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2.4.2 Remote Re-Pressurization Technique
Occasionally the concrete stress cell may be located at some distance from an accessible
surface and would require a pinch tube which is longer than is practical (i.e., over three
meters).
In this case it is possible to use the Geokon remote pinching apparatus as shown in Figure
7.
Figure 7 - Model 4850 with Remote Pinching Apparatus
A short pinch tube is pinched by a hydraulic piston on the end of a hydraulic line leading
to a hydraulic pump.
While the concrete is curing it will be a good idea to take simultaneous readings of
temperature and pressure to develop a temperature correction factor. See Section 4.2 for
information on temperature corrections.
When the concrete has cured and cooled the tube is pinched by applying pressure with the
hydraulic pump. The pinching effect begins at around 4 MPa (600) psi when the pinched
tube begins to crush and continues to about 10 MPa (1450 psi) when the tube is
completely flattened. The maximum burst pressure of the hydraulic tube is 17 MPa (2500
psi).
Connect the stress cell to the Model GK-404 or GK-405, readout box, (channel B), while
pinching. Stop pinching as soon as the pressure in the cell starts to rise rapidly. At this
point the cell is now in good contact with the surrounding concrete.
Generally, the pressure inside the stress cell should be increased until it is equal to about
110% of the estimated concrete stress. A slight relaxation of the cell after the re-
pressurization procedure is normal and should drop the cell pressure to a value roughly
equal to the concrete stress. From this point on, the cell pressure should then be equal to
the absolute concrete stress.
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