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For many years one of the goals of those engaged in the control of concrete quality and in the service behavior of concrete has been the development of suitable nondestructive tests to supply the information desired. To be of greatest usefulness, such tests should be applicable to concrete in the structure. It will be seen that the dynamic tests included in the following discussion, despite present limitations in application and in interpretation, are bringing the desired goal within reach in many respects. For the purpose of this discussion, dynamic testing will be defined as that in which the load is applied and removed in a manner such that the effects of creep during testing are negligible, and which does not usually result in destruction or damage to the concrete. In general, it is found that values of Young's modulus of elasticity computed from dynamic tests are somewhat higher than those determined for slower applications of load in which both elastic and plastic deformations may occur. Tests complying with this definition and in sufficiently wide use to warrant consideration may be subdivided into three groups: 1. Those identified as sonic tests, generally involving determination of the resonant frequency of a specimen. 2. Those identified as pulse velocity tests, generally involving measurement of the velocity of a compressional pulse travelling through the concrete. 3. Those involving the measurement of rebound distance of a hammer after striking a blow of controlled intensity, or the diameter of indentation caused by such a blow.
Whitehurst, E. A.
Director, University of Tennessee, Knoxville, Tenn.