Rachel Detwiler, Braun Intertec
Three factors are important for the sulfate resistance of concrete — the severity of exposure, selection of cementitious materials, and concrete permeability.
Exposure
The severity of exposure needs to be determined based on the concentration of sulfate in the water or soil. Exposure conditions are defined as negligible, moderate, severe, or very severe as shown in the table. If there is any doubt as to whether sulfate attack could occur in a given situation, the concrete materials should be selected for the next more severe exposure.
Test Methods
In the last few years, there has been some debate regarding the determination of the concentration of sulfate in soil(1) due to the lack of a standard test method. The United States Bureau of Reclamation (USBR) originated the requirements for concrete exposed to sulfates based on its own test method.(2) ASTM C 1580, “Standard Test Method for Water-Soluble Sulfate in Soil,” is similar to the USBR test method. Both methods prescribe a quantity of water to extract the sulfate from the soil sample. The concentration of the sulfate in solution is used to determine the quantity of sulfate in the original sample. If a different test method is used, the limits on sulfate content should be adjusted to maintain the same degree of conservatism.
Once the exposure is characterized, the cementitious materials to mitigate sulfate attack may be selected. One approach is to specify Type II cement for moderate exposure and Type V cement for severe and very severe exposures. The requirement for sulfate resistance may be met either by the optional chemical requirement or by the optional physical requirement, as specified by the purchaser. The chemical requirement limits the calculated tricalcium aluminate (C3A) content of the cement since it is the hydration product of the C3A that is most vulnerable to chemical attack. The optional physical requirement limits the maximum expansion as measured using ASTM C 452 to 0.040 percent at 14 days for Type V cement. In ASTM C 452, “Standard Test Method for Potential Expansion of Portland-Cement Mortars Exposed to Sulfate,” mortar bars made with the cement and additional gypsum are stored in water at 73°F (23°C) and the expansion measured between ages of 1 and 14 days.
Another approach to mitigate sulfate attack is to use a blended cement or combination of cementitious materials such as portland cement and fly ash or slag. Appropriate combinations of these materials can be more effective than the use of Type II or Type V cements. ASTM C 1157 for blended cements limits the expansion at 6 months to a maximum of 0.10 percent for moderate sulfate resistance (MS) or 0.05 percent for high sulfate resistance (HS). A one-year limit of 0.10 percent applies for high sulfate resistance if the 6-month expansion exceeds 0.05 percent. The applicable test method is ASTM C 1012, “Standard Test Method for Length Change of Hydraulic-Cement Mortars Exposed to a Sulfate Solution,” which can be used for portland cements, blended cements, or combinations of cementitious materials. In this test, mortar bars are stored in a sulfate solution at 23°C (73°F). Typically, 50 g/L sodium sulfate solution is used but other concentrations and/or other sulfates may be used to simulate field conditions. The length changes of the bars are measured periodically.
Concrete Permeability
The use of sulfate-resistant cementitious materials is necessary but not sufficient to make sulfate-resistant concrete. The concrete must also have low permeability. Low permeability does not automatically follow from low w/cm or high strength. Good concreting practices, particularly proper curing, are essential. At least 7 days wet curing at moderate temperatures should be specified and enforced. Longer curing times are necessary at lower temperatures.
Summary
When concrete will be exposed to sulfates, the exposure must be characterized using the USBR test or ASTM C 1580. The appropriate requirements for cementitious materials and concrete quality can then be determined from the table. The cementitious materials must meet the requirements of ASTM C 150 for sulfate resistance with portland cement or ASTM C 1157 for combinations of cementitious materials. In addition, the w/cm must be limited and careful attention must be paid to good concreting practices, particularly curing.
References
- Rebel, B., Detwiler, R. J., Gebler, S. H., and Hooton, R. D., “The Right Sulfate Test Makes a Difference,” Concrete International, Vol. 27, No. 2, February 2005, pp. 49-52.
- Method of Test for Determining the Quantity of Soluble Sulfate in Solid (Soil or Rock) and Water Samples, United States Department of the Interior, Bureau of Reclamation, 1973.
- Adapted from Kosmatka, S. H., Kerkhoff, B., and Panarese, W. C., Design and Control of Concrete Mixtures, EB001, 14th Edition, Portland Cement Association, Skokie, IL, 2002, p. 28.
Editor’s Note
This article is the fourth in a series that describes tests for use with HPC. Previous articles appeared in Issue Nos. 36, 37, and 39.