Traseira Piñeiro, Laura
(2024)
Evaluation of encapsulated rejuvenators conventionally used for asphalt self-healing enhancement as architected cellular particles.
PhD thesis, University of Nottingham.
Abstract
Recent advancements in pavement engineering have introduced innovative materials and methods to enhance asphalt performance and promote road sustainability. Among these, encapsulated asphalt rejuvenators are gaining attention for their potential to extend the lifespan of pavements by promoting asphalt’s self-healing capabilities. Asphalts containing this type of additive are regarded as smart materials, as the capsules, that are added to the asphalt during the mixing process, progressively release the bitumen rejuvenator in response to traffic loading. Thus, bitumen ageing, which causes asphalt to become stiffer and prone to cracking, can be controlled, and the binder regains its ability to flow and close any microcracks, preventing asphalt cracking.
The first research studies focused on examining different categories of asphalt rejuvenators, evaluating not only their effectiveness in restoring the chemical composition of the aged bitumen, but also their stability during the encapsulation process and their thermal degradation during asphalt manufacturing. Concerning the encapsulation process, although different encapsulation methods were evaluated, the prevalent manufacturing technique involved the external gelation of sodium alginate in the presence of calcium. Subsequently, research delved deeper into the optimization of the encapsulated rejuvenators through experimental testing, by assessing their effect on the crack-healing of damaged samples. However, the working mechanism of the capsules is still subject to some uncertainties. Although the calcium-alginate polymeric structure has been studied primarily for its capacity to progressively release the rejuvenator, the impact of the encapsulation structure itself, a cellular material with energy absorption properties, on asphalt performance and durability has not yet been thoroughly explored.
Firstly, this thesis examined the effect of cellular capsules on the mitigation of asphalt ravelling, which is, along with asphalt cracking, one of the most prevalent distresses on the road network. It was observed that asphalt’s mass loss linearly decreased as the cellular capsule content was increased. When evaluating the effect of the compaction level and the aggregate gradation on the mass loss of asphalt specimens with and without capsules, a correlation was found between the efficiency of the cellular capsules in mitigating the mass loss and the air void content of the asphalt mixtures. Moreover, after assessing changes in the internal porous structure of the capsules under loading through a combination of experimental testing and imaging techniques, it was found that the capsules were more than a release mechanism, as their cellular structure experienced a progressive collapse under impact-loading while absorbing energy, which may induce a local re-compaction in the asphalt mixture.
Based on these results, the focus shifted towards a comprehensive evaluation of the cellular capsules’ impact on the performance of Stone Mastic Asphalt (SMA) manufactured at an asphalt plant. Variables throughout asphalt production that might influence the mechanisms governing the plastic deformation of the capsules containing an asphalt rejuvenator were evaluated in this assessment and then, results were contrasted with lab-produced asphalt under controlled conditions. Lastly, the capsules’ deformation was qualitatively evaluated using a Finite Element (FE) model to verify findings from the testing campaign. It was concluded that cellular capsules can resist mixing at an asphalt plant without compromising their performance, and the deformation of the capsules affected asphalt’s stability by up to 13%, reduced the particle loss by up to 25% and increased asphalt’s macrotexture by 10%. It was also hypothesised that, to maximize their energy absorption, the cellular capsules must be part of the aggregate skeleton.
Lastly, a fundamental study was conducted to test the hypothesis made, in which the use of lattice structures that absorb energy as an alternative to cellular capsules containing asphalt rejuvenators was evaluated as well due to the advantages they offer in the design, optimisation, and manufacturing processes. Therefore, several lattice structures with a wide range of stiffness, strength, and energy-absorption properties were designed and 3D-printed, and two lattice structures with distinct properties were selected for the fundamental study. Subsequently, the mechanical behaviour of a composite granular medium composed of granite aggregates and either the cellular capsules previously manufactured or 3D-printed lattices was investigated. Cyclic and monotonic experiments were conducted on this composite granular medium, to measure its mechanical response, the plastic deformation of lattice structures and cellular capsules, and the crushing behaviour of aggregates. Through a comparative analysis of results obtained from lattices with significantly different mechanical and energy-absorption properties, it was found that capsules induced physical changes to the aggregate skeleton of asphalt. Besides, despite design constraints, it was proved that lattice metamaterials could be successfully 3D-printed and tailored to meet specific requirements, and the Voronoi lattice emerged as a potential alternative to replace the capsules.
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