Recovery of Performance-Deficient Asphalt Mixtures: A Comparative Assessment of Additive Modification and Balanced Mix Redesign

Traditional volumetric asphalt mix design methods do not directly evaluate cracking and rutting resistance, which may result in mixtures with inadequate field performance despite satisfying conventional volumetric requirements. Balanced Mix Design (BMD) has been introduced to address this limitation by incorporating performance testing. However, it remains unclear whether additive modification alone can recover mixtures that do not satisfy BMD criteria, or whether volumetric redesign (binder content optimization) is necessary. Therefore, this study investigated the effectiveness of additive modification before and after BMD redesign using IDEAL-CT and IDEAL-RT performance indices. Three additive systems were evaluated: SBS polymer, nano-silica, and hybrid modification. Performance threshold values for CT and RT Indices were established using a percentile-based statistical approach derived from an experimental database of 18 asphalt mixtures prepared with different aggregate gradations, filler types, and asphalt binder contents. The 25th percentile (P25) was adopted as the minimum performance criterion. The experimental program consisted of three evaluation cases. The first case investigated the ability of additives to recover initially performance-deficient mixtures before and after BMD redesign. Results showed that additive incorporation alone was insufficient to recover deficient mixtures and, in some cases, reduced cracking resistance due to excessive stiffness. However, after BMD redesign, the modified mixtures achieved balanced cracking and rutting performance above the adopted threshold values. The second case, based on a single initially compliant mixture, suggested that additives may be more effective when applied to mixtures that already satisfy performance requirements; however, this observation is preliminary and requires validation with a broader range of passing mixtures. The hybrid modification system provided the best overall balance. The third case isolated the effect of SBS polymer after BMD redesign using paired t-tests and Cohen’s d analysis. Statistical results confirmed a significant and very large positive effect of polymer modification on rutting resistance, whereas the effect on cracking resistance depended strongly on filler type. Overall, the findings indicate that additive modification cannot reliably compensate for deficient mixture design, while the combination of BMD redesign and additive modification can successfully achieve balanced performance. In addition, filler characteristics were found to play a critical role in controlling modifier effectiveness. These findings are based on a limited number of mixtures and additive combinations, and the proposed thresholds (CT ≥ 52, RT ≥ 44) should be validated using independent datasets before general application.