Bridge - Half-Cell Potential (HCP)
Target of Investigation
The half-cell potential (HCP) method can be used to identify corrosion activity of steel reinforcement in reinforced concrete structures. However, the method cannot directly measure the degree of corrosion.
Description
HCP measurement is based on the coexistence of corroding areas (or anodic half-cells) and non-corroding areas (or cathodic half-cells) on rebar. The measurement is calculated as the potential differences, or voltages, across the steel-concrete interfaces. The potential difference between a standard portable half-cell, typically a Cu/CuSO4 electrode, and the reinforcing steel of a concrete element is measured.(1) When the reference electrode is moved along a line or grid on the surface of a member (figure 1), the spatial distribution of corrosion potential can be mapped. Any change in the potential between the reference electrode and the steel-concrete interface can be attributed to, among other things, the corrosion activity at the surface of the steel.
![HCP Measurements on a Concrete Deck Using a Rolling Probe. A worker is pushing a rolling probe on a concrete deck. The probe is a small cylindrical object attached to the end of a shaft and connected by cables to a device providing measurements, or read-outs. The worker is walking with one hand pushing the shaft and the other holding the read-out device attached to a strap around the worker’s neck.](https://infotechnology.fhwa.dot.gov
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The HCP measurement is a well-established and widely used electrochemical technique to evaluate active corrosion in steel reinforced and pre-stressed concrete structures.(3) HCP measurements should be taken on a clean concrete surface since presence of isolating layers (asphalt, coating, and paint) may make measurements erroneous or impossible. Using empirical comparisons, the HCP’s results can be linked to the probability of active corrosion.
Physical Principle
Figure 2 depicts the principle of the HCP method. Corrosion of steel in concrete is similar to the electrochemical mechanism of corrosion of a metal in an electrolyte. This implies that separate anodic and cathodic processes take place simultaneously on the same metal surface. At the corroding side (the anode), iron is dissolved and oxidized to iron ions, leaving electrons in the steel. At the cathodic side of the reaction, oxygen is reduced and hydroxyl ions are produced. The potential of the generated electrical field is measured by a reference electrode. The reference electrode is connected to the positive end of a voltmeter and steel reinforcement to the negative one. The reference electrode is usually galvanically coupled to the concrete surface using a wet sponge. The input impedance of the voltmeter should be in the range of 106Ω to 109Ω.
![Principle of HCP Measurement. The two-dimensional schematic depicts a half-cell probe, labeled “reference electrode,” resting on a thick concrete surface. The view of the surface is a cut-away from the side. The half-cell probe is connected to a voltmeter labeled “-414 mV.” The other connection to the voltmeter is from a metal rod embedded horizontally in the concrete. A corroded portion of the metal rod is labeled “anode.” Uncorroded portions of the rod are labeled “cathode.” The voltmeter connection is to a cathode portion of the rod. Current flows from the anode to the cathodes. Iso-potential lines are rising near the ends of the anode portion of the rod. Three of the iso-potential lines start at one end of the anode and return at the other end. These three lines are labeled “-500,” “-600,” and “-700.”](https://infotechnology.fhwa.dot.gov
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Data Acquisition
ASTM C876–09, Standard Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete, describes the HCP test method. The test procedure on a bridge deck can be summarized in the following steps:(4)
- Lay out a 2ft by 2ft grid on the deck.
- Locate the top reinforcing bar using a rebar locator (GPR or pachometer) at several points along the bridge deck. A minimum of two locations is required for every test area so that continuity of the reinforcement can be checked. However, the actual number of locations depends on the size of the bridge and the continuity of the reinforcement.
- Drill or core a 1 inch hole in the concrete to expose the reinforcement at each location where reinforcement has been located (figure 3).
![Drilling a Hole into the Concrete over a Shallow Bar. A worker is drilling a hole down into concrete with a large power drill.](https://infotechnology.fhwa.dot.gov
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- Securely connect a lead wire to the reinforcement (figure 4).
![Lead Wire Connected to the Reinforcement by an Electrical Clip. The photo is a close-up of a hole drilled into the concrete. A red wire extends into the hole. Protruding from the hole is the end of an electrical clip attaching the wire to the metal reinforcement in the concrete.](https://infotechnology.fhwa.dot.gov
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- Check electrical continuity of the steel reinforcement cage between the installed lead wire locations (taps) by connecting the two taps to a standard multimeter and measuring the resistance. The reinforcement is considered continuous when the measured resistance is 1Ω or less considering the resistance of the lead wire.
- Securely connect the exposed steel to the reference electrode.
- Connect the reference electrode to the negative terminal and the steel reinforcement to the positive terminal of the high impedance voltmeter.
- Wet the contact sponge of the half-cell with soapy water to ensure good electrical contact with the concrete deck.
- If time and access allows, pre-wet the deck prior to testing.
- Place the reference electrode (wheel/rod) on each test point. Once the voltage has stabilized, the measurement can be made by moving the wheel along the grid (figure 5).
![The Reference Electrode (Wheel/rod) Placed on a Test Point. A worker has rolled a half-cell probe to a test point on a concrete surface. The worker is holding the shaft attached to the cylindrical probe and is looking at the read-out device attached to a strap around the worker’s neck. Small white marks indicate other test points on the deck. The marks are in a grid pattern.](https://infotechnology.fhwa.dot.gov
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- The voltage values are stored in the data acquisition system or recorded by hand depending on the system being used.
- Once testing is complete, patch all tap locations with acceptable concrete materials (figure 6).
![Patching Test Holes. A worker is kneeling on a concrete surface, using a small trowel to patch test holes that have been drilled into the surface.](https://infotechnology.fhwa.dot.gov
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The ionic solution in the reference electrode must be replaced periodically according to the manufacturer recommendations.
Data Processing
No processing of HCP data is typically performed. Discrepancies in measurement accuracy may exist when there are extreme variations in the cover depth, and HCP measurements may need to be adjusted based on measuring deck electrical resistivity obtained at the same HCP test locations. However, in practice, data are generally gridded and mapped using contour-mapping software based on the measured HCP data.
Data Interpretation
Generally, x– and y-coordinates are plotted against measured voltage to produce a map showing areas of very high likelihood for active corrosion, very low likelihood for active corrosion, or a transition zone that spans the measurements in between. According to ASTM C876–09, Standard Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete, a measured potential more negative than -350mV corresponds to a 90 percent probability of active corrosion.(4) A measured potential less negative than -200mV corresponds to a 90 percent probability of no active corrosion. Corrosion activity is uncertain if the potential is in the range of -350mV to -200mV (figure 7). HCP data are typically used with other physical or chemical sampling or other NDE methods. These methods can be used jointly to determine the corrosion state that exists within a bridge deck. Many researchers have shown a good correlation between GPR and HCP maps or other more direct measures of a corrosive environment or active corrosion. It is important to understand that these values are not absolute and depend on the material and environmental conditions of the measured reinforced concrete element.
![Contour Map. HCP Measurements Representing Spatial Distribution of the Potential Values. The x-axis is Longitudinal Distance, ranging from 0 to approximately 85 ft. The y-axis ranges from 0 to 12 ft. Below the graph is a color scale indicating probability of corrosion. Varying shades of red and yellow colors (-720 to -350 mV) on the left of the scale indicate a 90 percent probability of corrosion. Shades of green and light blue colors in the middle of the scale represent an uncertain or transition area, ranging from -350 mV to -200 mV. Dark colors on the right represent a 90 percent probability of no corrosion, covering potentials greater than -200 mV. The majority of the colors on the map indicate a 90 percent probability of corrosion. There are a few pronounced dark areas indicating 90 percent probability of no corrosion.](https://infotechnology.fhwa.dot.gov
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Potential Values.(2)
Advantages
- Rapid equipment setup, data collection, and data analysis.
- Easy to use with minimal training.
- Uncomplicated data processing; reduces to plotting the raw data.
- Economical.
- Moderately repeatable measurements.
- Correlates well with other NDE methods (GPR and electrical resistivity) and, to some extent, chemical sampling (chloride testing) to identify deck areas where corrosive environment and corrosion processes exist.
Limitations
- Erroneous or impossible measurements due to isolating layers (asphalt, coating, and paint) on the deck surface or decks with coated rebars.
- Cannot be conducted if electrical continuity does not exist in the element being evaluated.
- Does not directly measure corrosion rate or the extent of past corrosion (rebar section loss due to corrosion).
- Unreliable measurements when the concrete is wet, dense, or polymer-modified.
References
- Gucunski, N., et.al., Comprehensive Bridge Deck Deterioration Mapping of Nine Bridges by Nondestructive Evaluation Technologies, Project SPR-NDEB(90)-8H-00, Center for Advanced Infrastructure and Transportation, Rutgers University, January, 2011.
- Strategic Highway Research Program, “SHRP2 NDToolbox,” (Website), Washington, DC, Accessed online: February, 2015. http://www.ndtoolbox.org/content/bridges.
- Gucunski, N., et. al., Nondestructive Testing to Identify Concrete Bridge Deck Deterioration, Report S2-Ro6A-RR-1, SHRP2 Renewal Research, Transportation Research Board, 2013.
- ASTM, Standard, Standard Test Method for Half Cell Potentials of Reinforcing Steel in Concrete, C876, American Society for Testing and Materials, West Conshohocken.