Keywords: Concrete, Defect, Imaging, Reconstruction, SAFT, Ultrasound
This paper was presented at the International Symposium Non-DestructiveTesting in Civil Engineering (NDT-CE) 26.-28.09.1995 in Berlin. NDT-CE, Full Program or the Ultrasound Part

Table of contents |Full Text|
1. Summary
2. Propagation properties of ultrasonic waves in concrete
3. Why using SAFT?
4. SAFT implementation
5. Experimental setup
6. Results of measurements
7. Concluding remarks

Summary

In ultrasonic testing of concrete the evaluation of internal structure is becoming possible through improved equipment and testing methods. Some of the topics where work is under progress are thickness measurement, search for voids, and location of ducts. A number of researchers have developed techniques ready for application all requiring access of the concrete element from one side only [1]. They are mostly adopted from other areas of non-destructive testing such as inspection of steel but needed modification to fit the special features of concrete. This contribution shows the use of ultrasonic SAFT-reconstruction to image 2-dimensional sections of concrete elements. SAFT (Synthetic Aperture Focusing Technique) is based on subsequent focusing of the data measured on an aperture to every point of the reconstructed area through superposition of the time records [2]. The reconstructed image is in terms of impedance differences caused by, e.g., the back wall, and inclusions. Main emphasis is put on demonstration of the applicability of 2-dimensional SAFT-imaging to the ultrasonic inspection of concrete. To explain why SAFT is especially suited for that purpose some of the propagation properties of ultrasonic waves in concrete are reviewed, and a comparison is made between SAFT and conventional A- and B-scan techniques. Some extensions to the fundamental algorithm are listed which are incorporated in the present implementation. Reconstruction results measured both in laboratory and in the field illustrate that SAFT is already applicable to testing concrete with grain size up to 16 mm and is a promising tool for the inspection of concrete with high resolution.

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