Combined effects of crumb rubber and tyre-derived steel wire on self-compacting concrete: experimental, statistical and machine learning approaches

The valorisation of end-of-life tyres in cementitious materials provides a sustainable strategy to reduce landfill burden and conserve natural resources. This study investigates the combined influence of crumb rubber (CR) as partial fine aggregate replacement (5–20%) and tyre-derived steel wire (SW) reinforcement (0.5–2.0%) in self-compacting concrete (SCC), a hybrid approach that remains insufficiently explored. Response Surface Methodology (RSM) based on a Central Composite Design was used to evaluate and optimise fresh, mechanical, and durability properties. Results showed that CR content was the most influential parameter, with higher replacement levels (≥15%) reducing flowability, compressive strength, and durability due to increased internal friction and pore connectivity. In contrast, higher SW dosages (around 2.0–2.3%) significantly improved split tensile and flexural strengths through crack-bridging action, partially offsetting the strength loss caused by rubber inclusion. Multi-objective optimisation identified an optimal mix of approximately 10% CR and 2.3% SW, which satisfied SCC workability criteria while maintaining balanced strength and durability. In addition, machine learning models were developed for compressive strength prediction, with XGBoost showing the best performance (R2 ≈0.96). The proposed experimental, statistical and ML framework offers practical guidance for designing sustainable rubberised SCC for non-prestressed structural and infrastructural applications.