MODIFIER INFLUENCE IN THE CHARACTERIZATION OF HOT-MIX RECYCLED MATERIAL

The hot-mix recycling operation for bituminous mixes commonly uses a modifier to restore the aged asphalt cement to a condition that resembles a virgin asphalt cement. In the laboratory this is done on the extracted asphalt cement, thus assuring a thorough mixing. In the actual recycling operation, however, the modifier is added to the material during the mixing process and merely coats the salvaged asphalt concrete particles that are being recycled. It takes a certain amount of time for the modifier to combine with the old asphalt, but the exact nature of this time period and its influence on the structural behavior of the mix has not been reported. This paper investigates the influence of this diffusion process on material behavior and shows that the diffusion process exerts a large influence on the material properties required for long-term performance predictions in computer models such as VESYS. Immediately after sample preparation, the mix may have high stiffness and excellent resistance to rutting. For the material investigated in this paper, a week after preparation the stiffness had decreased by a factor of two, and the resistance to rutting had decreased accordingly. This behavior is explained from a conceptual consideration of the diffusion process and is physically verified by extracting the outer and inner layers of the modifier-asphalt cement combination prepared in a simulated recycling operation and by comparing their consistency. Consideration of this phenomenon has serious implications in characterization for long-term performance predictions and in evaluating the effects of various laboratory conditioning procedures. (Author)