Abou-Foul, Mustafa
(2019)
Moisture transport through pores in porous asphalt pavement materials.
PhD thesis, University of Nottingham.
Abstract
Using a porous pavement system is the most feasible alternative to the traditional impervious asphalt and concrete pavement. The ability of porous asphalt (PA) to allow infiltration of storm water runoff make its installation very successful in the pathway, shopping areas, parking areas and driveways as well as areas with moderate traffic use. The benefits of the porous asphalt system are not only its capacity to reduce water runoff but also in its abilities to cool the surface temperature to avoid the Urban Heat Island (UHI) effect caused by a high pavement temperature and to reduce pollutant load and noise. However, there are critical challenges associated with porous asphalt related to its durability and functionality as PA tends to clog with time and loses its resistance to damage.
This research was mainly aimed at investigating the moisture transport properties of asphalt materials. Moreover, the thesis proposes a novel methodology for the study of water permeability and water evaporation of porous asphalt (PA) materials. The inspirations behind this work are related to the critical challenges associated with porous asphalt in regard to its durability and functionality.
The research was undertaken to study the effect of the porous asphalt pore space properties on the hydraulic conductivity of a wide range asphalt porosities. X-ray computed tomography was used to identify the 3D pore space of porous asphalt samples and a statistical model for the hydraulic conductivity of asphalt mixtures in the range of porosities studied is proposed. Results show that the hydraulic conductivity is related mostly to the air void content.
The study also investigated the water evaporation process from an asphalt mixture under laboratory conditions to understand the effect of the porous asphalt air void space structure on evaporation dynamic from asphalt materials. It was observed that the evaporation rate is related predominantly to the air void content, pore size distribution and tortuosity. Moreover, in order to be able to track the moisture distribution and transport inside the pore space of porous asphalt materials, the 3D pore structure was printed in transparent resin blocks, then, a water evaporation test was performed on the 3D printed samples using dyed water. The macro-porosity, pore size distribution, air void tortuosity, water conductivity and water retention curves of the 3D printed porous asphalt samples were quantified by means of image analysis. Furthermore, a model estimates the evaporation rates from porous asphalt materials for a wide range of porosities was developed and tested experimentally. Within the range of asphalt mixtures studied in the present work, the evaporation rate was found to be related predominantly to the porosity, pore size distribution and tortuosity. It was found that the period during which water evaporation occurs at the surface is relatively short during the drying of porous asphalt materials due to their relatively large pores that weaken the capillary forces. The results also illustrate that the beginning of stage-2 evaporation depends on the porosity and tortuosity of the porous asphalt material, among other parameters. This study results and analysis provide new insights into the dynamics of water evaporation from asphalt materials.
The present work was also aimed at exploring the effect of the average void diameter on the hydraulic conductivity of a porous asphalt mixture, with porosities ranging from 14% to 31%. The pore spaces of asphalt were generated using a novel virtual air pore generator tool optimized by a Differential Evolution (DE) algorithm and 3D printed in transparent resin blocks. The permeability tests were conducted using the 3D printed transparent resin samples to understand the effect of pore topology on the hydraulic conductivity. Moreover, the virtual air void space was compared to that of real asphalt sample pore systems quantified using X-ray Computed Tomography scans. In addition, the new air pore generator tool can generate a realistic 3D pore space for asphalt materials in terms of visual, topological, statistical and air void shape properties. It was found that, in the range of porous asphalt materials investigated in this research, the high dispersion in hydraulic conductivity at the same air void content is due the effect of the average void diameter. Finally, the relationship between average void diameter and the maximum aggregate size and gradation in porous asphalt materials was investigated.
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