The Overlay Tester (OT): Using the Fracture Energy Index concept to analyze the OT monotonic loading test data

In this study, the concept of the Fracture Energy (FE) Index was explored as a fracture parameter to characterize and differentiate the cracking resistance potential of hot-mix asphalt (HMA) subjected to the Overlay Tester (OT) test under monotonic loading in the laboratory; denoted herein as the OTM test. Mathematically, the FE Index was derived and defined as a parametric ratio of the total FE to the HMA tensile strength and tensile strain at peak failure load per unit crack length under the OT monotonic loading testing, with a tensile loading rate of 3.125 mm/min at a test temperature of 25 °C. Various HMA mixes, commonly used in Texas, with cracking resistance potential ranging from poor to good, were evaluated. Corresponding results indicated that the FE Index has promising potential to be used as a fracture parameter to discriminate and rank the cracking resistance potential of HMAs in the laboratory. While variability was generally within the acceptable tolerance (i.e., coefficient of variation of less than 30%), the FE Index exhibited sensitivity to the mix-design variables and was able to capture the effects of changes in the asphalt-binder content and temperature variations. Thus, this concept should be investigated further with more mixes and validated with field data for subsequent consideration in routine mix-designs and screening for adequate HMA cracking resistance in the laboratory.

► The Overlay Tester (OT) test is explored in a monotonic loading mode.
► The concept of Fracture Energy (FE) Index is introduced as a fracture parameter.
► FE Index successfully screens HMA mixes and is sensitive to mix design variables.
► A correlation is proposed between monotonic and repeated loading OT tests based on FE Index.
► Monotonic loading OT test has promising potential as a surrogate HMA fracture test.