Half cell potential testing

Steel embedded in good-quality concrete is protected by the high alkalinity pore water which, in the presence of oxygen, passivates the steel. The loss of alkalinity due to carbonation of the concrete or the penetration of chloride ions (arising from either marine or de-icing salts, or in some cases present

Figure 1.31 IR photograph (left) of delaminated area of the silo in Figure 1.30. Right is a normal photograph.

in situ from the use of a calcium chloride additive) can destroy the passive film. In the presence of oxygen and humidity in the concrete, corrosion of the steel starts. A characteristic feature for the corrosion of steel in concrete is the development of macrocells – the co-existence of passive and corroding areas on the same reinforcing bar with the corroding area as the anode and the passive surface as the cathode. The voltage of such a cell can reach as high as 0.5 V or more, especially where chloride ions are present. The resulting current flow (which is directly proportional to the mass lost by the steel) is determined by the electrical resistance of the concrete and the anodic and cathodic reaction resistance.

The current flow in the concrete is accompanied by an electrical field which can be measured at the concrete surface, resulting in equipotential lines that allow the location of the most corroding zones at the most negative values. This is the basis of potential mapping, the principal electrochemical technique applied to the routine inspection of reinforced concrete structures (Broomfield, 2007).

The use of the technique is described in an American Standard, ASTM C876-09, Standard Test Method for Half Cell Potentials of Reinforcing Steel in Concrete (ASTM, 2009). The standard has changed considerably from its earlier incarnation in 1980 and now contains numerous caveats and more guidance on factors influencing half cell potential values.

In use, a reference connection is made by exposing a reinforcing bar and connecting by brazing or by drilling a small hole and inserting a self-tapping screw into the bar (CAUTION – this method is not suitable for prestressed

372.0

or post-tensioned steel reinforcement, where only a mechanical connection such as a crocodile clip can be permitted). The reference connection is connected to the positive terminal of a high impedance digital millivoltmeter. A reference cell such as a copper/copper sulfate half cell is attached to the negative terminal of the meter. This cell has a porous plug at one end, which permits an electrical connection. A grid is marked on the surface of the concrete at suitable intervals, usually between 0.5 to 1 m spacings in a square grid pattern. The surface of the concrete to be tested is wetted with a wetting agent at each node to be measured on the grid intersections. The meter is touched to the concrete surface and the millivoltmeter reading noted. The cell is then removed and touched again and a second reading taken. The readings should be within 10 mV of each other. Caution should be exercised if readings drift – stable values should be obtained.

Factors affecting the potential field

When surface potentials are taken, they are measured remote from the reinforcement due to the concrete cover. The potentials measured are therefore affected by the ohmic potential drop in the concrete. Several factors have a significant effect on the potentials measured:

CONCRETECOVERDEPTH

With increasing concrete cover, the potential values at the concrete surface may indicate an average of local active and passive steel, making the location of corroding anodic areas more difficult.

CONCRETERESISTIVITY

The concrete humidity and the presence of ions in the pore solution affect the electrical resistivity of the concrete. The resistivity may change both across the structure and with time as the local moisture and salt content vary. This may create an error of ±50 mV in the measured potentials (John et al., 1987).

HIGHLYRESISTIVESURFACELAYERS

The macrocell currents tend to avoid highly resistive concrete. The measured potentials at the surface become more positive and corroding areas may be undetected.

POLARISATIONEFFECTS

Steel in concrete structures immersed in water or buried in the earth often have a very negative potential due to restricted oxygen access (Popovics et al., 1983). In the transition region of the structure (splash zone or above ground), negative potentials can be measured due to galvanic coupling with immersed rebars. These negative potentials are not related to corrosion of the

reinforcement. Corrosion rate measurements can be useful in assessing the significance of these high potential values, as can resistivity measurements.

Results and interpretation

According to the ASTM method, corrosion can only be identified with 90% certainty at potentials more negative than –350 mV. Experience has shown, however, that passive structures tend to show values more positive than –200 mV and often positive potentials. Potentials more negative than –200 mV may be an indicator of the onset of corrosion. The patterns formed by the contours can often be a better guide in these cases.

In any case, the technique should never be used in isolation, but should be coupled with measurement of the chloride content of the concrete and its variation with depth and also the cover to the steel and the depth of carbonation. The ASTM document also suggests a potential difference technique where differences in potential are plotted over relatively small areas to locate corroding steel. This can be helpful in dry weather, for example, when corrosion cells become less active and may not always follow the ASTM guideline values. However, peaks of potential at corroding areas may well still exist. It has to be remembered, when evaluating results, that

the technique measures what is happening on the day. _{Considerable variation}

in numeric values can be encountered over different seasons and weather conditions.

Figure 1.33 shows a survey of a car park deck slab using a 1 m grid. The corroding areas can be readily identified.

1.9.2 Resistivity

The electrical resistivity is an indication of the amount of moisture in the pores, and the size and tortuosity of the pore system. Resistivity is strongly affected by concrete quality, i.e. cement content, water/cement ratio, curing and additives used.

Equipment and use

The main device in use is the four-probe resistivity meter. These have been modified from soils applications and are used by pushing pins directly onto the concrete with moisture or gels to enhance the electrical contact. Millard et al. (1991) described two versions of the equipment. Some variations use drilled-in probes or a simpler, less accurate two-probe system. Further information can be found in Concrete Society Technical Report No 60 (Concrete Society, 2004).

Resistivity = 2 ʌ a V/I (Ÿcm)

where: R is the resistance by the ‘IR drop’ from a pulse between a surface

a is the electrode spacing

I is the current passed between outer probes (amps)

V is the potential measured between inner probes (volts).

Interpretation

Interpretation is empirical. The following interpretations of resistivity measurements have been cited when referring to depassivated steel:

> 20 kŸcm low corrosion rate

10–20 kŸcm low to moderate corrosion rate 5–10 kŸcm high corrosion rate

< 5 kŸcm very high corrosion rate.

Limitations

The resistivity measurement is a useful additional measurement to aid in identifying problem areas or confirming concerns about poor-quality concrete. Readings can only be considered when used with other measurements, such as chloride content, depth of carbonation, cover to reinforcement and half cell potential.

It should be understood that the resistivity does not tell you how fast the corrosion is or how much corrosion there is, it simply indicates the capacity of

the concrete to support a corrosion cell if one exists. For example, a concrete

showing a low half cell potential but also a low resistivity is not actively corroding, but could do if the conditions for corrosion existed. Conversely, a concrete showing a high half cell potential and a high resistivity is unlikely to show a significant rate of corrosion.

There are devices which can actually measure corrosion rate directly (Broomfield, 2007) such as the Gecor 8, although these have seen only limited use in the field.

A F K P U Z AE AJ _AO AT AY BD BI BN 32 28 24 20 16 12 8 4 -mV

Half-Cell Contour Plot St. John's Level 4 Extension

350-400 300-350 250-300 200-250 150-200 100-150 50-100 0-50

1.10 Summary

Reinforced concrete can suffer from a significant number of defects, although good-quality, properly designed, well-mixed and placed concrete is normally durable for many years. It is critical, before approaching the repair of a structure, to understand what has caused the problem and to design a repair appropriate to that problem. Failure to do so can often result in a very short-lived repair. This chapter has attempted to introduce something of the range of different types of defect that can occur and some of the non- destructive and semi-destructive methods available to determine the causes of such problems.

People engaging in concrete repair should beware of attempting to identify the cause of a problem just from its appearance. Laboratory testing and in particular petrographic examination are almost always essential as part of the diagnostic process.

Once the problem is understood, an appropriate repair system can be designed.

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