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Determining the corrosion activity in steel reinforcement using Half Cell Potential testing.

      1) Copper-copper sulphate half-cell
      2) Connecting Wires
      3) High Impedance Voltmeter

CANIN corrosion analyzer is based on electro-chemical process to detect corrosion in the reinforcement bars of structure. It represents a galvanic element similar to a battery ,producing an electrical current, measurable as an electric field on the surface of concrete. The potential field can be measured with an  electrode known as half cell. The electrical activity of the steel reinforcement and concrete leads them to be considered as one half of battery cell with the steel acting as one electrode and concrete as electrolyte. The name half cell surveying derives from the fact that the one half of the battery cell is considered to be the steel reinforcing bars and surrounding concrete. The electrical potential of a point on the surface of steel reinforcing bar can be measured comparing its potential that of copper – copper sulphate reference electrode/silver- silver nitrate reference electrode on the surface  

The positive terminal of the voltmeter is attached to the reinforcement and the negative terminal is attached to the copper-copper sulphate half cell. If there is any corrosion in the bars, the excess electrons in the bar would tend to flow from the bar to the half cell. Because of the way the terminals of the voltmeter are connected in the electrical circuit, the voltmeter indicates a negative voltage. The measured half cell potential is the open circuit potential, because it is measured under the condition of no current in the measuring circuit. A more negative voltage reading at the surface is to interpreted to mean that the embedded bar has more excess electrons, and there is, therefore, a higher likelihood that the bar is corroding.  

The half cell potential readings are indicative of the probability of corrosion activity of the reinforcing bars located beneath the copper-copper sulphate reference cell. However, this is true only if the reinforcing steel is electrically connected to the bar attached to the voltmeter.


When there is active corrosion, current flow through the concrete between anodic and cathodic sites is accompanied by an electric potential field surrounding the corroding bar. The equipotential lines intersect the surface of the concrete and the potential at any point can be measured using the half potential method.  

The apparatus includes copper-copper sulphate half-cell, connecting wires and a high impedance voltmeter. This half-cell is composed of a copper bar immersed in a saturated copper sulphate solution. It is one of the many half cells that can be used as a reference to measure the electrical potential of embedded bars. A high impedance voltmeter (normallygreater than 10MW) is used so that there is very little current through the circuit. The coppercopper sulphate half-cell makes electrical contact with the concrete by means of porous plug and a sponge that is moistened with a wetting solution

One of the instrument available in the market is CANIN corrosion analyzer, which is computer based device for making half cell potential measurement. This particular instrument stores data acquired at different test points and display equipotential contours.

The half cell potential readings are indicative of the probability of corrosion activity of the reinforcing bars located beneath the copper-copper sulphate reference cell

If stability cannot be achieved by pre-wetting, it may be because of stray electrical currents or excessive electrical resistance in the circuit. In either case, the half cell potential method should not be used.
   2)    For conducting this test access to the reinforcement is must.
   3)   The method cannot be applied to epoxy coated reinforcement or concrete with coated surfaces .

  1) Improper grid spacing : Excessive spacing can miss points of activity or provide insufficient data closer spacing increase the cost of survey
  2) Moisture content : concrete is sufficiently moist to complete the circuit necessary for a valid measurement free surface water between test points at the time of potential measurement.


As per ASTM C 876, two techniques can be used to evaluate the results=>  
the numeric technique : In the numeric technique, the value of the potential is used as an indicator of the likelihood of the corrosion activity. The potential measured at the surface of concrete can be interpreted as per table given below:- 

Phase of Corrosion Activity
Potential as measured by Copper Half Cell
1. Initial Phase – Corrosion activity not taking place
< - 200 mV
2.Transient Phase – Corrosion activity uncertain
- 200 mV to - 350 mV
3. Final phase – corrosion occurring positively
> - 350 mV

The numeric method should not be used in the following conditions –
(a) Carbonation extends to the level of reinforcement
(b) Comparison of corrosion activity in outdoor concrete with highly variable moisture or oxygen content.
(c) Evaluation of indoor concrete that has not been subjected to frequent wetting.
(d) To formulate conclusions about changes in corrosion activity due to repairs which changed the moisture or oxygen content at the level of the steel.
(ii) the potential difference technique: In the potential difference technique, the areas of active corrosion are identified on the basis of the potential gradients. In the equipotential contour plot, the closer spacing of the voltage contour indicates regions of high gradients. The higher gradient indicates, higher risk of corrosion. The potential difference technique is considered more reliable for identifying regions of active corrosion than is the use of numerical limits.



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