Characterisation of cracking resistance of polymer modified asphalt mixtures using fracture mechanics and image analysis

Abstract

Most distresses in asphalt pavements are directly related to fracture. Thus, it becomes clear that identifying and characterising fracture properties of asphalt mixtures are critical steps towards a better pavement design. This study provides a detailed analysis of the crack resistance of the modified asphalt mixtures with crumb rubber and plastic waste. These modified mixtures have been introduced in road construction based on the basic understanding of the mechanical properties and lack of intensive studies on fracture mechanics. This study used the fracture mechanic concepts combined with the image analysis technique to evaluate the cracking behaviour of the asphalt mixtures. The asphalt mixtures were examined using semicircular bending (SCB) test under different loading modes and levels, emphasising the impact of ageing and moisture. Digital image analysis technique was used to capture the crack initiation and propagation length. The crack length data were used to construct the resistance curve (R-curve) based on cumulative fracture energy for the control and modified asphalt mixtures. Crack mouth opening displacement (CMOD) data was used to analyse the crack initiation, crack propagation, and failure stages of the fatigue process. The digital image correlation (DIC) technique was successfully utilised in capturing the strain map of the SCB sample under monotonic and repeated load tests. Then, a two-dimensional finite element model using ABAQUS software was used to simulate the SCB test under monotonic load. The results showed that incorporating plastic and crumb rubber into the asphalt mixture increases the energy release rate by 10% and 20%, respectively. On the other hand, asphalt with crumb rubber expressed lower cracking resistance due to ageing and moisture conditioning than conventional and plastic modified mixtures. A different pattern was captured for the R-curve showing the impact of adding plastic and crumb rubber into the asphalt mixture. The R-curve showed that the rubberised mixture consumes 8% lower fracture energy at the crack initiation stage compared to the control and plastic modified asphalt mixtures. Moreover, the CMOD analysis showed that the rubberised asphalt mixture promoted crack propagation, whilst plastic waste modified asphalt mixture resisted cracking by delaying the crack initiation phase compared to the control asphalt. The digital image analysis of the modified asphalts indicates higher crack tortuosity and fractal dimension by 35% and 48%, respectively, compared to the control asphalt. This can be attributed to the increased cracking resistance of the modified asphalt mixtures. The DIC clearly emphasises the impact of crumb rubber in increasing the plastic deformation around the crack tip, while small fracture zones and high strain concentration were observed in the strain map of the mixture containing plastic waste. Additionally, the extended finite element method has successfully predicted the impact of change in notch length on the value of peak load and displacement.