Effects of Crumb Rubber Amendments on the Porosity, Water Holding Capacity, and Bulk Density of Three Green Roof Substrates

Extensive green roof systems are becoming increasingly popular with urban Low Impact Design (LID), having a number of tangible benefits, not the least of which is storm water mitigation, ameliorating urban storm water discharge from impervious surfaces while reducing high-flow periods which cause stream-bed erosion and carry pollutants to local waterways. The most important factor of storm water amelioration is the substrate or media, which are mineral-based manufactured aggregates, blended with specific granulometric distributions based on design intent or regional rainfall patterns, better optimizing water holding capacity while still providing enough air-filled porosity for healthy plant roots. Crumb rubber, a recycled tire product, is a potential green roof substrate amendment, and is currently available in large quantities throughout North America. Crumb rubber offers many potential benefits, including improvements in substrate air-filled porosity while reducing the weight of the green roof substrate for increasing the potential retrofit of older buildings. Three typical green roof substrates were amended with 8–12 sieve-mesh crumb rubber in increments of 0, 6, 12, 18, 24, and 30% by volume, and were analyzed for air-filled porosity and water holding capacity with the North Carolina State University Porometer system. Higher proportions of crumb rubber significantly reduced total porosity and water holding capacity, yet increased air-filled porosity compared to unamended control in one substrate and decreased bulk density in all substrates. This study was developed to determine the effect of crumb rubber amendments on the porosity and bulk density of green roof substrates to balance water holding performance yet, retain a root friendly environment, and increase the potential for green roof retrofit onto buildings with limited structural capacity.