International Conference --

N.D.T. in Civil Engineering, Liverpool 1993

"An Improved Method for Measuring Reinforcing Bars of Unknown Diameter in Concrete using a Cover Meter"

John C Alldred MA, MSc, Protovale (Oxford) Ltd

Abstract

All commercially-available cover meters are capable of indicating depth of cover to a reinforcing bar of known diameter. If the bar size is unknown, discrepancies between the assumed and actual diameter will give rise to errors in indicated cover which, whilst not necessarily serious, are always undesirable. Various methods have been devised in the past to estimate an unknown bar-size, of which the so-called "spacer method" has been the most frequently used.

This paper describes in detail an improved method, based on ratios of signal strengths rather than differences in indicated covers, which is shown to yield less ambiguous results than the previous method. The accuracy and limitations of the new method, with particular reference to the influence of neighbouring bars, are quantified and compared with previously-achievable results.

Introduction

Electromagnetic covermeters operate by detecting either or both of two of the properties of reinforcing steel-- electrical conductivity, and magnetic permeability. The signal strength received increases with bar diameter (approximately, but not exactly, linearly), and decreases rapidly with distance, ie depth of cover. The relationship between signal strength and cover is not a simple power law, but can sometimes be approximated by an inverse-fourth to inverse-sixth law. Instruments are normally internally-calibrated to directly display the cover to a bar of known diameter, and also sometimes include provision for the indication of original signal strength to allow a user to draw up their own conversion graphs for bar-sizes not included by the manufacturer. Since the signal varies more rapidly with cover than with diameter, a discrepancy between assumed and actual diameter will produce a small error in indicated cover which may or may not be acceptable.

If the bar diameter is unknown, the signal strength received will correspond to a list of possible pairs of values of diameter and cover (with the larger diameters being associated with larger covers, and vice versa), but the "correct" pair will be unknown. To resolve the ambiguities, more information is required, typically at least one further measurement on the same bar but with some difference in the conditions of measurement.

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