ABSTRACT
The first reports of the measurement of the velocity of mechanically generated pulses through concrete appeared in the USA in the mid-1940s. It was found that the velocity depended primarily upon the elastic properties of the material and was almost independent of geometry. The potential value of this approach was apparent, but measurement problems were considerable, and led to the development in France, a few years later, of repetitive mechanical pulse equipment. At about the same time, work was undertaken in Canada and the United Kingdom using electro-acoustic transducers, which were found to offer greater control on the type and frequency of pulses generated. This form of testing has been developed into the modern ultrasonic method, employing pulses in the frequency range of 20-150 kHz, generated and recorded by electronic circuits. Ultrasonic testing of metals commonly uses a reflective pulse technique with much higher frequencies, but this cannot readily be applied to concrete because of the high scattering which occurs at matrix/aggregate interfaces and microcracks. Concrete testing is thus at present based largely on pulse velocity measurements using through-transmission techniques. The method has become widely accepted around the world, and commercially produced robust lightweight equipment suitable for site as well as laboratory use is readily available. Nogueira andWillam (65) found that UPV methods, where the amplitude
of the signal was studied, could be used to estimate microcrack growth in concrete and hence to study mechanical damage, whilst Pavlakovic et al. (66) have used a guided wave technique to study damage in post-tensioned tendons in bridges. Krause et al. (67) have studied ultrasonic imaging with an array system to examine defects behind dense steel reinforcement, including cover to pipe ducts and ungrouted tendon ducts. Koehler (68) has further examined the use of specialized Synthetic Aperture Focussing Techniques (SAFT) to provide 3D visualization of defects in concrete structures, such as gravel pockets, and to locate tendon ducts. Krause and Wiggenhauser (69) also successfully used ultrasonic 2D and 3D methods to establish the position of tendon ducts in a bridge deck, and Popovics (70) has recently
reviewed some of these techniques together with tomography. Andrews (71) has suggested that there is much scope for new applications with the development of improved fidelity transducers and computer interpretation. Study of pulse attenuation characteristics has been shown by the authors to provide useful data relating to deterioration of concrete due to alkali-silica reaction (72) although there are practical problems of achieving consistent coupling on site. Hillger (73) and Kroggel (74) have both described the development of pulse-echo techniques to permit detection of defects and cracks from tests on one surface as well as the use of a vacuum coupling system, and the application of signal processing techniques to yield information about internal defects and features is the subject of current research as noted above. Another interesting development, described by Sack and Olson (75), involves the use of rolling transmitter and receiver scanners, which do not need any coupling medium, with a computer data acquisition system that permits straight line scans of up to 9m to be made within a timescale of less than 30 seconds. Although it is likely that many of these developments will expand into
commercial use in the future, the remainder of this chapter will concentrate upon conventional pulse velocity techniques. If the method is properly used by an experienced operator, a considerable
amount of information about the interior of a concrete member can be obtained. However, since the range of pulse velocities relating to practical concrete qualities is relatively small (3.5-4.8 km/s), great care is necessary, especially for site usage. Furthermore, since it is the elastic properties of the concrete which affect pulse velocity, it is often necessary to consider in detail the relationship between elastic modulus and strength when interpreting results. Recommendations for the use of this method are given in BS EN 12504-4 (76) and also in ASTM C597 (77).
