| ABSTRACT: |
CONTRIBUTION OF CAPACITANCE PROBES FOR THE NONDESTRUCTIVE INSPECTION
OF EXTERNAL PRESTRESSING DUCTS
J. Iaquinta
French Public Works Research Laboratory, Paris Cedex 15, France
Bridges include external prestressing, either originally or after a reinforcement of structure. The cables are
put in HDPE sheaths, where the residual space is, often imperfectly, injected under high pressure with a
cement grout. Detecting injection defects inside the opaque sheaths is visually impracticable from outside.
Moreover, existing test procedures are generally inappropriate, destructive (endoscope), cumbersome
(gamma-rays) and expensive.
Recently, the need for developing new and efficient diagnosis tools was emphasized by an avalanche of
strands ruptures affecting steel cables in contact with air or water at places not protected by grout.A series of
laboratory experiments showed that the auscultation with capacitance probes is promising, but this approach
is tricky because the system appears as an heterogeneous mixture of conductors and dielectrics. Indeed, the
main difficulty is to provide a correct interpretation of the data in terms of degradation of the coating along
with an occurrence of air or water inclusions, and the feasibility has still to be proved. In order to know if the
presence of the cable itself can disturb the measurements in such proportions that any inspection is destined
to failure, the problem is tackled here from a numerical point of view. Preliminary results indicate that
capacitance is highly sensitive to the location of the strands, but that it remains possible to identify
superficial defects. This conclusion is corroborated by tests performed with the actual probe.
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