Advanced NDT methods for the assessment of concrete structures

The development of methods for the Non-Destructive Evaluation (NDE) of concrete and masonry structures has made considerable progress in the past two decades. Technology and knowledge transfer from other areas such as materials testing and medicine has been the main source for innovation, supported through multidisciplinary research by scientists, technicians and support personnel. A versatile toolbox of methods for the investigation of RC structures has emerged from this effort. In the area of simulation and 3D-reconstruction of ultrasonic and radar data, progress was driven by more powerful computers and development of state of the art software for 3D re-construction and visualization of data. Bringing these developments into the state of practice, into everyday use on construction sites by trained engineers rather than researchers still remains a major challenge. To earn the trust of industry, the reliability of these Non-Destructive Testing (NDT) methods should be established and adequately verified to prove that their application is worth the additional expense and effort. Validation of NDT methods has to prove that the customer requirements are met by the test. A focused effort is needed to implement a validation methodology for NDT methods in civil engineering, which is widely accepted and used. In an effort towards this goal, a number of reference specimens have been developed over the years at BAM and other locations, acting as a standard for specific testing tasks. Those specimens are available to the NDT community for comparing their methods and instruments under independent conditions. The development and application of NDT methods for concrete structures requires more than just using off-the-shelf testing equipment. Existing test methods must be improved for more reliable use. New methods borrowed from other areas of testing such as medicine or geophysics should be further developed. Using an array of methods and combining their results, preferably through data-fusion algorithms has a lot of potential. Over the years, the NDT group at BAM has developed and accumulated valuable expertise in some of the above-mentioned areas. The focus of the research and developments efforts at the NDT-CE division at BAM has been on methods used to evaluate the geometrical properties of structure such as thickness, location of components as well as detection of the presence of voids, delaminations, ungrouted tendon ducts or cracks. the concrete, 12 transducers each are combined and form a large aperture for transmitting and receiving the signals. Scanning of lines and areas was possible with the A1220 sensors in reasonable time, due to the fact that no special coupling was required. With the scanning systems developed at BAM, automated testing utilizing these transducers became a reality. Based on these experimental developments, reconstruction processes developed in other areas could be utilized for ultrasonic data collected within a 2D aperture. The result of such tests is a 3D reconstruction of the volume below the test surface (Streicher et al. 2005). As an example of 3D imaging of concrete bridges a C-scan from a bridge is shown in Figure 2. It clearly shows tendon ducts in a depth of 10 cm over the entire length (10 m) of the tested web. 1 ULTRASONIC PULSE ECHO (UPE) It became clear very early that ultrasonic pulse echo on concrete requires very low frequency transducers (Neisecke 1991) to overcome the scattering problem in coarse aggregate concrete. Multi position experiments and synthetic aperture data analysis led to reliable thickness measurements of concrete slabs. In order to reduce testing times and to ease experiments, arrays of ten transducers were used. In the mid 90s a new type of transducer was developed in Russia (www.acsys.ru 2006) which did not require special coupling to the concrete surface (Fig. 1). Based on transversal excited ultrasound, testing became much easier to perform with the availability of these commercial products. The individual transducers have ceramic pin contact to