In-plant production of warm recycled mixtures produced with sbs modified bitumen: A case study

Asphalt mixtures produced at reduced temperatures through Warm Mix Additives (WMA) allow reductions of fuel consumption and harmful emissions, ensuring economic and environmental benefits. Considering that nowadays mixtures include more and more often reclaimed aggregates, the combination of recycling and WMA technologies represents a major challenge in road construction and needs further investigations for identifying drawbacks/advantages. Several concerns derive from regular in-plant productions since warm recycled mixtures have been mainly optimized through laboratory studies without evaluating possible issues of large-scale in-plant productions and lay-down processes. So far, few field constructions were realized, limited to small trial sections and without considering the use of modified bitumens. Given this background, the paper describes in-plant productions of warm recycled mixtures prepared with three chemical WMA additives for the construction of an extensive motorway segment. Dense-graded mixtures for binder and base courses as well as open-graded mixtures for wearing courses were produced. Three full-scale trial sections included warm mixtures whereas a further section, used as reference for comparison purposes, comprised analogous mixtures realized through hot recycling according to the current practice. The paper describes the construction steps and the controls carried out to verify technical standard requirements in terms of volumetric properties, compactability and Indirect Tensile Strength. The main objective was to attest the feasibility of large-scale productions to adequately reproduce the mix design previously implemented through laboratory studies when WMA technologies and recycling techniques are concurrently involved. Moreover, gas emissions monitoring at the asphalt plant during the real scale productions was conducted through Continuous Emissions Monitoring Systems in order to quantify the potential benefits in terms of pollution. Results demonstrate the suitability of WMA chemical additives to produce at low temperature mixtures with adequate performance, concurrently recording a significant reduction in pollutants without needing mix design modifications or implementation of expensive new technologies.