Experimental Investigation on The Mechanical and Physical Properties of Concrete Incorporating Alkali-Treated Crumb Rubber as Partial Replacement of Fine Aggregate

Replacing natural sand with crumb rubber from waste tyres can reduce the demand for river sand and the burden of waste tyres, but untreated rubber weakens concrete because it bonds poorly with cement paste. This paper reports an experimental study of M25 concrete in which 5, 10, 15 and 20% of the fine aggregate by volume was replaced with untreated or sodium hydroxide (NaOH)-treated crumb rubber, compared with a conventional control mix. Workability, fresh density, compressive strength at 7 and 28 days, splitting tensile strength, flexural strength, hardened density and water absorption were measured. All properties deteriorated as rubber content increased. At 20% replacement the 28-day compressive strength fell from 33.4 MPa for the control to 18.9 MPa with untreated rubber and 22.0 MPa with treated rubber. Relative to untreated mixes, treatment raised compressive, splitting tensile and flexural strength by up to 16.4%, 12.9% and 10.8% and reduced water absorption by up to 12.9%, but no rubberised mix matched the control. Two-factor analysis of variance showed significant effects of replacement level and rubber condition (p < 0.001) and no significant interaction. Using a minimum of 25 MPa at 28 days, treatment raised the acceptable replacement level from 10% to 15%. The results suggest that treated rubber concrete is better suited to non-structural and low-to-moderate strength applications.