Model of Flow Regimes in Porous Pavement and Porous Friction Courses

Abstract

Porous pavements (PPs) and porous friction courses (PFCs) are increasingly recognized as viable alternatives to traditional impervious pavements. PPs and PFCs passively provide safety, hydrologic, and rainfall-runoff treatment benefits. These benefits are a result of such permeable systems providing hydraulic conveyance and filtration of particulate matter (PM) transported by runoff. With respect to hydrology, these systems reintroduce infiltration, evaporation, and storage phenomena; with respect to traffic and road safety, the systems increase skid resistance in wet conditions, reduce hydroplaning, and reduce splash-and-spray phenomena; with respect to runoff treatment, the systems function as a filter for PM and PM-bound constituents. These phenomena and the models therefore are a function of the hydrodynamics (which are not necessarily laminar) in the porous medium and commonly are characterized by the hydraulic conductivity of the porous medium. The measured hydraulic conductivity (permeability) was examined in 12 common porous asphalt mixes. A Darcian model for hydraulic conductivity in the laminar flow regime was examined, as commonly done for PPs and PFCs. However, hydrodynamic regimes in porous asphalt that extended beyond laminar flow during testing were identified for these porous mixes. Therefore, the variability of the hydraulic conductivity as a function of applied hydraulic head in a hydrodynamic regime (based on the Reynolds number) was measured and modeled. From these results, a saturated seepage model was developed; it is valid for laminar and transitional regimes. Implications for the application of standardized permeameter testing of such porous mix designs are reported.