Performance assessment of sustainable cold asphalt mixtures with RAP and modified bitumen emulsion: a laboratory investigation of mechanical properties and moisture sensitivity

Recycled cold asphalt offers a sustainable and cost-efficient solution by incorporating reclaimed materials, reducing energy consumption, and minimizing environmental impacts. This study investigates the effectiveness of styrene-butadiene rubber (SBR) latex modification and varying reclaimed asphalt pavement (RAP) contents in enhancing the performance of cold recycled asphalt mixtures. The Modified Marshall method was used to prepare specimens with RAP contents (0, 25, 50, 75, and 100%) using both unmodified bitumen (UMB) and SBR-modified bitumen (SBRMB). Bitumen properties were evaluated through penetration, softening point, penetration index (PI), Fourier Transform Infrared Spectroscopy (FTIR), and surface free energy (SFE) tests. Mixture performance was assessed using Marshall stability (MS), indirect tensile strength (ITS), tensile strength ratio (TSR), resilient modulus (MR), resilient modulus ratio (RMR), and flow number (FN) tests. FTIR analysis revealed significant chemical changes in SBR-modified bitumen, including the formation of new polar bonds and enhanced intermolecular forces. The addition of SBR latex improved thermal stability by 18.5% and increased the PI from 0.41 to 2.28. SFE measurements showed a 16.2% increase in cohesion energy for polymer-modified mastics. Although increasing RAP content reduced MS by 24.2% at 100% RAP, SBR modification partially mitigated this decrease. ITS values improved by 15.2% with RAP addition, and moisture resistance indices (TSR and RMR) improved with both RAP and SBR incorporation. However, FN decreased significantly with increasing RAP content, with the highest RAP levels exhibiting a substantial reduction compared to the control sample. Statistical analysis using ANOVA demonstrated that mixture type was the most influential factor in determining performance, while the SBR modification effects were significant but less pronounced. The findings highlight the potential of combining SBR polymer and RAP to produce sustainable cold recycled asphalt mixtures.