- since 1996 -
|NDT.net Issue - 2009-04 - NEWS |
|Testing concrete has always been a problematic and cumbersome endeavor. Typical tests to determine concrete quality involve coring numerous samples in order to get suitable information through destructive compression testing. Content and flaw detection generally performed by ground penetrating radar (GPR) and ultrasonic systems can typically penetrate no deeper than eight inches. With the newly developed impact echo technology, concrete content determination is made accessible and efficient while dramatically reducing testing time.
ASTM recently announced its acceptance of impact echo technology under the ASTM C1383 standard. Impact echo technology is based on the detection of stress waves caused by an impact on the surface of the test medium. With the portable impact echo system (PIES), the technician is equipped with a patented piezo-electric crystal displacement sensor, a handheld palm computer and a set of impactors.
The test surface is struck causing a very slight elastic deformation that propagates throughout the material as compression (P) waves and shear (S) waves. The waves propagate through the material, reflecting and diffracting off the inside surfaces of the material. The waves are quickly detected by the sensor as they return to the surface of the material. These very slight deformations are translated into a voltage and read by the palm computer for the results of the test. A single test takes less than a second to perform, allowing the technician to run numerous tests on a vast area in very little time. Comparatively, impact echo technology can cover the same area of GPR systems in almost a tenth of the time.
With an understanding of impact echo technology and the testing process, the technician can select the parameters of the test. The user can increase or decrease the level of detail to find voids, honeycombing or most other inside imperfections in the concrete. As the stress waves bend and reflect off the inside conditions, great amounts of detail are gained with a single test. Furthermore, the stress waves propagate inside the concrete with very little deterioration. Very little accuracy is lost with increased thickness, allowing the unit to perform reliably even to a depth of 30 feet, far surpassing the limits of ultrasonic and radar technology.
The basic function of the PIES is to determine concrete thickness. As already discussed, this system can yield data on the solid concretes flaw characteristics. With the included programming, the information gathered from each test can also be used to calculate other properties of the sample material. Properties such as the static and dynamic elastic moduli and Poissons ratio can be determined without the need for destructive tests. The determination of these properties also allows insight into the concrete samples compressive strength. Because of the potential flaws inherent in the concrete, the compressive strength cannot directly be determined without comparison to destructive compression tests. Even so, the information from crushing two or three core samples is enough to accurately determine the compressive strength at any other point in the concrete. This method reduces an area requiring thirty core compression tests to three, reducing the time required, expenses accrued and damage to the finished structure.
The Qualitest PIES unit comes with everything needed in order to perform expert impact echo testing. The handheld computer comes prepared for testing and need only be connected to the system to begin. The handheld computer is protected by a dust-proof casing and the sensor power and conversion system is all supported by a light-weight power supply and signal digitizer. An impact echo technology workshop is included in the package as well as an instructional suite including detailed manuals and texts as well as video training.