Nondestructive Test Methods

Electrochemical Corrosion Testing

Corrosion of steel embedded in concrete is an electrochemical process that involves the formation of an electrical circuit between areas of active corrosion (anodes) and passive areas (cathodes).  The formation of corrosion products at the anodes is an expansive process that results in the cracking and eventual spalling of the concrete.  In the corrosion process, the concrete acts as an electrolyte allowing the flow of ions from anodes to cathodes. 

Due to the electrochemical nature of the process, information about the extent of corrosion activity can be developed from electrochemical testing of the concrete and reinforcing steel.  Corrosion testing typically includes measurements on both the reinforcing steel and concrete and is most commonly used to assess uncoated, bonded reinforcing steel.

Good quality concrete typically provides a high pH coating on the reinforcing steel that prevents corrosion from occurring.  The high pH coating will breakdown in the presence of chloride or over time due to exposure of the concrete to carbon dioxide.  Therefore assessment of corrosion typically starts with measurement of the chloride concrete of the content and assessment of the concrete pH.  Continuity between reinforcing steel elements also needs be verified as a part of the assessment process. 

Several electrochemical methods can be used to assess the state of corrosion in a concrete element.  These methods include half-cell potential measurement and corrosion rate measurement.

Half-Cell Potentials

The half-cell potential test is an electrochemical testing method for indicating whether active corrosion may be occurring in reinforcing steel in concrete structures. The method is described by ASTM C 876, “Standard Test Method of Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete.”

The half-cell potential test uses a standard voltmeter to measure the potential difference between the reinforcing steel and a copper-copper sulfate reference electrode in contact with the concrete surface. The potential difference provides an indication of the presence or absence of active corrosion of the reinforcing steel. This test method does not provide information regarding the actual rate of corrosion; rather, it indicates where active corrosion may be occurring with or without visual signs of distress.



Corrosion Rate

Corrosion rate testing is an electrochemical testing method used to evaluate the rate of active corrosion in reinforcing steel at a given test location. This rate is compared with statistical data to estimate the period of time following testing during which visible signs of corrosion damage may appear.

Although half-cell corrosion potentials can give an indication of the likelihood of active corrosion, the corrosion potential test does not provide definitive information on the rate of the active corrosion. It is important to have an indication of the rate of active corrosion because, although part of a structure may be experiencing corrosion, the rate of corrosion may be low enough that significant section loss and structural damage may not occur for many years. Thus, it may be possible to delay remediation until a future date. Conversely, the measured rate of corrosion may be very high, indicating need for expeditious repair and/or rehabilitation.

Corrosion rate testing implements the polarization resistance technique. The device applies a current to a given section of reinforcing steel and measures the corresponding increases in half-cell potential. The amount of current required to change the potential is proportional to the corrosion rate of the steel in the specific test area at the specific time of testing. WDP’s corrosion rate device confines the current to a known area of steel directly below the sensor. This confinement provides more precise measurement of the true polarization resistance as compared to devices which do not provide this confinement.



Concrete Resistivity Measurement

The half-cell corrosion potential and corrosion rate measurements are intended to assess the condition of embedded reinforcing steel.  The concrete resistivity test measures the resistance of the concrete to the flow of ions.  Higher levels of concrete resistivity correlate to a greater resistance of the concrete to the flow of ions, and thus a lower probability that reinforcing steel corrosion is occurring.

WDP measures a concrete resistivity using a modified four-point Wenner resistivity meter which is commonly used for soil resistivity measurements.  The modified probe uses four equally spaced (2 in. or 50 mm apart) probes on a straight line.  The measured resistivity values are compared to published data to assess the ability of the concrete to support corrosion activity.