On the thermal and visual pedestrians' perception about cool natural stones for urban paving. A field survey in summer conditions

While cool materials are widely acknowledged for lowering surface and air temperatures, mitigating Urban Heat Island, reducing emissions imputable to active cooling systems in buildings, concerns arise about their impact on pedestrians' thermal and visual perception. These materials are typically applied on roofs, urban paving and building envelope: when the application is on non-usable spaces for pedestrians, it represents a win-win solution, since it decreases thermal stresses with no penalties for pedestrians. Instead, if the cool surface is installed over a passage area, glare and thermal radiative stress could affect pedestrians' comfort perception. In this work, a naturally cool, light colored stone is considered in the form of aggregates with varying grain size for cool roofing and paving application. Therefore, given its intrinsic passive cooling effect, this paper wants to experimentally investigate if such sustainable and cost effective material can create sensible thermal/visual discomfort perceived by pedestrians. To this aim, pedestrians' perception is investigated by means of in-situ survey and continuous monitoring in summer variable weather conditions, by taking into account several paving systems, i.e. grassland, asphalt, natural stones, and the investigated cool stone aggregates. The study demonstrated how in hot and sunny weather conditions, pedestrians prefer grassland, while asphalt is the least favorite material in any case. Cool gravel based surface does not produce thermal discomfort but it produces some visual discomfort due to glare issue, only in sunny weather conditions. In fact, variable weather conditions significantly affect pedestrians' sensitivity and their preference, also in the same summer season.