Effects of elevated temperature and water re-curing on fracture process of hybrid fiber reinforced concretes

Hybrid fiber reinforced concrete has been introduced in constructions due to its superior mechanical performance compared to plain and mono fiber reinforced concrete. However, hybrid fiber reinforced concrete mechanical behavior after exposition at elevated temperature is of considerable importance for the safety of concrete structures and need more in-depth understanding. In this study, the effect of the elevated temperatures and water re-curing on the Mode I fracture performance of two hybrid fiber reinforced concretes was investigated. The Mix S exploited long hooked-end and medium hooked-end steel fiber, as well as high strength short wave-shaped steel fiber, for the hybrid reinforcement. Mix P contained polypropylene fibers substituting the high strength short wave-shaped steel fibers. The damage degree (DD) was estimated using the ultrasonic pulse velocity (UPV) method. The fracture process and development of fracture process zone (FPZ) length was studied by three-point bending tests and measuring the full-field displacement and strain on the surface of the specimen by the digital image correlation (DIC) technique. The results indicated an increasing damage degree with the elevated temperature, and a considerable reduction after water re-curing. The elevated temperatures above 200 °C deteriorate: modulus of rupture (MOR), residual strength, and fracture toughness. The dependency of the deterioration coefficient (DC) and the temperature from 20 °C to 800 °C was predicted by a linear / exponential relationship. Branching crack appeared when the elevated temperature exceeded 200 °C. Water re-curing enhanced the flexural performance of thermally damaged hybrid fiber reinforced concrete, reducing the crack branching and recovering the FPZ length. Mix S had a better flexural performance than Mix P at elevated temperature.