IntroductionUltra-High-Performance Concrete (UHPC), with ultra-high compressive strength and remarkable durability, holds great promise for constructing lightweight and high-performance structures. The ratio of live load effect to total load effect in the lightweight UHPC structure will be significantly increased, and higher cyclic stress amplitudes resulting in fatigue failure of the structure would occur accordingly. The combined tensile and bending stresses exist in certain components of practical engineering structures, such as UHPC bridge decks in the hogging moment region of the girder. Under overall load, the decks are subjected to a state akin to axial tension due to their limited thickness and considerable distance from the neutral axis of the girder. Yet, under the alternating localized wheel loads, the decks are subjected to cyclic bending stresses. Under the simultaneous application of the overall and local loads, UHPC bridge decks in the hogging moment region of the girder are actually subjected to combined tensile and bending stresses with variable amplitude. Many researchers have investigated the effects of steel fiber types, steel fiber content, and temperature on the bending fatigue properties of UHPC. However, there are limited studies on the axial tensile fatigue properties of UHPC, and investigations on the fatigue properties of UHPC under combined tensile and bending action are not found in the available literature. To determine the fatigue-resistant performance of UHPC with different tensile-bending stress ratios or the eccentricity ratio e/h of applied eccentric tension, an experimental study on the fatigue resistance performance of UHPC under combined tensile and bending action was conducted, and the corresponding fatigue strength was determined.MethodsWith the test parameter of eccentricity ratio e/h (0, 0.1, 0.4, and ∞, e/h=0 refers to axial tension and e/h=∞ to pure bending), 25 specimens with a commonly used steel fiber content of 2% were designed, manufactured, and tested. Among these, 9 specimens were tested for monotonic properties, and 16 specimens were tested for fatigue properties. The axially tensile and eccentrically tensile properties of UHPC were tested by C-shape specimens with corbel at both ends, and pure bending behaviors of UHPC were tested by four-point bending beams with identical cross-sections. The fatigue test was carried out under different eccentricity ratios and stress levels using a specially designed three-hinge truss device converting the tension loads to compression loads. Electrical resistance strain gauges were arranged at the extreme tensile edge of the specimens to measure the tensile strain before cracking. Two linear variable differential transformers (LVDTs) were continuously arranged in the extreme tensile edge of the specimen with a gauge length of 100 mm to measure the average tensile strain after cracking. The progression of crack width on the surface of specimen during fatigue testing was tracked using a hand-held crack detector with an accuracy of 0.01mm. The number of load cycles until fatigue failure under different stress levels was recorded, and further, the effects of eccentricity ratios and stress levels on fatigue life of UHPC were analyzed.Results and discussionIt was found that the development of fatigue tensile strain and crack width on the tensile surface of the specimens with fatigue failure reveals three stages of increasing rapidly with a decrease of strain rate, a steady growth trend with an almost constant rate, and increasing suddenly following a fatigue fracture. The tensile strain of the specimen without fatigue failure converges to a stable value after the rapid development in the first stage and remains unchanged in the second stage. The fatigue strength of UHPC increases with the eccentricity ratio rising, which can be attributed to the effect of stress redistribution between adjacent stripes in the tensile region caused by the increasing strain gradient on the cross-section. Compared with the corresponding strength of axial tensile specimens, for specimens with eccentricity ratios of 0.1, 0.4 and ∞, the initial cracking strength is increased by 10%, 38%, and 51%; the ultimate strength is increased by 14%, 57%, and 134%; while the fatigue strength is increased by 11%, 46%, and 105%, which demonstrate that the increased magnitude of fatigue strength lies between that of initial crack strength and ultimate strength. When the stress level of the specimen is close to its fatigue strength, the cumulative damage of the specimen is obvious, and the residual strength after fatigue is significantly lower than the corresponding static strength. When the stress level of the specimen is significantly lower than its fatigue strength, the cumulative damage of the specimen is slight, and the residual strength after fatigue is only slightly lower than its corresponding static strength.ConclusionsAs eccentricity ratios rises, the fatigue strength σ_f and the fatigue strength ratio σ_f/f_e increase, while the fatigue strength ratio σ_f/f_p decreases, which means the stress level index S_e=σ_max/f_e relative to the initial crack strength can more reasonably reflect the variation of fatigue strength of UHPC with eccentricity ratios under combined tension and bending action. For the UHPC with steel fiber content of 2% under combined tension and bending action, a fatigue equation for predicting the fatigue life of UHPC under different eccentricity ratios and various stress levels is proposed.