Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1990-05-21
1991-12-03
Weisstuch, Aaron
Chemistry: electrical and wave energy
Apparatus
Electrolytic
20415311, G01N 1704
Patent
active
050697749
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a surface mounting corrosion probe for use in monitoring the rate of corrosion of steel reinforcement embedded in concrete.
In normal circumstances steel embedded in concrete corrodes at a very low rate. This is due to the high alkalinity of the concrete environment which causes the surface of the steel to remain in an electrochemically passive state. A number of external factors may lead however to the ingress of aggressive ions, such as chloride ions, into the concrete. Degradation of the concrete may also occur as a result of the action of carbon dioxide and water present in the atmosphere. These factors can lead to an increase in the rate of corrosion of the reinforcing steel, endangering both the integrity and strength of the reinforced concrete structure. Corrosion of the steel reinforcemnt can lead to an accumulation of corrosion products which, being more voluminous than the original steel, exert pressure on the concrete covering the reinforcement, eventually leading to cracking and spalling. Loss of metal can also lead to localised thinning and weakening of the reinforcing steel, again weakening the concrete structure, necessitating expensive repairs or shortening the life of the structure.
It is desirable therefore to be able to determine non-destructively the rate of corrosion of steel reinforcement in any part of a given suspect structure, and various techniques have been developed to do this.
Electrochemical techniques have been applied to the assessment of the corrosion of reinforcing steel and rely on the measurement of the natural electrochemical potential of the reinforcing elements. Typically, a reference electrode is placed on the concrete surface of the structure and connected to the reinforcement via a voltmeter. A technique known as `potential mapping` or `half cell measurement` uses this approach, measuring the natural corrosion potential values over a grid of locations drawn on the surface of the structure. Subsequent analysis of the data then uses an empirical criterion to determine which parts of the embedded reinforcement are at risk from corrosion damage. This procedure is widely used and is subject to an ASTM specification (C876-80). The technique does not provide information as to the rate of corrosion attack, indicating only the likelihood of corrosion occurring.
Electrochemical techniques of `linear polarisation resistance measurement` and `a.c. impedance` have also been used in the past. These techniques measure the response of the electrochemical processes occurring naturally on the steel/concrete interface to an external perturbation. The measurement yields a value of a `polarisation resistance`, which can be shown to be inversely related to the rate of corrosion.
There are several problems associated with these techniques. The first is that a secondary (counter) electrode is required to supply the necessary perturbing current to achieve the measurement. A second problem arises from an uncertainty in the size of the area of steel being polarised during the measurement. Additionally the concrete itself is often highly electrically resistive, leading both to difficulties in the application of the external polarisation and to errors in the measurement of the exact value of the `polarisation resistance` of the interface as the measurement effectively determines the series sum of the concrete and interfacial resistances.
Measurement using existing procedures typically involves the insertion of counter electrode/reference electrode assemblies into the concrete structure at a number of locations, covering these assemblies with fresh concrete or mortar compound and using the above electrochemical techniques to obtain an estimate of the corrosion rate of either the steel reinforcement in the vicinity of the inserted probe or of the probe elements themselves. Such an approach involves considerable and expensive effort in coring the concrete, placing the probes, and making good the concrete surface. Furthermore, the location of the probes is fixed an
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Hladky Karel
John David G.
University of Manchester Institute of Science & Technology
Weisstuch Aaron
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