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"Extending the Life of Bridges, Civil & Building Structures"

Structural Faults & Repair,
Edinburgh, July 1997

"Borehole Probes for Locating
Tendon Ducts in Concrete"

John C Alldred MA, MSc, Protovale (Oxford) Ltd,
and Jon Basten-Pitt BSc, GBG, Cambridge

ABSTRACT

Rebar Locators and cover meters are suitable for locating the first layer of reinforcing steel in concrete, and can usually also locate the second layer of reinforcement. Unfortunately these instruments are not usually successful at locating a third layer of reinforcement or post-tensioning tendons below them; the weaker signal from steel deeper within the structure is masked by the much stronger signal from the steel nearer the surface.

The solution that has been investigated is to construct a slim probe that can be inserted into a borehole drilled into the concrete at a position that avoids and passes the first two layers of reinforcement. The probe is then connected to the electronics of an existing rebar locator or cover meter instead of its usual (surface) probe. By tailoring the polar response of the borehole probe to "look" sideways, the probe can be rotated within the hole to identify the nearest metal at that depth.

The first prototype probe constructed by GBG (formerly GB Geotechnics) used this sideways-looking technique, and could be used either with a Protovale Imp or more appropriately with a Protovale Rebar Plus rebar locator as the host electronics. Subsequent prototypes constructed by Protovale have added a forward-looking sensor, and are intended for use with either the Rebar Plus or a CoverMaster cover meter to yield more informative readings.

These probes have been used on a number of site investigations, and although as a result of being constrained to their borehole they can not enable the user to "image" the deeper reinforcement with the same ease that is possible with a probe which can be moved freely over the surface, they have nevertheless succeeded in providing sufficient additional information to allow post-tensioning tendon ducts to be reliably located.

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