There are many techniques used to assess the corrosion risk or activity of steel in concrete. The most commonly used is the half cell potential measurement that determines the risk of corrosion activity. Whilst the half cell potential measurement is effective in locating regions of corrosion activity, it provides no indication of the rate of corrosion. However, a low resistance path between anodic and cathodic sites would normally be associated with a high rate of corrosion than a high resistance path. Such resistivity measurements determine the current levels flowing between anodic and cathodic portions, or the concrete conductivity over the test area, and are usually used in conjunction with the half-cell potential technique. This is an electrolytic process as a consequence of ionic movement in the aqueous pore solution of the concrete matrix. An alternative technique to estimate the rate of corrosion, which is becoming increasingly popular, is the linear polarization resistance.
8.2. EQUIPMENT
Although other commercial devices like the less accurate two probe system are also available, the Wenner four probe technique is generally adopted for resistivity measurement of in situ concrete. The technique was first used by geologists to investigate soil strata. The technique can be used to determine resistivities quickly and with little or no damage to the concrete structures under study, Fig. 8.1.
FIG. 8.1. Schematic of Wenner 4 probe resistivity meter.
The equipment consists of four electrodes (two outer current probes and two inner voltage probes) which are placed in a straight line on or just below the concrete surface at equal spacings. A low frequency alternating electrical current is passed between the two outer electrodes whilst the voltage drop between the inner electrodes is measured. The apparent resistivity (ρ) in “ohm-cm” may be expressed as:
ρ = 2πaV/I (12)
where
V is voltage drop, I is applied current, a is electrode spacing.
The calculation assumes the concrete to be homogeneous and the inhomogeneity caused by the reinforcement network must be allowed for by properly placing the probes to minimize its effect.
8.3. GENERAL PROCEDURE
Resistivity measurement is a fast, simple and cheap in situ non-destructive method to obtain information related to the corrosion hazard of embedded reinforcement.
The spacing of the four probes determines the regions of concrete being measured. It is generally accepted that for practical purposes, the depth of the concrete zone affecting the measurement will be equal to the electrode spacing. If the spacing is too small, the presence or absence of individual aggregate particles, usually having a very high resistivity, will lead to a high degree of scatter in the measurement. Using a larger spacing may lead to inaccuracies due to the current field being constricted by the edges of the structure being studied. In addition, increased error can also be caused by the influence of the embedded steel when larger spacings are employed. A spacing of 50 mm is commonly adopted, gives a very small degree of scatter and allows concrete sections in excess of 200 mm thick to be measured with acceptable accuracy.
The efficiency of surface coupling is also important. In order to establish satisfactory electrical contact between the probes and the concrete, limited damage to the concrete surface sometimes can not be avoided. In some commercial devices, wetting or conductive gel is applied when the probes are pushed against the concrete surface to get better contact.
Prewetting of the surface before measurement is also advised. Small shallow holes may also be drilled into the concrete which are filled with a conductive gel. The probes are then dipped into each hole. However, this procedure is not practical for site use.
8.4. APPLICATIONS
The ability of corrosion currents to flow through the concrete can be assessed in terms of the electrolytic resistivity of the material. This resistivity can determine the rate of corrosion once reinforcement is no longer passive. The presence of ions such as chloride will also have an effect. At high resistivity, the rate of corrosion can be very low even if the steel is not passive. For example, reinforcement in carbonated concrete in an internal environment may not cause cracking or spalling due to the very low corrosion currents flowing.
The electrical resistivity of concrete is known to be influenced by many factors including moisture, salt content, temperature, water/cement ratio and mix proportions. In particular, the variations of moisture condition have a major influence on in situ test readings.
Fortunately, in practice, the moisture content of external concrete does not vary sufficiently to significantly affect the results. Nevertheless, precautions need to be taken when comparing results of saturated concrete, e.g. those exposed to sea water or measurements taken after rain showers, with those obtained on protected concrete surfaces. Another important influence is the ambient temperature. Concrete has electrolytic properties; hence, resistivity will increase as temperature decreases. This is particularly critical when measurements are taken during the different seasons, with markedly higher readings during the winter period than the summer period.
The principle application of this measurement is for the assessment of the corrosion rate and it is used in conjunction with other corrosion tests such as the half-cell potential measurement or linear polarization measurement methods. There are no generally accepted rules relating resistivity to corrosion rate. However, a commonly used guide has been suggested for the interpretation of measurements of the likelihood of significant corrosion for non-saturated concrete where the steel is activated, see Table 8.1.
TABLE 8.1. GUIDE FOR THE INTERPRETATION OF THE MEASUREMENTS DURING CORROSION ASSESSMENT
Resistivity (ohm cm) Likely Corrosion Rate Less than 5,000 Very high
5,000 – 10,000 High
10,000 – 20,000 Low / Moderate Greater than 20,000 Negligible
In practice, it is necessary to calibrate the technique, either through exposing the steel to assess its condition, or by correlating the resistivity values with data collected with other techniques. For instance, the values given in Table 8.1 apply when the half-cell potential measurement shows that corrosion is possible.
9. ELECTROMAGNETIC METHODS OF TESTING CONCRETE