The method of half-cell potential measurements normally involves measuring the potential of an embedded reinforcing bar relative to a reference half-cell placed on the concrete surface. The half-cell is usually a copper/copper sulphate or silver/silver chloride cell but other combinations are used. The concrete functions as an electrolyte and the risk of corrosion of the reinforcement in the immediate region of the test location may be related empirically to the measured potential difference. In some circumstances, useful measurements can be obtained between two half-cells on the concrete surface. ASTM C876 - 91 gives a Standard Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete.
3.2. EQUIPMENT FOR HALF-CELL ELECTRICAL POTENTIAL METHOD The testing apparatus consists of the following (Fig. 3.1):
FIG. 3.1. A copper-copper sulphate half-cell.
Half-cell: The cell consists of a rigid tube or container composed of dielectric material that is non-reactive with copper or copper sulphate, a porous wooden or plastic plug that remains wet by capillary action, and a copper rod that is immersed within the tube in a saturated solution of copper sulphate. The solution is prepared using reagent grade copper sulphate dissolved to saturation in a distilled or deionized water.
The rigid tube should have an inside diameter of not less than 25 mm; the diameter of the porous tube should not be less than 13 mm; the diameter of the immersed copper rod should not be less than 6 mm and its length should be at least 50 mm.
Present criteria based on the half-cell reaction of Cu → Cu++ + 2e indicate that the potential of the saturated copper-copper sulphate half-cell as referenced to the hydrogen electrode is -0.316 V at 72oF (22.2oC). The cell has a temperature coefficient of about 0.0005V more negative per oF for the temperature range from 32 to 120oF (0 to 49oC).
Electrical junction device: An electrical junction device is used to provide a low electrical resistance liquid bridge between the surface of the concrete and the half-cell. It consists of a sponge or several sponges pre-wetted with a low electrical resistance contact solution. The sponge can be folded around and attached to the tip of the half-cell so that it provides electrical continuity between the porous plug and the concrete member.
Electrical contact solution: In order to standardize the potential drop through the concrete portion of the circuit, an electrical contact solution is used to wet the electrical junction device. One solution, which is used, is a mixture of 95 mL of wetting agent or a liquid household detergent thoroughly mixed with 19 L of potable water. At temperatures less than 10oC approximately 15% by volume of either isopropyl or denatured alcohol must be added to prevent clouding of the electrical contact solution, since clouding may inhibit penetration of water into the concrete to be tested.
Voltmeter: The voltmeter should be battery operated and have ± 3% end of scale accuracy at the voltage ranges in use. The input impedance should be not less than 10 MW when operated at a full scale of 100 mV. The divisions on the scale used should be such that a potential of 0.02 V or less can be read without interpolation.
Electrical lead wires: The electrical lead wire should be such that its electrical resistance for the length used does not disturb the electrical circuit by more than 0.0001 V.
This has been accomplished by using no more than a total of 150 m of at least AWG No. 24 wire. The wire should be suitably coated with direct burial type of insulation.
3.3. GENERAL PROCEDURE FOR HALF-CELL ELECTRICAL POTENTIAL METHOD Measurements are made in either a grid or random pattern. The spacing between 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 with greater than 150 mV indicates an area of high corrosion activity. A direct electrical connection is made to the reinforcing steel with a compression clamp or by brazing or welding a protruding rod. To get a low electrical resistance connection, the rod should be scraped or brushed before connecting it to the reinforcing bar. It may be necessary to drill into the concrete to expose a reinforcing bar. The bar is connected to the positive terminal of the voltmeter. One end of the lead wire is connected to the half-cell and the other end to the negative terminal of the voltmeter. Under some circumstances the concrete surface has to be pre-wetted with a wetting agent. This is necessary if the half-cell reading fluctuates with time when it is placed in contact with the concrete. If fluctuation occurs either the whole concrete surface is made wet with the wetting agent or only the spots where the half-cell is to be placed. The electrical half-cell potentials are recorded to the nearest 0.01 V correcting for temperature if the temperature is outside the range 22.2 ± 5.5oC.
Measurements can be presented either with a equipotential contour map which provides a graphical delineation of areas in the member where corrosion activity may be occurring or with a cumulative frequency diagram which provides an indication of the magnitude of affected area of the concrete member.
Equipotential contour map: On a suitably scaled plan view of the member the locations of the half-cell potential values are plotted and contours of equal potential drawn through the points of equal or interpolated equal values. The maximum contour interval should be 0.10 V.
An example is shown in Fig. 3.2.
FIG. 3.2. Equipotential contour map.
Cumulative frequency distribution: The distribution of the measured half-cell potentials for the concrete member are plotted on normal probability paper by arranging and consecutively numbering all the half-cell potentials in a ranking from least negative potential to greatest negative potential. The plotting position of each numbered half-cell potential is determined by using the following equation.
1 100
fx plotting position of total observations for the observed value, % r rank of individual half-cell potential,
¦
n total number of observations.The ordinate of the probability paper should be labeled “Half-cell potential (millivolts, CSE)” where CSE is the designation for copper-copper sulphate electrode. The abscissa is labeled “Cumulative frequency (%)”. Two horizontal parallel lines are then drawn intersecting the –200mv and –350mv values on the ordinate across the chart, respectively. After the half-cell potentials are plotted, a line is drawn through the values. The potential risks of corrosion based on potential difference readings are shown in Table 3.1.
TABLE 3.1. 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%
However, half-cell electrode potentials in part reflect the chemistry of the electrode environment and therefore there are factors which can complicate these simple assumptions.
For example, interpretation is complicated when concrete is saturated with water, where the concrete is carbonated at the depth of the reinforcing steel, where the steel is coated and under many other conditions. In those situations an experienced corrosion engineer may be required to interpret the results and additional testing may be required such as analysis for carbonation, metallic coatings and halides. For example, increasing concentrations of chloride can reduce the ferrous ion concentration at a steel anode thus lowering (making more negative) the potential.
3.4. APPLICATIONS OF HALF-CELL ELECTRICAL POTENTIAL TESTING METHOD This technique is most likely to be used for assessment of the durability of reinforced concrete members where reinforcement corrosion is suspected. Reported uses include the location of areas of high reinforcement corrosion risk in marine structures, bridge decks and abutments. Used in conjunction with other tests, it has been found helpful when investigating concrete contaminated by salts.
3.5. RANGE AND LIMITATIONS OF HALF-CELL ELECTRICAL POTENTIAL INSPECTION METHOD
The method has the advantage of being simple with equipment also simple. This allows an almost non-destructive survey to be made to produce isopotential contour maps of the
surface of the concrete member. Zones of varying degrees of corrosion risk may be identified from these maps.
The limitation of the method is that the method cannot indicate the actual corrosion rate.
It may require to drill a small hole to enable electrical contact with the reinforcement in the member under examination, and surface preparation may also be required. It is important to recognize that the use and interpretation of the results obtained from the test require an experienced operator who will be aware of other limitations such as the effect of protective or decorative coatings applied to the concrete.
4. SCHMIDT REBOUND HAMMER TEST