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Permeable Friction Course (PFC) is a layer of porous asphalt approximately 50 mm thick placed as an overlay on conventional impervious roadway surfaces. The use of PFC improves driver safety due to reduced splash/spray, improved visibility, and decrease in hydroplaning during rainfall events. In addition, PFC improves the stormwater runoff quality from the roadway and has recently been approved as a new stormwater best management practice (BMP) in the State of Texas. However, over time the pore space in the PFC can become clogged with trapped sediment, suggesting a decrease in driver safety and water quality benefits. Therefore, to ensure proper utilization of these benefits, accurate measurement of the drainage properties of PFC is required. In particular, measurement of the hydraulic conductivity is used to determine whether the drainage benefits of PFC are expected to persist. This paper discusses methods for measuring the porosity of core specimens and hydraulic conductivity under non-Darcy flow behavior characterized by the Forchheimer equation. Laboratory and field experimental results of four years of data (2007 to 2010) for three roadways around Austin, TX are presented. The accurate measurement of in-situ hydraulic conductivity using a new nondestructive field test is essential to maintain proper drainage of the PFC layer and to determine when the pore space has become clogged with sediment. In addition, an overview of numerical modeling results of nonlinear two-dimensional flow through PFC is outlined and results are presented in order to properly determine the hydraulic conductivity. Statistical analysis of porosity data show a decrease in porosity with time suggesting clogging of the pore space. Porosity values range from 12% to 23%. However, there has been no observed statistical decrease in hydraulic conductivity with time, suggesting the driver safety and water quality benefits of the PFC layer have persisted after six years of operation. Hydraulic conductivity values on PFC core specimens range from 0.02 cm/s to nearly 3 cm/s. Furthermore, no maintenance of the PFC layer has been conducted during that time.

A permeable friction course (PFC) is a 50 mm layer of porous asphalt placed on top of conventional, impermeable pavement. This paper describes a multi-year research effort on the water quality and hydraulic aspects of PFC. Water quality monitoring of three field sites near Austin, Texas showed a 90% reduction of total suspended solids (TSS) compared to conventional pavement. Significant reductions were also observed for total copper, total lead and total zinc, though concentrations of dissolved constituents were not significantly different. The hydraulic properties of PFC are of interest to assess the drainage capacity of the pavement and the effects of clogging. The properties investigated in this study were the porosity and the hydraulic conductivity. Porosity was measured from core specimens and found to range from 0.12 to 0.23. Hydraulic conductivity was also measured from core specimens and ranged from 0.1 to 3 cm/s. A new field method for measuring the in-situ hydraulic conductivity of PFC was developed and compared to the laboratory measurements. Predictions of the water depth on PFC roads are needed to assist designers in selecting a pavement thickness and to evaluate the effects of clogging. A Permeable Friction Course Drainage Code (PERFCODE) was developed to make these predictions. Measured porosities and hydraulic conductivities were used as inputs to PERFCODE. Outputs were the variation of water depth through a storm and the runoff hydrograph. The modeled hydrograph is compared to runoff hydrographs obtained by field measurement.