Numerical Modelling of the Hydraulic Performance and Clogging Development of Porous Asphalt Mixes Using CFD and DEM
Creators: name: Zhang, Zhiyuan
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400512
subject: Transport engineering
classificationCode: 400505
subject: Construction materials

description: The hydraulic performance of porous asphalt (PA) pavements is significantly affected 
by pores connectivity, shape, content, and distribution. The variety of pore patterns has 
been proven to impact the porous structure significantly. However, for any type of 
permeable pavement, clogging is a challenge and a potential issue that cannot be 
perfectly addressed. The accumulated clogging materials, such as debris, sediments, 
and organic matter, can enter the pores, progressively blocking the water flow path and 
affecting the infiltration capacity negatively. The functionality of porous asphalt will 
be eventually lost once the pavement is fully clogged. It is necessary to systematically 
analyse the relationship between pore characteristics and hydraulic performance, and 
evaluate the magnitude of the clogging mechanism inside the pore structure. 
Compared with laboratory test, numerical modelling can provide more details, visualise 
motions, and reveal the movement at any flow time inside a porous structure, which is 
hard to observe during laboratory experiments. The discrete element method (DEM) 
has been widely used to reconstruct the granular structure in the geomechanical field; 
commonly, soil granules have been represented by spheres. However, there is a 
substantial difference between spherical particles and actual aggregate in terms of 
shapes, which might lead to inaccurate structure reconstruction, especially affecting the 
prediction of hydraulic conductivity. Based on the literature review, limited 
investigation has been conducted into the hydraulic performance of asphalt pavements 
considering a more realistic, thus complex, structure of the asphalt concrete. In most 
cases, the porous structure is created using the DEM approach; however, in those 
studies, spherical balls representing the aggregate are used to virtually reconstruct a PA 
pavement specimens. 
In addition to using simple spheres, previous studies had limitations in reconstructing 
realistic particle size distributions (PSD). Due to the characteristics of the spherical 
balls used in past models, compaction of the asphalt sample proved difficult, especially 
when the size difference between the largest and smallest particle diameters is large. 
Due to the sphere pattern and PSD, the analysis of virtual PA samples made by DEM 
using spheres has significant limitations. Two different particle arrangements, cubic 
and hexagonal arrangements, can also be incorporated in a DEM model. The results 
from past studies showed that the minimum porosity of spherical aggregates can reach 
47.64% for cubic arrangement and 25.95% for the hexagonal arrangement. The design 
used in this study for the virtual reconstruction of PA specimens follows the guidelines 
of the Victorian Department of Transport and Planning, Australia (Section 417 - Open 
Graded Asphalt, 2018) for open-graded asphalt mixes (OGA). According to the 
specification, OGA's porosity shall be in the range of 18% to 25%, which is outside of 
the porosity tolerance range for PA samples made by spherical aggregates. To control 
the air voids content, aggregates in a DEM model would need to be more realistic (i.e. 
shape and size) so that the compaction process can be similar to the experiments 
conducted in the laboratory. 
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This thesis aims to investigate the relationship between the hydraulic performance of 
PA mixes and their porous structure through numerical simulation using the DEM and 
computational fluid dynamics (CFD). This advanced numerical model combining DEM 
with CFD method is developed to mitigate the adverse effects brought by spherical
shape aggregate and deeply understand the relationship between the volumetric 
properties of the pore structure of PA pavements and the infiltration capacity. To solve 
the issue caused by spherical aggregates, a photogrammetry approach is applied to 
capture the realistic aggregate shape as scanned in the laboratory via a simple mobile 
phone. The 3D model so reconstructed follows the PSD of the OGA mix, as specified 
by the Victorian standard. Firstly, the investigation is carried out in 2D to understand 
the foundations of combining the processes of DEM and CFD . Experimental PA slices 
were prepared and reconstructed computationally using image morphology for 
calibration and validation purposes. The result shows that, in a 2D simulation, DEM 
can perfectly reconstruct the pore structure similarly to the structure captured from the 
laboratory-made slices. However, it also pointed out that limitations still exist under 2D 
analysis and a single-phase CFD simulation in pore structure calculations of hydraulic 
conductivity. 
To investigate the effects of the volumetric properties and the pore structure of PA 
pavements on the infiltration performance and run-off potential, a 3D numerical model 
using DEM for sample generation and CFD for hydraulic simulation is developed. This 
study scans various aggregates at different gradations to build a comprehensive 3D
shape template database. Then, DEM software is applied to randomly generate non
spherical particles and compact the asphalt mix to the design porosity of 20% and 25%. 
To provide a higher-quality and accurate calculation for the water motion inside 
connected pores, a multi-phase with volume of fluid (VOF) model is applied to replace 
the single-phase CFD simulation. With the VOF model support, the multi-phase CFD 
simulation provides detailed information about the water infiltration inside the pore 
structure dynamically. The effect of the volumetric properties of pore structure on 
hydraulic conductivity can be further evaluated via the voids distribution curve of pore 
location over depth. Moreover, the relationship between general air voids and effective 
voids can also be analysed via VOF-CFD simulation, which builds the fundamental 
understanding of the effect of air voids on the hydrology behaviour of PA pavements. 
To investigate the clogging mechanism in pore structure and its impact on the hydraulic 
performance, the study on the short-term hydraulic conductivity using DEM and CFD 
approach is conducted. The clogging potential at different porosities is different. 
Various gradations of sediments are applied to test the clogging potential at 20% and 
25% porosity. To better understand the clogging mechanism, pre-tests about clogging 
simulation with sediment gradations from 0.25 mm to 1.0 mm in PA samples with 20% 
and 25% porosity are applied. The research shows that it is harder for particles larger 
than 0.6 mm to travel inside the pores and move deeper into the asphalt layer when the 
porosity is less than 25% (i.e. surface accumulation). Based on the finding and balance 
of computing power, the mixed sediments gradation of 80% 0.5 mm and 20% 0.75 mm 
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particles is used for the analysis of the clogging mechanism and reduction in hydraulic 
conductivity. 
Lastly, it should be noted that the study of 2D modelling with single-phase simulation 
(Chapter 4) is published in the Journal of Hydrology in April 2023. The study of 3D 
modelling with multi-phase CFD simulations (Chapter 5) has been submitted to 
Construction and Building Materials in October 2023, and it is currently under review. 
The study “Investigation for the effect of sediment clogging development on the short
term hydraulic conductivity using CFD and DEM method” (Chapter 6) prepared in 
December 2023 is currently under review.