comparison of alternative asphalt concrete rut characterization

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BERTHELOT, CROCKFORD & LYTTON 25This research found the rapid triaxial tester to be capable of characterizing the fundamentalthermomechanical constitutive behavior of asphalt concrete mixes required for mechanistic roadmodelingapplications. The rapid triaxial tester distinguished significant differences in the compressionmodulus, compression Poisson’s ratio, and phase angle among the Radisson SPS-9A asphalt concretemixes across load frequencies, stress states and test temperatures. The rapid triaxial compressive complexmodulus, Poisson’s ratio and phase angle can be mathematically transformed to provide linearviscoelastic materials constitutive relations that can be input into viscoelastic road models to directlypredict permanent deformation of road structures. As a result, the ability to accurately measureviscoelastic constitutive properties under stress states, temperatures and load frequencies representative ofthose in the field may pose significant promise for reliably predicting rutting behavior of asphalt concretepavements.5.0 COMPARITIVE ANALYSISGood repeatability and accuracy of characterization results is essential for materials specification androad-modeling purposes. Figure 8 illustrates the mean coefficient of variation obtained from each asphaltconcrete 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 triaxialcharacterization methods was relatively low. However, the SHRP Level III shear characterization and therapid triaxial compression Poisson’s ratio produced a relatively high coefficient of variation with theexception of the SHRP shear phase angle measurements. Given that the operator of the SHRP shear testerin this research had considerable materials testing experience with complex feedback control systems andthat system control problems with the SHRP shear tester could not be detected through detailed and timeconsuming system diagnostics, the high coefficient of variation could only be attributed to experimentalerror. It is believed that the relatively short 50 mm sample height may have had a significant influence inthe high variability of the SHRP shear tester measurements as witnessed in the high number of samplesthat failed during SHRP shear testing.This study found the coefficient of variation of the rapid triaxial compression Poisson’s ratiomeasurements to be slightly higher than the other rapid triaxial measurements Since the rapid triaxialtester radial transducer instrumentation was not designed for testing at 5°C and 20°C, the Poisson’s ratiomeasurement 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 beremedied 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 testtemperatures only. As can be seen, the coefficient of variation of the rapid triaxial compressive Poisson’sratio 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 testincreased significantly, further illustrating the uncertainty in the shear modulus measurements at hightemperature 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 RadissonSPS-9A test sections after three years of service. As can be seen in Table 24, the coefficient of variationof the 1.5 m straight edge rut depth measurements across the SPS-9A asphalt concrete mixes relativelyhigh.
BERTHELOT, CROCKFORD & LYTTON 2650Coefficient of Variation (%)454035302520151050Marshall FlowMarshall StabilityHv eem StabilityUnconfined Compressive StrengthSHRP Bulk ModulusSHRP Constrained Compression ModulusTriaxial Compression ModulusSHRP Total Simple Shear StrainSHRP Complex Shear ModulusSHRP Shear Phase AngleSHRP RSCSR Cumulative StrainSHRP RSCSR Cumulative Strain RateTriaxial Extension ModulusTriaxial Compression Poisson's RatioTriaxial Extension Poisson's RatioTriaxial Phase AngleFigure 8 Asphalt Concrete Characterization Coefficient of Variation of Characterization MethodsGrouped by Radisson Specific Pavement Studies-9A Asphalt Concrete Mix, Stress State and TestTemperatureTable 24 Mean Rut Measurements in Radisson Specific Pavement Studies -9A Test Sections afterThree Years of ServiceSPS-9A TestSectionMean RutMeasurementsUsing 1.5 m RutBar (mm)RutMeasurementsCoefficient ofVariation Using1.5 m Rut Bar(Percent)900901 4.3 29.65900902 5.9 23.45900903 4.9 22.78900959 3.5 30.65900960 5.4 16.85900961 7.3 23.31900962 3.4 30.31Mean 5.0 25.29
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