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Half Cell corrosion mapping is an effective method for assessing the severity of corrosion in reinforced concrete structures. This rapid, cost-effective and non-destructive survey method provides key information in corrosion evaluations. Corrosion of steel reinforcement is a major factor in the deterioration of highway and bridge infrastructure and half cell corrosion mapping can be used to detect corrosion, as well as being the first step towards its rehabilitation. It can also be used as a quality assurance tool in the repair and rehabilitation of concrete structures and as a predictor of remaining service life.
The most practical way to determine the severity of corrosion in reinforcement steel is to measure its corrosion potential. This is done with the use of a standard portable half-cell, normally a copper/copper sulphate (Cu/ CuSO4) standard reference electrode. The reference electrode is connected to the negative end of a voltmeter and the positive end of the voltmeter is connected to the reinforcement steel. Depending on the measured half cell corrosion potential value, the probability of active corrosion is determined.
ASTM C876 provides a half cell potential range . Depending on the measured half cell corrosion potential, the probability of active corrosion is determined.
How to Perform Half Cell Corrosion Mapping
1- Measurement Points
The first things is to know the test locations. In general, a schematic grid will help you with smoothly taking the measurements, and managing test results. There is no general rule about the grid spacing. A finer mesh will be more precise, but also more expensive. Wide spacing can reduce the resolution of test results, and may result in inaccurate half cell potential measurements. A suitable spacing should be selected with regards to the area under investigation.
2- Rebar Connection / Connectivity Test
Half cell potentials at each test point is measured to an identical reference point. An electrical connection to the rebar network is required at this reference location. normally, concrete cover should be locally removed (this can be achieved by drilling into concrete), to establish a sound electrical connection to rebar. To do so, one needs to locate the rebar first (this can be done by using a rebar locator). Rebar network normally provides a connected mesh. However, in larger area, such as deck slabs, one needs to check if the steel rebar network is connected.
3- Electrical Connection to Voltmeter
The connecting lead cable from the negative terminal should be connected to the reference electrode, while the other lead wire should be connected to the rebar network at the reference location. The voltmeter should be able to measure direct current (dc) voltage, and have the capacity to be battery operated. The voltmeter should have a variable input impedance ranging from 10 to 200 MΩ may be used to determine the input impedance required to obtain precision readings.
4- Pre-wetting the surface
If the surface of concrete is too dry, the pre-wetting is required. Pre-wetting can either be achieved by spraying the water over the location to be tested, or through using a wet sponge. ASTM C 876 describes how suitable pre-wetting condition can be achieved.
5- Perform Measurements
At each location, the value of half cell potential should be recorded to the nearest 0.01 V (ASTM C 876). For each point, record the coordinates of the test point, as well as the corrosion potential value. If the weather condition, including the temperature has changes, the values of the half-cell measurement should be adjusted for temperature variations.
Presentation of Half-Cell potential measurements
A Half-Cell potential equipotential contour map is widely used to report Half Cell potential data. Results can also be reported as a cumulative frequency distribution of corrosion potential.