Permeable pavements : hydraulic and mechanical investigations

Lu, Guoyang; Oeser, Markus (Thesis advisor); Grabe, Jürgen (Thesis advisor); Wang, Dawei (Thesis advisor)

Aachen : Institut für Straßenwesen (2019)
Book, Dissertation / PhD Thesis

In: Aachener Mitteilungen Straßenwesen, Erd- und Tunnelbau 68
Page(s)/Article-Nr.: V, 174 Seiten : Illustrationen, Diagramme

Dissertation, RWTH Aachen University, 2019

Abstract

Conventionally, pavements are designed as sealed structures to inhibit the penetration of water into the structure, where it may cause damage. However, with the rapid increase of sealed areas due to urban development and industrial activity, the natural retention capacity of urban areas has exhibited a significant decrease. To recover the natural hydrological cycle and to mitigate risk of urban flooding, permeable pavements can be implemented. During a rainfall event, water can quickly infiltrate through the pavement structure into the subsoil; this relieves the requirements to urban drainage facilities and replenishes the natural water cycle. A void-rich pavement structure, such as Porous Asphalt (PA) and Pervious Concrete (PC) pavements based on open-graded aggregate distribution, is currently the most feasible and effective way to ensure a high permeability. Apart from the high hydraulic conductivity, porous structures also contribute to a reduction of traffic noise emissions by acting as an acoustic absorber. However, the open porous design results in a weakened pavement structure; the low shear-stress resistance of the porous structure can be identified as a main cause of the high susceptibility to grain ravelling. The durability as well as adhesion failure of porous pavement mixtures represent the most prominent obstacles restricting a more widespread application of permeable pavements. Based on the recent development of novel polyurethane-bound pervious mixtures (PUPM), the widespread application of fully permeable pavement (FPP) structures has become viable, which entails significant environmental benefits for the transport infrastructure. However, the saturation state has a major influence on the performance and durability of FPP. Due to the open porous structural design of pervious pavement material (PPM), surface runoff is allowed to infiltrate through pavement surface into the subsoil. In this case, moisture-induced damage is one of the most significant contributors to the premature deterioration of such permeable pavement mixtures. The pore-water pressures generated by intermittent dynamic vehicle loading can decrease the material strength significantly. The build-up and dissipation of pore-water pressure is recognized as a critical factor influencing the bearing capacity of FPP structures. The majority of research on the deterioration of permeable pavements has focused on phenomenological approaches, while the underlying mechanisms are still mostly unclear. To fully understand the moisture induced deterioration mechanisms of permeable pavements, comprehensive studies of the material characteristics, the hydraulic properties, the mechanical response and numerical investigations of FPP are required. The main objective of this thesis is to characterize the mechanical and functional properties of PUPM by combining applicable standards and modified testing methods. The water distribution and the flow characteristics in the vertical and the horizontal directions in PUPM will be quantitatively studied. The effect of the hydraulic gradient and mixture design on the flow will be characterized. Both laboratory and in-situ testing will be conducted to evaluate the response of permeable pavements to various loading states. Lastly, a numerical investigation will be conducted to explain the fundamental deterioration mechanism of FPP based on the novel PUPM material. This thesis provides an important reference for the facilitation of a wider application of PU binder in permeable pavements by providing an extensive understanding of the functionality of PUPM. Research on the hydraulic properties of PPM illustrates the inapplicability of Darcy’s law in the analysis of directional water transport in PPMs. Modified flow models are developed based on the pore microstructures, which exhibit more consistent results compared to the experimental data. Based on laboratory and in-situ measurements, it is found that the accumulated pore water pressure increases with the number of load cycles and also increases for each layer as the saturation increases. These findings support the quest for an in-depth understanding of stress states in FPP and the corresponding degradation mechanisms. During the numerical investigation, a modified stress-dependent moisture-sensitive cross-anisotropic elastic (SMAE) model was successfully applied as a constitutive law to characterize the coupled hydro-mechanical interaction in the unbound granular base (UGB) material. Based on the predictions for the stress state of the UGB layer, an optimized design of FPP with a PUPM surface layer is proposed. Further FEM analyses were performed based on the coupled stress-dependent moisture-sensitive cross-anisotropic elastoplastic (SMAEP) model. The results obtained by the SMAEP model predict a reduction of the tensile stress along the depth of the base course, and an increase of the compressive stress from the centre of the UGB layer to the top of the UGB layer, which is more suitable for the FPP system. To further understand the material characteristics and the deterioration mechanism of the FPP system, prospect researches are addressed. The constitutive model for the PUPM and the fracture behaviour are suggested for further in-depth studies. The analysis of the hydraulic properties, based on the characteristics of the three-dimensional (3D) pore structure is highly recommended. To further consider the dilation effect within the base course of FPP, a more comprehensive theoretical model based on hypoplasticity should be explored for the UGB layer. Moreover, it is suggested that the life cycle analysis (LCA) method is to be modified in a theoretical and experimental manner.