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Non-Destructive Testing in Civil Engineering, Berlin 1995

Determination of Reinforcing Bar Diameter and Cover by Analysing Traverse Profiles from a Cover Meter

Abstract

This paper investigates the analysis of the width of the response of the instrument as the search probe is scanned across the bar. Previously, it had been assumed that the variation of traverse profile width with bar diameter was insufficiently distinct to allow a meaningful interpretation; but in this paper it is shown that the traverse width is a simple function of the effective width of the search probe and the distance from the surface of the concrete to the centre of the bar, but independent of bar diameter. As a result, by using the cover meter initially to indicate just signal strength, the distance to the bar's centre can be determined directly without the need for prior knowledge of the bar size. By next using the instrument as a conventional cover meter, the bar diameter can be deduced as being that value at which the indicated cover plus half the diameter equals the previously-determined distance to centre.

As with previous methods employing such a two-stage process, a high level of confidence can be placed on the value obtained for the cover to a bar of unknown size, but the accuracy of the deduced diameter would be expected to be dependent on disturbing influences such as the proximity of neighbouring bars within the structure. However quantitative experiments have shown that it is possible to compensate for the signals received from neighbouring bars - both parallel and transverse to the bar being measured - thereby enabling reliable results to be obtained even in significantly congested situations.

The technique can use any commercially-available cover meter that is capable of indicating signal strength. Due to its simplicity, the technique can easily be applied manually on-site; by using a graph to convert traverse width to distance to centre, or by using a specially-prepared "ruler" scale to measure the traverse width, no calculations are required. This technique also lends itself to applications where continuous traverses are being recorded on a data-logger for subsequent post-processing; and also to similar applications using remote-controlled robots as being constructed to survey difficult-access situations, when there is usually sufficient computer processing bandwidth available to allow real-time analysis of the cover meter signals.

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