Challenges in Translating Rubberized Mortar Properties to Concrete: Segregation and Shear Slump

Rubberized cementitious composites are gaining focus in sustainable construction. Crumb rubber, sourced from waste tires, is used as a partial replacement for sand in concrete. However, rubber's low density, elasticity, and hydrophobic nature reduce workability and mechanical strength, owing to voids and poor compaction. To address these issues, supplementary cementitious materials (SCMs) like silica fume and fly ash, along with polypropylene (PP) fibres, were incorporated in this study. A control mortar mix with a 1:4 cement-to-sand ratio was established, and 10% of the sand was replaced by crumb rubber. SCMs were added at 5% by mass of cement, PP fibres at 0.1% by volume, and a water-reducing agent at 2% by mass. The results demonstrated that the flow-ability was reduced by 50%, reflecting the stiffening effect of rubber, but the compressive strength remained comparable to that of the control specimens due to the beneficial impact of silica fume, fly ash, and fibres. Density decreased by 19% due to the rubber's low density. However, when this mix was applied to concrete, it resulted in shear slump and segregation, as the rubberized mortar was insufficient to cover the coarse aggregates. This suggests that sand replacement in concrete should be done by volume rather than mass to prevent these issues and ensure proper aggregate coverage.