International Congress

Concrete in the Service of Mankind University of Dundee, 1996

Development of cover meters for stainless reinforcement -- two successful approaches

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

Abstract

Although stainless steel, by being inherently non-corroding, is more tolerant of a reduction in the concrete cover whose alkaline environment is otherwise required to inhibit corrosion of high-tensile steel reinforcement, quality control procedures will still require the position and location of the reinforcement to be non-destructively determined.

Reinforcing bar locators and cover meters have been available for forty years, but the original design - and the majority of subsequent instruments - all rely on the magnetic properties of the steel, and so are incapable of even registering the presence of non-magnetic stainless steel. Some of the more recent cover meter designs respond either partially or predominantly to the electrical conductivity of the steel, and so might appear more suitable; but the exceptionally low conductivity of stainless steel yields a drastically reduced signal even to these instruments, so that the detection range is severely limited and cover indications are meaningless. It was therefore felt necessary to develop portable site instrumentation which could reliably locate stainless steel reinforcement and indicate its depth beneath the concrete surface.

In fact, two development projects were undertaken. The first started from a modest performance specification, and was successfully implemented by combining and improving the technologies used in two existing instruments: the "Imp Stainless-steel Wall-tie Locator" and the "Rebar Plus" rebar locator and cover indicator. The second project specified a full-function cover meter, which includes an automatic and instantaneous determination of bar diameter and steel grade which is necessary because the signal strength received from a bar - and hence the deduced cover - varies more rapidly with bar size than is the case with high-tensile steel.

Both instruments provide a clear audible signal as well as visual indication, as experience shows that this is vital to achieve rapid concrete testing. Both are also suitable for measuring high-tensile bars (again, a practical necessity); and the second instrument also provides automatic and instantaneous estimation of high-tensile bar diameter.

This paper describes the development of both instruments, and discusses the performance achieved under site conditions, with particular reference to the ability to resolve and measure complex structures of reinforcement.

Return to Home Page

Back to Technical Library: Papers
[Reply]

Information request