Grouting defects in ducts reduce the corrosion protection around the tendons. Hence, it is important to be able to detect grouting defects using non-destructive methods. The following methods are suitable in principle:
· radiography (X-ray method)
· ultrasonic methods (reflection or transmission)
· impact echo method (elastic waves).
These methods are discussed in the following sections.
Radiography is the only method for detecting grouting defects in ducts that is currently widely used in practice. For this purpose, an imaging beam (gamma or X-rays) is directed at the concrete surface and is received on a detector which is arranged on the posterior side of the concrete member (Fig. 6.20). Flaws such as cavities (e.g., grouting defects) as well as the position of ducts can be detected from the intensity of rays received. Newer equipment permits concrete members to be screened up to a thickness of about 1.5 m.79 The dimensions of the images are ca. 0:3 x 0:5 m. The necessary exposure times vary depending on the concrete cover, from 10 min up to 1±2 hours. Images of an incompletely and a completely grouted duct are shown in Fig. 6.21. On the left side, the incomplete grouting at the top of the duct is shown by the blackened area. If steel ducts are used, it is more difficult to analyse grouting defects since the radiation penetrating through the duct is reduced. The advantage of the radiographic method is its ability to provide information about the internal area of the duct. However, the disadvantages are the need to conform to the laws for radiation protection, the considerable weight of the equipment and the long exposure time required. In addition, radiography works as a `point-wise method’ which does not allow a quick scan of members, so radiography is only used in exceptional circumstances.
Ultrasonic and impact echo methods
With the ultrasonic and impact echo methods, elastic waves are generated on the concrete surface. In the impact-echo method, waves are scattered and reflected by faults (see Fig. 6.22) and so allow predictions concerning possible defects to be made. Flaws can be identified by means of deviations from the normal wave velocity. In contrast to the ultrasonic method, signals are evaluated in the frequency domain. It is difficult to apply the ultrasonic method to concrete because of the strong attenuation of the ultrasonic waves due to scattering and absorption caused by pores and grains in the concrete. To overcome this, ultrasonic waves in the range of 10±200 kHz are used. The associated wave- lengths range from 0.3 to 0.015 m. Ultrasonic measurements can detect voids but are not always effective in detecting grouting defects in field conditions.
Large-aperture procedures for scanning the members are necessary. The Synthetic-Aperture-Focusing Technique (SAFT) is effective and has been applied for some time now. The probe transmits waves into the concrete, and the reflected waves (e.g., reflected from tendons) received by the transducers are recorded by a computer. Every image-point, which represents a possible reflector or diffuser, must be considered. The image-point records the amplitude according to the time the wave takes to travel. High values denote the existence of a reflector at a point. In laboratory conditions, it is possible to detect grouting defects using SAFT. Figure 6.23 shows a reconstructed SAFT image using ultrasonic shear-waves with a polarisation parallel to the tendon. Between 800 and 1400 mm, the grouting defect becomes visible due to the absence of the echo from the tendon.
The detection of grouting defects and locating tendons should be possible using the scanning impact-echo method (SIEM).80 However, reliable and consistent results have not yet been achieved.83 At present, it is not possible to use the ultrasonic and impact-echo methods to detect grouting defects in prestressed members on site, but research based on SAFT aimed at improving these methods is in progress.