Simulating repeated recycling of hot mix asphalt

Addition of reclaimed asphalt pavement (RAP) to asphalt mixes is a standard procedure today in many countries. With increasing use of RAP it becomes more probable that asphalt material is recycled multiple times. This paper deals with the change of binder and asphalt mix properties after multiple recycling steps. A reference dense hot mix asphalt (HMA) containing no recycled asphalt was aged in the laboratory to produce artificial RAP. This was afterwards used in a proportion of 40% together with virgin mineral aggregates and bitumen to produce a new asphalt mixture with binder properties and volumetric characteristics that matched the reference mixture as closely as possible. This mixture was aged again and used for a second recycling mixture until finally three recycling cycles have been carried out. In addition, a second series simulating 100% recycling without addition of mineral aggregates or bitumen, but solely adding a rejuvenating agent, was performed. In the case of 40% recycling, most of the binder and mixture properties remained virtually constant except for the water sensitivity, where a significant decrease in the indirect tensile strength ratio (ITSR) was detected. Analysis of the binder by means of Fourier transform infrared spectroscopy (FTIR) showed that the potential functional groups in the RAP were almost completely oxidised and could not provide information on the ageing degree of RAP. With 100% recycling the binder in the mixture hardened more during each recycling stage. Despite this, the mixture demonstrated good properties with regard to water sensitivity, fatigue behaviour and plastic deformation. Obviously the binder characteristics of penetration and softening point ring and ball are not sufficient to assess the performance of recycled pavements. There is evidence that the mixture properties should be taken into account in the design of recycled pavements, at least when additives like rejuvenators or polymers are added.