The effect of the water–cement ratio on the residual fracture mechanical properties of concrete after heat loading

Abstract The fire resistance of concrete structures depends not only on the degradation of compressive and tensile strength but also on changes in fracture mechanical behaviour, which remain less well understood at elevated temperatures. This study examines how the water–cement (w/c) ratio influences the residual compressive strength, fracture energy, and double‑K fracture toughness parameters of concrete after exposure to temperatures between 20 °C and 800 °C. Three mixtures with identical aggregate matrix but different w/c ratios (0.68, 0.45, and 0.35) were tested using CMOD‑controlled three‑point bending and compressive strength tests. The results demonstrate that the w/c ratio is a key parameter governing the residual mechanical performance after heat exposure. Lower w/c ratios yielded higher residual strength, greater fracture energy, and improved thermal resistance, owing to their denser microstructure and more stable interfacial transition zones. All mixtures showed an increase in fracture energy after 100 °C, but the temperature corresponding to the maximum value shifted significantly: 200 °C for the high w/c mixture and 500–600 °C for the lower w/c mixtures. Initiation fracture toughness was sensitive to w/c ratio below 200 °C, while unstable fracture toughness decreased nearly linearly and was largely unaffected by w/c ratio. These findings highlight the dominant role of the water–cement ratio in determining the post‑fire fracture behaviour of concrete.