Effect of air voids on the fracture resistance of HMA in the indirect tensile cracking (IDEAL-CT) test

ABSTRACT

Air voids (density) is a key volumetric property of asphalt mixes and play an unignorable role in anti-cracking. While lower voids were expected to result in more durable asphalt mixtures, contradictory trends were found in lab testing. In this study, several plant-produced mixtures were compacted to three levels of air voids (7%, 9%, and 11%) and tested to evaluate the sensitivity to the crack resistance. The results indicate that the higher the air voids, the better the cracking resistance as measured by the CT_index. To understand the cause, a stepwise regression analysis was conducted to model the IDEAL-CT parameters with volumetric and gradation parameters. The air void, percent passing the 0.6 mm (P_0.6) and binder content (P_b) are found to have significant effects on CT_index and post-peak slope. The analysis of the CT change rate shows that the P_b, dust to binder ratio (D/B), and P_0.6 can be used to predict the effect of the voids with a correlation factor (R²) of 0.96. For further understanding, the asphalt film thickness was calculated and found that compared with using film thickness as the single fitting parameter, combining the film thickness with D/B in modelling can dramatically increase the R² of correlation from 0.67 to 0.92.

KEYWORDS:

• Asphalt mortar film thickness
• cracking resistance
• indirect tensile cracking test (IDEAL)
• air void

Disclosure statement

No potential conflict of interest was reported by the author(s).