Evaluation of the effect of SBS polymer modifier on cracking resistance of Superpave mixtures

A laboratory investigation was conducted to evaluate the effects of SBS polymer modification on the cracking resistance and healing characteristics of Superpave mixtures. Asphalt mixtures with low and high design asphalt contents using unmodified and SBS modified asphalt cements were produced in the laboratory. Tests performed with the Superpave IDT included resilient modulus tests, creep tests, repeated load fracture tests, healing tests, strength tests at two loading rates, as well as longer-term creep tests to failure. The test results showed that the benefit of SBS modifiers to mixture cracking resistance appeared to be derived primarily from a reduced rate of micro-damage accumulation. The reduced rate of damage accumulation was reflected in a lower m-value without a reduction in fracture limit. However, SBS modifier does not appear to influence healing or aging of asphalt mixture. Therefore, it was shown that the benefits of the SBS modifier were clearly identified by using the HMA fracture model, which accounts for the combined effects of m-value and fracture energy limit on cracking resistance. It was also determined that the residual dissipated energy as determined from Superpave IDT strength tests appears to be uniquely associated with the presence and benefit of SBS modification, and may provide a quick way to make relative comparisons of cracking performance. Longer-term creep tests showed that time to crack initiation appeared to provide another parameter uniquely related to the effects of SBS modification. Cost analyses indicate that the construction cost of AC layer would be reduced by using SBS modifier. Consequently, the use of SBS modified mixtures was clearly justified. In conclusion, the key to characterizing the effects of SBS modifier on the cracking resistance of asphalt mixture is in the evaluation of the combined effects of creep and failure limits.