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Direct Measurement Methods
This section describes NDT methods used for measuring rebar corrosion directly from an exposed surface of a concrete structure.

Half-cell Potential

Basic Concept: This method is sometimes called the corrosion potential or rest potential method. The objective of this method is to measure the voltages that are present over rebar in concrete. The half-cell is a hollow tube containing a copper electrode and immersed in copper sulfate solution. The bottom of the tube is porous and is covered in a sponge material. The copper sulfate permeates this sponge that can then be placed on a concrete surface allowing an electrical potential (voltage) to be measured. The objective of the method is to measure the voltage difference between the rebar and the concrete over the rebar. Large negative voltages (-350mV) indicate that corrosion may be taking place. Voltages smaller than about -200 mV generally mean corrosion is not taking place.

 Half-cell measuring circuit for detecting rebar corrosion.

Figure 39. Half-cell measuring circuit for detecting rebar corrosion.

Figure 39 shows the circuit used and the concrete and rebar. The half-cell is the reference electrode.

Data Acquisition: Half-cell surveys simply require making an electrical connection to the rebar and then taking readings over the area of interest by pressing the sponge of the reference electrode against the concrete and observing the voltmeter reading. The readings are usually taken on some type of predetermined grid system. A device called a potential wheel is sometimes used. This provides a wheel for the tip of the half-cell and allows continuous readout of corrosion potentials, making the method quite rapid. To ensure that sufficient electrolyte is present at the surface, it is usual for all of the reading locations to be pre-wet using a fine spray of weak detergent mixture. A half-cell instrument is shown in figure 40.

Data Processing: Since the survey directly measures the quantity of interest (namely voltage), no data processing is required. The data may be plotted on a map and contoured for ease of interpretation and for presentation purposes.

Data Interpretation: Experience has shown that for potentials whose magnitude is greater than -350 mV, there is a 90% probability that corrosion is active. If the magnitude of the potential is less than -200 mV, then there is a 90% probability that corrosion is not active.

Figure 40 shows typical results from a half-cell survey. As can be seen, the voltages extend from about -50 mV to over -350 mV, suggesting that corrosion is taking place at these high negative voltage locations.

Half-cell instrument.  (Hammond Concrete Testing)

Figure 40. Half-cell instrument. (Hammond Concrete Testing)

Example Half-cell Potential results.

Figure 41. Example Half-cell Potential results.(Hammand Concrete Testing)

Limitations: Several factors must be kept in mind when conducting and interpreting half-cell measurements. It is important to consider the oxygen and chloride concentration and the resistivity of the concrete, all of which can influence the readings. Adding to these complications are the advances in concrete and repair technologies, such as dense material overlays, concrete sealers, corrosion inhibitors, chemical admixtures, and cathodic protection systems. It is important to understand and consider these complicating factors during a half-cell survey and to supplement the results with other nondestructive surveys. With most of the current equipment, an electrical connection has to be made with the rebar. However, some recent developments involve the measurement of potential gradients using two reference electrodes, eliminating the need to make direct electrical contact with the rebar.

Linear Polarization Resistance (LPR)

Basic Concept: Linear Polarization Resistance (LPR) provides a relationship between the voltage (potential) and current density of a material. The polarization resistance of a material is defined as the slope of the potential-current density (ΔE/Δi) curve at the free corrosion potential. Thus Rp = ΔE/Δi as ΔE tends to zero. There are a number of ways to carry out the LPR measurement. Perhaps the simplest is to use two nominally identical electrodes. A small potential difference (e.g., 20 mV) is applied to these electrodes, and the resulting current is measured. This current is proportional to the inverse of the polarization resistance and, hence, is directly proportional to the corrosion rate.

Data Acquisition: A more sophisticated approach is to use a potentiostat and a three-electrode arrangement. The test electrode is polarized by a small amount (10 to 20 mV) from its free corrosion potential, and the required current is measured. Again, this is directly proportional to the corrosion rate. Figure 42 shows a corrosion-monitoring sensor that can be installed at a site and measures the instantaneous corrosion rate of rebar in concrete using the LPR principles. This device, which is installed as a fixture at the site of interest, provides the instantaneous corrosion rate of the electrodes in the concrete environment. The probes are monitored frequently or continuously to track changes in the corrosion rate.

Linear Polarization Resistance probe. (Rohrback Cosasco Systems, Inc.)

Figure 42. Linear Polarization Resistance probe. (Rohrback Cosasco Systems, Inc.)

The main challenge of applying well known corrosion rate measurement techniques such as the Linear Polarization Resistance (LPR) technique to actual rebars embedded in concrete has been confinement of the applied potential (or current) perturbation to a well-defined rebar area to minimize measurement errors. The development of guard ring devices has addressed this issue.

The guard ring is maintained at the same potential as the counter electrode to prevent the current from the counter electrode from flowing beyond the confinement of the guard ring. The counter, reference, and guard ring electrodes can be conveniently located in a sensor placed directly above the rebar of interest (figure 43), necessitating only one electronic lead attachment to the rebar for corrosion rate measurements, as in the case of the simple potential measurements.

Guard ring devices will display a certain corrosion rate reading expressed as thickness loss per time unit (e.g., mm/year) after the polarization cycle is completed, but there are many simplifying assumptions in the derivation of this corrosion rate. Important limitations include the assumption of uniform corrosion over the rebar surface (this rarely applies to chloride- induced rebar corrosion), simplified models for the electrochemical reactions and ge transfer processes, assumed values for the Tafel constants, as well as possible inaccuracies in "IR" (voltage) drop resistance corrections.

Guard ring setup for Linear Polarization Resistance measurements.

Figure 43. Guard ring setup for Linear Polarization Resistance measurements.

Data Interpretation: An example Tafel graph is shown as figure 44. This shows the voltage-current response of a corroding electrode, which tends to be linear over a small range of potential either side of the free corrosion potential. This is because both the anodic and cathodic currents are exponentially related to potential, and the difference between two such exponential curves is nearly linear over a small range of potential. Figure 44 shows the anodic and cathodic currents. A further fundamental source of inaccuracies is that no allowance is made for the effects of macrocell corrosion that are inherent to actual rebar grids. In addition, the applicability of these types of measurements to cracked concrete is presently not clear.

Limitations: For the steel in a concrete system, it is imperative that sufficient time is allowed for a current value to stabilize at a certain potential (or vice versa). For example, in the potentiostatic LPR technique, it will typically take several minutes for the current to reach a stable level after the polarizing voltage is applied. Shorter polarization could lead to significant measurement errors.

Unfortunately, the guard ring technology does not lend itself to quickly assessing large concrete surface areas. To reduce evaluation times to acceptable, practical levels, it may be advisable to map the corrosion potential values, followed by selective application of the guard ring device to critical areas.

Example Tafel graph.

Figure 44. Example Tafel graph.

Galvanostatic Pulse Technique

Basic Concept: In this technique, an anodic current pulse is imposed onto the rebar for a short period of time, using a counter electrode positioned on the surface of the concrete. The resultant rebar potential change (E) is recorded by means of a reference electrode, also located on the concrete surface. Typical current pulse duration (t) and amplitude have been reported at 3s and 0.1 mA respectively.

Data Acquisition and Interpretation: The slope of the potential-vs-time curve (E/t) measured during the current pulse can be used to provide information on the rebar corrosion state. Passive rebar reportedly has a relatively high slope, whereas rebar undergoing localized corrosion has a very small slope. In the latter case, the rebar potential only shifts by a few millivolts under the applied current pulse. It is also possible to use the potential data to obtain a measure of the concrete resistivity (for a given depth of cover). The technique is reportedly very rapid and may facilitate more unambiguous information on the rebar corrosion state than is possible by simple potential mapping. Figure 45 shows the GalvaPulse© instrument which is manufactured by Germann Instruments.

GalvaPulseĀ© instrument. (Germann Instruments)

Figure 45. GalvaPulse© instrument. (Germann Instruments)

Electrochemical Noise

Basic Concept: Unlike other electrochemical techniques, noise measurements do not rely on any "artificial" signal imposed on the rebar probe elements. Rather, natural fluctuations in the corrosion potential and current are measured to characterize the severity and type of corrosive attack. For these measurements, three nominally identical rebar probe elements can be conveniently embedded in the concrete.

Data Acquisition: There may be some reluctance to using electrochemical noise for rebar corrosion measurements in the field due to a perceived "over sensitivity" of the equipment and fears of external signal interference. Although such concerns may be justified in certain cases, and this technology is relatively new to the rebar field, it has recently been used successfully in rebar corrosion measurements in the Vancouver harbor and in clarifier tanks of the paper and pulp industry in British Columbia. In these applications, the rebar noise probes were embedded in large (up to 4 meters long) concrete prisms. These prisms were partially submerged and exposed to seawater and to the effluent solution in the clarifiers.

Electrochemical noise data showed increased corrosion activity associated with the tidal cycle in the Vancouver harbor. In this case, the highest corrosion activity on the probe element, located in cracked concrete, occurred as the tide level approached that of the crack. Under these conditions, seawater is available to penetrate the concrete as corrosive electrolyte, together with a high degree of aeration, a combination expected to stimulate electrochemical corrosion processes. Noise data from electrodes placed in cracked and uncracked concrete and exposed to pulp and paper effluent confirmed that, as expected, the presence of cracks facilitated more rapid diffusion of corrosive species to the rebar surface and led to higher electrochemical corrosion activity. To date, no significant interference or other problems have been encountered in the noise measurements of these two field exposure programs.

Data Processing and Interpretation: Apart from the interpretation of the "raw" noise data, it has become customary to conduct further statistical data processing, spectral analysis, and chaos theory analysis. Such data treatment serves to reduce the volume of data and to assist in distinguishing different forms of corrosion from one another.

Advantages: A distinct advantage of the electrochemical noise techniques is that the initiation and propagation of corrosion pits can be clearly identified. Distinct pit initiation transients, comprised of a sharp signal increase with passive film breakdown and a more gradual recovery of the signal to baseline levels as the passive film repairs itself, were observed for a carbon steel rebar probe exposed to chloride containing concrete pour solution. These distinct signatures of the initiation of corrosion pits were evident long before the attack was observable by visual means, indicating the "early warning" capabilities of this technology.

Acoustic Emissions

Basic Concept: Acoustic emissions (AE) monitoring of concrete has been used to detect rebar corrosion and has been shown to detect film cracking, gas evolution, and microcracking. Although attenuation of the AE signal in concrete has been a concern in the past, placement of the AE transducers on the reinforcing steel and using the steel as a sound propagation medium should allow the onset of steel corrosion to be detected. It is also possible to use the AE method to calculate the location where the steel corrosion is occurring. This appears to be a promising technique that can be used as a bridge inspection method to quantify the condition of steel-reinforced concrete where corrosion is occurring.

The AE method measures the high-frequency acoustic energy that is emitted by an object that is under stress. Slow crack growth in ductile materials produces few events, whereas rapid crack growth in brittle materials produces a significant number of high amplitude events. Corrosion product buildup and subsequent microcracking of the concrete represents the latter phenomenon.

Data Acquisition and Processing: A typical AE monitoring system uses piezoelectric sensors acoustically coupled to the test object with a suitable acoustic coupling medium, (grease or adhesive). The output of the sensors is amplified and filtered by pre-amplifiers and then fed to the monitor via shielded coaxial cables. The monitor further filters and amplifies the AE signals, processes the data, and displays the results. Both results and raw data are typically recorded for archival purposes or for post-test analysis, for instance, to determine location of the AE signal.

Magnetic Field Disturbance (MFD)

This technique can be used to detect flaws in steel, and, in particular, rebar.

Basic Concept: The method involves passing a strong magnetic field through a concrete structure to magnetize embedded steel. Sensors are used to measure the field produced by the metal in the structure. Anomalies are observed where flaws occur in the steel.

The flaws produce a distinctive anomaly depending on their size and the distance between the flaw and the sensor. The method can be used when the cables are encased in grout-filled ducts.

Advantages and Limitations: Laboratory experiments have shown that MFD can detect fractures and corrosion of reinforcing strands in air and concrete, as well as in filled plastic and metal ducts in concrete. Although there have been problems involving background structural disturbances, the MFD system has shown that it is one of the most feasible methods for detecting corrosion and failure of both strands and plain steel in pre-tensioned and post-tensioned structures.