Modification of CT-index Calculation Method based on Failure Work-Displacement Curve from Ideal-CT

Abstract: Cracking is one of the primary distress types for asphalt pavement. To enable rapid and accurate evaluation of asphalt mix cracking resistance, the Indirect Tensile Asphalt Cracking Test (Ideal-CT) was developed in 2017. Due to its simplicity, practicality, and efficiency, Ideal-CT has gained wide acceptance in the paving industry. However, several critical issues remain unresolved. One key challenge is that the current CT-index calculation method, which is based on load-line displacement (LLD) curves, heavily depends on the 75% peak load point (P75) in the post-peak range. This makes the CT-index incompetent in some cases, as it lacks a comprehensive consideration of the whole stage of cracking propagation, to identify the impact of key mix design factors such as air voids and polymer modification on the cracking resistance of asphalt mixes.
The primary objective of this study is to develop a new cracking performance index CT-MARC from the Ideal-CT test to address the above-mentioned challenges associated with the existing CT-index parameter. To achieve this, the failure work-displacement curve, rather than load-line displacement curve, is drawn and used to better characterize and understand the cracking evolution process of the asphalt mixture sample during the test. The newly developed CT-MARC is developed from the curve and defined as the average failure work over the generated displacement in the post-peak stage. Mechanically, it represents the average work that is needed to create one unit length of the crack during the cracking propagation stage.
A series of asphalt mixes with varying aggregate structures and sources, binder PG grades, polymer modifications, RAP/RAS contents, binder contents, and other volumetric design parameters are prepared to evaluate the sensitivity of the CT-MARC to the key mix design properties. The comparisons between the CT-MARC and CT-index are also evaluated. The results indicate that the new CT-MARC parameter prevails over the existing CT-index. Specifically, while CT-index increases with increase in air void of the mix, CT-MARC decreases as mixture air void increases, effectively capturing the effect of mixture compaction/density on its performance. Additionally, CT-MARC accurately captures the impact of polymer modification on the cracking resistance of asphalt mixes while CT-index could not. Furthermore, CT-MARC also shows the ability to capture the effects of change in aggregate structure and source, binder PG and content, RAP/RAS content on mix cracking performance. Lastly, the good correlation between the CT-MARC and the fatigue life (Nf) measured from the binder time sweep testing of the corresponding binder blends underscores the ability of the developed parameter in evaluating the cracking performance of asphalt mixtures with different design properties.