Investigating parameters governing UHPFRC tensile fatigue behaviour via direct tensile tests on dog-bone shaped specimens

Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) offers superior fatigue service life endurance due to its strain-hardening behavior, enabled by the effective crack bridging effect provided by the fibers. This makes it a promising solution for fatigue sensitive structures, expressway deck slabs being among the most widespread. Unlike conventional RC slabs that require frequent repairs, the use of UHPFRC can result in extended service life and reduced life-cycle costs. However, assessing the long-term fatigue performance of UHPFRC under actual loading conditions is crucial for practical implementation. This study investigates the effect of the through-thickness fiber distribution on the tensile fatigue behavior of UHPFRC using dog-bone-shaped specimens. Monotonic tests were first conducted to establish fatigue loading levels (40 %, 60 %, 80 % of the tensile strength from their results), followed by fatigue tests to evaluate strain accumulation and crack propagation, as affected by fiber distribution. Fatigue tests revealed that specimens at a 40 % stress level endured over 6.4 million cycles, whereas 60 % and 80 % specimens failed after 82,888 and 26,412 cycles, respectively. The propagation of fatigue damage was analyzed through the evolution of tensile stiffness modulus, crack propagation, and damage evolution curves. A key consideration is that at low-fatigue loading levels (40 % of tensile strength), displacement sensors may capture creep deformation effects, while at higher loading levels (60 % and 80 %), bending effects may influence the measurements due to non-uniform stress distribution also related to uneven through-thickness distribution of the fibers. Strain gauges and displacement sensors provided complementary insights into damage evolution, emphasizing the necessity of an integrated data analysis approach that accounts for their respective characteristics.