Development and Assessment of Monotonic Cracking Index for Plastic-Modified Asphalt: A Comparative Study with Established Performance Metrics

Traditional performance indices used to evaluate the cracking performance of neat asphalt binders do not effectively assess plastic-modified asphalt binders. To address this, new rheological tests such as linear amplitude sweep, frequency sweep, and binder yield energy have been developed. Among these, the binder yield energy test (BYET) provides a simple and efficient means of measuring yield energy, flexibility, and peak stress under monotonic shear loading. However, its focus on performance up to peak stress overlooks the critical post-peak stress response, which is useful for assessing crack resistance. This study introduces the monotonic cracking index (MCI), derived from the BYET, which incorporates pre- and post-peak stress responses to evaluate the cracking behavior of plastic-modified asphalt binders. Binders modified with recycled high-density polyethylene and recycled polypropylene were prepared by adding these plastics to a PG 64-22 base binder at 2%, 4%, and 6% by weight of the binder to evaluate the effects of plastic content on cracking resistance. The results show strong correlations between the MCI and advanced performance indicators, including the Glover-Rowe (G-R) parameter, crossover temperature (Tδ=45°), and rheological index (R-value), but an inverse correlation with fatigue life (Nf). While Nf results indicate a higher loading cycle before failure, other indices indicate increased susceptibility to cracking with the addition of rHDPE and rPP, highlighting the potential of MCI to accurately assess the cracking performance of plastic-modified binders.