Warm chemical additive to improve water resistance of asphalt mixtures containing steel slags: a multi-scale approach

Warm mix asphalt (WMA) represents a suitable technology to enhance environmental sustainability of road paving. WMA is an asphalt concrete that can be produced and compacted at lower temperatures than usual. The additional inclusion of recycled aggregates (e.g. steel slags) in substitution of natural ones is a widespread solution effectively leading to further environmental benefits. In this sense, the assessment of moisture susceptibility could delineate an interesting investigation topic since some issues related to bitumen-steel slags chemical affinity were evinced. Given this background, the present paper shows an experimental characterization aimed at investigating moisture susceptibility of WMA containing electric arc furnace steel slags through a multi-scale analysis approach. In particular, a chemical additive containing anti-stripping agents and adhesion promoters was used to obtain warm mixes. To achieve the above-mentioned goal, stripping tests were performed on bitumen-aggregate systems and analyzed with a digital imaging processing method. Moreover, warm asphalt mixes were subjected to freeze-thawing procedures in order to study the water effects on stiffness characteristics and indirect tensile strengths. A reference unmodified binder and traditional natural aggregates (limestone) were also tested for comparison purposes. Experimental results showed analog trends both at bitumen-aggregate and mixture scale, demonstrating that warm modification by chemical additive is a promising technology able to prevent stripping problems mitigating issues related to bitumen-steel slags affinity.