Eric Sumsion and Dr. Spencer Guthrie, Department of Civil and Environmental Engineering

Introduction

Corrosion affects reinforced concrete structures throughout the world. Corrosion, induced by concrete carbonation and penetration of chloride ions from deicing salts and/or seawater, reduces the load-bearing area of reinforcing steel, weakens the bond between steel and concrete, and cracks the concrete surrounding the steel. As a result of these effects, corrosion can cause premature failure of reinforced concrete structures. If corrosion can be identified early in a structure’s life, corrective methods can be taken to protect the structure from further damage. Assessing rebar corrosion is not easy, however, because it takes place beneath the surface of the concrete. Electrochemical impedance spectroscopy (EIS) is one of several methods available for measuring the corrosion state of reinforced concrete (1). EIS is performed by applying an alternating current to a reinforcing steel system and measuring the impedance over a suite of frequencies (2-5). Given a set of impedances, corrosion rates of the steel can be inferred. Unfortunately, EIS is difficult to perform outside the controlled environment of a lab. In the field, existing EIS devices are difficult to use and typically produce inconsistent and unintelligible results. A new device has been recently developed by Dr. Brian Mazzeo, Dr. Spencer Guthrie, Paul Bartholomew, and the author that may be able to produce consistent results in the field that are comparable to those produced by laboratory EIS devices. The new device is able to establish connection with the concrete surface without the aid of a conductive solution, which removes the tested system from its in-situ state and introduces variability to the testing. Additionally, the new device does not require the use of a reference electrode.

Procedures

Two sets of reinforced concrete slabs were created in order to test the new EIS device. First, a set of nine concrete slabs, each containing five pieces of reinforcing steel at various depths, was constructed using a concrete mix typical of those used for bridge decks in Utah. Sodium chloride was added to six of the slabs during concrete mixing (three slabs with 2 lbs of chloride ions per cubic yard of concrete and three slabs with 4 lbs of chloride ions per cubic yard of concrete) in order to cause the steel to corrode. These slabs were placed outside and allowed to weather for two years so that they might accurately simulate the condition of in-service concrete bridge decks. In the second set, four additional slabs were created with changes in the water/cement ratio of the concrete mix in order to account for differences in concrete structure. Sodium chloride was added to two of those slabs at a concentration of 10 lbs of chloride ions per cubic yard of concrete.

Each of the slabs was tested with the experimental EIS device and with a VersaSTAT, an established commercial EIS device. The VersaSTAT was used with a copper-copper sulfate reference electrode and a titanium mesh surrounded by a sponge filled with conductive solution as a counter electrode, as shown in Figure 1. Tests were performed over the same suite of frequencies, ranging from 1 mHz to 100 kHz, in order to determine whether or not the two devices achieved similar results at each frequency.

Results

Unexpectedly, the results of testing with the two devices were significantly different for all thirteen of the slabs. Changes in chloride concentration, water/cement ratio, and rebar depth did not improve the similarity of the data sets. Repeated testing verified these results. Indeed, similarities between the two sets of data were entirely absent. For none of the tested frequencies did the devices produce comparable results.

Conclusions

The experimental EIS device and the VersaSTAT did not produce similar results, regardless of the corrosion state of the reinforced concrete tested. This may be a result of the differences between the two systems in the device/concrete interface. Moreover, the experimental EIS device may be sensitive to different reinforced concrete properties than the VersaSTAT.

The author plans to conduct additional testing in the future to determine which properties of reinforced concrete affect the experimental device.

References

1. Esmaeilpoursaee A. An Analysis of the Factors Influencing Electrochemical Measurements of the Condition of Reinforcing Steel in Concrete Structures. PhD thesis. Department of Mechanical Engineering, University of Waterloo, Waterloo, Ontario, Canada, 2007.
2. Silverman, D.C. Primer on the AC Impedance Technique. Electrochemical Techniques for Corrosion Engineering. Houston, Texas, 1986, pp. 73-79.
3. Koleva, D.A., J.H.W. de Wit, K. van Breugel, L.P. Veleva, E. van Westing, O. Copuroglu, and A.L.A. Fraaij. Correlation of Microstructure, Electrical Properties and Electrochemical Phenomena in Reinforced Mortar. Materials Characterization, Vol. 59, 2008, pp. 801-815.
4. Vedalakshmi, R., and N. Palaniswamy. Analysis of the Electrochemical Phenomenon and the Rebar-concrete Interface Using the Electrochemical Impedance Spectroscopic Technique. Magazine of Concrete Research, Vol. 62, No. 3, March 2010, pp. 177-189.
5. Lemoine, L., F. Wenger, and J. Galland. Study of the Corrosion of Concrete Reinforcement by Electrochemical Impedance Measurement. Corrosion Rates of Steel in Concrete, ASTM STP 1065. American Society for Testing and Materials, Philadelphia, 1990, pp. 118-133.