Investigating the Use of Crumb Rubber Modified Asphalt to Improve Ageing Resistance in Local Government Roads

Andrew Kidd

doi:10.25907/01105

The use of crumbed rubber recycled from end-of-life tyres as a bitumen modifier for use in asphalt has seen renewed interest as a means of addressing the waste tyre challenge in Australia. Local research has initially focused on the use of higher crumb rubber (CR) proportions in crumb rubber modified (CRM) binders for asphalt surfaces. The use of low-dosage CRM binders in conventional dense graded asphalt (DGA) is not as established, and the ability to provide improved ageing resistance in pavement surfaces warranted further investigation. The focus of this research was local roads in the Brisbane City Council road network, with the aim of determining the viability of low-dosage CR contents in DGA mixtures for these applications. It was also necessary to compare the relative performance of both field and terminal wet-blended CRM binders to guide implementation strategies for their asphalt operations. The research aims were achieved by comparing the properties of a standard 10 mm nominal DGA design using 0%, 5%, 10% and 15% CR contents (by weight of the binder fraction) for both blending approaches. A multigrade (M1000) mixture was also assessed, to determine if CRM DGA could be used in more heavily trafficked sections of the Brisbane road network. The primary mechanism of pavement deterioration for local roads is usually environmental ageing, rather than load related. It was therefore critical to understand the ability of CRM DGA to withstand the effects of thermo- and photo-oxidative ageing to prolong surface life. CRM binders and DGA were subjected to artificial temperature, pressure and UV radiation ageing methods, where the relative changes in their engineering properties were examined as a measure of their ability to resist the ageing process. Analysis of variance and other statistical methods determined the significance of the relative effects, and research aims were addressed by comparing the relative effects of CR content, blending approach, and ageing for the CRM binders and DGA. Preliminary investigations considered the composition of the CR particles for both the virgin CR feedstocks, and the recovered particles post-blending for the respective CRM binders. It was determined that the fractional composition of the resulting CR particles post-blending did not result in a uniform loss of constituent materials. Rather, the volatiles, natural rubber and synthetic polymers formed the majority of the dissolved components, while the carbon black and inert fillers then constituted the majority of the remaining CR particles. These findings had significant implications regarding how the CR particles affect volumetric design. Accordingly, a new volumetric design approach for CRM DGA was developed, by removing the undigested CR particles from the binder fraction for determination of volumetric properties. It was concluded that conventional methods were fundamentally incorrect, and an alternative dissolution based volumetric design method resolved issues specific to CRM DGA. It was concluded through analysis of the CRM material properties that the useful temperature interval was significantly increased, through improved low, intermediate and high temperature properties. Relative improvements increased with increasing CR contents, where 15% CR marked the transition from low to high modification effects. Accordingly, it was concluded that 15% CRM DGA mixtures may be suitable for use in more heavily trafficked pavement surfaces, based on the improved mechanical performance properties. Analysis of aged material performance determined that CRM DGA significantly reduced thermo- and photo-oxidative ageing. Increasing CR contents similarly improved ageing resistance effects, through improved retention of binder and mixture ductility, cracking and fatigue resistance properties. However, it was concluded through statistical analysis that 5% CRM DGA mixtures provided, at best, no relative dis-improvement, and a minimum 10% CR content was required to improve ageing resistance. Similarly, as local roads do not require the more significantly improved performance properties provided by 15% CR dosages, it was concluded that 10% CR dosages provided the best balance of improved ageing resistance, against increased costs, for local roads. It was also concluded that the effect of blending approach did not statistically significantly affect ageing severity. Accordingly, either field or terminal blends are suitable for CRM DGA, and decisions as to their adoption should be based on economics and operational logistics for local manufacturing. Cost model scenarios for the various CR contents and blending approaches supported the above findings, where it was determined that 10% and 15% CRM DGA mixtures were economically viable, with respect to improvements for environmental ageing resistance, on a whole of surface life cycle basis. Finally, analysis of the M1000 materials determined the binder was prone to premature ageing effects, and the research supports discontinuing the use of M1000 in surface applications. This highlighted a gap in the current Australian specifications and testing frameworks for the evaluation of aged bituminous materials performance, where long-term ageing is not considered. Consequently, it was recommended that practitioners consider long-term ageing methods, consistent with international specifications and best practice, in order to limit binder modifiers or specific binder feedstocks and structural compositions that may be more prone to severe oxidative ageing effects.

Investigating the Use of Crumb Rubber Modified Asphalt to Improve Ageing Resistance in Local Government Roads

Files and links (1)

Abstract

Details

Metrics