Tribological Evaluation for an In-Depth Understanding of Improved Workability of Foamed Asphalt

Asphalt binder acts as a lubricant between two aggregates during mixing and compaction. The frictional behavior of asphalt binder as a lubricant in an asphalt-aggregate system plays an important role in mixing and compaction (workability) of warm-mix asphalt (WMA). This study evaluates the frictional behavior of foamed WMA through tribological characterization using a ball-on-three-plates apparatus assembled inside a dynamic shear rheometer (DSR). The tribological test was conducted on six foamed asphalts with varying foaming water contents (FWCs) and on a controlled unfoamed asphalt binder at four different temperatures: 25°C, 60°C, 100°C, and 135°C. These binders were also characterized for rheological and chemical properties. Test results show that the foaming process alters the frictional resistance of the foamed asphalts; specifically, the foaming process reduces the coefficient of friction in elastohydrodynamic and hydrodynamic lubrication regimes. Reduced frictional resistance helps in the formation of a better coating during mixing and improves the sliding ability between aggregates during compaction at reduced temperature.