GPM is used to estimate the corrosion rate of steel in steel-reinforced concrete structures. The corrosion rate is very important for determining how fast a concrete member is deteriorating; this determination is not possible with either the ER or HCP technologies. GPM is also advantageous in corrosion risk assessment over HCP measurement in situations when concrete is wet, dense, or polymer-modified, and thus the access to oxygen is limited.(1)
GPM is an electrochemical NDE technology used for rapid assessment of rebar corrosion, primarily to estimate the corrosion rate for steel rebars. GPM is based on the polarization of rebar using a small current pulse and measurement in the potential change.(2) GPM uses an electrode device positioned on a concrete surface and a reference electrode attached directly to the steel reinforcement to determine the half-cell potential, corrosion rate, and resistance between the reinforcement and device (figure 1).
Figure 1. Photo. Galvanostatic Pulse Measurement with Close Up of Control Unit.
The GPM corrosion assessment is based on the measurement of the current required to change the potential difference between the reinforcement and a standard reference electrode. The current is a result flow of electrons from the anodic and cathodic sides in concrete. The anodic side is a corroding rebar where there is surplus of electrons on the base metal as a result of releasing metal ions into an electrolytically conducting liquid. Those excess electrons flow to a cathodic side, concrete with oxidizing agents in the liquid, inducing a corrosion current between the two sides. Knowing the current and voltage allows an inspector to determine the polarization resistance, which is related to the rate of corrosion.
The GPM test is conducted by first applying to counter electrode a number of anodic DC pulses (5-400μA). The first, stronger pulse is between 3 to 60 seconds, and its intensity will primarily depend on the properties of the object under investigation. Several smaller consecutive pulses of gradually increased intensity may follow the first pulse. The objective of applying DC pulses is to obtain a sufficiently high polarization, at which point potential variations are recorded by means of a reference electrode (figure 2). Noncorroding or passive reinforcement possesses no current and thus will have a high resistance to current flow which is reflected in a potential shift of around 100 mV.(3) Active corrosion is likely when the potential shift gets smaller. This can be also explained as an actively corroding system can quickly compensate for a removal of electrons due to the corrosion reaction or polarization achieved by the current pulse remains low in an actively corroding area.
Figure 2. Composite Graph. Galvanostatic Pulse Measurement of Corrosion Activity.
GPM can be conducted at selected locations of a bridge element (for example, a bridge deck), or the element can be fully mapped with respect to the anticipated corrosion rates. In the second case, a 2-ft by 2 ft grid should be marked on the element using a chalk or washable paint. Since GPM requires the device to be connected to the reinforcing steel, a connection should be made prior to testing by drilling a hole in the concrete element and establishing a connection with a rebar using an alligator clip, screw, or other means (figure 3). Therefore, a rebar locator and drilling equipment should accompany the GPM system. The electrical continuity of the rebar mesh should be checked to identify the need for additional connections to rebars. This is achieved by measuring resistance between two connections to rebars, typically on the diagonally opposite sides of the element, like a bridge deck. The holes need to be patched with approved patch material after the completion of the GPM survey. The surface of the deck should be pre-wetted before the survey, or the test locations lightly sprayed shortly before the measurement.
Figure 3. Photo. Connection to an Exposed Rebar.
The actual measurement is conducted by placing a reference electrode on the test location (figure 4). The sponge at the bottom of the electrode should be wet to ensure electrical coupling with the concrete element. One strong and several shorter pulses are applied to achieve sufficiently high polarization of the reinforcement. At that time, potential variations are being measured by the reference electrode and data recorded on a handheld computer through a DAQ software. As described in the physical principle of GPM, potential changes equal or higher than 100 mV are an indication of passive reinforcement, while lower are indication of corrosion activity. Since the corrosion rate is proportional to the amount of current required to change the potential, it can be calculated.
Figure 4. Photo. Reference electrode with wet sponge
Data processing for GPM basically means plotting raw data in a way users can understand. No other data processing is required. The equipment usually comes with contour mapping software where the raw data is gridded and mapped. The slope of the potential-vs-time curve measured during the current pulse can be used to provide information on the rebar corrosion state. It is also possible to use the potential data to obtain a measure of the concrete resistivity (for a given depth of cover).
The electrical resistance and corrosion rate maps from a GPM survey of a bridge deck are shown in figures 5 and 6, respectively. As illustrated in figure 5, the measured electrical resistivity varies between 5 and 63 kΩ. The lower the resistivity, the higher the probability of a corrosive environment as a result of intrusion of moisture, chlorides, and salts in concrete. On the other hand, a highly resistive concrete will be an indication of a non-corrosive environment.
Figure 5. Contour Map. Concrete Resistance Map as Measured by a GPM System.(4)
Figure 6. Contour Map. Map for Corrosion Rate.(4)
The calculated corrosion rate can be further correlated to the cross-section loss. In figure 6, the cross-section loss is calculated and presented in the map for the assumption that a corrosion rate of 1 μA/cm2 corresponds to a cross-section loss of about 11.6 μm/year.
- Easy to learn, requiring low to medium level of expertise for equipment setup and data collection.
- Economical test.
- Repeatable test.
- Sufficient accuracy for practical applications.
- Works on any concrete surfaces.
- Determines half-cell and resistance.
- Relatively small equipment.
- Needs direct access to reinforcement.
Unstable measurements when concrete cover resistivity is high.
Requires pre-wetting of the measurement area.
Time consuming and labor intensive on large bridge decks.
Results may be inaccurate in aged structures because the change in microstructure of concrete and loss of moisture from the concrete make the potential of rebar and resistivity of concrete more unpredictable.
- Sørensen, H.E. and Frølund, T., "Monitoring of Reinforcement Corrosion in Marine Concrete Structures by the Galvanostatic Pulse Method" International Conference on Concrete in Marine Environments, Hanoi, Vietnam, Proceedings, October 2002.
- Elsener, B., Hug, A., Bürchler, D. and Böhni, H., "Evaluation of localized corrosion rate on steel in concrete by galvanostatic pulse technique," in: Corrosion of Reinforcement in Concrete Construction (Page, C. L., Bamforth, P., Figg, J. W., Ed.), Society of Chemical Industry, London, England, 1996, pp 264.
- Bäßler, R., Burkert, A., Frølund, T. and Klinghoffer, O., "Usage of GPM-Portable Equipment for Determination of Corrosion Stage of Concrete Structures," paper 3388, NACE Corrosion Conference, San Diego, CA, Proceedings, 2003.
- Strategic Highway Research Program, "SHRP2 NDToolbox," (Website), Washington, DC, Accessed online: February 2015.