Simple Indexes to Identify Fatigue Performance of Asphalt Concrete

Using indexes based on engineering properties is among the ways to evaluate the quality of an asphalt concrete mix in terms of its resistance to fatigue cracking. These indexes typically are developed based on measuring asphalt concrete or asphalt binder properties in a laboratory test. Among such tests, the semicircular bend (SCB) test has shown potential as a promising test to determine fatigue resistance of asphalt mixtures. In the work presented, this test was explored extensively to determine proper fatigue performance identifiers from the test. Numerous tests were conducted to determine the effect of asphalt mix composition on the SCB fatigue test results. A major index considered from this test for evaluation is the flexibility index (FI), which is defined as the ratio of fracture energy and the post peak slope at inflection point derived from a typical load-deformation or stress–strain monotonic test. This index has been proposed based on extensive research at the University of Illinois. In the work presented in this study, tests were conducted on both short- and long-term aged asphalt mixes to evaluate FI and its reliability as a fatigue resistance indicator. The mix components considered as test variables included binder content, air void, and binder stiffness. The results showed a significant reduction in FI fatigue resistance after long term aging. Reduced FI is believed to associate with reduced fatigue life. The sensitivity of FI and other engineering properties to mix parameters were investigated in this research. It was found that FI could not be used alone and as a sole indicator of fatigue performance. It was determined that two indexes are needed for this purpose—the first presents the mix toughness and ductility, and the second is a strength- or stiffness-based index.