Published in Concrete News - 1.1MB
Newsletter from FORCE Technology on Concrete Inspection and Analysis - No. 4 Jan 2002.

About the UPE method Principle The principle of the ultrasonic pulse echo method is transmission and detection of shear horizontal ultrasonic waves. Shear waves emitted from transducers propagate into the concrete and are reflected at interfaces between materials of different acoustic impedance (acoustic imped-ance is the product of density and wave velocity). Thus for a solid concrete member the waves will be reflected at the back (concrete/air interface) and will be registered by receiving transducers at the concrete surface. If the concrete contains defects such as voids or cracks then some or all of the energy in the wave package will be reflected and the depth, position (and size) of the defects can be determined. The depth penetration of the waves is normally between 500 and 1000 mm depending on the quality of the concrete and the maximum size of the aggregates. Application on site Experience has shown that the UPE unit used by FORCE Technology can detect voids of 100 mm in 800 mm thick concrete. It may also be used to detect reinforcing bars and pre-stressing cable ducts. This ultrasonic equipment is unique state-of- the art technology. In areas of good access approximately 1500 tests can be made and analyzed in a working day.

Severe corrosion on the outer reinforcement on a burried concrete tank for storage of fuel oil has caused delamination and spalling off the concrete cover in some parts of the walls. FORCE Technology was asked to establish an overview of the actual damage on the walls, which should form the basis for decision of the most appropriate rehabilitation method and for estimation of the extent of repair. Simple acoustic control by tapping with a hammer was not applicable due to a rather thick concrete cover. Therefore, the ultrasonic pulse echo method (UPE) was chosen for a survey of the delaminations on the walls of the concrete tank.

Fig 1: The photo shows a burried concrete fuel oil tank in the Copenhagen area. Reinforcement corrosion has caused delamination and spalling of the concrete cover.

After excavation it was necessary to remove dirt from the concrete walls by high-pressure water cleaning to ensure a good contact between the concrete surface and the NDT equipment. The delamination survey was then carried out by UPE measurements in a 50 cm x 50 cm grid on all walls.
The A1220 Ultrasonic Flaw Detector from the Russian company »Spectrum« produces so-called B-scans. The figure below shows an example of a B-scan. The large diagram with squares in different shades of grey shows a graphical mapping of the result from UPE measurements along a horisontal line on one of the concrete walls. A single measurement is performed at a frequency of 33 kHz for every 50 cm on the line. The y-axis in the diagram depicts the measurement points and the x-axis depicts the depth below the surface of the concrete wall. The grey shade of each pixel (small square) indicates the intensity of the pulse-echo reflection in the corresponding position in the concrete wall. Higher amplitude echoes appear darker.
It is evident that these B-scans are not very easy to interprete by anyone else but an NDT expert. A more simple presentation of the test results is needed. Therefore, in this case the risk of delamination in the concrete has been evaluated for each measurement point as an integer from 0 and 3 representing the following degree of damage:


Examples of the four different degrees of damage are shown by the green histogrammes to the right in the figure, where the echo intensity vs. depth is shown for four different measurement points (indicated by red lines). The evaluation of the degree of damage was based mainly on the amplitude of the bottom echo (see figure below). The criteria for the evaluation can briefly be summarised as: 0 = distinct bottom echo, 1 = suspect bottom echo, 2 = very weak bottom echo, 3 = no bottom echo. Examples of B-scan (left) and echo intensity histograms (right) from UPE measurements on a horisontal line on the concrete wall. See text for further details.

Fig 3: 2D surface plot of the results from UPE measurements on the west wall. This type of presentation gives a good visual impression of the intensity and distribution of damaged areas on the wall.

A 2D surface plot based on the degree of damage on one of the walls is shown on the figure above. This surface plot shows the risk of delaminations in each grid point by colour codes (degree of damage). The surface plot shows that a total of 43 measurement points out of a total of 200 are characterized as having high or medium risk of delamination. Accordingly, the UPE measurements predicts that approximately 20% of the total concrete surface can be expected to suffer from delamination. During the rehabilitation it was found that the actual delaminated areas on the concrete walls was in good correlation with the predictions from the UPE measurements. Delaminations were found in nearly every area with measurement points marked red, and in some areas with points marked orange. No visible damage could be observed in areas with measurements points marked yellow and green. of presentation gives a good visual impression of the intensity and distribution of damaged areas on the wall. A 2D surface plot based on the degree of damage on one of the walls is shown on the figure above. This surface plot shows the risk of delami-nations in each grid point by colour codes (degree of damage). The surface plot shows that a total of 43 measurement points out of a total of 200 are characterized as having high or medium risk of delami-nation. Accordingly, the UPE measurements predicts that approximately 20% of the total concrete surface can be expected to suffer from de-lamination. During the rehabilitation it was found that the actual delaminated areas on the concrete walls was in good correlation with the predictions from the UPE measurements. Delaminations were found in nearly every area with measurement points marked red, and in some areas with points marked orange. No visible damage could be observed in areas with measurements points marked yellow and green.

For further information, please contact:
Peter Shaw
phone: +45 4326 7208 e-mail: psh@force.dk
Email: psh@force.dk
Internet: www.force.dk
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