Cambridge Ultrasonics

Concrete

Demand for concrete inspection and monitoring

Concrete is the most popular man-made material used in the world. In its manufacture it contributes substantially to CO₂ emissions. Between 1945 and 1980 there was very substantial use of concrete for new structures throughout the world. Typical design lives were 30 years to 120 years so many structures are now reaching the end of their design lives. Design codes for concrete structures have become more demanding consequently the cost of concrete structures has gone up faster than inflation. Now the world cannot afford to replace its stock of concrete structures and there is strong commercial pressure to continue to use concrete structures beyond their design life-times.

Unfortunately, many concrete structures deteriorate more quickly than anticipated and must be decommissioned before the end of their design-lives. Probably the best examples are to be found in the Persian Gulf where the shortage of fresh water necessitated the use of sea water in making concrete. Unfortunately, the chlorides in sea water cause accelerated corrosion of the steel forms used in concrete and some jetties used for loading crude oil onto tankers deteriorated in a few years instead of lasting 50 years.

When faced with using a structure beyond its design life or when faced with rapid deterioration it is essential that good inspection methods are available to assess the structural integrity of the material. Unfortunately, there are few good methods available commercially for inspecting concrete.

There is a growing demand for high quality methods for inspecting and monitoring concrete structures. Cambridge Ultrasonics has developed suitable technology.

Cambridge Ultrasonics and concrete inspection - international R&D

1987 marked the beginning of Cambridge Ultrasonics' interest in concrete inspection. The work was initially supported by the the Transport Research Laboratory and by the Marine Technology Directorate, acting on behalf of several North Sea oil majors that owned large concrete structures. The work was more recently supported by the European Commission. Several commercial organisations have also participated in the development or supported it since, including: Taywood Engineering and Sonatest of the UK, ISoTest of Italy, Alcatel Espace of France, SP of Sweden, Technical University Darmstadt of Germany, BMW, IETCC of Spain, Nesco of Spain and Queens University Belfast of UK. The common theme was following Cambridge Ultrasonics' recommended technical approach.

Technical issues

Concrete is a difficult material to inspect when the ultrasonic wavelength is comparable to the size of the aggregate particles (stones) in the concrete. The wavelength is the limit of the smallest object that can be resolved. Inspection engineers want to find objects almost as small as aggregate particles. The situation is similar to finding small cracks in cast iron and stainless steel or many applications of medical imaging in living tissue. When the wavelength is much longer than the aggregate particles then there is little or no scattering of ultrasound by the aggregates and the propagation path is transparent to ultrasound but when the wavelength is approximately the same as the aggregates then scattering from aggregates becomes strong and random - the propagation path becomes increasingly opaque to ultrasound.

Transducer arrays and attenuation

A solution to imaging under conditions of random (stochastic) scattering lies in using multi-path ultrasonic beams and averaging the multi-path signals. This is most easily done using arrays. Cambridge Ultrasonics' visualization system was important in giving a qualitative understanding of how pulses of ultrasound travel through heterogeneous materials such as concrete and this understanding was used to advantage in the design of prototype equipment. Visualization showed that random scattering causes plane waves to lose coherence and to cause the pulse to be somewhat dispersed but visualization also showed that ultrasonic energy propagated as a clearly defined pulse across many scatterers, in fact little or no energy is lost by scattering. The loss of coherence causes large aperture, piezoelectric receivers to register zero signal when a scattered pulse arrives, despite the arrival of a large amount of energy. No signal is registered because the de-coherent waves create random charges, both positive and negative in sign, on both surfaces of piezoelectric receivers. The conducting plates on the surfaces of the piezoelectric receivers average the charges to zero. This has been the main cause of the so called high attenuation of ultrasound in concrete and why conventional, large aperture (receiver) ultrasound equipment has failed to work on concrete. In fact ultrasonic absorption is small in concrete if scattering is ignored.

In 1994 Cambridge Ultrasonics was asked to develop new equipment and arrays for the Institut für Massivbau in the Technical University of Darmstadt, for a German national project co-ordinated by the German Concrete Society. The project made breakthroughs in detecting post-tensioned tendons and back-walls at ranges of 1 m - previously thought to be impossible. Under the guidance of Cambridge Ultrasonics the Institut für Massivbau went on to make images of the interior of concrete.

In 2003 Cambridge Ultrasonics with financial support by the Commission of the European Union and other partners developed and tested two new ultrasonic systems: a portable inspection system to see inside concrete using an array and a monitoring system for concrete structures that used a distributed array of intelligent sensors. The monitoring system was able to detect the onset of micro-cracking in concrete, a precursor of significant structural deterioration.