Study on the mixing process improvement for hot recycled asphalt mixture

Existing studies have shown that the mixing process of hot recycled asphalt mixture has a considerable effect on its performance. The conventional mixing process applied widely in pavement industry, mixed the RAP and virgin aggregate first. Then asphalt binder was added for the second mixing. This conventional mixing process, however, ignored the asphalt-adsorbing capacity of different aggregates, may resulting in the heterogeneity of recycled asphalt mixture after mixing. Adjusting the mixing sequence of RAP, virgin asphalt binders and aggregate is confirmed as an effective way to solve the above-mentioned issue. However, the change in the demand for virgin asphalt binders for each mixing step, which is raised from aggregate gradation and different RAP content, is not taken into consideration in this improved method. In view of this weakness, this study proposed an improved mixing technique for hot recycled asphalt mixture, which comprehensively considered the mixing sequence of materials and determined the amount of asphalt binders added for different RAP content based on the specific surface area of aggregates. And the prediction formula for the amount of virgin asphalt added each step is further proposed from the perspective of aggregate specific surface area. Several laboratory experiments were conducted to verify the feasibility of this improved technique. The results indicated that, the improved mixing technique could contribute to enhancing the water stability, low temperature cracking resistance and fatigue performance of hot recycled asphalt mixtures. And the higher the content of the RAP used in recycled mixture, the stronger the improvement of the improved mixing process.