Investigation on effects of aggregate roughness on bond strength of aggregate-bitumen systems

Abstract

It is critical to understand bonding-behavior of aggregate with bitumen. The surface chemistry and roughness of aggregates play an important role to create sufficient bond with bitumen. Usually, asphalt-bond-strength (ABS) test is used to directly quantify bonding-behavior of aggregate-bitumen system, further, surface-free-energy (SFE) approach has also been used by some researchers. AASHTO T-361 recommends to use aggregates polished with 280-silicon-carbide (SiC) powder to determine bond-strength with bitumen. However, increase or decrease in polishing-level can alter aggregate-roughness and may affect bond-strength. Further, there can be numerous roughness-scale for aggregates such as, arithmetic (Ra), root-mean-square (Rq) and box-roughness (Sa), which needs detailed study. Therefore, one of the strong motivations of this study was to evaluate effects of aggregate-roughness on bond-strength using ABS approach. Similarly, aggregate SFE may change with change in its roughness, thereby affecting aggregate-bitumen bonding. The present study evaluates effects of aggregate-roughness on aggregate-bitumen bond-strength using ABS and SFE approaches. Basaltic substrates and viscosity-grade binder (VG30) were selected for this study. To generate different roughness-levels, aggregate substrates were polished with 220SiC and 1000SiC powders. At each polishing-level, Ra and Sa parameters were evaluated using zeta-profilometer. The ABS and SFE measurements were conducted on combination of aggregate and bitumen. The results showed that bond-strength increased with increase in aggregate-roughness, indicating better bonding for rough aggregates. Further, Sa parameter which remained unaddressed in earlier research studies was recommended to quantify aggregate roughness rather than Ra parameters, because it measures roughness over area, which is one of the major contributions of present study.