Influence of gradation on asphalt mix rutting resistance measured by Hamburg Wheel Tracking test

Abstract

Aggregate gradation is a key characteristic affecting the asphalt mix rutting resistance. The relationship between mix gradation and rutting behavior has been comprehensively studied, but only a few studies have incorporated the testing results from the Hamburg Wheel Tracking (HWT) test. This study used HWT test results to discuss the influence of four mix design variables, namely asphalt binder content, 4.75 mm key sieve passing, coarse particle gradation variation (>4.75 mm) and fine particles gradation variation (<4.75 mm) on the mix rutting resistance. A total of 26 asphalt mixes were tested using HWT test at 50 °C. Fine Aggregate (FAc) parameter derived from Bailey method was applied to quantitatively describe the gradation characteristic and was correlated with the creep slope from HWT tests. The results showed that both excessive and inadequate asphalt binder contents will impair the rutting performance. Increasing the mix 4.75 mm passing ratio from 20% to 35% slowly increases the rutting depth, with the passing ratio exceeding 41%, the mix exhibits significantly reduced rutting resistance and stripping behavior is observed. Mix skeleton with a uniform gradation in each size shows the minimum rutting resistance because smaller particles could damage the integrity of the mix skeleton by pushing the coarse aggregate open. Skeleton composed by 13.2–9.5 mm aggregates outperforms the skeleton composed by 9.5–4.75 mm aggregates. A finer gradation in aggregate fine fraction (<4.75 mm) has a positive influence on the rutting resistance as the HWT creep slope negatively correlates with the mix FAC parameter (R2 = 0.73).