International Concrete Abstracts Portal

Over the last 3 years, an impact-echo technique has proven to be an effective evaluation tool in identifying defects and locating embedded items in concrete structures. The advantages of the method are that it is nondestructive, relatively easy to calibrate, and testing can be conducted from one surface of the element under test. Operating experience with the impact-echo technique has shown that, within known limitations, the technique can provide a rapid nondestructive means for performing certain concrete condition surveys.

This paper summarizes recent developments in the impact-echo technique for locating defects in concrete structures. First, the impact-echo principle is reviewed and frequency analysis of impact-echo signals is explained for those unfamiliar with the technique. Advancements in signal processing that have been aimed at simplifying and automating data interpretation are discussed, and examples from the use of an artificial-intelligence technique for automated interpretation of signals from plate-like structures are shown. Impact-echo instrumentation is reviewed and an impact-echo field system is described. The newest application for the method is testing bar-like structures, such as columns and beams. Numerical and experimental controlled-flaw studies established the basis for the use of impact-echo for this application. Representative results obtained from impact-echo studies carried out on reinforced concrete columns with circular and rectangular cross sections are presented. Finally, an overview of ongoing research is presented.

A research program was undertaken to evaluate commercially available nondestructive testing techniques to locate voids in grouted tendon ducts. A laboratory scale mockup was used to evaluate several methods. On the basis of these results, impact-echo, ultrasonic, pulse-echo, and ultrasonic pitch-catch systems were selected for further evaluation. A full-scale mockup of a section representing an icewall of an offshore drilling structure was fabricated. It contained grouted tendon ducts with voids of various sizes and configuration. A test was carried out in which three teams of researchers from Canada and the U.S. evaluated nondestructively the mockup without knowledge of the locations or nature of the voids. This paper presents the results of the preceding evaluation and makes recommendations for further research.

The use of nondestructive testing in the laboratory is well-documented and standard specifications are available. However, when these nondestructive testing methods are used on site, additional factors have to be taken into consideration to enable meaningful interpretation of measurements obtained. This aspect of knowledge has not received sufficient attention for standard specifications to be drafted. Suggestions are put forward in this paper on precautions to be taken when applying nondestructive testing on site. The methods of testing discussed include the magnetic method of concrete cover or bar size determination, the rebound hardness, ultrasonic pulse velocity and the penetration resistance (Windsor Probe) test. The methods of calibration in the laboratory are reviewed and the ways to check on equipment and its calibration during site work are proposed. The information to be recorded and the interpretation of data are discussed. The need for trained personnel to carry out site testing, as well as experienced professionals to interpret test results, is emphasized.

An investigation was carried out to evaluate the effectiveness of epoxy systems injected into cracks in concrete in order to repair damaged concrete structures. It was found that the usual parameters (viscosity, elastic modulus) characterizing epoxy system are not sufficient for such an evaluation, since concrete microstructure and crack width are determinant factors. Successful1 injection into larger macrocracks (>0,8 mm) is independent of the epoxy system viscosity. However, for narrower cracks (< O,3 mm), the effectiveness of the injection strongly depends on the viscosity of the epoxy system, so that a threshold value of viscosity can be determined for each microcrack width. The injection of epoxy systems into cracks of porous concretes causes, in addition to the crack filling, an impregnation of the material surrounding the crack area. This impregnation causes a strength increase, which is higher for more porous concretes and higher epoxy system viscosity. The mechanical behaviour of injected concretes do not depend significantly on the mechanical properties of the epoxy system, whereas the rheological properties of the epoxy system can affect the performances of injected concretes.