Thermo-mechanical performance of concrete incorporating different walnut shell aggregate gradations and rubber fibres under elevated temperatures

Despite growing interest in agricultural waste shell materials as a sustainable alternative to natural coarse aggregate, the combined influence of walnut shell aggregate (WSA) particle-size gradation (PSG), replacement level and waste tyre rubber fibres (RuF) on the performance of concrete after exposure to elevated temperatures remains largely unexplored. In this study, along with a control mix, 24 concrete mixtures incorporating coarse-sized WSA (WC) and medium-sized WSA (WM) at replacement levels of 5–30%, with and without 2.5% RuF, were investigated. The mechanical, stress–strain, durability and microstructural properties of the concretes after exposure to temperatures up to 400°C were evaluated. The results showed that the performance of WSA concrete (WSAC) was strongly influenced by the PSG, with WM mixtures generally exhibiting higher compressive strength and better residual performance than the corresponding WC mixtures. The incorporation of RuF reduced brittleness and improved the residual mechanical performance of the WSAC after thermal exposure. Strong correlations were observed between compressive strength and density (R2 = 0.880) and between compressive strength and water penetration depth (R2 = 0.946), highlighting the influence of density and permeability on concrete performance. Among all the mixtures, the concrete containing 10–15% WM with 2.5% RuF provided the best balance between reduced density and residual mechanical performance, retaining approximately 60–72% of its original compressive strength after exposure to 400°C. These findings indicate the importance of WSA PSG and RuF incorporation in developing lower-density concrete with improved residual performance after moderate thermal exposure.