Hydraulic properties of polyurethane-bound permeable pavement materials considering unsaturated flow

Constant urban and infrastructural growth leads to increased sealing of areas in municipal areas. This surface sealing results in distortions of the natural water cycle, which again can lead to noticeable changes in the natural ecosystem. To counteract the effects of surface sealing, innovative permeable road structures provide a promising environment-friendly opportunity. An innovative highly permeable road surface material based on alternative binders using polyurethane (PU) to replace the conventionally used bituminous binders has been developed in the course of an ongoing research project. It is intended that the high permeability of the road's surface material and underlying support layers leads to a quick drainage through the road construction down to the subsoil. The aim of this study is to understand and quantify the relevant hydraulic properties of PU-bound road surface material, i. e. the water retention behaviour and the hydraulic conductivity in saturated and unsaturated states. A predictive model to determine the surface material's water retention behaviour was used as a first estimation and very low air entry values were observed. Results from laboratory experiments on the material's saturated hydraulic conductivity as well as the water retention behaviour in the low suction range are presented. Additionally, empirical hydraulic models to describe the retention behaviour and models for the saturated and unsaturated hydraulic conductivities are outlined and applied.