• TABLE OF CONTENTS
HIDE
 Title Page
 Acknowledgement
 Table of Contents
 List of Tables
 List of Figures
 Abstract
 Introduction
 Literature review
 Equipment and facilities
 Effect of enclosed concrete test...
 Materials and plate testing...
 Procedures
 Pavement response at ambient...
 Low-temperature pavement respo...
 Response prediction at low...
 Conclusions and recommendation...
 Appendix
 Reference
 Biographical sketch
 Copyright






Title: Low-temperature response of asphalt concrete pavements
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00089974/00001
 Material Information
Title: Low-temperature response of asphalt concrete pavements
Physical Description: Book
Language: English
Creator: Roque, Reynaldo
Publisher: Reynaldo Roque
Publication Date: 1986
 Record Information
Bibliographic ID: UF00089974
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000935236
oclc - 16396428

Table of Contents
    Title Page
        Page i
    Acknowledgement
        Page ii
    Table of Contents
        Page iii
        Page iv
        Page v
    List of Tables
        Page vi
        Page vii
        Page viii
        Page ix
    List of Figures
        Page x
        Page xi
        Page xii
        Page xiii
        Page xiv
        Page xv
    Abstract
        Page xvi
        Page xvii
    Introduction
        Page 1
        Page 2
        Page 3
        Page 4
    Literature review
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
        Page 40
        Page 41
    Equipment and facilities
        Page 42
        Page 43
        Page 44
        Page 45
        Page 46
        Page 47
        Page 48
        Page 49
        Page 50
        Page 51
        Page 52
        Page 53
        Page 54
        Page 55
        Page 56
        Page 57
        Page 58
        Page 59
        Page 60
        Page 61
        Page 62
        Page 63
        Page 64
        Page 65
        Page 66
    Effect of enclosed concrete test pit on pavement response
        Page 67
        Page 68
        Page 69
        Page 70
        Page 71
        Page 72
        Page 73
        Page 74
        Page 75
        Page 76
        Page 77
        Page 78
        Page 79
        Page 80
        Page 81
        Page 82
        Page 83
        Page 84
        Page 85
        Page 86
        Page 87
        Page 88
        Page 89
        Page 90
        Page 91
        Page 92
        Page 93
        Page 94
        Page 95
        Page 96
        Page 97
        Page 98
        Page 99
        Page 101
        Page 102
        Page 103
        Page 104
        Page 105
        Page 106
        Page 107
        Page 108
        Page 109
        Page 110
        Page 111
        Page 112
    Materials and plate testing procedures
        Page 113
        Page 114
        Page 115
        Page 116
        Page 117
        Page 118
        Page 119
        Page 120
        Page 121
        Page 122
        Page 123
        Page 124
        Page 125
        Page 126
        Page 127
        Page 128
        Page 129
        Page 130
        Page 131
        Page 132
        Page 133
        Page 134
        Page 135
        Page 136
        Page 137
        Page 138
        Page 139
        Page 140
        Page 141
        Page 142
        Page 143
        Page 144
        Page 145
        Page 146
        Page 147
        Page 148
        Page 149
    Procedures
        Page 150
        Page 151
        Page 152
        Page 153
        Page 154
        Page 155
        Page 156
    Pavement response at ambient temperatures
        Page 157
        Page 158
        Page 159
        Page 160
        Page 161
        Page 162
        Page 163
        Page 164
        Page 165
        Page 166
        Page 167
        Page 168
        Page 169
        Page 170
        Page 171
        Page 172
        Page 173
        Page 174
        Page 175
        Page 176
        Page 177
        Page 178
        Page 179
        Page 180
        Page 181
        Page 182
        Page 183
        Page 184
        Page 185
        Page 186
        Page 187
        Page 188
        Page 189
        Page 190
        Page 191
        Page 192
        Page 193
        Page 194
        Page 195
        Page 196
        Page 197
        Page 198
        Page 199
        Page 200
        Page 201
        Page 202
        Page 203
        Page 204
        Page 205
        Page 206
        Page 207
        Page 208
        Page 209
        Page 210
        Page 211
        Page 212
        Page 213
        Page 214
        Page 215
    Low-temperature pavement response
        Page 216
        Page 217
        Page 218
        Page 219
        Page 220
        Page 221
        Page 222
        Page 223
        Page 224
        Page 225
        Page 226
        Page 227
        Page 228
        Page 229
        Page 230
        Page 231
        Page 232
        Page 233
        Page 234
        Page 235
        Page 236
        Page 237
        Page 238
        Page 239
        Page 240
        Page 241
        Page 242
        Page 243
        Page 244
        Page 245
        Page 246
        Page 247
        Page 248
        Page 249
        Page 250
        Page 251
        Page 252
        Page 253
        Page 254
        Page 255
        Page 256
        Page 257
        Page 258
        Page 259
        Page 260
        Page 261
        Page 262
        Page 263
        Page 264
        Page 265
        Page 266
        Page 267
        Page 268
        Page 269
        Page 270
        Page 271
        Page 272
        Page 273
        Page 274
        Page 275
        Page 276
        Page 277
        Page 278
        Page 279
        Page 280
        Page 281
        Page 282
        Page 283
        Page 284
        Page 285
        Page 286
        Page 287
        Page 288
        Page 289
        Page 290
        Page 291
        Page 292
        Page 293
        Page 294
        Page 295
        Page 296
        Page 297
        Page 298
        Page 299
        Page 300
        Page 301
        Page 302
        Page 303
        Page 304
        Page 305
        Page 306
        Page 307
        Page 308
        Page 309
        Page 310
        Page 311
        Page 312
        Page 313
        Page 314
        Page 315
        Page 316
        Page 317
        Page 318
        Page 319
        Page 320
        Page 321
        Page 322
        Page 323
        Page 324
        Page 325
        Page 326
        Page 327
        Page 328
        Page 329
        Page 330
    Response prediction at low temperatures
        Page 331
        Page 332
        Page 333
        Page 334
        Page 335
        Page 336
        Page 337
        Page 338
        Page 339
        Page 340
        Page 341
        Page 342
        Page 343
        Page 344
        Page 345
        Page 346
        Page 347
        Page 348
        Page 349
    Conclusions and recommendations
        Page 350
        Page 351
        Page 352
        Page 353
        Page 354
        Page 355
        Page 356
        Page 357
        Page 358
    Appendix
        Page 359
        Page 360
        Page 361
        Page 362
        Page 363
        Page 364
        Page 365
        Page 366
        Page 367
        Page 368
        Page 369
        Page 370
        Page 371
        Page 372
        Page 373
        Page 374
        Page 375
        Page 376
        Page 377
        Page 378
        Page 379
        Page 380
        Page 381
        Page 382
        Page 383
        Page 384
        Page 385
        Page 386
        Page 387
        Page 388
        Page 389
        Page 390
        Page 391
        Page 392
        Page 393
        Page 394
        Page 395
        Page 396
        Page 397
        Page 398
        Page 399
        Page 400
        Page 401
        Page 402
        Page 403
        Page 404
        Page 405
        Page 406
        Page 407
        Page 408
        Page 409
        Page 410
        Page 411
        Page 412
        Page 413
        Page 414
        Page 415
        Page 416
        Page 417
        Page 418
        Page 419
        Page 420
        Page 421
        Page 422
        Page 423
        Page 424
        Page 425
        Page 426
        Page 427
        Page 428
        Page 429
        Page 430
        Page 431
        Page 432
    Reference
        Page 433
        Page 434
        Page 435
        Page 436
        Page 437
        Page 438
        Page 439
        Page 440
        Page 441
        Page 442
    Biographical sketch
        Page 443
        Page 444
        Page 445
        Page 446
    Copyright
        Copyright
Full Text










LOW-TEMPERATURE RESPONSE
OF ASPHALT CONCRETE PAVEMENTS






By

REYNALDO ROQUE


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY


1986













ACKNOWLEDGMENTS


I would like to express my gratitude to Dr. Byron E. Ruth, Chairman

of my Graduate Supervisory Committee, for his guidance, encouragement,

and friendship. I would also like to thank Dr. F. C. Townsend,

Dr. J. L. Davidson, Professor W. H. Zimpfer, Dr. M. C. McVay,

Dr. D. L. Smith, and Dr. J. L. Eades for serving on my Graduate

Supervisory Committee. I consider myself fortunate to have had such a

distinguished committee.

A very special thanks goes to the Florida Department of

Transportation (FDOT) for providing the financial support, testing

facilities, materials, and personnel that made this research possible.

I would also like to thank the many individuals at the Bureau of

Materials and Research of the FDOT who contributed to this research

project by giving so generously of their time. I especially want to

thank the personnel in the Pavement Performance Division, Bituminous

Materials and Research Section, the Pavement Evaluation Section and the

Soil Materials and Research Section for their help and consideration.

I would also like to thank Candace Leggett for her

conscientiousness and diligence in typing this dissertation.

Finally, I would like to thank my wife Maria for encouraging me to

return to school, and for her encouragement and patience throughout my

Ph.D. program.














TABLE OF CONTENTS

Pane

ACKNOWLEDGMENTS................................................ ii

LIST OF TABLES............................................... ..... vi

LIST OF FIGURES ..................... ............................ x

ABSTRACT......................... .. ........... ... .... ....... xvi

CHAPTERS

I INTRODUCTION... .............. .............................. 1

II LITERATURE REVIEW .................................. ...... 5

2.1 Introduction........................................... 5
2.2 Distress In Asphalt Concrete Pavements.................. 6
2.2.1 Modes of Distress.................................. 6
2.2.2 Cracking Mechanisms................................. 9
2.3 Properties of Asphalt Cement and Asphalt Concrete
As Related to Low-Temperature Pavement Response
and Cracking........................................... 15
2.3.1 Asphalt Cement Properties.......................... 16
2.3.2 Asphalt Mixture Properties.......................... 20
2.4 Properties of Foundation Materials...................... 30
2.5 Prediction of Thermal- and Load-Induced Stresses,
Strains, and Failure In Asphalt Concrete Pavements...... 35

III EQUIPMENT AND FACILITIES..................................... 42

3.1 Description of Test Pit Facility....................... 42
3.2 System for Hot Mix Asphalt Distribution................. 43
3.3 Pavement Cooling System................................ 43
3.4 Measurement System for Pavement Response................ 47
3.4.1 Measuring Instruments............................ 49
3.4.2 Data Acquisition System............................ 55
3.5 Loading Systems: Rigid Plate Load vs. Flexible
Dual Wheels............................................. 62

IV EFFECT OF ENCLOSED CONCRETE TEST PIT ON PAVEMENT RESPONSE.... 67

4.1 Introduction........................................... 67
4.2 Preliminary Analysis .................................... 67
4.3 Effect of Test Pit Constraints.......................... 72
4.3.1 Analytical Model ..................... ............. 72














4.3.2 Effect of Constraints on Subgrade Response.......... 73
4.3.3 Effect of Constraints on Limerock Base Response..... 77
4.3.4 Effect of Constraints on Three-Layer System......... 85
4.4 Methodoloqy to Account for the Effect of Test Pit
Constraints on Pavement Response Prediction............. 91
4.4.1 Rigid Plate Loading on the Subgrade................. 91
4.4.2 Rigid Plate Loading on the Reinforcing Base
Layer............ ........ ................ ......... 92
4.4.3 Predicting Pavement System Response in the
Test Pit.................... .................. .. 104

V MATERIALS AND PLATE TESTING PROCEDURES....................... 113

5.1 Introduction............... ............................. 113
5.2 Laboratory Tests...................................... 113
5.2.1 Fairbanks Sand Subgrade............................. 113
5.2.2 Crushed Limerock Base............................... 113
5.2.3 Asphalt Cement and Asphalt Concrete................. 115
5.3 Material Placement and Compaction...................... 122
5.4 Material Properties In Situ ........................... 126
5.4.1 Plate Load Test Procedures......................... 126
5.4.2 Plate Tests Immediately After Placement............ 129
5.4.3 Plate Tests After Pavement Removal .................. 141

VI PROCEDURES.................................................. 150

6.1 Dynamic Plate Load Tests at Ambient Temperatures........ 150
6.2 Low-Temperature Pavement Response Tests.................. 151
6.2.1 Introduction...................................... 151
6.2.2 Pavement Cooling and Initial Dynamic Load Tests..... 153
6.2.3 Creep Test Procedures............................... 155

VII PAVEMENT RESPONSE AT AMBIENT TEMPERATURES.................... 157

7.1 Initial Plate Load Tests at Fast Loading Rate........... 159
7.1.1 Dynamic Load Test Results........................... 159
7.1.2 Elastic Layer Simulation and Evaluation of
Results................. .... .............. .. ........ 163
7.2 Initial Plate Load Tests at Slow Loading Rate........... 170
7.2.1 Dynamic Load Test Results........................... 170
7.2.2 Evaluation of Results............................... 170
7.3 Additional Plate Load Tests at Fast Loading Rate........ 182
7.3.1 Dynamic Load Test Results........................... 182
7.3.2 Elastic Layer Simulation and Evaluation of
Results................................... ... .. .... 186
7.4 Summary ... ......................... .................... 213














VIII LOW-TEMPERATURE PAVEMENT RESPONSE........................... 216

8.1 Preliminary Tests With the Rigid Plate.................. 216
8.2 Reinstrumentation for Tests With the Dual Wheels......... 221
8.3 Results of Tests With the Dual Wheel Loading System..... 221
8.3.1 Introduction............................ ...... 221
8.3.2 Pavement Response During Cooling.................. 223
8.3.3 Dynamic Load and Creep Response at Different
Temperatures........................................ 255
8.3.4 Combined Effect of Thermal and Load Response........ 320

IX RESPONSE PREDICTION AT LOW TEMPERATURES.................... 331

9.1 Dynamic Load Response......................... ........ 331
9.2 Thermal Resoonse...................................... 347
9.3 Creep Response........................................ 348

X CONCLUSIONS AND RECOM"ENOATIONS .............................. 350
10.1 Conclusions........................ ............... 350
10.1.1 Pavement Testinq and Evaluation Method............. 350
10.1.2 Thermal and Load Response of Asphalt Concrete
Pavements ....... ............... ............ ...... 356
10.2 Recommendations...................................... 356

APPENDICES

A RELATIONSHIPS BETWEEN ASPHALT CONCRETE PROPERTIES
AND ASPHALT CEMENT PROPERTIES............................... 359

B PAVEMENT TEMPERATURES DURING COOLING ........................ 361

C MEASURED THERMAL STRAINS DURING COOLING..................... 368

D DYNAMIC LOAD RESPONSE MEASUREMENTS .......................... 375

E CREEP TEST DATA............................................ 396

REFERENCES....................... ... ...... .......... .............. 433

BIOGRAPHICAL SKETCH.............................................. 443














LIST OF TABLES


Table Page

2.1 Primary Types and Causes of Distress In Asphalt
Concrete Pavements......................................... 7
2.2 Modes, Manifestations, and Mechanisms of Types of
Distress............ ..... .................... ............... 8

4.1 Sand Subgrade Modulus for Different Layer Depths
and Poisson's Ratio.................................... 75
4.2 Effect of Concrete Floor on Surface Deflections
for Different Base Stiffnesses .............................. 79
4.3 Predicted Deflections Using AXSYM........................... 83
4.4 Tabulated Deflection Basins to Show Effect of
Test Pit Floor on Pavements of Different Stiffness........... 87
4.5 Effect of Test Pit Walls on Surface Deflections
for Pavements of Different Stiffness......................... 90
4.6 Computer Runs to Determine Poisson's Ratio Effect............ 99
4.7 Measured and Predicted Surface Strains....................... 109

5.1 Laboratory Test Results: Fairbanks Sand.................... 114
5.2 Laboratory Test Results: Ocala Formation Limerock........... 116
5.3 Source of Materials and Job Mix Formula for Asphalt
Concrete.................................................. 117
5.4 Test Pit Asphalt Concrete Properties......................... 118
5.5 Rheology and Penetration of Asphalt Recovered From
Test Pit During Initial Placement: September, 1982.......... 120
5.6 Rheology and Penetration of Asphalt Recovered From
Test Pit After All Testing: September, 1985..................... 121
5.7 Load Increments Used for Plate Load Tests: Fairbanks
Sand Subgrade.............................................. 127
5.8 Load Increments Used for Plate Load Tests:
Limerock Base............................ .................... 128
5.9 Modulus Values Immediately After Placement:
Fairbanks Sand Suhgrade.................................... 134
5.10 Modulus Values Immediately After Placement:
Limerock Base.............................................. 139
5.11 Modulus Values Without Accounting for Test Pit
Constraints............................................ 142
5.12 Modulus Values After Pavement Removal:
Fairbanks Sand Subqrade.................................... 145
5.13 Modulus Values After Pavement Removal: Limerock Base........ 146

6.1 Summary of Order of Testing................................. 152

7.1 Summary of Dynamic Plate Load Tests at Ambient
Temperatures................................................. 158








Table Page

7.2 Surface Deflections: Fast Loadina Rate..................... 162
7.3 Surface Strains: Fast Loading Rate......................... 162
7.4 Surface Deflections: Slow Loading Rate...................... 173
7.5 Surface Strains: Slow Loading Rate.......................... 173
7.6 Measured Surface Deflections at 20.6 C (69 F):
Center Plate Loading Position............................... 184
7.7 Measured Surface Strains at 20.6 C (69 F):
Center Plate Loading Position .............................. 184
7.8 Measured Surface Deflection at 25.6 C (78 F):
South Plate Loading Position................................ 185
7.9 Measured Strains at 25.6 (78 F):
South Plate Loading Position................................ 185

8.1 Summary of Average Pavement Temperatures During Testing...... 259

B.1 Pavement Temperatures During Cooling: Test Position 1....... 362
B.2 Pavement Temperatures During Cooling: Test Position 2....... 364
B.3 Pavement Temperatures During Cooling: Test Position 3...... 366

C.1 Measured Thermal Strains During Cooling: Test Position 1.... 369
C.2 Measured Thermal Strains During Cooling: Test Position 2.... 371
C.3 Measured Thermal Strains During Cooling: Test Position 3.... 373

D.1 Measured Deflections, Test Position 1, 0 C (32 F)........... 376
D.2 Measured Strains, Test Position 1, 0 C (32 F)................ 377
0.3 Measured Deflections, Test Position 1, 6.7 C (44 F).......... 378
0.4 Measured Strains, Test Position 1, 6.7 C (44 F).............. 379
D.5 Measured Deflections, Test Position 1, 13.3 C (56 F)......... 380
D.6 Measured Strains, Test Position 1, 13.3 C (56 F)............. 381
D.7 Measured Deflections, Test Position 1, 0 C (32 F),
Repeat Test................................................. 382
D.8 Measured Strains, Test Position 1, 0 C (32 F),
Repeat Test................................... ....... 383
D.9 Measured Deflections, Test Position 2, 0 C (32 F)............ 384
D.10 Measured Strains, Test Position 2, 0 C (32 F).............. 385
D.11 Measured Deflections, Test Position 2, 6.7 C (44 F).......... 386
0.12 Measured Strains, Test Position 2, 6.7 C (44 F).............. 387
0.13 Measured Deflections, Test Position 2, 13.3 C (56 F)......... 388
D.14 Measured Strains, Test Position 2, 13.3 C (56 F) ........... 389
D.15 Measured Deflections, Test Position 3, 0 C (32 F)............ 390
0.16 Measured Strains, Test Position 3, 0 C (32 F)................ 391
D.17 Measured Deflections, Test Position 3, 6.7 C (44 F).......... 392
D.18 Measured Strains, Test Position 3, 6.7 C (44 F).............. 393
D.19 Measured Deflections, Test Position 3, 13.3 C (56 F)......... 394
D.20 Measured Strains, Test Position 3, 13.3 C (56 F)... .... .... 395

E.1 Measured Permanent Deflections For Different Times of
10,000-lb. Static Load Application, Position Number 1,
13.3 C (56 F).............................. ....... ........... 397
E.2 Measured Creep Strains For Different Times of 10,000-1b.
Static Load Application, Position Number 1, 13.3 C (56 F).... 398








Table Page

E.3 Measured Dynamic Deflections at 10,000 lbs. After
Different Times of Static Load Application, Position
Number 1, 13.3 C (56 F)...................................... 399
E.4 Measured Dynamic Strains at 10,000 Ibs. After Different
Times of Static Load Application, Position Number 1,
13.3 C (56 F).............................................. 400
E.5 Measured Permanent Deflections For Different Times of
10,000-lb. Static Load Application, Position Number 1,
6.7 C (44 F) ............... ......................... ...... .. 401
E.6 Measured Creep Strains For Different Times of 10,000-lb.
Static Load Application, Position Number 1, 6.7 C (44 F)..... 402
E.7 Measured Dynamic Deflections at 10,000 Ibs. After
Different Times of Static Load Application, Position
Number 1, 6.7 C (44 F)....................................... 403
E.8 Measured Dynamic Strains at 10,000 lbs. After Different
Times of Static Load Application, Position Number 1,
6.7 C (44 F)........... ................ ........ .... 404
E.9 Measured Permanent Deflections For Different Times of
10,000-lb. Static Load Application, Position Number 1,
0 C (32 F)..................... .............. 405
E.10 Measured Creep Strains For Different Times of 10,000-lb.
Static Load Application, Position Number 1, 0 C (32 F)....... 406
E.11 Measured Dynamic Deflections at 10,000 lbs. After
Different Times of Static Load Application, Position
Number 1, 0 C (32 F)........................................ 407
E.12 Measured Dynamic Strains at 10,000 lbs. After Different
Times of Static Load Apolication, Positon Number 1,
0 C (32 F) .............................. .... ... .... ...... 408
E.13 Measured Permanent Deflections For Different Times of
10,000-lb. Static Load Application, Position Number 2,
6.7 C (44 F)......................... .... .................... ...... 409
E.14 Measured Creep Strains For Different Times of 10,000-lb.
Static Load Application, Position Number 2, 6.7 C (44 F)..... 410
E.15 Measured Dynamic Deflections at 10,000 Ibs. After
Different Times of Static Load Application, Position
Number 2, 6.7 C (44 F)............................ .......... 411
E.16 Measured Dynamic Strains at 10,000 Ibs. After Different
Times of Static Load Application, Position Number 2,
6.7 C (44 F) .................. ...... ......... .... .... .... 412
E.17 Measured Permanent Deflections For Different Times of
10,000-lb. Static Load Application, Position Number 2,
0 C (32 F)................................................ 413
E.18 Measured Creep Strains For Different Times of 10,000-lb.
Static Load Application, Position 2, 0 C (32 F).............. 414
E.19 Measured Dynamic Deflections at 10,000 Ibs. After
Different Times of Static Load Application, Position
Number 2, 0 C (32 F)... ........ ........................... 415
E.20 Measured Dynamic Strains at 10,000 Ibs. After Different
Times of Static Load Application, Position Number 2,
0 C (32 F)................................ .... ............. 416


viii








Table Page

E.21 Measured Permanent Deflections For Different Times of
10,000-lb. Static Load Application, Position Number 2,
13.3 C (56 F)................................................ 417
E.22 Measured Creep Strains For Different Times of 10,000-lb.
Static Load Application, Position Number 2, 13.3 C (56 F).... 418
E.23 Measured Dynamic Deflections at 10,000 Ibs. After
Different Times of Static Load Application, Position
Number 2, 13.3 C (56 F)...................................... 419
E.24 Measured Dynamic Strains at 10,000 Ibs. After Different
Times of Static Load Application, Position Number 2,
13.3 C (56 F)................................................ 420
E.25 Measured Permanent Deflections For Different Times of
10,000-lb. Static Load Application, Position Number 3,
0 C (32 F)............................................ ..... 421
E.26 Measured Creep Strains For Different Times of 10,000-lb.
Static Load Application, Position Number 3, 0 C (32 F)....... 422
E.27 Measured Dynamic Deflections at 10,000 Ibs. After
Different Times of Static Load Application, Position
Number 3, 0 C (32 F) .................................... 423
E.28 Measured Dynamic Strains at 10,000 Ibs. After Different
Times of Static Load Application, Position Number 3,
0 C (32 F)................................. .................. 424
E.29 Measured Permanent Deflections For Different Times of
10,000-lb. Static Load Application, Position Number 3,
6.7 C (44 F)............................................ ... 425
E.30 Measured Creep Strains For Different Times of 10,000-lb.
Static Load Application, Position Number 3, 6.7 C (44 F)..... 426
E.31 Measured Dynamic Deflections at 10,000 Ibs. After
Different Times of Static Load Application, Position
Number 3, 6.7 C (44 F)...................................... 427
E.32 Measured Dynamic Strains at 10,000 Ibs. After Different
Times of Static Load Application, Position Number 3,
6.7 C (44 F) .............. .................. ............ 428
E.33 Measured Permanent Deflections For Different Times of
10,000-lb. Static Load Application, Position Number 3,
13.3 C (56 F) ....................................... 429
E.34 Measured Creep Strains For Different Times of 10,000-lb.
Static Load Application, Position Number 3, 13.3 C (56 F).... 430
E.35 Measured Dynamic Deflections at 10,000 Ibs. After
Different Times of Static Load Application, Position
Number 3, 13.3 C (56 F).............................. ...... 431
E.36 Measured Dynamic Strains at 10,000 Ibs. After Different
Times of Static Load Application, Position Number 3,
13.3 C (56 F)............................ ................... 432













LIST OF FIGURES


Figure Page

3.1 Hopper for Asphalt Hot Mix Distribution.................... 44
3.2 Layout of Test Pit Cooling System.......................... 46
3.3 Insulated Test Pit Cover Completely Installed................ 48
3.4 Insulated Test Pit Cover With Panels Removed.............. 48
3.5 LVDT Support System for Plate Loading........................ 50
3.6 LVDT Support System for Dual Wheel Loading................... 51
3.7 LVDT Prepared for Tests at Low Temperatures................. 53
3.8 Test Pit Pavement Completely Instrumented and Ready
for Testing With Dual Wheels
a) Frontal View .......................................... 54
b) Diagonal View........................................... 54
3.9 Two-Inch Strain Gages Mounted on Asphalt Concrete........... 56
3.10 Schematic Diagram of Data Acquisition System................. 59
3.11 Data Acquisition System In Test Pit Facility................. 60
3.12 Typical Deflection Recording on Digital Oscilloscope......... 60
3.13 Typical Deflection Output on X-Y Plotter.................... 61
3.14 Rigid Plate Loading System........... ....................... 63
3.15 Flexible Dual Wheel Loading System......................... 64

4.1 Measured and Predicted Deflection Basins in the Test Pit..... 69
4.2 Effect of Concrete Floor at Different Depths on
Predicted Deflection Basins................................. 71
4.3 Effect of Different Base Layer Stiffness on
Predicted Deflection Basins...... ............................ 80
4.4 Effect of Test Pit Walls on Limerock Base Response.......... 82
4.5 Comparison of AXSYM and Elastic Layer Theory Solutions....... 84
4.6 Three-Layer Systems as Modeled for Analysis.................. 86
4.7 Pavement System Models to Determine Wall Effect.............. 89
4.8 Equivalent Systems Based on Maximum Plate Deflection
on Subgrade.............. ........... ...................... 94
4.9 Comparison of Response of Equivalent Systems Based
on Maximum Plate Deflection on Subgrade .................... 95
4.10 Comparison of Test Pit System and Burmister System........... 97
4.11 Stress Distribution Under Riqid Plate on
Semi-Infinite Mass: 50 psi Average Pressure................. 101
4.12 Measured vs. Predicted Deflection Basins at
18.3 C (65 F).................... ........................... 107

5.1 Viscosity Temperature Relationships for Asphalt
Recovered from the Test Pit................................. 123
5.2 Location of Plate Load Tests: Fairbanks Sand Subgrade....... 130
5.3 Location of Plate Load Tests: Limerock Base................. 130
5.4 Applied Stress vs. Deflection: 12-in. Plate on
Fairbanks Sand Subgrade .................................... 132








Figure Page

5.5 Applied Stress vs. Deflection: 30-in. Plate on
Fairbanks Sand Subgrade.................................... 133
5.6 Applied Stress vs. Deflection: 16-in. Plate on
Limerock Base..............................................136
5.7 Deflections Used to Calculate Limerock Moduli.............. 138
5.8 Location of Plate Load Tests: Fairbanks Sand Subgrade....... 144
5.9 Location of Plate Load Tests: Limerock Base................. 144

7.1 Test Pit Diagram: Elevation ............................... 160
7.2 Test Pit Diagram: Plan...................................... 161
7.3 Measured Deflection Basins: Fast Loading Rate............... 164
7.4 Measured Strain Distributions: Fast Loading Rate............ 165
7.5 Load-Deflection Relationships: Fast Loading Rate............ 166
7.6 Load-Strain Relationships: Fast Loading Rate................ 167
7.7 Test Pit Pavement System as Modeled for Elastic Layer
Analysis ..................................................... 169
7.8 Measured Deflection Basins: Slow Loading Rate............... 171
7.9 Measured Strain Distributions: Slow Loading Rate............ 172
7.10 Load-Deflection Relationships: Slow Loading Rate............ 175
7.11 Load-Strain Relationships: Slow Loading Rate............. 176
7.12 Deflection Basin Comparison for Fast and Slow Loading
Rates: 10,000 Ibs.............. ........... ................ 177
7.13 Deflection Basin Comparison for Fast and Slow Loading
Rates: 7,000 Ibs............................ .............. 178
7.14 Deflection Basin Comparison for Fast and Slow Loading
Rates: 4,000 Ibs............................................ 179
7.15 Deflection Basin Comparison for Fast and Slow Loading
Rates: 1,000 Ibs.............. ............................. 180
7.16 Location of Plate Loading Positions and Strain and
Deflection Measurements ......... ............ .............. 183
7.17 Measured Deflection Basins at 20.6 C (69 F).................. 187
7.18 Measured Deflection Basins at 25.6 C (78 F)............... 188
7.19 Measured Strain Distributions at 20.6 C (69 F).............. 189
7.20 Measured Strain Distributions at 25.6 C (69 F).............. 190
7.21 Deflection Basin Comparison at 10,000 Ibs.................. 191
7.22 Deflection Basin Comparison at 7,000 Ibs..................... 192
7.23 Deflection Basin Comparison at 4,000 Ibs.................... 193
7.24 Deflection Basin Comparison at 1,000 Ibs.................. 194
7.25 Strain Distribution Comparison at 10,000 Ibs................ 195
7.26 Strain Distribution Comparison at 7,000 Ibs................. 196
7.27 Strain Distribution Comparison at 4,000 Ibs................. 197
7.28 Strain Distribution Comparison at 1,000 Ibs................. 198
7.29 Measured vs. Predicted Deflections at 10,000 Ibs.:
E2 = 53,000 psi, 20.6 C (69 F)............................. 201
7.30 Measured vs. Predicted Strains at 10,000 Ibs.:
E2 = 53,000 psi, 20.6 C (69 F).............................. 202
7.31 Measured vs. Predicted Deflections at 10,000 Ibs.:
Eq = 75,000 psi, 20.6 C (69 F)............................. 203
7.32 Measured vs. Predicted Strains at 10,000 Ibs.:
E2 = 75,000 psi, 20.6 C (69 F)........................... 204
7.33 Measured Load-Deflection Relationships at 20.6 C (69 F)...... 209
7.34 Measured Load-Strain Relationships at 20.6 C (69 F).......... 210








Figure Page

7.35 Measured vs. Predicted Deflections at 4,000 Ibs.:
E2 = 40,000 psi, 20.6 C (69 F)............ ................... 211
7.36 Measured vs. Predicted Strains at 4,000 lbs.:
E2 = 40,000 psi, 20.6 C (69 F)............................... 212

8.1 Layout of Strain Gages and Cables in the Test Pit........... 222
8.2 Location of Test Positions in the Test Pit.................. 224
8.3 Thermocouple and Strain Gage Location During Cooling:
Test Position 1.... ....................... .......... 225
8.4 Thermocouple and Strain Gage Location During Cooling:
Test Position 2............................................ 226
8.5 Thermocouple and Strain Gage Location During Cooling:
Test Position 3........................................... ... 227
8.6 Measured Cooling Curves: Test Position 1.................... 228
8.7 Measured Cooling Curves: Test Position 2................... 229
8.8 Measured Cooling Curves: Test Position 3.................... 230
8.9 Change in Temperature Gradient During Cooling............... 232
8.10 Measured Longitudinal Strains vs. Temperature:
Test Position 1.............................. ...... ..... 233
8.11 Measured Longitudinal Strains vs. Temperature:
Test Position 2................ ............... .......... 234
8.12 Measured Longitudinal Strains vs. Temperature:
Test Position 3.................... ................... 235
8.13 Measured Transverse Strains vs. Temperature:
Test Position 1................................... ........... 236
8.14 Measured Transverse Strains vs. Temperature:
Test Position 2........................................... 237
8.15 Measured Transverse Strains vs. Temperature:
Test Position 3........ ............... ...... ......... 238
8.16 Longitudinal Strain Distributions During Cooling:
Test Position 1.............................................. 242
8.17 Longitudinal Strain Distributions During Cooling:
Test Position 2........................................... 243
8.18 Longitudinal Strain Distributions During Cooling:
Test Position 3........... ............................... 244
8.19 Transverse Strain Distributions During Cooling:
Test Position 1............................................. 245
8.20 Transverse Strain Distributions During Cooling:
Test Position 2............................. ....... ..... 246
8.21 Transverse Strain Distributions During Cooling:
Test Position 3............................................. 247
8.22 Comparison of Measured Thermal Strains for Different
Cooling Cycles: Six Feet from South Wall.................... 250
8.23 Comparison of Measured Thermal Strains for Different
Cooling Cycles: 8.33 Feet from South Wall ................. 251
8.24 Longitudinal Strain Distributions During Cooling............ 253
8.25 LVDT and Strain Gage Location During Load Tests:
Test Position 1..................................... ......... 256
8.26 LVDT and Strain Gage Location During Load Tests:
Test Position 2...................................... 257
8.27 LVDT and Strain Gage Location During Load Tests:
Test Position 3.............................................. 258








Figure Page

8.28 Load-Unload Times for Dynamic Loading with Dual Wheels...... 261
8.29 Measured Longitudinal Deflections at 0.0 C (32 F):
Test Position 3............................................. 263
8.30 Measured Longitudinal Strains at 0.0 C (32 F):
Test Position 3............... ........................... 264
8.31 Measured Transverse Deflections at 0.0 C (32 F):
Test Position 3.............................................. 265
8.32 Measured Transverse Strains at 0.0 C (32 F):
Test Position 3............ ........ ... ............... 266
8.33 Measured Longitudinal Deflections at 5.7 C (44 F):
Test Position 3............................................. 267
8.34 Measured Longitudinal Strains at 6.7 C (44 F):
Test Position 3............................................. 268
8.35 Measured Transverse Deflections at 6.7 C (44 F):
Test Position 3............................................. 269
8.36 Measured Transverse Strains at 6.7 C (44 F):
Test Position 3...................................... ........ 270
8.37 Measured Longitudinal Deflections at 13.3 C (56 F):
Test Position 3.................................... ..... 271
8.38 Measured Longitudinal Strains at 13.3 C (56 F):
Test Position 3............................................. 272
8.39 Measured Transverse Deflections at 13.3 C (56 F):
Test Position 3.............................................. 273
8.40 Measured Transverse Strains at 13.3 C (56 F):
Test Position 3.............................................. 274
8.41 Comparison of Measured Longitudinal Deflections at
Different Temperatures: Test Position 1, 10,000 Ibs......... 278
8.42 Comparison of Measured Longitudinal Strains at
Different Temperatures: Test Position 1, 10,000 Ibs......... 279
8.43 Comparison of Measured Transverse Deflections at
Different Temperatures: Test Position 1, 10,000 Ibs......... 280
8.44 Comparison of Measured Transverse Strains at
Different Temperatures: Test Position 1, 10,000 lbs......... 281
8.45 Comparison of Measured Longitudinal Deflections at
Different Temperatures: Test Position 2, 10,000 Ibs......... 282
8.46 Comparison of Measured Longitudinal Strains at
Different Temperatures: Test Position 2, 10,000 Ibs....... 283
8.47 Comparison of Measured Transverse Deflections at
Different Temperatures: Test Position 2, 10,000 Ibs......... 284
8.48 Comparison of Measured Transverse Strains at


Different Temperatures: Test Position 2, 10,000 Ibs.........
8.49 Comparison of Measured Longitudinal Deflections at
Different Temperatures: Test Position 3, 10,000 Ibs.........
8.50 Comparison of Measured Longitudinal Strains at
Different Temperatures: Test Position 3, 10,000 Ibs.........
8.51 Comparison of Measured Transverse Deflections at
Different Temperatures: Test Position 3, 10,000 Ibs.........
8.52 Comparison of Measured Transverse Strains at
Different Temperatures: Test Position 3, 10,000 Ibs.........
8.53 Comparison of Predicted Longitudinal Deflections at
Different Temperatures: 10,000 lbs.........................


286

287

288

289

290


xiii








Figure Page

8.54 Comparison of Predicted Longitudinal Strains at
Different Temperatures: 10,000 Ibs......................... 291
8.55 Permanent Longitudinal Deflections at 0.0 C (32 F):
Test Position 3.............................................. 294
8.56 Longitudinal Creep Strains at 0.0 C (32 F):
Test Position 3.................. ...................... 295
8.57 Permanent Transverse Deflections at 0.0 C (32 F):
Test Position 3........................................ 296
8.58 Transverse Creep Strains at 0.0 C (32 F):
Test Position 3............................................. 297
8.59 Comparison of Permanent Longitudinal Deflections at
Different Temperatures: Test Position 1...................... 300
8.60 Comparison of Longitudinal Creep Strains at Different
Temperatures: Test Position 1.............................. 301
8.61 Comparison of Permanent Longitudinal Deflections at
Different Temperatures: Test Position 2.................... 302
8.62 Comparison of Longitudinal Creep Strains at Different
Temperatures: Test Position 2............................... 303
8.63 Comparison of Permanent Longitudinal Deflections at
Different Temperatures: Test Position 3.................... 304
8.64 Comparison of Longitudinal Creep Strains at Different
Temperatures: Test Position 3............................. 305
8.65 Comparison of Dynamic Load Response Immediately Prior
to Creep Tests for Different Test Positions:
0.0 C (32 F)... ................................. ...... .. 308
8.66 Comparison of Permanent Longitudinal Deflections for
Different Test Positions: 0.0 C (32 F).................... 309
8.67 Comparison of Dynamic Load Response at Different Times:
0.0 C (32 F), Test Position 1............................... 310
8.68 Comparison of Dynamic Load Response at Different Times:
0.0 C (32 F), Test Position 2.............................. 311
8.69 Comparison of Dynamic Load Response at Different Times:
0.0 C (32 F), Test Position 3................................ 312
8.70 Comparison of Dynamic Load Response After Creep Tests
for Different Test Positions: 13.3 C (56 F)................ 317
8.71 Comparison of Dynamic Load Response After Creep Tests
for Different Test Positions: 6.7 C (44 F).................. 318
8.72 Comparison of Dynamic Load Response After Creep Tests
for Different Test Positions: 0.0 C (32 F).................. 319
8.73 Comparison of Load-Deflection Relationships for
Different Test Positions: 0.0 C (32 F)...................... 325
8.74 Comparison of Load-Deflection Relationships for
Different Test Positions: 6.7 C (44 F)...................... 326
8.75 Comparison of Load-Deflection Relationships for
Different Test Positions: 13.3 C (56 F)................... 327

9.1 Comparison of Measured and Predicted Longitudinal
Deflection Basins at 0.0 C (32 F)............................ 335
9.2 Comparison of Measured and Predicted Longitudinal
Strain Distributions at 0.0 C (32 F)........................ 336
9.3 Comparison of Measured and Predicted Transverse
Deflection Basins at 0.0 C (32 F)............................ 337


xiv








Figure Page

9.4 Comparison of Measured and Predicted Transverse
Strain Distributions at 0.0 C (32 F)....................... 338
9.5 Comparison of Measured and Predicted Longitudinal
Deflection Basins at 6.7 C (44 F).......................... 339
9.6 Comparison of Measured and Predicted Longitudinal
Strain Distributions at 6.7 C (44 F)......................... 340
9.7 Comparison of Measured and Predicted Transverse
Deflection Basins at 6.7 C (44 F)............................ 341
9.8 Comparison of Measured and Predicted Transverse
Strain Distributions at 6.7 C (44 F)......................... 342
9.9 Comparison of Measured and Predicted Longitudinal
Deflection Basins at 13.3 C (56 F)........................... 343
9.10 Comparison of Measured and Predicted Longitudinal
Strain Distributions at 13.3 C (56 F)....................... 344
9.11 Comparison of Measured and Predicted Transverse
Deflection Basins at 13.3 C (56 F)........................... 345
9.12 Comparison of Measured and Predicted Transverse
Strain Distributions at 13.3 C (56 F)....................... 346













Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy


LOW-TEMPERATURE RESPONSE
OF ASPHALT CONCRETE PAVEMENTS

By

REYNALDO ROQUE

May, 1986


Chairman: Byron E. Ruth
Major Department: Civil Engineering

The variable performance of asphalt concrete pavements indicates

that existing design procedures may be inadequate. However, before

improved design procedures can be developed, the mechanisms that lead to

pavement cracking must be fully understood. Therefore, this research

program was developed to monitor and define the response and failure

characteristics of pavements subjected to thermal and dynamic loading

conditions.

An asphalt concrete pavement was tested under controlled

temperature conditions in an enclosed concrete test pit. Thermal

contraction strains were measured as the pavement was cooled from room

temperature to temperatures below freezing. Load-induced deflections

and strains in the pavement were measured for dynamic load tests

performed at temperatures ranging from -6.7 C (20 F) to 25.6 C (78 F).

Finally, permanent deflections and creep strains were measured under

static loads at temperatures ranging from 0.0 C (32 F) to 13.3 C (56 F).








Load-induced deflections and strains measured at temperatures

ranging from 0.0 C (32 F) to 21.1 C (70 F) were accurately predicted

using linear elastic layer theory when suitable layer moduli were used

for input. Asphalt concrete moduli determined from correlations with

measured asphalt viscosity resulted in accurate prediction of measured

deflections and strains at all temperatures and load levels tested.

Suitable moduli for the subgrade and base layers were determined from

plate load tests performed on these materials in situ. The use of

proper analytical tools to evaluate the plate load test data was

critical in the determination of these moduli.

Thermal and load response measurement of the four-inch asphalt

concrete pavement indicated that temperature differentials produced by

rapid cooling caused the asphalt concrete layer to contract and bend in

such a way that it separated and uplifted from the base. The uplift

effect resulted in load-induced deflections, strains, and stresses at

0.0 C (32 F) that were in some cases more than double those expected for

pavements exhibiting elastic behavior. The exact mechanism that led to

the uplift phenomenon could not be determined from the measurements

obtained. Because of the uplift effect, measured creep strains and

failure could not be evaluated. Based on the findings from this

investigation, recommendations are presented for improved testing

procedures.


xvii














CHAPTER I
I INTRODUCTION


The Florida Department of Transportation (FDOOT) has had variable

performance with asphalt concrete pavements. Some pavements have

developed cracking within less than five years, while others have given

satisfactory performance after many years. In all cases, cracking was

observed with little or no distortion in the pavement layers and with no

apparent deficiencies in the asphalt concrete mixtures. The problem is

not unique to Florida. Pavement condition surveys of existing highways

and test roads around the United States indicate that traffic-associated

cracking is of major concern to highway engineers. Cracking is one of

the first indicators of distress observable in asphalt pavements and

often leads to other forms of distress.

The variable performance observed for pavements designed using

current design procedures indicates that these procedures are

deficient. Existing design procedures are empirically derived based on

correlations of certain material or pavement system parameters with

observed field performance. These procedures consider two forms of

cracking: low-temperature thermally-induced cracking and traffic-load-

induced fatigue cracking. The most commonly proposed approach to limit

thermal cracking is to limit the asphalt stiffness as measured for a

minimum design temperature, where the limiting stiffness is usually

obtained from correlations with observed field performance. Design

thickness requirements to provide adequate fatigue life are established








by attempting to limit pavement deflections or strains under a given

design load. These procedures neglect that pavement deflections,

stresses, and strains cover a wide spectrum of values dependent on

temperature and climatic fluctuations. The variable properties of

individual asphalts at low temperatures and the combined effect of

thermally- and load-induced stresses are not considered. In addition,

the two modes of cracking considered cannot explain certain types of

failures commonly encountered in practice.

Therefore, it is necessary to develop improved design procedures to

reduce the cracking potential of pavements. However, this will be

difficult to accomplish until the mechanisms that lead to cracking are

fully understood. Although the causes or factors involved with cracking

are known, the actual mechanisms that lead to cracking have not been

identified with definitive measurements on full-scale pavements.

Investigations sponsored by the FDOT led to the establishment of the

hypothesis that cracking of asphalt concrete pavements is a brittle

failure induced by short-term repetitive loads and thermal stresses that

occur during cool weather when the asphalt stiffness is high. This

suggests that asphalt concrete pavements should be designed for a

critical condition where stresses or creep strains induced in the

pavement are of sufficient magnitude to produce cracking. This critical

condition may be a result of the combined effects of asphalt age

hardening, base and subgrade support, asphalt concrete modulus,

vehicular loads, pavement cooling rate, and temperature.

The research work done in Florida, along with the variable

performance of existing pavements, indicated that a research program

should be developed to monitor and define the behavior of pavements








subjected to thermal and dynamic loading conditions. This resulted in

the formulation of a research program to test full-scale asphalt

concrete pavement systems under controlled conditions.

An FDOT test pit facility was developed for this purpose, since

there are obvious problems associated with trying to monitor this type

of behavior in the field. The test pit facility made it possible to

construct a layered system of materials to simulate a flexible pavement

system in the field. In order to provide temperature control, a cooling

system with an insulated cover was installed in the test pit.

Thus having developed the capability to simulate temperature and

loading conditions encountered in actual roadways, this research program

was initiated in an attempt to satisfy the following objectives:

1. To measure and evaluate the response of asphalt concrete

pavements to changes in temperature, and determine the effect

of this response on the dynamic load response and failure

characteristics of the pavement.

2. To measure and evaluate the dynamic load response of asphalt

concrete pavements at different temperatures and load levels.

3. To measure and evaluate load-induced creep strains and

permanent deflections induced in asphalt concrete pavements at

different temperatures.

4. To compare the response measurements listed in items one, two,

and three to values predicted by theoretical stress-strain

distributions and parameters obtained from laboratory tests.

In order to meet these objectives, a complete series of tests was

performed on a pavement section that was typical for Florida. A

measurement and data acquisition system was installed that was capable








of obtaining static and dynamic deflection and strain measurements at

ten different points in the pavement at any given time. Thermal strains

developed in the asphalt concrete layer during cooling were measured for

several cooling cycles. Dynamic load tests were performed at

temperatures ranging from -6.7 C (20 F) to 25.6 C (78 F), using both

rigid plate and flexible dual wheel loading systems. Finally, permanent

deflections and creep strains were measured for specified durations of

static loads at temperatures ranging from 0.0 C (32 F) to 13.3 C

(56 F). Dynamic load tests were also performed at different times

during creep tests to observe the effect of creep on the dynamic load

response of the pavement. Although the results of these tests were not

entirely definitive, they emphasized the need to consider the combined

effect of temperature and load in the analysis of asphalt concrete

pavements.














CHAPTER II
LITERATURE REVIEW


2.1 Introduction

The research presented in this document focuses on defining and

predicting low temperature response and failure of asphalt concrete

pavements. This includes the response of pavements to changes in

temperature and the effect these changes have on the load response and

failure limits of the asphalt concrete layer. Two elements of the

analysis system considered here make it unique: the use of measured

theological parameters of the asphalt at low temperatures to predict the

response and failure characteristics of the asphalt concrete; and the

fact that cracking is considered a short-term phenomenon that occurs

when the combined effect of temperature and traffic loads exceed the

failure limit of the asphalt concrete pavement. Although this approach

is totally different from traditional approaches, a review of the

literature will serve two purposes:

1) to establish the need and develop the rationale behind the

proposed method of analysis; and

2) to give an overview of existing knowledge of asphalt concrete

pavement response to temperature changes and traffic loads,

including an assessment of our ability to predict response and

failure.








2.2 Distress In Asphalt Concrete Pavements

2.2.1 Modes of Distress

The modes of distress in asphalt concrete pavements are well

recognized and the causes of distress, at least in general terms, are

also known. Tables 1 and 2 (1,2) are two examples of tables listing the

types and causes of distress in asphalt concrete pavements. These

tables show that failures can be grouped into three major categories:

cracking, rutting, and disintegration.

Pavement surveys around the country and the world indicate that of

these three categories, cracking is the major problem in terms of amount

and cost. Based on extensive observations by himself and others, Finn

(3) stated that traffic-associated cracking is the number one priority

item for improving and extending the performance of asphalt pavements.

Pedigo et al. (4) reviewed a great deal of work that has been done on

pavement distress and reached similar conclusions. Finn also stated

that traffic associated cracking is one of the first indicators of

distress observable in asphalt pavements, and that cracking is often

observed with little or no distortion. In reviewing the results of the

AASHO Road Test, he found that cracking led to other forms of distress

(such as rutting), and that more cracking occurred when the pavement was

cold than warm. However, there was a lack of information as to when and

where the first cracks occurred and how these cracks propagated.

Furthermore, asphalt properties were not measured at low temperatures.

Information of this type is lacking, even today. Measurements of

the environmental and loading conditions at the time of initial

cracking, along with relevant material properties, are crucial to the

development of damage criteria. Such information could not be found in








Table 2.1:


Primary Types and Causes of Distress in Asphalt Concrete
Pavements. After Ruth, 1985 (1)


Type Of Distress


Causes or Contributing Factors


- Consolidation
Shear failure
Low stability
Abrasion
Traffic
High temperatures


2. Thermal Cracking





3. Load Associated or
Fatigue cracking


4. Combined Thermal and
Induced Cracking

5. Heaving (Localized
or Extensive: Frost
Boils, Ice Lenses)




6. Settlement and Slope
Failures


- Thermal contraction
Shrinkage
Low temperatures
Fast rate of cooling
Excessively hard asphalts
Lack of snow cover (insulation)

- Traffic volume and loads
Deflection basin characteristics:

1. Layer moduli
2. Layer thickness
3. Asphalt viscosity

Climate Microclimate

1. Temperature
2. Drainage moisture variations

Material quality

- Combine factors in items 2 and 3


- Expansive soils
Frost susceptible soils
Drainage
Permeability
Capillarity
Depth and rate of frost
penetration

- Quality of in situ materials
Quality of construction
Drainage and moisture conditions
Mining activity
Karst terrain sinkholes and
cavity collapse


1. Rutting














Table 2.2:


Modes, Manifestations, and Mechanisms of Types of Distress.
After McCullough, 1971 (2)


Mode Manifestation Mechanism

Fracture Cracking Excessive loading
Repeated loading (i.e., fatigue)
Thermal changes
Moisture changes
Slippage (horizontal forces)
Shrinkage
Spalling Excessive loading
Repeated loading (i.e., fatigue)
Thermal changes
Moisture changes


E


distortionn Permanent deformation
Time-dependent deformation
(e.g., creep)
Densification (i.e., compa
Consolidation
Swelling
Faulting Excessive loading
Densification (i.e., compa
Consolidation
Swelling


Disintegration Stripping
Chemical reactivity
Abrasion by traffic
Raveling and scaling
Chemical reactivity
Abrasion by traffic
Degradation of aggregate
Durability of binder


Excessive loading


action)



:tion)


Adhesion (i.e., loss of bond)


Adhesion (i.e., loss of bond)








the literature. Thus, although the causes of cracking are well known,

the actual mechanisms that lead to cracking have not been verified with

definitive measurements of actual failures on full-scale pavements.

Several mechanisms have been proposed that cannot account for basic

material response and failure characteristics, variability of

environment, and loading conditions encountered in actual pavements.

These have led to empirical design procedures, which are valid only for

the conditions from which they were derived.



2.2.2 Cracking Mechanisms

Traditionally, cracking has been broken down into traffic-load

induced and thermally-induced, with little consideration for the

combined effects of the two mechanisms. Low-temperature transverse

cracking has been recognized as the most common non-traffic associated

failure mode and is a serious problem in Canada and parts of the United

States (5,6,7). This type of failure is generally considered a

temperature phenomenon caused by low temperatures. As the pavement

temperature decreases the asphalt concrete wants to contract, but

contraction is resisted by the friction between the asphalt concrete

layer and the base and by the length of the roadway in the longitudinal

direction. This resistance results in tensile stresses in the pavement,

which are greatest in the longitudinal direction.

Several researchers have postulated that cracking occurs when these

thermally induced tensile stresses exceed the tensile strength of the

asphalt concrete (8, 9, 10). This mechanism has been confirmed by

laboratory and field investigations (7-13), and provides the basis for

the hypotheses that have been presented for low temperature cracking.








The theological properties of the asphalt at low temperatures are

generally recognized as the most important factor in low-temperature

transverse cracking (5, 11, 14, 15, 16, and others). Many researchers

have associated low temperature cracking with properties such as asphalt

stiffness, viscosity, temperature susceptibility, and glass transition

temperature. These properties, of course, are all related to the

asphalt's ability to flow and thus relax stresses. All researchers have

found that the stiffer and more temperature susceptible the asphalt, the

greater the potential for cracking.

Probably the most commonly proposed approach to control thermal

cracking is to limit the asphalt stiffness as measured for a minimum

design temperature. McLeod (17) concluded that low temperature pavement

cracking is likely to occur whenever the stiffness of the pavement

attains a value of 6.9 E9 Pa (1.0 E6 psi) at a pavement depth of two

inches, at the minimum temperature encountered, and for a loading time

of 20,000 seconds. Fromm and Phang (18) proposed a value of 1.4 E8 Pa

(20,000 psi) at 10,000 seconds loading time. Gaw (19) reported that the

St. Anne test pavements cracked at an asphalt binder stiffness of 1.0 E9

Pa (145,000 psi) and a mixture stiffness of 2.0 E10 pa (2,900,000 psi)

at 1800 seconds loading time. Many researchers have found good

agreement between measured stiffness and observed cracking of pavements

in the field and confirmed that pavements using softer asphalts exhibit

less cracking (7, 12, 20, 21, 22).

Ruth (14) concluded that cracking would be reduced by using

asphalts with lower viscosities and improved theological behavior at low

temperature. Fabb (23) concluded that low viscosity and low temperature







susceptibility are conducive to reducing the temperature at which

fracture occurs.

The advantage of using a softer binder, particularly one with a low

temperature susceptibility, was demonstrated by Hills and Brien (8).

Fromm and Phang (16) also reported that less temperature susceptible

asphalts were associated with pavements exhibiting less cracking.

Schmidt (24) suggested that the glass transition temperature of the

asphalt might be a more definitive measure of non-load associated

cracking than measured viscosities, since at temperatures lower than the

glass transition temperature the asphalt behaves elastically, while at

higher temperatures it exhibits viscoelastic response. Thus, below the

glass transition temperature there is almost no potential for stress

relaxation.

Other factors have been found to influence low temperature

transverse cracking, but to a lesser degree than asphalt properties.

Tuckett et al. (25) found that higher asphalt contents reduced thermal

cracking. Fabb (23) reported that increasing binder content reduced

thermal fracture, but only slightly. He also concluded that the

properties and grading of the aggregate had little or no effect on the

resistance of the asphalt concrete to thermal cracking. Cooling rate

was found to have little effect on the failure temperature by Fabb (23)

and Fromm and Phang (16). However, they only compared relatively high

cooling rates. Finally, results of the St. Anne Test Road indicated

that only half the frequency of low temperature cracking occurred in

10-inch pavements than did in 4-inch pavements (26).

The concept of fatigue is probably the most recognized concept that

has been suggested for use in the evaluation of traffic-load associated








failure (27-31). Fatigue distress is the phenomenon of fracture under

repeated stresses which are less than the tensile strength of the

material. Fatigue characterization of materials has been studied

extensively and there are innumerable references on this topic (e.g. 31-

39).

The philosophy behind the approach to the analysis and design of

asphalt concrete pavements considered in this thesis is totally

different from conventional approaches based on fatigue. In fact, the

fatigue concept is considered erroneous, and will not be covered in much

detail. Design procedures based on fatigue assume that there is some

average pavement condition for which an equivalent amount of damage will

be incurred under each passing wheel load. These procedures neglect

that deflections, strains, and stresses cover a wide spectrum of values

dependent on temperature and climatic fluctuations. They cannot

properly account for the variable properties of individual asphalts at

low temperatures.

Several researchers have proposed modifications to fatigue life

predictions based on temperature, recognizing that the fatigue life of

materials tested in the laboratory is dependent on temperature.

However, the basic concept of fatigue damage has remained unchanged.

Rauhut and Kennedy (40) proposed one such modification and discuss

modifications proposed by other researchers. They also recognize that

fatigue damage is difficult to evaluate since there is limited knowledge

as to fatigue life relations for real pavements, reliable test data

exists for only a limited number of mixtures, and there is insufficient

information to define how fatigue life varies with temperature and








mixture characteristics. Furthermore, they point out that no laboratory

fatigue test comes close to simulating actual field conditions.

One very significant point is that fatigue life is highly dependent

on the type of fatigue test performed. While illustrating the effect of

temperature on fatigue life, Pell and Cooper (34) showed that as the

temperature is lowered, fatigue life increases under stress controlled

tests, but decreases under strain controlled tests.

Recently, investigators have found that rest periods markedly

increase the fatigue life of bituminous mixtures (41). This seems to

indicate that asphalt concrete has the potential to heal, or that the

actual failure mode is not a true fatigue phenomenon. Both of these

ideas negate the validity of conventional fatigue approaches.

Ruth and Maxfield (42) found the concept of fatigue did not apply

to specimens from test roads in Florida. They found that the failure

strains for in-service cores were the same as for fabricated cores.

They concluded that fracture of asphalt concrete is related to a process

of cumulative creep strain and that fracture strain is primarily depen-

dent on asphalt properties and loading conditions. Ruth et al. (43)

pointed out that during warm weather, temperatures are high enough to

eliminate stress or strain accumulation that leads to fatigue failure.

Pavements designed using conventional fatigue approaches have given

marginal performance. Ruth et al. (43) has stated that for similarly

designed asphalt concrete pavements in Florida some have developed

cracking within less than five years, while others give satisfactory

performance after many years. Roberts et al. (44) stated that very few

highways have served without maintenance even for five or ten years, and

in many cases roads with low traffic volumes have experienced premature








failure. Rauhut and Kennedy (40) stated that the occurence of fatigue

cracking in the field is quite variable, even for apparently identical

sections.

In addition, traditional approaches are unable to explain certain

types of failures observed in the field. It is well recognized that

immediate and disastrous failures may occur with weakened subsoil

conditions after just a few passes of a heavy vehicle (45). Molenaar

(46) has pointed out that traditional approaches cannot explain

longitudinal cracks observed to occur at the pavement surface. This

type of cracking is very common in practice.

Several studies indicate that the combined effects of thermally and

load induced stresses may cause cracking. Haas and Topper (13)

indicated that even if the thermal stresses are not sufficient to cause

cracking the addition of load associated stresses may result in pavement

failure. From and Phang (16) reported a case where heavily loaded

trucks were carried during the winter months in one direction only.

They found that there was a greater incidence of transverse cracking on

the heavily loaded side, thereby illustrating the combined effect of

thermal and load stresses. Ruth et al. (43) hypothesized that this type

of mechanism may be the cause of some early pavement failures in Florida

and elsewhere. However, almost all studies presented in the literature

consider only the load effect or the thermal effect.

Ruth et al. (43) were the first to present an approach that

combines the effect of thermal and dynamic stresses as the main cause of

failure. They considered pavement cracking to be caused by brittle

failure induced by short term repetitive loads and thermal stresses that

occur during cool weather when the asphalt stiffness gets very high.







Their idea is to design the pavement for a critical condition based on

material properties, loads, and environment. They developed a pavement

analysis model that considers cracking as a result of asphalt properties

(including age hardening), vehicular loads, pavement cooling rate and

temperature. The analysis program was used to evaluate the effect of

different asphalt viscosities, cooling rates, and pavement thicknesses

on pavement performance. Predictions of cracking temperatures for a

Pennsylvania DOT test road were obtained which identified the two

cracked sections in the test road. Analysis of typical highways in

Florida indicated that some pavements may give marginal performance,

which was indirectly substantiated by observed early cracking of pave-

ments, particularly those located in northern Florida.



2.3 Properties of Asphalt Cement and Asphalt Concrete As
Related To Low-Temperature Pavement Response and Cracking

The response and failure of asphalt concrete pavements have been

shown to be highly dependent on the properties of the asphalt cement.

Thus, proper characterization of asphaltic materials is extremely

important. The characterization of bituminous materials for use in

conventional design methods is based mostly on empirical procedures

which rely on correlations of their results with field performance. The

Marshall and Hveem Stabilometer tests are most commonly used for this

purpose (27). These tests are performed at high temperatures and relate

mainly to the problems of stability, workability, and durability. Fun-

damental properties cannot be obtained directly from these tests.

Several researchers have attempted to correlate Marshall results with

fundamental properties (47), but it will be pointed out later that such








an approach can lead to serious error, particularly when predicting

properties at lower temperatures.

As explained earlier, the analysis method used in this dissertation

considers cracking to be caused by brittle failure induced by short term

repetitive loads and thermal stresses that occur when the asphalt

stiffness is high. Therefore, the emphasis here is placed on the behavior

at relatively low temperatures, roughly in the range from -10 C (14 F), or

approximately the glass transition temperature, to 25 C (77 F). This

temperature range is referred to as the near transition region (48).


2.3.1 Asphalt Cement Properties

The response of asphalt to an applied stress is time dependent,

where the strain increases at a given rate with time. At lower temper-

atures many asphalts are also shear susceptible, with the change in

creep strain rate not being proportional to the change in applied

stress. Finally, the behavior of all asphalts is highly dependent on

temperature.

In general, as the temperature is lowered, asphalts become more /

viscous and eventually exhibit glassiness, where different elasto- /

viscous behavior is observed, their coefficients of expansion change,\

and brittle fracture may develop (48). Jongepier and Kuilman (49)

explained the behavior of asphalt as a viscoelastic liquid. At low

temperatures, asphalt behaves like an elastic solid, while at high

temperatures its behavior is comparable to a viscous liquid. At inter-

mediate temperatures the behavior is influenced by both viscous and

elastic components. Asphalt cements show a characteristic common to

other amorphous materials; the glass transition phenomenon. Schweyer








and Burns (50) found that the glass transition temperature for a wide

variety of asphalts is between -10 C and 5 C (14 and 41 F).

The viscoelastic response of asphalts and asphalt mixtures is often

approached using mechanical models that combine Hookean springs and

Newtonian dashpots in various combinations (51). As mentioned above,

asphalt behavior at lower temperatures is often non-Newtonian (shear

susceptible), making these models unsuitable for complete description.

In any case, asphalt behavior is commonly described in terms of

theological parameters, and at low temperatures, asphalts can be

described in terms of three theological parameters: consistency or

viscosity, shear susceptibility, and temperature susceptibility (52).

However, the measurement of viscosity at low temperatures is rather

problematic because the asphalt behaves closer to an elastic material

with relatively low creep deformation rates. This means that creep or

viscosity tests will require an extremely long time of loading to obtain

measurable deformations at low stress levels. High stress levels are

usually necessary to obtain measurements within reasonable time

intervals, but if the material is shear susceptible the results may not

be representative of the material's behavior within the range of

interest.

Schweyer presented a pictorial review of an extensive number of

devices that have been used over the years to measure theological

properties (53). A more concise literature survey of the different

methods to measure low temperature rheology of asphalts is presented in

reference 48. In general, the traditional transient rheometers are not

directly adaptable to low temperature work (54). However, several

special testing devices have been used to conduct investigations of








asphalt properties at low temperatures. These have led to improved

understanding of low temperature asphalt behavior.

For example several investigators have concluded that low

temperature asphalt properties cannot be predicted from properties

measured at higher temperatures. Schweyer et al. (55) reported that

different asphalts demonstrate very different low temperature

theological properties. They emphasized that temperature susceptibility

in the near transition region can and should be evaluated by absolute

viscosity measurements rather than by empirical tests. They also stated

that temperature susceptibility cannot be predicted from behavior

exhibited at higher temperatures. In a comprehensive study of different

asphalts at the Asphalt Institute, Puzinauskas (56) reached the

following conclusions:

generally, viscosity at low temperature is affected more by

heating than viscosity at high temperature;

the low temperature viscosity of asphalt cements was found to

vary extremely and the variation increases with decreasing

temperature; and

shear effects become more pronounced with increasing viscosity or

with decreasing temperature.

Schmidt (57) investigated the reliability of standard ASTM tests to

predict low temperature stiffness of mixtures made with a wide variety

of asphalts. He concluded that low temperature thermally induced

cracking should not be implied from high temperature viscosity measure-

ments on diverse types of asphalts. Although many researchers have

found reasonable correlation between measured stiffness and observed

field cracking, relatively noor agreement has been obtained by







researchers estimating low temperature stiffness by means of tests at

higher temperatures. Pink et al. (54) and Keyser and Ruth (58) also

emphasized the importance of experimental measurements rather than the

use of empirical extrapolations to determine low temperature properties.

Probably the most significant advancement to the understanding of

low temperature response and failure properties of asphalts and asphalt

mixtures was the development in the 1970's of the Schweyer constant

stress rheometer (59). This device has the capability of measuring

theological properties at -10 C (14 F) and lower. Furthermore, Schweyer

established theological concepts that led to a definitive theological

model and methods to evaluate important parameters that relate to low

temperature behavior, including shear susceptibility.

The proposed theological model is the Burns-Schweyer model (55).

The model is a Burgers model with a modified dashpot to incorporate a

self-generating feedback system to regulate the rate of viscous flow.

Thus, the model accounts for viscous behavior for both Newtonian and

shear susceptible materials, as well as for elastic, and delayed elastic

behavior. Details pertaining to the measurement and evaluation of

theological parameters using the Schweyer rheometer may be found in

references 50, 55, 59, and 60. Ruth and Schweyer (61) showed that the

Burns-Schweyer model gives accurate prediction of the theological

properties of asphalts, including those that are very shear

susceptible. Keyser and Ruth (58) concluded that the Schweyer rheometer

is an excellent device for low temperature measurements of asphalt

properties and that the concepts developed by Schweyer provide values

more closely related to shear and strain rates encountered in the

laboratory and in actual pavements.








Therefore, the Schweyer rheometer has the unique advantage of

direct measurement of low temperature asphalt properties at any given

temperature. This instrument and the theological concepts presented by

Schweyer were key elements in the development of the analysis method

considered in this dissertation.


2.3.2 Asphalt Mixture Properties

Tests on asphalt mixtures are usually conducted to determine their

failure characteristics. However, the increased trend toward

mechanistic approaches and the application of elastic theory to pavement

evaluation and design, has initiated a concerted effort to define the

stress-strain response of bituminous mixtures (62). The variability of

materials in asphalt mixtures and the nature of pavement structures are

unlimited, making it nearly impossible to uniquely characterize their

stress-strain properties (63). Furthermore, a compromise between a

rigorous design solution and practicality is necessary. Material

characterization should be based on conditions that are believed to be

critical with respect to pavement response and failure.

As mentioned earlier, the approach to pavement failures in this

dissertation considers pavement cracking to be caused by brittle

fracture induced by short term repetitive loads and the thermal stresses

that occur during cool weather when the asphalt stiffness is high.

Using this approach it is necessary to predict the following: the

strains and deflections induced by applied dynamic stresses (wheel

loads); the short term creep strains induced by these dynamic stresses;

and the stresses and strains induced by temperature changes. References

cited later will show that by modelling the asphalt concrete layer as an








elastic continuum, researchers have obtained fairly accurate predictions

of measured strains and deflections in asphalt concrete pavements under

dynamic wheel loads, particularly at low ambient temperatures. Thus,

for a given set of temperature and loading conditions, it appears that

the stress-strain response of an asphalt mixture can be characterized

using an elastic modulus or E-value. The obvious point should be made

that asphalt mixtures are viscoelastic and describing their stress-

strain behavior by using an E-value is an idealization. The idealized

E-value will depend on how it is defined and the test method used to

measure it. Therefore, it is.necessary to identify laboratory

procedures that yield E-values that are suitable for proper pavement

response prediction. Suitable parameters for creep response, thermal

expansion and contraction, and failure limits are also necessary.

A variety of test methods has been used to test asphalt concrete

mixtures for characterization including compression (unconfined and

triaxial), bending (flexure), tension (direct and indirect), and shear

tests. Some of the different laboratory procedures and the different

idealized stiffness values they yield are described in reference 27.

There are certain advantages and disadvantages for each test, but these

are beyond the scope of this review. One problem with all the tests is

the effect of time or rate of loading, which makes suspect the elastic

equations used to analyze the test data. The effect of creep on stress

redistribution is almost always ignored because of the complexities

introduced into the analysis of test data. Many researchers (64, 65, 66

and others) have recommended using the indirect tensile test as the most

suitable for routine characterization in terms of practicality,

simulation of actual conditions, economy, and ease of testing.








Many researchers have concentrated on defining the relative effects

of different variables on the response characteristics of asphalt

mixtures. These studies are usually limited to those variables that are

considered to have a significant effect on the material behavior within

the researchers' scope of interest.

Deacon (38) summarized the major variables affecting the stiffness

and range of linear response for asphalt concrete mixtures and divided

them into three major categories: 1) loading; 2) mixture; and 3) envi-

ronmental related variables. He stated that four mixture related

variables have a considerable effect on the stiffness of asphalt paving

mixtures: air void content, asphalt content, asphalt viscosity, and

filler content. Temperature, mainly through its effect on asphalt

viscosity, was recognized as the major .factor in the behavior of

bituminous mixtures. Deacon also stated that the range of linear

response increases with increasing load frequency, decreasing

temperature and void content, and increasing asphalt content, asphalt

viscosity or filler content.

Bazin and Saunier (37) recognized the difficulty in changing any

one parameter without changing another, so they studied variation in

modulus for very different mixtures. They found that for correct binder

dosages and normal voids (4 to 8 percent), all other parameters had

little influence on modulus when compared to the effect of variation in

binder type, temperature, and time of loading. They suggest a linear

relation between log of modulus and void content of mix and proposed the

time-temperature superposition principle. This principle suggests that

there is an equivalency between time of loading and temperature.








Several researchers have investigated the stress dependence of

dynamic modulus. Cragg and Pell (67) reported stress dependence in

dynamic modulus but indicated that the change was small when compared

with temperature induced changes in modulus. Gonzalez et al. (64)

concluded that instantaneous modulus of elasticity decreased with

increasing temperatures and increasing number of applications but was

not affected by magnitude of applied stress. Kallas and Riley (68)

reported stress independent moduli for stresses between 17 and 70 psi

and temperatures from 4.4 C to 38 C (40 F to 100 F). Monismith et al.

(39) reported stress independent moduli over the range of 100 to 125 psi

using repeated flexure tests. Constant dynamic moduli were reported by

Pell and Taylor (33) for stresses below 125 psi at a temperature of 10 C

(50 F). They also indicated that low voids, low temperatures or high

loading rates, and adequate quantities of binder and filler were

conducive to linear response.

The effect of loading frequency on modulus has also been the

subject of several investigations. Yeager and Wood (69) described the

behavior of test specimens subjected to different frequencies. For

faster frequencies there is little time for flow and the mixture

behavior is more elastic. Slower load rates result in larger total

strains and lower calculated moduli. They reported a seven-fold

increase in modulus at 4.4 C (40 F) and a stress of 50 psi as the rate

of loading was increased from 1 to 12 cycles per second. Barksdale (70)

studied stress pulses applied by moving wheel loads and developed

relationships of stress pulse times as a function of vehicle velocity

and depth beneath the pavement surface. He developed charts that have







been subsequently recommended for determining loading times for dynamic

laboratory tests (71).

Investigations have also been conducted to evaluate different test

methods and differences in tensile and compressive properties. Kallas

(72) compared dynamic moduli of dense graded mixtures with normal air

voids at 4.4, 21.1, and 37.8 C (40, 70, and 100 F) and frequencies of 1,

4, and 16 cycles per second in tension, tension-compression, and

compression. He reported small differences in dynamic modulus in

tension, tension-compression, and compression for temperatures between

4.4 and 21.1 C (40 and 70 F) and frequencies between 1 and 16 cycles per

second. However, at 1 cycle per second and temperatures between 21.1

and 37.8 C (70 and 100 F), the dynamic tension or tension-compression

moduli averaged 1/2 to 2/3 of moduli in compression. The viscous

component of response was found to be considerably greater in tension

than in compression for frequencies of 1 to 16 cycles per second and

temperatures from 21.1 to 37.8 C (40 to 100 F). Wallace and Monismith

(73) compared moduli results from diametial indirect tension and

triaxial tests and analyzed the effect of anisotropy on both testing

procedures. They estimated that a more relevant assessment of modulus

could be obtained from diametral tests.

Although direct characterization is one approach to obtain mixture

parameters, the cost is prohibitive for most agencies (27), particularly

considering the variable properties of asphalts with temperature and

stress, and the fact that these properties can vary significantly for

different asphalts. Therefore, several investigators have established

relationships to predict mixture response parameters based on

temperature, time of loading, and material characteristics from







conventional tests. Miller et al. (62) reviewed some of the methods

that have been presented for predicting modulus from physical and

mechanical properties of the mixture, which usually take the form of

nomographs or master equations. Two of the better known methods are:

1) the different versions of the Shell Nomographs, and 2) the Asphalt

Institute Bituminous Mix Modulus Predictive Equation. The Shell

Nomographs were developed from Van der Poel's original stiffness charts

for asphalt cements for a given load rate and temperature. Relation-

ships were developed to translate bitumen stiffness to mixture stiffness

and these have been modified by several investigators. Modulus

prediction is based on asphalt stiffness as derived from softening point

and penetration, temperature, and frequency of loading. The asphalt

stiffness is then modified for mixture characteristics to obtain the

modulus of the mixture. The Asphalt Institute Equation is based on

tests performed on different mixtures at varying temperatures and fre-

quencies. Modulus prediction is based on load frequency, void content,

filler content, asphalt viscosity at 21.1 C (70 F), temperature, and

asphalt content.

Note that these methods cannot account for the variable properties

of individual asphalts at low temperature. Asphalt properties at

different temperature are inferred from consistency measurements at high

temperatures, and in the case of the Shell Nomographs, these measure-

ments are entirely empirical in nature. The temperature and shear

susceptibility of individual asphalts is ignored. As pointed out

earlier in this review, several researchers have shown that these

properties are a major factor in the response of the mixture, and they

can vary significantly from asphalt to asphalt.








Ruth et al. (43) were the first to present modulus relationships

based on a direct evaluation of measured asphalt viscosities at

different temperatures. The dynamic modulus relationship was developed

for dense-graded mixtures and a loading time of 0.1 seconds. It

requires the asphalt viscosity at a specific temperature as determined

from measurements with the Schweyer rheometer. Viscosity measurements

are made at several temperatures and stress levels (comparable to those

induced in laboratory tests on mixtures), in order to develop asphalt

viscosity-temperature relations for input. Therefore, this prediction

method properly accounts for the effects of temperature and shear

susceptibility on mixture response, and thus shows the most promise for

accurate modulus determination. Furthermore, the tests required are

simple and inexpensive.

The evaluation of these relationships is one goal of the current

investigation. It has been recognized that there is much variability in

characterization parameters reported in the literature (63). Part of

the problem is that resilient modulus tests have not been standardized

and certain aspects of the test that have an important effect on results

have not been defined (66). One reason for the lack of standardization

is that little if any work has been done to identify test procedures and

data interpretation methods that yield modulus values which give

accurate prediction of measured response on full-scale asphalt concrete

pavements under varying temperature and loading conditions. In other

words, although the relationships between asphalt viscosities and

laboratory measured dynamic moduli seem adequate, it has not been

verified that laboratory generated moduli can be used to predict

response on full-scale pavements, particularly at low temperatures.








Ruth and Maxfield (42) also developed relationships between asphalt

viscosity and what he defined to be a static modulus. The static

modulus was calculated from results of constant stress creep tests from

which he also calculated a pseudo mix viscosity for creep strain

prediction. Ruth found good correlation between asphalt viscosity and

pseudo mix viscosity, which also correlated well with static modulus.

Thus he developed relationships for both static modulus and creep

strains that are based on the low temperature viscosity of the

asphalt. The creep strain prediction model is a function of the asphalt

viscosity (function of temperature), the applied stress and the failure

stress. The static modulus is representative of the material's long

term response, as for the case of thermally induced stresses.

It should be noted that creep prediction models presented by other

researchers were almost exclusively developed to predict rutting. Their

results will not be discussed because they have not considered low

temperature creep response.

With the emphasis on fatigue, only a limited amount of work has

been done to define the failure limits of asphalt concrete mixtures in

terms of stresses or strains. Even for low temperature transverse

cracking, failure parameters are usually presented in terms of a

limiting asphalt stiffness or a fracture temperature, determined from

correlations with observed cracking in the field.

Several researchers have reported that tensile strength and failure

strain of asphalt mixtures are dependent on rate of loading and

temperature, and all have determined that the tensile strength increases

as the temperature decreases and the rate of loading increases (9, 12,

36, 74, 75, 76). Ruth (14) reported that as the temperature decreases








the failure stress increases but remains constant below some transition

temperature which is dependent on asphalt properties. Heukelom (74)

presented evidence that the tensile strengths of mixtures are related to

asphalt properties. Various researchers have reported failure stresses

for conditions of low temperatures and fast loading rates. Finn (77)

reported fracture strengths of 290 to 580 psi for asphalts in bulk under

low temperature conditions and rapid loading. For asphalt mixtures

under the same conditions he reported strengths generally ranging from

400 to 700 psi. Ruth and Olson (78) reported failure stresses between

380 and 440 psi and chose a value of 400 psi as typical.

Ruth and Olson (78) reported that as the temperature decreases the

tensile strain at failure decreases. Tons and Krokosky (76) reported

that increasing asphalt content within practical limits had little

effect on strain at ultimate strength. They also stated that rate of

loading had little effect at low temperatures. Epps and Monismith (36)

showed that the strain at failure is related to the stiffness of the

mixture, the strain at failure decreasing with increasing mixture

stiffness. Pavlovich and Goetz (79) computed limiting strains from

axial deformations in direct tension tests and determined that

temperature is the most significant factor affecting limiting strains.

Strain rate has some effect hut not as much as temperature. They found

the limiting strain at 60 C (140 F) was 300 to 500 times greater than

that at -27.5 C (-17.5 F). Salam and Monismith (80) presented an

equation to determine the strain at failure for asphalt mixtures based

on the strain at failure of the asphalt, the asphalt stiffness, and the

mixture stiffness. The asphalt properties used are penetration and ring








and ball softening point, which are empirical in nature and performed at

high temperatures.

Ruth and Maxfield (42) tested different mixtures and determined

that measured failure strains are primarily a function of asphalt

viscosity. Ruth and Potts (81) found that the energy required to

fracture a specimen decreased with increasing viscosity. These findings

led to the development of relationships between asphalt viscosities and

strain and energy at failure. The relationships are unique, since they

can directly account for the properties of individual asphalts at

different temperatures.

Investigations on the thermal expansion and contraction charac-

teristics of asphalt concrete mixtures have led to the following

conclusions (82, 83):

1. Different asphalts produce different amounts of expansion and

contraction.

2. The amount of shrinkage during cooling is more than the amount

of expansion during heating.

3. There are two different coefficients of expansion between -10 F

and 140 F, and the transition temperature between the two was

found to vary between 70 F and 86 F. Values in the low and

hiqh range have been called the solid and fluid thermal

coefficients, respectively.

4. The greater the asphalt content, the greater the thermal

expansion and contraction.

5. In the fluid state, the amount of expansion (contraction)

depends on the degree of restraint, while in the solid state

the expansion is the same for free and friction conditions.








6. Jones et al. (83) developed the following equation for

predicting the cubic coefficient of expansion in the solid

state:


mix


= ac Bac + Vagg Bagg
Vix
mix


where, Bmix


Bac


Bagg


Vmix =

Vac =

Vaaa
aa =


cubic thermal coefficient of expansion for
asphalt concrete

cubic thermal coefficient of expansion for
asphalt (glassy state)

cubic thermal coefficient of expansion for
aggregate

volume of. asphalt plus aggregate

volume of asphalt

volume of aggregate.


Assuming isotropic properties the linear coefficient of thermal

contraction (a) is:

mix
mix ~


2.4 Properties of Foundation Materials

The characterization of soils and granular base and subbase

materials, for both analysis and performance, is required data for all

structural pavement design methods, and is often treated with

considerable simplification. Geotechnical engineers often feel that

structural engineers have little interest in those parts of their work

below the ground level, and their feelings are certainly justified in

the case of pavements (84). Of the 185 papers presented to the past

five International Conferences on the Structural Design of Asphalt

Pavements only 15 have been concerned in any detail with the mechanical







properties of soils and granular materials, and much of this work has

concentrated on the prediction of rutting.

Soil behavior depends on many factors including water content, dry

density, stress level, stress states, stress path, structure, stress

history, and soil moisture tension (85). Although the relative effect

of these variables can be investigated in the laboratory, the cost of

even a limited number of tests may be prohibitive for most highway

projects.

Reference 27 describes tests that are typically used to

characterize soils and granular materials for pavement analysis,

including CBR, plate load tests, and triaxial testing. These tests are

usually performed for a limited set of conditions that in all proba-

bility do not encompass the variable conditions encountered during the

life of the pavement. Furthermore, the parameters obtained from such

tests are dependent on sample preparation, testing procedures, and data

interpretation methods, all of which have not been standardized.

Several researchers have presented relationships of resilient
modulus as a function of dry density and moisture content for specified

soils, based on laboratory testing. However, even these types of rela-

tionships are of limited value since they may ignore certain variables

that may have a significant effect on response. Also there is little

evidence that response parameters determined from conventional tests, or

any other test, provide for accurate prediction of response of full-

scale pavements.

A detailed review of work that has been done to define the relative

effects of different factors on soil response, or the relationships

developed to predict response parameters measured in the laboratory,








will not be presented here. This information is beyond the scope of the

work presented in the dissertation. However, some of the more recent

work done will be reviewed in an attempt to present a general view of

the state of knowledge in the area of characterizing soils for pavement

response prediction.

References cited later show that stresses and strains within the

asphalt concrete layer can be predicted fairly accurately by using an

effective modulus for the soil in an elastic layer analysis. They also

state that it is unlikely that stresses within the soil layers can be

predicted using a similar approach. Even for the simpler case of

predicting stresses in the asphalt concrete layer, the problem of deter-

mining a suitable effective modulus remains.

It has become evident in recent years that it is difficult or even

impossible to predict the behavior of pavements solely from laboratory

test data (86). Therefore, there has been much more emphasis on full-

scale pavement testing. A great deal of work is being done to determine

response parameters from nondestructive testing devices such as

Dynaflect and Falling Weight Deflectometer, as evidenced by the many

papers presented on this subject in the latest International Conference

on the Bearing Capacity of Roads and Airfields (87), and the 1985

sessions of the Transportation Research Board. This approach involves

the calculation of effective layer moduli based on measured surface

deflections. The approach has advantages and disadvantages. One

advantage is that many tests can he performed quickly and easily over

several miles of roadway under in situ conditions. Therefore, modulus

relationships can be developed for widely varying conditions. However,

surface deflections alone may not yield unique solutions and the








peculiarities of the different testing devices may result in loading

conditions that are not directly comparable to moving wheel truck

loads. In addition, the method is only suitable for existing pavements

and the results obtained will represent only the behavior during the

particular time tested.

Maree et al. (86) presented an approach to determine layer moduli

based on a device they developed to measure deflections at different

depths within the pavement structure. They suggested that effective

moduli for use in elastic layer theory can be determined from correction

factors or shift factors established from field measurements using their

device at different times of the year and under different conditions.

They determined effective moduli from tests performed at different load

levels and under different environmental conditions using their

multidepth deflectometer. They found that moisture condition has a

significant effect on response and its effect can be as important as

stress state. They also demonstrated that the modulus of individual

materials depends on the modulus of the underlying layers. The stress

dependence of several pavement materials was also demonstrated from the

field data. They found that most granular materials behave in a stress-

stiffening way, while most subgrade materials (e.g. weathered shale)

behave in a stress-softening way. Comparison of field and laboratory

data showed that although the trends apparent in the field were also

apparent in the laboratory, the constant-confining pressure triaxial

tests overestimated the modulus of the base materials tested.

Accurate prediction of stresses and strains within the foundation

layers is necessary for the prediction of rutting and stability failures

within these layers. Luhr and McCullough (88) state that moduli of








unbound materials should vary both horizontally and vertically in the

pavement structure according to equations equating modulus with state of

stress (i.e. MR = Aod for fine-grained materials and MR = kik2 for

granular materials). They stated that although this variation can be

satisfactorily represented by finite element models (F.E.M.), these

models have the following disadvantages: 1) they usually require large

amounts of computer time; 2) the variability in pavement performance

data may make their precision superfluous; and 3) F.E.M.'s are usually

too complex and consume too much time to be used routinely in pavement

management systems. They concluded that elastic layer theory with an

equivalent effective modulus gives reasonable response prediction.

Brown and Pappin (89) seem to disagree. They developed a contour model

to predict non-linear behavior and lack of tensile strength in soils and

incorporated this model into the finite element program SENOL. Develop-

ment and limited verification of their model involved both laboratory

tests and full-scale testing of pavements under controlled conditions.

Based on a parametric study and limited measurements they determined the

following:

deflections and strains in the asphalt layer can be reasonably

predicted using an equivalent effective modulus in elastic layer

theory;

it is unlikely that elastic layer theory can be used to predict

stresses and strains within the unbound layers;

they suggest that the K-o approach is less satisfactory than a

linear elastic solution; and

the simplest approach for design calculations is the use of a

linear elastic system provided adeaqate equivalent moduli are used.








They suggest that detailed nonlinear analysis using the SENOL model

would lead to adequate selection of these moduli.

It seems clear that more work is needed to determine suitable

parameters for soil response and to develop models for more accurate

prediction of stresses and strains within the soil layers. Finite

element models should lead to more satisfactory results but the stress-

strain relationships still have to be improved. However, accurate

measurements on full-scale pavements to develop such models are lacking.



2.5 Prediction of Thermal- and Load-Induced Stresses,
Strains, and Failure In Asphalt Concrete Pavements

Several investigators have developed models to predict thermal

stresses and strains in asphalt concrete. Hills and Brien (8) developed

a simple calculation procedure to predict thermally induced stresses.

Their solution was based on a restrained, infinitely long pseudoelastic

beam exposed to a uniform temperature drop. Lateral restraint was

neglected. Limited experimental work showed that the method gave

reasonable predictions. Haas and Topper (13) used the basic equation

presented by Hills and Brien to develop a procedure to calculate thermal

stresses that recognizes the temperature and stiffness gradients that

exist in actual pavements. Shahin (90) modified the Hills and Brien

equation to accommodate coefficients of contraction not constant with

temperature.

Christison et al. (7) used five different analyses for thermal

stress computation and compared predicted and observed values. He

concluded that the pseudoelastic beam yields reasonable results but

points out certain difficulties with this analysis: 1) the predicted

stress depends on the time interval used in the calculation; and 2) the








method does not allow for stress relaxation subsequent to the time

interval in which stresses are computed.

Monismith et al. (9) developed a thermal stress equation for an

infinite viscoelastic slab and complete restraint. Stress is calculated

as a function of depth, time, and temperature. A relaxation modulus is

required from uniaxial creep tests.

Ruth et al. (43) presented a stress equation that considers rate of

cooling, creep rate, and variation in modulus with temperature. The

model requires the asphalt viscosity-temperature relationship from which

all calculations are made. The method is unique in that it can account

for individual asphalt properties.

The prediction of thermal cracking in asphalt pavements is usually

based on comparing the accumulated thermal stress with the tensile

breaking strength of the asphalt concrete. Several investigators have

compared stresses as predicted by the various models with observed

cracking.

Burgess et al. (12) reported that the method of Hills and Brien

correlated well with cracking observed in the St. Anne Test Road.

Christison (7) compared results from pseudoelastic beam, viscoelastic

beam, and viscoelastic slab analyses to cracking observed at St. Anne.

He determined that thermal cracking could be predicted by using the

computed stresses from either analysis at 1/2-inch depth.

Haas and Topper (13) indicated that Monismith's method appeared to

predict unusually high stresses, which may be due to his assumption of

infinite lateral extent.

Models to predict thermal stress cracking have also been

presented. The computer program COLD (91) was developed based on the








Hills and Brien equation. It uses a stress criteria of 200 psi. Shahin

and McCullough (92) developed a model to predict the amount of thermal

cracking. The model predicts temperatures, thermal stresses, low-

temperature cracking, and thermal fatigue cracking. They indicated that

comparisons of predicted cracking with measurements at the Ontario and

St. Anne Test Roads were reasonable. Lytton and Shanmugham (93)

developed a mechanistic model based on fracture mechanics to predict

thermal cracking of asphalt concrete pavements. The model assumes

cracking is initiated at the surface of the pavement and propagates

downward as temperature cycling occurs. The prediction of transverse

cracking and cracking temperature can also be attempted with the Haas

model (94) and the Asphalt Institute Procedure (95). Keyser and Ruth

(58) found absolutely no correlation between actual cracking in 6- to 9-

year old pavements in Quebec, and cracking predicted using the latter

two models.

Ruth et al. (43) developed a thermal cracking model based on their

stress equation. The model computes stresses, creep strains, and

applied creep energy, which are all used as failure parameters.

Predictions of cracking temperatures for a Pennsylvania D.O.T. test road

were obtained which identified the two cracked sections in the test

road.

It should be noted that although indirect comparisons have been

made with observed field cracking, little if any measurements exist of

actual contraction and deformation of asphalt concrete pavements during

cool ing.

Over the years, various solutions have been presented to predict

stresses, strains, and deflections within the pavement system due to








applied wheel loads. In 1885, Boussinesq presented the mathematical

solution for a concentrated load on a boundary of a semi-infinite

body. Love extended this solution to solve for a distributed load on a

circular area. Burmister (96) was the first to present a solution for a

multi-layered elastic system, and developed solutions for specific two-

and three-layered systems. Schiffmann (97) extended Burmister's

solutions to include shear stresses at the surface. Elastic solutions

for layered systems have been presented in the form of tables, graphs,

and equations that include a wide range of parameters (98). Also,

several computer programs have been developed for elastic layer

analysis.

With the advent of the finite element method, more sophisticated

models have been introduced, including viscoelastic analysis (99), Brown

and Pappin's contour model (89), and Yandell's mechano-lattice analysis

(100). Many of the finite element models have been developed for the

problem of rut prediction, and more recently to predict response within

the soil layers more accurately.

For the purpose of predicting stresses, strains and deflections

within the asphalt concrete layer, there seems to he general agreement

that elastic layer analysis is suitable. Although there is some

question of its ability to predict response in the underlying layers,

there has been considerable verification of its ability to predict

response in bound layers.

Avital (101) discusses a series of computer programs available for

the analysis of multi-layered systems. Barksdale and Hicks (32) present

a general description of multi-layered systems and finite element

approaches, along with extensive references on their detailed








development. They recommend the use of elastic layered systems for

pavement analysis since only two variables are needed (modulus and

Poisson's ratio).

The moderators for the last International Conference on the

Structural Design of Asphalt Pavements (84) indicated that use of linear

elastic theory for determining stresses, strains, and deflections is

reasonable as long as the time-dependent and nonlinear response of the

paving materials are recognized. They noted that the papers presented

at the conference confirm that multilayer elastic models generally yield

good results in layers containing binders. It is interesting to note

that of the 16 papers relating to analyzing pavements, 14 used

multilayer elastic theory and the two that used viscoelastic procedures

reduced their viscoelastic idealization to equivalent elastic layered

systems.

Ros et al. (102) measured strains and pressures at different levels

in a variety of trial sections subjected to standard loads at varying

speeds and temperatures. They found good correspondence between

measured values and values computed with an elastic layer program

(BISAR, Ref. 103), using properties determined in situ and in the

laboratory. Correlation was especially good at hich asphalt

stiffness. Halim et al. (104) performed tests on reinforced and

unreinforced flexible pavements in a test pit and found that elastic

layer theory (BISAR) provides a reliable tool to predict flexible

pavement response. They suggested that use of a calibration factor for

stress-dependent materials is more efficient and less time consuming

than more sophisticated models. Waterhouse (105) measured axial

stresses vertically below the central axis of a circular load and








determined that elastic theory gave reasonable prediction of stress

distribution. However, as pointed out in section 2.4 by Brown and

Pappin (89), it is unlikely that elastic theory can predict stresses

within the soil layers, although they did find good correspondence

within the asphalt layer and at the surface of the subgrade using

elastic theory. Several other researchers have also found good

correspondence with measured results using elastic layer theory (e.g.

106, 107).

As mentioned earlier, the approach to pavement failure considered

in this dissertation is based on the idea that pavement cracking is

caused by brittle failure induced by short term repetitive loads and

thermal stresses that occur during cool weather when the asphalt

stiffness is high. Therefore, pavement design and analysis methods

should be based on the theological properties of the asphalt which are

used to predict thermal and dynamic load stresses and strains at thermal

conditions typical of the lowest temperatures expected. The predicted

values can then be compared to the failure limits of the material for a

direct evaluation of failure. This method was proposed by Ruth et al.

(15).

Avital (101) implemented a computer program (CRACK) to handle this

interaction of load and temperature induced stresses. The program

combines an elastic layer computer program for response prediction under

dynamic load and a thermal program for thermal stress and creep strain

prediction. The program requires temperature conditions, traffic

volumes, theological properties of the asphalt, and the pavement struc-

ture characteristics (layer thickness, modulus, and poisson's ratio).





41

This approach is totally different from conventional fatigue

approaches. Therefore, the different distress prediction models that

have been developed will not be presented here. These models usually

attempt to predict pavement life by using different empirically based

sub-models that predict fatigue cracking, rutting, and thermal cracking

separately. The most sophisticated of these are the different versions

of the VESYS model presented by Kenis et al. (108).













CHAPTER III
EQUIPMENT AND FACILITIES


3.1 Description of Test Pit Facility

A test pit facility located at the Office of Materials and Research

of the Florida Department of Transportation (FDOT) was used in this

research project. The facility included a 15 ft. long, 13'-4" wide, 6'-

2" deep concrete pit with a test area of 8 ft. by 12 ft. The test pit

made it possible to construct a layered system of materials to simulate

a flexible pavement system in the field. It had the following features:

control of water level in the test pit;

a hydraulic loading system that could apply both static and

dynamic loads anywhere within the 8 ft. by 12 ft. test area;

two linear displacement transducers for deflection measurements;

a 20,000 lb. capacity load cell; and

a four-channel continuous plotter used for both the transducers

and the load cell.

A detailed description of this facility is given in Research Report S-1-

63, Civil Engineering Department, Engineering and Industrial Experiment

Station, University of Florida. The facility was formerly used for the

evaluation of Florida base course materials using a variation of the

plate bearing test. However, the following modifications were necessary

to make it suitable for testing complete flexible pavement systems:

a proper method for distributing hot asphalt concrete mix within

a reasonable time period to allow for proper compaction and to

avoid premature cooling of the mix;








a cooling system with proper insulation to provide the capability

of testing at different temperatures; and

a proper measurement and data acquisition system for deflection,

strain, and temperature measurements.

Therefore, a good deal of work was done to design, procure materials,

and construct equipment to overcome these deficiencies.



3.2 System for Hot Mix Asphalt Distribution

A triangular shaped hopper that spans the 8 ft. width of the test

pit and distributes material through an adjustable opening at its bottom

was designed to distribute the hot mix. A picture of the hopper is

shown as Figure 3.1. The hopper was made of steel angle and plate and

stands approximately 2'-6" high. It had a level capacity of 33 cf which

made it possible to place a four-inch lift in the test pit in only one

pass. The hopper was designed for completely manual operation and ran

on steel angle rails that were installed in the test pit. A concrete

pad was placed immediately north of the hopper to allow use of a dump

truck for asphalt concrete distribution.



3.3 Pavement Cooling System

Different alternatives were evaluated to select a pavement cooling

system for installation in the test pit area. An air cooling unit was

chosen as the most suitable system. A system of this type is clean,

essentially maintenance free, and provides for reasonably accurate

temperature control. A local mechanical engineer was hired to design

and prepare equipment specifications for a suitable air cooling system.
















7 -/- '. .-


Figure 3.1: Hopper for Asphalt Hot Mix Distribution


,,
~pph~
-


' ; nIR~fFPh29i
~a


,-c- .-
6-' ~c~~--. .
,*ra :~








The system was designed to cool six-inch thick pavements to a

temperature of -10 C (15 F) at a rate of 3.3 C (5 F)/hour, as measured

at a depth of 1/4-inch from the surface of the pavement. Temperature

control was achieved by manually controlling the cooling unit. A fully

automatic system with greater cooling capacity was originally

considered, but its cost was prohibitive. In any case, automatic

controls are of limited value, since temperature gradients would be

present in the pavement as long as the unit was running.

The cooling system consists of a direct expansion, low temperature

refrigeration system. The evaporator (Larkin ELT-300) was located

directly in the test pit and cooled the pavement by recirculating cold

air across the test pit surface. Temperature control was achieved by

lowering the pavement temperature below the required test temperature,

stopping the refrigeration system, and allowing the test pit temperature

to drift upward. The condensing unit (Larkin CS 0750L1), which housed a

7.5 HP compressor, sat outside the building's north wall. Refrigerant

hoses and electrical cables from the condensing unit to the evaporator

unit were connected through holes drilled in the north wall of the test

pit. Drainage was accomplished with a heated drain pipe, which was

passed through a hole drilled in the test pit wall. A layout of the

test pit cooling system is shown in Figure 3.2.

An insulated cover for the test pit area was designed and

constructed. The cover had a solid wood frame which enclosed the test

pit area. The top of the cover consisted of five removable wood panels

that spanned the 8-ft. width of the test area and were supported by a

ledger on the cover's frame. One panel had a one-ft. diameter hole to

allow for placement of the loading ram. The panel dimensions are such













Condensing
Unit


North Wall of
Building

Evaporator
Location
During
Operation


Bay Door


-Flexible
Refrigerant
Hose


i i Evaporator
S__Location
-- When not
L.J Testing
Test Pit
Reaction
Beam -/


NOTH


Insulated Test
Pit Cover


Figure 3.2: Layout of Test Pit Cooling System








that the panel with the hole can be moved to three different positions,

thereby providing for three different loading positions during cooling.

All sections of the cover, including the frame and the panels, were

insulated with six inches of polystyrene, as recommended by the

contractor. Once the frame and panels were in place, all the joints

were sealed with clay and tape to reduce moisture migration and

infiltration into the test pit during operation. A nylon sheet was

placed over the entire cover to further reduce infiltration. A picture

of the cover, completely installed, is shown in Figure 3.3. Figure 3.4

shows the cover removed for access to the test pit.


3.4 Measurement System for Pavement Response

The test pit facility was formerly used exclusively to evaluate

Florida base course materials using a variation of the plate bearing

test. These tests required a loading system, capability for two

deflection measurements, and a recording device for the load and two

displacements. Considerably more extensive measurements were required

for complete evaluation of asphalt concrete pavements at different

temperatures. Static and dynamic deflection and strain measurements at

different points in the pavement were required to define the pavement

response during loading, and the contraction of the pavement during

cooling. Temperature measurements were also required. Therefore, a

measurement and data acquisition system was designed and installed for

this purpose.
































Insulated Test Pit Cover Completely Installed


Insulated Test Pit Cover With Panels Removed


Figure 3.3:


Figure 3.4:








3.4.1 Measuring Instruments

Linear variable differential transformers (LVDT's) were purchased

to obtain static and dynamic deflection measurements at different points

on the pavement surface. Schaevitz model DCD-200 LVDT's, with a range

of 0.20 in. (0.5 cm) and an output of 50 V/in. (19.7 V/cm) were

used. Two dual-output power supplies were purchased to operate these

units. All LVDT's were individually calibrated using a micrometer and a

voltmeter.

Two LVDT support systems were designed and constructed: one for

use with the plate loading system, and the other for use with a dual

wheel loading system designed for use in the test pit. Figure 3.5 shows

a plan view of the test pit area with the LVDT support system used for

plate testing. The system consisted of wood LVDT mounts supported by

1.5 in. diameter pipes that spanned the eight-foot width of the test

pit. The mounts could be positioned to obtain deflection measurements

at any specified distance from the load, and the entire system could be

moved for testing at different positions. The LVDT's were spring-loaded

and could be adjusted vertically by way of an adjustment screw on the

wood mount.

Figure 3.6 shows the LVDT support system used for loading with the

dual wheels. The longitudinal support was a laminated two-by-four-inch

beam which was located underneath the axle of the dual wheel system.

The entire length of the beam was grooved to accept the vertically

adjustable LVDT mounts shown in Section B-B (Figure 3.6). Thus the

mounts could be positioned at any specified distance from the load.

The same mounts used for plate testing (Figure 3.5) were used to obtain

transverse deflections with the dual wheel system. These mounts were





50









PLAN VIEW


Wood LVDT
Mounts



Loading Plate -


A
Ledger to
Support Pipes


~~1~~~~

=




-5---
=
=


11/2"Pipe



-N


A
_4

----------


Section A-A
Hole for LVDT Wires
0i o Hold-Down

jo,
01 1 I [j_- Clamp


Adjustment-.
Screw

SHold-Down
Si* Clamp


Spring-I
LVDT'S


Figure 3.5: LVDT Support System for Plate Loading














PLAN VIEW


Note: See Figure 3.5
for Section A-A.


SECTION B-B
Vertically Adjustable
LVDT Mount





Groovi
for
=-== Mountir
pring-Loaded
LVDT


Figure 3.6: LVDT Support System for Dual Wheel Loading







supported on 1 1/2-inch diameter pipes that rested on longitudinal beams

as shown in Figure 3.6. The supports could all be moved for testing at

different positions.

During initial cooling trials, the LVDT's malfunctioned at low

temperatures, even when covered with heavy insulation. The low

operating temperature of the cooling unit would eventually penetrate the

insulation and cause these units to malfunction. Therefore, a heating

system was installed to maintain the LVDT's at fairly constant temper-

ature during cooling. Variable output heater wire (0 to 4 watts/ft. of

wire) was wrapped around the individual LVDT's, which were then covered

with a 1/2-inch layer of rubber foam insulation. A voltage regulator

was used to control the output of the heater wire, which was adjusted as

necessary to maintain the LVDT's at constant temperature of about 25 C

(77 F) during cooling. A picture illustrating how the LVDT's were

prepared for testing is shown in Figure 3.7. A picture of the entire

LVDT support system with the dual wheel loading system in place and

ready for testing is shown in Figure 3.8.

Strain measurements were obtained with two-inch bonded wire strain

gages (Micro-Measurements EA-06-20CBW-120). Two methods were used to

position the gages at a given location. For surface strain measure-

ments, the gages were mounted at specified points on the pavement after

it was placed and compacted. Several gages were also installed for

measurements at the bottom of the asphalt concrete. These gages were

first mounted on asphalt concrete cores (4-inch diameter and 2 1/2-

inches high) and then were positioned at specified locations on the

compacted base material. The cores were prepared in the laboratory,

using an asphalt concrete mixture similar to the one used for the rest

of the pavement.




















wI


Figure 3.7:


LVDT Prepared for Tests at Low Temperatures







































a) Frontal View


C~~-~- ;r
~~

i.
*-I,


b) Diagonal View


Figure 3.8: Test Pit Pavement Completely Instrumented and Ready
for Testing With Dual Wheels


~t rx








The strain gages were mounted with epoxy directly on the asphalt

concrete surface. The following procedure was used to prepare the

surface and mount the gages. The asphalt concrete surface was prepared

by sanding; first with a belt sander, and then with progressively finer

sandpaper until the surface was "glass-smooth". Clear tape was then

used to lift off all loose particles from the surface. Cleaning with

the tape was repeated until the tape was completely clean when lifted

off the surface. A thin layer of epoxy was then applied to the clean

surface and the gage was positioned, taking care to remove any air

bubbles trapped underneath the gage. A thin layer of epoxy was also

applied to the surface of the gage, for protection and to aid in

bonding. A sheet of cellophane was placed on top of the gage and clear

tape was used to hold the gage in position until the epoxy set. The

cellophane was used to prevent possible damage from the tape adhering

directly to the gage. Once the epoxy set, the tape was removed and the

strain gage wires were soldered to the gage. The completely installed

gage was covered with a piece of masking tape followed by a piece of

duct tape for protection. A picture of the two-inch strain gages

mounted on asphalt concrete cores is shown in Figure 3.9.


3.4.2 Data Acquisition System

A data acquisition system was designed and installed which was

capable of monitoring and recording ten dynamic deflection measurements,

ten dynamic strain measurements, 20 temperature measurements, and load

magnitude and time of loading. As mentioned earlier, only one recording

device, a Gould model 2400 strip chart recorder, was available in the

test pit facility, since this was all that was needed to evaluate base






































Figure 3.9:


Two-Inch Strain Gages Mounted on Asphalt Concrete








course materials. This high speed recorder had four channels, but only

three channels with amplifiers were available for use. Two channels

were used in conjunction with LVDT's to obtain deflection measurements,

and the third channel was used with the load cell to monitor and control

load magnitude and time of loading.

Five dual-beam digital oscilloscopes were purchased to record

additional'deflection and strain measurements. The oscilloscopes used

were Nicolet Explorer Series 2090 with model 201 plug-in units. These

instruments had the capability of monitoring and recording displacements

or strains continuously with time. All five oscilloscopes had a

temporary recording system for indefinite storage of measurements taken

within a specified time interval (i.e. one sweep of the oscilloscope).

In addition, three of the five oscilloscopes were equipped with a floppy

disc recording system for permanent data storage. Permanent storage of

data obtained with the other two oscilloscopes was accomplished by using

an X-Y plotter (Hewlett-Packard Model 7046B). Once data were temporar-

ily stored for a given series of loading cycles, they were immediately

output to a calibrated X-Y plotter for permanent recording.

Four oscilloscopes, two with floppy disc recording systems and two

without, were used in conjunction with eight LVDT's to obtain deflection

measurements. These eight measurements, plus the two on the strip chart

recorder provided for ten simultaneous deflection measurements at

different points on the pavement.

The following equipment was used in conjunction with the strain

gages to obtain strain measurements:

a Vishay/Ellis (V/E) 21 AK switch, balance, and calibration

module;








a V/E 20 AJMLH strain gage indicator; and

a digital oscilloscope with floppy disc recording system.

These units could handle 10 strain gages, in either a 1/4-, 1/2-, or

full-bridge arrangement. However, only one gage could be monitored at

any given time with the strain gage indicator. Output from the

indicator was sent to the digital oscilloscope for continuous recording

with time.

Deflection and strain measurements stored on floppy discs, were

later output to calibrated X-Y plotters. The plotters were calibrated

for an average LVDT output, since each LVDT had a slightly different

calibration. The measurements, as determined from the X-Y plotter

output, were then adjusted for the calibration factor of the particular

LVDT used. All cables going to the LVDT's and the strain gages were

passed through an access hole drilled through the side of the insulated

test pit cover.

A schematic diagram of the data acquisition system is shown in

Figure 3.10. A picture of the system is shown in Figure 3.11. Figure

3.12 is a picture of a typical recording of deflections on a digital

oscilloscope and Figure 3.13 shows a typical output recorded on an X-Y

plotter.

Temperature measurements were obtained with a Fluke Model 2240A

Datalogger. This unit used thermocouple wires to receive and record

temperature measurements for up to 20 different positions at one time.

It could record temperatures at specified time intervals or could be

triggered to record at any given time. The unit automatically records

the date and time of the readings.













Record on
Floppy Discs


J To and From
Load Cell
To and From
LVDT'S 9&10


Remote Control
for simultaneous
Operation of
Digital Oscilloscopes


Input Voltage
to LVDT'S


Output Voltage from LVDT'S
to Digital Oscilloscopes


Figure 3.10: Schematic Diagram of Data Acquisition System
















. ......
_.....~... 1*-


I i I


Figure 3.11:


Data Acquisition System in Test Pit Facility


Figure 3.12: Typical Deflection Recording on Digital Oscilloscope


- ---------------------L-~*lwr*uurr~llll I~-






































Figure 3.13:


Typical Deflection Output on X-Y Plotter







3.5 Loading Systems: Rigid Plate Load vs. Flexible Dual Wheels

Two loading devices were used during the course of this research:

a rigid plate loading and flexible dual wheel loading. Rigid plate

loading was accomplished with a 12-inch diameter steel plate. A cage

was used in conjunction with the plate to evenly distribute the load

over the plate's area, and to increase the plate's rigidity. The plate

was always set on a thin layer of hydrocal (plaster) for levelling and

to evenly distribute the load. A diagram of the rigid plate loading

system is shown in Figure 3.14.

A set of small jet aircraft wheels (Piper Aircraft 31T) were used

for flexible dual wheel loading. The wheels were purchased from a

second-hand dealer for use in the test pit. An axle, which was

compatible with the existing loading system, was designed and machined

for the wheels. The wheels carried a pressure of 100 psi and were

designed to operate at 3,750 Ibs. However, the wheels easily carried

5,000 Ibs. each for a total of 10,000 Ibs. on the dual wheel system.

The actual loaded area was determined from wheel imprints made in the

laboratory at different load levels. A picture of the dual wheel system

is shown in Figure 3.15. Note that it was necessary to attach cables to

the wheels to prevent them from rotating about their vertical axis.

A hydraulic loading system, which could apply static and dynamic

loads, was used with both rigid plate and dual wheel loading. Loading

cycles could be preset for any combination of loading time and rest

period. The time required for the load to be fully applied could not be

controlled, and was dependent on the distance the loading ram had to

travel. Therefore, the load was applied faster with the rigid plate

than with the flexible wheels.


















,Swivel Plate


Loading Cage to
Distribute Load -



Hydrocal to
Level Plate


12" Diameter
Steel Plate


Surface


Figure 3.14: Rigid Plate Loading System






































Figure 3.15:


Flexible Dual Wheel Loading System








After extensive experience with the rigid plate and the dual wheel

devices, several advantages and disadvantages were observed for each.

These are as follows:

I. Rigid Plate Loading.

A. Advantages

loading time could be controlled very accurately, since the

load came on almost instantaneously;

the loading area was circular and constant; and

the position of the load was always known because it was

difficult for the plate to move during loading; and

there was less wear and tear on the loading system since very

little ram movement was required for loading.

B. Disadvantages

very high shear stresses were induced at the edge of the

plate, causing it to sink and forcing a plane of failure;

analysis of a rigid plate on a multi-layer system was a major

problem, since there was no computer program available that

accurately predicted stresses and strains under a rigid

load. An analytical procedure was developed using an elastic

layer analysis, but it proved to be extremely tedious.
II. Flexible Dual Wheel Loading.

A. Advantages

this loading was close to flexible type loading (constant

pressure) and could be modeled more easily with existing

computer programs;

dual wheel loading was more representative of actual truck

loads in the field; and








the wheels were easier to position than the plate, since they

did not require setting with hydrocal.

B. Disadvantages

the load did not come on instantaneously since the wheels had

to deform before a load was applied. Furthermore, the time

required for the load to come on, depended on load

magnitude. Therefore, it was difficult to set the loading

time and to evaluate creep strain accumulation for dynamic

loading conditions. For this reason, all creep tests were

performed using static loads.

up to 2 1/2 inches of ram movement was required for loading,

which caused greater wear and tear in the loading system;

the loading area varied with load and was not circular (more

difficult to model); and

the wheels tended to roll during loading so that the exact

position of load was not known.



It was evident that neither of the loading devices was perfect, but

the disadvantages associated with the plate were overwhelming. The

loading was not representative of wheel loads and the analysis procedure

required tremendous amounts of time. Therefore, the dual wheel loading

system was used for the majority of tests performed.














CHAPTER IV
EFFECT OF ENCLOSED CONCRETE
TEST PIT ON PAVEMENT RESPONSE


4.1 Introduction
The test pit used in this research was made up of 8-inch concrete

walls and a 12-inch concrete slab, which enclosed a volume of 8 ft. by

12 ft. by 6 ft. deep. The layered pavement system was placed and tested

within this volume. Most analytical solutions and computer programs,

consider the layered system (or soil mass) to be infinite in lateral

extent and semi-infinite in depth. Three-dimensional finite element

programs could model the test pit, but as discussed later, these pro-

grams were found to be either too expensive or inaccurate. Therefore, a

study was undertaken to evaluate the effects of the test pit floor and

walls on the response of the layered system to an applied load, and to

establish a methodology to account for these effects.



4.2 Preliminary Analysis

An initial attempt was made to predict the measured response of the

asphalt concrete pavement with an elastic layer computer program. The

pavement deflection and strain measurements used for this analysis were

obtained at a temperature of 18.3 C (65 F), using the plate loading

system at 10,000 Ibs. and 0.1 sec. loading time (see Section 7.1).

Parameters for the sand subgrade and limerock base were determined from

plate load tests performed in the test pit. Conventional analytical

solutions, which consider the pavement lavers to be infinite in lateral







extent and semi-infinite in depth, were used to obtain these

parameters. A sand subgrade modulus of 14,000 psi was calculated using

Boussinesq's theory for a rigid circular load on a semi-infinite mass.

Using Burmister's two-layer theory for similar conditions, a limerock

modulus of 75,000 psi resulted (see Table 5.11 and accompanying

discussion). This procedure is commonly used to obtain modulus values

from plate bearing tests performed in the field. The modulus values

obtained were essentially equivalent moduli, which represent the

behavior of all the materials below the tested surface. These values

are sometimes used to predict the response of the entire pavement system

using elastic layer analysis.

An asphalt concrete modulus of 145,000 psi was calculated for a

temperature of 18.3 C (65 F), using previously established correlations

with measured asphalt viscosity (see Appendix A). Therefore, the

following moduli, Poisson's ratios, and layer thicknesses were used in

an elastic layer computer program (BISAR) to predict pavement response:

Layer Poisson's
Material Thickness (in.) Modulus (psi) Ratio

Asphalt Concrete 4 1/8 145,000 0.35

Limerock Base 6 3/4 75,000 0.40

Sand Subgrade semi-infinite 14,000 0.40

Figure 4.1 shows the measured deflections and the deflections

predicted for the system above (Predicted 1). Although the shapes of

the deflection basins matched reasonably well outside the loaded area,

the measured deflections were grossly overpredicted. It appeared that

the effect of the floor was not properly accounted for by simply using

an effective layer modulus for the sand subgrade. A second computer run










DISTANCE FROM LOAD CENTER (INS.)
6 12 18 24 30 36


Measured


"-'ePredicted C


Predicted (


_~/


EAC = 145,000 psi d = 41/8" V= 0.35
ELR = 75,000 psi d =63/4" V= 0.40
ESAND = 14,000 psi d =oo = 0.40


SEAC = 145,000 psi d = 41/" V= 0.35
ELR = 75,000 psi d = 63/4" V= 0.40
ESAND = 14,000 psi d =48" V= 0.40
ECONC = 3,500,000 d = V= 0.20


20-

Ij


/


Figure 4.1: Measured and Predicted Deflection Basins in the Test Pit


/








was made with a semi-infinite concrete embankment (E = 3,500,000 psi

and P = 0.20) underneath the sand layer. The sand layer was assigned a

finite depth of 48 in. and all other parameters were unchanged. The

predicted deflection basin for this case is also shown in Figure 4.1

(Predicted 2), and shows that the concrete foundation had a very

significant effect on the pavement's response. However, even with the

concrete embankment at a depth of 48 in., the measured response was

overpredicted, which made it clear that there were other factors

affecting the pavement response that were not being accounted for in the

analysis.

A series of program runs was made to determine the depth at which

the concrete floor had no effect on response. The depth of the sand

layer was varied from 48 in. to infinity, while maintaining all other

parameters constant. The results of this analysis are shown in Figure

4.2. Note that even at a depth of 120 in. (10 ft.) the concrete floor

had a significant effect on the predicted pavement response.

The following conclusions were drawn from this preliminary

analysis:

the effect of the concrete floor must be directly accounted for

in the analysis procedure. It cannot be accounted for by simply

using an equivalent layer modulus determined from plate load

tests;

the walls may have an effect on the response of the pavement.

The measured deflections were overpredicted, even when a concrete

embankment was introduced. Therefore, the wall effect needed to

be evaluated and accounted for in the analysis; and











DISTANCE FROM LOAD CENTER (INS.)
12 18 24


(No Concrete)






EAC = 145,000 psi d = 41/8" V = 0.35
ELR = 75,000 psi d = 63/4" V = 0.40
ESAND 17,000 psi d = Variable V= 0.30
ECONC = 3,500,000 psi d =o V= 0.20


Figure 4.2: Effect of Concrete Floor at Different Depths on Predicted Deflection Basins


Measured







the effect of the floor and walls must also be accounted for when

analyzing plate test data. Layer moduli determined from

analytical solutions for systems of semi-infinite depth, are not

suitable for pavement response prediction.

Therefore, a study was undertaken to evaluate the effect of the test pit

floor and walls on the response of the subgrade, the limerock base, and

the complete pavement system.



4.3 Effect of Test Pit Constraints

4.3.1 Analytical Model

Although the elastic layer theory computer program can model a

floor, it cannot model walls. The program considers all materials

infinite in the lateral direction. In addition, the program can only

handle flexible loads and the plate loading system is rigid. Therefore,

several available finite element computer programs were considered to

evaluate the effect of the test pit constraints on pavement perfor-

mance. The AXSYM computer program was chosen for this purpose.

AXSYM is a three-dimensional finite element program written by

E. L. Wilson at the University of California at Berkeley. The program

is for solution of axisymmetric stress-deformation problems using

nonlinear stress-strain characteristics. It is specifically designed

for analysis of vertically loaded circular footings resting on or

beneath the surface of a soil mass. Only linear elastic stress-strain

characteristics were used in conjunction with the program. The basic

difference between the AXSYM model and the test pit is that AXSYM models

the walls as a circular tank, whereas the test pit is rectangular. This







was not considered a major problem and the effects could be defined and

approximated using this model.

Two other programs were also investigated; YBFE1 and SAPIV. YBFE1

is a two-dimensional (plane strain) finite element program developed for

soil-structure interaction problems. Preliminary program runs using

YBFE1 revealed that this program was unsuitable for predicting flexible

pavement response. The error was probably introduced by the plane-

strain nature of the model and the type of finite element used. SAPIV

is a three-dimensional finite element program developed for structural

dynamics but can be used to model homogeneous masses by means of a brick

or plate element. This program would have been most accurate in

modeling the test pit, but preliminary attempts at running the program

showed that an excessive amount of computer space was needed. This

space was not available on the current version of SAPIV at the

University of Florida. In addition, the cost of running the program was

prohibitive for the purposes of this project.



4.3.2 Effect of Constraints on Subgrade Response

Closed form solutions are available for the vertical displacement

of a rigid circle on both a semi-infinite mass and on a finite layer.

The following equations may be used to calculate these displacements:

Semi-infinite: Pz = (1-2) ava

Finite Layer: Pz I Pavg(a)

where z vertical displacement of rigid circle (ins.)

u Poisson's ratio

Pavg average pressure on the rigid circle (psi)








a radius of circle (ins.)

E Young's modulus (psi)

IP influence coefficient: function of P and depth of
finite layer.

Subgrade moduli were calculated with these equations to show the effect

of assuming different finite layer depths and different Poisson's

ratios. Plate deflections measured in the test pit were used in the

calculations. The modulus values calculated are shown in Table 4.1.

As expected, the modulus increases with decreasing Poisson's ratio

and increases with increasing layer depth. However, the main purpose of

this comparison is to show that by assuming an infinite layer as opposed

to a finite layer, errors in excess of 20 percent may result in subgrade

modulus calculations. Similarly, errors in excess of 20 percent may

result in the modulus if a Poisson's ratio of 0.5 is assumed as opposed

to 0.3. Therefore, when calculating modulus for pavement response

prediction using plate load data, it is necessary to use the finite

layer solution. Poisson's ratio values are difficult to determine, but

values of 0.3 to 0.4 are usually considered reasonable for granular

materials. A Poisson's ratio of 0.3 was assumed for the sand subgrade

in the test pit, since laboratory tests by other researchers indicated

that this was a typical value for the Fairbanks Sand.

AXSYM was used to determine the effect of the test pit walls on the

response of the sand subgrade. The sand subgrade was assumed to be a

finite layer of 48-inch thickness. A subgrade modulus of 15,420 psi was

calculated for an assumed Poisson's ratio of 0.3 (see Table 4.1). The

following computer runs were made with these parameters to determine the

wall effect:




















Table 4.1: Sand Subgrade Modulus for Different Layer Depths and
Poisson's Ratios


Modulus Values: Sand Subgrade

Poisson's Depth of Finite Layer (in.)

Ratio Semi-Infinite 24 36 48

0.2 17,890 14,590 16,020 16,610

0.3 16,960 13,410 14,530 15,420

0.4 15,650 12,100 13,290 13,940

0.5 13,980 10,260 11,510 12,160


Note:
(a) Calculated using average 12-in. plate deflection at 15
loading cycle.


psi on 5th







wall at 15 ft., frictionless;

wall at 8 ft., frictionless;

wall at 4 ft., frictionless; and

wall at 4 ft., full friction.

It should be noted that rigid plate loading and finite element grids of

similar geometry were used in all runs.

The plate deflections as well as the deflection basins predicted by

the program, were identical for all cases, indicating that the wall had

absolutely no effect on the response. However, the deflections

predicted by the AXSYM program were about 14 percent less than predicted

by closed form solution (6.55 E-3 vs. 7.59 E-3 in.). It seemed like the

finite element grid used in the AXSYM runs was not fine enough.

Therefore, the number of elements was doubled and the program was

rerun. Although the program solution was closer, it underpredicted

deflections by about ten percent (6.83 E-3 vs. 7.59 E-3 in.). However,

one interesting point is that deflections remained unchanged away from

the loaded plate for the increased element grid.

Based on these results it seemed apparent that the type of finite

element used by AXSYM could not properly handle the high stress

concentrations at the edge of the rigid circle. Therefore, one cannot

put reliance on the rigid plate deflections predicted by AXSYM, except

on a relative basis. It also seems that the error introduced by the

high stress concentrations on these elements does not affect the

deflections away from the loaded area.

Several runs were made with the elastic layer computer program to

verify the accuracy of the AXSYM Drogram away from the rigid loaded

area. A finite layer of 48 in. was used with a modulus of 17,000 psi








and a Poisson's ratio of 0.3. For the AXSYM program, a flexible load

was used and the walls were placed at 8 ft. and assumed frictionless.

The elastic layer program predicted deflections that were identical to

the deflections determined by closed form solution. The AXSYM solution

was identical to the elastic layer solution outside the loaded area and

was within 3 percent of the elastic layer solution underneath the load.

One additional AXSYM run was made to insure that the rigid and

flexible plate AXSYM solutions gave the same results away from the

loaded area, since the comparison of AXSYM and the elastic layer

solution was done for a flexible load. This comparison showed that the

AXSYM rigid and flexible plate solution predict identical deflections

beyond 2 in. of the loaded area.

The following conclusions were made after having verified the

deflections predicted by AXSYM:

the walls have absolutely no effect on the response of the sand

subgrade to a load applied at its surface; and

the floor has a definite effect on the sand subgrade response. A

finite layer closed form solution should he used to determine the

subgrade modulus from plate bearing test data in the test pit.


4.3.3 Effect of Constraints on Limerock Base Response

The effect of the concrete floor on the response of the limerock

base was investigated by a series of runs with the elastic layer

solution computer program. Two systems were analyzed: a 6.75 in.

limerock base over a semi-infinite subgrade; and a 6.75 in. limerock

base over a finite subgrade of 48 in. on a concrete embankment. Three

limerock base moduli were used for each system: 30,000, 60,000, and








100,000 psi. All other material properties were the same for all runs

and are given in Table 4.2.

The predicted deflection basins for each system are presented in

Table 4.2. The deflection differences for systems with and without a

concrete embankment (or floor) are also shown in Table 4.2. These

differences indicate that the effect of the concrete floor was to reduce

the deflections by an amount that is relatively independent of the

stiffness of the limerock base (approximately 2.5 E-3 in.).

A comparison of the deflection basins for the three cases studied

is presented in Figure 4.3. Clearly, the effect of the concrete floor

is considerable and must be accounted for in the analysis.

The following AXSYM runs were made to determine the effect of the

walls on the limerock base response:

wall at 7 ft., no friction, rigid plate;

wall at 7 ft., no friction, flexible plate;

wall at 4 ft., no friction, rigid plate; and

wall at 4 ft., full friction, rigid plate.

The following pavement system was used in the analysis:

Modulus (psi) Poisson's Ratio Thickness (in.)

Limerock: 90,000 0.40 6.75

Sand: 14,530 0.30 36

This system was underlain by a rigid base. A relatively high limerock

modulus was chosen for the analysis, since this stiffer system would be

affected to a greater degree by wall friction. A pressure of 50 psi on

a 12-inch diameter area was applied in all cases.













Table 4.2: Effect of Concrete Floor on Surface Deflections for Different Base Stiffnesses


Surface Deflections (in.)

Modull Poisson's Thickness Distance From Center of the Plate (in.)
f of Layers (psi) Ratios (in.) 0 4 6 9 12 18 24 30 36 48

2 30,000 0.4 6.75 2.43E-2 2.17E-2 1.70E-2 1.13E-2 8.74E-3 5.89E-3 4.39E-3 3.50E-3 2.91E-3 2.19E-3
15,420 0.3 SM-INF

3 30,000 0.4 6.75
15,420 0.3 48.0 2.18E-2 1.92E-2 1.45E-2 8.83E-3 6.27E-3 3.48E-3 2.06E-3 1.26E-3 7.79E-4 2.84E-4
3,500,000 0.2 SM-INF
Difference 2.5E-3 2.5E-3 2.5E-3 2.47E-3 2.47E-3 2.41E-3 2.33E-3 2.24E-3 2.24E-3 1.91E-3

2 60,000 0.4 6.75 1.89E-2 1.72E-2 1.44E-2 1.08E-2 8.68E-3 6.00E-3 4.46E-3 3.52E-3 2.90E-3 2.17E-3
15,420 0.3 SM-INF

3 60,000 0.4 6.75
15,420 0.3 48.0 1.64E-2 1.47E-2 1.20E-2 8.33E-3 6.27E-3 3.64E-3 2.17E-3 1.32E-3 8.10E-4 2.92E-4
3,500,000 0.2 SM-INF
Difference 2.5E-3 2.5E-3 2.4E-3 2.47E-3 2.41E-3 2.36E-3 2.29E-3 2.20E-3 2.09E-3 1.88E-3

2 100,000 0.4 6.75 1.59E-2 1.47E-2 1.28E-2 1.02E-2 8.49E-3 6.05E-3 4.53E-3 3.55E-3 2.92E-3 2.16E-3
15,420 0.3 SM-INF

3 100,000 0.4 6.75
15,420 0.3 48.0 1.35E-2 1.23E-2 1.04E-2 7.81E-3 6.12E-3 3.74E-3 2.28E-3 1.39E-3 8.52E-4 3.03E-4
3,500,000 0.2 SM-INF
Difference 2.4E-3 2.4E-3 2.4E-3 2.39E-3 2.37E-3 2.31E-3 2.25E-3 2.16E-3 2.07E-3 1.86E-3











DISTANCE FROM CENTER OF PLATE, IN.
12 18 24


5E-3





10E-3
z
0
U
-J
u.
o 15E-3


// 48" Subgrade Concrete Slab
// ----Semi-Infinite Subgrade


i/ Base Modulus:
'/ 0 30,000 psi
0 60,000 psi
A 100,000 psi










of Different Base Layer Stiffness on Predicted Deflection Basins


/ 1'
/O








Figure 4.4 shows a comparison between deflection basins for the

wall at 7 ft. with no friction and the wall at 4 ft. with full

friction. This comparison gives a direct indication of the effect of

having the wall at 4 ft. as opposed to having no wall. As shown in the

figure, the effect of the wall was to shift the deflection basin upward

by a small amount. The actual deflections for each case, given in Table

4.3, show that the deflection difference between the two basins is about

0.23 E-3 in., or about 2.2 percent of the plate deflection. This effect

is relatively insignificant, especially considering that the accuracy of

our measurements was somewhere in this range.

An elastic layer program run was made to evaluate the accuracy of

the AXSYM solution for this two-layer case. The elastic layer program

run gave a deflection basin identical to the AXSYM flexible plate run

with wall at 7 ft. The basin from the AXSYM rigid plate run with wall

at 7 ft. was also identical to these basins outside of the loaded

area. Figure 4.5 shows the deflection basins plotted for these three

runs. Note that the rigid plate deflections seem low relative to the

flexible plate, again showing AXSYM's inability to handle the high

stress gradients induced at the edge of the plate. There is also some

discrepancy under the load between the AXSYM flexible plate solution and

the elastic layer solution, but this is small. In general, it seemed

that the AXSYM solution was accurate and could be used to evaluate the

wall effects on a relative basis.

Therefore, the following conclusions were made concerning the

effect of the test pit constraints on the limerock base response:

the floor effect is significant and must be accounted for, but

the effect is independent of limerock base modulus; and

the wall effect is insignificant and can be ignored.






















6 1


DISTANCE FROM LOAD CENTER (INS.)

18 24 30


AXSYM., Rigid Plate, Wall @ 4 ft., Full Friction

AXSYM., Rigid Plate, Wall @ 7 ft., No Friction



Limerock: Modulus = 90,000 psi, d = 63/4", V= 0.40

Sand: Modulus = 14,530 psi, d = 36", V= 0.30

Underlain by Rigid Layer


Figure 4.4: Effect of Test Pit Walls on Limerock Base Response


LI
z

z 5E.
0

0
w
Uj
-j
u.
u 10E.
0
w

B
15
0 5E
0r
iSE














Table 4.3: Predicted Deflections Using AXSYM


Deflections (E-3 in.)
Position Distance From Load Center

of Wall 0.0 6.0 7.5 9.0 12.0 16.0 20.0 24.0 30.0 36.0 42.0 48.0

4 ft. (NF)* 10.19 10.17 8.06 7.09 5.46 3.82 2.61 1.74 0.90 0.41 0.17 0.10

A-4 ft. (FF)* 10.16 10.14 8.03 7.06 5.43 3.78 2.56 1.68 0.82 0.32 0.08 0.0

B-7 ft. (NF)* 10.38 10.36 8.25 7.27 5.65 4.01 2.79 1.92 1.06 0.54 0.25 0.08

B A 0.22 0.22 0.22 0.23 0.22 0.23 0.23 0.24 0.24 0.22 0.18 0.08

* F No Friction
FF Full Friction




University of Florida Home Page
© 2004 - 2010 University of Florida George A. Smathers Libraries.
All rights reserved.

Acceptable Use, Copyright, and Disclaimer Statement
Last updated October 10, 2010 - - mvs