Techshore Inspection Services-Kochi-official blog of Techshore Kerala: Techshore Inspection Services : Half Cell Electrical Potential method

Principle

The half-cell electrical potential method is used to measure the potential of an embedded reinforcing bar relative to a reference half-cell placed on the concrete surface. The half-cell usually used is a copper/copper sulphate or silver/silver chloride cell but other combinations are also used. The concrete functions as an electrolyte and the risk of corrosion of the reinforcement near the test location may be related empirically to the measured potential difference. ASTM C876 - 91 gives a Standard method for testing Half-cell potential of uncoated reinforcing steel in concrete.

Equipment

The testing apparatus consists of the following components:

Copper-Copper Sulphate Half-Cell:

The cell consists of a rigid tube made of dielectric material which is non-reactive with copper or copper sulphate, a porous wooden or plastic plug which remains wet by capillary action, and a copper rod which is immersed within the tube in saturated copper sulphate solution. The solution is prepared by dissolving reagent grade copper sulphate crystals in distilled or de ionized water.

The inside diameter of rigid tube should not be less than 25 mm and that of porous tube should not be less than 13 mm. The immersed copper rod should have inner diameter of not less than 6 mm and length 50 mm.

Electrical junction device:

An electrical junction device is used to provide a low electrical resistance liquid path between the concrete surface and the half-cell. It consists of a sponge pre-wetted with a low electrical resistance contact solution. The sponge can be attached to the tip of the half-cell so that it provides electrical continuity between the porous tip and the concrete member.

Electrical contact solution:

Electrical contact solution is used to wet the electrical junction device so that the potential drop through the concrete portion of the circuit is standardized. It is a prepared by thoroughly mixing a 95 ml of wetting agent or a liquid household detergent with 19 L of potable water.

Voltmeter:

A battery operated voltmeter is used with ± 3% end of scale accuracy at the voltage ranges in use. The input impedance should be not less than 10 MW while operating at a full scale of 100 mV.

Electrical lead wires:

The electrical lead wire should be such that its electrical resistance for the required length does not disturb the electrical circuit by more than 0.0001 V.

Procedure

Measurements are made in a grid pattern. The spacing between the measurements is generally chosen such that adjacent readings are less than 150 mV with the minimum spacing so that there is at least 100 mV between readings. An area of high corrosion activity is indicated by the area with greater than150 mV. A direct electrical connection is made to the reinforcing steel using a compression clamp.

To get a low electrical resistance connection, the rod must be scraped before connecting it to the reinforcing bar. The reinforcement bar is connected to the positive terminal of the voltmeter. A lead wire is used to connect the half-cell to the negative terminal of the voltmeter. Before the test begins, the concrete surface has to be pre-wetted with a wetting agent. Areas in the member where corrosion activity may be occurring can be represented using an equipotential contour map.

The potential risks of corrosion based on potential difference readings are shown in Table 1 below:

Table 1. Risk of Corrosion against the potential difference readings

Potential difference levels (mv)	Chance of re-bar being corroded
Less than –500	Visible evidence of corrosion
-350 to -500	95%
-200 to -350	50%
More than -200	5%

Sunday, February 5, 2017

Techshore Inspection Services : Half Cell Electrical Potential method

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