Flexibility Index Threshold Optimization for Various Asphalt Concrete Mixes and Climatic Conditions

Abstract

Cracking is a major distress that adversely affects the performance of asphalt concrete (AC); a material used as a surface layer on most United States (US) paved roads. The Illinois Flexibility Index Test (I-FIT) is a simple, rapid, repeatable, and cost-effective test used to assess AC cracking potential. Flexibility Index (FI) is the main resultant parameter of the I-FIT. Test conditions (25°C and 50 mm/min), however, yield dissimilar FI values for AC mixes containing different binder grades, which may make the use of a single FI threshold inappropriate. This research effort developed a framework for establishing FI thresholds for unaged AC specimens that were tested using the current I-FIT temperature and loading rate. Five laboratory-designed AC mixes with the same aggregate skeleton and binder content—but different binder grades—were used to isolate the binder’s influence. A reference AC mix was selected and its FI and secant modulus used as target parameters. An equivalent-stiffness approach, utilizing loading rate or temperature variations to attain the target secant modulus, was used to account for the impact of binder grades on FI of the remaining four AC mixes. The equivalent-stiffness approach yielded a narrower FI range compared to the conventional I-FIT. Statistical analysis of FI results showed no significant differences in the mean FI values of the control mix and of the four AC mixes tested using equivalent-stiffness approach. FI thresholds based on conventional I-FIT (25°C and 50 mm/min) could be established once the FI obtained from equivalent stiffness testing matches or exceeds the target FI.