Understanding the Illinois Flexibility Index Test (I-FIT) using Indiana Asphalt Mixtures

Abstract

Asphalt mixture cracking has become the dominant type of asphalt pavement distress in Indiana. In recent years, efforts have shifted away from the standard Superpave mixture design method in search of methods to improve the resistance of asphalt pavements to cracking-related distress. This paper presents a review of the applicability of the Illinois Flexibility Index Test (I-FIT) to evaluate the cracking potential of Indiana asphalt mixtures. In this study, two cracking indices were compared: the flexibility index (FI) and the cracking resistance index (CRI), both derived from the load-displacement curve. The applicability of quality assurance (QA) tests of laboratory-compacted and field-compacted samples was also explored to evaluate the cracking potential, and I-FIT was investigated for its applicability as a performance-related quality control (QC)/QA test. The results show that the FI values obtained from field-compacted samples were consistently higher than those of the lab-compacted samples. Both the FI and CRI values were significantly affected by variations in specimen thickness and air void content, with higher FI values observed with an increase in the air void content and a decrease in specimen thickness. The CRI values were less sensitive to I-FIT variability and more repeatable compared to the FI values. Finally, as an illustrative example, the cumulative distribution function of the FI values for a population of asphalt concrete (AC) mixtures was used to establish the different ranges for the quality thresholds of the materials.