Quantification and Evaluation of Long-term Draindown and Its Influence on the Raveling Susceptibility of Open-graded Friction Course

A porous pavement is a type of sustainable pavement that allows stormwater to infiltrate through the pavement into the natural soil bed. An open-graded friction course (OGFC) is a type of porous asphalt mixture that is commonly used as a wearing course typically having a thickness less than 1.5 inches that is constructed over a conventional asphalt surface. This porous wearing course is used to improve the frictional resistance of pavements and minimize hydroplaning on highways. Raveling is a defect commonly seen in OGFC because of the limited amount of fine aggregates in the mix. This reduction in fine particles from the aggregate matrix causes a reduction in the number of contact points between aggregate particles. This reduction in contact points limits the amount of asphalt binder that is able to bind coarse aggregate particles together. The fewer the contact points between aggregate particles; the more likely raveling is to occur on the surface of the pavement (Shaowen and Shanshan, 2011). Previous studies have been conducted on the draindown that occurs during production and hauling of the asphalt binder due to gravitational forces. However, only a few studies have been conducted on the draindown of asphalt binder after installation, over the service life of the pavement. For the purpose of this research study, “long-term draindown†is defined as the downward migration of asphalt binder through the pore structure of an open graded friction course over the service life of the structure after construction. OGFC can also exhibit clogging of the pore structure caused by the gravity-induced draindown (long-time draindown) of asphalt binder from the top to the bottom of the OGFC pavement layer over the life of the pavement. This can cause a reduction in the binder film thickness surrounding the aggregate particles near the surface and potentially lead to an increase in the raveling susceptibility of the OGFC mix design while clogging the accessible air voids of the structure. The decrease in binder thickness surrounding the aggregate particles and oxidation of the remaining binder film near the surface of the pavement can lead to an increase in raveling susceptibility of OGFC. This research study evaluated and quantified long-term draindown and its influence on the raveling susceptibility of OGFC. In order to effectively evaluate the effects of long-term draindown on the raveling susceptibility of OGFC and identify the underlying mechanism; new laboratory test methods were designed and compared to the existing test method that is currently used to evaluate the raveling susceptibility of OGFC. The new laboratory test methods focused on simulating the forces commonly applied to the surface of OGFC during its service life and showed more of a correlation to the direct shear and indirect tensile strength tests. This indicates that the two new test methods show more of a cohesive failure with minimal fracturing of the aggregate particles compared to the existing test method (Cantabro). The long-term draindown was quantified using image analysis and a Draindown Factor [DF] was calculated. The DF indicated that long-term draindown does have an influence on the raveling susceptibility of the surface of OGFC mixture. The mixture becomes more susceptible to raveling as draindown increases (i.e., higher the DF)