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The effect of the rate of application of load was investigated in compression tests of a concrete having a nominal static strength of 2500 psi. The test specimens were 3 by 6-in. cylinders, and the loads were applied at stress rates ranging from about 10 to 2 × 107 psi per sec. The lower rates of loading were obtained using a conventional hydraulic testing machine, and the higher rates were achieved by loading the specimens in a drop-hammer machine. The duration of impact in the drop-hammer machine was controlled by placing rubber buffers of appropriate thickness and hardness on top of the concrete specimens. The compressive strength of the concrete increased with the rate of loading. The maximum ratio of dynamic to static compressive strength was about 1.8 for the maximum rate of loading obtained with the drop-hammer machine. There was a significant increase in the secant modulus of elasticity as the rate of loading increased. The resistance of concrete to impact, as measured by its ability to absorb strain energy, also increased with the rate of application of load.
Acting Chief, Nat. Bureau Standards, Washington, D. C.