Assessment of Low-Temperature Properties of Styrene–Butadiene–Styrene-Modified Asphalt Binders and Mastics Based on Relaxation Characteristics

This study investigates the low-temperature relaxation behavior of asphalt binders and asphalt mastics using a ductilometer-based uniaxial tensile relaxation test under a constant displacement rate. Although the low-temperature performance of SBS-modified asphalt binders has been widely studied, the relaxation mechanisms of asphalt mastics containing SBS-modified binders, particularly considering the effects of the SBS content and short-term aging, remain insufficiently understood. Polymer-modified binders were prepared from 50/70 penetration-grade bitumen blended with an industrially produced SBS copolymer concentrate containing 9% SBS. Three binders with SBS contents of 3%, 5%, and 7% were manufactured and used to prepare asphalt mastics with mineral filler. Tensile relaxation tests were performed at −12 °C on unaged and RTFOT-aged specimens. The relaxation behavior was evaluated using a modified generalized Maxwell model to describe the viscoelastic response of the materials. The novelty of this study lies in the systematic comparison of SBS-modified binders and corresponding asphalt mastics using a unified experimental and modeling approach. Unlike previous studies focused mainly on polymer-modified binders, this research extends the analysis to binder–filler systems and provides new insight into the influence of the SBS content and aging on stress relaxation mechanisms. The results demonstrate that increasing the SBS content improves the relaxation capacity of both the binders and mastics, enhancing their ability to dissipate thermally induced stresses. Moreover, a higher SBS content reduces the sensitivity of materials to short-term aging, indicating improved resistance to low-temperature cracking.