posted on 2024-05-28, 23:43authored byZhiyuan Zhang
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.