Rubber–water interaction and estimation of crumb rubber absorption in concrete mixes

Waste tires generate one of the largest sources of solid waste, and crumb rubber (CR) as aggregate in concrete is a viable option to valorise it. While the interaction of CR with water is complex, there are no standardised tests to measure this interaction and quantify non-reactive water in the cementitious matrix retained by CR (absorption). Conventional methods are not suitable for CR owing to its properties. Consequently, reported absorption values range widely from 0% to over 43%, and some authors even suggest treating CR as fully hydrophobic (negligible absorption). This way undermines the reliability of rubberised concrete (RC) mix design and hinders accurate estimations of its properties. This study proposes two complementary methodological approaches to estimate the water absorption of CR aggregates: (a) thermogravimetric analysis (TGA); and (b) a simplified mechanical method using California Bearing Ratio moulds. TGA results demonstrated that CR aggregates exhibit water absorption approximately 2 to 3 times greater than natural sand, challenging the widespread assumption of CR’s hydrophobic behaviour in RC. The mechanical method provided an indirect indicator associate to the water absorption of the CR within the cementitious matrix. The findings underscore the need to consider CR absorption and refine methods for measuring water absorption of CR.