Quantifying the Fatigue Material Properties of UHPFRC with Steel Microfibers at Cracks

Abstract

The addition of steel fibers to concrete in ultrahigh-performance concrete (UHPC) to form ultrahigh-performance fiber-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 fibers bridge a crack and consequently allow tensile stresses across the crack. Tests also have 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 fibers. To quantify the behavior of UHPFRC post cracking, the fatigue behavior of steel microfiber concrete at a crack was 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, was developed and can be applied to any UHPFRC that exhibits debonding. Three distinct cyclic behaviors were identified and quantified: no incremental set, such that there is no quantifiable damage due to cyclic loading; the incremental set is constant, such that there is quantifiable damage; and a rapid unstable increase in the incremental set.