Measurement and Modeling of Hydraulic Characteristics of Permeable Friction Course (PFC)

Abstract

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.