Effects of mix design variables on flexibility index of asphalt concrete mixtures

Abstract

ABSTRACT The semicircular bending (SCB) test has been commonly used to evaluate the fatigue resistance performance of asphalt concrete (AC) mixtures. In this study, flexible index (FI) was obtained based on the Illinois Flexibility Index Test (I-FIT, AASHTO TP124), which was then correlated with mix design variables, including reclaimed asphalt pavement (RAP) content, nominal maximum aggregate size (NMAS), and binder grade. As part of the SCB test, two notch widths were evaluated as well. As a result, the effect of binder grade on FI is significant at the 0.001 level. For the three binder grades (PG 64-22 and PG 67-22, PG 76-22) tested, the higher binder grade generally results in a higher FI. The effect of NMAS on FI is significant at the 0.05 level. Mixtures with the NMAS of 12.5 mm have a lower FI than those with the NMAS of 9.5 mm. The difference in RAP content (25% versus 30%) did not affect a significant difference on FI, which likely due to the marginal difference of the two RAP content levels tested. The binder content has a significant effect on the resulting FI. Mixtures with a slightly higher binder content would have a significantly higher FI. Based on elasticity analysis, 1% increase in the binder percentage would affect approximately 7.53% increase in FI. Other control variables, including air void percent, void in the mineral aggregate, and void filled with asphalt, were found to be insignificant. This may be attributable to the fact that those variables were controlled during the specimen fabrication and their variances are relatively small. Finally, the effect of notch width is significant at the 0.001 level. The larger notch width (4 mm) resulted in a lower FI compared to the smaller notch width (1.5 mm).