The Concrete Laboratory (CL) has used nondestructive testing (NDT) in the evaluation of concrete and concreting materials. In recent years, more emphasis has been placed on the inspection and structural evaluation of completed Corps of Engineers structures. The need for a capability to predict, prior to placement, certain properties of concrete mixtures after hardening has been realized. This report discusses the history, progress to date, and needs for future research and development in the field of NDT.

Development of the resonant frequency technique as a method for evaluation of concrete structures is in progress. It is desirable that structures be evaluated in place, nondestructively, and in real time. The availability of digital Fourier analyzers and mathematical functions such as spectra, coherence, and transfer relationships permits the analysis of the behavior of large structures under dynamic conditions in place and in real time. An impact system has been tested which was proven more economical and versatile than the sinusoidal system currently in use. With transient loading, all frequencies are applied to the structure simultaneously rather than being swept through one at a time. Because the resonant frequency of a structure is directly related to its dynamic Young's modulus and hence its mechanical integrity, the resonant frequency technique is useful in the field as well as in the laboratory. Factors that influence the soundness and safety of a structure, such as modulus, continuity, and boundary conditions (i.e. foundation and other restraints) also directly affect the resonant frequency. Some limited work has been done with both mathematical and physical modeling to develop measurement criteria that will improve prediction of the expected resonant frequency for structures with varying geometries and varying degrees of foundation restraint. The characteristic vibrational signature of a structure may be evaluated at selected time intervals in an effort to predict service life. Improvement in modal analysis and prediction of frequencies by mathematical modeling is needed. Field work has demonstrated the feasibility of the technique. (Author).