Research and development service for novel ultrasonic inspection systems for concrete

See also:

Second generation ultrasonic inspection

The research and development service offered by Cambridge Ultrasonics has application to improving the inspection methods for large concrete structures, such as: off-shore oil production structures, bridges, dams and nuclear power stations. New inspection methods should also find application on smaller concrete structures, for example: multi-storey car-parks; sports stadiums; spillage confinement bases for LPG vessels; large refractory blocks; linings of catalytic crackers in oil refineries; foundations for raised pipelines; concrete structures suffering wave erosion in sea environments, such as jetties, and structures built using sea water instead of fresh water.

Cambridge Ultrasonics and the Institut fur Massivbau Darmstadt (IfMD) worked successfully together three years ago on a project sponsored by the German Institute of Standards and the German Concrete Association that resulted in a major breakthrough in pulse-echo inspection of concrete. the project demonstrated for the first time that pulse-echo inspection could be applied to concrete to probe to depths of approximately 1 m with good reliability. Performance like this was hitherto thought to be impossible. This project and another collaborative European project, developing the method of resonance spectroscopy, have created a first generation of inspection instruments. Cambridge Ultrasonics has strong ideas on how to make improved second-generation instruments.

Benefits to owners of concrete structures

The benefits to owners of structures of developing new inspection systems are:

Benefits to providers of inspection services and manufacturers of inspection equipment

The benefits to inspection companies and equipment manufacturers of developing new inspection systems are:

The need for improved inspection methods

The consequences of catastrophic failure of large concrete structures are so great and potentially so damaging to public-relations that there is a strong need to use best possible procedures for maintaining structures. The financial consequences of a serious, but non-catastrophic, failure are usually so considerable that this eventuality, too, must be avoided. Unfortunately, existing commercial inspection systems for concrete structures do not yet provide a level of performance that meets requirements. One reason for this situation is that the market for inspection equipment is too small at present to finance the development costs of new equipment.

First generation of advanced inspection systems

Cambridge Ultrasonics and the IfMD have created a first generation of new instruments for inspecting concrete that made significant advances in this technology. The Darmstadt pulse-echo system can detect back-wall echoes in concrete at ranges in excess of one metre and can pin-point the position of stressing tendons to within approximately one centimetre at a range up to half a metre with a reliability better than 90%. The Cambridge resonance spectroscopy system has inspected the quality of a range of industrial components including pre-cast concrete components with success-rates in the range 82% to 100%. The Darmstadt system already satisfies many requirements for site inspection but it is only a laboratory instrument. The Cambridge system has not yet been evaluated for in-service monitoring of concrete structures, which is thought to be its best application to concrete inspection.

We have worked on a range of transducers and signal processing but so far the methods we favour are as follows:

Institut fur Massivbau Darmstadt

The IfMD, part of the Technical University Darmstadt, is one of the leading German civil engineering research institutes working on large structures. Related projects have been supported by the German Concrete Association, the German Institute of Standards and several other German construction companies as well as the European Commission. The IfMD will concentrate upon specifications, laboratory evaluation tests, providing laboratory test samples and performing tests on site.

Advantages - single test surface

One of the advantages of the methods we have developed is that they are all capable of working from a single test surface - there is no need to get access to the opposite surface to receive signals (contrast with conventional transmission testing for measuring strength). It is much cheaper to inspect a structure if the inspection can be made from a single surface - by way of contrast, if access is also required from the underside of, say, a road deck then the cost of inspection is greater due to the extra time spent and the cost of additional equipment.

One simple way of reducing the cost of inspection is to limit access to a single surface but this eliminates two commercial inspection methods including:

The methods we advocate are capable of working from a single test surface and so have an intrinsic capability to offer quick and cost-effective inspections.

Advantages - intrinsically safe equipment

Some methods for inspecting concrete require special safety precautions:

Disadvantages of existing commercial ultrasonic inspection systems

Established commercial ultrasonic inspection systems are often considered unsuitable for inspecting large concrete structures because:

Hammer tapping is well-known and the method of impact-echo has become popular in the last few years as a more sophisticated variant of hammer tapping. The objections raised by inspection engineers to these methods are that they are not sufficiently reliable or that interpretation difficult in the majority of cases. In a recent informal comparison of the Darmstadt pulse-echo system and an impact-echo system the Darmstadt system was easily able to detect a tendon duct in a relatively small concrete test sample whereas the impact-echo system could not.

Recent improvements - the first-generation instruments

Transducers have been developed with the following qualities:

Signal processing algorithms have been developed with the following qualities:

The pulse-echo system improves the quality of signals received from concrete to such an extent that they are now of a quality comparable to signals from commercial ultrasonic systems used on steel. It should be relatively straightforward for inspection engineers trained to use ultrasound on steel to use the second-generation system for testing concrete Pulse-echo systems are ideal for answering the question, “What lies under the surface of this concrete?” The resonance spectroscopy system works best when used to inspect concrete samples repeatedly, for regular maintenance or during mass-production. It answers questions like, “Is this item of good quality or bad quality?” The two systems have different capabilities and have different applications. Used appropriately they cover a wide range of applications.

Offshore concrete structures

Concrete structures used in the offshore oil and gas industry have exhibited good durability in service. Although many of these structures are nearing the end of their design lives they may be suitable for extended service but issues such as structural integrity and possible changes in material properties must be kept under constant and increasingly important review.

Onshore concrete structures, unfortunately, serve only as a source of concern because many have shown a worrying tendency towards unexpectedly rapid deterioration, sometimes progressing to structural failure and collapse. Repair and refurbishment is expensive but it is possible that some assessments are over-conservative and the number of repairs might be safely reduced if better inspection techniques were available. Offshore concrete is generally higher in quality, with thicker cover and the marine environment can substantially improve concrete properties by self-healing of micro-fissures and long-term gains in strength. On the other hand, marine conditions can be detrimental - for example, sub-sea corrosion of reinforcement bars does not necessarily result in spalling of concrete, rendering detection more difficult. The offshore loading and damage conditions are also markedly different, especially long-term fatigue and impact damage from ship collision and dropped objects.

There must therefore be some concern about whether problems exist offshore that are not being detected and whether there is sufficient confidence in present inspection and assessment methods to allow the selection of optimal maintenance strategies and techniques over extended service lives.

There is a need to improve the maintenance of offshore structures in two ways:

Concrete bridges

Some years ago a concrete bridge crossing a motor-way in England collapsed whilst it was being lifted by a crane. Fortunately, there were no injuries but the motor-way was closed for longer than was scheduled for the original operation. Disruption was caused to local roads and towns. The bridge collapsed because its stressing tendons were heavily corroded, with the majority of the tendons badly affected. When the bridge was lifted many of the tendons broke causing the bridge to collapse. The unsafe condition of the tendons had not been detected by visual inspection.

There are many thousands of bridges in the World built to the same design. After the incident a ban was placed on constructing new bridges of the same type in the United Kingdom. An important question must surely be, "Have inspection methods improved since the bridge collapsed so that the public can be confident in using the remaining stock of bridges?"

There is a need to improve the maintenance of bridges in two ways:

Concrete dams

Many of the dams in the World are more than thirty years old and there are questions about their future. Inspection methods capable of great penetrating range are needed when inspecting dams. For this reason few existing NDT methods work on dams. There is a need to improve the inspection needs of dams in two ways:

Concrete confinement walls of nuclear power station

The concrete confinement wall of a nuclear power station is a defensive barrier for protecting the environment against releases of radioactive material when active management methods fail. Close control of quality is used during the construction of these walls and the concrete is generally inspected carefully once it is in place. X-radiography is often used but the equipment is heavy, cumbersome and slow. Without doubt X-ray inspection offers good results but its disadvantages are:

There are needs to use equipment that:

The techniques we advocate should improve the inspection of nuclear power stations in two ways:

Publications