NDT-CE Symposium Berlin 1995: A Concluding Review - Page 3
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 Problems
|Cracks, Fracture||Durability||Maintenance or Replacement|
|Internal Condition, Voids and similar Defects||Hardening||Subsurface Detection, Position Finding|
|Water Content||Defects in generel||Thermal Conductivity|
Fig. 6: Symposium contributions to the subject "Testing Problems"
As in the past years concrete still dominates the scene together with distinct measurement techniques It is quite new, however, that the strength of concrete can be determined at single dynamic actions directly on constructions /753/. The gain of strength during the hardening process is recorded by ultrasonic pulse velocity tests /1165/1173/1215/. The loss of concrete strength exposed to high temperatures of up to 800 ºC was tested by sclerometric methods and ultrasonic testing devices /903/. For high strength concrete the break-off method was used /963/. The simultaneous consideration of the rebound hammer and the ultrasonic pulse velocity for in-situ measurements of the compressive strength has for years been well documented. Nevertheless it is undoubtedly the most commonly applied technique for concrete testing. Exchange of experience is therefore always necessary and interesting /977/. An improvement in the accuracy of sclerometer testing may be possibly obtained by combinating it with pull-out tests /947/. As for steel reinforcement its ultimate resistance and yield limit were determined by eddy current and a portable microprocessor instrument /1199/.
Cracks and Fractures
To distinguish between an uncracked and a cracked stage during the service life of a reinforced concrete beam longitudinal vibrations by a hammer impact were analyzed with respect to the damping constant and the decaying velocity. Cracks in bricks /797/ or in concrete members and structures are detected and localized by strain waves /1275/ and independent of the surrounding by transient waves of the impact-echo method /453 online/. The impact acoustic method works in a similar manner. In order to find cracks and delaminations impact acoustics by hammering are induced on the concrete's surface /513/. Flaws in concrete samples or structures are detected by ultrasonic resonance spectroscopy /41/ or by the holographic technique /1249/. As for eddy current as a tool to detect fractures of reinforcements in concrete the influence of concrete cover, test frequency and transducer's geometry was investigated /1047/. Fractures of prestressing steel strands can be located by the remanent magnetism method which is based on the magnetic stray field that is formed at fractures /23/211/.
|Contrary to measurements across structures the echo methods need only access to one face. The ultrasonic pulse-echo-method /261/281 online/1217/ measures for example the thickness of concrete. The impact-echo method allows to determine the thickness of hollow cylindrical concrete structures even if there is a contact with soil or rock /495/. The pavement thickness can be evaluated from a survey vehicle moving at normal driving speed using the ground penetrating radar system /655/. Radar is also effective in measuring the thickness of masonry walls and it provides information on the composition (voids or fill) behind the walls /629/. The thickness of layered systems like soils and pavements can be determined by a seismic method called "spectral-analysis waves " /437/.||
Fig 4: Wall thickness measurement for specimen 1 using the pulse echo method and A-scan evaluation. (in 281 online)
Internal Condition of a Material, Voids and similar Defects
|The ultrasonic impulse-echo technique combined with image processing gives information on the internal condition of concrete /495/. If time and intensity of ultrasonic (velocity) impulses give no sufficient answer on defects like voids or areas of poor concrete compaction ("honeycombs") then an analysis of the frequency spectrum gives more information /393/. With normally good resolution the ultrasonic impuls-echo technique is able to detect voids and honeycombs in concrete /281 online/419 online/ as well as the method of ultrasonic tomography /297/ Voids in post tension ducts may give access to water entry and therefore to corrosion.|
Fig 5: Void detected at 13.4 cm (in 419 online)
Multi-ring electrodes were installed in drilled boreholes in order to estimate the moisture content near the concrete surface and to check the effectiveness of coatings to prevent corrosion of reinforcements /621/. In concrete mixers and in non-metallic moulds the moisture content can be measured by electromagnetic waves (radar) at a frequency of about 1 GHz /1033/. The nuclear magnetic resonance technique (NMR) offers the possibility to investigate the amount of water, the transport of water or to determine the water/cement ratio /181/. For the manufacturing of bricks the water content of the raw material is very important. During production a special microwave moisture meter registers the instantaneous values of water content /1005/. The TDFR (time domain reflectometry) and radiometric methods are suitable for measuring the moisture or water content of bulk materials. An alternative to these methods is the technique of dual-energy computerized tomography that was applied for compacted soil /445/.
The relative permittivity of water is about 10 times higher compared with that of most building materials. This is the physical basis for several moisture measurement techniques, for instance the simple dielectric method /995/ or that of electromagnetic waves with evaluation of phase and amplitude which can also be applied in boreholes /159/. Mostly with higher accuracy overall values of moisture - that means only one value for a total cross section with no differentiation of distinct depths - can be measured by the radar technique /663/. Surely extraordinary considerations are necessary to investigate moisture distributions in walls of buildings under very cold conditions /1003/. Obviously the best resolutions in space and time are produced by the microwave transmission methods /151/157/ and extreme high resolutions are offered by gamma-radiation measurements /705/. For bulk materials the time domain reflectometry (TDR) /575/ and the microwave reflection method /173/ have been used successfully. The NMR-method (nuclear magnetic resonance) works quite differently which is a powerful tool to determine the amount of moisture or a moisture depth profile in porous materials like concrete /167/.
The electrical resistivity of concrete governs to a great extent the corrosion rate of reinforcements. Therefore such measurements allow - in combination with those of the dielectric properties - to differentiate the effect of porosity /961/ from the corrosion influence of moisture /277/. In general corrosion can be recognized by the electrical potentials measured /567/887/ and can be monitored by the AC impedance method /1091/. The corrosion interface responses to a small galvanostatic pulse perturbation and can thus be measured /121/. The penetration of chloride increases the corrosion risk for the reinforcement significantly. A system has been developed to monitor the depth of the critical chloride content /109/. Possible places of corrosion of underground pipelines have been located by measurements of electric fields /1099/ and those of coated reinforcement in concrete by the remanent magnetism method /865/.
Cover-measuring instruments are mostly specified to measure the concrete cover in those cases where the diameter of the reinforcing bar is known. In spite of this in /1283/ a procedure is reported which does not need prior knowledge of the bar size in order to determine the distance to the bar's centre. The same holds true for the eddy current technique /927/.
Durability is an important property in respect to the service life of a structure. As for frost well distributed air voids are essential. Instead of testing the air content of fresh concrete now a new device - the air void analyzer- allows to determine the volume and the size of air voids in hardened concrete /261/ Gas permeability measurements allow to qualify the durability of concrete /939/. A characteristic value is the coefficient of permeability /985/. The electrochemical noise analysis proved to be useful for checking the efficiency of repair materials in respect to corrosion of steel bars /1109/.
In general /511/ or specifically at rock/concrete interface /495/ delaminations can be identified by use of the impact-echo method. Damage detection in cladding panels has been performed by dynamic testing using a laser interferometer /319/. If there are very small delaminations and special materials like carbon fiber reinforced plastics the impulse thermography is a suitable procedure of evidence /689/.
Density measurements are a domain of radiometric or radiographic test methods. A special radiation density gauge, for example, can determine the density of fresh concrete, the consolidation of subsurface layers and so on with immediate evaluation /1227/.
Due to the great expenditure of work far too few measurements of heat transmission have so far been carried out. To determine the thermal conductivity a high speed method with specimens is reported /959/. In-situ measurements, however, for example by a heat flux sensor require time and need a comprehensive understanding of steady state or varying climate conditions and of the way a sensor interacts with its surrounding as well /1019/. A dynamic analysis of in-situ values reduces the duration of the measurements to some days and by using a special Monte-Carlo technique the thermal properties of distinct layers of a wall can be determined /383/. Probably the best way to solve the problems is to determine heat and moisture in place and then to calculate numerically their distribution and movement /365/.
In /437/ the influence of underground obstacles such as utility units, subways or foundations on seismic measurements is discussed. Special problems often require a combination of different testing methods. The presence of a container in a foot of an arch bridge could for instance be detected /859/.
Subsurface Detection, Position Finding
The capabilities of sub-surface radar to map voids and open spaces within a masonry weir /637/ or to detect scour holes around a bridge support /69/ were investigated. The measurements were carried out with nominal antenna frequencies in the 100 MHz to 500 MHz range. Magnetic fields show the position and depth of underground pipelines
/1099/. The seismic tomography is able to visualize the inside of two- or three-dimensional objects from observations collected around their surfaces or between boreholes /1125/.
Maintenance or Replacement
As for large structures the decision between maintenance and replacement are economically of great importance. Infrared thermography and ground penetrating radar when used either separately or in conjunction - are best suited to investigate various pavement conditions of bridges, highways and airports /1127/. Extreme situations are produced after warlike actions. Infrared thermography, dynamic testing, endoscopy and ultrasound measurements must help to decide whether to recover or demolish a building or to reduce the traffic or else /475/911/.
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