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Instruments and Means for Reinforced Concrete Structures Inspection A.V. Kovalev, A.A. Samokrutov, V.G. Shevaldykin, V.N. Kozlov, I.Yu. Pushkina (MSIA 'SPECTRUM', RUSSIA), S. Hubbard (JME Ltd., UK) Contact

Introduction

Referring to the practice of the 20^th century and analyzing the modern tendencies it is possible to say with reasonable confidence that the reinforced concrete was, still is and will be one of the basic construction materials. This material is used to construct buildings, bridges, tunnels, nuclear reactors and other structures, its extensive use requires development of corresponding diagnostics methods and means. So it is necessary to build instruments and develop procedures providing estimation of a structures condition and to determine its service life.

The NDT methods

The most common NDT methods for this specific task are radiographic testing (RT) and ultrasonic testing (UT). The RT method provides high quality images of internal structure of constructions, but, as a rule, to get these images it is required to have two side access to the examined object' surfaces. The RT method although versatile has strict safety requirements.

In figure 1 the diagrams demonstrating the potentialities of RT method are shown. Presented information demonstrates the possibility of RT examination in case of concrete with various thickness. The aim of test to detect the internal heterogeneity, the degree of reinforcement corrosion and other flaws.

Fig 1:

At present, the UT method for diagnostics of buildings, concrete and reinforced concrete structures, rocks and like materials is becoming more popular. With the help of ultrasound it is possible to estimate the strength of the concrete, to find our internal flaws, to carry out technological tests and to determine the geometrical parameters of construction structures (CS). At the same time, the UT method is practically the only one providing the one side examination of CS. This fact very often is of special importance.

Short review of the state of the available on market ultrasonic (US) means
Methods of US through and surface sounding for CS inspection has wide application all over the world. Corresponding equipment is manufactured by a number of European and American companies. But the instruments available on the market do not solve rather essential problem of thickness gauging and defectoscopy of concrete objects by one sided access. From published materials it is known about attempts to develop such type of equipment, these attempts are made by well known instruments making firms, by some European NDT centers within the frames of common European projects, but meanwhile all the results are staying at the stage of the prototypes' laboratory test.

Review of instruments for US inspection of CS produced by MSIA "Spectrum" (past history, area of application, special features)

In Russia, since 1970 work purposely oriented for the development of equipment for one sided US examination of concrete objects have been carried out. At MSIA "Spectrum", at the end of eighties, the first prototypes of such instruments had been developed. The most interesting instruments were UT-201 (see fig. 2) and UE-201. Approximately at the same time started commercial production of these machines.

Fig 2: The general view of the concrete thickness gauge UT-201.

Fig 3: The example of results obtained by means of the UE-201

These instruments use US low frequency operating range from 50 to 100 kHz.

The UT-201 provided measurement of monolithic concrete and reinforced concrete thickness at the depth up to 500 mm with the accuracy not worse that 10%.

UE-201 provided visualisation of the monolithic CS internal structure at the depth of 700 mm. The instrument also made 3D reconstruction of distribution of the US reflection coefficients in the volume of 250 x 700 x 1000 mm. The operator was able to follow the results of instruments operation as B- or C-scans. The results of UE-201 in the form of B-scan is shown in figure 3.

The instruments, mentioned above, in principle provided the solution of the task to be solved - the thickness gauging and tomography of CS in the case of one side access. But during there trial operation it was found the main disadvantage - that was acoustic contact required couplant liquid. This proved to be the most time consuming part of the test, especially if the surface was not smooth.

Utilising collected information and knowledge during the middle of the nineties, in Russia was developed a new generation of US instruments for concrete and reinforced concrete inspection. The main advance of this generation was the use of dry-point acoustic contact (DPAC). The basic idea of this design was to provide acoustic contact from the probe and examined object surface via severe, point like contact area. This special feature increase the rate and reliability of the CS examination and eliminates the necessity to specially prepare the object surface.

During all stages of the new generation instruments development a number of serious scientific problems were investigated and solved. These were:

• To analyze the nature of disturbances typical for US inspection of heterogeneous structures (concrete is a classical sample of such a structure). The results of this analysis were used at the design stage to provide reliable detection of the valid signals over the noise background;
• To develop portable, wide band and highly efficient US probes implementing DPAC;
• To determine and work out algorithms of signals processing for 3D reconstruction of CS internal structure;
• The work out procedures of the new instruments' implementation. Some procedures give advise how to use the instruments to solve practical tasks and others give recommendations for metrological verification.

Successful solution of the above mentioned tasks gave a start to a new generation of the US instruments for concrete inspection.

Fig 4: General view of the UK1401.

Fig 5: General view of the A1220

Between these new instruments the following may be listed: US tester for concrete UK1401 (see. Fig. 4), thickness gauge - defectoscope A1220 (see Fig. 5), ultrasonic tomograph A1230 (see fig. 6). For all these instruments the operating frequency range is in frequency domain from 30 to 100 kHz and in all of them the DPAC technology used.

Fig 6: General view of the A1230

Instruments technical parameters and some examples of there practical use

UK1401
The ultrasonic tester UK1401 is designed for gauging the time of flight or velocity of ultrasonic longitudinal waves propagated in various solid materials. This unit is an electronic module with built-in two US probes. The instrument realizes the known method of surface sounding. Its special features - simplicity of operation, high reliability and accuracy as well as robust construction.

With the help of this instrument it is practically possible to solve the problem of CS strength evaluation both during fabrication and in use, search of surface defects like cracks and there depth estimation.

One of the interesting practical applications of the instrument is the evaluation of the carrying capacity of the concrete pillar electrical lines holders. Together the Russian railway roads specialists and "Spectrum" specialists developed and introduced the guidelines describing how to carry out this examination, the final results were highly efficient. Due to combined implementation of the instrument and in details described inspection procedure the number of incidents of failure of these holders was reduced.

The other sample of instrument use is its implementation during construction activity. When the built-in memory and link with an external PC via the IR-adapter are used it is possible to monitor the process of concrete aging. This is especially useful when the building is constructed by methods of continuous casting. A result of such inspection is shown in figure 7. In figure 8 it is shown the result presenting the strength variations along the vertical wall construction. These results were obtained on construction site at MSIA "Spectrum" this year.

Fig 7: Relationship between strength of the concrete and time(obtained with the help of the UK1401).

Fig 8: Distribution of concrete strength along the vertical wall construction (obtained with the help of the UK1401).

It is necessary to underline that the UK1401 is one of the most used instruments in the construction and other branches of industry.

A1220
The instrument A1220 is a more sophisticated and comprehensive in comparison with the ultrasonic tester UK1401. This is a low frequency US flaw detector (see fig. 5). It comprises electronic module and a 24 elements (4 x 6) antenna array (AA). Also implementing the technology of DPAC. The AA elements are spring loaded and have an independent suspension system providing measurements over curved and rough surfaces.

The A1220 is designed for thickness gauging and location of foreign inclusions, hollow spaces and cracks, etc. inside concrete, reinforced concrete, rocks, plastics and like materials using one side access. The operation of the A1220 is based on the US pulse-echo method. The results of examination can be presented on the internal-in screen like A-scan - traditional linear presentation of echo-signal, or B-scan (in some countries it is called C-scan) presenting the image of the cross-section. Both types of scans are shown in figure 9. When the A1220 is used in combination with the external PC the collected data may be presented like 3D reconstruction image of internal object structure.

A-scan	B-scan
Fig 9: View of presentation of A-scan and B-scan on the A1220 screen.

The instrument provides inspection at the depth up to 1 m. At the depth of some 500 mm it is possible to detect hollow channels with diameter of 12 mm.

In the figures 10 and 11 the pictures of the test block with artificially fabricated defects and the 3D image of this very block are correspondingly presented.

Fig 10: Picture of test block with artificial defects

Fig 11: The 3D reconstructed image of concrete test block with artificial defects.

A1230
Sometimes the practical task imposed on an instrument have very tough requirements. Such requirements imply the use of the limiting values of UT method such as sensitivity and space resolution and others in the case of one side access. The realization of such possibilities is given by the US tomograph A1230 (see fig. 6). The A1230 is more sophisticated than the A1220.

This instrument provides reconstruction of the image of internal structure of reinforced concrete objects and give 3D presentation of the collected data in real time mode. The instrument operation is based on the algorithm of back reconstruction (Synthetic Aperture Focusing Technique SAFT-C) described by the authors [1]. The tomograph comprises antenna array (AA) and block of calculations, as a rule this is a standard portable PC. In comparison with the A1220, the tomograph provides a greater resolution and gives the chance to measure deeper objects and does not have shadow dead zones.

The ways of potential development
At the present stage, at MSIA "Spectrum" the works over further development of areas of application of UT method are carried out, especially in the field of DPAC technology use.

The appointed targets are as follows:

• increasing of instruments sensitivity due to use of specialized data processing algorithms;
• introduction of new methods providing separation of signals from reinforcement and hollow spaces;
• search of ways how to obtain the information about space distribution of strength properties of the concrete;
• improvement of instruments design to reduce there weight and size.

Some aspects of the international cooperation
The development of world and science does not recognize any frontiers. The problem of maintenance and inspection of the concrete objects (like bridges, tunnels, roadways coating, etc.) is of great importance all over the world. The use of new instruments requires the combination of all possible efforts and experience to introduce them in practice in the best manner. The final results and how wide the area of application of new products will be strongly depend on well developed service assistance and worked out and detailed methodological assistance. MSIA "Spectrum" has long and fruitful cooperation with its English partner, company "JME Ltd.". The companies combine their efforts in different areas and the new direction of collaboration is the use of instruments for concrete examination. The "JME Ltd." company has long practice in providing services in NDT examination, including UT methods.

The new instruments UK1401 and A1220 and the way of there introduction in practice are the subject of special attention of the companies.

The "JME Ltd." company has arranged the trials of these new products (UK1401 and A1220) in Denmark, USA, France, in UK and at other facilities abroad. The specialists of the company help to adapt Russian instruments to the requirement of the western market.

Good practical results have been obtained with the help of these two instruments and they will be presented during the report at the Conference.

Conclusions

1. UT methods give a chance to solve the problems of concrete and reinforced concrete inspection.z
2. At present in Russia has developed and produced a variety of US means for concrete inspection, in this instruments the technology of dry acoustic contact (DPAC) is used. This technology does not have analogs in the world practice. It provides inspection of construction structure as in the case of one side access and in through sounding.
3. The work to further improve the existing developments and the development of the methodic recommendations for the use of instruments are ongoing.
4. MSIA "Spectrum" and company "JME Ltd." combined there efforts to realize gained scientific and practical experience of each party to introduce new products at the European market and to other countries of the world.
5. The obtained practical results and reports from customers in Russia and potential customers abroad make this area of activity very promising. The final results will increase the probability of safe use of potentially dangerous technogenic objects from concrete and will decrease the potential threat to life of people living near such objects.

References

1. V.N. Kozlov and others, 'Thickness measurements and flaw detection in concrete using ultrasonic echo-method'. Nondestructive Testing Evaluation, 1997, Vol. 13, pp. 73-84.

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