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

Structural Faults & Repair, London, July 1995

"Improvements to the Orthogonal Method for Determining Reinforcing Bar Diameter using a Cover Meter"

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

SYNOPSIS

A method for determining the diameter and cover of a bar of unknown size, by taking measurements with the search probe first parallel and then orthogonal to the bar, was first described by Tam in 1977. The actual evaluation of the two unknowns required an iterative graphical method. In this paper it is shown that the ratio of the orthogonal to parallel signal strengths leads directly to a determination of bar diameter which is independent of cover; after this rapid and simple diameter-determining stage, the cover meter is then used in the normal way to indicate the cover to a bar of (now) known size.

As with any attempted measurement, the result is liable to be influenced by the presence of neighbouring bars, and so the effects of both transverse and parallel bars nearby are quantitatively presented; any errors so caused would also be applicable to the original orthogonal method.

This new method can be used with any model of cover meter that has a directional search head and can display signal strength accurately; the (simple) calculation involved could also be carried out automatically, as is the case in the new Protovale CM9 cover meter.

HISTORICAL INTRODUCTION

Over the period of time since cover meters became generally available, there have been numerous attempts to devise methods for deducing the diameter of reinforcing bars. Many have attempted a simultaneous solution for two unknowns (cover and diameter), based on measurements of the same bar under two sets of conditions; and the Orthogonal Detection Technique as first described by Tam is an example of this [Ref.1]. In that method, a measurement of signal strength was first made with the cover meter search head aligned parallel to the bar (the conventional alignment), and a calibration graph consisting of a family of curves of signal strength versus cover used to obtain a list of pairs of possible values of bar size and cover -- this stage is the same as the normal procedure for the determination of cover to a bar of known size. Next, a second measurement of signal strength was made, this time with the axis of the search head aligned perpendicular to the bar (the orthogonal reading). A second calibration graph is required containing a family of curves of orthogonal signal strength versus cover for each bar size. Since the orthogonal signals (apart from being weaker) vary more rapidly with bar diameter than do the parallel signals, it will be found that the intersection of the orthogonal signal strength on the second graph over the range of covers in the first stage will span a narrower range of possible diameters. The first graph is inspected again, this time over this narrower range of diameters, yielding a narrower range of possible covers. This process is repeated, alternating between the two graphs, with the range of possible covers and diameters reducing at each iteration, until (hopefully) a single pair of diameter/cover values is reached.

This technique does not seem to have enjoyed much popularity, possibly because the necessary graphs are not always available, and the procedure appears time-consuming until practice produces familiarity with it; despite the fact that, with patience, the accuracies claimed of ±1 bar size and ±5mm can be achieved.

It is now suggested that this 18-year-old technique could be resurrected, if a simpler procedure for analysing the two readings can be found.

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