Influence of bitumen grade and air voids on low-temperature cracking of asphalt

Low-temperature cracking is one of the main causes of pavement distress in colder areas with below-freezing temperatures. Resistance of asphalt to this cracking depends on a number of factors. A complex knowledge about this phenomena is a key factor during the design process, the building and the use stage of roads. Among the leading drivers are the type of bitumen and air voids in the bituminous mixture. In this research project, both the bituminous binders and mixtures were tested to determine the influence of these components on the low-temperature performance of asphalt pavements. A wide range of bituminous binders were tested, including 20/30, 35/50, 50/70, 70/100 and 100/150 grades. This approach allowed for an in-depth evaluation of the impact of bitumen type and air voids on the low-thermal susceptibility of asphalt mixture. The basic tests for bitumen were performed such as the penetration value at 25 °C (EN 1426), the softening point with the Ring & Ball method (EN 1427) and the group composition with SARA analysis using the Thin Layer Chromatography with Flame Ionization Detector (TLC/FID) method. Both neat bitumen and bitumen after Rotating Thin Film Oven Test (RTFOT) short-term simulated ageing (EN 12607–1) were tested. Next, bituminous mixtures based on mentioned bitumen with 2.2%, 3.5% and 6.5% air voids were prepared and subjected to uniaxial tension in Thermal Stress Restrained Specimen Test (TSRST) to determine their low-temperature performance. The test results point to the binder type as the key determinant of the low-temperature performance of asphalt. Softer bitumens of a higher penetration have a better low-temperature performance and the group composition is the determinant of the TSRST failure temperature. The effect of air voids were of secondary importance in this respect.