2.1. Materials

The present study utilized three asphalt binders, 50/70 paving-grade bitumen, 45/80–55 polymer-modified bitumen, and 45/80–80 highly modified bitumen (HiMA), used in Poland [44] typically for producing asphalt surface courses intended for low, medium, and heavy trafficked roads, respectively. The specific asphalt binders were selected for their distinct functional characteristics despite their similar penetration ranges. The asphalt binders were commercially sourced from Orlen Asfalt (Poland). The basic characterization of these base asphalt binders is presented in Table 1.

Properties of the base asphalt binders used in the study.

Although the base asphalt binders were comparable in terms of their penetration at 25 °C, their remaining properties differed greatly due to the specific polymer modifications. The polymer-modified asphalt binders had greatly increased plasticity ranges defined as the difference between their softening points and Fraass breaking points. Additionally, the dynamic viscosities of the asphalt binders were significantly affected by the modification.

The fluxing agent was produced using pure fatty acid methyl esters derived from rapeseed oil (RME) without adding anti-ageing additives, based on the experience from previous studies [25,29]. RME, widely used as an additive to bio-fuels, contains anti-ageing additives that may block hardening and stiffness recovery in bitumen and asphalt mixtures. Selected RME properties used in the study are shown in Table 2.

Selected properties of the pure RME for producing the Bioflux fluxing agent.

The foaming process of bitumen can be improved by adding oils or oil esters which lower the viscosity of the binder. This makes the foaming process more efficient. Nevertheless, the binder liquefaction effect is not favorable due to the properties of asphalt mixtures because of lowering their stiffness. Therefore, it is reasonable to use liquefication additives which, during the production process of an asphalt mixture, reduce its consistency, and when used in the road structure, its original properties are slowly restored in the cross-linking process. It is possible through the oxypolymerization reaction and the availability of double unsaturated bonds in oils. Vegetable oils comprise a variable number of double bonds depending on the composition, and RME is needed to be activated because of its natural limits. Cross-linking efficiency depends on the number of double bonds and their position in the aliphatic chain of fatty acids. Thus, the pure RME was subjected to an oxidation reaction in the presence of oxidation promoters for activation:

The reaction was conducted in a laboratory reactor with a 0.3 ratio of reactor diameter to RME height and with an initial temperature of 25 °C. It was oxidized for two hours with an airflow value of 500 L/h per 1 kg of the product. The final product was kept in a sealed steel container at 5 °C until use.