Effectiveness of recycled rubber aggregate in ASR mitigation of cement-based composites

Rubber derived from end-of-life tires offers a sustainable solution for reducing the environmental impact of cement-based materials. In this study, recycled rubber aggregate (RRA) was used as a partial volumetric replacement (15% and 30%) of natural fine sand in cement-based mortars to evaluate its effectiveness in mitigating alkali–silica reaction (ASR). The experimental program included mechanical testing, ASR expansion measurements, and microstructural analyses. The results showed that increasing RRA content led to a reduction in compressive strength by up to 24.1% and flexural strength by up to 19.8% after 28 days of curing. Despite this reduction, a significant improvement in ASR resistance was observed. For mortars containing highly reactive aggregates, ASR expansion decreased from approximately 0.73% in the reference mixture to 0.47% with 30% RRA, corresponding to a reduction of up to 33%. Microstructural observations confirmed that RRA acts as a stress-relieving inclusion, limiting crack propagation and reducing ASR gel formation. Physicochemical analyses (XRD, TGA, and FTIR) indicated that alkaline treatment induces surface oxidation of RRA without affecting the stability of mineral components. The findings demonstrate that ASR mitigation is not solely due to dilution of reactive aggregates, but also to the elastic and microstructural buffering effects of RRA. Furthermore, a simple predictive model is proposed to estimate the required rubber content for effective ASR mitigation