Comparison of rutting resistance of unmodified and SBS-modified Superpave mixtures by accelerated pavement testing

Superpave asphalt mixtures have been used in highway pavements in the US since the late 1990s. Modified binders have also been used in some of the Superpave mixtures in an effort to increase the cracking and rutting resistance of these mixtures. Due to the short history of these mixtures, it is still too early to assess the long-term performance of these Superpave mixtures and the benefits from the use of the modified binders. This paper presents the results of a full-scale pavement-testing program to evaluate the rutting resistance of Superpave mixtures with and without polymer modification using a Heavy Vehicle Simulator. Results from the HVS tests showed that the pavement sections with two 5-cm lifts of SBS-modified mixture clearly outperformed those with two 5-cm lifts of unmodified mixture, which had two to two and a half times the rut rate. The pavement sections with a lift of SBS-modified mixture over a lift of unmodified mixture practically had about the same performance as the sections with two lifts of SBS-modified mixture when tested at ambient temperature, and had only about 20% higher rutting than those with two lifts of modified mixture when tested at 50 °C. The test section with two lifts of SBS-modified mixture and tested at 65 °C still outperformed the test sections with two lifts of unmodified mixture and tested at 50 °C. Rutting of the unmodified mixture was observed to be due to a combination of densification and shoving, while that of the SBS-modified mixture was due primarily to densification.