Performance of steel fiber reinforced concrete beams under sustained service loads

In order to evaluate the performance of steel fiber reinforced concrete beams under sustained service loads, the influences of two key parameters, sustained load level and steel fiber content, are investigated by series of bending tests.. Specimens Group I are subjected to a sustained load equal to 50% of the ultimate load (i.e. 0.5) and mix with steel fibers varying as 0%, 0.5%, 1.0%, 1.5% and 2.0%. Specimens Group II mix with steel fibers with the content of 1.0% and differ in the sustained load level varying as 0.35, 0.5, 0.59, 0.65 and 0.80. Considering the influences of degradation of material strength, reduction in reinforcing steel bar area and bond properties with service time, an evaluation model for the serviceability limit state of cracking and deflection is established using time-dependent uncertainty coefficients. The model is used to analyze the time-dependent reliability indices of beams bearing sustained service load. Research results show that reliability index increases with the increasing of steel fiber content in specimens Group I and decreases with the increasing of sustained load level in specimens Group II. In early stage, the reliability index decreases rapidly, whereas with longer period of sustained loading, it decreases gradually. Under long-term sustained loads, the incorporation of steel fiber improves the cracking characteristics ductility of the material, causing effective crack and deflection control in steel fiber reinforced concrete beam.