The following letter was received from Doug Hooton of the University of Toronto concerning the Q & A about the Rapid Chloride Permeability Test that appeared in Issue No. 6, November/December 1999.
As Chairman of ASTM C 09.66 on Concrete Resistance to Fluid Penetration, I have examined a number of rapid tests, and ASTM C 1202 deserves to be seen more positively as a rapid index test of concrete quality. The five “Cons” that were raised in the article merit comment.
- “The test has poor correlation with ponding tests when different mixes are compared.” The reason for poor correlation with the AASHTO T259 test is largely due to insufficient data collection and poor analysis techniques specified in AASHTO T259.(1,2) When more appropriate analysis procedures are used on T259 results (using depth of chloride penetration rather then integrated chloride values), the results agree fairly well with ASTM C 1202 results (corrected for temperature rise).
- “The test is not a direct measure of chloride permeability. It only measures electrical conductivity of the concrete.” While true, conductivity or its reciprocal—resistivity—is a useful index of the connected pore structure in the concrete. One could argue that there are easier ways of determining this property.
- “Chloride ions only carry a small proportion of the current during the test, so the test is not specific to chloride.” This is also true. The current is carried by all the ions in the concrete’s pore solution. However, except in the case of admixtures such as calcium nitrite that leave conductive ions in pore solution, this is not a real concern for most concretes since, after 28 days, most pore solutions are mainly alkali hydroxides. ASTM C 1202 has a warning statement with respect to calcium nitrite.
- “It has been claimed that the test yields erroneous results when applied to silica fume concrete.” This claim(3) is based on AASHTO T259 data which when re-analysed (See Item 1), does not bear out in fact.(1,2) As well, other ponding (chloride bulk diffusion) tests and migration test results have also been found to relate very well to ASTM C 1202 results, regardless of silica fume’s presence.(4,5) In fact, the relative effect of silica fume addition on concrete’s chloride penetration resistance is predicted by ASTM C 1202 with as much accuracy as can be expected with any rapid test.
- “Self-heating of the specimen during the test affects test results.” While true for specimens that have charges in excess of about 2000 coulombs, this is above the values typically required for good quality concrete in bridge structures. A solution proposed for this problem of temperature-affected increases in conductivity is to take the 30-minute charge passed (before temperature rises) and multiply it by 12 to approximate the six-hour value.(1,4)
References
- McGrath, P. F. and Hooton, R. D., “A Re-Evaluation of the AASHTO T259 – 90 Day Salt Ponding Test,” Cement and Concrete Research, Vol. 29, 1999, pp. 1239-1248.
- Hooton, R. D., “Discussion of Durability Aspects of Precast Prestressed Concrete – Parts 1 and 2,” PCI Journal, Vol. 42, No. 3, May-June 1997, pp. 65-66.
- Sherman, M. R., MacDonald, D. B., and Pfeifer, D. W., “Durability Aspects of Precast Prestressed Concrete – Parts 1: Historical Review and 2: Chloride Permeability Study,” PCI Journal, Vol. 41, No. 4, July-August 1996, pp. 62-95.
- McGrath, P. F. and Hooton, R. D., “Influence of Binder Composition on Chloride Penetration Resistance of Concrete,” Proceedings of the Fourth International Conference on Durability of Concrete, Sydney, ACI SP-170, Vol. 1, Aug. 1997, pp. 331-347.
- Hooton, R. D., Pun, P., Kojundic, T., and Fidjestøl, P., “Influence of Silica Fume on Chloride Resistance of Concrete,” Proceedings, PCI/FHWA International Symposium on High Performance Concrete, New Orleans, Oct. 20-22, 1997, pp. 245-256.