Factors affecting shear strength between open-graded friction course and underlying layer

• Shear strength of open-graded friction course and underlying layer was tested.
• Three factors affecting the shear strength were considered.
• Surface texture depth was measured and correlated to shear strength.
• Statistical analysis was performed to compare the factors.

This study investigates the factors that affect the bonding strength between open-graded friction course (OGFC) and underlying layer through laboratory testing. The direct shear strength test was performed to obtain the shear strength between OGFC and different underlying layers. Three factors were considered in the study: mixture type of underlying layer, tack coat application rate, and temperature. Two types of dense graded surface mixture and one type of stone matrix asphalt (SMA) were selected as underlying layer. The shear strength was evaluated at four tack coat application rates and three test temperatures. To investigate the friction between OGFC and underlying layer on the shear strength, the surface texture depth of underlying layer was also measured. Results from the study showed that underlying layer mixture type, tack coat application rate, and temperature all played a significant role in the shear strength, with temperature as the most significant factor followed by surface texture depth of underlying layer. At low temperatures, high stiffness of asphalt made both tack coat rate and surface texture depth of underlying less significant. At intermediate to high temperatures, surface texture depth of underlying layer had a significant effect on the shear strength, indicating that selection of appropriate underlying layer with adequate friction with OGFC is critical for a good bonding shear strength. Surface texture depth of underlying layer can be used as an indicator of the friction between OGFC and underlying layer.