Investigation of Bonding between Open Graded Friction Courses and Underlying Asphalt Pavement Layers

Open Graded Friction Course (OGFC) asphalt was created to enhance roadway safety and has been used since the 1950s in the United States to improve the frictional resistance of asphalt pavements (Kandhal and Mallick, 1998). The use of OGFCs increased across the United States in the 1970s in light of the Federal Highway Administration (FHWA) program to increase skid resistance on roadways (Kandhal, 2002). OGFC mix consists mainly of a high proportion of coarse aggregate having a gap gradation, small amounts of fine aggregate and asphalt binder (Shaowen and Shanshan, 2011). OGFC aggregate gradations are engineered to increase the air voids and allow water to infiltrate into the pavement instead of flowing over the pavement surface. OGFC is a pavement surface layer that is typically 0.75 to 1.25 inches thick with a designed air void content of above 15%. The quality of the bond between asphalt pavement layers is critical to the overall performance of a multi-layer pavement structure. This is also true for pavements comprised of an OGFC layer as a poor bond between the OGFC layer with the underlying layer can result in delamination and accelerated raveling. There are several variables that affect the bond between pavement layers including underlying surface condition, tack coat material, and application rate, OGFC mixture gradation (or texture), OGFC binder viscosity and content, compaction effort, and ambient conditions. This research investigated the variables affecting the bond between the OGFC and the underlying pavement layer to gain a thorough understanding of the influence of each variable. This evaluation was conducted on composite specimens consisting of an OGFC layer compacted on top of a dense graded Surface Type A (STA) asphalt substrate. It was found that the tack coat is not only beneficial for bonding, but also it is effective in permeability reduction. There was a substantial reduction in permeability after the tack coat application and compaction of OGFC on the top of STA. The interface shear strength (ISS) test results indicate that UltraTack had the highest shear strength among the five different tack coats included in this study. In regards to aggregate gradation it was found that the bond strength increased with the increase in percent passing No. 4 sieve for the composite specimen with NMAS of 12.5 mm. It was assumed that with higher percent passing No. 4 sieve the aggregate potentially has more contact points at the interface which can improve the adhesion bond and increase the bond strength. It was also found that there is a direct relationship between the number of gyrations and the ISS\u27the higher the gyration number, the greater the bond strength. The ISS results differences were generally not significant between 30 and 45 gyrations, but it was significant greater than the specimens made with 15 gyrations