Structural Coefficient of Open-Graded Friction Course

Open-graded friction courses (OGFCs) are noted for their contributions to driving safety and noise reduction on flexible pavements. The inclusion of OGFCs in pavement thickness design has varied across the United States. Some states equate OGFC with dense-graded materials, whereas other states give no structural value. The demand for more economical pavement cross sections fuels the need to quantify the structural value of OGFC to ensure optimized pavement thicknesses. Two full-scale pavement sections—one with OGFC as the surfacing material and one without—are evaluated to determine the structural coefficient of the OGFC layer. The sections were built with the same total thickness at the National Center for Asphalt Technology Test Track in 2009 and were subjected to more than 9 million equivalent single-axle loads. Deflection data from extensive falling weight deflectometer testing were used to find the OGFC structural coefficient (a_OGFC) equal to 0.15. This coefficient translated into a required 12% thickness increase to achieve the same pavement structural number as the control section. To confirm this finding, direct strain was measured in each section to compute the pavement thickness required to achieve equivalent strain levels. The increase in thickness was within 0.05 in. of that predicted through the a_OGFC approach. Performance monitoring indicated similar performance levels in terms of fatigue cracking (none) and rutting (approximately 5 mm). Additional monitoring and research is needed to validate the OGFC structural coefficient further and develop standards for incorporating OGFC into mechanistic–empirical pavement design.