Research Article Transportation Research Record 1–9 Ó National Academy of Sciences: Transportation Research Board 2020 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0361198119899611 journals.sagepub.com/home/trr Flexibility Index Threshold Optimization for Various Asphalt Concrete Mixes and Climatic Conditions Uthman Mohamed Ali 1 , Imad L. Al-Qadi 1 , and Hassan Ozer 1 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 con- ditions (25°C and 50mm/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 bin- der’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 approachyielded 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 conven- tional I-FIT (25°C and 50mm/min) could be established once the FI obtained from equivalent stiffness testing matches or exceeds the target FI. According to the Federal Highway Administration (FHWA), 94% of the nation’s 2.7million miles of paved roads and highways are surfaced with asphalt materials (1). These include full depth asphalt, conventional asphalt, and concrete pavements with asphalt overlays. Approximately 400million tons of asphalt concrete (AC) worth more than $30billion was produced in 2017 (2). It is, therefore, crucial that this widely used paving material performs satisfactorily during its intended service life. Cracking constitutes six of the 15 common AC distresses listed in FHWA’s ‘‘Distress Identification Manual’’ (3). It impacts AC pavement performance as well as ride quality and requires costly maintenance or rehabilitation. Cracking manifests on the surface in the form of fati- gue, reflective, and longitudinal cracking, as well as ther- mal cracking. The distress is believed to have increased with the adoption of Superpave mix design, which led to dry mixes resistant to rutting, but more susceptible to cracking (4). In addition, the use of recycled materials, such as reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS), exacerbated the problem and led to brittle mixes that are more prone to cracking. Several laboratory tests were developed to evaluate the cracking susceptibility of AC by characterizing its fracture and fatigue resistance. These include dissipated creep-strain energy (DCSE), four-point bending fatigue, disk-shaped compact tension (DCT), Texas overlay tes- ter (OT), and semi-circular bend (SCB) geometry tests (5). The Illinois Flexibility Index Test (I-FIT), which uses SCB geometry, is a simple, cost-effective, repeatable, and easy-to-implement test used to determine the cracking potential of AC at intermediate temperatures (6). Flexibility Index (FI) is the main output of the I-FIT, and it is based on the fracture energy (FE) and post-peak slope of an AC mix. Researchers investigated the impact of temperature and loading rates on the FI during I-FIT development. They selected 25°C and 50 mm/min because it expedited 1 Illinois Center for Transportation, University of Illinois at Urbana- Champaign, Urbana, IL Corresponding Author: Address correspondence to Uthman Mohamed Ali: uthmanm2@illinois.edu