Improved overlay tester for fatigue cracking resistance of asphalt mixtures.
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2017-01-01
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Abstract:Premature cracking of the asphalt concrete (AC) layer in flexible pavement is one of the major concerns of the pavement community. Over the past decade, AC mixes have been designed using the Hamburg wheel-tracking device to improve their rutting potential that might have impacted the cracking resistance and flexibility of the AC mixes. Several highway agencies have either implemented or considered implementing performance tests to predict the cracking potential of AC mixes in the laboratory setting. One such test, the Overlay Tester (OT) test, measures the number of cycles to failure of the AC specimens by simulating the opening and closing of the cracks induced by daily temperature variations and high tensile strain generated by the traffic load. The OT test is routinely used by the Texas Department of Transportation (TxDOT) to screen mixes with well and poor crack resistant potential. The variability of the number of cycles to failure that is used as the performance index is expressed as a major concern in reliably characterizing the cracking potential of the AC mixes.
The performance of the OT, in general, and the number of cycles to failure, in particular, were evaluated in this study. The consistency of the number of cycles to failure and potential parameters that can be measured from the OT were comprehensively investigated with two different AC mix types. Fundamentally, the cracking potential of an AC mix can be characterized in two stages: a) crack initiation and b) crack propagation. An alternative cracking methodology and performance indices were implemented for the OT considering these two stages. The consistency and repeatability of the proposed performance indices, critical fracture energy and crack progression rate, seem to be better than the acceptable repeatability level defined as a coefficient of variation (COV) of less than 20%. The proposed cracking methodology and preliminary failure limits seem to characterize and discriminate satisfactorily the cracking resistance of several AC mix types commonly used in Texas. A parametric study was conducted on key variables (e.g., glue type and gluing method) considered in the current OT specifications (Tex-247-F) using synthetic specimens. The results of this parametric study were then used to improve the specimen preparation and testing processes. The improved OT test method was preliminarily validated with OT tests on field cores from pavement sections with known field performance. Given its promise in this study, the improved OT test method is recommended as a routine test during the mix-design process of AC mixes.
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