The low temperature properties of conventional and modified asphalt binders evaluated by the failure energy and secant modulus from direct tension tests

Abstract

In our previous direct tension test (DTT) studies, SBS polymer modification was found to increase the failure stress values with increasing polymer levels. The predicted critical cracking temperatures (Tcritical) in Superpave MP la specification were found to be 3 to 6°C lower than the bending beam rheometer (BBR) low temperature parameter according to Superpave MP1 specification. In this study, the DTT results were analyzed and compared in terms of the DTT failure energy and secant modulus instead of failure stress or failure strain values. As expected, the DTT secant modulus was found to increase with increasing hardness of the non-modified asphalt binder materials; however, the secant modulus decreases upon Styrene Butadiene Styrene (SBS) polymer modification. If the secant modulus were used to evaluate the stiffness of asphalt binder materials at low temperature, the SBS modified asphalt binders would not only have better low-temperature properties than predicted by the BBR low-temperature parameter in Superpave MP1 but also better than predicted by the Tcritical in Superpave MP1a specification. The failure energy of SBS modified asphalt binder at Tcritical was found to be invariably higher than the Tcritical failure energy of non-modified asphalt binders, even though the Tcritical of PMA was already 3–6°C lower than the Tcritical of the non-modified asphalt binder. The elastic polymer network in the PMA probably contributes to higher DTT failure stress, failure strain and failure energy values.