Flexibility index threshold optimization for various asphalt mixes and geographic locations

Majority of the highways and paved roads in the United States use asphalt concrete (AC) as a surface layer. However, the performance of this material is adversely affected by cracking. Several tests were developed to predict AC susceptibility to cracking. The Illinois Flexibility Index Test (I-FIT) is a simple, rapid, repeatable, and cost-effective test used to assess the cracking potential of AC. The test is performed at 25°C and 50mm/min loading rate. The main resultant parameter of the test is flexibility index (FI). A high FI generally implies better resistance to cracking. Asphalt concrete’s binder impacts FI results. Soft binders, such as PG 58-28, yield relatively higher FI values compared to similar AC containing stiffer binder (e.g. PG 70-22). Illinois Department of Transportation (IDOT) established a single FI threshold of eight for all unaged AC mixes. In addition, IDOT proposes an FI threshold of four for long-term aged specimens. Several AC mixes met the unaged threshold but failed the criterion for long-term aging condition and vice versa. The goal of this thesis is to minimize impact of binder grade on FI results. To isolate the impact of binder grade on FI values, for both unaged and long-term aged specimens, ensure testing of AC with various binders at equivalent stiffness (equi-stiffness), the same secant modulus. The secant modulus is defined herein as the ratio of 50% of the peak load to its corresponding displacement. Two approaches, using various test temperatures or loading rates, were investigated. Five laboratory-designed AC mixes containing various binder grades (PG 52-40, PG 58-34, PG 64-22, PG 70-22, and PG 76-22) were tested. The AC mixes have the same aggregate type and gradation, as well as binder content to isolate the effect of binder grade on I-FIT results. The AC mix with PG 64-22 was selected as the control mix. When testing was performed at the same secant modulus for the five AC mixes, by varying loading rate, the resultant FIs were more consistent than when temperature varied. In addition, the variation in FI values increased with aging AC specimens for three days at 95°C using a forced-draft oven. In addition to identifying an FI threshold that corresponds to a geographic location (based on binder grade used), the proposed approach would better correlate unaged and aged FI values for the same AC mix