Up to 30% of RAP in open graded wearing course with innovative additives of hydrophobizing property

Abstract: The experimental study carried out aims to research an innovative mix design for open graded wearing course of an environmentally sustainable highway pavement with a high RAP content. The study is based on the recycled materials come from the milling of the highway pavement in place, to which a minimum percentage of virgin aggregates is added to produce, providing the same performance, a reduction in the natural resources used and, at the same time, ensure a strong boost in eco-sustainability. The environmental impact factor is thus improved by working on the reduction of raw material consumption, waste in favour of the circular economy, energy and CO2 emissions. The main objective was to design the application of a bituminous binder containing additives with multifunctional properties, i.e. a blend of polymers specialised in confining the RAP lumps, stably bonding the grains of the mix and increasing resistance to oxidation generated by sunlight and operating climatic conditions. The performance of the mix is guaranteed by a selection of new-generation additives that preserve the workability of the mix design at low temperatures during production in plant, mixing in the transport and the asphalt paving phase. Specifically, the following were investigated: a high-performance synthetic-based regenerating liquid additive and a hydrophilizing and antioxidant additive with thermo-resistance characteristics suitably developed to give the bitumen greater hydrophobicity. The former gives the mix plasticising and adhesion-activating properties of a phosphoric nature, the latter allows for greater compatibility and greater cohesion properties in the mix with the aggregates, hence, greater resistance to weather washout. The experimental investigation carried out studied asphalt mixes with 25 to 30 % RAP content, considering the implementation of synthetic-based additive in dosages between 0.05 and 0.15 % and hydrophilizing additive in dosages of 1 and 3 % on the weight of the bituminous binder The experimental activity of mechanical characterisation continued after the verification of chemical compatibility and possible coalescence problems. The mechanical performance of the open-graded mix asphalt with high RAP content were performed on cylindrical specimens. Specifically, laboratory tests of a destructive nature were performed, such as the Cantabro Test, Scuffing Resistance Test, and Indirect Tensile Strenght. In addition, rheological characterisation tests were carried out using the DSR on bituminous binders with the additive formulation described in the previous points and used for the packaging of each mix studied. The results obtained demonstrated an excellent mechanical response of the mix designs containing RAP percentages up to 30%, with results comparable to the virgin pavement mix. Although the milled (RAP) material content is high, the additives used guarantee both the mechanical characteristics and an increase in the binder's resistance to atmospheric agents and ultraviolet radiation (photo-oxidation phenomenon), providing an extension of the pavement useful. In addition, the selected additives have proven to allow a reduction of CO2 emissions into the environment; the implemented additives allow the reduction of the packaging temperature by about 20°C compared to standard mixes without loss of workability during the laying phase.