Influence of specimen geometry on mode I fracture toughness of asphalt concrete

Abstract

Determination of fracture toughness (KIc) for the asphalt mixture concrete requires employment of suitable testing methods and specimens. Disc-shape specimens are among the favorite test geometries for conducting the fracture toughness experiments on asphalt concrete materials since they can be manufactured easily from those samples obtained by the gyratory compactor machine. Semi-circular bend (SCB) and edge notch disc bend (ENDB) specimens are two suitable and well-known specimens in this regard, and both of them are tested using three-point bend setup. In order to compare the consistency of fracture toughness results obtained from these two specimens, an experimental study is performed herein on a typical asphalt concrete mixture. In the experiments, the thickness of SCB and ENDB specimens and also the loading rate and test temperature are considered as variable. It is found that the KIc values obtained from both SCB and ENDB testing methods are in good agreement. The corresponding value of KIc obtained from the SCB specimen being slightly smaller than the data obtained from the ENDB specimen. Using a two term KI + T strain-based fracture model, that the critical normal strain value for both specimens is shown to be nearly the same and constant for both SCB and ENDB specimens. In addition, it is found that the fracture toughness value and load carrying capacity of asphalt mixture increases by increasing the specimen thickness (from 30 to 70 mm) and the loading rate (from 1 to 6 mm/min), and by decreasing the test temperature (from −15 °C to −25 °C).