An unidentified vehicle recently struck the bottom of a railroad overpass and damaged one of the concrete beams in the overpass. The damaged beam was taken intact to the University of Texas where the Spectral-Analysis- of-Surface-Waves (SASW) method was used to nondestructively delineate the damaged zones. SASW measurements performed on the beam revealed a significant velocity contrast between damaged and undamaged zones. These measurements were consistent with visual inspection of the beam and also indicated the presence of cracking that was not visibly detectable. In addition, SASW measurements taken while repairing the beam revealed how surface wave velocity measurements can be used to monitor improvements in the integrity of a beam after each repair step.

Acoustic emission (AE) has the potential to be an effective tool in evaluation of concrete structures under the action of loads causing cracking. In conventional testing, several AE parameters are investigated to elucidate microfracturing behavior in concrete. To identify internal cracks, the AE location technique is available, which is based on measuring arrival time differences. By employing multi-channel AE observations, the location of a crack responsible for an AE source can be determined. To obtain quantitative information on crack kinematics, the procedure is further studied and a technique for kinematic characteristics of internal cracks is developed. The AE source is mathematically represented by a moment tensor, by which the classification of cracks into tensile and shear cracks and the determination of crack directions can be made. To implement the procedure into a conventional AE system, software named SiGMA (Amplified Green’s function for moment tensor malysis) has been developed. The analysis is readily available on an AE waveform analyzer system consisting of a digital waveform-recorder and a microcomputer (controller). The procedure is applied to a uniaxial compression test of a plate specimen with a through-thickness slit and to a tensile test of a reinforced concrete rigid frame. The crack locations, the classification of crack types, and the determination of the directions of crack motion are in good agreement with experimental findings. The results show the procedure certainly provides a new technique for kinematic identification of internal cracks.

Dilatational or P-wave speed in concrete is needed in impact-echo testing if the dimensions of structural elements or the location of flaws is to be determined. Previously the P-wave speed had to be determined from cores or from performing a test on a portion of the structure having known dimensions and no flaws. In cases where neither approach was possible, an estimate had to be made of the wave speed. This paper presents the details of a method for independently determining P-wave speed in concrete using a Rayleigh-wave speed measurement between two points on the surface. Such a procedure increases the power, versatility, and ease of use of the impact-echo method. In this paper the Rayleigh wave speed procedure is explained. Systematic errors involved in the measurement procedure areexamined, and the accuracies that can be expected using the procedure in conjunction with the impact-echo test procedure are discussed. Appropriate uses of the procedure are given, and the limitations of the method are stated. It is shown that the Rayleigh wave technique is an easy to use technique for estimating wave speeds. Typically, speeds within about 4% of the actual wave speed can be obtained.

This paper describes the application of geophysical tomography and scientific visualization techniques for evaluating the internal condition of massive concrete structures. The resulting output is a three dimensional representation of the structure showing the spatial distribution of ultrasound data. As various aspects of ultrasound data (e.g. velocity and attenuation) are related to concrete quality, the location and orientation of areas of inferior material or discontinties can be identified. In addition, specific features within the image can be highlighted and quantified. Results are presented from a preliminary study carried out to assess the potential application of this technique for evtiating the internal condition of large concrete elements. A large concrete block was constructed with a number of internal defects such as cracks, areas of poorly compacted concrete and uncemented aggregate, and large voids. A large number of measurements (ultrasonic pulse velocity) were taken to provide a network of velocities across a section. Algebraic tomographic techniques were then applied to reconstruct a two dimensional image. By taking a series of contiguous sections and stacking them together, a three dimensional model of the sample or structure was then created. Finally the three dimensional data set was visualized using advanced graphics techniques such as vohune rendering. Overall, the initial results are promising, and indicate that the presence and location of internal defects can be determined

In the past few years, a great progress was done in the field of acoustical imaging. One aspect of this progress has been the application of this evolving technique to the non-destructive evaluation of civil engineering structures. This paper gives a brief review of the basic theory of acoustical imaging. It also describes a case study to highlight the practical aspects and the possibility of using this technique to assess the internal conditions of concrete structures.