Experimental investigation on mechanical properties of plain and rubberised concretes with steel–polypropylene hybrid fibre

Abstract Researchers are currently investigating the incorporation of recycled materials from worn automobile tyres such as crumb rubber (CR) aggregate in concrete to resolve the issue of waste tyres and save the environment by making it green and sustainable. Nevertheless, CR incorporation causes a large drop in mechanical properties. In this study, hybrid fibre-reinforced rubberised concrete (RuC) was used to address the issues on sustainability and strength decline simultaneously. All the fibre-reinforced RuCs contained 20% CR (1–2 mm in size) by volume as a fine aggregate replacement. Hybridisation of micro steel (MS) and polypropylene (PP) fibre improved the mechanical properties of concrete. The variables studied were various contents of MS fibres (0%, 0.75%, 0.825%, 0.9%, 1%) and PP fibres (0%, 0.1%, 0.175%, 0.25%, 1%) with and without CR. The effects of MS, PP and (PP–MS) hybrid fibres at the same volume fraction of 1.0% on the workability, bulk density, elastic modulus and compressive, splitting tensile and flexural strengths of fibre-reinforced concrete were investigated. Experimental results showed that the concrete mix with hybrid fibres (0.1% PP + 0.9% MS) had higher compressive and splitting tensile strengths and elastic modulus than other concrete mixes. Moreover, the mixes with 1.0% MS significantly enhanced the flexural strength of concrete. The mixes with 1% PP negatively influenced the mechanical properties of concrete. The mechanical properties of RuC decreased when fine aggregate was replaced by CR. Despite this decrease, a successful mix of CR and hybrid fibre was obtained.