Evaluation and Characterization of Recycling Agents for Asphalt Concrete

The increasing use of recycled materials such as reclaimed asphalt pavements and /or recycled asphalt shingles in pavement construction has led to an overall increase in demand for recycling agents in the market. Currently the only method for their classification is ASTM D4552 that uses viscosity and percentage of saturates to differentiate between different products. This method has proven to be incomplete as the chemistry of the recycling agents plays a very important role along with viscosity. This study was conducted to investigate the differences between various types of recycling agents used in the field today. Chemical as well as rheological properties were taken into consideration to evaluate these products. Binder and mixture materials were collected from a Delaware field project and different recycling agents available in the market were selected. Analyses were conducted at three different levels – (a) Recycling agents themselves, (b) Rejuvenated binder blends {base binder + asphalt from recycled material + recycling agent}, and (c) Rejuvenated mixture. For the recycling agents and the binder blends, a rheological analysis using a dynamic shear rheometer was conducted alongside a chemical analysis using Fourier transform infra-red spectroscopy. The mixtures were evaluated by performance tests including the Hamburg wheel tracking test for rutting and the Illinois flexibility index test for cracking. The results indicate that testing of recycling agents themselves might be misleading as their interactions with asphalt binder can alter their chemistry. The differences between just the softening agents that only reduce the modulus and true “rejuvenators” that alter the blend to reduce the stiffness and increase the phase angle were established by exploring the rejuvenating mechanism for the various recycling agents. The challenges faced by some traditional tools for binder characterization were also made apparent when used for rejuvenated blends. A new parameter, called Binder Embrittlement Parameter (BEP), that includes both chemical and rheological changes was developed to better identify good recycling agents. The effect of dose and type of recycling agent was analyzed again for mixture blends by analyzing rutting and cracking data obtained. Finally, the recycling agents were ranked according to their effectiveness using binder as well as mixture results