Linear rheological behavior of bituminous paving materials

The linearity limits of various unmodified and modified bituminous binders, as well as asphalt mixtures incorporating these binders, have been investigated using a dynamic shear rheometer and a purpose-built, dynamic, direct tension-compression, servohydraulic testing apparatus. Both continuously graded, as well as gap-graded, asphalt mixtures have been tested together with a fines/filler mortar (mastic). Cylindrical specimens of these mixtures were subjected to sinusoidal loading at temperatures between 10 and 40degreesC, first, under stress sweep conditions and, second, under frequency sweep conditions. The results show that there are binder and bulk strain dependent linearity criteria for both binders and asphalt mixtures at high complex modulus values as well, as a high-temperature binder strain dependent linearity criterion for elastomeric modified binders. The linearity bulk strain criterion for the asphalt mixtures was found to be in the order of 100 microstrain with the binder strain criterion for the binders being at least 100 times greater at just over 10,000 microstrain and the polymer network binder strain criterion at 1,000,000 microstrain. In general, the linear viscoelastic rheological characteristics of the asphalt mixtures were similar with differences in the complex modulus and phase angle only being found at intermediate temperatures and frequencies and are attributed to the rheological characteristics of the binders. However, the extra stiffening effect of the aggregate skeleton did increase the elastic response of the asphalt mixtures relative to that of the binders, particularly at high temperatures and low frequencies.