Abedraba Abdalla, Mariam
(2024)
In situ application of porous particles in delaying asphalt
ravelling.
PhD thesis, University of Nottingham.
Abstract
Asphalt is one of the most widely used materials for road surfacing due to its cost-effectiveness, durability, and flexibility. However, as it is susceptible to temperature changes and traffic loading, it can deteriorate and require regular maintenance. Common surface distress types like cracking and ravelling, if not treated, can lead to significant pavement damage. To address these issues, recent advances in pavement engineering have introduced innovative materials in asphalt design aimed at improving asphalt performance and promoting more sustainable road infrastructure.
Among these advances, encapsulated asphalt rejuvenators are gaining attention due to their potential to extend pavement lifespans by promoting asphalt’s self-healing capabilities. Specifically, the calcium-alginate polymeric porous capsules manufactured through ionic gelation have been the most effective and are preferred by many researchers. Added during the asphalt mixing process, these smart materials release the rejuvenator under traffic load, mitigating bitumen ageing by helping to restore the binder’s flow properties, allowing it to close microcracks and prevent further asphalt cracking.
Research then focused on optimising these capsules by experimentally testing their impact on crack healing and ravelling mitigation. With that, a new mechanism was proposed, suggesting that the encapsulation inner structure, similar to cellular materials with energy absorption properties, might have an impact on asphalt ravelling mitigation by progressively collapsing under impact-loading, leading to local re-compaction in the asphalt mixture. This theory was further explored by characterising and testing various 3D-printed lattice structures with different relative densities as alternatives to rejuvenator capsules in a composite granular medium under cyclic and monotonic loading. The results revealed increased aggregate strain, improved energy absorption, and reduced aggregate crushing.
Therefore, this thesis identified the need to explore the impact of cellular structures on asphalt’s mechanical performance by using alternative cellular materials with air-filled cavities instead of encapsulated rejuvenators. For this, polymeric cellular foams, widely used in structural and energy-absorbing applications, were considered due to their ability to endure significant deformations. The first phase of the thesis focused on characterising the mechanical and morphological properties of a commercially available, high-temperature resistance, closed-cell polyethylene terephthalate foam (PET Airex-T92) across three densities (T92.130, T92.200, and T92.320), to evaluate the influence of varying densities on the cellular material’s energy absorption capabilities.
Results indicated that both PET T92 and oil-filled porous capsules shared a similar polyhedral cell structure and deformation stages, with higher-density materials offering greater stiffness, strength, and energy absorption.
Based on the characterisation of T92 foam, the focus shifted to examining its impact on the mechanical performance of Stone Mastic Asphalt (SMA), specifically ravelling resistance. The foams were added in varying volume percentages, and it was found through CT scanning that they could withstand the mixing temperature and compaction. Notably, adding 1% of T92.320 by the total volume of the asphalt reduced aggregate loss by 32.92%, maintained stiffness similar to the reference mixture, increased rut depth by 4% and doubled fatigue life. These results led to the hypothesis that incorporating a lower concentration of a softer cellular material, strategically distributed, to act as part of the aggregate skeleton could enhance load distribution by acting as a stress-absorbing medium and reducing stress concentrations at aggregate contact points.
Lastly, full-scale field tests were conducted to evaluate the long-term performance of asphalt with cellular materials. Due to the high cost of PET T92 foam, encapsulated rejuvenators were used instead. The first step involved developing a large-scale manufacturing machine for capsule production, the first of its type within the field in terms of scale and speed of production. Subsequently, a trial road with eight sections, varying in binder content and including oil-filled porous capsules, was constructed. Skid resistance, rut depth, and macrotexture were periodically assessed, with extracted core samples analysed before and after two years of ageing to gain further insights. Results showed that macrotexture evolution, along with rutting, showed a similar trend with and without capsules. However, cores containing capsules extracted on day one and after 2 years of field ageing from the road trial exhibited greater resistance to mass loss compared to asphalt without capsules, likely to equate to increased surface course life. Moreover, CT scans of the cores after two years of ageing confirmed the presence of capsules within the asphalt, further supporting the sustained effectiveness of the capsules in reducing aggregate loss over time.
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