Impact of low temperature and NaCl attack on the fracture properties of engineered cementitious composites (ECC)

Engineered cementitious composites (ECC) is a potential reinforcement and repair material for hydraulic structures due to its excellent mechanical properties. However, the fracture properties of ECC under the coupling effect of low temperature and chloride ions attack are still unclear. In this study, fracture experiments were performed on ECC specimens under different working conditions (immersed in water and NaCl solutions for different time (0, 30, 60, and 90d) and undergoing different freeze-thaw cycles (0, 25, 50, and 100 cycles) in NaCl solution) at experimental temperatures of 20 °C and −20 °C. The relationship between the depth of chloride ions erosion and the nominal fracture energy was analyzed. The effect of the coupled of temperature and chloride ions attack on the fracture performance of ECC under different working conditions was investigated. The results showed that: Under the loading condition of −20 °C, the fracture parameters of ECC under NaCl solution immersion were all lower than those of the water immersion group, indicating that extravasated chloride ions negatively impacted ECC's fracture properties. When ECC was loaded at −20 °C, the initiation fracture toughness and unstable fracture toughness of ECC under NaCl solution immersion were lower than those of the water immersion group, but the fracture energy was larger than that of the water immersion group, and when the specimens were immersed for 90 days, the drop in the fracture energy of ECC loaded at −20 °C compared to 20 °C for the NaCl solution immersion group was 59.1% less than 62.7% for the water immersion group; While loading at 20 °C revealed a linearly declining relationship between erosion depth and fracture energy, loading at −20 °C revealed no discernible pattern. The crack initiation fracture toughness of ECC gradually decreased with the increase of the freeze-thaw cycles, but the specimens' crack initiation fracture toughness following NaCl freezing at −20 °C was consistently larger than that at 20 °C. Following 100 freeze-thaw cycles on the specimens, the increase in initiation fracture toughness at −20 °C compared to 20 °C for the water freeze-thaw group was 128.4% less than that of the salt freeze-thaw group, which was 135.8%, while the decrease in fracture energy was 19.9% less than that of the salt freeze-thaw group, which was 46.3%. It shows that the repair effect of ice on ECC after NaCl freeze-thaw cycles is stronger than that of water freezing at the low temperature.