Hot Mix Asphalt Recycling: Practices and Principles

Hot mix asphalt recycling has become common practice all over the world since the 1970s because of the crisis in oil prices. In the Netherlands, hot recycling has advanced to such an extent that in most of the mixtures more than 50% of reclaimed asphalt (RA) is allowed. These mixtures with such a high RA content are produced in a batch plant to which a parallel drum is attached. In this drum RA is pre-heated to approximately 130°C. Since 2007 another hot mix recycling techniques became available in which RA is mixed in cold and moist condition in contrary to conventional methods. It is a so called double barrel drum mixer. In this method virgin aggregates are superheated in the inner drum and mixed with cool and moist RA and fines and virgin bitumen in the outer drum. In both cases, double drum and partial heating methods, the virgin aggregates have to be pre-heated to higher temperatures than with mixtures without RA in order to achieve a mixing temperature of around 170°C. Dependent on RA amount, moisture, pre-heating, etc. in the batch plant the virgin aggregates have to be pre-heated to around 300° C and in the double barrel drum to around 500°C. These high temperatures have led to concerns about the quality of the produced mixtures. Since 2008, a new Dutch specification system for asphalt mixtures is in place in line with the European standards (EN13108 series). The new regulation gives contractors freedom to select their own material such as bitumen grade and the amount of recycling; however, in return it makes them responsible for the quality of the mixture. The mixture should fulfill the requirement of fundamental performance characteristics of the mixtures such as resistance to fatigue and permanent deformation. In this research two major questions have been investigated. One of the questions is whether these high temperatures have a negative effect on the bituminous binder, while the other important question is whether the RA binder will blend totally with the virgin binder that is added. The focus of this research was on four objectives. The first objective was to develop a laboratory mixing method to simulate the real recycling process in the field. The second objective was to assess the effect of the double barrel drum on the mixture quality in comparison with conventional batch plant. Third, it was aimed to measure the blending degree between two binders. And finally, to increase the understanding of the mechanism behind blending RA binders with virgin bitumen, with focus on their micro structure. To cover all research topics, this dissertation is organized in two parts in which the first part is devoted to laboratory and field mixture evaluation while the second part is presenting the exploratory research on the fundamental aspects of blending. Research in part 1 is conducted in two phases, laboratory simulation and field experimentation. The conventional partial warming recycling method (PW) the upgraded double drum mixing method (UPG) were simulated in the lab and the quality of mixtures were compared with the standard mixing method (SM) in the lab at different RA content and different moisture content. This research showed that higher percentages of RA results in higher stiffness and lower fatigue life. However in the UPG method with 4% moisture and 60% RA, the mixture became remarkably lower in stiffness and durable against fatigue. This might be because of the lack of blending or the effect of foaming of bitumen. It was concluded that the UPG method could not effectively be simulated in the lab. In the next experimental phase of the study, three identical mixtures were produced with 50% RA and 4.3% bitumen. One mixture was produced in a batch plant (BB) while the second one was mixed using a double drum mixer by the same contractor (A). The third mixture was produced in the laboratory (L) using a lab pugmill mixer. The comparison between three mixtures shows that mixture L has a higher stiffness than A and BB. Mixture BB has as slightly higher stiffness than A. Furthermore, mixture A has the lowest stiffness which is most probably due to the system of cold and moist RA feeding into the double drum system. Besides the 4PB fatigue and stiffness test, monotonic uniaxial tension (UT) and compression (UC) tests were performed to be used in material modeling and to determine a fatigue endurance limit. The limit value of the stress ratio parameter (Rlimit) was determined which is useful in the determination of the endurance limit in a three-dimensional state. It shows that different mixing methods lead to different endurance limits. It turns out that the plant produced mixture has a higher endurance limit than the laboratory mixture. In this research an infrared thermography method was used in every material preparation stage. The temperature homogeneity of the mixtures in the lab and in the field was investigated. It proved to be a useful method in visualizing the temperature exchange during mixing and compaction. In Part 2, the effect of superheating aggregates is studied by simulating RA and real aggregates with glass beads and artificial aged binder. The stage extraction method was evaluated in this research with respect to size and shape of aggregates. The blending and diffusion mechanism between old and new bitumen is studied at the microstructure level by means of Nano indention and Nano-CT scanning. The morphology of different types of bitumen was detectable by these techniques; however the blending zone couldn’t be characterized.