COMPARISON OF ALTERNATIVE TEST METHODS FOR PREDICTING ASPHALT CONCRETE RUT PERFORMANCE Prepared By: Curtis Berthelot, Assistant Professor Bill Crockford, President Robert Lytton, Professor Presented in: Canadian Technical Asphalt Association Proceedings 44 th Annual Conference Vol XLVL, pp 405-434 1999 ACKNOWLEDGMENTS The authors would like to acknowledge the assistance provided by Ms. Stella White P.Eng. of Saskatchewan Highways and Transportation. In addition, the participation and support provided by Pounder Emulsions Division of Husky Asphalt Marketing, Imperial Oil, Koch Materials, Moose Jaw Asphalt, and the National Science and Engineering Research Council is greatly appreciated.
Transcript
COMPARISON OF ALTERNATIVE TEST METHODS FOR PREDICTING ASPHALT CONCRETE RUT PERFORMANCE
Prepared By:
Curtis Berthelot, Assistant Professor
Bill Crockford, President
Robert Lytton, Professor
Presented in: Canadian Technical Asphalt Association Proceedings
44th Annual Conference Vol XLVL, pp 405-434
1999
ACKNOWLEDGMENTS
The authors would like to acknowledge the assistance provided by Ms. Stella White P.Eng. of Saskatchewan Highways and Transportation. In addition, the participation and support provided by Pounder Emulsions Division of Husky Asphalt Marketing, Imperial Oil, Koch Materials, Moose Jaw Asphalt, and the National Science and Engineering Research Council is greatly appreciated.
BERTHELOT, CROCKFORD & LYTTON 1
ABSTRACT This study sought to investigate methods for characterizing the rutting behavior of asphalt concrete mixes. Seven Specific Pavement Studies-9A (SPS-9A) asphalt concrete mixes (two Marshall and five SuperpaveTM) were characterized with respect to Marshall stability and flow, Hveem stability, unconfined compressive strength, SHRP Level III shear tester, and triaxial frequency sweep properties. Traditional phenomenological asphalt concrete characterization methods distinguished some significant differences between the Radisson SPS-9A asphalt concrete mixes. However, they did not provide material constitutive relations necessary for mechanistic road response modeling. The SHRP shear tester also distinguished some significant differences between the asphalt concrete mixes and can provide material constitutive relations. However, the SHRP shear tester is expensive to own and operate, produces relatively high variability, and was found to be impractical for use by the road industry. The rapid triaxial tester determined the most significant difference between the asphalt concrete mixes and was found to be an efficient mechanistic testing apparatus complementary to the SHRP gyratory compactor. Its results also best correlated with the relative rutting behavior of the SPS-9A test sections after three years of service. The complex properties measured by the rapid triaxial tester could be mathematically transformed into linear viscoelastic prony series that could be used for viscoelastic mechanistic road modeling.
BERTHELOT, CROCKFORD & LYTTON 2
1.0 BACKGROUND Asphalt concrete rut prediction has traditionally been based upon purely-empirical and phenomenological-empirical materials testing and road modeling techniques. However, changing traffic types, sizes, and volumes;, advanced road materials;, and the aging diversity of the road network are beyond the inference space of traditional empirical and phenomenological-empirical rut prediction models. As a result, these traditional models can instill false confidence in rut predictions that are not commensurate with actual rutting observed in the field. To overcome these limitations, road researchers are developing mechanistic-empirical road modeling techniques [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. Early mechanistic-empirical road models employed elastic constitutive theory [15, 16, 17, 18]. Although mechanistic based, elasticity theory assumes no induced damage or healing during loading and unloading. These assumptions are known to be incorrect for road materials based upon observations of road deterioration and measurements of dissipated energy during loading and unloading, both in the lab and in the field. Recent advances in inelastic road modeling and improved computational capabilities now enable the damage and healing behavior of asphalt concrete to be directly characterized and encoded into rut prediction models [19, 20, 21, 22, 23, 24]. Although inelastic material constitutive behavior is theoretically more appropriate for modeling roads, it introduces increased road-modeling complexities and requires more advanced material characterization methods. 2.0 OBJECTIVE The objective of this study was to evaluate alternative asphalt concrete characterization methods that can be used for asphalt concrete specifications and rut performance modeling. 3.0 SCOPE This study includes an evaluation of alternative rut characterization methods including Marshall stability and flow, Hveem stability, unconfined compressive strength, SHRP Level III shear tester, and triaxial frequency sweep properties. Seven Radisson Specific Pavement Studies-9A (SPS-9A) asphalt concrete mixes (two Marshall and five SuperpaveTM) were used as the field test sections for this study. The Radisson SPS-9A test site layout and pavement structure cross section are illustrated in Figure 1 and the respective asphalt cement binder grades are summarized in Table 1. The characterization results obtained from the alternative testing methods across the seven Radisson SPS-9A asphalt concrete mixes were compared with respect to: repeatability, ability to distinguish significant differences between the Radisson SPS-9A asphalt concrete mixes, ability to correlate with observed rutting in the field, and the suitability for the road industry for material specifications and road modeling purposes.
Marshall stability and flow testing of asphalt concrete has been widely used by the road industry and is standardized in ASTM D 1560 and AASHTO T245. Marshall stability and flow measurements were taken on triplicate gyratory compacted samples at seven percent air voids from each Radisson SPS-9A asphalt concrete mix. Table 2 summarizes the Marshall stability measurements and Table 3 summarizes the Marshall flow measurements of the Radisson SPS-9A asphalt concrete mixes.
Table 2 Marshall Stability Measurements and Duncan's Pairwise Comparison of Radisson Specific Pavement Studies-9A Asphalt Concrete Mixes
SPS-9A Test Section
Mean Marshall Stability
(N)
Marshall Stability
Coefficient of Variation (Percent)
Duncan's Grouping*
900901 3958 10.84 C 900902 7562 6.55 A 900903 5115 15.72 B C 900959 7176 13.02 A 900960 5975 8.63 B 900961 4685 14.22 C 900962 7828 9.30 A Mean 6042 11.18
* Asphalt concrete mixes with same letter are not significantly different.
Table 3 Marshall Flow Measurements and Duncan's Pairwise Comparison of Radisson Specific Pavement Studies9A Asphalt Concrete Mixes
SPS-9A Test Section
Mean Marshall Flow
(0.25 mm Increments)
Marshall Flow Coefficient of
Variation (Percent)
Duncan's Grouping*
900901 12.0 8.33 C 900902 14.3 4.03 B C 900903 14.3 4.03 B C 900959 16.3 18.70 A B 900960 14.3 4.03 B C 900961 12.0 8.33 C 900962 18.7 17.22 A Mean 14.6 9.23
* Asphalt concrete mixes with same letter are not significantly different. A statistical analysis was performed at a 95 percent confidence level to evaluate the significance of the Marshall stability and flow measurements of the Radisson SPS-9A asphalt concrete mixes. A one-way analysis of variance concluded that Radisson SPS-9A asphalt concrete mix type had a significant effect on the mean Marshall stability measurements. Duncan's pairwise comparison (25) determined that the mean Marshall stability measurements of mixes 900902, 900959, and 900962 were not significantly
BERTHELOT, CROCKFORD & LYTTON 5
different; mixes 900903 and 900960 were not significantly different; and mixes 900901, 900903, and 900961 were not significantly different. A one-way analysis of variance concluded that Radisson SPS-9A asphalt concrete mix type had a significant effect on the mean Marshall flow measurements. Duncan's pairwise comparison determined that the mean Marshall flow of mixes 900959 and 900962 were not significantly different; mixes 900902, 900903, 900959, and 900960 were not significantly different; and mixes 900901, 900902, 900903, 900960, and 900961 were not significantly different. The Marshall stabilometer is widely used throughout the road industry due to its simplicity, cost effectiveness, repeatability, and concomitant empirical experience. The Marshall stabilometer was found to distinguish some significant differences between the Radisson SPS-9A asphalt concrete mixes. Limitations of the Marshall stabilometer include: it is unable to provide feedback controlled on-sample multi-axial mechanistic measurements; it specifies a small sample size (62.5 mm by 101 mm); it applies boundary tractions under curvilinear platens resulting in nonlinear stress-strain fields; it does not have the ability to apply confinement tractions to the sample; and it is designed for testing asphalt concrete at 60°C at a specified displacement rate of 1.3 mm/min. As a result, the Marshall stabilometer cannot be used to characterize the fundamental thermomechanical constitutive behavior of asphalt concrete across stress and strain states, stress and strain rates, and temperatures representative of those experienced in the field as required for mechanistic road-modeling. The Marshall stabilometer was found not to provide proper ranking of the Radisson SPS-9A asphalt concrete mixes concomitant to field rutting performance after three years of service. 4.2 Hveem Stability Characterization
Hveem stability characterization of asphalt concrete has been widely used by the road industry and is specified in ASTM D1561 and AASHTO T246. Hveem stability testing was performed on triplicate gyratory compacted samples at seven percent air voids from each Radisson SPS-9A asphalt concrete mix. Table 4 summarizes the Hveem stability measurements across the Radisson SPS-9A asphalt concrete mixes. A statistical analysis was performed at a 95 percent confidence level to evaluate the significance of the Hveem stability measurements across the Radisson SPS-9A asphalt concrete mixes. A one-way analysis of variance concluded that Radisson SPS-9A asphalt concrete mix type had a significant effect on the mean Hveem stability measurements. Duncan's pairwise comparison determined that the mean Hveem stability of mix 900902 was significantly different from all other mixes; however, mixes 900960 and 900962 were not significantly different; mixes 900903, 900959, and 900961 were not significantly different; and mixes 900901 and 900961 were not significantly different. The Hveem stabilometer was found to distinguish some significant differences between the Radisson SPS-9A asphalt concrete mixes and produced a low mean coefficient of variation. A drawback to the Hveem stabilometer is it requires a high capacity load frame, which increases the owning and operating costs and renders it impractical for field testing. In addition, like the Marshall stabilometer, the Hveem stabilometer does not provide feedback controlled on sample multi-axial mechanistic measurements and it specifies a small sample size (62.5 mm diameter by 101 mm tall). As a result, the Hveem stabilometer cannot be used to characterize the fundamental thermomechanical constitutive behavior of asphalt concrete across stress and strain states, stress and strain rates, and temperatures representative of those experienced in the field as required for mechanistic road-modeling. The Hveem stabilometer was found
BERTHELOT, CROCKFORD & LYTTON 6
not to provide proper ranking of the Radisson SPS-9A asphalt concrete mixes concomitant to field rutting performance after three years of service. Table 4 Hveem Stability Measurements and Duncan's Pairwise Comparison of Radisson Specific
Pavement Studies-9A Asphalt Concrete Mixes SPS-9A Test
Section Mean Hveem
Stability Hveem
Stability Coefficient of
Variation (Percent)
Duncan's Grouping*
900901 28.61 1.94 D 900902 40.02 1.44 A 900903 32.53 3.32 C 900959 30.95 7.38 C 900960 34.78 3.71 B 900961 30.80 2.70 C D 900962 35.32 3.68 B Mean 33.29 3.45
* Asphalt concrete mixes with same letter are not significantly different. 4.3 Unconfined Compressive Strength Characterization
Unconfined compressive strength testing of asphalt concrete is standardized in ASTM D1074 and AASHTO T167. Unconfined compressive strength testing was performed on triplicate gyratory compacted samples at seven percent air voids from each Radisson SPS-9A asphalt concrete mix at 5°C, 20°C, 40°C, 70°C, and 100°C. Tables 5 and 6 summarize the mean unconfined compressive strength measurements across the Radisson SPS-9A asphalt concrete mixes and test temperatures respectively. A statistical analysis was performed at a 95 percent confidence level to evaluate the significance of the unconfined compressive strength measurements across the Radisson SPS-9A asphalt concrete mixes and test temperatures employed in the analysis. A two-way analysis of variance concluded that Radisson SPS-9A asphalt concrete mix type, test temperature and the interaction effects of mix type and test temperature had a significant effect on the mean unconfined compressive strength measurements.. Duncan's pairwise comparison determined that the mean unconfined compressive strength of mixes 900959 and 900962 were significantly different from all other mixes; however, mixes 900901, 900902, 900903, and 900960 were not significantly different; and mixes 900901, 900902, 900903 and 900961 were not significantly different. Duncan's pairwise comparison across the test temperatures grouped by Radisson SPS-9A asphalt concrete mix determined that the mean unconfined compressive strengths at 5°C, 20°C, and 40°C were significantly different from those at all other temperatures; however, there was no significant difference between 70°C and 100°C.
BERTHELOT, CROCKFORD & LYTTON 7
Table 5 Unconfined Compressive Strength Measurements and Duncan's Pairwise Comparison of Radisson Specific Pavement Studies-9A Asphalt Concrete Mixes Grouped by Test Temperature
SPS-9A Test Section
Mean Unconfined
Compressive Strength
(kPa)
Mean Unconfined
Compressive Strength
Coefficient of Variation (Percent)
Duncan's Grouping*
900901 1725 17.66 B C 900902 1679 11.57 B C 900903 1726 16.16 B C 900959 2576 5.40 A 900960 1874 7.89 B 900961 1589 13.10 C 900962 1222 9.60 D Mean 1770 11.31
* Asphalt concrete mixes with same letter are not significantly different.
Table 6 Unconfined Compressive Strength Measurements and Duncan's Pairwise Comparison across Test Temperatures Grouped by Radisson Specific Pavement Studies -9A Asphalt Concrete
Mix Test
Temperature Mean
Unconfined Compressive
Strength (kPa)
Mean Unconfined
Compressive Strength
Coefficient of Variation (Percent)
Duncan's Grouping*
5°C 6134 10.90 A 20°C 1601 6.37 B 40°C 713 11.46 C 70°C 298 14.51 D
100°C 106 13.31 D Mean 1770 11.31
*Test temperatures with same letter are not significantly different. Unconfined compressive strength testing was found to distinguish some significant differences between the Radisson SPS-9A asphalt concrete mixes. However, because confinement tractions are not applied to the sample and the compressive tractions required to fail the sample are significantly greater than those typically experienced under field loading conditions, the stress state generated during the unconfined compressive strength test does not accurately represent the true stress state experienced under field loading conditions. Unconfined compressive strength testing also requires a heavy capacity load frame, which increases owning and operating costs and renders it impractical for field testing. Like the Marshall and Hveem stabilometers, unconfined compressive strength testing does not provide feedback control on sample multi-axial measurements and therefore cannot characterize the thermomechanical constitutive behavior of asphalt concrete across stress and strain states, stress and strain rates, and temperatures representative of those experienced in the field as required for mechanistic road-modeling. The
BERTHELOT, CROCKFORD & LYTTON 8
unconfined compressive strength test was found not to provide proper ranking of the Radisson SPS-9A asphalt concrete mixes concomitant to field rutting performance after three years of service. 4.4 SHRP Shear Tester Characterization
The Strategic Highway Research Program developed a prototype suite of tests to characterize the thermomechanical behavior of asphalt concrete mixes and they are specified in AASHTO TP7-94 (25). The SHRP suite of shear tests include: volumetric-hydrostatic test, uniaxial strain at constant radius test, simple shear at constant height test, frequency sweep shear at constant height test, and repeated shear at constant stress ratio test. The repeated shear at constant height is specified in AASHTO TP7-94 (25) as optional procedure F. However, it was not performed in this study. Asphalt concrete samples used for the SHRP Level III shear tester characterization were prepared using SHRP specified gyratory compaction protocols as outlined in AASHTO TP4 (25). Figure 2 shows an asphalt concrete sample mounted in the SHRP shear tester for the volumetric-hydrostatic and uniaxial strain at constant radius tests. Figure 3 shows an asphalt concrete sample mounted in the SHRP shear tester for the simple shear at constant height, frequency sweep shear at constant height and repeated shear at constant stress ratio tests.
The SHRP volumetric-hydrostatic test evaluates the bulk elastic properties of asphalt concrete under different states of hydrostatic stress and temperature. SHRP volumetric-hydrostatic testing was performed on triplicate gyratory compacted samples at seven percent air voids from each Radisson SPS-9A asphalt concrete mix at test temperatures of 4°C, 20°C, and 40°C, and corresponding peak hydrostatic tractions of 830 kPa, 690 kPa, and 550 kPa, respectively. Tables 7 and 8 summarize the SHRP bulk modulus measurements across the Radisson SPS-9A asphalt concrete mixes and stress states-test temperatures considered in this analysis respectively. A statistical analysis was performed at a 95 percent confidence level to evaluate the significance of the SHRP bulk modulus measurements across the Radisson SPS-9A asphalt concrete mixes and stress states-test temperatures employed in the analysis. A two-way analysis of variance concluded that Radisson SPS-9A asphalt concrete mix type had a significant effect on the mean SHRP bulk moduli measurements. However, test temperature-stress state, and the interaction effects between Radisson SPS-9A asphalt concrete mix type and test temperature-stress state did not have a significant effect on the mean SHRP bulk measurements. Duncan's pair wise comparison of the mean SHRP bulk modulus of the Radisson SPS-9A asphalt concrete mixes grouped by test temperature-stress state determined that: mixes 900901, 900902, 900903, 900960, and 900962 were not significantly different; mixes 900901, 900902, 900903, 900959, and 900962 were not significantly different; and mixes 900901, 900903, 900959, 900961, and 900962 were not significantly different. Duncan's pairwise comparison of the mean SHRP bulk modulus across test temperatures-stress states grouped by Radisson SPS-9A asphalt concrete mix determined that
BERTHELOT, CROCKFORD & LYTTON 10
no significant difference existed between the mean SHRP bulk modulus measurements at 4°C (830 kPa), 20°C (690 kPa), and 40°C (550 kPa).
Table 7 SHRP Bulk Modulus Measurements and Duncan's Pairwise Comparison of Radisson Specific Pavement Studies-9A Asphalt Concrete Mixes Grouped by Test Temperature-Stress State
SPS-9A Test Section
Mean SHRP Bulk Modulus
(MPa)
Mean SHRP Bulk Modulus Coefficient of
Variation (Percent)
Duncan's Grouping*
900901 256.091 70.93 A B C 900902 409.220 67.79 A B 900903 374.032 53.25 A B C 900959 205.111 25.80 B C 900960 450.165 48.23 A 900961 185.729 43.39 C 900962 363.446 27.46 A B C Mean 320.542 48.12
* Asphalt concrete mixes with same letter are not significantly different.
Table 8 SHRP Bulk Modulus Measurements and Duncan's Pairwise Comparison of Test Temperatures-Stress States Grouped by Radisson Special Pavement Studies -9A Asphalt Concrete
Mix Test
Temperature (Hydrostatic
Traction)
Mean SHRP Bulk Modulus
(MPa)
Mean SHRP Bulk Modulus Coefficient of
Variation (Percent)
Duncan's Grouping*
4°C (830 kPa) 347.698 55.07 A 20°C (690 kPa) 273.068 43.66 A 40°C (550 kPa) 339.899 45.63 A
Mean 320.542 48.12 *Test temperatures-stress states with same letter are not significantly different. 4.4.2 SHRP Uniaxial Strain at Constant Radius Characterization
The SHRP uniaxial strain at constant radius test employs the SHRP shear tester to characterize the elastic and plastic behavior of asphalt concrete under constrained uniaxial loading. SHRP uniaxial strain at constant radius testing was performed on triplicate gyratory compacted samples at seven percent air voids from each Radisson SPS-9A asphalt concrete mix at test temperatures of 4°C, 20°C, and 40°C, and corresponding uniaxial tractions of 655 kPa, 550 kPa, and 345 kPa, respectively. Tables 9 and 10 summarize the SHRP uniaxial constrained modulus measurements across the Radisson SPS-9A asphalt concrete mixes and stress states-test temperatures considered in this analysis respectively. A statistical analysis was performed at a 95 percent confidence level to evaluate the significance of the SHRP uniaxial constrained compression modulus measurements across the Radisson SPS-9A asphalt concrete mixes and test temperatures-stress states employed in the analysis. A two-way analysis of
BERTHELOT, CROCKFORD & LYTTON 11
variance concluded that Radisson SPS-9A asphalt concrete mix type had a significant effect on the the mean SHRP uniaxial constrained compression modulus measurements. However, test temperature-stress state and the interaction effects of Radisson SPS-9A asphalt concrete mix type and test temperature-stress state had no significant effect on the mean SHRP uniaxial compression moduli measurements.. Duncan's pairwise comparison of the mean SHRP uniaxial constrained compression modulus of the Radisson SPS-9A asphalt concrete mixes grouped by test temperature-stress state determined that: mixes 900903, 900959, and 900961 were not significantly different; mixes 900902, 900903, 900960, and 900962 were not significantly different; and mixes 900901, 900902, 900960, and 900962 were not significantly different. Duncan's pairwise comparison of the mean SHRP uniaxial constrained compression modulus across the test temperatures-stress states grouped by Radisson SPS-9A asphalt concrete mix determined that no significant difference existed between the mean SHRP uniaxial constrained compression modulus measurements at 4°C (655 kPa), 20°C (550 kPa), and 40°C (345 kPa). Table 9 SHRP Uniaxial Constrained Compression Modulus Measurements and Duncan’s Pairwise Comparison of Radisson Specific Pavement Studies -9A Asphalt Concrete Mixes Grouped by Test
Temperature-Stress State SPS-9A Test
Section Mean SHRP
Uniaxial Constrained Compression
Modulus (MPa)
Mean SHRP Uniaxial
Constrained Compression
Modulus Coefficient of
Variation (Percent)
Duncan's Grouping*
900901 73.570 31.05 C 900902 108.728 58.20 B C 900903 156.457 37.73 A B 900959 209.890 37.22 A 900960 112.784 18.42 B C 900961 220.411 22.69 A 900962 131.344 57.83 B C Mean 144.741 37.59
* Asphalt concrete mixes with same letter are not significantly different.
Table 10 SHRP Uniaxial Constrained Compression Modulus Measurements and Duncan’s Pairwise Comparison of Test Temperatures-Stress States Grouped by Radisson Specific Pavement
Studies -9A Asphalt Concrete Mix Test
Temperature (Uniaxial Traction)
Mean SHRP Uniaxial
Constrained Modulus
(MPa)
Mean SHRP Uniaxial
Constrained Modulus
Coefficient of Variation (Percent)
Duncan's Grouping*
4°C (655 kPa) 159.235 43.49 A 20°C (550 kPa) 139.528 33.44 A 40°C (345 kPa) 135.460 35.84 A
Mean 144.741 37.59 *Test temperatures-stress states with same letter are not significantly different.
BERTHELOT, CROCKFORD & LYTTON 12
4.4.3 SHRP Simple Shear at Constant Height Characterization
The SHRP simple shear at constant height test employs the SHRP shear tester to characterize the elastic, viscoelastic, and plastic behavior of asphalt concrete mixes under states of mixed mode shear and uniaxial creep loading. SHRP simple shear at constant height testing was performed on triplicate gyratory compacted samples at seven percent air voids from each Radisson SPS-9A asphalt concrete mix at test temperatures of 4°C, 20°C, and 40°C, and corresponding SHRP simple shear tractions of 345 kPa, 105 kPa, and 35 kPa, respectively. Tables 11 and 12 summarize the total SHRP simple shear strain measurements across the Radisson SPS-9A asphalt concrete mixes and stress states-test temperatures considered in this analysis respectively. A statistical analysis was performed at a 95 percent confidence level to evaluate the significance of the total SHRP simple shear strain measurements across the Radisson SPS-9A asphalt concrete mixes and test temperatures-stress states employed in the analysis. A two-way analysis of variance concluded that Radisson SPS-9A asphalt concrete mix type, test temperature-stress state, and the interaction effects between Radisson SPS-9A asphalt concrete mix type and test temperature-stress state had a significant effect on the mean total SHRP simple shear strain measurements. Duncan's pairwise comparison of the mean total SHRP simple shear strain across the Radisson SPS-9A asphalt concrete mixes grouped by test temperature-stress state determined that: mixes 900961 and 900962 were significantly different from all other mixes; however, mixes 900901, 900903, and 900960 were not significantly different; and mixes 900901, 900959, 900960, and 900962 were not significantly different. Duncan's pairwise comparison across the test temperatures-stress states grouped by Radisson SPS-9A asphalt concrete mix determined that significant difference existed between the mean total SHRP simple shear strain measurements at 5°C (345 kPa), 20°C (105 kPa) and 40°C (35 kPa).
Table 11 Total SHRP Simple Shear Strain Measurements and Duncan’s Pairwise Comparison
(reference) of Radisson Specific Pavement Studies -9A Asphalt Concrete Mixes Grouped by Test Temperature-Stress State
SPS-9A Test Section
Mean Total SHRP Simple Shear Strain
(mm/mm)
Mean Total SHRP Simple Shear Strain Coefficient of
Variation (Percent)
Duncan's Grouping*
900901 0.007145 6.21 B C 900902 0.002038 55.49 D 900903 0.008073 20.44 B 900959 0.005119 31.53 C 900960 0.006430 15.87 B C 900961 0.011165 22.63 A 900962 0.006000 31.96 C Mean 0.006567 26.30
* Asphalt concrete mixes with same letter are not significantly different.
BERTHELOT, CROCKFORD & LYTTON 13
Table 12 Total SHRP Simple Shear Strain Measurements and Duncan’s Pairwise Comparison of Test Temperatures-Stress States Grouped by Radisson Specific Pavement Studies -9A Asphalt
Concrete Mix Test
Temperature (Simple Shear
Traction)
Mean Total SHRP Simple Shear Strain
(mm/mm)
Mean Total Simple SHRP Shear Strain Coefficient of
Variation (Percent)
Duncan's Grouping*
4°C (345 kPa) 0.004491 18.50 A 20°C (105 Pa) 0.006959 29.68 B
40°C (35 kPa) 0.008251 30.73 C Mean 0.006567 26.30
* Test temperatures-stress states with same letter are not significantly different.
4.4.4 SHRP Frequency Sweep Shear at Constant Height Characterization
The SHRP frequency sweep shear at constant height test employs the SHRP shear tester to evaluate the complex shear behavior of asphalt concrete. SHRP frequency sweep shear testing was performed on triplicate gyratory compacted samples at seven percent air voids from each Radisson SPS-9A asphalt concrete mix at 4°C, 20°C, and 40°C, respectively. Since the Radisson SPS-9A test site is located on Highway 16 where the design traffic loading is a truck tire traveling at a posted speed limit of 100 kilometers per hour, detailed analysis of the SHRP frequency sweep shear data was restricted to 10 Hz cyclic frequency. Tables 13 and 14 summarize the complex SHRP shear modulus measurements at 10 Hz cyclic frequency across the Radisson SPS-9A asphalt concrete mixes and test temperatures respectively. A statistical analysis was performed at a 95 percent confidence level to evaluate the significance of the complex SHRP shear modulus measurements at 10 Hz cyclic frequency across the Radisson SPS-9A asphalt concrete mixes and test temperatures employed in the analysis. A two-way analysis of variance concluded that Radisson SPS-9A asphalt concrete mix type and test temperature had a significant effect on the mean complex SHRP shear modulus at 10 Hz cyclic frequency. However, the interaction effects between Radisson SPS-9A asphalt concrete mix type and test temperature was found not to have a significant effect on the SHRP complex shear modulus measurements at 10 Hz cyclic frequency. Duncan's pairwise comparison of the mean complex SHRP shear modulus measurements at 10 Hz cyclic frequency of the Radisson SPS-9A asphalt concrete mixes grouped by test temperature determined that: mix 900902 was significantly different from all other mixes; however, mixes 900901, 900903, 900959, 900960, 900961, and 900962 were not significantly different. Duncan's pairwise comparison of the mean complex SHRP shear modulus at 10 Hz cyclic frequency across the test temperatures grouped by Radisson SPS-9A asphalt concrete mix determined that significant difference existed between the mean complex SHRP shear modulus measurements at 4°C, 20°C, and 40°C.
BERTHELOT, CROCKFORD & LYTTON 14
Table 13 Complex SHRP Shear Modulus Measurements and Duncan’s Pairwise Comparison at 10 Hz Cyclic Frequency of Radisson Specific Pavement Studies-9A Asphalt Concrete Mixes Grouped
by Test Temperature SPS-9A Test
Section Mean Complex
SHRP Shear Modulus
(MPa)
Mean Complex SHRP Shear
Modulus Coefficient of
Variation (Percent)
Duncan's Grouping*
900901 827.791 8.39 B 900902 1491.639 55.21 A 900903 855.529 11.15 B 900959 1029.969 24.37 B 900960 798.400 11.26 B 900961 655.615 31.59 B 900962 681.342 8.38 B Mean 905.755 21.48
*Asphalt concrete mixes with same letter are not significantly different. Table 14 Complex SHRP Shear Modulus Measurements and Duncan’s Pairwise Comparison at 10 Hz Cyclic Frequency across Test Temperatures Grouped by Radisson Specific Pavement Studies-
9A Asphalt Concrete Mix Test
Temperature Mean Complex
SHRP Shear Modulus
(MPa)
Mean Complex SHRP Shear
Modulus Coefficient of
Variation (Percent)
Duncan's Grouping*
4°C 1981.779 15.56 A 20°C 526.286 14.97 B 40°C 209.170 33.90 C Mean 905.745 21.48
* Test temperatures with same letter are not significantly different. Tables 15 and 16 summarize the mean complex SHRP shear phase angle measurements at 10 Hz cyclic frequency across the Radisson SPS-9A asphalt concrete mixes and test temperatures, respectively. A statistical analysis was performed at a 95 percent confidence level to evaluate the statistical significance of the mean SHRP shear phase angle measurements at 10 Hz cyclic frequency across the Radisson SPS-9A asphalt concrete mixes and test temperatures employed in the analysis. A two-way analysis of variance concluded that Radisson SPS-9A asphalt concrete mix type and test temperature had a significant effect on the mean SHRP shear phase angle measurements. However, the interaction effects between Radisson SPS-9A asphalt concrete mix type and test temperature was found not to have a significant effect on the mean SHRP shear phase angle measurements. Duncan's pairwise comparison of the mean SHRP shear phase angle measurements at 10 Hz cyclic frequency for each Radisson SPS-9A asphalt concrete mix grouped by test temperature determined that: mixes 900901, 900903, 900959, 900960, and 900961 were not significantly different; and mixes 900902 and 900962 were not significantly different. Duncan's pairwise comparison of the mean SHRP shear phase angle measurements at 10 Hz cyclic frequency across test temperatures grouped by Radisson SPS-9A asphalt
BERTHELOT, CROCKFORD & LYTTON 15
concrete mix determined that significant difference existed between the mean SHRP shear phase angle measurements at 4°C, 20°C, and 40°C. Table 15 Mean SHRP Shear Phase Angle Measurements and Duncan’s Pairwise Comparison at 10 Hz Cyclic Frequency of Radisson Specific Pavement Studies -9A Asphalt Concrete Mixes Grouped
by Test Temperature SPS-9A Test
Section Mean SHRP Shear Phase
Angle (Degrees)
Mean SHRP Shear Phase
Angle Coefficient of
Variation (Percent)
Duncan's Grouping*
900901 50.99 3.02 A 900902 44.58 9.22 B 900903 52.27 4.43 A 900959 49.76 9.74 A 900960 51.27 5.24 A 900961 51.58 3.21 A 900962 45.57 2.21 B Mean 49.43 5.30
*Asphalt concrete mixes with same letter are not significantly different.
Table 16 Mean SHRP Shear Phase Angle Measurements and Duncan’s Pairwise Comparison at 10 Hz Cyclic Frequency across Test Temperatures Grouped by Radisson Specific Pavement Studies -
9A Asphalt Concrete Mix Test
Temperature Mean SHRP Shear Phase
Angle (Degrees)
Mean SHRP Shear Phase
Angle Coefficient of
Variation (Percent)
Duncan's Grouping*
4°C 33.97 4.19 A 20°C 53.99 3.88 B 40°C 60.33 7.82 C Mean 49.43 5.30
* Test temperatures with same letter are not significantly different. 4.4.5 SHRP Repeated Shear at Constant Stress Ratio Characterization
The SHRP repeated shear at constant stress ratio test employs the SHRP shear tester to characterize the relative potential for asphalt concrete mixes to exhibit tertiary creep under repeated loads. SHRP repeated shear at constant stress ratio characterization was performed at the permanent deformation effective temperature (TeffectivePD) of 40°C at 98 percent reliability. SHRP repeated shear at constant stress ratio testing was performed on triplicate gyratory compacted samples at seven percent air voids and was continued for 20,000 load cycles or until the accumulated strain reached five percent. Figure 4 illustrates the cumulative SHRP shear strain measurements across the Radisson SPS-9A asphalt concrete mixes. As seen in Figure 4, none of the Radisson SPS-9A asphalt concrete mixes exhibited tertiary creep at the end of 20,000 load cycles.
BERTHELOT, CROCKFORD & LYTTON 16
Table 17 summarizes the cumulative repeated shear strain at 20,000 load cycles of the Radisson SPS-9A asphalt concrete mixes. A statistical analysis was performed at a 95 percent confidence level to evaluate the significance of the cumulative shear strain measurements at 20,000 load cycles across the Radisson SPS-9A asphalt concrete mixes. A one-way analysis of variance concluded that Radisson SPS-9A asphalt concrete mix type had a significant effect on the mean cumulative shear strain measurements at 20,000 load cycles. Duncan's pairwise comparison determined that the mean cumulative shear strain measurements at 20,000 load cycles of mix 900961 was significantly different from all other mixes; however, mixes 900901, 900903, 900959, and 900960 were not significantly different; mixes 900903, 900959, 900960, and 900962 were not significantly different; and mixes 900902, 900959, 900960, and 900962 were not significantly different.
Figure 4 Cumulative SHRP Repeated Shear Strain versus Number of Load Cycles of Radisson Specific Pavement Studies -9A Asphalt Concrete Mixes
BERTHELOT, CROCKFORD & LYTTON 17
Table 17 SHRP Repeated Shear Measurements and Duncan's Pairwise Comparison of Cumulative Shear Strain at 20,000 Load Cycles across Radisson Specific Pavement Studies -9A Asphalt
Concrete Mixes SPS-9A Test
Section Mean SHRP
Repeated Shear at CSR
Cumulative Strain
(mm/mm)
SHRP Repeated Shear
at CSR Cumulative
Strain Coefficient of
Variation (Percent)
Duncan's Grouping*
900901 0.028803 22.01 B 900902 0.002730 67.35 D 900903 0.021173 34.84 B C 900959 0.015782 27.81 B C D 900960 0.014654 33.01 B C D 900961 0.046602 42.95 A 900962 0.009083 14.59 C D Mean 0.019832 34.65
* Asphalt concrete mixes with same letter are not significantly different. CSR = Constant Stress Ratio Although the SHRP shear tester is a highly capable characterization apparatus designed to independently control axial, confinement, and shear tractions, several limitations were found to exist in the SHRP shear tester and the SuperpaveTM shear testing protocols as specified in AASHTO TP7. The coefficient of variation of the SHRP shear tester measurements was significantly higher than the phenomenological and rapid triaxial test methods. The SHRP shear tester is expensive to own and operate, and the shear testing protocols are quite sophisticated, rendering it impractical for use by most public road authorities, road contractors, road consultants, and materials suppliers. In addition, sample preparation requires sample saw cutting and gluing end platens which is time consuming and not conducive to production testing. This research also found problems in regards to several samples failing during SHRP frequency sweep shear testing as illustrated in Figure 5. Another limitation to the SHRP Level III shear testing protocols outlined in AASHTO TP7 is the specified three unique applied tractions and concomitant test temperatures. As a result, the non-linear stress sensitivity and temperature sensitivity of asphalt concrete mixes cannot be directly and independently characterized using the current SuperpaveTM shear tester protocols.
BERTHELOT, CROCKFORD & LYTTON 18
Figure 5 Failed SHRP Shear Test Sample
4.5 Rapid Triaxial Characterization
Triaxial testing has been used by the road industry to characterize all types of road materials [26, 27]. However, drawbacks to traditional triaxial test apparatus include: limited stress state and test temperature testing capabilities; sample preparation and instrumentation is time consuming and therefore, not conducive to production testing; limited feedback control capabilities; limited dynamic testing capabilities; and direct measurement of Poisson’s ratio is difficult. To overcome these drawbacks, this study employed a new prototype full feedback controlled rapid triaxial test apparatus shown in Figure 6 to characterize the Radisson SPS-9A asphalt concrete mixes. Dynamic frequency sweep testing for characterizing road materials and is standardized in ASTM D3497. The real and imaginary components of the complex modulus may be expressed as:
"E'EE* i+= where: E* = complex stiffness modulus (Pa) E’ = real component of stiffness modulus (Pa) E” = imaginary component of stiffness modulus (Pa)
BERTHELOT, CROCKFORD & LYTTON 19
Figure 6 Rapid Triaxial Tester
The applied traction waveform and strain response of a dynamic frequency sweep test may be expressed as:
( ) tp1111 TtT ωie=
)t(ip1111 )t( φωεε −= e
where: T11(t) = time dependent boundary traction in X1 coordinate direction (Pa) T11p = peak applied boundary traction in X1 coordinate direction (Pa) = time dependent axial strain response in X)t(11ε 1 coordinate direction = peak axial strain response in Xp11ε 1 coordinate direction ω = angular load frequency (radians per second) t = load period (seconds) φ = phase angle (radians) By substitution, the complex modulus of a time dependent material may be expressed as:
)t(p11
tp11
P11
P11 T)t()t(T
*Eφω
ω
εε −==
i
i
e
e
BERTHELOT, CROCKFORD & LYTTON 20
For an elastic material, strain response is instantaneous with the applied traction, therefore, the dynamic elastic modulus may be expressed as the absolute value of the complex modulus:
p11
p11*D
TE
ε== E
In addition to axial deformations, four radial LVDT’s were used to monitor radial deformations and Poisson’s ratio was measured directly as the ratio of the lateral and uniaxial strain.
( )( )( )
( )( )tt
tt
t11
33
11
2211 ε
εεε
ν −=−=
where: = Poisson’s ratio in X11ν1 coordinate direction
= strain in X11ε1 coordinate direction
= strain in X22ε2 coordinate direction
= strain in X33ε3 coordinate direction
Triaxial frequency sweep testing was performed at three fully reversed compression-extension boundary tractions illustrated in Figure 7; test temperatures of 4°C, 20°C, 40°C, 70°C, and 100°C; and load frequencies of 0.1 Hz, 1.0 Hz, 5.0 Hz, 10.0 Hz. However, since the Radisson SPS-9A test site is located on Highway 16, detailed analysis of the triaxial frequency sweep data analysis presented herein was restricted to compressive data at stress state three and 10 Hz cyclic frequency as illustrated in Figure 7. However, it should be noted that statistical differences were found to exist across all triaxial measurements at all stress states and cyclic frequencies-. Tables 18 and 19 summarize the triaxial complex compression modulus measurements at 10 Hz cyclic frequency and stress state three across the Radisson SPS-9A asphalt concrete mixes and test temperatures respectively. A two-way analysis of variance concluded that Radisson SPS-9A asphalt concrete mix type, test temperature and the interaction effects of Radisson SPS-9A asphalt concrete mix type and test temperature had a significant effect on the mean triaxial complex compression modulus measurements at stress state three and 10 Hz cyclic frequency. Duncan's pairwise comparison of the mean triaxial complex compression modulus measurements at stress state three and 10 Hz cyclic frequency of the Radisson SPS-9A asphalt concrete mixes grouped by test temperature determined that: mixes 900959, 900901, and 900962 were significantly different from all other mixes; however, mixes 900902 and 900903 and mixes 900902, 900960, and 900961 were not significantly different. Duncan's pairwise comparison of the mean triaxial complex compression modulus at stress state three and 10 Hz cyclic frequency across the test temperatures grouped by Radisson SPS-9A asphalt concrete mix type determined that the mean triaxial compression modulus measurements at 5°C, 20°C, 40°C, 70°C, and 100°C were significantly different.
BERTHELOT, CROCKFORD & LYTTON 21
350
650
50
250
150
50
450
250
50
T11:650/50T22=T33:350
T11:450/50 T22=T33:250
T11:250/50 T22=T33:150
time (sec)
App
lied
Bou
ndar
y T
ract
ions
(kPa
)
time (sec)
time (sec)
T11:p
T22=T33:q
Stress State 1
• • •
Stress State 3
Stress State 2
• • •
• • •
Figure 7 Triaxial Compression-Extension Frequency Sweep Boundary Tractions
Table 18 Mean Triaxial Complex Compression Modulus and Duncan's Pairwise Comparison at
Stress State Three and 10 Hz Cyclic Frequency of Radisson Specific Pavement Studies -9A Asphalt Concrete Mixes Grouped by Test Temperature
SPS-9A Test Section
Mean Triaxial Complex
Compression Modulus
(MPa)
Mean Triaxial Complex
Compression Modulus
Coefficient of Variation (Percent)
Duncan's Grouping*
900901 1245.862 6.46 B 900902 1150.703 7.92 C D 900903 1178.431 4.96 C 900959 1325.216 5.97 A 900960 1121.963 5.26 D 900961 1117.473 5.17 D 900962 941.386 5.96 E Mean 1154.429 5.96
*Asphalt concrete mixes with same letter are not significantly different.
BERTHELOT, CROCKFORD & LYTTON 22
Table 19 Mean Triaxial Complex Compression Modulus and Duncan's Pairwise Comparison at Stress State Three and 10 Hz Cyclic Frequency across Test Temperatures Grouped by Radisson
Specific Pavement Studies -9A Asphalt Concrete Mix Test
Temperature Mean Triaxial
Complex Compression
Modulus (MPa)
Mean Triaxial Complex
Compression Modulus
Coefficient of Variation (Percent)
Duncan's Grouping*
5°C 2460.274 3.51 A 20°C 1938.216 4.31 B 40°C 721.486 7.70 C 70°C 376.704 5.19 D
100°C 275.466 9.07 E Mean 1154.429 5.96
*Test temperatures with same letter are not significantly different. Tables 20 and 21 summarize the triaxial compression Poisson’s ratio measurements at 10 Hz cyclic frequency and stress state three across the Radisson SPS-9A asphalt concrete mixes and test temperatures respectively. A two-way analysis of variance concluded that Radisson SPS-9A asphalt concrete mix type, test temperature and the interaction effects of Radisson SPS-9A mix type and test temperature had a significant effect on the mean triaxial compression Poisson's ratio measurements at stress state three and 10 Hz cyclic frequency. Duncan's pairwise comparison of the mean triaxial compression Poisson's ratio measurements at stress state three and 10 Hz cyclic frequency across the Radisson SPS-9A asphalt concrete mixes grouped by test temperature determined that: mixes 900901 and 900961 were not significantly different; mixes 900902, 900903, and 900960 were not significantly different; and mixes 900959 and 900962 were not significantly different. Duncan's pairwise comparison of the mean triaxial compression Poisson's ratio measurements at stress state three and 10 Hz cyclic frequency across the test temperatures grouped by Radisson SPS-9A asphalt concrete mix determined that the mean triaxial compression Poisson’s ratio measurements at 5°C, 20°C, 40°C, 70°C, and 100°C were significantly different Tables 22 and 23 summarize the triaxial phase angle measurements at 10 Hz cyclic frequency and stress state three across the Radisson SPS-9A asphalt concrete mixes and test temperatures respectively. A two-way analysis of variance concluded that Radisson SPS-9A asphalt concrete mix type, test temperature and the interaction effects of Radisson SPS-9A asphalt concrete mix type and test temperature had a significant effect on the mean triaxial phase angle measurements at stress state three and 10 Hz cyclic frequency. Duncan's pairwise comparison of the mean triaxial phase angle measurements at stress state three and 10 Hz cyclic frequency of the Radisson SPS-9A asphalt concrete mixes grouped by test temperature determined that: mixes 900961, 900901, 900960 and 900959 were significantly different from all other mixes; however, mixes 900903 and 900962 were not significantly different; and mixes 900902 and 900903 were not significantly different. Duncan's pairwise comparison of the mean triaxial phase angle measurements at stress state three and 10 Hz cyclic frequency across the test temperatures grouped by Radisson SPS-9A asphalt concrete mix determined that the mean triaxial phase angle measurements at 5°C, 20°C, 40°C, 70°C, and 100°C were significantly different.
BERTHELOT, CROCKFORD & LYTTON 23
Table 20 Mean Triaxial Compression Poisson's Ratio and Duncan's Pairwise Comparison at Stress State Three and at 10 Hz Cyclic Frequency of Radisson Specific Pavement Studies -9A Asphalt
Concrete Mixes Grouped by Test Temperature SPS-9A Test
Section Mean Triaxial Compression
Poisson's Ratio
Mean Triaxial Compression
Poisson's Ratio Coefficient of
Variation (Percent)
Duncan's Grouping*
900901 0.265 4.42 A 900902 0.224 15.01 B 900903 0.232 22.20 B 900959 0.177 23.34 C 900960 0.221 9.49 B 900961 0.267 15.37 A 900962 0.193 14.75 C Mean 0.226 14.94
* Asphalt concrete mixes with same letter are not significantly different. Table 21 Mean Triaxial Compression Poisson's Ratio and Duncan's Pairwise Comparison at Stress State Three and 10 Hz Cyclic Frequency across Test Temperatures Grouped by Radisson Specific
Pavement Studies -9A Asphalt Concrete Mix Test
Temperature Mean Triaxial Compression
Poisson's Ratio
Mean Triaxial Complex
Compression Poisson’s Ratio Coefficient of
Variation (Percent)
Duncan's Grouping*
5°C 0.045 42.74 E 20°C 0.166 9.32 D 40°C 0.303 7.51 B 70°C 0.269 6.59 C
100°C 0.344 8.55 A Mean 0.226 14.94
*Test temperatures with same letter are not significantly different.
BERTHELOT, CROCKFORD & LYTTON 24
Table 22 Mean Triaxial Phase Angle and Duncan's Pairwise Comparison at Stress State Three and at 10 Hz Cyclic Frequency of Radisson Specific Pavement Studies -9A Asphalt Concrete Mixes
Grouped by Test Temperature SPS-9A Test
Section Mean Triaxial Phase Angle
(Degrees)
Mean Triaxial Phase Angle
Coefficient of Variation (Percent)
Duncan's Grouping*
900901 16.81 6.20 B 900902 14.71 3.89 E 900903 15.02 3.45 D E 900959 14.06 3.17 F 900960 16.05 4.32 C 900961 17.76 1.87 A 900962 15.39 5.60 D Mean 15.69 4.07
* Asphalt concrete mixes with same letter are not significantly different. Table 23 Mean Triaxial Phase Angle and Duncan's Pairwise Comparison at Stress State Three and
at 10 Hz Cyclic Frequency across Test Temperatures Grouped by Radisson Specific Pavement Studies -9A Asphalt Concrete Mix
Test Temperature
Mean Triaxial Phase Angle
(Degrees)
Mean Triaxial Phase Angle
Coefficient of Variation (Percent)
Duncan's Grouping*
5°C 7.63 7.16 E 20°C 14.73 3.46 D 40°C 19.67 4.19 A 70°C 18.48 2.35 B
100°C 17.93 3.20 C Mean 15.69 4.07
*Test temperatures with same letter are not significantly different. In the past, triaxial testing of road materials has specified a 2:1 sample height to diameter dimensional ratio. One of the benefits to the rapid triaxial tester employed in this study is that it characterizes 150 mm tall by 150 mm diameter SHRP specified gyratory compacted samples without the need for sample trimming, gluing end platens to the sample, or attaching membranes to the sample. This greatly decreases sample preparation and testing time. In addition, since end platens do not have to be glued to the samples, spatial stress/strain gradients and end shear effects are reduced. The ability to mechanistically characterize 150 mm diameter by 150 mm tall gyratory compacted samples is beneficial given the road industry’s adoption of the SHRP gyratory compactor for laboratory compaction of asphalt concrete. The rapid triaxial tester could therefore, be employed as a complementary mechanistic characterization tool to the SHRP gyratory compactor for asphalt concrete mix design and analysis such as the SuperpaveTM Level I system. The rapid triaxial tester has also been demonstrated in the field for production testing applications such as construction quality control/quality assurance [28].
BERTHELOT, CROCKFORD & LYTTON 25
This research found the rapid triaxial tester to be capable of characterizing the fundamental thermomechanical constitutive behavior of asphalt concrete mixes required for mechanistic road-modeling applications. The rapid triaxial tester distinguished significant differences in the compression modulus, compression Poisson’s ratio, and phase angle among the Radisson SPS-9A asphalt concrete mixes across load frequencies, stress states and test temperatures. The rapid triaxial compressive complex modulus, Poisson’s ratio and phase angle can be mathematically transformed to provide linear viscoelastic materials constitutive relations that can be input into viscoelastic road models to directly predict permanent deformation of road structures. As a result, the ability to accurately measure viscoelastic constitutive properties under stress states, temperatures and load frequencies representative of those in the field may pose significant promise for reliably predicting rutting behavior of asphalt concrete pavements. 5.0 COMPARITIVE ANALYSIS Good repeatability and accuracy of characterization results is essential for materials specification and road-modeling purposes. Figure 8 illustrates the mean coefficient of variation obtained from each asphalt concrete characterization method from triplicate test samples grouped by SPS-9A asphalt concrete mix, stress state, and test temperature. As seen in Figure 8, the coefficient of variation of the traditional phenomenological and rapid triaxial characterization methods was relatively low. However, the SHRP Level III shear characterization and the rapid triaxial compression Poisson’s ratio produced a relatively high coefficient of variation with the exception of the SHRP shear phase angle measurements. Given that the operator of the SHRP shear tester in this research had considerable materials testing experience with complex feedback control systems and that system control problems with the SHRP shear tester could not be detected through detailed and time consuming system diagnostics, the high coefficient of variation could only be attributed to experimental error. It is believed that the relatively short 50 mm sample height may have had a significant influence in the high variability of the SHRP shear tester measurements as witnessed in the high number of samples that failed during SHRP shear testing. This study found the coefficient of variation of the rapid triaxial compression Poisson’s ratio measurements to be slightly higher than the other rapid triaxial measurements Since the rapid triaxial tester radial transducer instrumentation was not designed for testing at 5°C and 20°C, the Poisson’s ratio measurement coefficient of variation at 5°C and 20°C were found to be higher than at other temperatures. As a result, the high coefficient of variation in the rapid triaxial Poisson’s ratio measurements can be remedied by employing more sensitive linear variable differential transducers at lower temperatures. Figure 9 shows the coefficient of variation across the characterization methods at the high test temperatures only. As can be seen, the coefficient of variation of the rapid triaxial compressive Poisson’s ratio decreased significantly when the cold test temperature was excluded from the analysis. However, the coefficient of variation of the SHRP shear modulus obtained form the frequency sweep shear test increased significantly, further illustrating the uncertainty in the shear modulus measurements at high temperature across the Radisson SPS-9A asphalt concrete mixes. Table 24 summarizes the mean observed rut measurements taken with a 1.5 m rut bar of the Radisson SPS-9A test sections after three years of service. As can be seen in Table 24, the coefficient of variation of the 1.5 m straight edge rut depth measurements across the SPS-9A asphalt concrete mixes relatively high.
BERTHELOT, CROCKFORD & LYTTON 26
0
5
10
15
20
25
30
35
40
45
50
Mars
hall Sta
bility
Mars
hall Flo
w
Hveem S
tabili
ty
Unconfined C
ompre
ssive S
trength
SHRP B
ulk M
odulus
SHRP C
onstrain
ed Com
pressio
n Modulu
s
SHRP T
otal S
imple
Shear S
train
SHRP C
omple
x Shear M
odulus
SHRP S
hear Phase A
ngle
SHRP R
SCSR
Cum
ulativ
e Str
ain
SHRP R
SCSR
Cum
ulativ
e Str
ain R
ate
Triaxia
l Com
pressio
n Modulu
s
Triaxia
l Exte
nsion M
odulus
Triaxia
l Com
pressio
n Pois
son's R
atio
Triaxia
l Exte
nsion P
oisson's
Ratio
Triaxia
l Phase A
ngle
Co
eff
icie
nt
of
Va
ria
tio
n (
%)
Figure 8 Asphalt Concrete Characterization Coefficient of Variation of Characterization Methods Grouped by Radisson Specific Pavement Studies-9A Asphalt Concrete Mix, Stress State and Test
Temperature Table 24 Mean Rut Measurements in Radisson Specific Pavement Studies -9A Test Sections after
Figure 9 Asphalt Concrete Characterization Coefficient of Variation of Characterization Methods Grouped by Radisson Specific Pavement Studies-9A Asphalt Concrete Mix, Stress State and High
Test Temperature Table 25 summarizes the Duncan’s pairwise comparison results of the alternative asphalt concrete characterization methods. As can be seen in Table and 25, the rapid triaxial tester best ranked the relative rutting behavior of the Radisson SPS-9A asphalt concrete mixes with the exception of mixes 900902 and 900962 which appear to be reversed in order.
BERTHELOT, CROCKFORD & LYTTON 28
Table 25 Characterization Method Duncan's Pairwise Comparison of Radisson Specific Pavement Studies -9A Asphalt Concrete Mixes
Triaxial Complex Compression Modulus at 10 Hz and SS3
900962 [E]
900961 [D]
900960 [D]
900902 [C,D]
900903 [C]
900901 [B]
900959 [A]
Triaxial Compression Poisson’s Ratio at 10 Hz and SS3
900959 [C]
900962 [C]
900960 [B]
900902 [B]
900903 [B]
900901 [A]
900961 [A]
Triaxial Phase Angle at 10 Hz and SS3
900959 [F]
900902 [E]
900903 [D,E]
900962 [D]
900960 [C]
900901 [B]
900961 [A]
Mean Field Rutting After Three Years Service {mm}
900961 {7.3}
900902 {5.9}
900960 {5.4}
900903 {4.9}
900901 {4.3}
900959 {3.5}
900962 {3.4}
SHRP = Strategic Highway Research Program RSCSR = Repeated Shear at Constant Stress Ratio SS3 = Stress State Three 6.0 SUMMARY AND CONCLUSIONS This study sought to investigate alternative methods for characterizing the rutting behavior of asphalt concrete mixes that can be used for material specifications and road modeling Seven Specific Pavement Studies-9A asphalt concrete mixes (two Marshall and five SuperpaveTM) built at the Radisson SPS-9A test site were characterized with respect to Marshall stability and flow, Hveem stability, unconfined compressive strength, SHRP Level III shear tester, and triaxial frequency sweep properties. The coefficient of variation of the traditional phenomenological and rapid triaxial characterization methods was relatively low. However, the SHRP Level III shear characterization produced a relatively high
BERTHELOT, CROCKFORD & LYTTON 29
coefficient of variation with the exception of the SHRP shear phase angle measurements. The high coefficient of variation could only be attributed to experimental error. It is believed that the relatively short 50 mm sample height may have had a significant influence in the high variability of the SHRP shear tester measurements as witnessed in the high number of samples that failed during SHRP shear testing. This study determined that traditional phenomenological asphalt concrete characterization methods distinguished some significant differences between the Radisson SPS-9A asphalt concrete mixes. However, they did not provide material constitutive relations necessary for mechanistic road response modeling. The SHRP shear tester also distinguished some significant differences between the asphalt concrete mixes and can provide material constitutive relations necessary for mechanistic road response modeling. However, the SHRP shear tester is expensive to own and operate, produces relatively high variability, and was found to be impractical for use by most public road authorities, road consultants, and road contractors. The rapid triaxial tester determined the most significant difference between the asphalt concrete mixes and was found to be an efficient mechanistic testing apparatus to complement the SHRP gyratory compactor. The measured materials properties obtained from the rapid triaxial tester were also found to best correlate predict correlate with the relative rutting behavior of the SPS-9A test sections after three years of service. The complex properties measured by the rapid triaxial tester could be mathematically transformed into linear viscoelastic constitutive relations which could be used for viscoelastic mechanistic road modeling. This ability to accurately measure viscoelastic constitutive properties under stress states, temperatures and load frequencies representative of those in the field may pose significant promise for reliably predicting rutting behavior of asphalt concrete pavements. DISCLAIMER
The viewpoints expressed herein are those of the authors, and are not necessarily endorsed by the agencies involved with this research. 7.0 REFERENCES
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