Response of reinforced concrete beams at high strain rates

A closed-loop servohydraulic testing machine was used to conduct high rate tests on reinforced concrete beams. Seven pairs of singly reinforced beams (without shear reinforcement) were tested under displacement control. From each pair, one beam was tested at a "static" rate (piston velocity = 0.00071 cm/sec) and the other at a "high" rate (piston velocity = 38 cm/sec). The total number of cracks reduced significantly at the high rate. Peak load and energy absorption capacity were found to increase with the rate of straining. The load-deflection curves for beams failing in flexure at the high rate did not show a sharp "yield point" or a "yield plateau." A standard sectional analysis using rate-dependent constitutive relationships does not adequately predict the shape of the high-rate load-deflection curve. Localized yielding of steel at higher strain rates is believed to be a cause for this. Computations to support this hypothesis are presented. For three of the pairs tested, final failure mode shifted from shear failure at the static rate to flexural failure at the high rate. This phenomenon is the opposite of the brittle transition in the mode of failure reported by some other researchers. It is tentatively proposed herein that this apparent contradiction can be explained on the basis of the rate sensitivity of the different steels used in these studies.