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Reinforcement Electropotential (Half-cell Survey)

Introduction


The incident level of problems caused by corrosion of steel reinforcement in concrete has led to the need for a test which can effectively identify those areas where corrosion is active without exposure of the steel.

Corrosion of steel reinforcement in concrete is normally prevented by the alkaline nature of the concrete which imparts chemical passivity to the steel. If the concrete is carbonated however, or if chloride ions are present at the steel surface, the corrosion reaction can commence providing a sufficient supply of oxygen and moisture are both present.

Depassivation of steel by either or both of the above mechanisms will promote anodic and cathodic activity at sites throughout the reinforcement. Metal is dissolved at the anodes where the oxidation of metal ions takes place and metal oxide corrosion products are subsequently formed. Simultaneously electrons are released and these are conducted through the metallic reinforcement (flow of electrical current) to a cathodic site where a reduction process occurs. The reaction typically involves the electrons, water and oxygen which together produce hydroxyl ions within the pore solution of the concrete.

Associated with the flow of current is a variation in electrical potential along the length of the corroding reinforcement, which may be detected with a suitable half-cell electrode.

Using the half-cell in conjunction with a suitable voltmeter the measurement of corrosion potential is possible.

For steel in concrete, anodic regions normally correspond to more negative potentials whilst cathodic regions correspond to less negative potentials.

Work carried out by the California Division of Highways, and subsequent research at the Transport & Road Research Laboratory at Crowthorne have demonstrated that the electrical potential of steel reinforcementwhen referred to a standard 'half-cell' is dependent upon the corrosion activity of the steel. Of the two types of cell covered in research work, saturated calomel and saturated copper/copper sulphate, the copper cell has generally been adopted for site work in the United Kingdom.

The purpose of the 'half cell' test is to measure the electrical corrosion potential. An assessment of the probable extent of corrosion activity can be made from the results obtained. The test is described in ASTM C876-91 and reviewed in a number of publications.

Equipment and test method


The availability of proprietary equipment is at present limited and Technotrade Structural Services Ltd. has therefore developed apparatus for the work

The essential details are a transparent tube full of saturated copper sulphate solution with a copper electrode immersed in the solution. This electrode passes through a seal at one end of the cell and is then connected to a portable computer by a flexible wire. The other end of the cell is closed with a porous disk which is in contact with a damp sponge to provide the electrical contact to the concrete surface. After locating a suitable area of reinforcement near the surface, a short length is exposed and an electrical connection made using a self-tapping screw. An insulating washer ensures that the connection is between the screw and bright steel. The lead from this connection is attached to the positive terminal of the computer. The negative terminal is then connected to the halfcell. By convention, potentials are considered negative when measuring the steel with respect to the copper. Present criteria indicate that the potential of the copper sulphate 'half cell' as referred to a hydrogen electrode is -316mV at 72°F (22°C) and the temperature correction is -0.9mV per °C.

Measurements are taken by pressing the sponge against the surface of the concrete and noting the meter reading. Two readings are taken at each location to provide protection against rogue results.In order to ensure that sufficient electrolyte is present at the surface all locations are prewetted using a fine spray of weak detergent mixture. A check of electrical continuity is made by repeating a small number of readings with the half cell connected at two different points.

Results


Experience by others in the USA has shown that for potentials more negative than -350mV there is a 90% probability that corrosion is active. If the potential is less negative than -200mV there is a 90% probability that corrosion is not active. Between these values it is not considered possible to predict the level of corrosion activity with any confidence, but our experience in the UK has been that, while these figures are a useful guideline, there has been a significant correlation between increasing (negative) potential in the -200 to -350mV zone and the occurrence of reinforcement corrosion, i.e. the -350mV value may perhaps be considered to represent the presence of significant reinforcement corrosion although it is probable that the onset of corrosion occurs at rather lower (more positive) potentials. Recent work for the American Federal Highways Administration suggests that corrosion of steel commences at a potential in the region of -240mV. In every case, it is wise to expose a representative number of areas of steel to provide a correlation with the half cell survey values. For the purpose of this survey the test results have been presented in tabular form as recorded on site and as an equipotential contour map which provides a graphical indication of areas where corrosion is most likely.

References


ASTM C876-91 "Standard test method for ‘half cell’ potentials of reinforcing steel in concrete."

Vassie P. R. "Evaluation of techniques for investigating the corrosion of steel in concrete." TRRL Report SR 397 Crowthorne, 1978. (Transport and Road Research Laboratory).

Stratfull R. F. "Half cell potentials and the corrosion of steel in concrete." Highway Research Record 1973, 433, 12-21.

Pfeifer D. W., Landgren J. R. et al. "Protective Systems for New Prestressed and Substructure Concrete." Report No. FHWA/RD-86/193. Federal Highways Administration, USA. April 1987.

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