Laboratory study on the use of recycled plastic granule from agricultural waste as drainage layer in green roofs

Although green roofs are mentioned among the most sustainable building envelope solutions, there is the need to replace current green roof materials by more environmentally friendly and low-cost products, proposing innovative technological solutions developed for Mediterranean area. In addition, Italy is one of the major consumers of agricultural plastic products but recycling of a substantially increased amount of plastic waste may lead to a supply surplus of secondary material. To mitigate this risk, applicability of the current secondary material needs to be increased. In this paper, the use of polyethylene granules from the recycling of agricultural plastic waste used for greenhouse covering as drainage layer in green roofs is evaluated. The physical properties (i.e. particle size, specific weight, minimum and maximum density and permeability) of five recycled plastic granules different in recycling process are determined through laboratory tests in compliance with ASTM Standards and compared with those of perlite, the granular material commercially used for the drainage layer. The results showed that the recycled plastic granules have physical properties comparable to those of perlite and, therefore, they are suitable for the drainage layer in green roofs. Among plastic materials, those coming from the waste of polyethylene recycling have better characteristics as possible drainage layer because they have lower specific weight and lower minimum and maximum density compared to the granule produced according to UNI 10667-1, thanks to the air gaps inside the granule. In addition, they have a lower permeability thanks to higher particle size and to the cavities on the granule surface that allow water storage and evaporation, providing support to vegetation development. Future research will evaluate the thermal properties of recycled granules in both laboratory, both in dry and saturated conditions, and full-scale set-ups, using recycled granule as drainage layer.