NDT-CE Symposium Berlin 1995: A Concluding Review - Page 4
Author:G. Schickert DGZfP, Berlin (D)
International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE) 26.-28.09.1995
Keywords: Catalogue Raisonne, Cooperation, Compendium, General Report, Research Activities
Contributions on Testing Methods
Ultrasonic Pulse Velocity
The ultrasonic pulse velocity method in generally applied to test the strength of concrete /1199/1207/ in combination with taking cores, and this for thicknesses of concrete of up to 4.5 m. Also tested are the integrity of masonry /401/ or of piles /305 online/ or reinforced concrete posts /1207/. In particular the method was applied to investigate a masonry weir /637/. Air voids, poor compaction and areas of low strength are better detectable by ultrasonic pulse velocity measurements if the results are put together in a grid constellation or in 3D-visualization /895/. The ultrasound waveform has various aspects, such as velocity, attenuation and frequency. These have been analyzed to determine their sensitivity to internal defects including uncompacted and low strength concrete. Iterative inversion methods make it possible to reconstruct three dimensional images of the objects in a sense of a ultrasonic tomography /297/. The hardening process of plaster, cement and concrete can be monitored by the pulse travel time /1181/1215/ and intensity and by analysing the frequency of the ultrasonic pulses /393 online/1145/. As for fresh concrete during construction the strength development of hardening is measured by putting the probes into the fresh concrete at some distance to the formwork /1165/.
|For safety reasons the unreduced operation of ground-anchors of bridges is evident. Ultrasonic measurements /453 online/ gave an answer. This testing method also has been adopted for quality control of basalt aggregates saturated with deleterious zeolites /1169/.|
Fig 4: Ultrasonic diagram of reflector in 5020 mm distance (in 453 online).
|Since the last Berlin NDT-CE Symposium in 1991 especially the application of the ultrasonic pulse-echo method has successfully been improved. The paper /281 online/ gives an overview, compares different details and last but not least proves the benefits of cooperation. If the ultrasonic velocity for a special material like concrete at a distinct date and place is known, the ultrasonic pulse-echo method has great advantages to measure the thickness of concrete structures /261/411 online/1217/. Only single sided access is necessary to determine the thickness or to detect voids and honeycombs /419 online/. At curtain facades the built-in position of anchoring elements could be located /867/. A very interesting technique to determine position and diameter as well of reinforcing bars is the use of the ultrasonic pulse-echo method together with a special processing algorithm /391/1135 online/. New testing probes (source and receiver) and the use of laser detection /281 online/ yield remarkable improvements and do not only permit measurements of thickness of concrete structures but also the determination of their internal condition /1135 online/.||
Fig 7: different stages of imperfect filling (in 419 online).
|Example of 281 online: LSAFT image of the metal duct calculated from a B-scan at specimen 1 (surface A). The B-scan was measured along the x-axis between 1 and K (horizontal axis in this figure, compare figure 1). 0 means point I/K-5/6, negative values show to the direction to the top of the specimen.|
Figure 7: LSAFT image
The acoustic emission technique is sometimes more precisely called the stress wave emission technique since frequencies may exceed by far the acoustic range /805/. The rate of acoustic emission corresponds to the most important deformation stages of concrete or concrete structures /787/, respectively. This was used to quantify the interface properties of fiber reinforced cement-based materials /31/ and to estimate the state of the internal condition of concrete and of with steel or carbon fiber reinforced concrete members and for identification of the fracture or of rebar corrosion sources /597/613/777/. An analysis of the recorded waveforms provides further information /605/. The acoustic emission technique was also used /805/ to evaluate the cracking behaviour of concrete composites .
Electrical and Electro-Chemical Methods
The method of eddy current testing allows to determine fractures in reinforcement /1047/ or to measure the thickness of concrete cover regardless of the diameter of the reinforcing bars and vice versa to determine the bar diameter /197/. Electrical measurements are associated with impedance and dielectric properties of cement pastes and concretes whereby in their turn these values are connected with the danger of corrosion of reinforcements /961/. Potential measurements are applied to investigate the corrosion state of steel reinforcement For a correct interpretation of the measured values, however, it is important to distinguish between actively corroding steel marked by high galvanic current and passive steel bars /393 online/. Potential measuring by means of a half-cell is also known as electrochemical potential noise (EPN) /73/. The electrochemical noise analysis offers a possibility to check the effectiveness of measures to prevent the corrosion of steel /1109/. The effect of concrete moisture on the sensitivity of half-cell-readings is studied in /277/. With respect to determine the rate of corrosion of steel reinforcement in concrete a new transient method with a small galvanostatic pulse perturbation is reported /121/. Measurements of the electrical conductivity allow to estimate the chloride diffusivity of young concrete which is also important for life time prediction of concrete structures /1173/. The electro-chemical impedance spectroscopy is applied for monitoring the cathodic protection of reinforced concrete /135/.
The intefferometry proves to be an efficient contactless testing method which can be applied independently of the surface quality of a material. The gradients of deformations are measured which allow conclusions on defects under the surface and on stress distributions. The testing method can work with a CD camera /245/. The laser speckle interferometry is more sensitive and therefore often applied in fracture mechanics /31/229/. The electronic speckle pattern interferometry (ESPI) /253/ allows to measure all three components of a deformation on the front face of an object. As detector for example an argon laser has been used for a measuring area of about 0.5 m2 /219/. The same technique was applied to find out the mutual behaviour of original natural stones and stone replacement materials in historical buildings /639/. In a similar way the electronic speckle interferometry was applied to investigate the interaction between bricks and mortar joints under compression and to detect strain distribution in masonry /253/. In /237/ the holographic interferometry reveals the successive stages in debonding due to increasing loads between small steel wires and the surrounding cementitious matrix.
The impact-echo method almost proved to be an allround technique /511/. In many cases it is very efficient. Locating size, depth of delaminations, cracks, voids within the material or voids in grouted tendon ducts have been investigated, also bond properties, reinforced masonry and structural wood. /511/.Transient stress waves are used for investigating masonry structures /401/1173/ and for detecting cracks in concrete structures /495/ like bridge decks and bridge beams, airport runways and columns. The impact acoustic method that induces the impact by hammering on the surface of concrete structures has a strong relation to the vibration of the surface. Defects such as cracks and delaminations shift the recorded spectra to the lower frequencies /513/. A further development of the impact-echo method uses a rolling transducer assembly. It can scan rapidly and display the results graphically of flat concrete structures with over 3000 test points per hour compared with rates of only 30 ... 60 points at point-by-point testing /43/. Finally it could be shown that the impact-echo response in laminates of polystyrene bedded concrete and ferrocement is influenced by the laminate configuration and the geometry of the steel wire mesh /503/.
It is quite obvious that magnetic methods relate to reinforcements. To be determined are their integrity and their distance to the material's surface, partly using remote-con
trolled robots /721/745/. Sometimes magnetic testing methods require the magnetization of the object. Often the pulse magnetization proved to be the best technique /1117/. The remanent magnetism method has proved to be suitable to locate fractures of prestressing steel strands /23/ and beyond this to detect areal corrosion /143/. The method works for concrete covers of up to 18 cm /23/211/. Extraordinary is the use of magnetic dust dispersed in the air to investigate patterns of steel reinforcement on bridges /1125/.
A distinction should be made between the microwave reflection method which in e. g. is used to measure the moisture in bulk materials /173/ and the microwave transmission method. The latter exists in two versions, the techniques with one and with two boreholes. Both are suitable for continuous and profile measurements in-situ. The first method mentioned uses a microwave field which is radiated into the wall. Inside the 12 mm borehole the electromagnetic field is scanned /151/. In the second method the microwaves are transmitted in a distinct depth inside one of the two boreholes. Accordingly the measured moisture area is well defined /157/L106/. The super high frequency electromagnetic diagnosis works with still higher frequencies than for microwaves which has been applied to test multi-layer coatings made of different materials /1063/. Microwaves can also monitor the water content of clay during the production of bricks /1005/. At first surprising, however, is the use of microwaves for stress measurements in ceramics and cementitious materials /1041/.
Moist areas in structural and historical buildings or masonry, respectively, are detectable by radar /205/663/, in the same way also voids, the condition of mortar fill, the thickness of a material /487/663/ and the position of reinforcements /663/673/737 online/. Radar also offers a considerable potential for sub-surface application to detect /69/ and to quantify /61/ scour holes in river beds. The ground penetrating radar has been used successfully to evaluate large rigid and flexible pavement areas of bridges, highways and airports /93/467/655/. A vehicle containing the radar system is e. g. driven at 10 km/h on bridge decks in order to identify the concrete condition under the waterproof membrane and the membrane itself /713/. If radar antennas (source and receiver) are moved around an object like a pillar of a cathedral then real tomography images are possible which for example show joints between stones and damaged areas inside /681/. Ground penetrating radar was applied for mapping voids and open spaces inside an old stone masonry weir /637/. Especially to test underground pipes and sewers including their surroundings a ground probing radar technology has been developed. It works with a flexible robot and intelligent sensors. Position, kind and cause of damage are detected as well as their severity /429/. Radar also proves to be useful in-situ to determine the water content of fresh concrete /1033/.
Infrared thermography (IR) is a well-known and approved contactless measuring method which among others can be used to observe time dependent processes like the failure process in wood /919/937/. IR can quickly, efficiently and economically evaluate large pavement areas like highways and airports /467/. The IR images are transformed into plans of bridge decks and show damaged zones as well as those in good condition /713/. Defects and dowels in multilayer building materials or structures can be detected especially in combination with image processing /353/ or numerical simulation /697/. Infrared thermography also helps to investigate chromatic alterations
of ancient bricks in historical buildings /929/ The active thermal method - that means heat is induced - allows to detect subsurface defects (defectoscopy) within different materials /927/. A special form of this kind of testing is the impulse, also called transient thermography which works well to test the bending or adhesion between carbon fiber reinforced plastics and the respective structure /689/ or to determine the built-in position of special anchoring elements /867/.
Natural frequencies, mode shapes and damping are the fundamental parameters of dynamic tests which allow to evaluate the stiffness of structures and by this to find changes which the buildings may undergo through their service life /1007/1153/821/. It is not absolutely necessary to induce an impact wave. Even for big structures or bridges ambient vibration is sufficient /559/. Dynamic tests may also locate crack areas in reinforced concrete beams /327/. A dynamic analysis helps to assess the stability of a TV-tower /877/. In comparison to radar measurements, however, the overall dynamic modal testing of bridges may be more time consuming since radar in addition offers information on geometric features and the composition behind the object /629/.
A laser interferometer was used to monitor - over quite a distance - flaring parts of a structure ranging from pinnacles on the Houses of Parliament in London to cladding panels /977/ Another interferometer with helium-neon laser measured deformations and cracks of reinforced concrete members /1249/ Furthermore lasers are indispensable for high-sensitive laser speckle interferometry /229/. Promising for future in-situ applications is a new high energy laser application with analysis of the chemical elements of the material by spectroscopy. Thus the depth of carbonized concrete zones or salt attack are detectable /529/.
Traditional Testing Methods
It is difficult to decide what is a traditional, i.e. a well known and proven method and what is not. However first and for most it is important to learn new applications and experiences. In this regard sclerometer tests /977/ are combined with pull-out testing /947/ and the break-off test method is reported in connection with newly developed high strength concrete /963 /.
Gamma-radiation is used for moisture measurements. The high resolution of this technique allows to investigate in-situ the moisture diffusion in materials like wood and clay /705/. A portable high energy X-ray betatron opens expanded possibilities of in-situ application /279/. Radiometric gauges are used for density measurements /1227/. Extreme conditions existed for radiographic measurements 100 m under the sea-level /851/
Usually the NDT method of tomography is related to radiation. But also the ultrasonic tomography is entering the field of NDT-CE /297/. The computerized tomography has been used to measure the water content in soils /445/. In order to determine the position of steel reinforcement or prestressing bars in concrete in principle the computer tomography is suitable. This, however, requires a multi angle technique and therefore an unhindered access to the object. These disadvantages are overcome by the concept of "non-linear backprojection" which needs only a small number of views and is applicable for in-situ measurements of concrete bridges or structures /1267/.
Nuclear Magnetic Resonance (NMR)
NMR offers a powerful technique to measure time dependent moisture distributions inside a material. Mostly it is necessary, however, to take samples with a diameter of 2 cm /189/. Beyond this the cement hydration and different molecular binding states of water can be registered including the possibility to evaluate pore size distribution and the permeability in porous materials /181/. A single side access NMR instrumentation can measure moisture profiles up to a maximum depth of 26 mm with a depth resolution of 1 mm /167/.
Time Domain Reflectometry (TDR)
Up to now TDR has mostly been used to determine moisture in bulk materials such as layers of course sand or crushed concrete of road structures /559/. This system is also fit for monitoring tasks and therefore has certainly a remarkable potential of future development.
Cracks in prestressing steel are more or less detectable if vibrations caused by a hammer impact are analysed in a similar manner of common acoustical measurements /621/.
Ultrasonic Resonance Spectroscopy
This method allows to assess the mechanical properties of materials with relatively low acoustic absorption - such as concrete - and to decide whether the sample has any flaws /43/.
Measuring gas permeability is a possible tool to value both, the durability of the concrete cover and to assess concrete performance /939/ The permeability of the surface layer of concrete can be measured by creating a vacuum on the surface /985/.
To determine the thickness of relative thin coatings in principle the heat transmission method is suitable /993/.
Often used are endoscopes. The stereo-photogrammetric technique was applied to determine the deformations of culvert systems due to soil pressure /1307/. The method of optical shapemetry works similar to the principle of the interferometry technique. It uses a slide projector and a CD-camera to get information on the profile or shape of the object /1305/. Fiber optic sensors are reported in /1013/1025/.
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