Multiphysics Modelling and Mitigation of Ageing in Asphalt Pavements

Abstract

Bitumen binders oxidise with air and cause ageing deterioration of asphalt pavements in the form of hardening, cracking and an overall decline in the mechanical performance. The degree of oxidative ageing is governed by many coupled internal and external physics and variables, making it difficult to predict. Mathematical models were established to represent the multiple physics that contribute to oxidative ageing of asphalt pavements. By coupling them together, these models can simultaneously simulate heat transfer, oxygen diffusion and oxidation kinetics to predict the oxidative ageing of asphalt pavements. The challenge lies in the non-linearity and circular dependency of these physics, making them difficult to solve and converge in numerical applications.The current study establishes a partial differential equation (PDE) based Finite Element (FE) modelling framework to solve these multiple physics using a weak form method. The framework is validated using field measurements of within-pavement temperatures and oxidative ageing products. A laboratory-based experimental investigation is conducted to select anti-ageing compounds (AACs)to mitigate oxidative ageing of bitumen binders effectively. Thin-film oven ageing and Fourier transform infrared spectroscopy (FTIR) tests are used primarily to select potential AACs. Detailed rheological and chemical tests are followed to detect any effects of these AACs on the long-term ageing performance of bitumen binders. Results indicate that the weak-form PDE-based model can effectively address the circular dependency among ageing-related Multiphysics. The model can reliably predict hourly profiles of temperature, oxygen pressure and oxidation products growth along the pavement depth, in different climate zones and for extended periods of field ageing.The air void content and distribution play a vital role in limiting oxidative ageing. Dense asphalt pavements with a low air voids content (<5%) experience little to no ageing, whereas asphalt pavements with air void content of 5-9% experience a growing oxidative ageing rate with an increasing air void content. Pavements with high air voids (> 9%) will have a full access to oxygen from the atmosphere, thus the average carbonyl content is high and uniform across asphalt pavement depth with no clear ageing gradient. A normalised carbonyl index (NCI) was proposed to quantify the oxidative ageing of bitumen modified by AACs. The activation energy of oxidation is found to be a suitable parameter to evaluate and assort the effectiveness of different AACs. Bitumen samples modified with 12% (1 furfural: 5 Irganox 1076), 15% Irganox 1076 and 3.5% (3 DLTDP: 4 furfural) demonstrated the best anti-ageing behaviour by retarding carbonyl content growth and decreasing the fatigue damage among selected AACs without sacrificing the stiffness of binder. The study represents a step forward in the understanding of long-term behaviour of asphalt pavements.