Factors Affecting Stresses in Concrete Pavement....................................................................................38 4 DESIGN AND CONSTRUCTI ON OF TEST SECTIONS...................................41 4.1. Description of the Experiment...................................................................41 4.2 Construction of Concrete Test Track.........................................................42 4.3 Removing of Concrete Slabs.....................................................................44 4.4 Dowel Bar Placement................................................................................48 4.5 Instrumentation Layout..............................................................................49 4.6 Placement of Strain Gauges.......................................................................53 4.7 Placement of Thermocouples.....................................................................54 4.8 Placement of Concrete...............................................................................56 4.9 Concrete Finishing and Sawing Joints.......................................................57 5 TESTING OF TEST SLABS.................................................................................61 5.1 Concrete Mix Characteristic......................................................................61 5.2 Concrete Testing........................................................................................61 5.3 HVS Loading.............................................................................................61 5.3.1 Slab 1C...........................................................................................66 5.3.3 Slab 2C...........................................................................................67 5.3.4 Slab 2E...........................................................................................67 5.3.5 Slab 2G...........................................................................................68 5.4 Temperature Data.......................................................................................68 5.5 Impact Echo Test.......................................................................................72 5.6 FWD Test...................................................................................................76 6 OBSERVED PERFORMANCE OF THE TEST SLABS.....................................78 6.1 Slab 1C.......................................................................................................78 6.2 Slab 1G.......................................................................................................78 6.3 Slab 2C.......................................................................................................83 6.4 Slab 2E.......................................................................................................87 6.5 Slab 2G.......................................................................................................87 7 ANALYSIS OF DATA..........................................................................................90 7.1 Estimation of Model Parameters................................................................90 7.2 Analysis of Dynamic Strain Data..............................................................94 7.2.1 Analysis of Measured Dynamic Strains for Detection of Cracks..94 7.2.2 Comparison of Measured Strain s with Computed Strains.............95 7.3 Analysis of Static Strain Data....................................................................97 7.4 Impact Echo Test Results.........................................................................102

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viii 7.5 Analysis of Performance of Concrete Mixes...........................................102 7.5.1 Computation of Stresses in the Test Slabs...................................102 7.5.2 Relating Stress/Strength Ratio to Observed Performance...........110 7.5.3 Required Concrete Properties for Performance...........................118 8 CONCLUSIONS AND RECOMMENDATIONS..............................................122 8.1 Summary of Findings...............................................................................122 8.2 Conclusions..............................................................................................124 8.3 Recommendations....................................................................................125 APPENDIX A HVS TESTING AND DATA COLLECTION SCHEDULE..............................126 B FWD DATA.........................................................................................................137 LIST OF REFERENCES.................................................................................................142 BIOGRAPHICAL SKETCH...........................................................................................147

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ix LIST OF TABLES Table page 4-1 Properties of the concrete used on the initial concrete test track...........................43 4-2 Strain gauge locations and identification numbers................................................52 5-1 Mix designs of concrete used in test slabs.............................................................62 5-2 Fresh concrete properties.......................................................................................63 5-3 Compressive strength, elastic m odulus and flexural strength data........................64 7-1 Stress analysis for slab 1C (Mix 1)......................................................................111 7-2 Stress analysis for slab 1G (Mix2).......................................................................112 7-3 Stress analysis for slab 2C (Mix3).......................................................................113 7-4 Stress analysis for slab 2E (Mix 4)......................................................................114 7-5 Stress analysis for slab 2G (Mix 5)......................................................................115 A-1 Schedule of testing and data collection for test slab 1C......................................126 A-2 Schedule of testing and data collection for test slab 1G......................................129 A-3 Schedule of testing and data collection for test slab 2C......................................132 A-4 Schedule of testing and data collection for test slab 2E.......................................134 A-5 Schedule of testing and data collection for test slab 2G......................................136

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x LIST OF FIGURES Figure page 3-1 Loading positions used in the stress analysis.........................................................28 3-2 Distribution of maximum principal stre sses due to a 12-kip load at the slab corner for the condition of no lo ad transfer at the joints........................................29 3-3 Distribution of stresses in the xx dire ction due to a 12-kip load at the slab corner for the condition of no lo ad transfer at the joints........................................30 3-4 Distribution of stresses in the yy dire ction due to a 12-kip load at the slab corner for the condition of no lo ad transfer at the joints........................................31 3-5 Distribution of maximum principal stre sses due to a 12-kip load at the slab corner for the condition of good lo ad transfer at the joints....................................32 3-6 Distribution of stresses in the xx dire ction due to a 12-kip load at the slab corner for the condition of good lo ad transfer at the joints....................................33 3-7 Distribution of stresses in the yy dire ction due to a 12-kip load at the slab corner for the condition of good lo ad transfer at the joints....................................34 3-8 Distribution of maximum principal st resses on the adjacent slab due to a 12-kip load at the slab corner for th e condition of good load transfer at the joints.......................................................................................................................35 3-9 Distribution of stresses in the xx di rection on the adjacent slab due to a 12-kip load at the slab corner for th e condition of good load transfer at the joints.......................................................................................................................36 3-10 Distribution of stresses in the yy di rection on the adjacent slab due to a 12-kip load at the slab corner for th e condition of good load transfer at the joints.......................................................................................................................37 3-11 Distribution of maximum principal stre sses due to a 12-kip load at the midedge for the condition of no lo ad transfer at the joints..........................................38 4-1 Layout of concrete slabs on test track....................................................................42 4-2 Placement of concre te on test track.......................................................................43

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xi 4-3 Finished concrete test track....................................................................................44 4-4 Making 3-inch deep saw cuts at the joints.............................................................45 4-5 Separation of concrete slab (12 ft 16 ft) into small pieces (3 ft 4 ft)................45 4-6 Separated concrete slab using diamond bladed saw..............................................46 4-7 Removal of separated pieces using the lifter.........................................................46 4-8 Correcting the damage portion of asphalt base......................................................47 4-9 Removal of concrete pieces adjacent to the surrounding slabs..............................47 4-10 Drilling dowel bar holes.........................................................................................48 4-11 Dowel bars epoxied to an adjacent sl ab before placement of the test slab............49 4-12 Strain gauge arrangement in a half bridge circuit..................................................50 4-13 Connection of the active and dummy stra in gauges in the half bridge circuit.......50 4-14 Instrumentation layout for test slab 1C..................................................................51 4-15 Instrumentation layout for test slabs 1G, 2C, 2E and 2G......................................52 4-16 Strain gauges fixed to the asphalt base using nylon rods.......................................53 4-17 Strain gauges protected by a PVS pipe before placement of concrete..................54 4-18 Thermocouples fi xed to a wooden rod...................................................................55 4-19 Placement of concrete around ther mocouples attached to a rod............................55 4-20 Formwork for the free edge of test slab 1C...........................................................57 4-21 Placing concrete around strain gauges...................................................................58 4-22 Placement of concrete for a test slab......................................................................59 4-23 Placement of concrete around dummy gauges in wooden blocks.........................59 4-24 Leveling of concrete surface..................................................................................60 4-25 Making a 3-inch deep saw cut at the joint.............................................................60 5-1 Comparison of compressive stre ngth of the concrete mixes used.........................65 5-2 Temperature differentials at slab 1C......................................................................69

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xii 5-3 Temperature differentials at slab 1G......................................................................70 5-4 Temperature differentials at slab 2C......................................................................70 5-5 Temperature differentials at slab 2E......................................................................71 5-6 Temperature differentials at slab 2G......................................................................71 5-7 Schematic representation of test set-up for wave speed measurement..................73 5-8 Waveforms from impact echo test for P-wave speed measurement......................73 5-9 Sketch of steel template for ma rking impact and receiver locations.....................74 5-10 Receiver and impact locations on test slab for impact echo test............................75 6-1 Shrinkage cracks on test slab 1C...........................................................................79 6-2 Corner crack on slab 1C.........................................................................................79 6-3 Cracked slab 1C at the end of HVS testing............................................................80 6-4 Crack map of slab 1C.............................................................................................81 6-5 Corner crack at the southern end of slab 1G..........................................................82 6-6 Transverse cracks at th e mid-edge of slab 1G.......................................................82 6-7 Crack propagation at the mid-edge of slab 1G with additional loading................83 6-8 Crack map of slab 1G............................................................................................84 6-9 Transverse cracks at mid-edge of slab 2C.............................................................85 6-10 Cracks on slab 2C at the end of HVS testing.........................................................85 6-11 Crack map of slab 2C.............................................................................................86 6-12 Transverse crack on lab 2E at the middle of the slab............................................87 6-13 Crack map of slab 2E.............................................................................................88 6-14 Transverse crack on slab 2G at the mid.................................................................88 6-15 Crack map of slab 2G............................................................................................89 7-1 Measured and computed deflection basi ns caused by a 9-kip FWD load at slab center for slab 1G...................................................................................................91

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xiii 7-2 Measured and computed deflection basi ns caused by a 9-kip FWD load at slab center for slab 2C...................................................................................................92 7-3 Measured and computed deflection ba sin caused by a 9-kip FWD load at slab joint for slab 1G.....................................................................................................93 7-4 Measured and computed deflection ba sins caused by a 9-kip FWD load at a free edge for slab 1G..............................................................................................93 7-5 Measured dynamic strains from gauge 3 on slab 2C.............................................95 7-6 Measured dynamic strains from gauge 4 on slab 2E.............................................96 7-7 Maximum measured compressive strain from gauge 4 on slab 2E.......................96 7-8 Measured and computed st rains for gauge 1 on slab 1C.......................................98 7-9 Measured and computed st rains for gauge 2 on slab 1C.......................................98 7-10 Measured and computed st rains for gauge 4 on slab 1C.......................................99 7-11 Measured and computed st rains for gauge 5 on slab 1C.......................................99 7-12 Measured and computed st rains for gauge 6 on slab 1C.....................................100 7-13 Measured and computed st rains for gauge 7 on slab 1C.....................................100 7-14 Measured strains at slab 1G in th e first method of applying a static load...........103 7-15 Measured strains at slab 2G in the second method of applying a static load.......103 7-16 Comparison of maximum measur ed dynamic and static strains..........................104 7-17 Grid lines for impact echo test and location of corner crack on slab 1G.............105 7-18 P-wave Speed along line 3 at corner of slab 1G..................................................106 7-19 Measured P-wave speed along li ne 4 at corner of slab 1G..................................106 7-20 Measured P-wave speed along li ne 8 at corner of slab 1G..................................107 7-21 Measured P-wave speed along line 10 at corner of slab 1 G...............................107 7-22 Measured P-wave speed along li ne 15 at corner of slab 1G................................108 7-23 Measured P-wave speed along li ne 16 at corner of slab 1G................................108 7-24 Stress/ flexural strength ratio versus HVS passes................................................116

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xiv 7-25 easured strains from slab 2C n the first 6 hours...................................................118 7-26 Computed stress/strength ratio versus compressive st rength of concrete using ACI equations for relating fc, E and flexural strength.........................................119 7-27 Relationship between compressi ve strength and elastic modulus.......................120 7-28 Relationship between flexural st rength and compressive strength......................120 7-29 Computed stress/strength ratio as a function of compressive strength using the developed relationship between fc, E and flexural strength...........................121 B-1 FWD test at center of slab 2C..............................................................................137 B-2 Test at center of slab 1G......................................................................................138 B-3 FWD test at joint 1G-1F......................................................................................139 B-4 FWD test at free edge-1G....................................................................................140 B-5 FWD test at a confined edge................................................................................141

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xv Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ANALYSIS AND VERIFICATION OF STRE SSES AND STRAINS AND THEIR RELATIONSHIP TO FAILURE IN CONCRETE PAVEMENTS UNDER HEAVY VEHICLE SI MULATOR LOADING By Mampe Arachchige Wasantha Kumara May 2005 Chair: Mang Tia Major Department: Civil and Coastal Engineering Research was performed to evaluate the behavior and performance of concrete pavement slabs at an early age under heavy vehicle simulator (HVS) loading. A concrete pavement test track was constructed at the accelerated pavement testing facility of the Florida Department of Transportation (FDOT). The test sections were instrumented with strain gauges and thermocouples to collect strain and temperat ure data. The finite element model FEACONS IV was used to analyze pavement behavior. Model parameters were determined by matching the deflection basins caused by the Falling Weight Deflectometer (FWD) load and the computed deflection basin, using FEACONS IV the finite element model. The measured maximum strains caused by a moving HVS wheel load were found to match fairly well with the measured maximum strains caused by a static wheel load of the same magnitude The difference between static and dynamic strains for the same magnitude load was small and fluctuated between positive and negative values.

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xvi The FEACONS program was used to calculate the maximum stresses in each test slab due to the HVS loads at various times in this study. The applicable pavement parameters (effective modulus of subgrade reaction, joint stiffness, and edge stiffness), concrete elastic modulus, HVS load, and temper ature differential in the concrete slab for each particular condition were used in each analysis. The computed stress-to-strength ratio can be used to explain the observed perf ormance of the test slabs used in the slabreplacement study. The properties needed to ensure adequate performance of concrete pavement at early age were determined. Impact echo tests were used successfully in this study to detect cr acks in a concrete slab. This was manifested by a sudden drop in the apparent measured speed of P waves across the location of cracks. Cracks in the conc rete slab were also successfully detected from observed changes in the measured strain s from strain gauges that had been installed in the concrete.

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1 CHAPTER 1 INTRODUCTION 1.1 Background Full-slab replacement is a common met hod for repairing badly deteriorated concrete pavement slabs. In Florida, this ty pe of repair work is typically performed at night, and the repaired slabs are opened to traffic by the next mo rning. It is essential that this repair work be finished in a minimal am ount of time. High-early -strength concrete is typically used in this applic ation in order to have suffici ent strength within a few hours after placement. The Florida Department of Transportation (FDOT) currently specifies that slabreplacement concrete must have a minimu m 6-hour compressive strength of 15.2 MPa (2200 psi) and a minimum 24 hour compressi ve strength of 20.7 MPa (3000 psi) ( 1 ). The California Department of Transportation (Caltr ans) has conducted research on the use of fast-setting hydraulic cement concrete (FSH CC) in slab replacement using the HVS. Fatigue resistance of the FSHCC was found to be similar to the fatigue resistance of the normal Portland cement concrete ( 2 ). Caltrans developed st andard special provisions (SSP) for slab and lane/shoulder replacement. However, there is no SSP for slab replacement with dowel bars. The current specification for slab replacement with no dowel requires a minimum modulus of rupture at opening to traffic of 2.3 MPa (333 psi) and 4.3 MPa (623 psi) at 7 days ( 3 ). A high cement content is usually used to achieve high early strength. However using a high cement content will increase he at development and drying shrinkage in the

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2 concretes and will increase the tendency for shrinkage cracking. Much of the observed early cracking of replacement slabs in Florid a may be attributed to shrinkage cracking. Question arises as to whether it is possible to reduce the requ ired early strength, so that cement content can be reduced. Because of the lack of research in this area, there are uncer tainties on the optimum concrete mixtures to be used in this applic ation. Questions arise as to required curing time and required early-age properties of conc rete for this application. Performance of the concrete-replacement slabs needs to be ev aluated using high-early-strength concrete under realistic pavement conditions, so that appropriate materials and construction requirements can be specified for this applicat ion. Analysis of stresses and strains in concrete slab during its early age and devel opment of a relationship between failure and various concrete properties is essential to determine the optimum c oncrete mix and curing time. 1.2 Problem Statement Questions arise as to whether the specif ied strength requirement (compressive strength or flexural strength) of concrete at particular tim e intervals as provided by the specification is sufficient to ensure performa nce. Since the stresses that develop in a concrete slab are affected by many factors (such as temperature condition, concrete properties such as elastic modulus and coe fficient of thermal expansion, and pavement conditions such as the subgrade modulus), the effects of these factor s on the performance of concrete replacement slabs need to be specified. If strength requirements are specified, how do other relevant para meters affect these requirements?

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3 1.3 Research Hypothesis Stress to strength ratio is an important indicator of potential performance of concrete-replacement slabs. Specifying onl y water/cement ratio, cement content, and minimum required strength is not sufficient to ensure good performa nce of a concretereplacement slab. 1.4 Objectives The main objective of this study was to study the factors affec ting performance of concrete-replacement slabs in Florida using acc elerated pavement testing by means of the HVS. Conduct a literature review on analysis me thods and experimental work on slab replacement Perform stress analysis of concrete-replacement slabs Design the experiment to test selected pa vement test sections and evaluate the performance of test sections using HVS loading Identify a suitable crack-detection met hod for evaluating concrete pavement Verify the models developed for analyz ing stresses and strains on concrete pavement subjected to temp erature and load effects Determine the relationships among the material and pavement parameters and the failure of concrete pavements

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4 CHAPTER 2 LITERATURE REVIEW 2.1 Structural Analysis of Concrete Pavements 2.1.1 Foundation Models As in numerous other engineering applic ations, the response of the supporting soil medium under the pavement is an important consideration. To accura tely evaluate this response, we must know the complete stressstrain characteristics of the foundation. Accurately describing the stress-strain charact eristics of any given foundation medium is usually hindered by the complex soil c onditions, which are markedly nonlinear, irreversible, and time-dependent. Furthermore, these soils are gene rally anistropic and inhomogeneous. Idealized models were de veloped to simulate soil response under predefined loading and boundary conditions. Certain assumptions about the soil medium were used for these idealizations. The assumptions are necessary for reducing the analytical rigor of such a complex boundary value problem. Two of the most frequently applied assumptions are linea r elasticity and homogeneity. 2.1.1.1 Dense-liquid foundation model In the dense-liquid foundation model (also known as the Winkler foundation model), the foundation is seen as a bed of evenly spaced, independent, linear springs. The model assumes that each spring deforms in response to the vertical stress applied directly to the spring, and does not transmit any shear stress to the adjacent springs. The relation between an external load, p, applied on any point is given by Equation 2-1

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5 P=kw (2-1) Where k is the modulus of subgrade reaction, w= displacement of foundation. No transmission of shear forces means that there are no deflections beyond the edges of the plate or slab. The liquid idealiz ation of this foundation type was derived for its behavioral similarity to a medium usi ng Archimedes’ Buoyancy principle. It was applied to analyze pavement support systems in studies by Westergaard ( 4, 5, 6 ). In the field, the k -value is determined using data obtained from a plate-loading test performed on the foundation usin g a 30-inch-diameter plate ( 7 ). The load is applied to a stack of 1-inch-thick plates, until a specified pressure ( p ) or deflection ( ) is reached. The k -value is then computed as the ratio of the pressure to the corresponding deflection, p k (2-2) Another method for obtaining a k-value for use in analysis is by back calculation from measured deflections of the slab surf ace obtained from nondestructive tests, using devices such as falling wei ght deflectometers (FWD). 2.1.1.2 Elastic-solid foundation The elastic-solid foundation model (some times referred to as the Boussinesq foundation) treats the soil as a linearly elastic, isotropic, homogenous material that extends semi-infinitely. It is considered a more realistic mo del of subgrade behavior than the dense-liquid model, because it takes into account the effect of shear transmission of stresses to adjacent support elements. Conse quently, the distribution of displacements is continuous; that is, deflection of a point in the subgrade is due to stress acting at that particular point, and also is influenced by decreasing by stresses at points farther away.

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6 Analyzing loaded slab supported on a solid foundation is math ematically more difficult. Various solutions were available in the literature, such as the Boussinesq solution, Equation 2-3. s sE pr w0 2) 1 ( 2 (2-3) Where, w = displacement of foundation surface at the center of loaded area, p = contact pressure, 0r = radious of the loaded area, s = Poisson’s ratio of foundation, and Es= elastic modulus of foundation. Because of its mathematical complexity, the solid-foundation model is less attractive than the dense-liq uid foundation model. Unlik e the dense-liquid foundation model, where the governing equations are differential, the elastic foundation model requires solving integral or integro-differentia l equations. The continuous nature of the displacement function in the elastic-solid model also means that this model cannot accurately simulate pavement behavior with di scontinuities in the structure, especially for slabs supported on natural soil subgrades. The model is unsuitable for predicting slab response at edges, corners, cracks, or joints with no physical load transfer. The elastic-solid foundation model considers the shear force interaction of different elements in the foundation. Although it improves on Winkler foundation model by considering shear forces in the foundation, fi eld tests showed inexact solutions for many foundation materials. Foppl (8) reported that the surface di splacements of foundation soil outside the loaded region decreased fast er than the prediction by this model. 2.1.1.3 Improved models using a modified Winkler foundation Dense-liquid and elastic-solid foundation models represent two extremes of actual soil behavior. The dense-liquid model assume s complete discontinuity in the subgrade

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7 and is better for soils with relatively low shear strengths (natural soils ). In contrast, the elastic-solid model simulates a perfectly conti nuous medium and is better for soils with high shear strengths (treated ba ses). The elastic response of a real soil subgrade lies somewhere between these two extreme f oundation models. In real soils, the displacement distribution is not continuous; neither is it full y discontinuous. Deflection under a load can occur beyond the edge of th e slab, and goes to zero at some finite distance. To bridge the gap between the dense-liquid and elastic-solid foundation models, researchers developed improved f oundation models in one of two ways: Starting with the Winkler foundation and (t o bring it closer to reality) assuming, some interaction among spring elements Starting with the elastic-solid foundation, assuming simple expected displacements or stresses A big problem with these models, is the l ack of guidance in se lecting the governing parameters (which have limited or no physical meaning). Hetenyi foundation: Hetenyi (9,10) suggested achieving interaction of independent spring elements by embedding an el astic beam in two-dimensional cases and by embedding a plate in the material of th e Winkler foundation in three-dimensional cases. It is assumed that the beam or pl ate deforms only in bending Equation 2-4 shows the relation between contact pressure p and deflection of foundation surface w for threedimensional cases. w D kw ps 2 (2-4) where 2= the Laplace operator and Ds =the flexural rigidity of an imaginary plate in the Winkler foundation, representing interact ion of independent spring elements.

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8 Pasternak foundation: Pasternak (11) considered shear interactions in the spring elements of a Winkler foundation by connecting the ends of the springs with a beam or plate consisting of incompressible vertical el ements that deformed only by transverse shear. Under this assumption, Equation 25 shows the relation between the contact pressure p and deflection of foundation surface w. w G kw pb 2 (2-5) wherebG =shear modulus of foundation. “Genelized” foundation by Venckovskii : In this foundation m odel, in addition to the Winkler hypothesis, Venckovskii (12) assumed that the applied moment nMis proportional to the angle of rotation. Equations 2-6 and 2-7 describe this analytically. kw p (2-6) dn dw k Mn 1 (2-7) where n is any direction at the point in the pl ane of the founda tion surface, and k and 1k are the corresponding proportionality factors. 2.1.1.4 Improved models by using a modified elastic-solid foundation Reissner foundation: Assuming that the in-pla ne stresses throughout the foundation layer (2-8) ar e negligibly small. 0 xy y x (2-8) and that the horizontal displacements at th e upper and lower surfaces of the foundation layer are zero, Reissner (13) obtained the relationship in Equa tion 2-9 for the elastic case p c c p w c w c2 1 2 2 2 14 (2-9) where ,1H E cs 32HG c ) 1 ( 2 sE G (2-10)

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9 To apply the Reissner model to th e case in which elastic modulus bE varies linearly with the depth of foundation, Horvath ( 14 ) developed a modified Reissner model. p C p w C w C2 3 2 2 1 (2-11) where 2 1, C Cand 3C are constants which are functions of elastic modulus bE and thickness H of the foundation. Beam-column-analogy foundation: From an elastic continuum, Horvath ( 15 ) developed a Pasternak-type, beam -column-analogy foundation model as w GH w H E ps22 (2-12) With this model, Horvath ( 16 ) analyzed the mat-supported Chemistry Building at Massachusetts Institute of Technology in Cambridge, Massachusetts. The comparison of computed and observed settlements showed that this model provided good agreement with observed behavior. 2.1.2 Analytical Solutions for Concrete P avement Response to Traffic Loading A complete theory of structural analys is of rigid pavement was suggested by Westergarrd ( 4, 5, 6, 17, 18, 19 ) using the classical thin-plate based theoretical models. Westergaard modeled the paveme nt structure as a homogenous isotropic, elastic, thin slab resting on a Winkler (dense -liquid) foundation. He iden tified the three most critical loading positions; the interior (also called center), edge, and corner and he developed equations for computing critical stresses and deflections for those loading positions. Westergaard’s original equa tions have been modified several times by different authors, mainly to bring them into better agreement with measured responses of actual pavement slabs. Ioannides et al ( 7 ) performed an extensive study on Westergaard’s original equations and the modified formulas.

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10 Interior loading: Westergaard defines interior loading as the case when the load is at a considerable distance from the e dge. Equation 2-13 gives the maximum bending stress at the bottom of the slab for the interior load of radius 0r 6159 0 ln 2 ) 1 ( 32 cr L h p (2-13) where p = uniformly distributed pressure, h = slab thickness, E = elastic modulus of concrete, = Poisson’s ratio of concrete, k= modulus of subgrade reaction, L is the radius of relative stiffness defined as 4 1 2 31 12 k Eh L (2-14) and, 0r rc when h r 724 10 (2-15) h h r rc675 0 6 12 2 0 when h r 724 10 (2-16) The modified radius cr was introduced to account for the effect of shear stresses in the vicinity of the load, which is neglected in the classical thin-plate theory. Equation 2-17 gives the deflection for interior loading ( 18 ). 2 0 0 2673 0 2 ln 2 1 1 8 L r L r kL p w (2-17) Corner loading: Westergaard proposed Equations 2-18 and 2-19 for computing the maximum bending stress and deflection, when th e slab is subjected to corner loading. 6 0 0 22 1 3 L r h p (2-18) L r kL p w 2 88 0 1 10 2 (2-19)

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11 Edge loading: Westergaard ( 4, 5, 6 ) defined edge loading as the case when the wheel is at the edge of the slab, but at a considerable distance from any corner. Two possible scenarios exist for this loading case: (1) a circular load with its center placed a radius length from the edge, and (2) a semi-circula r load with its straight edge in line with the slab. Equations 2.20 and 2-21 by Ioannides et al. ( 7 ) include modifications made to the original Westergaard equations. For the circular loading, the maximum bending stress and deflection are computed as L r kr Eh h p 2 2 1 18 1 2 1 3 4 84 1 100 ln 3 1 30 2 0 3 2 (2-20) L r k Eh p w0 34 0 76 0 1 2 1 2 (2-21) The maximum bending stress and deflection for a semi-circular loading at the edge is given by L r kr Eh h p 2 2 1 3 4 84 3 100 ln 3 1 30 2 0 3 2 (2-22) L r k Eh p w0 317 0 323 0 1 2 1 2 (2-23) Westergaard made the following simplif ying assumptions in his analysis, The foundation acts like a bed of spri ngs (dense liquid foundation model) There is a full contact between the slab and foundation All forces act normal to the surface where sh ear and frictional forces are negligible The semi-infinite founda tion has no rigid bottom The slab is of uniform thickness, and the neutral axis is at its mid-depth

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12 The load is distributed unifor mly over a circular contact ar ea ( for the edge loading, load is distributed uniformly over a semicirc ular contact area where the diameter of the semicircle is alone the edge of the slab) For corner loading, the circumference of th e circular area is tangential to the edge of the slab The concrete pavement acts as single se mi-infinitely large, homogenous, isotropic elastic slab with no discontinuities. Despite limitations associated with the simplifying assumptions, Westergaard’s equations are still widely used today to com pute stresses in pavements and to validate other models developed us ing different techniques. Because of simplifications associated w ith the above assumptions; the Westergaard theory has some limitations Stresses and deflections can be computed only for the interior, edge and corner loading conditions Shear and frictional forces on slab surface are ignored The Winkler foundation extends only to the edge of the slab The theory does not account for uns upported areas resulting from voids or discontinuities Multiple wheel loads cannot be considered Load transfer between joints or cracks is not considered. The thin plate based theoretical models for structural analysis of concrete pavement did not develop much furt her after Westergaard findi ngs. Pickett and Ray ( 20 ) made Westergaard’s solution easy to use and popular for the design of concrete pavement using influence chart. Further developments ha ve received less attention because of the complexities of the mathematics involved. Hogg and Hall ( 21 ) took the subgrade as a semi-i nfinite elastic-solid and they developed an analytical model for determini ng the stresses and deflec tions of a concrete

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13 slab under the action of a single load by using the elastic properties of subgrade. This model is effectively an infinite thin slab model because the derivation considers a single interior load far away from any edge or corner of a slab. Reissner ( 13, 22, 23 ) developed a thickplate theory to analyze two problems: (1) the problem of torsion of a re ctangular plate, and (2) the problems of plain bending and pure twisting of an infinite plate with a circular hole. The Reissner theory is regarded as stress-based shear deformable theory as it is based on assumed stress variation through the plate thickness. Hu ( 24 ) further extended Reissner’s th eory and developed another set of basic equations for thick plates that are simpler to solve than the original equations. Mindlin ( 25 ) proposed another formulation to acc ount for shear deformation based on a proposed displacement field through the plate thickness. A theoretical solution to the problem of a rectangular thick plate with four free edges and supported on Pasternak foundati on was developed by Shi et al. ( 26 ). The Fundamental equations for the problem were established by applying Reissner thick-plate theory and solved by applying the met hod of superposition. Fwa et al. ( 27 ) further extended this solution into analysis of concre te pavement and found differences existed in both stresses and deflections between thick-pl ate solutions and Westergaard’s solutions. 2.1.3 Numerical Solutions for Concrete Pave ment Response to Traffic Loading It has been virtually impossible to obtain analytical closed-form solutions for many pavement structures because of comple xities associated with geometry, boundary conditions, and material propert ies. With the evolution of high speed computers, the analysis of such complex problems using num erical technique was possible. The most commonly used numerical techniques for analyzi ng concrete pavement structures are: (1)

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14 discrete element method (DEM), (2) finite element method (FEM), and finite difference method (FDM) 2.1.3.1 Discrete element method (DEM) The first use of DEM for concrete pave ment analysis was made by Hudson and Matlock ( 28 ). In this analysis, the subgrade was idealized as a Winkler foundation. The effects of joints in this model were taken into consideration by reducing the original bending stiffness of the slab at those locations where a joint existed. The model developed by Hudson and Matlock ( 28 ) was later modified and improved by Vora and Matlock ( 29 ) to include element of different sizes, anisotropic skew slabs, and semiinfinite elastic solid subgrade. The major disadvantages of DEM formulations are that elements of varying sizes are not easily incor porated into the analysis, and that special treatment is needed at the free edge wher e stresses cannot be determined uniquely. 2.1.3.2 Finite element method FE techniques have been used to suc cessfully simulate different pavement problems that could not be modeled using the simpler multi-layer elastic theory. Further, it provides a modeling alternative that is we ll suited for applications involving systems with irregular geometry, unusual boundary conditions, or non-homogenous composition. Three different approaches were used for FE modeling of pavement system: plane-strain (2D), axisymmetric, and three-dimensional (3D) formulation. In the FE method, the level of accuracy obtained depends upon different fact ors, including the degree of refinement of the mesh (element dimensions), the or der and type of element, and location of evaluation. Various finite element models have been developed for analyzing the behavior of concrete pavement systems. Most of the fi nite element models use an assemblage of

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15 two-dimensional plate bending elements to mode l behavior of a concrete slab. A plate with medium thickness is thick enough to car ry the load by bending action but is thin enough such that the transverse shear deformation can be considered negligible. The subgrade is usually assumed to behave like eith er a Winkler (dense liquid) or an elastic solid foundation. The Winkler foundation can be modeled by a series of vertical springs at the nodes, which means that the deflec tion at any point of the foundation surface depends only on the forces at that point and does not depend on the forces or deflections at any other points. The stiffness of the foundation is represented by the spring constant. The use of an elastic solid foundation assu mes a homogeneous, elastic, and isotropic foundation with a semi-infinite depth. The de flection at any point depends on the forces at that point and also on the forces or deflections at othe r points. The following section briefly describes the basics and applications of a few finite element computer programs. KENSLAB( 30 ): The slab is treated in this model is composed of two bonded or unbonded layers with uniform thickness. The two layers can be either a high modulus asphalt layer on top of a concrete slab, or a cement-treated base. Rectangular thin-plate elements with three degrees of freedom per node (a vertical deflec tion and two rotations) are used to represent the slab. Load transfer through doweled joint or aggregate interlock can be considered in this model. Three type s of foundation are included in this model, namely the Winkler foundation, the semi-i nfinite elastic -solid foundation and layered elastic-solid foundation. Thr ee contact conditions between slab and foundation can be considered: full contact, partial contact wit hout initial gaps, and partial contact with initial gaps. Load transfer effects can be considered in analyzing the pavement slab system.

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16 ILLI SLAB ( 31 ): This model can be used to an alyze a jointed or continuouslyreinforced concrete pavement with a base or subbase and with or without an overlay, which can be either fully bonded or un-bonded to the concrete slab. A concrete slab is modeled as an assemblage of rectangular pl ate bending elements w ith three degree of freedom at each node. When a base or subbase layer and/or an overlay are used, they are also modeled as assemblages of plate bending elements. If there is no bond between the layers, the overall stiffness matrix for the mu ltiple layers is obtained by simply adding up the stiffness matrices of the concrete slab, the base or subbase a nd the overlay. For the case of perfect bond between layers, full strain compatibility at the interface is assumed. Thus, an equivalent layer can be obtain ed based on a transformed-section concept. Load transfer across the joints is modeled in various ways depending on the transfer devices used. Dowel bars are mode led as bar elements with two degrees of freedom at each node. The two displacement components are a vertic al displacement and a rotation about a horizontal transverse axis. The bar element is capable of transferring both a vertical shear force and a moment. If the loads are transferred across a joint only by means of aggregate interlock or keyway, th ey are modeled by vertical spring elements with one degree of freedom at each node. Only vertical forces are transferred across the joint by the spring element. The moment tran sfer can be neglected for such a joint. JSLAB ( 32 ): The JSLAB program was developed using a similar model as ILLISLAB. The pavement slab, the base or subba se layer, and the overlay are modeled as rectangular plate bending elemen ts based on the classical theory of thin plates with small deflections. These layers can be bonded or unbonded. The subgrade is modeled as a Winkler foundation represented by vertical springs. The effect of temperature gradient in

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17 the concrete slab is incorpor ated. The temperature is assumed to vary linearly along the slab depth. The subgrade stiffness is set to be zero at the locati ons where loss of support occurs. Dowel bar at the joints are modeled as bar elements with the ability to transfer both moment and shear forces across the joints. Th e effects of looseness of dowel bars can also be considered. Aggregate interlock and keyway are modeled by spring elements transferring shear forces only. WESLIQUID and WESLAYER ( 33 ): The finite element model used in the WESLIQUID and WESLAYER programs are also ba sed on the classical theory of a thin plate with small deflections. The slab is m odeled as an assemblage of rectangular plate bending elements with three degrees of freedom at each node in both programs. The difference between these two models is that the WESLIQUID model considers the sublayers as a Winkler foundation, while th e WESLAYER model uses an elastic layered foundation. The Winkler foundation is modeled by a series of vertical springs. For the elastic foundation, the Boussinesq’s soluti on is used to compute the deflections at subgrade surface for the case of a homogeneous elastic foundation and the Burmister’s equations are used to compute those for th e case of a layered el astic foundation. The two programs are able to take into account the effects of loss of support from the sublayer to the pavement slab. The loss of support can be due to linear temperature gradient in the slab or due to voids in the sublayer. Load is transferred across a joint by both shear forces and moment transfer. Shear forces are transferred either by dowel bars, key joint or aggregate interlock. The two models have three options for specifying shear transfer and one for moment transfer. The

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18 three methods of determining shear transfer ar e (1) efficiency of shear transfer, (2) spring constant and (3) diameter and spacing of dowel s. Moment transfer across joints or cracks is specified by the efficiency of moment transfer which is defined as a fraction of the full moment. FEACONS ( 34, 35, 36 ): The FEACONS (Finite Element Analysis of CONcrete Slabs) program was developed by the University of Florida for the analysis of concrete pavement behavior for the FDOT. FEACON S program was modified several times to upgrade its capabilities. The la test version, FEACONS IV program can be used for analysis of plain jointed concrete pave ments subjected to load and temperature differential effects. In the FEACONS progr am, a concrete slab is modeled as an assemblage of rectangular plate bending elem ents with three degree of freedom at each node. The three independent displacements at each node are (1) lateral deflection, w, (2) rotation about the x-axis, x, and (3) rotation about the y-axis, y. The corresponding forces at each node are (1) the downward force, fw, (2) the moment in the x direction, f x, and (3) the moment in the y direction, f y. The FEACONS IV progr am has the option of modeling a composite slab made up of a conc rete layer bonded to another layer of a different material such as an econocrete. Th e subgrade is modeled as a liquid or Winkler foundation which is modeled by a series of vertical springs at the nodes. A spring stiffness of zero is used when a gap exists between the slab and the springs due to subgrade voids. Either a linea r or nonlinear load-d eformation relationship for the springs can be specified. Load transfers across the joints between two adjoining slabs are modeled by shear (or linear) and torsional spri ngs connecting the slabs at th e nodes of the elements along

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19 the joint. Looseness of the dowel bars is m odeled by a specified slip distance, such that shear and moment stiffnesses become fully effective only when the slip distance is overcome. Frictional effects at the edges are modeled by shear springs at the nodes along the edges. 2.1.3.3 Finite difference method (FDM) FEM has overwhelming advantages over the FDM when applied to the analysis of pavement structures. However, FDM may be mo re suitable or conveni ent to use in some cases. The FDM is known to utilize a smalle r amount of memory than the FEM, it is likely that the FDM technique may be partic ularly useful in problems requiring large computer effort ( 7 ) The FDM in its application to the slab s-on-grade problem replaces the governing differential equation and the boundary conditions by finite di fference equations. These equations describe the variati on of the primary variable (i.e., deflection) over a small but finite spatial increment. The most important criterion that governs the adequacy of the finite difference approximation is the level of refinement of the finite difference grid. 2.2 Review of Concrete Pavement Failures in Slab Replacement Many forms of functional or structural di stresses have been reported from the newly replaced concrete slabs with in short time after construction. A survey on I-10 of 100 replacement slabs ranging in age between 1 to 3 years, showed that 35% of the slabs had developed cracks and spalls. In these slabs, fatigue damage is clearly ruled out as a cause of early cracking. Inve stigators of this study hypothesi zed that the micro cracks are developed in the slabs as a results of shortc oming in pavement design, concrete mix or construction ( 37 ).

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20 High early strength concrete has been used for slab replacement concrete to allow earlier use of the paved sections for m oving construction equipment and speeding up construction. High early strength concrete of ten uses high quantities of cement content. Increasing the cement content in concrete mixt ure tends to increase the heat development in the mixture. For the investigation of ef fect of cement type, curing method, and joint type on the performance of high early strength concrete in slab replacement, forty two (42) test sections were cons tructed on the out side lane of I-10 Fourteen different combinations of above factors were included in the design of test sections with 3 slabs for each design. Frequent condition surveys of 42 sections on I-10 showed that mid slab cracking occurred in 39 of the 42 slabs. The cracks developed at different times ranging from 24 hours to one year ( 37 ). Doweled joints perform better than undow eled joints. A reduction of 20% in deflection and lower stresses are expected in doweled joints ( 38 ). An extensive crack survey on Florida’s I-10 showed that dowelled pavement s ections had 30% less faulting and fewer corner cracks compared to undoweled sections ( 38 ). However, type of joint did not showed any relations to the rate of transverse and longitudinal cracks. A survey was conducted on deteriorated sect ions of I-75 to inve stigate the impact of dowel misalignment on rate of cracking( 39 ) Results of this study showed no correlation between misalignment of dowels and the rate of cracking. Okamoto et al ( 40 ) have identified the expected ranges of variations in concrete modulus of rupture have a signi ficantly greater effect on ear ly age fatigue life than the usual variations in other pavement mate rial properties including subgrade support, subbase thickness, subbase strength and layer thickness. A laborator y study of modulus

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21 of rupture coefficient of varia tion at ages of 1, 3, 7, 14, and 28 days for four different mixes ranged from 0 to 23.9 and averaged 7.0 percent ( 41 ). The study used two type of cements and two type of aggregates. Guide lines for the concrete strength for early opening of concrete pavement were determined based on the type of traffic (construction traffic, commercial and public traffic). A stress ratio of 0.5 was used as the opening criteria. Stress ratio was computed incorpor ating the effects of outside subbase support and strength variabilities. As a part of the Strategic Highway Resear ch Program (SHRP), fast track full depth repair test sections were constructed to demonstrate and validate the technologies that allow early opening of full-depth Portland Cement Concre te (PCC) pavement repairs to traffic and to document the information needed to apply this technology ( 42 ). The experimental factors included material type, strength at opening, and repair lengt h. A total of 11 different high-early strength concrete mixes with ope ning times ranging from 2 to 24 hours were evaluated at 2 field sites (I-20, Augusta, Georgia and SR-2, Vermilion, Ohio). The monitoring program consisted of conducting annu al visual distress surveys to monitor the development of cracking, faulting, and spalling. The results of long-term monitoring showed that full-depth repairs made with highearly-strength PCC can provide good longterm performance; however, adverse temperature conditions during in stallation can cause prematur e failures. The study also showed that the fatigue damage due to early opening is negligible, especially for repairs 3.7 m (12 ft) or shorter (obser ved longitudinal cracking on long span slabs). Based solely on fatigue considerations, full-depth repairs co uld be opened to traffic at lower strengths than those typically reco mmended; however, opening at strengths much less than previous recommendations is not advisable b ecause of the risk of random failures caused

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22 by single heavy load at early age. Ther efore, no changes to the opening criteria suggested in the SHRP C-206 manua l of practice are recommended (PCC modulus of rupture of 2.0 MPa (300 psi) by third-point testing, Compressive strength of 13.8 MPa (2,000 psi)). 2.3 Accelerated Pavement Testing Full-scale and accelerated pave ment testing (FS/APT) began as early as 1909 with a test track in Detroit, as identified by Metcalf ( 43 ). Results from FS/APT research activities created significant advances in pa vement engineering pract ice. Historically, probably the most notable of these in te rms of the effect on highway pavement engineering is the Road Test conducted by th e Association of State Highway Officials (AASHO) in the late 1950s. For airfield pavements, tests at the U.S.Army Corps of Engineers (USACE) Waterways Experiment Station (WES) since 1940 essentially defined the state of engineering practi ce. During the 1970s and 1980s, worldwide FS/APT activities and results in other countri es were significantly more productive than those in the United States, with important contributions being made by Australia, Denmark, South Africa, France, Britain and the Netherlands, among others ( 44 ). Current efforts are marked by the rene wed and resurgent interest in FS/APT programs worldwide since the mid-1980s. In the United States alone, major investments in FS/APT programs have been committed by FHWA, USACE (both at WES and at the Cold Regions Research and Engineering Laboratory [CRREL]), and the states of Minnesota, California, Texas, and Louisiana. In addition, the Federal Aviation Agency (FAA) is currently commissioning the largest APT machine in the world. The state of Florida and the National Center for Asphalt Technology (NCAT), in collaboration with

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23 the Alabama Department of Transportation, have both initiated major FS/APT efforts, which are likely to be the first ne w APT programs of the 21st century ( 44 ). APT is an alternative evaluation approach for full-scale test roads. Here, the precise weight of applied loads can be contro lled. Therefore, the pavement researcher has accurate information on the load and th e number of load repetition throughout the duration of the test. Pavement condition surveys and pavement response measurement can be conducted at different time intervals as desired. APT allows the applied loads to be precisely located on the pavement section a nd the wheel load can be run directly over the embedded strain gauges in concrete fo r dynamic and statistic load measurement. Accelerated pavement testing of concre te pavements presents unique challenges and a different technology than APT on flexib le pavements. It is recommended that APT experiments be designed primarily to provide validation data for mechanistic analysis, rather than for purely empirical comparisons. It is recommended that where possible, control sections be used to monitor slab behavior under environmental and internal changes, and that replicate sec tions be included in experiment designs. It is very useful to also establish long-term mainline monitori ng sections with the same variables as the APT sections ( 45 ). There were very few studies on rigid pavement evaluation using the HVS. The following section briefly descri bes the APT tests using HVS conducted on rigid pavement. Evaluation of Rapid Setting Concrete using HVS at Palmdale, California ( 46 ): As part of the Caltrans Long Life Paveme nt Rehabilitation Strategies (CLLPRS), a concrete blend of fast setting strength hydrau lic cement concrete and PCC was evaluated using HVS. Two full scale test sites with 210 m in length were constructed using this

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24 concrete blend on either side of State Route 14 about 5 miles south of Palmadale, California. The test site in the northbound direction incl uded sections with different concrete thicknesses on granular base. Jo int Deflection Measuri ng Devises (JDMD) and Edge Deflection Measuring Devises (EDMD) were installed to measure the surface deflection of each test section. The JDMD was installed to measure joint deflection and the EDMD was positioned to record the edge deflection in the middle of the slab. Multi depth deflectometers were installed to meas ure the surface and in depth deflections on few sections. Each test section was instru mented with thermocouples to measure the temperature at surface, mid-depth and bottom of the concrete. Some sections were instrumented with strain gauges. Strain ga uges were placed along the HVS wheel path at middle and corner of the test sections. Each location had two embedded gauges; one placed 40 mm from the bottom and the other placed 40 mm from the surface. Visual observations, deflections at joint a nd middle of the slab and load transfer efficiency were recorded with respect to HVS repetitions for each test sections. The researchers also observed the crack development of the test sections. Corner cracks were observed in many sections. The longitudinal cracks appeared on areas out side of the wheel path, progressed towards the wheel path and ended up as a corner cracks. Similarly, the transverse cracks appeared on the wheel path, progressed towards the joints and ended up as corner cracks. The report only summarizes the results and observations of this study. Cumulative Fatigue Analysis of Concrete Pavement Using APT results ( 47 ): The goal of this study was to use Accelerated Pavement Testing (APT) of field slabs in order to examine Miner’s hypothesis along w ith various fatigue damage models for

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25 concrete pavements. Mechanistic-empirical design procedures for concrete pavements use a cumulative damage analysis process to predict fatigue cracking in slabs. According to Miner's hypothesis, concrete should fr acture when the cumulated fatigue damage equals unity. In mechanistic-empirical desi gn procedures, this va lue corresponds to 50 percent chance of fatigue failure (50 percent cracked slabs). Several test sections constructed usin g fast-setting hydraulic cement concrete (FSHCC) in Palmdale, California, consisting of combinations of joint spacings, shoulder type, dowelled joints, and widened lanes, were constructed and evaluated using the Heavy Vehicle Simulator (HVS). These instrumented slabs were loaded with dual wheel and aircraft wheel loads ra nging from 40 kN (9,000 lb) to 150 kN (33,750 lb) with no wander, and were monitored past the concrete fatigue failure. Results indicated the test slabs cracked at cumulative damage levels significantly different from unity for all fatigue damage models, and in most cases, by several orders of magnitude. According to the results of this study, the use of Mine r’s hypothesis to characterize the cumulative fatigue damage in the concrete, did not accurately predict the fatigue failure of the concrete slabs. As such, the authors suggest alternative methods for incremental failure prediction should be explored. Heavy Vehicle Simulator Experiment on A Semi-Rigid Pavement Structure of a Motorway( 48): For the evaluation of a semi ri gid pavement design of the A2 motorway in Poland at Poznan two different te st sections were built The two pavement structures were exposed to accele rated loading by the HVS-Nordic. The pavements at the test sites were instrumented with strain ga uges, soil pressure cells and deflection gauges in order to assess the pavements response unde r the load and to compare these response

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26 measurements with values calculated from theoretical pavement models (South African Mechanistic Pavement Design Method (SAMDM)) During the construction of the test pavement s as well as during the load application deflection measurements with the Falling We ight Deflectometer (FWD) were performed periodically. A method was developed to co mbine the analysis of the results of the response measurements and the results of the deflection measurements with the FWD. By back calculation from the deflection result s E-moduli of the pavement layers were determined which consequently were used fo r a forward calculation of stresses, strains and deflections within the pavement. Thus a comparison with the response measurements was possible. By means of a se nsitivity analysis most realistic ranges of the E-moduli of the pavement layers, especially of the cement treated base layers as the main bearing element of the pavement, were determined.

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27 CHAPTER 3 STRESS ANALYSIS CONVENTIONAL CONCRETE PAVEMENT SLABS 3.1 Method of Analysis The Finite Element Analysis of CONc rete Slabs version IV (FEACONS IV) program was used to analyze the anticipated st resses on the test slabs when loaded by the HVS test wheel. The FEACONS program was de veloped at the University of Florida for the FDOT for analysis of concrete pavements subject to load and thermal effects. This program was chosen for use since both th e University of Florida and FDOT have extensive experience with this program and the reliability of this program has been demonstrated in previous studies ( 34, 35, 36, 38, 49, 50, 51 ). In the FEACONS program, a concrete slab is modeled as an assemblage of rectangular plate bending elements with three degrees of freedom at each node. Th e three independent displacements at each node are (1) lateral deflection, w, (2) rotation about the x-axis, x and (3) rotation about the y-axis, y. The corresponding forces at each n ode are (1) the downward force, fw (2) the moment in the x direction, f x, and (3) the moment in the y direction, f y. The FEACONS program was used to analyze the stresses in the test slabs when subjected to a 12-kip (53-kN) single wheel load with a tire pressure of 120 psi (827 kPa) and a contact area of 100 square inches (645 cm2), and applied along the edge of the slab, which represents the most crit ical loading location. Analys is was done for two different load positions: load at the corner of the slab and load at the middle of the edge (Figure 31).

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28 Figure 3-1. Loading positions us ed in the stress analysis The elastic modulus of the concrete was assumed to be 5,000 ksi (34.45 GPa) and the modulus of subgrade reaction was assumed to be 0.4 kci (272 MN/m3). The thickness of the concrete slabs was 9 inches (23 cm). Other pavement parameter inputs needed for the analysis are the joint shear stiffness (whi ch models the shear load transfer across the joint), the joint torsional stiffness (which m odels the moment transfer across the joint) and the edge stiffness (which models the load transfer across the edge joint). The values for these parameters are usually determin ed by back-calculation from the deflection basins from NDT loads (such as FWD) applied at the joints and edges. In the absence of data for determination of these parameters, tw o conditions were used in the analysis. One condition was for the case of no load transfer In this case, all the edge and joint stiffnesses were set to be zero. The ot her condition was for the case of good load transfer. In such a case, t ypical joint and edge stiffness values for good joint and edge conditions were used in the analysis. A shear stiffness of 500 ksi (3445 kPa), a torsional stiffness of 1000 ksi (6.89 MPa), and an edge stiffness of 30 ksi (207 kPa) were used for this condition.

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29 3.2 Results of Analysis Figure 3-2 shows the distribution of the ma ximum principal stresses at the top of the test slab caused by a 12 kip (53-kN) wheel lo ad at the slab corner, for the condition of no load transfer at the joints and edges. Fi gures 3-3 and 3-4 show the distribution of the stresses in the x (longitudinal) and y (lateral) direction, resp ectively, for the same loading and load transfer condition. Figure 3-2. Distributi on of maximum principal stresses due to a 12-kip load at the slab corner for the condition of no lo ad transfer at the joints

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30 Figure 3-3. Distribution of st resses in the xx direction due to a 12-kip load at the slab corner for the condition of no lo ad transfer at the joints

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31 Figure 3-4. Distribution of st resses in the yy direction due to a 12-kip load at the slab corner for the condition of no lo ad transfer at the joints Figure 3-5 shows the distribution of the maxi mum principal stresses in the test slab caused by a 12-kip wheel load at the slab corn er, for the condition of good load transfer at the joints and edges. Figures 3-6 and 3-7 s how distribution of the stresses in the x (longitudinal) and y (lateral) direction, re spectively, for the same loading and load transfer condition. Figure 3-8 shows the distribution of the ma ximum principal stresses on the adjacent slab caused by a 12-kip (53-kN) load at the slab corner, for the condition of good load transfer at the joints and edge s. Figures 3-9 and 3-10 show th e distribution of the stresses

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32 in the xx and yy directions, respectively, on the adjacent slab, for the same loading and load transfer condition. Figure 3-11 shows the distribution of the maximum principal stresses on the test slab caused by a 12-kip (53-kN) load at mid e dge, for the condition of no load transfer across the joints and edges. Figure 3-5. Distributi on of maximum principal stresses due to a 12-kip load at the slab corner for the condition of good lo ad transfer at the joints

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33 Figure 3-6. Distribution of st resses in the xx direction due to a 12-kip load at the slab corner for the condition of good lo ad transfer at the joints

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34 Figure 3-7. Distribution of st resses in the yy direction due to a 12-kip load at the slab corner for the condition of good lo ad transfer at the joints

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35 Figure 3-8. Distributi on of maximum principal stresses on the adjacent slab due to a 12kip load at the slab corner for the cond ition of good load transfer at the joints

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36 Distance, direction xx. inch 020406080100120140160180 Distance, direction YY, ft. 0 2 4 6 8 10 12 -20 0 0 0 0 20 20 20 40 40 60 80 0 0 0 0 0 0 Figure 3-9. Distribution of st resses in the xx direction on th e adjacent slab due to a 12kip load at the slab corner for the cond ition of good load transfer at the joints

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37 Distance, direction xx, inch 020406080100120140160180 Distance, direction YY, ft. 0 2 4 6 8 10 12 0 0 -20 -40 -60 -20 0 0 Figure 3-10. Distribution of st resses in the yy direction on th e adjacent slab due to a 12kip load at the slab corner for the cond ition of good load transfer at the joints

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38 Figure 3-11. Distributio n of maximum principal stresses due to a 12-kip load at the midedge for the condition of no lo ad transfer at the joints 3.3 Results of Previous Parametric Stud ies of Factors Affecting Stresses in Concrete Pavement Variation of temperature and/or moisture in a concrete pavement slab can cause the slab to curl and lose partial contact with subgrade. During the day, when the top of the slab is warmer than the bottom, the slab tends to curl up at the cente r. During the night, when the top is cooler than the bottom, the sl ab tends to curl up at the edges and joints. When loads are applied during these curling conditions, the maximum stresses in the slab could be substantially higher than those when the slab is fully contact with the subgrade. Therefore it is necessary to conduct a parame tric analysis of structural response of concrete pavement under critical thermalloading conditions. Ti a et al. conducted a comprehensive parametric analysis using the FEACONS program ( 34 ). The FEACONS

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39 program estimations were comparable to those computed by Influence chat and Westergard equation at the zero temperature differentials (Temperature at top minus temperature at bottom). The parameters stud ied include (1) The temperature differential in the Slab, (2) the concrete slab length, (3) the subgrade modulus (ks), (4) the elastic modulus of concrete (Ec), (5) the thickness of the concrete sl ab (Tc), and (6) the joint load-transfer characteristics. Effects of Temperature Differential and Slab Length: The maximum stress increases as the temperature differential incr eases. When a temperature differential is present in the slab, the maximum thermal-load induced stress increases with an increase in the slab length. The study showed that the maximum stresses increases at a higher rate as the slab length increases from 12 ft to 15 ft and at a slower rate as the slab length exceeds 15 feet. The effect of the slab le ngth on the maximum stresses decreases as the temperature differential in the slab decreases. Effects of Subgrade Modulus and Slab Length: The maximum stresses in the slab caused by a 20-kip (89 kN) single axle load at the edge center were computed for the condition of temperature differential of 200F and for the condition of zero temperature differential. The slab length was varied from 12 feet to 24 feet while the subgrade modulus was varied from 0.1 kci to 1.4 kci. The results showed that, with a temperature differential of 200F, the maximum stress increases as the subgrade modulus increases for a pavement with a slab length of 12 feet. Fo r a pavement with slab length of 15 feet, the maximum stress remains approximately constant regardless of the change of subgrade modulus. For a pavement with 20or 24-foot slabs, the maximum stresses decreases as

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40 the subgrade modulus increases. When the temp erature differential is not considered, the maximum stress decreases as the subgrade modulus increases. The maximum stress increases as the slab length increases from 12 feet to 20 feet and remains unchanged as the slab lengt h increases from 20 feet to 24 feet. Effect of Elastic Modulus of Concrete: The maximum stresses in the slab caused by a 20 kip single axle load at the edge center were computed for the temperature differential of +20F and for the condition of zero temperature gradient. The results showed that, with or with out the consideration of temperature differential in the slab, the maximum computed stress increases linearly as the concrete modulus increases. However, the rate of incr ease in the maximum stress is much greater with the presence of a temperat ure differential in the slab. Effects of Concrete Slab Thickness: The maximum stresses in the slab caused by a 20-kip single axle load were computed fo r the condition of temper ature differential of +20F and for the condition of zero thermal grad ient. The concrete slab thickness was varied from 6 inches to 20 inches. The results showed that, with or with out consideration of the temperature differential, the maximum stress decreases as the concrete slab thickness increases.

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41 CHAPTER 4 DESIGN AND CONSTRUCTION OF TEST SECTIONS 4.1. Description of the Experiment The experiment was designed to test the pe rformance of concrete slabs made with different concrete mixtures us ing the HVS. The concrete test track to be used for this study was constructed at the APT facility at the FDOT State Materials Research Park on September 25, 2002, by a concrete contractor under the supervision of FDOT personnel. This concrete test track consists of two 12-foot wide lanes. Each test lane consists of three 12 ft 16 ft test slabs, placed between six 12 ft 12 ft confinement slabs. Figure 4-1 shows the layout of the concrete slabs on this test track. Slabs were numbered as shown in the figure for the identification purpos e. The thickness of the concrete slabs is 9 inches. The plan for the testing program was to rem ove the 12 ft X 16 ft slabs at the time of test and with the HVS parked over the test sl ab area, and to place in these locations the replacement concrete slabs to be evaluate d. The HVS was used to apply repetitive moving loads along the edge of th e test slabs, which is the most critical wheel loading position on the concrete slabs. Analysis of the potential st ress distributions within th e concrete test slabs when subjected to the HVS loads was performed using the FEACONS finite element program as described in chapter 3 of this disserta tion. Based on the results of the analysis, optimum locations of the strain gauges to be placed on the test slabs were determined.

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42 Testing was continued until visible cracks de veloped. Static and dynamic strains at gauge locations were recorded every hour. The temperatures were recorded in 10 minutes intervals at the corner and center of the slab using the thermocouples placed in the concrete slab in 2 inch intervals from the surface of the concrete slab during the testing period. The ambient temperature and the temperature in AC layer which was overplayed by the testing track were also recorded in the same time intervals. Impact Echo was used to measure P wave velocity al one grid lines marked on the slab at the corner and the mid-edge of the slab. Impact echo measurements were collected when the HVS stopped static measurement and daily maintenance. This measurement can be used to detect the crack apart from the visual observations. Slabs to be removed and replaced, 3.7m X 4.9m (12' X 16') Permanent slabs, 3.7mX 3.7m (12' X 12') 1 2ft 1 2ft 12ft 12ft 16ft 12ft 16ft 12ft 16ft 12ft 12ft 2A2B 2C2D 2E2F 2G 2H2I 1A1B1C1D1E1F1G1H1I Figure 4-1. Layout of conc rete slabs on test track 4.2 Construction of Concrete Test Track The concrete test track was constructed over the existing two-inch thick asphalt surface of the APT test area at the FDOT State Materials Research Park. A debonding agent (a white-pigmented curing compound) was applied on the asphalt surface before placement of the concrete slab. Concrete meeting FDOT’s specifications for Florida Class 1 concrete was used. Since the 12 ft X 16 ft slabs were to be removed before testing, tie bars (for tying the adjacent la nes together) were placed only in the 12 ft X 12

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43 ft confinement slabs. Figure 4-2 shows the placement of concrete for the test track. Figure 4-3 shows the finished concrete test tr ack. Samples of concrete were taken from two randomly selected trucks (truck no. 2 and 7). The slump, air content and temperature (Table 4-1) of the fresh concrete were meas ured. The water cement ratio of the concrete was estimated from the amount of water and cement used. Compressi ve strength tests (Table 4-1) were run on the hardened c oncrete at 24 hours, 7 days and 28 days. Figure 4-2. Placement of concrete on test track Table 4-1. Properties of the concrete used on the initial concrete test track Strength, psi Truck No. Slump, inch Temp.F Air,%W/CSample .24 hrs 7 days 28 days 1 1310 3940 5270 2 1450 3730 5590 2 3 93 2.5 0.5 Average 1380 3840 5430 1 --5040 2 --5270 3 --4980 7 3.25 90 3.25 0.45 Average 5100

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44 Figure 4-3. Finished concrete test track After placement and finishing of the concrete on the test track, 3” deep saw cuts were made (Figure 4-4) to form the joints for the slabs. A diamond-bladed saw was used for these cuts to ensure a smooth, straight vertical surface. 4.3 Removing of Concrete Slabs For the slab-replacement test, slabs we re removed by a contractor under the supervision of FDOT personnel. A 12 ft 16 ft slab was separated in to 3 ft 4 ft pieces using a diamond-bladed saw (Figures 4-5 a nd 4-6). Each piece was removed using the weight lifter (figure 4-7). Sp ecial care was taken to protect the surrounding slabs. Steel plates were kept along the joint to protect the adjacent slab at the separation of slab pieces closer to the surrounding slabs (Figure 4-8). Damaged places of the asphalt base were patched using cold mix and compacted using a vibrator (Figure 4-9).

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45 Figure 4-4. Making 3-inch deep saw cuts at the joints Figure 4-5. Separation of concrete slab (12 ft 16 ft) into small pieces (3 ft 4 ft)

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46 Figure 4-6. Separated concrete slab using diamond bladed saw Figure 4-7. Removal of sepa rated pieces using the lifter.

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47 Figure 4-8. Correcting the dama ge portion of asphalt base Figure 4-9. Removal of concrete pie ces adjacent to the surrounding slabs

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48 4.4 Dowel Bar Placement Dowel bars were placed in drilled hole s (about 10 inches) made on the adjacent slabs at the mid depth of the slab in one-f oot intervals starting six inch from the edge. Dowel bar holes drilling were made (Figure 410). The dowel bars were fixed to the adjacent slabs using an epoxy and the open en ds of the bars to be embedded in the concrete of the test slab were sprayed w ith a lubricant to allow movement in the longitudinal direction. Figure 4-11 shows th e dowel bars epoxied to an adjacent slab before placement of th e concrete test slab. Figure 4-10. Drilling dowel bar holes

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49 Figure 4-11. Dowel bars epoxied to an adjace nt slab before placement of the test slab 4.5 Instrumentation Layout Strain gauges were placed to monitor the strains in the concrete test slab. Wheatstone half-bridge circuits were used. One strain gauge was used as an active gage to monitor the load-induced strain, while another one was used as a dummy gauge for temperature compensation. The Wheatstone ha lf-bridge circuit used (Figures 4-12 and 413). The active gauge with a resistance of RA is subjected to a temperature-induced strain (y) and a load-induced strain (x) simultaneousl y. The dummy gauge with a resistance of RD, is subjected only to a temper ature-induced strain (y). Th e effect of the temperatureinduced strain “(1+y)” is canceled out in th is half bridge circuit, and only the loadinduced strain is measured.

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50 Figure 4-12. Strain gauge arrange ment in a half bridge circuit Figure 4-13. Connection of the active and dummy strain gauges in the half bridge circuit The locations for the strain gauges were se lected based on the computed anticipated stress distribution on the test slab. Two st rain gauges were placed on the wheel path in the x direction at a distance 30 inches and a distance of 96 inches from the edge (Figure 4-14). Five additional strain ga uges were placed outside of th e wheel path (Figure 4-14). The dummy gauges and a set of thermocouples we re placed in concrete blocks made of same concrete mixture for temperature compensation of the strain gauges. The locations for the thermocouple for the slab are also shown in Figure 4-14. The first slab, Slab 1C, was instrumented as shown in Figure 4-14. The first crack of the slab 1C developed in the north bound of the slab and there were no any gauges on north portion of the slab. Th erefore, Instrumentation layout was improved and the Slabs

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51 1G, 2C, 2E and 2G were instrumented as shown in Figure 4-15. The strain gauge locations and the assigned numbers are shown in Table 2. The main difference between the second instrumentation plan and the first pl an was that strain gauge number 4 in the first plan was moved to the northern end of th e slab at 30 inches from the northern edge in the second plan. This strain gauge would replicate strain gauge No 3. Figure 4-14. Instrumentati on layout for test slab 1C N

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52 Figure 4-15. Instrumentation layout for test slabs 1G, 2C, 2E and 2G Table 4-2. Strain gauge locat ions and identification numbers Slab No Gauge No Direction Location 1 XX 3” from south end, outside wheel path 2 YY 3” from south end, outside wheel path 3 XX 30” from south end, on wheel path 4 XX 30” from south end, outside wheel path 5 XX 96” from south end, on wheel path 6 XX 96” from south end, outside wheel path 1C 7 YY 96” from south end, outside wheel path 1 XX 3” from south end, outside wheel path 2 YY 3” from south end, outside wheel path 3 XX 30” from south end, on wheel path 4 XX 96” from south end, on wheel path 5 XX 96” from south end, outside wheel path 6 YY 96” from south end, outside wheel path 1G, 2C, 2E, 2G 7 XX 30” from north end, on wheel path 30” N

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53 4.6 Placement of Strain Gauges Embedment strain gauges were installed at the locations described in table 4-2. Strain gauges were embedded in the concrete wi th a cover depth of 1 inch at the top. This was achieved by fixing them between two Nylon rods using nut and bolt as shown in Figure 4-16. The Nylon rods were then fixed to the base (asphalt) layer. A PVC cylinder was placed around the strain gauges during place ment of concrete, as shown in Figure 417. The concrete was placed in the cylinder manually to prevent disturbance from the concrete handling instruments. Figure 4-16. Strain gauges fixed to the asphalt base using nylon rods

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54 Figure 4-17. Strain gauges protected by a PVS pipe before placement of concrete 4.7 Placement of Thermocouples Two set of thermocouples were placed at co rner and the center of the slab as shown in figure 4-15. One set of thermocouple was placed in a concrete block where dummy gauges were placed. A set of thermocouple c onsists of 6 gauges (k type junctions) and gauges in concrete were placed at 0.5, 2.5, 4.5, 6.5, 8.5 inches from the surface and the gauge in asphalt was placed 1 inch below the asphalt surface. This was achieved by fixing the thermocouples to a wooden rod (Figur e 4-18). Before placement of the rest of the concrete, fresh concrete was placed manually around the wooden rod with the thermocouples attached (Figure 4-19). Th is procedure ensured the proper position of thermocouples and prevent disturbance fr om the concrete handling instruments.

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55 Figure 4-18. Thermocouples fixed to a wooden rod Figure 4-19. Placement of concrete ar ound thermocouples attached to a rod

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56 4.8 Placement of Concrete Two different concrete mixes were used fo r the slab replacement. The mix design details will be provided in the chapter 5 of this dissertation. Before placement of concrete on test track, the samples of conc rete were collected from a random truck to determine the concrete properties for quality as surance and .evaluation of mixes. First, samples of concrete were taken from the conc rete truck before the accelerating admixture was added for conductance of the slump, unit we ight and air content tests. Samples of concrete were again taken af ter the addition of the accelera ting admixture for slump test and for fabrication of test specimens for compressive strength, elastic modulus and shrinkage evaluation. Concrete mix properties and characteristics will be presented in the chapter 5 of this dissertation. The first test slab (1C) to be replaced was confined with three adjacent slabs and had one free edge. Figure 4-20 shows the form work for the free edge of Slab 1C. The other four test slabs (1G, 2C, 2E& 2G) to be placed were free at both longitudinal edges. So formwork was used for on both edges of these test slabs. A debonding agent (a white-pigmented curing compound) was applied on the asphalt surface before placement of the concrete on the asphalt PVC pipes were placed around the strain gauges to protect them from concrete handling instruments. The concrete was pl aced manually around the strain gauges inside the PVC pipes. Figure 4-21 shows a picture of how the concrete was placed in two PVC cylinders where the strain gauges were held in position. After the concrete was placed to the same thickness on both inside and outside of the PVC pipe, the PVC pipe was then pulled out manually. Figure 4-22 shows the placement of concrete for a test slab. Concrete was placed manually in wooden bloc ks where the dummy strain gauges were

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57 located. Figure 4-23 shows the placemen t of concrete around the dummy gauges. Vibrators were used to consolidate the conc rete in the test slab and the dummy gauge blocks 4.9 Concrete Finishing and Sawing Joints A vibrating leveling bar was used to leve l off the concrete. Figure 4-24 shows the leveling of the concrete surface of the test sl ab. The concrete surface was finished with additional hand troweling. A broom was passed over the co ncrete surface to produce a rough surface texture before it hardened. Af ter placement and finishing of the concrete, 3” deep saw cuts were made to form the joints for the slabs. Figure 4-25 shows a picture of this sawing operation Figure 4-20. Formwork for the free edge of test slab 1C

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58 Figure 4-21. Placing concrete around strain gauges

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59 Figure 4-22. Placement of concrete for a test slab Figure 4-23. Placement of concrete around dummy gauges in wooden blocks

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60 Figure 4-24. Leveling of concrete surface Figure 4-25. Making a 3-inch deep saw cut at the joint

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61 CHAPTER 5 TESTING OF TEST SLABS 5.1 Concrete Mix Characteristic Two different cement contents were used for concrete mixes in five test slabs. Two slabs, 1C and 2G, used a concrete mix with a high cement content (850 lb per yd3/ 9 bags per yd3)and the other three slabs (2 C, 1G, 2E) used a concrete mix with a low cement (725 lb per yd3/ 7.7 bags per yd3). Table 5-1 shows the mix design details for the five test slabs. 5.2 Concrete Testing Samples of concrete were taken from th e concrete truck before the accelerating admixture was added for conductance of the sl ump, unit weight and air content tests. Samples of concrete were again taken after the addition of the acce lerating admixture for slump test and for fabrication of test specime ns for compressive strength, elastic modulus and shrinkage evaluation. Fresh concrete prope rties for the test slabs are shown in table 5-2. The compressive strength, elastic modulus flexural strength data for the five mixes placed are shown in Table 5-3 and the compressive strength data are plotted in Figure 5-1 5.3 HVS Loading Testing of the replacement test slabs was performed using a Heavy Vehicle Simulator (HVS), Mark IV model. HVS lo ading was applied using a 53.4 kN (12,000 lb) super single tire with a contact pressure of 827 Pa (120 ps i), and traveling at six (6) mph in a uni-directional mode with no wander along the longitudinal edge of the test slab.

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62 Table 5-1. Mix designs of c oncrete used in test slabs Slab Material Target (wt./yd3) Actual (wt./yd3) Moisture, % Aggregate source Cement 850 lb 844 lb D57 stone 1785 lb 1775 lb 2.0 Pit # 08-012 DOT sand 1114 lb 1111 lb 6.1 Pit # 76-349 Air entrain. admixture ( Darex ) 927 oz Superplasticizer (Adva-540) 51 oz Accelerator (Daraccel) 385oz Water 20.6 Gal 1C Mix 1 W/C 0.30 Cement 725 lb 720 lb D57 stone 1771 lb 1754 lb 1.2 Pit # 08-012 DOT sand 1173 lb 1169 lb 4.2 Pit # 76-349 Superplasticizer (Adva-540) 48.5oz Accelerator (Daraccel) 385 oz Water 19 Gal 1G Mix 2 W/C 0.30 Cement 725 lb 718 lb D57 stone 1775 lb 1760 lb 1.4 Pit # 08-012 DOT sand 1173 lb 1166 lb 6.1 Pit # 76-349 Superplasticizer ( Adva-540 ) 51oz Accelerator (Daraccel) 385 oz Water 18.6 Gal 2C Mix 3 W/C 0.30 Cement 725 lb 725 lb D57 stone 1775 lb 1778 lb 1.6 Pit # 08-012 DOT sand 1173 lb 1175 lb 4.4 Pit # 76-349 Superplasticizer (Adva-540) 48oz Accelerator (Daraccel) 384 oz Water 21.1 Gal 2E Mix 4 W/C 0.30 Cement 850 852 lb D57 stone 1785 lb 1780 lb 1.9 Pit # 08-012 DOT sand 1114 lb 1048 lb Pit # 76-349 Superplasticizer (Adva-540) 55oz Accelerator (Daraccel) 384 oz Water 24.3 Gal 2G Mix 5 W/C 0.30

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63 Table 5-2. Fresh c oncrete properties Test Property Mix 1 (Slab 1C) Mix 2 (Slab 1G) Mix 3 (Slab 2C) Mix 4 (Slab 2E) Mix 5 (Slab 2G) Slump-Pre Accelerator 3.5” 6.75” 9.25” 8.5” 10.5” Slump-W / Accelerator 2.5” 3.75” 10” 2.5” 7.75” Temperature, F 95 89 85 87 85 Unit weight, pcf 141.2 144.4 142.7 137.3 143.6 Air, % 1.25 1.75 1.75 5.25 0.75 RH % 90 81 98 66 76

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64Table 5-3. Compressive st rength, elastic modulus and flexural strength data Time, Mix 1,(Slab 1C) Mix 2, (Slab 1G) Mix 3,(Slab 2C) Mix 4,(Slab 2E) Mix 5,(Slab 2G) Comp1. 103 psi E2*. 103 psi R3* 103 psi Comp1. 103 psi E2. 103 psi R3 103 psi Comp1. 103 psi E2. 103 psi R3 103 psi Comp1. 103 psi E2. 103 psi R3 103 psi Comp1. 103 psi E2. 103 psi R3 103 psi 4 hr 980 1267 235 710 480 1388.5 630 1730 670 1569 6 hr 1700 1577 309 1100 274 860 220 1250 260 1210 1775250 8 hr 2260 1854 357 1520 1170 2629.5 1560 2620 1830 2514 1 day 4750 2749 517 3340 3302.5 477 2770 3223.0 434 3440 2920525 3850 2789530 3days 5280 3300 545 4803 3883 4340 4650 2953 7days 5960 3540 579 5540 614 5020 585 4980 3300650 5530 600 9days 3579 582 3826.0 28days 6653 3950 612 6520 3952.0 666 6510 666 5810 760 6400 760 1 Compressive strength, 2 Elastic modulus, 3 Flexural strength, estimated data

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65 Time, hrs 024487296120144168192600624648672 Compressive Strength, psi 0 1000 2000 3000 4000 5000 6000 7000 Slab1C Slab1G Slab2C Slab2E Slab2G Figure 5-1. Comparison of compressive strength of the concrete mixes used The testing plan was to apply the HVS loading six hours after the start of the placement of concrete. However, due to m echanical problem of the HVS, HVS loading was not started until eight hours af ter concrete placement for the first test slab (Slab 1C). HVS loading was continued until visible cracks developed. In the cases where the test slabs performed well with no cracking, the wh eel load was increased to 67 kN (15,000 lbf) and then to 80 kN (18,000 lbf).

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66 The strain gauges were connected to a Vishay System 6000 (Model 6100) for strain reading and data acquisition ( 52 ). This system has the ability to take individual strain readings at very high frequency. This enable d the recording of the dynamic strains as the wheel passed over the pavement. For the first test slab, dynamic strain readings were recorded at 8, 9, 12 and 24 hours after conc rete placement and every day thereafter until visible cracks developed. For the other four test slabs, th e data acquisition system was programmed to collect dynamic strain data au tomatically for 30 seconds at every hour. Strain gauge readings due to a static wheel load were also taken for four wheel loading positions, namely corner (pt1), mid-edge (pt2), 30 inches from the south joint (pt3) and 30 inches from north joint (pt4). 5.3.1 Slab 1C HVS loading was originally planned to start at 6 hours afte r the start of the placement of the concrete. However, due to mechanical problem with the HVS, HVS loading was not started until 8 hours after the start of concrete placement for Test Slab 1C. The schedule for testing and data collection for Test Slab 1C is presented in appendix A1. HVS loading using a12-kip (53 kN) super single wheel wa s applied along the edge of the slab for 7 days with a total load repeti tions of 86,000. After stopping for one day for HVS maintenance, the HVS load was then ra ised to 15 kips (67 kN) and applied for 5 more days with an additional 59,000 load repe titions. The HVS load was then raised to 18 kips (80 kN) and applied for 2 more days with an additional 11,300 load repetitions. Strain gauge readings due to static loads were taken for two loading positions, namely corner and mid edge.

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67 5.3.2 Slab 1G The schedule for testing and data collecti on for Test Slab 1G is presented in appendix A2. HVS loading was started at 6 hou rs after the start of the placement of the concrete. HVS loading using a12-kip (53 kN ) super single wheel was applied along the free edge of the slab for 9 days with a tota l load repetitions of 107,152. The HVS load was then raised to 15 kips (67 kN) and app lied for 5 more days with an additional 55,067 load repetitions. For Test Slab 1C, strain gauge readings due to static loads were taken for two loading positions, namely corner (pt 1) and mi d-edge (pt 2). For Test Slabs 1G, 2C, 2E and 2G, two more static loading positions were used. They were at the locations of strain gauges No. 3 and No. 4, which were on the whee l path and at 30 inches from the southern and northern joints of the slab, respectivel y. They were named as pt 3 and pt 4 respectively. 5.3.3 Slab 2C The schedule for testing and data collecti on for Test Slab 2C is presented in appendix A3. HVS loading was started at 6 hou rs after the start of the placement of the concrete. HVS loading using a12-kip (53 kN ) super single wheel was applied along the free edge of the slab for 8 days with a tota l load repetitions of 93,323. Strain gauge readings due to static load were taken for 4 positions, pt 1, pt 2, pt 3 and pt 4, as described in the previous section, each day before continuing with dynamic loading. 5.3.4 Slab 2E The schedule for testing and data collecti on for Test Slab 2E is presented in appendix A4. HVS loading was started at 6 hou rs after the start of the placement of the concrete. HVS loading using a12-kip (53 kN ) super single wheel was applied along the

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68 free edge of the slab for 9 days with a tota l load repetitions of 59,923. HVS loading was shutdown for 3 days due to a mechanical pr oblem of the HVS. It resumed loading on 7th day and continued until 10th day. Strain readings due to static loads were taken at 4 positions each day. 5.3.5 Slab 2G The schedule for testing and data collecti on for Test Slab 2G is presented in appendix A5. HVS loading was started at 6 hou rs after the start of the placement of the concrete. HVS loading using a12-kip (53 kN ) super single wheel was applied along the free edge of the slab for 9 days with a total load repetitions of 80,000. Strain readings due to static load were take n at 4 positions each day. 5.4 Temperature Data Two sets of thermocouple wires were used to monitor the temperature distribution in the slab. Each set of thermocouples used in slab replacement consists of 6 gauges which were fixed on a wooden rod in equi distance. Thermocouples installed in Whitetopping pavements were at various dept hs, in one-inch increment, in the test sections to monitor its temperature distri bution. For the both studies, one set of thermocouples was installed at the slab corner at the side which would be loaded by the HVS wheel. One set of thermocouples was in stalled at the slab center. An extra thermocouple was installed in a concrete block (1 ft 1ft 9 in.) which placed under the shade of the HVS for the slab replacement st udy. The thermocouple readings were taken at every 10 min. intervals. Temperature differentials between the top and bottom of the slab were computed and plotted against time for Slab s 1C, 1G, 2C, 2E, and 2G in Figures 5-2, 5-3, 5-4, 5-5 and 5-6, respectively. It can be seen that th e temperature differentials fluctuated between

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69 positive values in the daytime to negative values at night. For Slabs 1C and 1G, the maximum positive temperature differential was around +15 F while the maximum negative temperature differential was around -10 F. For Slab 2C, the maximum positive temperature differential was around +22 F while the maximum negative temperature differential was around -16 F. For slab 2E, the maximum positive temperature differential was around +17.5 F while the maximum negative temperature differential was around -11 F. For Slab 2G, the temperature data collection was suspended several times due to lightening strikes on the thermo couple data acquisition system. Thus, Figure 5-6 shows only the available temperature differential data -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 168/12/20038/13/20038/14/20038/15/20038/16 /20038/17/20038/18/20038/19/20038/20/2003TimeTemp. Differential 0F Center Corner Figure 5-2. Temperature di fferentials at slab 1C

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70 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 169/15/039/17/039/19/039/21/039/23/039/25/03TimeTemp. Differential 0F Center Corner Figure 5-3. Temperature di fferentials at slab 1G -16 -12 -8 -4 0 4 8 12 16 20 24 10/13/0310/15/0310/17/0310/19/0310/21/0310/23/0310/25/0310/27/03 TimeTemp. Differential 0F Center Corner Figure 5-4. Temperature di fferentials at slab 2C

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71 -15 -10 -5 0 5 10 15 20-1500500250045006500850010500Time, minTemp. diff, F Center corner Loading starts12.50PM 3.20 AM Stop loading Figure 5-5. Temperature di fferentials at slab 2E -20 -15 -10 -5 0 5 10 15 20 25 3/28/2004 3/30/2004 4/1/2004 4/3/2004 4/5/2004 4/7/2004 4/9/2004 TimeTemp. Differentials, oF center corner Figure 5-6. Temperature di fferentials at slab 2G

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72 5.5 Impact Echo Test The impact echo test was used to detect cracks and flaws in concrete. Impact echo test is a non-destructive test on concrete and masonry structures that is based on the use of stress waves (P waves, R waves and S wa ves) that propagate through concrete and masonry and are reflected by inte rnal flaws and external surfaces ( 53 ). An impact echo instrument consists of two transducers, a m echanical impactor, a data acquisition system and a computer. A small steel ball is used to make a mechanical impact against a concrete and masonry surface and that impact generates low frequency stress waves that propagate into the structure. The reflected stress waves from flows and external surfaces can be detected by the transducers. P wave (surface wave) velocity is measured by using two transducers as shown in Figure 5-7. These two transducers are rigidly cl amped in a spacer bar that holds them at a fixed distance, L (typically 300 mm), apart from one another. The impactor is applied at about 150 mm from one of the transducers. This arrangement of transducers and impactor can separate the P wave from the R and S waves. The objective is to measure the precise arrival times of the P wave fronts at th e two transducers. If these times are t1 and t2, then the wave speed is given by 1 2t t L Vs (5-1) P Wave speed in a homogeneous, semi-inf inite, elastic solid is a function of Young’s modulus of elasticity, the mass densit y, and Poisson’s ratio of the material. The impact echo test and the determinat ion of the P wave speed were done according to ASTM C 1383 standard test met hod. An example of the P waves recorded from an impact echo test for P wave speed measurement (Figure 5-8).

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73 Figure 5-7. Schematic representation of test set-up for wave speed measurement Figure 5-8. Waveforms from impact ec ho test for P-wave speed measurement The corner and the middle edge of the slab were chosen for the impact echo P wave speed measurement. The results of stress anal yses indicated that th ese were two areas of highest stresses when the test slab was load ed by the HVS wheel, and thus were possible locations for crack development. P wave speeds at the marked locations were measured at regular time intervals to detect the possibl e development of cracks. Initiation of cracks tends to reduce the apparent elastic modulus of the material, and subsequently will reduce the wave speed through the material. Steel template (Figure 5-9) was made to

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74 conveniently mark the hammer (impact) and transducer (receive r) locations on the concrete slab. The locations of the impact and receiver on the test slab for the impact echo test for P-wave speed measurement are shown in Figure 5-10. Impact echo tests for P-wave speed measurement were run on Test Slabs 1C and 1G successfully. However, due to problems en countered with the data acquisition system of the impact echo equipment during the later part of this study, this test was not used for the rest of the test slabs. Figure 5-9. Sketch of steel template for marking impact and receiver locations

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75 Figure 5-10. Receiver and impact locations on test slab for impact echo test

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76 5.6 FWD Test FWD tests were conducted on the slab repl acement pavement sections to determine the modulus of subgrade reaction, edge coeffici ent and join coefficient, which are needed in the modeling of the concrete slab in the FEACONS program .Similar test were conducted for the Whitetopping pavement to determine the moduli of underline layers and the joint coefficient (spring constant). FWD tests were run at midday between 12 PM and 3.00PM and at early morning between 6AM and 8.00AM. At mid day, the temperature differential tends to be positive a nd slab tends to curl down at the edges and joints. This is the best time to run the FWD test for evaluation of joints and edges because the slab is more likely to be at full contact with the subgrade at the edges and joints. At midnight to early morning, the temperature diffe rential tends to be ne gative and the slab tends to curl down at the center of the slab. Th is is an ideal time to run the FWD test at the center of the slab for evaluation of the c ondition of the concrete slab base and the subgrade. Two FWD tests were performed at early morning by placing the FWD load at the slab center on Slabs 1G and 2C. and th e center of the HVS supporting portion of the 4inch Whitetopping pavement. Three set of readings for each loading position were taken for three different loads (around 5000, 9000 a nd 12000 lbs). Each test was duplicated during the same time interval. The test resu lts and the configurat ion of geophones and the load on the slab for Slabs 1G and, 2C are shown in Table B-1,and Table B-2 respectively in Appendix B. Three FWD tests were conducted to estimate the joint and edges stiffness at midday by placing the load at the slab jo int (IG and 1F), slab free edge (1G) and confined edge (1E and 2E). The test result s and the configurati on of geophones and the load on the slab for joint, free edge and conf ined edge are shown in Tables B-3, B-4 and

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77 B-5, respectively. The test wa s duplicated during the same ti me interval and tested for three load cases

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78 CHAPTER 6 OBSERVED PERFORMANCE OF THE TEST SLABS 6.1 Slab 1C Shrinkage cracks were observed at a few pl aces on the finished surface of Slab 1C within a few hours after the placement of the concrete. Figure 6-1 shows the shrinkage cracks induced on the slab. The first lo ad related crack was detected on the 15th day of loading, which was one day after the load had been increased to 18 kips. The first detected crack was a corner crack, which occu rred on the wheel path at the south end of the slab. Figure 6-2 shows a pi cture of this corner crack. This corner crack extended from the joint at 25 inches from the edge to the edge at 35 inches from the joint. HVS loading was continued for another day, at which point another corner crack, which extended from the joint at 47 inches from the edge, was observed. Figure 6-3 shows a picture of the cracked Slab 1C at the end of HVS testing. Figure 6-4 shows the initial and final crack map of the slab 1C 6.2 Slab 1G For Slab 1G, cracking was detected after 11 days of HVS loading and 2 days after the HVS load was raised to 15 kips, with a to tal of 107,152 load repetit ions at 12 kips and 20,464 load repetitions at 15 kips. At the no rthern end of the slab, there was a corner crack, which extended from the joint at 30 inches from the edge to the edge at 33 inches from the joint. This crack happened to run ove r the location of strain gauge no. 3. Figure 6-5 shows a picture of this corner crack.

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79 Figure 6-1. Shrinkage cracks on test slab 1C Figure 6-2. Corner crack on slab 1C

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80 Figure 6-3. Cracked slab 1C at the end of HVS testing There was also a similar crack on the southern end of the slab, which extended from the edge at 34 inches from the joint. However, this crack did not propaga te all the way to the joint. In addition to the corner cracks, ther e were two transverse cracks at the mid-edge of the slab (89 and 100 inches from the norther n joint of the slab). Figure 6-6 shows the transverse cracks at the mid-edge of the sl ab. These cracks propagated towards the center of the slab with additional HVS loading. Figure 6-7 shows the propagation of the midedge cracks with additional load ing. The portions of the crac ks that are marked as 1, 2 and 3, as shown in the picture, indicate the amount of crack propagation on the 1st, 2nd and 3rd day after crack initiation. Figure 6-8 sh ows the initial and final crack map of the slab 1G

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81 96" 96" 25" 47" 144" N 12" 35" (A) 96" 96" 25" 47" 144" N 12" 35" (B) Figure 6-4. Crack map of slab 1C. (A) Initial crack, (B ) Final cracks

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82 Figure 6-5. Corner crack at the southern end of slab 1G Figure 6-6. Transverse cracks at the mid-edge of slab 1G

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83 Figure 6-7. Crack propagation at the mid-edge of slab 1G with additional loading 6.3 Slab 2C For slab 2C, cracks were first detected after 3 days of HVS loading, with a total of 33,176 load repetitions at 12 kips. There were two transverse cracks at about 1 foot off from the center of the slab in the wheel path Figure 6-9 shows a picture of these two transverse cracks. Additional hairline cracks we re detected after 7 days of loading, with a total of 82,243 load repetitions at 12 kips. Figure 6-10 shows a picture of the cracks on Slab 1C after 7 days of HVS loading. Fi gure 6-11 shows the initia l and final crack map of the slab 2C

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84 144" N 12" 34" 33" 89" 96" 92" 96" 30" (A) 144" 12" 34" 33" 89" 96" 92" 96" 30" (B) Figure 6-8 Crack map of slab 1G. (A) Initial cracks (B) Final cracks

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85 Figure 6-9. Transverse crack s at mid-edge of slab 2C Figure 6-10. Cracks on slab 2C at the end of HVS testing

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86 144" N 12" 96" 96" 83" 84" (A) 144" 12" 96" 96" 83" 84" (B) Figure 6-11. Crack map of slab 2C. (A) Initial cracks, (B) Final cracks

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87 6.4 Slab 2E For slab 2E, cracks were first detected at the 7th day after the start of HVS loading, with a total of 40,809 load repe titions at 12 kips. The HVS loading was halted for 3 days due to a mechanical problem with the HVS after 3 days of loading. A transverse crack was first detected one day after HVS loading at 12 kips was resumed (or at the 7th day of test). This crack extended from the loadi ng edge to the opposing edge at the middle of the slab. Figure 6-12 shows the transverse crack that developed on Slab 2E. Figure 6-13 shows the crack map of the slab 2E. Figure 6-12. Transverse crack on la b 2E at the middle of the slab 6.5 Slab 2G For slab 2G, the first cracks (t ransverse)was detected at the 6th day of loading with 50,933 passes of HVS wheel load at12 kips. The transverse crack initia ted 9 inch south to the mid edge of the slab was extended to the opposite edge with load ings The transverse

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88 crack initiated 36 inches north to the mid edge of the slab propagated towards the opposite edges with loading. Figure 6-14 show s the transverse cracks that developed on Slab 2G. Figure 6-15 shows the initial a nd final crack maps of the slab 2G 144" 12" 96" 96" Figure 6-13 Crack map of slab 2E Figure 6-14. Transverse crack on slab 2G at the mid.

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89 144" 12" 96" 96" 36" 9" (A) 144" 12" 96" 96" 36" 9" (B) Figure 6-15. Crack map of slab 2G. (A) Initial cracks, (B) Final cracks

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90 CHAPTER 7 ANALYSIS OF DATA 7.1 Estimation of Model Parameters As presented in Chapter 3 of this disser tation, the FEACONS program was used to perform stress analyses to determine the optim um locations for strain gauges. In those previous analyses, some assumed values for the various pavement parameters were used with the purpose of determining the locat ions of maximum stresses rather than determining the magnitudes of the maximum st resses accurately. However, in analyzing the performance of the test slabs under the HVS loading, the temperature-load induced stresses on the test slabs needed to be de termined accurately. This necessitated the accurate estimation of the various pavement parameters needed by the FEACONS program to perform the stress analyses eff ectively. The modulus of subgrade reaction of the test slab was estimated by back-calcula tion of the FWD deflection basins using the FEACONS program. The deflection basins cause d FWD loads applied at the slab center was used in this case. The effect of joint and edge stiffness was assumed to be negligible for the deflection at the slab center. Loading area of the FWD is circular with a 12-inch diameter. A twelve inch by twelve inch square loading area was used in the finite element mesh to model the loading plate. The othe r pavement parameters used in the FEACONS analyses were as follows. 1. Concrete thickness 9 inches 2. Concrete modulus of elasticity 4000 ksi 3. Poison’s ratio 0.2 4. Slab sizes – 12 ft 16 ft 5. Applied load -12 kips

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91 6. No temperature effect The computed and measured deflections at the locations of the geophones for Slab 1G are shown in Figure 7-1. The measured de flections in the X dir ection were noted to be different from those in the Y direction. The differences in deflections in the two directions may be due to cracks on the slab. The computed deflection basin was obtained by using a modulus of subgrade reaction of 0.9 kci. The computed deflection for Slab 2C is shown in the Figure 7-2. For Slab 2C, the modulus of subgrade reaction was estimated to be 1.1 kci. The lower modulus of subgrad e reaction for the slab 1G could be due to deterioration of the asphalt layer prior to the placement of the concrete slab at Slab 1G. FDOT personal had noticed the deterioration of the asphalt layer at Slab 1G and had used a cold asphalt patch to repa ir the damaged area before placement of the replacement concrete slab. -70 -60 -50 -40 -30 -20 -10 0 -20-10010203040506070 distance, inchdeflection, um MeasuredDir. Y Measured Dir.-X Calculated Figure 7-1. Measured and computed deflec tion basins caused by a 9-kip FWD load at slab center for slab 1G

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92 -60 -50 -40 -30 -20 -10 0 -20-100102030405060 7 distance, inchdeflection, um Measured Dir.-X Measured Dir. Y Calculated Figure 7-2. Measured and computed deflec tion basins caused by a 9-kip FWD load at slab center for slab 2C Results of FWD tests at the slab joint were used to estimate the joint coefficients. The estimated modulus of subgrade reaction an d the values of the other known pavement parameters were used in the FEACONS progr am to compute the analytical deflections. The computed and measured defections at the slab joint for Slab 1G are shown in Figure 7-3. The linear and torsional coefficients of the joint were 200 ksi and 600 K-in/in. These coefficients gave a fairly good match between the computed and the measured deflection at the joint. Results of FWD tests at the free edge of the test slab were used to estimate the edge coefficients. The estimated subgrade modulus and the other known pavement parameters were used in the FEACONS program to calcu late the deflections caused by a 9-kip FWD

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93 load at the slab edge. An edge stiffness of 10 ksi gave a fairly good match between the computed and measured deflection at the fr ee edge. Figure 7-4 shows the computed and -100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 -20-10010203040506070 distance, inchdeflection, um Measured Calculated Figure 7-3. Measured and computed deflecti on basin caused by a 9-kip FWD load at slab joint for slab 1G -120 -100 -80 -60 -40 -20 0 -20-1001020304050607 0 distance, inchdeflection, um Measured Calculated Figure 7-4. Measured and computed deflec tion basins caused by a 9-kip FWD load at a free edge for slab 1G

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94 measured deflections for the free edge of Slab 1G. Similarly, the edge coefficient for a confined edge was estimated by using the resu lts of FWD tests run at a confined edge. The edge coefficient for a confined edge was determined to be 23 ksi 7.2 Analysis of Dynamic Strain Data Dynamic strains in the test slabs cau sed by the moving HVS wheel load were collected at 7 strain gauge lo cations. Two of the gauges were placed in the transverse direction (YY) while the other 5 gauges were installed in the longitudinal direction (XX), as described in Section 4.1.5 of this dissert ation. The main purposes for the measurement of these dynamic strains were (1) to detect crac k development in the sl ab, (2) to verify the stresses and strains as computed by th e FEACONS computer program, and (3) to evaluate the stress distribution in the c oncrete slab caused by moving wheel loads. 7.2.1 Analysis of Measured Dynamic St rains for Detection of Cracks The changes in the measured dynamic st rains could be used to detect the development of cracks and the locations of the cracks. Figure 7-5 shows the plots of dynamic strains as measured by Gauge 3 as a HVS wheel passed over it, before and after a crack developed on Slab 2C. From the plot s for Day 2 and Day 3, when the crack had not yet developed, it can be seen that the measured strains starte d to increase gradually after the load passed over the slab joint (at around the time of 18.7 seconds on the plot) and approached the location of the gauge. The measured strain peaked when the load was directly over the gauge (at around the time of 20.3 seconds on the plot). After the load passed over the gauge, the strain would quickly reversed from positive to negative as can be seen from the plots. A change in the plots of dynamic strains can be observed after Day 3, when a crack was observed. From the plots for Days 4, 5, 7 and 8, it can be seen that the dynamic

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95 strain did not start to increase until the load passed over the crack (a t around the time of 19.6 seconds). It can also be seen that, due to the formation of the crack, the magnitudes of the maximum positive and negative strains ar e noticeably higher than those before the crack. -1 0 1 0 30 50 70 90 1 81 8.51 91 9.52020.52121 .52222.523Time, SecDynamic Strain Day 3 Day 2 Day 7 Day 4 Day 5 Day 8Before Crack A fter Crack Figure 7-5. Measured dynamic st rains from gauge 3 on slab 2C For Slab 2E, cracks were first detected one day after HVS loading at 12 kips was resumed (or at the 7th day of test). Figure 7-6 show s the plots of measured dynamic strains from Gauge 4, as a HVS wheel passed over it, before and after crack initiation. A plot of the maximum measured compressive strain from Gauge 4 versus time is shown in Figure 7-7. Magnitude of the measured co mpressive strain drastically increased (indicating the initiati on of crack on the 7th day) although the crack was not observed until the 8th day. 7.2.2 Comparison of Measured Strains with Computed Strains The FEACONS program was used to comput e the strains in the concrete caused by the HVS wheel load. The pavement parameters as needed by the FEACONS model were determined as described in Section 7.1. Th e stress at each gauge location was computed

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96 using the FEACONS model for the case of a stat ic load at several specified locations on the wheel path. These static load locations were converted to a tim e scale that indicates the time the HVS test wheel passed over these locations. The computed stresses were used to calculate the strains us ing the elastic modulus and Poiss on’s ratio of the concrete. Figure 7-6. Measured dynamic st rains from gauge 4 on slab 2E 30 40 50 60 70 80 90 100 3/23/33/43/53/63/73/83/93/10 Time, dateCompressive Strain,10E-06 in/in Figure 7-7. Maximum measured compressi ve strain from gauge 4 on slab 2E 0 20 40 60 80 100 120 0 2 4 6 8 10 12 14Time, secMeasured Strain, 10^-6 in/in Day 7(3/8/04) 12.50PM Day 8 (3/9/04), 3.00AM Compressive strain

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97 Figures 7-8 through 7-13 show the compar ison of theoretical strains using the FEACONS model and the measured dynamic strain s at gauge locations 1, 2, 4, 5, 6and 7 on Slab 1C. Gauge number 3 had picked up lot of noise, and thus the comparison between theoretical and measured is not presented for this gauge. It can be seen from these figures that the computed strains are generally fairly close to the measured values. The closeness of th e analytical strains to the measured strains indicates that the FEACONS program can model the response of the concrete slab fairly well and can be used to analyze the behavior of the slab under othe r critical load and thermal conditions. 7.3 Analysis of Static Strain Data The FEACONS program models the wheel lo ads as static loads no matter whether the loads are moving or stationary. Questions arise as to whether it is valid to model a moving load as a static load as done in the FEACONS program, and the possible difference between the stresses caused by a static load and those cause by a moving load of the same magnitude. This question wa s investigated by comparing the measured strains from the installed strain gauges wh en a test slab was loaded by a moving HVS wheel load, with those when the slab was load by a static wheel load of the same magnitude. However, the attempt to apply a static wheel load of a fixed and specified magnitude ran into some technical challenges. In the initial attempt, the HVS wheel was placed on the specified location and the load was gradually increased to the specified magnitude. When the load was noted to have reached the specified magnitude, the load was then released immediately. The reason that the static load had to be applied in this manner was due to the fact that it required some time before a static HVS load could be stabilized to a specified leve l. Figure 7-14 shows the res ponses of Gauges 1 and 2 when

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98 110 115 120 125 130 135 140 145 150 155 160 2828.52929.53030.53131.5 Time, Sec.Strain E-6 in/in measured strain computed strain 10 per. Mov. Avg. (measured strain) Figure 7-8. Measured and computed strains for gauge 1 on slab 1C 80 90 100 110 120 130 140 4040.54141.54242.54343.54444.545Time, SecStrain, E-6 in/in strain measured computed strain 10 per. Mov. Avg. (strain measured) Figure 7-9. Measured and computed strains for gauge 2 on slab 1C

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99 120 130 140 150 160 170 180 190 200 210 220 3434.53535.53636.53737.538Time, SecStrain,E-6 in/in strain measured computed strain 2 per. Mov. Avg. (strain measured) Figure 7-10. Measured and computed strains for gauge 4 on slab 1C -280 -260 -240 -220 -200 -180 -160 3434.53535.53636.53737.53838.5Time, Sec.Strain, E-06 in/in measured strain computed strain 2 per. Mov. Avg. ( measured strain) Figure 7-11. Measured and computed strains for gauge 5 on slab 1C

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100 140 160 180 200 220 240 260 3434.53535.53636.53737.538Time, Sec.Strain, E-06 in/in measured strain computed strain 5 per. Mov. Avg. (measured strain) Figure 7-12. Measured and computed strains for gauge 6 on slab 1C 355 360 365 370 375 380 385 390 395 2727.52828.52929.53030.53131.532Time, Sec.Strain, E-06 in/in measured strain computed strain 10 per. Mov. Avg. (measured strain) Figure 7-13. Measured and computed strains for gauge 7 on slab 1C

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101 a static wheel load was placed at the slab corn er of Slab 1G and gradually increased to an intended magnitude of 12-kip, and then releas ed immediately. As can be observed from Figure 7-14, the measured strains increased as the applied load was increased. As the load was released when it r eached the intended magnitude, the measured strains can be observed to drop suddenly. There were two main concerns with this method of loading. First, there was not a time during the test when the applied load was truly static. Second, it cannot be ascertained that the exact intended load magnitude was reached when the load was released. Because of these concerns, a second method for applying the static load was used. In the second method for applying a static load, the HVS wheel was first placed at a location away from the intended load locati on. The wheel load was increased until it reached and stabilized at th e intended load level. The HVS wheel was then moved slowing to the intended test location, and kept at the inte nded load location for about 10 seconds. Figure 7-15 shows the plot of measured strains versus time from Gauges 1 and 2 for a static load applied at the corner of Slab 2G on the 3rd day of loading. It shows that the strain during the static loading period remained fairly constant except for some noises picked up by the data acquisition system. It can also be seen that the static loading did not cause any residual strains in the concrete. After the wheel load was moved away from the load location, the measured strains fr om gauges 1 and 2 can be seen to return to their original values before loading The measured strains due to static loads were compared with those due to moving loads of the same magnitude. Figure 716 shows the comparison of the maximum measured strains from Gauges 1 and 4 on Slab 2G caused by a 12-kip static load with the

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102 maximum measured strains due to a moving load of the same magnitude. It can be seen that the difference between the measured static and dynamic strains is small and fluctuates between positive and negative values This means that there is no significant difference between the pavement response from a static load and that from a moving load of this speed. Thus, it is proper to model a m oving load of this speed as a static load, as done in the FEACONS analysis. 7.4 Impact Echo Test Results The impact echo test was used to measure the P wave speed along the grid lines marked on the corner and the mid-edge of the test slab. These impact echo tests were performed during the times when the HVS was stopped for maintenance. The grid lines used on Slab 1G are shown on Figure 7-17. On the same figure is also shown the location of the corner crack which appeared la ter on that slab. The plots of measured Pwave speed along Lines 3, 4, 8,10,15,16 at the co rner of the Slab 1G are shown in Figures 7-18 through 7-23, respectively. The plots show generally an increase in P-wave speed with time as the concrete gained strength and its elastic modulus increased. However, after the corner crack was form ed across the grid lines on 9/ 27/03, the P-wave speed can be seen to decrease drastically. 7.5 Analysis of Performance of Concrete Mixes 7.5.1 Computation of Stresses in the Test Slabs The FEACONS program was used to calculate the maximum stresses in each test slab due to the HVS loads at various times. The applicable pavement parameters (i.e. effective modulus of subgrade reaction, joint stiffnesses and edge stiffness), concrete elastic modulus, HVS load, and the temperature differential in the concrete slab for each

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103 particular condition were used in each analys is. The coefficient of thermal expansion of the concrete was assumed to be 4.5 X 10-6/F Figure 7-14. Measured strains at slab 1G in the first method of applying a static load -60 -55 -50 -45 -40 -35 -30 010203040 Time, secstrain, 10E-06 in/in Gauge 1 Gauge 2 Figure 7-15. Measured strains at slab 2G in the second method of applying a static load -220 -200 -180 -160 -140 -120 -100 020406080100Time, sec.Strain, 10E-06 in/in Gauge 1 Gauge 2 Gauge 2 Gauge 1 Gauge 1 Gauge 2

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104 Applied load : 12-kip Slab Number: 2G Time, days 0123456 Compressive Strain, in/in 0 10 20 30 40 50 60 70 Dynamic Gauge1 Static Gauge1 Dynamic Gauge4 Static Gauge4 Figure 7-16. Comparison of maximum measured dynamic and static strains

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105 Figure 7-17. Grid lines for impact echo test and location of corner crack on slab 1G 8 ft 1 6 7 10 9 8 3 2 5 4 3 2 1 18 5 9 7 8 6 4 13 17 14 16 15 12 11 10 Receiver Positions Crack path

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106 2500 2700 2900 3100 3300 3500 3700 3900 16-Sep18-Sep20-Sep22-Sep24-Sep26-Sep28-Sep30-Sep2-Oct Time ,daysVelocity, m/ s Figure 7-18. P-wave Speed along li ne 3 at corner of slab 1G 2400 2600 2800 3000 3200 3400 3600 3800 16-Sep18-Sep20-Sep22-Sep24-Sep26-Sep28-Sep30-Sep2-Oct Time, daysVelocity, m/ s Figure 7-19. Measured P-wave speed along line 4 at corner of slab 1G

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107 2000 2250 2500 2750 3000 3250 3500 3750 4000 16-Sep18-Sep20-Sep22-Sep24-Sep26-Sep28-Sep30-Sep2-Oct Time, daysVelocity, m/ s Figure 7-20. Measured P-wave speed along line 8 at corner of slab 1G 2500 2700 2900 3100 3300 3500 3700 3900 16-Sep18-Sep20-Sep22-Sep24-Sep26-Sep28-Sep30-Sep2-Oct Time, daysVelocity, m/s Figure 7-21. Measured P-wave speed along line 10 at corner of slab 1 G

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108 2400 2600 2800 3000 3200 3400 3600 3800 4000 16-Sep18-Sep20-Sep22-Sep24-Sep26-Sep28-Sep30-Sep2-Oct Time hrsVelocit y Figure 7-22. Measured P-wave speed along line 15 at corner of slab 1G 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 16-Sep18-Sep20-Sep22-Sep24-Sep26-Sep28-Sep30-Sep2-Oct Time, daysVelocity, m/s Figure 7-23. Measured P-wave speed along line 16 at corner of slab 1G

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109 The concrete elastic modulus is an impor tant material property that affects the stress/strain behavior of the c oncrete slab, and is a needed input to the FEACONS model. The elastic moduli of the concrete at the vari ous ages were obtained from the results of elastic modulus tests, and are shown in Table 5-3. When an elastic modulus from direct measurement was not available, it was first estimated from the compressive strengths of the concrete at the correspondi ng age by the following equation (54): E = 33 w1.5 fc 0.5 (7-1) Where E = elastic modulus, in psi w = unit weight, in pci fc = compressive strength in psi The computed elastic modulus (E) from the above equation was then adjusted by multiplying by the ratio of the measured and the computed elastic modulus values as computed from the available data. In order to evaluate the likelihood for the co ncrete to crack at the various times and conditions, the maximum computed tensile stress es were divided by the flexural strength of the concrete at the corresponding age to obt ain the stress-strength ratio. The flexural strengths of the concrete at the various ages were obtained from th e results of flexural strength tests as shown in Table 5-3. When the flexural strength from direct measurement was not available, it was first estimated from the compressive strength of the concrete at the corresponding age by the following equation: R = 7.5 fc 0.5 (7-2) where R = flexural strength, in psi fc = compressive strength in psi

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110 The computed flexural strength (R) from the above equation is then adjusted by multiplying by the ratio of measured flexural strength to computed flexural strength as computed from the available data. The ratios between the computed stress and the flexural strength were computed at curing times of 4, 6, and 8 hours and 1, 3, 7, 9, and 28 days, at which the samples were tested for their compressive strength, elastic modulus and flexural strength in the laboratory. The computation of stress to st rength ratios for Slabs 1C, 1G, 2C, 2E and 2G are shown in Tables 7-1 through 7-5, respectively. Figure 7-24 shows the plots of stress to strength ratio versus the number of 12-kip HVS wheel load passes for these five test slabs. 7.5.2 Relating Stress/Strength Ratio to Observed Performance The computed stress/strength ratios for the mixes as shown in Figure 7-24 can be used to explain the observed performance of the different test slabs. Slab 1C and Slab 2G used the same mix design (with a cement content of 850 lbs/yd3) and had similar strength and stress/strength ratio at late r ages. However, the mix used in Slab 2G had a much lower early strength and a much higher stress /strength ratio at early age. The computed stress/strength ratio for Slab 2G at early age (6 hours) was high er than 1.0. This explains why Slab 2G failed prematurely, while Slab 1C performed well. Slabs 1G, 2C and 2E show the same trend in stress/strength ratio at early age of the concrete. Slab 1G performed well, and can be explained by its computed stress/strength ratio less than 1 throughout. Cracks developed on Slab 2C on the 3rd day of loading due to a high temperature gradient (+20 F) present in the concrete slab, which induced higher stresses in the slab.

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111Table 7-1. Stress analysis for slab 1C (Mix 1) Flexural Strength, psi Time, hrs Temperature Differential, F Accumulated HVS passes Applied Load, kips Compressi ve Strength, psi Elastic Modulus, ksi Computed Stress, psi Computed Measured or Adjusted Computed Stress / Strength 4 3.2 012 9801711227.02352580.88 6 4.9 012 17002254253.03093400.74 8 1.8 012 22602599246.03573920.63 24 7.4 531112 47503767321.05175690.56 72 7.0 2909012 52803972324.05455990.54 168 6.5 7468012 59604220326.05796370.51 216 6.5 8600112 60264243327.05826400.51 216 6.5 8600115 60264243390.05826400.61 312 *6.5 14500015 61584290391.05896470.60 312 *6.5 14500018 61584290455.05896470.70 360 *6.5 15630018.62244313457.05926510.70 672 66534459 612673 data is not available. Assumed the temperature differential of the last day of data collection

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112Table 7-2. Stress analys is for slab 1G (Mix2) Flexural Strength, psi Time, hrs Temperatue Differential, F Accumulated HVS passes Applied Load, kips Compressiv e Strength, psi Elastic Modulus psi Computed Stress psi computed Measured/ Adjusted Computed Stress / Strength 4 -6.1 01271012672062002200.94 6 6.3 012110015772572492740.94 8 7.7 101512152018542752923220.85 24 14.9 913412334027493624334770.76 72 13.9 4498712480333003815205720.67 168 13.7 9518712554035403965586140.64 216 12.4 11599612563335793855636190.62 216 12.4 11599615563335794535636190.73(15 kips) 264 9.5 13912815572736184285686240.69(15 kips) 672 65203950606666

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113Table 7-3. Stress analys is for slab 2C (Mix3) Flexural Strength, psi Time, hrs Temperatue Differential, F Accumulated HVS passes Applied Load, kips Compressive Strength, psi Elastic Modulus, psi Computed Stress psi computedMeasured/Adjusted Computed Stress / Strength 4 1.4 0124801030217 1641641.32 6 -3.4 0128601388216 2202200.98 8 -7.2 9901211701627208 2572570.81 24 9.5 89141227702629319 3954340.73 72 20.2 331761238833223435 4675140.85 168 20.2 822431250203826472 5315850.81 672 65104366 605666

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114Table 7-4. Stress analysis for slab 2E (Mix 4) Flexural Strength, psi Time, hrs Temperatue Differential, F Accumulated HVS passes Applied Load, kips Compressive Strength, psi Elastic Modulus, psi Computed Stress psi computedMeasured/Adjusted Computed Stress / Strength 4 -1.8 0126301221221 1881881.18 6 0 01212501730234 2652600.90 8 0 10001215601920243 2962960.82 24 17.2 76951234402620372 4405250.71 72 15 307801243402920372 4945950.63 168 5 408091249803230299 5296500.46 192 192

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115Table 7-5. Stress analysis for slab 2G (Mix 5) Flexural Strength, psi Time, hrs Temperatue Differential, F Accumulated HVS passes Applied Load, kips Compressive Strength, psi Elastic Modulus, psi Computed Stress psi computedMeasured/Adjusted Computed Stress / Strength 4 3.2 0126701174228 1941861.22 6 4.9 01212101569252 2612501.01 8 1.8 10001218301775248 3213080.81 24 7.4 70001238502514301 4655300.57 72 7.0 200001246502789309 5115630.55 168 6.5 700001255302953311 5586000.52

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116 HVS Passes 0.02.0e+44.0e+46.0e+48.0e+41.0e+51.2e+51.4e+5 Stress / Flexural Strength Ratio 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Slab1C Slab1G Slab2C Slab2E Slab 2G Figure 7-24. Stress/ flexural st rength ratio versus HVS passes This can be seen from the sharp increase in the stress/strength ratio for Slab 2C on the third day. Though the maximum computed tensile stress occurred at the bottom of the slab, the cracks were observed to initiate from the top of the sl ab. The two observed cracks on the top of the slab occurred at the locations of maximum tensile stresses at the top of the slab (though their magn itudes are slightly lower than that at the bottom of the slab). It is believed that some residual tensil e stresses might be induced at the top of the slab during curing. These residu al tensile stresses, when added to the load induced tensile

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117 stresses can cause the maximum tensile stress es at the top of the slab to exceed the maximum tensile stresses at the bottom of the slab. The evidence of possible residual tensile st resses at the slab surface can be seen from the measured strains from slab 2C in the first 6 hours after concrete placement as shown in Figure 7-25. The high tensile strain as recorded by gauge 5 and 6 indicated that the slab expanded at the top dur ing early age. Residual tensile stresses at the top of the slab would result as the concrete cooled down and tried to contract. The crack that developed in Slab 2E wa s postulated to be caused by the locking up of the dowel bars at the joint. It was noted that the crack on Slab 2E was different from the cracks on the other slabs. While the cr acks on other slabs pr opagated gradually from the loading edge to the joints or the opposite edge, the crack that developed on Slab 2E was a single transverse crack that cut across th e entire slab and occurred in a short time. That crack occurred right after the HVS loading was resumed after three days of shutdown due to a mechanical problem. From the appearance of the deep transverse crack across the middle of the slab, it was postulated that the crack might be caused by the high stresses from the locking up of the dowel bars at both joints. When the slab tried to contract at night but could not due to the locked dowel ba rs at the joint, the tensile stresses would be induced, which could crack the slab across the center. A analytical studies by other researchers (55, 56), have shown that in the case of dowel locking, tensile stresses can be induced at the center of the slab. The magnitude of these tensile stresses depend on depend on, temperature ch ange, temperature grad ient, shrinkage and slabbase shear transfer

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118 -50 0 50 100 150 200 05000100001500020000 Time, sec.Strain, 10^-06 in/in Gauge 1 Gauge 2 Gauge 4 Gauge 5 Gauge 6 Gauge 7 Figure 7-25 Measured strains fr om slab 2C n the first 6 hours 7.5.3 Required Concrete Properties for Performance The results from this experimental study s how that loading a c oncrete slab at the early age when the induced stress may be higher than the strength of the concrete will adversely affect the performance of the slab What should the required properties of the concrete for slab replacement be? Basically, the concrete used should be such that the anticipated maximum tensile stresses should be less than the flexural strength of the concrete at the time when the slab is open to traffic. Using the pavement conditions of the test slabs in this study (9 inch slab on a strong foundation) and an applied wheel load of 12 kips, the stress/strength ratios of concrete of different compressive strengths are computed for different temperature differentials in the slab. In doing these com putations, the elastic modulus of the concrete was assumed to be related to the compressiv e strength by Equation 7-1, and the flexural strength was related to the compressive streng th by Equation 7-2, as presented earlier. The coefficient of thermal expansion of concrete was assumed to be 4.5 X 10-6/F.

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119 Figure 7-26 shows the plot of the computed stress/stre ngth ratio as a function of compressive strength by using the assumed re lationships as given by Equations 1 and 2, for a 9-inch concrete slab with similar foundatio n as that of the test slab and subjected to a 12-kip wheel load. 9 inch slab subjected to 12-kip wheel load Compressive Strength,x103 psi 02000400060008000 Stress / Flexural Strength Ratio 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Temperature Differential in Concrete -100F Temperature Differential in Concrete 00F Temperature Differential in Concrete 100F Temperature Differential in Concrete 200F Figure 7-26. Computed stress/strength ratio versus compressive strength of concrete using ACI equations for relating fc, E and flexural strength It is to be noted that if the relations hips among the compressive strength, elastic modulus and flexural strength are different, the plot of computed stress/strength ratio versus compressive strength would be different Using the limited data from this study, the relationship between compre ssive strength and the elas tic modulus was developed, and plotted in Figure 7-27. The relationshi p between the flexural strength and the compressive strength was also devel oped and plotted in Figure 7-28.

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120 y = 39.785x0.5262R2 = 0.9562 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 02000400060008000 Compressive strength, psiElastic Modulus, ksi Figure 7-27. Relationship between comp ressive strength and elastic modulus y = 6.0762x0.5301R2 = 0.9806 0 100 200 300 400 500 600 700 01000200030004000500060007000 Compressive strength, psiFlexural strength, psi Figure 7-28. Relationship between flexur al strength and compressive strength The following regression equations were developed from the available data:

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121 E = 39.785fc 0.5262 (7-3) R2 = 0.9562 Where fc = compressive strength, psi E = elastic modulus, ksi R = 6.0672 fc 0.5301 (7-4) R2 = 0.9806 Where R = flexural strength, psi fc = compressive strength, psi Figure 7-29 shows the plot of computed st ress/strength ratio versus compressive strength by using Equations 7-3 and 7-4 for relating comp ressive strength to elastic modulus and flexural strength. 9 inch slab subjected to 12-kip load Compressive Strength,x103 psi 02000400060008000 Stress / Flexural Strength Ratio 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Temperature Differential in Concrete -100F Temperature Differential in Concrete 00F Temperature Differential in Concrete 100F Temperature Differential in Concrete 200F Figure 7-29. Computed stress/strength ratio as a function of compre ssive strength using the developed relationship between fc, E and flexural strength

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122 CHAPTER 8 CONCLUSIONS AND RECOMMENDATIONS 8.1 Summary of Findings Five 9-inch thick concrete replacement slabs were constructed and tested by a Heavy Vehicle Simulator (HVS), which appl ied a 12-kip super single wheel load in a uni-directional mode along the edge of the slab beginning at 6 hours after the placement of concrete. Two of the test slabs (1C and 2G) used a concre te with a cement content of 850 lbs per cubic yard of concrete, while the ot her three test slabs (1G, 2C and 2E) used a concrete with a cement content of 725 lbs per cubic yard of concrete. The results of the experiments indicated that Sl abs 1C and 1G performed well, while Slabs 2C, 2E and 2G cracked prematurely under the 12-kip wheel loads. The FEACONS (Finite Element Analysis of CONcrete Slabs) computer program was used to model the response of the test slabs and to compute the stresses in the concrete slabs due to the applied loads and th e temperature differentials in the concrete slabs. The good performance of Slabs 1C a nd 1G, which had different cement contents and different strengths from one another, was at tributed to the fact that the temperatureload induced stresses were much lower than th e flexural strengths of the concretes. The premature cracking of Slabs 2C and 2G, whic h also had different cement contents and different strengths from one anot her, was attributed to the fact that the temperature-load induced stresses exceeded the estimated flexural strength of the conc rete during the early age of the concretes.

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123 The premature cracking of Slab 2E coul d not be explained by the computed temperature-load induced stresses. From the appearance of the deep transverse crack across the middle of the slab, it was postula ted that the cracking might be caused by the locking-up of the dowel bars at both joints. Impact echo tests were used successfully in this study to detect cracks in a concrete slab. This was manifested by a sudden drop in the apparent measured speed of P waves across the location of cracks. Cracks in the conc rete slab were also successfully detected from observed changes in the measured strain s from strain gages that had been installed in the concrete. The predicted strains in the concrete sl ab as calculated by the FEACONS program matched fairly well with the measured strains from the installed strain gages. The measured maximum strains caused by a m oving HVS wheel load were found to match fairly well with the measured maximum strain s caused by a static wheel load of the same magnitude. This indicates that it is proper to model a moving load of this type by a static load as used in the FEACONS program. Plots of stress to flexural strength ratio versus compre ssive strength of concrete were developed for a typical 9-inch concrete replacement slab subjected to a 12-kip wheel load and different temper ature differentials in the concrete slab. When the ACI equations were used to relate the compress ive strength to elastic modulus and flexural strength of concrete (as presented in Fi gure 7-27), a compressive strength of 1600 psi (flexural strength of 300 psi) or above at the time of the loadi ng of the concrete slab, with a temperature differential of 10 F, would be required to ensu re that the induced stress would not exceed the flexural strength (or the stress to strength ratio of less than 1).

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124 When the relationships between the compress ive strength, elastic modulus and flexural strength as developed from the limited test da ta from this study were used (as shown in Figure 7-30), a compressive strength of 1100 psi (flexural strength of 248 psi) or above at the time of the loading of the concrete slab, with a temperature differential of 10 F, would be required to ensure th at the stress to strength ratio would be less than 1. 8.2 Conclusions The results from slab replacement study show that the performance of a concrete replacement slab depends not just on the cem ent content of the concrete mix, as two concrete slabs with the same concrete mix design can have drastically different performance. The performance of a concrete replacement slab will depend on whether or not the concrete will have sufficient strength to resist the anticipat ed temperature-load induced stresses in the concrete slab. The strength development of a concrete depends not only on the mix design but also the conditi on under which the concrete is cured. The anticipated temperature-load induced stress es are a function of the slab thickness, effective modulus of subgrade reaction, modulus of the concrete, coefficient of thermal expansion of the concrete, anticipated loads and anticipated temperature differentials in the concrete slab. The antic ipated stress must be lower th an the anticipated flexural strength of the concrete (stre ss-strength ratio less than 1) at all times to ensure good performance. Since FDOT accept concrete ba sed on compressive strength, the required compressive strength is estimated from flexural strength-compressive strength relationship for Florida concrete Based on the limited test results from this study, it appears that for a 9-inch slab placed on an adequate foundation (as in the case of the asphalt base used in this study)

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125 and a maximum temperature differential of +10 F in the concrete slab, a minimum required compressive strength of 1100 (flexural strength of 248 psi) to1600 psi (flexural strength of 300 psi) for the concrete at the time of application of traffic loads may be adequate. It may be feasible to lower th e minimum required compressive strength of 2200 psi at 6 hours, as specified by the curr ent FDOT specifications, to 1600 psi at 6 hours, subject to further testing and verification. 8.3 Recommendations Due to the limited scope of this study a nd the limited amount of testing performed in this study, no recommendation for change s in FDOT specifications for concrete replacement slab should be made at this point. It is recommended th at further testing and research in this subject area be conducted, w ith particular focus on the following areas: The use of maturity meter to accurately determine the strength of the in-place concrete, and to determine the time when the concrete will have sufficient strength to be open to traffic. Determination of the relationships between compressive strength, flexural strength and elastic modulus of typical concretes used in replacement slabs in Florida. Accurate determination of these relationships is needed in order to determine the required strength of the concrete before th e pavement slab can be open to traffic. Determination of temperature distributions in typical concrete pavement slabs in Florida. This information is needed in order to accurately determine the maximum temperature-load induced stresses in the concrete slabs. The strength of the concrete needs to be higher than this maximum induced stress to avoid cracking. It was unfortunate that detailed inform ation on the mix designs and properties of the aggregates used in the concrete was not available in this study. This detailed information should be obtained in future te st slabs, so that the concrete mix design used can be verified and effects of materials can be fully evaluated.

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126 APPENDIX A HVS TESTING AND DATA COLLECTION SCHEDULE Table A-1. Schedule of testing and data collection for test slab 1C Order of Testing Date Time Load Remarks/# HVS Passes Initial Strain Readings 8/12/2003 10:10 AM Curing Strains (1st 6 hours) 8/12/2003 11:20 AM Static Strain #1 (Corner) 8/12/2003 6:43 PM 12 kips Static Strain #2 (Corner) 8/12/2003 6:47 PM Static Strain # 3 (Center) 8/12/2003 6:50 PM Initial Dynamic Load 8/12/2003 6:53 PM Strain reading at 6 hours after mixing of concrete was missed. Testing time was moved from originally scheduled 4:50 to 6:43 PM due to mechanical problems Dynamic Strain 9h 8/12/2003 7:48 PM Static Strain #4 (Corner) 9 hr 8/12/2003 7:54 PM Static Strain # 5 (Center) 9 hr 8/12/2003 7:56 PM Dynamic Strain 10.5 h 8/12//2003 9:14 PM Dynamic Strain 11 h 8/12/2003 9:50 PM Dynamic Strain 12 h 8/12/2003 10:48 PM Dynamic Strain 13 h 8/12/2003 11:46 PM Static Strain # 5 (Corner ) 13 h 8/12/2003 11:52 PM Static Strain # 6 (Center ) 13 h 8/13/2003 11:55 PM Dynamic Strain 15 h 8/13/2003 1:45 AM Static Strain Day 1 (Corner) 8/13/2003 9:28 AM 5311, 7:35 AM 8/13/03 Static Strain Day 1 (Center) 8/13/2003 9:35 AM Dynamic Strain Day 1 8/13/2003 10:51AM Static Strain Day 2 (Corner) 8/14/2003 8:52 AM 16950, 7:24 AM 8/14/03 Static Strain Day 2 (Center) 8/14/2003 8:56 AM Dynamic Strain Day 2 8/14/2003 10:50 AM Static Strain Day 3 (Corner) 8/15/2003 10:17 AM 29090, 7:20 AM 8/15/03 Static Strain Day 3 (Center) 8/15/2003 10:19 AM Dynamic Strain Day 3 8/15/2003 11:17 AM Static Strain Day 4 (Corner) 8/16/2003 8:23 AM 40044, 9:03 AM 8/16/03

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127 Table A-1 continued Order of Testing Date Time Load Remarks/# HVS Passes Static Strain Day 4 (Center) 8/16/2003 8:27 AM Dynamic Strain Day 4 8/16/2003 8:46 AM Static Strain Day 5 (Corner) 8/17/2003 8:21 AM 50900, 7:33 AM 8/17/03 Static Strain Day 5 (Center) 8/17/2003 8:26 AM Dynamic Strain Day 5 8/17/2003 8:36 AM Static Strain Day 6 (Corner) 8/18/2003 8:52 AM 62540, 7:23 AM 8/18/03 Static Strain Day 6 (Center) 8/18/2003 8:55 AM Dynamic Strain Day 6 8/18/2003 10:13 AM Static Strain Day 7 (Corner) 8/19/2003 8:31 AM 74680, 7:01 8/19/03 Static Strain Day 7(Center) 8/19/2003 8:35 AM Dynamic Strain Day 7 8/19/2003 10:54 AM HVS Maintenance (Day 8) 8/20/2003 New Load Electrical problems resulted in shutdown Static Strain Day 9 (Corner) 8/21/2003 8:40 AM 15kips 86001, 7:31 AM 8/19/03 Static Strain Day 9 (Center) 8/21/2003 8:46 AM Dynamic Strain Day 9 8/21/2003 10:52 AM Dynamic Strain Day 10 8/22/2003 10:03 AM 98440, 7:22 AM 8/22/03 Static Strain Day 10 (Corner) 8/22/2003 10:10 AM Static Strain Day 10(Center) 8/22/2003 10:13 AM Static Strain Day 11 (Corner) 8/23/2003 9:17 AM 110609, 9:08 AM 8/23/03 Static Strain Day 11 (Center) 8/23/2003 9:21 AM Dynamic Strain Day 11 8/23/2003 9:26 AM Dynamic Strain Day 12 8/24/2003 8:43 AM 122371, 9:00 AM 8/24/03 Static Strain Day 12 (Corner) 8/24/2003 8:49 AM Static Strain Day 12(Center) 8/24/2003 8:54 AM Static Strain Day 13 (Corner) 8/25/2003 8:55 AM 134088, 7:15 AM 8/25/03 Static Strain Day 13(Center) 8/25/2003 8:58 AM Dynamic Strain Day 13 8/25/2003 10:51 AM Load changes 8/25/2003 11:00 AM 18 kips Pressure of super single tire adjusted to New Load Static Strain Day 14 (Corner) 8/26/2003 8:50 AM 145000, 7:23 AM 8/26/03

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128 Table A-1 continued Order of Testing Date Time Load Remarks/# HVS Passes Static Strain Day 14 (Center) 8/26/2003 8:56 AM Dynamic Strain Day 14 8/26/2003 10:51 AM Dynamic Strain Day 15 8/27/2003 6:07 AM 156300, 6:54 AM 8/27/03 Static Strains Day 15 (Corner) 8/27/2003 6:10 AM Static Strains Day 15 (Center) 8/27/2003 6:19 AM Cracks detected on concrete slab on wheel path

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129 Table A-2. Schedule of testing and data collection for test slab 1G Order of Testing Date Time Load # HVS Passes / Remarks Curing Strain_1 9/16/2003 9:48:16 AM No Load Curing Strain_2 9/16/2003 9:55:45 AM “ Static Strain_6hrs_Pt 1 9/16/2003 3:02:07 PM 12 kips Static Strain_6hrs_Pt 2 9/16/2003 3:04:23 PM Static Strain_6 hrs_Pt 3 9/16/2003 3:06:52 PM Dynamic Strian_6 h 9/16/2003 3:11:35 PM Dynamic Strain_6h 9/16/2003 3:13:39 PM Dynamic Strain_12h 9/16/2003 9:10:49 PM Static Strain_12h_Pt 1 9/16/2003 9:15:56 PM Static Strain_12h_Pt 2 9/16/2003 9:21:44 PM Static Strain_12h_Pt 3 9/16/2003 9:25:45 PM Dynamic Strain_24h 9/17/2003 9:00:37 AM Static Strain_24h_Pt 1 9/17/2003 9:12:03 AM 9134 @ 9:13 AM 9/17/03 Static Strain_24h_Pt 2 9/17/2003 9:15:18 AM Static Strain_24h_Pt 3 9/17/2003 9:17:58 AM Static Strain_Day 3_Pt 1 9/19/2003 9:09:13 AM Static Strain_Day 3_Pt 2 9/19/2003 9:12:58 AM Static Strain_Day 3 _Pt 3 9/19/2003 9:16:33 AM Static Strain Day 3_ Pt 4 9/19/2003 9:19:29 AM 4th point added for static strain data collection Dynamic Strain_Day 3 9/19/2003 9:23:26 AM 33221 @ 9:46 AM 9/19/03 Dynamic Strain_Day 4 9/20/2003 9:01:56 AM Static Strain_Day 4_Pt 1 9/20/2003 9:05:31 AM Static Strain_Day 4_Pt 4 9/20/2003 9:09:44 AM Static Strain_Day 4_Pt 2 9/20/2003 9:15:53 AM Static Strain_Day 4_Pt 3 9/20/2003 9:19:16 AM 44987 @ 9:28 AM 9/20/03 Dynamic Strain_Day 5 9/21/2003 8:48:06 AM Static Strain_Day 5_Pt 1 9/21/2003 8:56:01 AM Static Strain_Day 5_Pt 4 9/21/2003 9:01:16 AM Static Strain_Day 5_Pt 2 9/21/2003 9:04:17 AM Static Strain_Day 5_Pt 3 9/21/2003 9:07:06 AM 57314 @9:15 AM 9/21/03 Dynamic Strain_Day 6 9/22/2003 8:54:05 AM 69942 @ 9:01 AM 9/22/03 Static Strain_Day 6_Pt 1 9/22/2003 9:05:54 AM Static Strain_Day 6_Pt 2 9/22/2003 9:09:24 AM Static Strain_Day 6_Pt 3 9/22/2003 9:18:15 AM Static Strain_Day 6_Pt 4 9/22/2003 9:21:06 AM Dynamic Strain_Day 7 9/23/2003 8:52:38 AM 82331 @ 9:01 AM 9/23/03 Static Strain_Day 7_Pt 1 9/23/2003 9:07:57 AM Static Strain_Day 7_Pt 2 9/23/2003 9:10:26 AM Static Strain_Day 7_Pt 3 9/23/2003 9:14:16 AM Static Strain_Day 7_Pt 4 9/23/2003 9:19:31 AM Dynamic Strain_Day 8 9/24/2003 9:33:20 AM 95187 @ 9:37 AM 9/24/03

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130 Table A-2 continued Order of Testing Date Time Load Remarks/# HVS Passes Static Strain_Day 8 _Pt 1 9/24/2003 9:39:18 AM Static Strain_Day 8_Pt 2 9/24/2003 9:46:12 AM Static Strain_Day 8_Pt 3 9/24/2003 9:50:08 AM Static Strain_Day 8_Pt 4 9/24/2003 9:53:31 AM Dynamic Strain_Day 9 9/25/2003 8:41:22 AM 107152 @ 9:00 AM 9/25/03 Static Strain_Day 9_Pt 1 9/25/2003 9:07:04 AM Static Strain_Day 9_Pt 2 9/25/2003 9:09:59 AM Static Strain_Day 9_Pt 3 9/25/2003 9:13:31 AM Static Strain_Day 9_Pt 4 9/25/2003 9:17:27 AM Dynamic Strain_Day 9_15 kips 9/25/2003 10:50:47 AM Dynamic Strain_Day 10_15kips 9/26/2003 9:02:21 AM 115996 @ 9:04 AM 9/26/03 Static Strain_Day 10_12kips_Pt 1 9/26/2003 9:09:27 AM Static Strain_Day 10_12kips_Pt 2 9/26/2003 9:11:26 AM Static Strain_Day 10_12kips_Pt 3 9/26/2003 9:14:14 AM Static Strain_Day 10_12 kips_Pt 4 9/26/2003 9:15:55 AM Static Strain_Day 10_15 kips_Pt 1 9/26/2003 10:30:46 AM 15 kips Static Strain_Day 10_15 kips_Pt 1i 9/26/2003 10:33:10 AM Static Strain_Day 10_15 kips_Pt 2 9/26/2003 10:36:49 AM Static Strain_Day 10_15 kips_Pt 3 9/26/2003 10:40:27 AM Static Strain_Day 10_15 kips_Pt 4 9/26/2003 10:45:14 AM Dynamic Strain_Day 11 9/27/2003 9:20:15 AM 127616 @ 10:00 AM 9/27/03 Static Strain_Day 11_Pt 1 9/27/2003 9:30:30 AM Static Strain_Day 11_Pt 2 9/27/2003 9:46:08 AM Static Strain_Day 11_Pt 3 9/27/2003 9:50:07 AM Static Strain_Day 11_Pt 4 9/27/2003 9:52:44 AM Cracks detected on Strain Gage position # 3 Dynamic Strain_Day 12 9/28/2003 8:59:08 AM 139128 @ 9:20 AM 9/28/03 Static Strain_Day 12_Pt 1 9/28/2003 9:03:17 AM Static Strain_Day 12_Pt 4 9/28/2003 9:06:30 AM Static Strain_Day 12_Pt 2 9/28/2003 9:12:58 AM Static Strain_Day 12_Pt 3 9/28/2003 9:16:06 AM Dynamic Strain _Day 13 9/29/2003 8:38:39 AM 150160 @9:01 AM 9/29/03 Static Strain_Day 13_Pt 1 9/29/2003 9:05:30 AM Static Strain_Day 13_Pt 2 9/29/2003 9:09:31 AM

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131 Table A-2 continued Order of Testing Date Time Load Remarks/# HVS Passes Static Strain_Day 13_Pt 3 9/29/2003 9:13:23 AM Static Strain_Day 13_Pt 4 9/29/2003 9:16:25 AM Dynamic Strain_Day 14_15 kips 9/30/2003 8:49:43 AM 162219 @ 8:54 AM 9/30/03 Static Strain_Day 14_15kips_Pt 1 9/30/2003 9:00:19 AM Static Strain_Day 14_15kips_Pt 2 9/30/2003 9:03:33 AM Static Strain_Day 14_15 kips_Pt 3 9/30/2003 9:09:22 AM Static Strain_Day 14_15 kips_Pt 4 9/30/2003 9:11:53 AM

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132 Table A-3. Schedule of testing and data collection for test slab 2C Order of Testing Date Time Collected Load # HVS Passes / Remarks Curing Strain 10/13/2003 9:28:34 AM 12 kips Static Strain pt 1_6h 10/13/2003 3:14:01 PM Static Strain pt 2_6h 10/13/2003 3:17:23 PM Static Strain pt 3_ 6h 10/13/2003 3:21:17 PM Static Strain pt 4_ 6h 10/13/2003 3:28:35 PM Dynamic Strain_6h 10/13/2003 3:35:22 PM Static Strain pt 1 _12 h 10/13/2003 9:38:31 PM Static Strain pt 2_12 h 10/13/2003 9:25:46 PM Static Strain pt 3_12 h 10/13/2003 9:30:17 PM Static Strain pt 4_12 h 10/13/2003 9:34:51 PM Dynamic Strain_12 h 10/13/2003 9:42:14 PM Dynamic Strain_24 h 10/14/2003 8:51:38 AM 8914 @9:01 AM 10/14/03 Static Strain pt 1_24 h 10/14/2003 9:22:11 AM Static Strain pt 2_24 h 10/14/2003 9:26:48 AM Static Strain pt 3_24 h 10/14/2003 9:29:21 AM Static Strain pt 4_24 h 10/14/2003 9:31:59 AM Dynamic Strain_Continuous_Day1 10/14/2003 11:49:04 AM Dynamic Strain_Day 2 10/15/2003 8:47:38 AM 21159 @9:02 AM 10/15/04 Static Strain pt 1_Day 2 10/15/2003 10:44:42 AM Static Strain pt 2_ Day 2 10/15/2003 10:40:12 AM Static Strain pt 3_ Day 2 10/15/2003 10:47:39 AM Static Strain pt 4_Day 2 10/15/2003 10:50:08 AM Dynamic Strain_Continuous_Day 2 10/15/2003 11:19:20 AM Dynamic Strain_Day 3 10/16/2003 8:53:53 AM 33176 @9:01 AM 10/16/04 Static Strain pt 1_Day 3 10/16/2003 9:03:36 AM Cracks detected in mid slab area Static Strain pt 2_Day 3 10/16/2003 9:06:21 AM Static Strain pt 3_Day 3 10/16/2003 9:09:16 AM Static Strain pt 4_Day 3 10/16/2003 9:11:35 AM Dynamic Strain_Continuous_Day 3 10/16/2003 11:21:48 AM Dynamic Strain _Day 4 10/17/2003 8:50:06 AM 45433 @9:03 AM 10/17/04 Static Strain pt1_Day 4 10/17/2003 9:09:41 AM Static Strain pt 2_Day 4 10/17/2003 9:12:01 AM Static Strain pt 3_Day 4 10/17/2003 9:14:38 AM Static Strain pt 4_Day 4 10/17/2003 9:17:08 AM Dynamic Strain_Day 4 10/17/2003 2:41:10 PM Dynamic Strain_Continuous_Day 4 10/17/2003 4:09:11 PM Dynamic Strain_Day 5 10/18/2003 9:14:57 AM 57476 @ 9:40 10/18/04 Static Strain pt 1_Day 5 10/18/2003 9:24:34 AM Static Strain pt 2_Day 5 10/18/2003 9:28:38 AM Static Strain pt 3_Day 5 10/18/2003 9:33:14 AM Static Strain pt 4_Day 5 10/18/2003 9:36:17 AM

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133 Table A-3 continued Order of Testing Date Time Collected Load # HVS Passes / Remarks Dynamic Strain_Continuous_Day 5 10/18/2003 10:45:12 AM Static Strain pt 1_Day 6 10/19/2003 8:56:26 AM 69647 @ 9:16 10/19/04 Static Strain pt 2_Day 6 10/19/2003 8:59:43 AM Static Strain pt 3_Day 6 10/19/2003 9:07:30 AM Static Strain pt 4_Day 6 10/19/2003 9:12:17 AM Dynamic Strain_Continuous_Day 6 10/19/2003 9:57:17 AM Dynamic Strain_Day 7 10/20/2003 8:38:37 AM Additional hairline cracks detected in wheel path Static Strain pt 1_Day 7 10/20/2003 9:00:32 AM 82243 @ 8:57 10/20/04 Static Strain pt 2_Day 7 10/20/2003 9:03:27 AM Static Strain pt 3_Day 7 10/20/2003 9:05:41 AM Static Strain pt 4_Day 7 10/20/2003 9:08:05 AM Dynamic Strain_Continuous_Day 7 10/20/2003 3:22:43 PM Dynamic Strain_Day 8 10/21/2003 8:48:52 AM 93323 @ 9:02 10/21/04 Static Strain pt 1_Day 8 10/21/2003 9:01:00 AM Static Strain pt 2_Day 8 10/21/2003 9:03:11 AM Static Strain pt 3_Day 8 10/21/2003 9:06:31 AM Static Strain pt 4_Day 8 10/21/2003 9:08:53 AM

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134 Table A-4. Schedule of testing and data collection for test slab 2E Order of Testing Date Time Collected Load # HVS Passes / Remarks Curing Strain 3/2/2004 11:24:00 AM 12 kips Initial Static Strains 3/2/2004 4:27:00 PM Initial Static Strains #2 3/2/2004 4:33:00 PM Initial Dynamic Strains 3/2/2004 4:38:00 PM Initial Dynamic strains (2 Passes) 3/2/2004 4:47:00 PM Initial Dynamic strains 3/2/2004 4:48:00 PM Initial Static strain 3/2/2004 5:06:00 PM Initial Dynamic strain 3/2/2004 5:19:00 PM Dynamic Strain 3/2/2004 6:25:00 PM Static 3/2/2004 6:30:00 PM Dynamic 3/2/2004 7:25:00 PM Static 3/2/2004 7:30:00 PM Dynamic 3/2/2004 8:28:00 PM Static 3/2/2004 8:33:00 PM Dynamic 3/2/2004 9:25:00 PM Static 3/2/2004 9:30:00 PM Dynamic 3/2/2004 10:27:00 PM Static 3/2/2004 10:32:00 PM Dynamic Day 2 3/3/2004 8:28:00 AM 7695 Static Day 2 3/3/2004 10:35:00 AM All Day Dynamic Day 2 3/3/2004 10:42:00 AM Static 1033 Day 3 3/4/2004 10:48:00 AM Dynamic Strain 1033 Day 3 3/4/2004 11:00:00 AM 19010 All day Dynamic Day 3 3/4/2004 12:15:00 PM Dynamic Strain Day 4 3/5/2004 8:51:00 AM 30780 3/5/2004 8:54:00 AM HVS Shuts down due to Computer problems 3/8/2004 12:09:00 PM HVS Resumes testing Static Strain Day 7 (After Restart from problem) 3/8/2004 12:44:00 PM 30780 Dynamic Strain Day 7 (After restart) 3/8/2004 12:52:00 PM All Day Dynamic Day 7 3/8/2004 1:00:00 PM 3/9/2004 Crack appears in center of slab going through the center gages: full length, total penetration Dynamic Strain Day 8 3/9/2004 8:46:00 AM 40809 Static Strain Day 8 3/9/2004 10:40:00 AM All Day Dynamic Day 8 Part 1 3/9/2004 11:04:00 AM 52624 All Day Dynamic Day 8 Part 2 3/9/2004 1:36:00 PM

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135 Table A-4 continued Order of Testing Date Time Collected Load # HVS Passes / Remarks Dynamic Day 9 (Before Restart) 3/10/2004 8:52:00 AM 59923 Static Strain Day 9 3/10/2004 10:47:00 AM Dynamic Day 9 (After Restart) 3/10/2004 10:59:00 AM All Day Dynamic Day 9 Part 1 3/10/2004 11:07:00 AM All Day Dynamic Day 9 Part 2 3/10/2004 3:52:00 PM 3/11/2004 9:07:00 AM End of test

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136 Table A-5. Schedule of testing and data collection for test slab 2G Order of Testing Date Time Collected Load # HVS Passes / Remarks Curing Strain 3/30/2004 11:12:06 AM 12 kips 0 Initial Static 3/30/2004 4:35:19 PM Initial Dynamic 3/30/2004 4:41:40 PM Statics Day 1 3/31/2004 10:10:12 AM Dynamic Day 1 3/31/2004 10:15:41 AM Dynamic Day 2 4/1/2004 10:14:35 AM Statics Day 2 4/1/2004 10:33:11 AM 10091 Continuous Day 2 4/1/2004 10:58:40 AM Statics Day 3 4/2/2004 10:55:39 AM 20000 Continuous Day 3 4/2/2004 11:01:50 AM Dynamic Day 3 4/2/2004 8:17:09 AM Dynamic Day 4 4/3/2004 8:46:53 AM 30316 Continuous Day 4 4/3/2004 10:53:30 AM Continuous Day 5 4/4/2004 5:39:56 PM Statics Day 5 4/4/2004 5:23:20 PM 40950 Dynamic Day 6 4/5/2004 8:33:06 AM Cracks noticed on slab early in the morning Statics Day 6 4/5/2004 10:57:51 AM 50933 Continuous Day 6 4/5/2004 12:34:54 PM Dynamic Day 7 4/6/2004 8:58:54 AM Cracks extend towards the middle of the slab Statics Day 7 4/6/2004 10:49:41 AM 60000 Continuous Day 7 4/6/2004 11:03:51 AM Dynamic Day 8 4/7/2004 8:21:53 AM Very extensive cracks towards the middle Statics Day 8 4/7/2004 10:12:07 AM 70000 Continuous Day 8 4/7/2004 10:16:36 AM Dynamic Day 9 4/8/2004 8:47:19 AM Statics Day 9 4/8/2004 10:47:29 AM 80000 Continuous Day 9 4/8/2004 11:13:19 AM End of test

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137 APPENDIX B FWD DATA Figure B-1. FWD test at center of slab 2C

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138 Figure B-2. Test at center of slab 1G

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139 Figure B-3. FWD test at joint 1G-1F

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140 Figure B-4. FWD test at free edge-1G

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141 Figure B-5. FWD test at a confined edge

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142 LIST OF REFERENCES 1. Florida Department of Transportation (FDOT). Standard specification for Road and bridge Construction. Florida Department of Transportation, Maps and Publications, Tallahassee, FL, 2004 2. Roesler, J.R., J. Harvey, D. Hung, L.D. Plessis, and D.Bush. Evaluation of LongerLife concrete Pavement for California Using Accelerated Pavement Testing. Accelerated Pavement Testi ng International Conference, Reno Nevada October 1999 3. California Department of Transportation,CalTrans. Slab Replacement Guidelines. California Department of Transportation, Publication Distribution Unit, 1900 Royal Oaks Drive, Sacramento, CA, 2004 4. Westergaard, H. M., Analysis of Stresses in Concrete Roads. Proceedings of Highway Research Board, Vol. 5, 1926, pp 90-112 5. Westergaard, H. M., Analyt ical tools for Judging Results of Structural Tests of Concrete Pavements. Public Roads, Vol. 14, No. 10, 1933, pp 185-188 6. Westergaard, H. M., New Formula for Stress es in Concrete Pavement of Airfields. American Society of Civil Engineers, ASCE, Vol. 113, 1947, pp 425-444 7. Ioannides, A. M., M. R. Thompson, and E. J. Barenberg. Westergaard Solutions Reconsidered. Transportation Research Record, No. 1043, National Research Council, Washington D.C., 1985, pp 13-23 8. Foppl, A. Vorlesungen uber Technische Mechanik, Vol. 3, 4th ed., B. B. Teubner Leipzig, Germany ,1909, pp. 228 9. Hetenyi, M. Beams of Elastic Foundations, Th e University of Mi chigan Press, Ann Arbor, Michigan, 1946 10. Hetenyi, M. A General Solution for th e Bending Beams on an Elastic Foundation of Arbitrary Continuity. Journal of Applied Physics, Vol. 21, 1950, pp 55-58 11. Pasternak, P. L. On a New Method of An alysis of an Elastic Foundation by Means of Two Foundation Constants. Gps. Izd. Lit.po. Strait I Arkh, 1954

PAGE 159

143 12. Venckovskii, B. K. Bending of Annular and Circular Plates on a Generalized Foundation under the Combined Action of Late ral and Radial Forces. Raschety na Prochnost, Collection of Papers, Vol. 3, Mashgiz, Moscow, USSR, 1958 13. Reissner, E. A Note on De flections of Plates on a Viscoelastic Foundation. Journal of Applied Mechanics, ASME, Vol. 80, 1958, pp 144-145 14. Horvath, J. S. New Subgrade Model Applied to Mat Foundations. Journal of Geotechnical Engineering, Vol. 109, No 12, 1983, pp 1567-1587 15. Horvath, J. S. Subgrade Models for Soil-Structure Interaction Analysis Proc., Foundation Engineering: Current Principles and Practice, Evanston, Illinois, June 1989 16. Horvath, J. S. Beam-Column-Analogy Model for Soil-Structure Interaction Analysis. Journal of Geotechnical Engineering, Vol. 119, No. 2, 1992, pp 358-364 17. Westergaard, H. M. Theory of Concrete Pavement Design. Proceedings of Highway Research Board, Vol. 7, Part 1, 1927, pp 175-181 18. Westergaard, H. M. Stresses in Concrete Runways of Airports. Proceedings of Highway Research Board, Vol. 19, 1939, pp 197-202 19. Westergaard, H. M. Stresses Concentrat ion in Plates Loaded Over Small Area. Transactions, ASCE, Vol. 108, 1943, pp 831-856 20. Pickett, G., and G. K. Ray. Influe nce Charts for Concrete Pavements, Transactions, ASCE, Vol. 116, 1951 21. Hogg, A., and A. Hall. Equilibrium of a Thin Plate Symmetrically Loaded, Resting on an Elastic Subgrade of Infinite Depth. Philosophical Magazine., Series 7,Vol 25, 1938, pp. 576-582 22. Reissner, E. Effect of Transverse Shear Deformation on Elastic Plates. Journal of Applied Mechanics, Vol. 12, 1945, pp 69-77 23. Reissner, E. On a Variational Theorem in Elasticity. Journal of Mechanics and Physics, July 1950, pp 90-95 24. Hu, H. C. Variational Principal and It s Application in Elastics. Science Press, China, 1981, pp 465-477 25. Mindlin, R. D. Influence of Rotatory Inertia and Shear on Flexural Motion of Isotropic Plates. Journal of Applied Mechanics, Vol. 23, 1951, pp 31-38 26. Shi, X. P., S. A. Tan, and T. F. Fwa. Rectangular Thick Plate With Free Edges on Pasternak Foundation. Journal of Engineering Mechanics, Vol. 120, No. 5, 1994, pp 971-988

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144 27. Fwa, T. F., X. P. Shi, and S. A. Ta n. Analysis of Concrete Pavements by Rectangular Thick –Plate Model. Journal of Transporta tion Engineering, ASCE, Vol. 122, No. 2, 1996, pp 146-154 28. Hudson, W. R., and H. Matlock. Analysis of Discontinuous Or thotropic Pavement Slabs Subjected to Combined Loads. Highway Research Record, Vol. 131, 1966, pp 1-48 29. Vora, M. R., and H. A. Matlock. A Disc rete-Element Analysis for Anisotropic Skew Plates and Grids Research Report No S6-18, Center for Highway research, University of Texas, Austin, 1970 30. Huang, Y. H. Pavement Analysis and Design, 1st ed., Prentice Hall, Upper Saddle River, NJ, 1993 31. Tabatabaie, A. M. and E. J. Barenberg. Finite Element Analysis of Jointed of Cracked Concrete Pavements. Transportation Research Record, No. 671, TRB, National Research Council, Washington D.C. 1978, pp. 11-17 32. Tayabji, S. P., and B. E. Colley. Analysis of Jointed Concrete Pavements. Federal Highway Administration, National T echnical Information Service, 1981. 33. Chou, Y. T., Structural Analysis Com puter Programs for Rapid Multicomponent Pavement Structures with Discon tinuities-WESLIQUID and WESLAYER. Technical Report 1, 2, and 3, US Army Engineering Waterways Experiment Station, Vicksburg, MI, May 1981. 34. Tia, M., C. L. Wu, B. E. Ruth, D. Bloo mquist, and B. Choubane. Field Evaluation of Rigid Pavements for the Developmen t of A Rigid Pavement Design SystemPhase1V. Final Report, State Projec t# 99700-7359-010, Florida Department of Transportation, August 1989. 35. Tia, M., M. Armaghani, C.-L. Wu, S. Lei, and K. L. Toye. FEACONS 111 Computer program for an Analysis of Jointed Concrete Pavements. Transportation Research Record, No 1136, National Research Council, Washington D.C., 1987, pp12-22 36. Kumara, W. M. Tia, C.-L. Wu, B. C houbane. Evaluation of Applicability of Ultrathin Whitetopping in Florida. Transportation Research Record, No 1823, National Research Council, Washington D.C.,2002, pp39-46 37. Armaghani, J. M., Factors Affecting Performance of Concrete Pavements.Proc., 5th International Conference on Copncre te Pavement Design and Rehabilitation, Purdue University, West Lafayette, IN, April 1993. 38. Armaghani, J. M, Comprehensive Analysis of Concrete Pavement Response to Temperature and Load Effects. Ph.D. Di ssertation, University of Florida, Gainesville, FL, 1987

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145 39. Armaghani, J. M.,T. J. Larsen, and L. L. Smith. Design-Related Distress in Concrete Pavements, Florida’s Interstate 75. Concrete International, Design and Construction, ACI, Vol.10, No 8, Aug. 1988, pp 43-49 40. Okamoto, P. A., C. L. Wu, S. M. Tarr, and L. W. Cole. Early Opening of PCC Pavements to Traffic”,Proc., 5th International Conference on Copncrete Pavement Design and Rehabilitation, Purdue University, West Lafayette, IN, April 1993, 41. Okamoto, P.A., P. J. Nussbaum, K. D. Smith T. P.Darter, T. P. Wilson, C. L. Wu, and S. D. Tayabji. Guidelines for Ti ming Contraction Joint Sawing and Earlist Loading for Concrete Pavements. FHWA-RD-91-079,080, Federal Highway Administration Washington D.C., 1991 42. Yu1,H.T., J. Mallela, and M. I. Darter. Long-Term Performance of Fast-Track FullDepth Repairs. Presented at the Transportation Research Board 81st Annual Meeting, Washington D.C., 2002 43. Metcalf, J. B, Application of FullScale Accelerated Pavement Testing. Transportation Research Record, NCHRP Synthesis of Highway Practice 235, National Research Council, Washington D.C.,1996 44. COETZEE, N. F., W. Nokes, C. Monismith, J. Metcalf, and J. Mahoney. FullScale/Accelerated Pavement Testing: Current Status and Future Directions. Presented at the Transportation Research Board 82nd Annual Meeting, Washington D.C., 2003 (21) 45. Harvey, J., L.D. Plessis, and J. Roesler. Accelerated Pavement Testing on Concrete Pavements: A Review of Some Lessons Learned. Proc., 2nd International Conference on Accelerated Pavement Testing, Minneapolis, Minnesota September 2004 46. Plessis, L. D., D. Bush, F. Jooste, D. Hung, C. Scheffy, J. Roesler, L. Popescu, and J. Harvey. HVS Test Results on Fast -Setting Hydraulic Cement Concrete Palmadale, California Test Sections, S outh Tangent, Draft Report, California Department of Transportation, July 2003. 47. Rao, S., and J. Roesler. Cumulative Fatigue Damage Analysis of Concrete Pavement Using Accelerated Pavement Testing Results. Proc., 2nd International Conference on Accelerated Pavement Testing, Minneapolis, MN September 2004 48. Blab, R., L. G. Wiman, and J. Litzka. Heavy Vehicle Simulator Experiment on A Semi-Rigid Pavement Structure Of A Motorway. Proc., 2nd International Conference on Accelerated Pavement Testing, Minneapolis, MN September, 2004 49. Choubane, B., and M. Tia. Nonlinear Te mperature Gradient effect on Maximum Wraping Stresses in Rigid Pavements. Transportation Research Record, No. 1370, National Research Council, Washington D. C. 1992, pp. 11-19

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146 50. Choubane, B., and M. Tia. Analysis and Ve rification of Thermal Gradient Effect on Concrete Pavement. Journal of Transportation Engineering, Vol. 121, No. 1, 1995, pp. 75-81 51. Zang, J., T.F. Fwa, K. H. Tan and X. P. Shi. Model for Nonlinear Thermal Effect on Pavement Wraping Stresses. Journal of Transportation Engineering, Vol. 129, No. 6, 2003, pp. 695-702. 52. System 6000Model 6100 Sc anner, Instruction Manual,” Vishay MicroMeasurements, Raleigh NC, 2002 53. Sansalone, M.J. and W. B. Streett. Imp act Echo, Nondestructive Evaluation of Concrete and Masonry, Bullbrier Press, Ithaca, NY 1997 54. ACI 318-02: Building Code Requirements for Structural Concrete and Commentary. American Concrete Institute, Detroit, MI., 2002 55. Davids, W.G., Z. M. Wang, G. Turkiyya h, J. Mahoney, and D. Bush.3D Finite Element Analysis of Jointed Plain C oncrete Pavement with EverFE2.2. Transportation Research Record, No 1853, National Research Council, Washington D.C., 2003, pp. 92-99 56. William, G.W. and S. N. Shoukry, 3D Fi nite Element Analysis of TemperatureInducedStresses in Dowel Jointed Concrete Pavements. International Journal of Geomechanics, Vol 1, No 3, 2001, pp. 291-308

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147 BIOGRAPHICAL SKETCH Wasantha Kumara was born on January 20, 1971 in Sri Lanka. He is the son of Mrs. D. Mampearachchi and the (late) Mr. D. Mampearachchi. He graduated with a Bachelor of Science degree from the Departme nt of Civil Engineering of the University of Moratuwa, Sri Lanka in 1997. He was awarde d best student in Building and Structural Engineering at the University of Moratuwa in 1997. After graduation, he was recruited as an assistant lecturer for the Department of Civil Engineering at the University of Moratuwa. He enrolled in the master’s prog ram at the University of South Florida in August 1999. He earned a graduate research as sistantship to complete his studies. He married Lakmini Wadanambi on December 21, 2000. He graduated with a Master of Science degree in Civil Engineering in Spri ng 2001.He was awarded the Georgia Brosch memorial scholarship for outstanding scholar ship and service by the Center for Urban Transportation Research at the University of South Florida. He was selected Student of the Year by the American Society of Highway Engineers at the E ngineers Week Banquet held in St. Petersburg in 2001. He enrolled in the Ph.D. program in the Department of Civil and Coastal Engineering at the University of Florida (U F) in August, 2001 to continue his studies on pavement materials. He earned a graduate rese arch assistantship to complete his studies at UF. The most precious thing of his life, the birth of his daughter Sahanya, happened on April 12, 2004. After graduation He will resu me his work at University of Moratuwa, Sri Lanka.