Solar roads are transportation infrastructures able both to generate electricity thanks to solar cells placed under a semi-transparent layer and to ensure heavy traffic circulation. In this paper, a novel transparent top layer made of glass aggregates bonded together using a polyurethane glue is presented. The goal is to design a composite material able to support traffic load, guarantee vehicle skid-resistance, allow the passage of sunlight, and protect the solar cells. For this purpose, the authors investigated the effect of different variables (thickness, glue content, and glass aggregate distribution) on the mechanical and optical performances of the material applying the factorial design method. The semi-transparent layer was characterized by performing the three-point bending test and measuring the power loss. Regarding the vehicle friction, experimental tests with the British Pendulum were conducted in order to measure the skid resistance of the surface and compare it with the specifications of a typical road infrastructure. According to the fraction factorial design and the British Pendulum test, the following mixture was developed: 42.8% of 4/6 mm; 42.8% of 2/4 mm, 14.4% of glue in volume, and a thickness of 0.6 cm. The first results are encouraging, and they demonstrate the feasibility of a semi-transparent layer for future applications in full scale.
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