Experimental study on tensile behaviors of cracked ultra-high performance concrete under freezing and thawing

A combination of external actions and autogenous shrinkage makes ultra-high performance concrete (UHPC) prone to cracking in service, which inevitably harms the durability and working life. This paper focuses on studying the effects of freezing and thawing (FT) cycles on the tensile behavior of cracked UHPC. Three sets of dog-bone shaped specimens with pre-tensile strain levels of 0%, 0.05%, and 0.1% were prepared. Two kinds of experimental environments including FT cycles and soaking in water were adopted for comparison. Uniaxial tensile test, non-destructive measurements, and loss-on-ignition were performed to characterize the performances, along with ImageJ technology to identify the sectional pore structures. The results indicated that, compared with non-cracked UHPC, long-term FT action caused more deterioration in the initial cracking strength, tensile strength, tensile strain, and especially strain energy of cracked UHPC. In comparison, the water action generally favored the development of tensile properties of UHPC independent of the cracking state. The crack distribution tended to be unsaturated under the FT environment, showing a larger cracking spacing. The FT action hampered the re-hydration reaction, leading to a slower improvement of mass and resonance frequency at the early stage, and the values were always lower than those of the non-cracked UHPC. In addition, the section near to pre-tensile crack showed more large pores under the FT environment, and a reverse trend occurred under the water environment. Thus, it could be concluded that the cracked UHPC was more sensitive to FT action.