Research on fracture behavior of lightly reinforced concrete beam under different loading rates

Abstract

Under impact loads, the crack resistance of reinforcement in concrete is the focus of impact dynamics research. In this study, the three-point bending tests of lightly reinforce concrete beams with notch were conducted with a drop hammer testing machine. The high speed camera and Digital Image Correlation (DIC) technology were applied to catch the fracture process and analyse the displacement field. The shape and geometry of specimen followed the RILEM recommendation, i.e., 150 mm × 150 mm in cross section, 800 mm in length, notch-depth ratio was around 1/3 and span was kept constant 600 mm. And the impact force, mid-span deflection, steel strain beneath the loading point and concrete strain at the notch tip were measured over four loading rates. The experiment results showed that the crack initiation strain rate increased linearlywith the loading rate, and the growth trend weakened when loading rate was 1.771×103 mm/s. Based on the test results, the empirical formula of crack initiation strain rate with respect to loading rate wasgiven, which was meaningful to simulate the crack initiation in concrete under dynamic loading. In addition, with the increase of loading rate, the steel yielded gradually, which resulted in the response time difference between impact force and steel strain decreased. With the recovery of elastic deformation of steel bars, the crack started to close and the macro crack became visible clearly. Meanwhile, the crack mouth opening displacement (CMOD) declined to a constant value after reaching the maximum value. Then, the crack mouth opening displacement rate was obtained from the fitting curve, and a linear growth relationship between CMOD rate and loading rate was founded. The failure process of the lightly reinforced beams was analyzed based on the CMOD rate, which provided an idea for comparing the fracture process of the beam under dynamic and static loads.