Effect of the repeated recycling on hot mix asphalt properties

A significant growth has been shown in recycling of the old asphalt pavement as a technically and environmentally preferred way of rehabilitating the existing pavements during the three decades. However, savings acquired by using this technology may be lost through excessive maintenance processes if the recycled pavement exhibits too much deterioration. The current design methods for recycled HMA hypothesize a state of complete blending between the recycling agent and RAP binder. In practice, the complete blending does not occur as the recycling agent does not penetrate the whole layer of the aged binder around RAP particles (Carpenter and Wolosick, 1980). As a result of this, the resultant binder within the recycled mix differs from the desired binder, leading to dissimilarity in properties of the recycled and virgin mixes. Consequently, if the recycled mix was subjected to ageing and recycling for second time, the respond of its resultant binder will not the same as if it was the desire binder. This in turn may make the performance of recycled mix of second cycle differs from that of first cycle. Therefore studying the effect of repeated recycling on performance of the recycled HMA was the aim of this research. First, three types of RAP (reclaimed asphalt pavement) were manufactured in the laboratory and were utilized to produce three types of recycled HMA. After testing the recycled mixes, they were aged again to the same ageing time and temperature, then were crushed to be used as RAP for next generation of recycling. This process was repeated three times. Bitumen 40/60 pen and 70/100 pen were used for the virgin and recycled mixes respectively. All virgin and recycled mixes were designed to have identical aggregate grading, bitumen content, air voids, and binder viscosity. Stiffness and fatigue characteristics were measured after each cycle by the Indirect Tensile Stiffness Modulus test (ITSM) and Indirect Tensile Fatigue Test (ITFT). The results showed that, in spite of, presence deterioration in stiffness or fatigue resistance after the first cycle, the repeated recycling had no further significant effect on deterioration of these properties. Because there was considerable degradation in performance of recycled mixes after the first cycle, certain factors that were believed to improve the efficiency of mixing of these types of mixtures were investigated. These factors included size of RAP agglomeration, mixing temperature, dry mixing time between superheated aggregate and RAP, warming of RAP, and mixing mechanism. The results showed the importance of all factors in improving the mechanical properties of recycled mixes. However, the most influential factors were mixing temperature and warming of RAP. Durability of recycled mixes to resist moisture damage was assessed by the water sensitivity test. The results demonstrated that the recycled mixes were not susceptible to moisture damage and can resist the harmful action of water better than the virgin mix. An interesting element in this research was the possibility of using the Hirsch model to estimate the rheological properties of effective binder within recycled mixes without applying recovery process.