Experimental analysis of natural gravel covering as cool roof and cool pavement

Building energy efficiency has become a key research issue in the last years, given the huge energy requirement of the construction sector and the potential for energy saving. In particular, passive solutions such as natural materials are always appreciated for their intrinsic high thermal performance and low environmental impact. In this view, natural stones represent a good solution as building envelope covering or city pavements. This paper concerns the experimental characterization of several gravel coverings for roofs and urban paving in terms of their albedo characteristics. A cheap and local natural high reflectance stone is chosen, which common use is as aggregate for concrete. The purpose of the work is to study its intrinsic cooling potential. To this aim, the in-field albedo of gravel samples is measured with varying grain size of the same natural stone named “Bargiolina”. These in-field measurements are compared to in-lab measurements about solar reflectance and thermal emissivity. The analysis shows a significant variation of the albedo with varying grain size of the same stone. Both in-lab and in-field measurements agree that the stone with the smallest grain size, i.e. thin sand, has the best optic-thermal performance in terms of solar reflectance (62%). This feature results in the reduction of the surface temperature when exposed to solar radiation. Moreover, a natural mixed stone is compared to the high reflectance stone, demonstrating that the chosen “Bargiolina” presents an intrinsic cool behavior. Therefore, this natural, low-cost, low impact and high performance material could be successfully applied as cool roof or cool paving, for its passive cooling contribution and its potential for reducing urban heat islands.