Influence of crumb rubber and water-cement ratio on the properties of fresh and hardened concrete

This paper investigates the influence of crumb rubber obtained from recycled waste tires on the properties of fresh and hardened concrete. Crumb rubber was used as a partial volumetric replacement of fine aggregate in proportions of 2.5% and 5.0%. The experimental program included testing of fresh concrete properties (workability, density, and air content) as well as hardened concrete properties (density, compressive strength, and water permeability). Tests were conducted for two water-cement ratios (w/c = 0.55 and w/c = 0.45). The results showed that the addition of crumb rubber reduces concrete density and compressive strength while increasing the air content in fresh concrete mixtures. An increase in the depth of water penetration was also observed in mixtures containing higher percentages of crumb rubber, particularly at higher water-cement ratios. However, the use of a lower water-cement ratio mitigates the negative effects of rubber addition due to the formation of a denser cement matrix microstructure. The obtained results indicate that crumb rubber can be used as an alternative material in concrete production, provided that the mixture composition and replacement percentage are carefully defined.