Effects of specimen size and loading conditions on the fracture behaviour of asphalt concretes in the SCB test

The semi-circular bending (SCB) test has gained popularity for cracking evaluation of asphalt concrete due to its simplicity in the test setup and sample preparation. The current standards for the SCB test, such as EN 12697-44:2010, AASHTO TP 105-13, ASTM D8044-16, and ASSHTO TP 124-18, utilise 150 mm as the standard diameter for test samples. Recently, the 100 mm diameter was investigated and its applicability for use in the SCB test was confirmed. However, investigation of factors affecting the fracture behaviour of 100 mm diameter samples, such as loading rate, temperature, and aggregate size, is still limited. As yet, no comparison of the SCB fracture performance of 100 mm diameter and 150 mm diameter samples (e.g. fracture toughness, fracture energy, cracking indexes) is available for the nominal maximum aggregate size (NMAS) of 10 mm or larger. Furthermore, conversion of results from the two diameters is not yet available. For applications of the 100 mm samples in parallel with the 150 mm samples, comparisons and conversion of the fracture properties obtained from the two sample sizes are significant. This study was performed to fulfil that purpose. Both SCB sample diameters were prepared with three different thicknesses of 30, 40 and 50 mm, using different NMAS of 10, 14 and 19 mm. The SCB samples were tested at four different loading rates of 0.5 mm/min, 1 mm/min, 5 mm/min and 10 mm/min, and at two temperatures of 25 °C and 10 °C. The study investigated fracture properties including fracture toughness, fracture energy, and cracking indexes such as the flexibility index and the cracking resistance index. The results showed that the fracture toughness for the two diameters was similar, while the fracture energy obtained from the 100 mm samples was roughly 21% less than that achieved from the 150 mm samples. The cracking resistance index results obtained from the 150 mm samples were approximately 8% higher than those obtained from the 100 mm samples. The flexibility index from both diameters showed low correlation. The overall result suggested that the SCB test with 100 mm samples can be used to evaluate the fracture behaviour of asphalt mixtures with NMAS up to 19 mm, and the fracture properties obtained from the two different sample sizes are convertible.