Effect of steel fiber content on failure strength and toughness of UHPC-CA at low temperature: an experimental investigation

As a potential and sustainable construction material, ultra-high performance concrete containing coarse aggregate (UHPC-CA) finds promising applications in engineering constructions, particularly in severe cold regions. This study conducts an experimental study to investigate the influence of fiber content (ranging from 0.0 % to 3.0 %) on the uniaxial compression and splitting-tension failures of UHPC-CA across a temperature range of -90 °C to 20 °C. Through microstructure analysis, the underlying mechanisms and quantitative analysis behind the low-temperature enhancement effect and fiber reinforcement effect on the nominal strength and toughness of UHPC-CA were elucidated. The test results show that as the temperature drops from 20 °C to -90 °C, nominal strengths of UHPC-CA monotonically increase (with a maximum increase 88.1% for splitting-tension strength while 72.6% for compressive strength), but the compressive toughness reduces with a maximum decrease 54.6%. Additionally, the incorporation of steel fibers can enhance the nominal strengths and compressive toughness, exhibiting the fiber reinforcement and toughening effects. Compared to the UHPC matrix, all the increases for nominal strengths and compressive toughness of UHPC-CA with 2.0 % steel fibers at room temperature are higher than those at low temperatures, demonstrating that the decreasing temperature can weaken the fiber reinforcement and toughening effects, which may attribute to fact that the formation of microcracks (due to the expansion of phase change of pore ice) in the surface between steel fiber and UHPC matrix at low temperatures, as observed in the microscopic test. However, as the fiber content exceeds 2.0 %, the splitting-tension strength decreases, thereby it is recommended that the optimal steel fiber content is around 2.0 % for UHPC-CA at low temperatures. Finally, considering the quantitative effects of steel fiber characteristics and temperature, the empirical prediction models for cryogenic nominal strengths were proposed and verified, which can well predict compressive and splitting-tension strengths of UHPC-CA with various steel fiber contents at low temperatures. This study can provide an experimental data reference for the safety design of UHPC structures and contribute to promoting the application of UHPC-CA in the engineering construction of cold regions.