Sensitivity of rutting and moisture resistance of asphalt mixes to gradation and design air void content

ABSTRACT

Bituminous mixes are generally designed with gradation and air void content ranges adopted by different agencies. The selected gradation and binder content are permitted to vary within specified tolerance limits. This leads to the possibility of a wide range of mixes with varying mix performance. Thus, it is essential to understand the influence of variation in these parameters on the performance of mixes. The present study aims to examine the sensitivity of the mix performance to variation within the specified gradation and air void ranges. Five aggregate gradations were considered for designing mixes for the volumetric design criterion of 4% air voids. For three of the five gradations, mixes were also designed for two additional design air void contents, 3% and 5%. Rutting resistance of the mixes was measured using a rut tester and moisture resistance was evaluated in terms of Tensile Strength Ratio (TSR). The densification characteristics represented by voids in mineral aggregate (VMA) values of the mixes prepared using the five aggregate gradations are similar to those of the corresponding aggregate packing characteristics (dry aggregate air voids, DAAV). A good relationship was observed between mix rutting and minimum voids in mineral aggregate and DAAV. Sensitivity analysis carried out with different aggregate gradations, design air void contents and tolerances, indicated that rut depth as well as TSR vary over a wide range for the combinations of mixes considered (all within the gradation, air void content and tolerance limits).Compared to the design air void content, aggregate gradation is found to have significantly larger effect on rut depth whereas both gradation and design air void content have similar effect on the TSR value. Using the results obtained from the present investigation, the Indian Marshall mix design criteria (MoRTH 2013) for stability and flow were revisited and new criteria are proposed.

KEYWORDS:

• Design air voids
• gradation envelope
• dry aggregate air voids
• minimum voids in mineral aggregates
• wheel tracking rut depth
• tensile strength ratio
• Marshall stability and flow
• film thickness

Disclosure statement

No potential conflict of interest was reported by the authors.