Sodium Hydroxide Treatment of Crumb Rubber as Fine Aggregate in Concrete: A Review

Concrete is the most widely used construction material, yet its production consumes large quantities of natural sand, while end-of-life vehicle tyres remain a difficult waste stream. Partial replacement of fine aggregate with crumb rubber from waste tyres addresses both problems, but untreated rubber lowers strength, stiffness and workability and increases water absorption, largely because of a weak bond between the hydrophobic rubber surface and the cement paste. This paper gives a simple review of published work on rubberised concrete, with emphasis on pre-treatment of crumb rubber with sodium hydroxide (NaOH). It covers the properties of waste tyre rubber, the reported effects of rubber on the fresh, mechanical, physical and durability properties of concrete, the nature of the rubber–cement interface, alternative methods of improving that interface, and the mechanisms and parameters of NaOH treatment. The literature agrees on the broad effects of rubber. NaOH treatment is the most widely studied and practical chemical treatment and generally reduces, but does not remove, the strength penalty. Gaps remain in controlled comparison of treated and untreated rubber, reporting of treatment conditions, physical-property data and statistical analysis.