Abstract

     Frost and thawing damage asphalt in cold climates. Water that enters the pores of asphalt at low temperatures may accelerate its degradation. Water can expand into pores, altering the void content and shape, thus affecting the asphalt’s internal structure and fracture properties. We can develop more durable asphalt types if we understand how void topology changes with freeze–thaw cycles. The purpose of this study is to establish a correlation between the mechanical properties of asphalts and the topological properties of voids. To determine their internal structure, various asphalt types representing dense asphalt and asphalt with voids were made and X-rayed before and after each freeze–thaw cycle. We also obtained the mechanical properties of asphalt and correlated them with the void properties. It was found that dense asphalt has the lowest degradation rate in wet conditions characterised by non-connected gaps, which was approximately constant with freeze–thaw cycles; however, dense asphalt was least durable under dry conditions compared with asphalts with more voids. Due to its high water retention rate, asphalt with a 10% void content degraded at an accelerated rate during the initial cycles. As a result of bigger voids, asphalt with a higher pore content plateaued in later cycles due to reduced water retention.

Introduction

     Asphalt mixture is a heterogeneous material made of aggregates, mineral filler, and a bituminous binder. It is compacted at temperatures ranging between 135 °C and 155 °C; asphalt mixtures may also include voids, up to more than 20 % of the total asphalt’s volume, depending on the gradation and compaction methodology. Asphalt is the most used material to build pavement surfaces. Asphalt undergoes different mechanical loads and environmental conditions during its lifetime, leading to crack and damage and reducing its lifespan [1], [2]. In particular, the extreme environmental condition in cold regions with large temperature fluctuations [3] and the water permeation are some of the most harmful deterioration mechanisms that may affect asphalt, leading to thermal stress [4], loss of adhesion at the interface between aggregate and bitumen [5], and loss of cohesion of the binder [6].

Conclusions

     Several types of asphalt with acceptable air void contents ranging from 3% to 14% were examined for their influence during freeze–thaw cycles, where a correlation between differences in air void topology and changes in mechanical properties was established. According to the results in this study, the topological properties evolution under freeze–thaw cycles was influenced by three different phenomena: (1) volume increase of the existing voids and formation of small cracks, (2) voids coalescence, and (3) cracking and new voids formation. The voids were analysed using X-ray CT scans after every freeze–thaw cycle, and the mechanical properties were measured from 3-point bending tests. The following conclusions were obtained:

     The three different asphalts had different behaviours when exposed to freeze–thaw cycles, attributed to the different aggregate size distributions and air void topologies. Hence, the durability of asphalt to freeze–thaw cycles could be controlled by carefully selecting the aggregate gradation, aggregate morphology and air void topologies.