Methods of Corrosion Potential Assessment of Concrete Structures
1. Cover meter survey
The necessity to provide adequate cover thickness to control corrosion needs no emphasis. A cover thickness survey is useful to determine existing cover thickness in a specified location, where a damage has been identified and elsewhere, for comparison on the same structure.
The cover thickness can be measured non-destructively using commercially known cover meters. The cover meters are also used to identify the location and diameter of rebar: COVERMASTER and PROFOMETER are commercially available instruments, which are used to measure the cover thickness and rebar size.
Table 1 shows how the cover reading are to be interpreted for corrosion assessment.
Fig: Cover meter of Profometer
Table -1: Interpretation of Cover Thickness Survey
|Sl. No.||Test Results||Interpretation|
|1||Required cover thickness and good quality concrete||Relatively not corrosion prone|
|2||Required cover thickness and bad quality concrete cover||Corrosion prone|
|3||Very less cover thickness yet good quality cover concrete||Corrosion prone|
2. Half Cell Potential Survey
Corrosion being an electrochemical phenomenon, the electrode potential of steel wire with reference to standard electrode undergoes changes depending on corrosion activity.
A schematic survey on well-defined grid points gives useful information on the presence or probability of corrosion activity. The same grid points are used for other measurements, namely, rebound hammer and UPV could be used for making the data more meaningful. The common standard electrodes used are:
i. Copper – Copper sulphate electrode (CSE)
ii. Silver – Silver chloride electrode (SSE)
iii. Standard Calomel electrode (SCE)
The measurement consists of giving an electrical connection to the rebar and observing the voltage difference between the bar and a reference electrode in contact with concrete surface. (Fig. 1. (a))
Generally the voltage potential becomes more and more negative as the corrosion becomes more and more active. However less negative potential values may also indicate the presence of corrosion activity, if the pH values are less.
Fig-1 (a): Half Cell Potential Test
The general guidelines for identifying the probability of corrosion based on half cell potential values are suggested as in ASTM C876 are given in Table 2.
Table-2: Corrosion Risk by Half Cell Potentiometer
In any case, the technique should never be used in isolation, but should be coupled with measurements of chloride content of the concretes and its variation with depth and also the cover to the steel and the depth of carbonation.
However, a systematic “potential mapping survey” is considered to be more useful for on-site identification of the Corrosion State of rebars. This will facilitate setting out potential profile or potential contour. A typical potential contour is shown in fig.1 (b) and (c).
Initially when potential surveying was introduced as per ASTM C 876, each reading was interpreted in isolation and the numerical value was directly correlated to the degree of corrosion. Subsequently, this approach was realised to be erroneous because non-corroded steel can exhibit a wide range of potential values.
It is now realised that potential values should be assessed not in isolation but as a group and the inter relationship of the potentials within a group should form the basis of interpretation.
Analysis of potential contour will generally consist of identifying the locations with accumulated potential lines indicating to the corroding areas beneath.
(b) Shaded Mapping
(b) Contour Plot
Fig: 1 (b) and (c) Typical Half Cell Potential Contours
Locating at a glance, the anodic areas identified by the gathering of isopotential lines having more severe potential gradient. Ascertaining whether or not a structure is actively corroding.
It is necessary to realise certain important parameters (listed below) which influence the measured potentials of the reinforcement.
- The potentials of rebar measured on the surface of, or within concrete may not be a true representation of the values at the surface of the steel
- The physical i.e. moisture content and chemical state of concrete i.e. presence of electrolyte ions can result in wide variation.
- The ohmic drop due to electrical resistance of the concrete also can induce variations
- With increased concrete cover, the potential values at the concrete surface over actively corroding and passing slab become similar.
The electrical rsistance of concrete plays an important role in determining the quality of concre from the point of view ‘corrosion susceptibility potential’ at any specific location. This parameter is expressed in terms of “Resistivity” in ohm-cm.
For general monitoring, a resistivity check is important because long-term corrosion can be anticipated in concrete structures where accurately measured values are below 10000 ohm-cm. further if resistivity values fall below 5000 ohm-cm, corrosion must be anticipated at a much earlier period (possibly within 5 years) in the life of a structure.
Table 2 indicates the general guidelines of resistivity values based on which areas having probable corrosion risk can be identified in concrete structures.
Table-2: Corrosion risk from resistivity
|Resistivity ohm-cm||Corrosion Probability|
|Greater than 20,000||Negligible|
|10,000 – 20,000||Low|
|5,000 – 10000||High|
|Less than 5,000||Very high|
The principle of resistivity testing in concrete is similar to that adopted in soil testing. However when applied in concrete, a few drawbacks should be realised.
The method essentially consists of using a 4-probe technique in which a known current is applied between two outer probes 100 mm apart and the voltage drop between the inner two elements at 50mm spacing is read off allowing for a direct evaluation of resistance, R.
Using a mathematical conversion factor, resistivity is calculated as per principle of four probe resistivity testing illustrated in figure2.
Fig -2: Resistivity Meter (4 Probe System)
The following drawbacks are important to note while analysing and interpreting the resistivity values:
- The value obtained represents only the average evaluation over the depth regulated by the chosen probe spacing and not that of concrete at steel interface.
- The resistivity of concrete varies with varying moisture condition
- The instrument should have adequate IR drop compensation for measurement.
Table-3: Corrosion Probability based on Resistivity and Potential Mapping
Measurement of corrosion rate:
In reinforced concrete structures, determination of actual rate at which the reinforcement is corroding assumes larger importance since the laboratory results are not directly applicable to field conditions.
Another form of the polarization method has been developed and is known as Linear Polarisation Resistance (LPR) method for the on-site study of corrosion rates of steel in concrete.
The fundamental principle of linear polarization is based on the experimentally observed assumption that for a simple model corroding system, the polarization curve for few millivolts around the corrosion potential obeys a quasi-linear relationship. The slope of this curve is the so called polarization resistance .
From this slope, the corrosion rate can be determined using stern-Geary equation
Where B is a constant which is a function of the Tafel slopes and ba, bc and determined from the formula below:
The value of B usually lies between 13 and 52 mV depending on the passive and active corroding system. For on-site measurements, the testing system consists of a potentiostat, counter electrode, reference electrode and the reinforcement as working electrode. This system is schematically illustrated in fig. 3.
Fig-3: Resistivity Testing for Concrete
A typical plot of linear polarization curve is shown in fig.4.
It is necessary that for measurements in concrete, the potentiostat should have electronic ohmic compensation (IR drop) or otherwise, the valies to be obtained by calculation or separate experiments.
Fig-4: Linear Polarization Curve.
|1||Cover Meter/ Profo-meter (in-situ Test)||Non-destructive method for measuring – Thickness of cover concrete – Reinforcement diameter – Reinforcement spacing|
|2||Half Cell Method (in-situ Test)||Non-destructive method for measuring/plotting corrosion potential for assessing probability of corrosion|
|3||Resistivity Measurement (in-situ Test)||Non-destructive method for assessing electrical resistivity of concrete.|
|4||Permeability a) Water b) Air||Assessment of in-situ permeability of concrete due to water and air.|
|5||Initial surface absorption (Lab Test)||An indicator of surface permeability|
Half cell potential measurement method is used for finding out the status of corrosion actively in the embedded steel.