Refining the indirect tensile (IDT) N flex Factor test to evaluate cracking resistance of asphalt mixtures for mix design and quality assurance

Abstract A procedure has recently been developed using the IDT strength test to evaluate the cracking resistance of asphalt mixtures. This procedure includes the determination of toughness and brittleness slope, which are then combined to calculate a cracking resistance parameter termed N flex Factor. Although a reasonable correlation was found between IDT N flex Factor with the FHWA ALF fatigue cracking data, the determination of N flex Factor has several limitations. This study was undertaken to refine the IDT N flex Factor test for evaluating the cracking resistance of asphalt mixtures for mix design and quality assurance. In the first phase, the test was performed using a closed-loop, servo-hydraulic control system with on-specimen instrumentation. Test results showed that test temperature had a significant effect on both Poisson’s ratio and IDT N flex Factor results, while the effect of loading rate was insignificant. It was also found that assuming a constant Poisson’s ratio was likely to bias the accurate determination of IDT N flex Factor; instead, a practical correlation was established between Poisson’s ratio and secant stiffness determined from the load-displacement data. In the second phase, the sensitivity of IDT N flex Factor to various mix design parameters was evaluated using a simplified test frame. In general, the N flex Factor was found sensitive to changes in asphalt binder content, asphalt binder grade, as well as inclusion of recycled asphalt materials. Thus, the IDT N flex Factor shows the potential for being used as a reliable cracking parameter for mix design and quality assurance.