Plastic Derived Bitumen Modifiers (w-Binder) from Pyrolysis in Sustainable Road Construction

Abstract

There are current largescale efforts within the paving industry to move towards the use of sustainable binder alternatives to bitumen, which is a nonrenewable and highly impacting resource. The use of waste plastics as binder materials within asphalt concrete is considered as a practical and cost-effective alternative, especially as the growth of new recycling capacities is becoming more crucial. Furthermore, plastic-derived bitumen modifiers from the thermochemical treatment of plastics could be a viable solution to the current limitations associated with plastic bitumen modifiers (PMB), while producing asphalt with enhanced rheological properties and failure resistances. This study provides a novel contribution to outlining the potential of highdensity polyethylene (HDPE) thermal pyrolysis waxes in the modification of bitumen (w-binder) and subsequent hot mix asphalt (HMA) mixtures (w-asphalt), as well as in reclaimed asphalt (RAP) rejuvenation. In the interest of product and process optimisation, it establishes key relationships between pyrolysis process parameters, the chemical and thermal properties/ mechanisms of the wax modifiers and the rheological and mechanical performance of the modified binders/mixtures. Finally, dense graded asphalt concrete modified with an optimal HDPE pyrolysis wax (6 wt% of the binder) and 20% RAP was produced and its resistance to key pavement deterioration modes was determined. The optimal wax was produced at higher pyrolysis temperatures and nitrogen flowrates (having the lowest vapour residence times.) Such process parameters had a crucial role in the resultant wax chemistry and thermal ageing behaviours. Oxidation and polymerization reactions were key mechanisms identified during wax thermal ageing and their effect on the resultant binder and mixture properties were highlighted. The asphalt mixtures produced had enhanced or unaffected resistance to the key failure modes studied, with the RAP + HDPE pyrolysis wax mixture showing superior performance. The HDPE pyrolysis wax acted as a sufficient rejuvenating agent to mitigate the otherwise adverse effects to fatigue resistance of high RAP content in HMA mixtures. This application of plastic pyrolysis wax could help to reduce the amount of nonrenewable materials used for HMA production, increasing the usage of recyclable and secondary materials within flexible pavements in the effort to approach a circular economy.