The addition of steel fibres to concrete in ultra-high performance concrete (UHPC) to form ultra-high performance fibre reinforced concrete (UHPFRC) has been shown to have a great benefit by substantially increasing the flexural capacities and ductilities at the ultimate limit state and reducing crack widths and increasing flexural rigidities at the serviceability limit state. This is because the fibres bridge a crack and consequently allow tensile stresses across the crack. Tests have also shown that tensile cyclic loads applied across a crack can reduce these benefits by allowing the crack to widen through a gradual debonding of the fibres. To quantify the behaviour of UHPFRC post cracking, the fatigue behaviour of steel microfibre concrete at a crack is studied through 33 tensile fatigue tests on precracked UHPFRC and 6 monotonic tests. An approach for processing the results based on the increase in crack width per cycle, that is the incremental set, has been developed and can be applied to any UHPFRC that exhibits debonding. Three distinct cyclic behaviours have been identified and quantified: where there is no incremental set such that there is no quantifiable damage due to cyclic loading; where the incremental set is constant such that there is quantifiable damage; and where there is rapid unstable increase in the incremental set.

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Tension‐stiffening controls the serviceability behavior of concrete structures as it is responsible for crack formation and, consequently, the deflection of beams. In fiber reinforced concrete, such as ultra‐high performance fiber reinforced concrete (UHPFRC), fibers bridge cracks and thereby transfer tensile stresses across the cracked region, allowing for tensile stresses to be carried by the concrete within the cracked region. Due to structures being designed for longer design lives, the consideration of long‐term effects such as fatigue is required. Much research has examined tension‐stiffening under fatigue when subjected to low cyclic loading, but very little has considered the effects of high‐cycle fatigue, especially for UHPFRC. This paper presents the results of nine UHPFRC tension‐stiffening tests under high‐cycle fatigue in which the crack formation and development under varying cyclic ranges were studied. Specimens were subjected to as many as 5.7 million cycles, and crack readings were taken during each test. The experimental results demonstrate the random nature of cracking on UHPFRC as well as the increase in the crack width under cyclic loads. Finally, this research described the extension of an existing partial‐interaction mechanics model to allow for the stress in the fibers and the increase in crack width due to high cycle fatigue.