Kyle Stanish, University of Cape Town, R. Doug Hooton, University of Toronto, and Michael D. A. Thomas, University of New Brunswick
Chloride-induced corrosion is a major cause of deterioration of reinforced concrete structures. The best method of minimizing the problem is by producing high quality concrete that is capable of resisting the ingress of chlorides. To ensure quality concrete, it is necessary to have a measure of the concrete’s ability to resist chloride ingress that can be used as a standard test.
The traditional test that has been used for this purpose is AASHTO T 277 (ASTM C 1202), commonly referred to as the Rapid Chloride Permeability Test (RCPT). This test, while providing a rapid indicator of concrete’s resistance to fluid penetration, does have a few drawbacks, principally: (i) the current passed is related to all the ions in the pore solution, not just chloride ions; (ii) the high voltage leads to temperature increases during the test, which affects the properties of the concrete; and (iii) a relatively high variability. To overcome some of these drawbacks, the FHWA sponsored an investigation of various alternative test methods.(1) Some of the results of this investigation were reported in HPC Bridge Views, Issue No. 13. This investigation recommended the use of a Rapid Migration Test, which has since been adopted as AASHTO provisional Standard TP 64. The Rapid Migration Test was originally proposed by Tang and Nilsson(2) in Sweden, and has been standardized by Nordtest, a Scandinavian organization, as NT Build 492.
The main difference between AASHTO TP 64 and Nordtest NT Build 492 is that NT 492 allows calculation of a nonsteady state, chloride diffusion coefficient. This was considered for the AASHTO test, but the theory behind the calculation has been questioned.(3)
For the Rapid Migration Test, a 50-mm (2-in.) long, 100-mm (4-in.) diameter concrete sample must be obtained. It is then saturated using the vacuum saturation procedure of the RCPT. Next, the sample is clamped inside a silicone rubber tube between two solutions: 10 percent sodium chloride on one side and 0.3 molar sodium hydroxide on the other. A typical test setup is illustrated in the figure, although other options are possible including using AASHTO T 277 cells.
Initially, a 60 volt potential is applied across the sample and the current measured. Based upon the initial current, the voltage is adjusted to bring it to a range suitable for that quality of concrete. The voltage is then applied for 18 hours. The applied voltage drives the chloride ions into the previously uncontaminated concrete. Upon removal, the concrete sample is split in half along its length. The broken faces are then sprayed with 0.1 molar silver nitrate solution—a colorimetric indicator. The silver nitrate reacts with any stable chloride ions that are present to form a white layer, while the uncontaminated area turns brown. The average depth of chloride penetration is obtained by taking measurements at 10 mm (0.4 in.) intervals across the diameter. The average value is then divided by the product of the applied voltage in volts and time in hours to rate the sample.
The results from this test have been shown to be unaffected by different cementitious materials and the presence of conductive admixtures. The specimen does not experience a temperature rise during the test. The test also has a lower variability than the RCPT.(3, 4) An approximate correlation between the results of the Rapid Migration Test and the RCPT is shown in the table. It is believed that the Rapid Migration Test has significant advantages and its use will lead to improved evaluation of concrete quality in a chloride environment.
Results
- Hooton, R. D., Thomas, M. D. A., and Stanish, K., Prediction of Chloride Penetration in Concrete, Publication No. FHWA-RD-00-142, Department of Transportation, Federal Highway Administration, McLean, VA, 2001, 419 pp.
- Tang, L. and Nilsson, L. “Rapid Determination of the Chloride Diffusivity in Concrete by Applying an Electrical Field,” ACI Materials Journal, Vol. 89, No. 1, January-February 1992, pp. 49-53.
- Stanish, K., Hooton, R. D., and Thomas, M. D. A., “A Novel Method for Describing the Chloride Ion Transport Due to an Electrical Gradient in Concrete – Part I: Theoretical Description, and Part II: Experimental Evidence,” Cement and Concrete Research, Vol. 43, pp. 43-57, 2004.
- Tang, L. and Sorensen, H. E., “Evaluation of the Rapid Test Methods for Chloride Diffusion Coefficient of Concrete, Nordtest Project No. 1388-98,” SP Report 1998:42, Swedish National Testing and Research Institute, Boras, Sweden, 1998.
Editor’s Note
This article is the second in a series that describes tests for use with HPC. The first article appeared in Issue No. 36.