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Ultrasonic echo and transmission techniques are used in civil engineering on a regular basis. New sensors and data processing techniques have lead to many new applications in the structural investigation as well as quality control. But concrete structures in the nuclear sector have special features and parameters, which pose problems for the methods and instrumentation currently available, e.g. extreme thickness, dense reinforcement, steel liners or special materials. Several innovative ultrasonic techniques have been developed to deal with these issues at least partly in lab experiments and pilot studies. Modern imaging techniques as multi-offset SAFT have been used e. g. to map delaminations. Thick concrete walls have successfully been inspected, partly through a steel liner. Embedded ultrasonic sensors have been designed which will be used in monitoring networks of large concrete structures above and below ground. In addition, sensitive mathematical methods as coda wave interferometry have been successfully evaluated to detect subtle changes in material properties. Examples of measurements and data evaluation are presented.
Static and dynamic pile tests are carried out to determine the load bearing capacity and the quality of reinforced concrete piles. As part of a round robin test to evaluate dynamic load tests, structure integrated fibre optic strain sensors were used to receive more detailed information about the strains along the pile length compared to conventional measurements at the pile head. This paper shows the instrumentation of the pile with extrinsic Fabry-Perot interferometers sensors and fibre Bragg gratings sensors together with the results of the conducted static load test as well as the dynamic load Tests and pile integrity tests.
Two research institutes are currently evaluating dynamic pile load testing methods on bored piles in a sandy environment. A test site has been prepared, which ensures comparable conditions at all pile locations and provides detailed knowledge on soil and other boundary conditions. A detailed site investigation program was performed at a dedicated area on the BAM test site south of Berlin, Germany. Undisturbed soil samples have been collected from boreholes. In addition several CPTs and geophysical surveys have been performed. The site consists mainly of well graded, partly well compacted medium sands. But at depth an inhomogeneous gravel layer was detected in some parts of the site. This has led to a redesign of the test piles. They are now shorter than originally intended to avoid gravel layers at depth and to ensure that the drop weight available for dynamic test is capable to mobilize the required load.
Ground penetrating radar (GPR) and infrared (IR) thermography techniques have been used in many civil engineering applications for the structural visualization and defect detection.
However, validation tests of the methods performance for the defection of defects in the nearsurface region with respect to the defects different material and depth below the surface are lacking. To overcome this, we performed GPR and IR thermography tests where the different material properties, shape and depth of defects were studied on concrete and the evaluation of seismic related damage propagation was assessed on stone masonry walls. The results showed that IR thermography, though being greatly affected by the presence of water in the specimen, outperformed GPR in the detection of defects very close to the surface. However, already at the depth of 3 cm and further up till almost 7.5 cm, the performance of GPR resembles the one of IR thermography for the detection of polystyrene (air) voids. On the plastered masonry walls, IR thermography could detect an air gap resulting from plaster delamination as small as 2 mm.
Moreover, structural cracking resulting from the induced lateral load could be detected at an early stage.
The influence of moisture on the reliability of detection of larger voids in brickwork masonry was investigated using three non-destructive techniques: radar, ultrasonic and complex resistivity (CR). Radar and ultrasonic travel time tomography, as well as CR tomography, were performed over a specific cross section of a specimen containing a large void at a known position to determine the influence of different levels of moisture content in the brickwork on the wave velocities and the CR magnitude. We defined a numerical estimator to quantitatively determine the void detection efficiency from the images obtained when exposing the specimen to moisture. The results showed radar to be the most reliable technique for void detection in both dry and wet masonry, while CR performed much better in detecting larger air voids in wet masonry.
Nuclear Power Plants have been in operation for ca. 50 years. Based on this experience, non-destructive testing tasks specific to thick and highly reinforced nuclear containment structures have materialized. The performance based Service life extension of existing NPPs also needs a measurement based decision to support continuing the service of the concrete part of the installations.
By nature, concrete is a very durable material and any natural Deterioration processes may take a long time to become critical to the structure. The experience of more than 50 years of service limits the testing tasks to a few ones which are not yet solved.
Research in NDT of concrete structures is performed by many research institutions all over the world with different technical and systematic approaches. Results are mainly obtained on laboratory specimens, sometimes additional field studies are reported. This research takes place independently without coordination and as a result, the outcomes mostly lack full comparability.
Software for data analysis has become indispensable and very powerful. This part of testing needs more attention when it comes to evaluate test results. Validation of NDE solutions is becoming a critical part in concrete structure testing.
Validation is by definition the proof that a customer´s requirements in the test are being met by the testing solution. This includes equipment, personnel qualification and data analysis. In the following tables, the research recommendation Validation is used in the sense, that proof of the performance of existing solutions needs to be adressed. In general, a validation methodology for NDE solutions for concrete testing in itself needs to be researched and established.
Comparability of research also needs an accepted and easily accessible reference. From experience, it is almost impossible to manufacture exact copies of test specimens at different locations. Round Robin tests are therefore needed to evaluate the performance of a test.
Data evaluation is generally done using dedicated software, sometimes Hardware dependant and not interchangeable between systems. Software is ever more increasingly becoming more powerful and sophisticated. An in-depth evaluation needs to address the comparability and validation of Software used for data analysis and evaluation. The vision of a unified software pool for NDE investigations would undoubtedly support research tremendously.
Quantitative NDE is mostly recommended to assess the condition of a structure. However, qualitative data can be very useful, especially for processes which change the material properties or deteriorate the structure (e.g. corrosion of reinforcement). The need for reliable baseline data is a key factor for such monitoring tasks.
Ultrasonic methods are valuable tools for quality assessment and structural imaging of concrete. In such applications, accurate and reliable determination of ultrasonic wave velocities is essential, as they are affected by various experimental and environmental factors. In this research coda wave interferometry (CWI) was used to determine the influence of temperature (0 to 50 °C) on ultrasonic wave velocity in concrete samples. A resolution of better than 10-4 was achieved in the measurement of relative velocity changes. Reversibility and repeatability as well as the influence of moisture were taken into account. In addition the influence of sensor (transmitter and receiver) positioning errors on the results of the interferometric algorithm was evaluated as well as the benefit of newly developed, permanently embedded sensors. The presented results can be used to evaluate and refine data from ultrasonic monitoring systems.
Parallel Seismic was introduced about four decades ago for foundation length estimation. As the borehole required by this test is in most cases more expensive than the test itself, the usage is limited so far. In addition the graphical Interpretation technique which is still state of the art tends to overestimate the length and neglects several geometrical Parameters. New Interpretation algorithms have been recently introduced rely on model based travel time curve Utting. They improve the accuracy and extend the maximum ränge (distance pile to borehole) of the method. Borehole or pile inclination as well as soil layering can be considered. If the location is properly chosen, several piles can be tested using a single borehole.
The capabilities and limitations of the methods have been tested using simulations and measurements at the BAM-TTS test site in Horstwalde south of Berlin and several commercial projects involving piles, pile secant walls and sheet piles. To improve the accuracy of the method as well as to provide additional Parameters to the geotechnical engineer it is recommended to use the borehole for Downhole Seismic measurements. Determination of compressional and shear wave velocities can be used to determine the dynamic elastic properties of the subsoil. The approach is evaluated again on the BAM-TTS test site. The affectivity of this kind of combiuation can be further enhanced by using a seismic cone.