Effect of ageing processes on solar reflectivity of clay roof tiles

Clay roof tiles are widely used as roofing materials because of their good mechanical and aesthetical properties. The exposure to atmospheric agents and, most of all, to pollutants and smog affects negatively the solar reflectance of a tile surface. The aim of this study is to analyze the influence of ageing on the solar reflectance of clay roof tiles. We studied samples provided by manufacturer in Greece and USA. Samples were coated with either organic or inorganic coatings. Natural ageing processes were used for samples with inorganic coating, and artificial ageing simulation was performed on all samples. Samples were naturally aged in a test farm in Arizona, with an exposure time of 3 years. In artificial ageing processes, the surface of the tiles was subjected to the application of two different mixtures simulating exposure to i) Arizona weathering agents such as clay, salts and soot and ii) Arizona, Florida and Ohio weathering agents through an average mixture made by clay, salts, particulate organic matter and soot. The amount of soiling mixture deposited on the surface of the samples was aimed at reproducing a 3 years exposure. Soiled samples were subjected to air blowing and rinsing under running water to simulate the wind and rain effects, respectively. The effects of both natural ageing and artificial soiling on the surface reflectivity of the clay roof tiles were assessed in the UV-Vis-NIR range (range from 300 to 2500 nm). The two different soiling conditions were found to affect significantly the solar reflectance of the samples, in particular the samples soiled with the average mixture present a decrease up to 0.20, while Arizona weathering condition affects the solar reflectance up to 0.05, and neither air blowing nor rinsing seem to permit a significant recovery of the surface properties. All solar reflectance measurements were computed by averaging the spectral reflectivity weighted by the AM1GH solar spectral irradiance.