Author: Li, Danning
Title: Multiscale investigation on the aging and recycling mechanisms of asphalt rubber pavement
Advisors: Leng, Zhen (CEE)
Degree: Ph.D.
Year: 2024
Subject: Pavements, Asphalt
Pavements, Asphalt -- Recycling
Product life cycle
Hong Kong Polytechnic University -- Dissertations
Department: Department of Civil and Environmental Engineering
Pages: xvii, 150 pages : color illustrations
Language: English
Abstract: Asphalt rubber, a paving binder produced by mixing crumb rubber modifier (CRM) from waste tires and asphalt cement, has been more and more widely used over the past years around the world, because of its various economic and environmental benefits. With the increasing application of AR, the recyclability of AR pavement at the end of its service life has become an inevitable question. However, a recycling framework dedicated for reclaimed asphalt rubber pavement (RARP) is still missing due to the intricate aging mechanism of AR binder and the complexities in the rejuvenation design and recycling process. To fill these research gaps, a multiscale approach combining experimental methods, micromechanical modeling, and life cycle analysis is applied in this study to achieve a deeper and more comprehensive understanding on the aging and recycling mechanisms of AR pavement.
To achieve the objectives of this study, two main factors affecting the aging of AR binder were investigated, namely the structural and mechanical evolutions of the multiphase system of AR, and the absorption of bitumen and its components by rubber. AR binders at different aging conditions were phase-separated to reveal the structural change of the multiphase system. A series of chemical and rheological tests were conducted and a micromechanical back-calculation program was developed to reveal the compositional and mechanical change of AR and its components under different aging conditions. The results proved the superior aging resistance of AR binder and the indispensable role of rubber absorption. The inner structure of AR binder evolved during aging with decreased swelling rubber proportion, whose mechanical contribution to the overall AR binder gradually decayed during aging.
The blending efficiency of RARP is an important premise before the rejuvenation design. Two mobilization indexes of aged rubber and bitumen were developed based on the Fourier transform infrared (FTIR) spectroscopy and thermal gravimetric analysis (TGA) tests, respectively. Both indexes were found to increase with the mixing temperature, and a positive correlation existed between them. The increased blending efficiency also led to a better cracking resistance of the RARP-included mixture. The foam-based warm mix asphalt (WMA) additive markedly improved the blending efficiency but negatively affected the cracking resistance RARP-included mixture.
The revealed aging evolution pattern of AR binder implied that the conventional rejuvenation strategy of recovering bitumen light fractions might not fully restore the microstructure of RARP binder. A chemistry-based method was proposed to indirectly estimate the rubber dissolution degree in RARP. Three rejuvenation schemes were proposed, including the traditional rejuvenation strategy of adding rejuvenator to recover the bitumen light components, the special incorporation of AR binder containing extra rubber content to offset the rubber degradation during aging, and a joint rejuvenation scheme combining both. The mortar level test results indicated that adding rejuvenator can effectively soften the aged RARP mortar, but it failed to slow down the rheological evolution in the secondary aging. The incorporation of extra rubber content notably improved the aging resistance but compromised the workability of RARP-included mortar. The compound rejuvenation scheme could reach a balance between the rejuvenation effectiveness, aging resistance, and workability. The three rejuvenation schemes were then extended to the mixture level. Recycling RARP into new AR mixture improved the rutting resistance but impaired the cracking resistance. Two selected rejuvenators, namely tall oil and aromatic extract, both compromised the rutting resistance of the RARP-included mixtures but showed the opposite effects in the fracture resistance. The supplementing of swelling rubber did not compromise the workability of RARP-included mixture, and it was more effective in improving the cracking and aging resistances. The life cycle assessment (LCA) and life cycle cost analysis (LCCA) indicated that the recycling of RARP can save a considerable amount of greenhouse gas (GHG) emissions and costs due to the reduced usage of virgin bitumen and aggregates. The use of rejuvenator increased the GHG emission and cost, while incorporating extra rubber could further reduce the cost.
Rights: All rights reserved
Access: open access

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