Instrumentation Innovation was approached in 1993 by the Institut für Massivbau, Darmstadt, one of the leading German research institutes on large concrete structures, and asked to design and build new pulse-echo equipment for testing concrete. The initiative has led to several important advances over the last few years in the technology of ultrasonic inspection of concrete. It is now possible to make inspections that were previously thought to be impossible. This breakthrough responds to the growing need to inspect large concrete structures such as: off-shore structures, bridges, nuclear power stations, tunnels and dams.
Ultrasound is an attractive probe for inspection purposes because:
- is not an ionising radiation and is, therefore, intrinsically safe.
- pection systems have low power
requirements so can be made rugged and portable.
The Darmstadt pulse-echo equipment works with reflected ultrasonic waves through a single test-surface - unlike conventional transmission equipment requiring access to two facing surfaces. A breakthrough in the technology is based upon using new transducers and signal processing.
Research transducers are now available:
- with high sensitivity,
- needing only modest drive powers,
- with wide frequency range - allowing
advanced computer signal-processing to be used,
- that can be arranged in arrays - allowing
a wide range of signal-processing algorithms to be used.
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Signals from the Darmstadt pulse-echo system showing (upper signal) the weak echoes from a stressing tendon dúct at 114 mm cover (lying mainly outside the ultrasonic beam) and the back-wall echo at 504 mm. In the lower signal the transducer array was positoned precisely over the tendon, causing a strong echo, but obscuring the the back-wall.
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The Darmstadt pulse-echo system is best suited for answering the question, "What lies under the surface of this concrete?
The equipment has worked well for the last two years and successfully completed
many evaluation experiments. The Institut für Massivbau was sponsored by the German Institute of Standards, the German Concrete Association and several
German construction companies.
The pulse-echo system has been able to detect a variety of targets at various ranges:
- 0 to 500 mm range - line targets as small
as 20 mm diameter such as voids and reinforcement bars have been detected with a success rate in excess of 90% and range error of only +/-5%.
- 500 mm to 2 m - larger targets such as
plane back-walls and honeycombing have been detected with a success rate in excess of 90%.
- Networks of fine cracks can be detected, for example, around reinforcement bars.
- Tendon ducts can be located to good precision.
- Works from a single surface in
reflection.
It is anticipated that further improvements can be made in the performance but the present performance already satisfies many inspection requirements. It is also worth noting that this system could be used by existing inspection engineers with little or no re-training, allowing the new technology to be introduced quickly.
Partners wanted in new project
Cambridge Ultrasonics and the Institut für Massivbau, Darmstadt are looking for other organisations to join a project and develop a second generation of ultrasonic instruments for testing concrete. The research projects of the last few years have been successful but have been aimed more at proving the feasibility of the technology in the laboratory rather than developing it to the point where site inspections can be done. The absence of equipment able to work on-site is now limiting full evaluation of the technique.
The kind of partners that are sought are: owners of large concrete structures, agencies of government involved in health and safety, inspection companies and manufacturers of inspection equipment. There are already five potential participants in the project within Europe. Partners from USA and Asia would also be welcome.
Details from David Androws on Tel: +44 (0)1954 231494, Fax: +44 (0)1954 232 282.
Or contact Dr. Otto Kroggel Email: kroggel@hrz1.hrz.th-darmstadt.de
