Workability and mechanical properties of crumb-rubber concrete

The addition of crumb-rubber aggregate has a significant effect on concrete mix apparent porosity which affects both the plastic state workability and hardened state mechanical properties. New experimental data are presented and analysed to better understand this behaviour. Crumb-rubber-modified concrete exhibits reduced slump while maintaining a high compaction factor in the plastic state. Structural rubber-modified concrete (f_cu > 17 MPa, ρ_d  =  > 2000 kg/m³) can be designed with crumb-rubber substitution up to 20%wt fine aggregate replacement or up to approximately 15%wt coarse aggregate or coarse + fine aggregate replacement. The dynamic modulus of elasticity can be predicted reliably using the empirical equation E_d  =  0·524f_cu + 15·5 which allows the ultrasonic pulse velocity test to be used as an indicative assessment of mix quality. ‘Good’ quality rubber-modified mixes can be produced using 10%wt fine aggregate and coarse + fine aggregate replacement mixes, and up to 20%wt coarse + fine aggregate replacement, and 30%wt fine aggregate replacement, is achievable depending upon the original mix workability.