Evaluation of strength characteristics of recycled asphalt pavement materials

Use of recycled asphalt pavement (RAP) can reduce the cost of pavement materials significantly. In Queensland, the Department of Transport and Main Roads is reluctant to increase RAP usages and has placed stricter limits, allowing a maximum RAP content of only 15% by mass of mix. This is due to the fact that the presence of aged and stiffer binder can increase the resilient modulus of the mixtures and reduce fatigue life if no allowance is made. Researchers' suggest adapting a low grade binder as an option when using higher percentage of RAP. This research was to assess whether using a softer binder could help to improve the fatigue life of high percentile RAP mixtures. The mix design tested was a dense grade 20mm base course. Experiments were carried out to find the mechanical properties of batches comprised of 0, 15% RAP with same binder and 30%, 45% and 60% of RAP with one step lower level binder. During batching all possible efforts were taken to maintain homogeneity between the samples, and effective binder volume was adjusted for air voids before estimating the actual modulus. The results were used to estimate the fatigue life using the Austroads' fatigue model. It was found that mixing higher percentile RAP together with the binder one grade lower than usual can be effective in reducing residual modulus and improving fatigue life of the pavement. However, there is also potential to overcompensate, yielding softer mixes which may be more susceptible to rutting. It was also found that 60% mix could give similar mechanical performance to a conventional mix. In summary, using the softer binder had the greatest impact on blended binder fatigue life so Australian guidelines can be improved to make the most effective use of recycled asphalts where further studies are required