Effects of Aggregate Shape on Performance of Gravel-Aggregate Hot-Mix Asphalt Using Digital Image-Based Approach

The present study investigated the application potential of crushed gravel (CG) aggregates in asphalt mixes. Three different types of aggregates; i.e., gravel (G), crushed gravel (CG), and crushed stone (CS); were used for shape characterization and mix performance evaluations. The CG aggregates were produced by crushing G aggregates using a single-stage jaw crusher. The aggregates of six different sizes; i.e., P19-R12.5 (passing through 19 mm and retaining on 12.5 mm), P12.5-R9.5, P9.5-R4.75, P4.75-R2.36, P2.36-R1.18, and P1.18-R0.600; were considered in this study. The aggregate was subjected to shape characterization using a digital image-based aggregate image measurement system (AIMS). The cumulative shape index (CSI) for each aggregate shape parameter was determined for three control (i.e., 100% G, 100% CG, and 100% CS) and three mix blends (i.e., 25% CG + 75% CS, 50% CG + 50% CS, and 75% CG + 25% CS). The performance of the asphalt mixes under shear, compression, and moisture damage was evaluated using an aggregate slip test (AST), stability (S), and moisture damage potential tests, respectively. Additionally, the correlations between mix performance and CSIs were drawn. The results showed that the crushing of G aggregates significantly improves the angularity, texture, and form2D, but decreases the sphericity. The CG aggregate angularity increases with the decrease in aggregate size. The gradation with 100% G had the least cumulative angularity index (CAI) and cumulative texture index (CTI) followed by the 100% CG and the 100% CS gradations. However, in the cases of cumulative sphericity index (CSpI) and cumulative form2D index (CF2DI), varying trends were observed. The performance tests revealed that all but the 100% G mix combinations satisfied the stability and moisture resistance criteria of asphalt mixes. Also, mixes with 100% CG aggregates offered a relatively higher rutting resistance than the 100% G aggregates. The CAI, CTI, and CSpI exhibited positive correlation with stability and rutting resistance, whereas the CF2DI showed negative correlation. Further, the AST test more effectively captured the effects of aggregate shape than stability. The study concluded that even 100% CG aggregates can be used in asphalt mixes without any detrimental effects on performance.