Green tunnel pavement: Polyurethane ultra-thin friction course and its performance characterization

During the construction process of tunnel asphalt pavement, a lot of asphalt fume can be generated. The asphalt fume usually cannot dissipate quickly during the tunnel's construction, resulting in significant harm to the workers’ health and extending traffic congestion. To solve the problem, a new tunnel pavement, polyurethane (PU) ultra-thin friction courses (PU/UTFC) was proposed, with excellent mechanical properties and wear resistance, and almost no harmful emissions due to its room temperature construction. For this purpose, two PU mixtures (PUM), PU-based open-graded friction courses (PU/OGFC) and PU concrete (PUC), with three different component ratios were investigated in terms of the mechanical properties, functional properties, environmental impact evaluation, and thermo-mechanical properties. Results have shown that PUM, compared to conventional asphalt mixtures, possesses excellent mechanical properties, functional properties, such as skid resistance, noise absorption performance and flame-retardant property, as well as sound environmental properties. Specifically, the increase in isocyanate index (R) can significantly enhance the mechanical properties and water resistance of PUM but will reduce its freeze-thawing resistance. In contrast to PU/OGFC, PUC has higher mechanical properties but lower water stability. The above two PU/UTFCs will resolve significant problems in asphalt pavement in the tunnel and possess a broader application prospect.