Q: We're building a wastewater treatment plant with two concrete tanks that hold wastewater having a pH between 5 and 5.5. Is concrete adequately resistant to acids of this strength? Or do we need to use a protective coating?
A.: The question isn't easy to answer simply. By itself, pH isn't a true indicator of the aggressiveness of water. The pH has no direct relationship to the amount of acid present and therefore it can't be used alone to predict the degree of attack by the water. The neutral point on the pH scale is 7.0; values below 7 indicate acidity and values above 7 alkalinity.Water with a pH in the range of 7 to 8 is alkaline, but if it can dissolve calcium hydroxide it may still be aggressive to portland cement concrete. The dissolving power of such water depends on the relationship of total dissolved solids, temperature, hardness, alkalinity, and the pH of the water (Ref. 1).The Aggressiveness Index (AI) is a useful tool for determining whether or not a particular water is potentially damaging to concrete. The AI is defined in the American Water Works Association (AWWA) Standard for Asbestos-Cement Distribution Pipe (Ref. 2):
AI = pH logl0(AH)where A = total alkalinity in mg/l CaCO3
H = calcium hardness as mg/l CaCO3According to the AWWA document, the AI can be used to classify the water's potential effect on concrete as:
AI < 10.0
10.0 < AI < 11.9
AI < 12.0
Calculating the AI requires more information than just the pH, but it can be more useful than relying on relationships between pH and concrete damage. Concrete exposed to highly aggressive water would probably require a protective coating. Moderately aggressive water could probably be handled by using good quality concrete and good construction practices:
- Enough cement and a low enough water-cement ratio to produce dense concrete
- Air entrainment to provide freezing and thawing protection and reduce permeability
- Fly ash or other pozzolan that will combine with calcium hydroxide to form insoluble lime silicates
- Design for watertight contraction and construction joints that minimize intermediate cracking
- Adequate consolidation during placing
- Moist curing
More information on the effect of chemicals on concrete is found in a pamphlet by the Portland Cement Association, "Effects of Substances on Concrete and Guide to Protective Treatments" (Ref. 3).
1. Green, J. K., and P. H. Perkins, Concrete Liquid Retaining Structures Design, Specification and Construction, Applied Science Publishers Ltd., London, England, 1979.
2. AWWA Standard for Asbestos-Cement Distribution Pipe, 4 inch through 16 inch, for Water and Other Liquids. AWWA C400-77, American Water Works Association, Denver, Colorado, 1977.
3. "Effects of Substances on Concrete and Guide to Protective Treatments," IS001.04T, Portland Cement Association, Skokie, Illinois, 1981.