Mechanical, elastic, and damage behavior of structural green concrete incorporating walnut shell aggregate

This study presents a comprehensive experimental evaluation of walnut shell (WS) aggregates in concrete, considering not only strength properties but also stress–strain behavior, elastic characteristics, density reduction, and damage response, which have received limited attention in previous studies. Walnut shell aggregates were used as partial replacements for both fine (FWS) and coarse (CWS) natural aggregates at substitution levels of 5–25%. The results showed that low replacement levels (10% FWS and 5% CWS) produced compressive and tensile strengths comparable to or slightly higher than those of conventional concrete, whereas higher replacement levels reduced strength but improved deformability and ductility, as indicated by broader stress–strain curves and increased Poisson’s ratios. The modulus of elasticity decreased with increasing walnut shell content and exhibited a nonlinear trend with compressive strength, while dry density reductions of up to 250 kg/m³ corresponded to approximately 10% dead-load savings. Microstructural correlations based on SEM analysis and the high water absorption of the bio-aggregates provided a preliminary analytical basis for assessing the behavior of walnut shell-incorporated green concrete within the investigated dataset. Low replacement levels of walnut shell aggregates (5% CWS and 10% FWS) provided the most favorable balance between strength, deformability, and density reduction. These findings suggest that walnut shell aggregates may be promising for selected lightweight and non-structural concrete applications, while any broader structural use remains exploratory and conditional upon future long-term durability validation.