Q. I recently read some papers on the corrosion of concrete in sewer manholes. Apparently, this is not a problem of gasses floating around in the sewer, but an issue of anaerobic bacteria (Thiobacillus) living on the surface of the concrete, creating sulfuric acid, which corrodes the concrete.
The prevailing response has been to specify liners. However, there is an antimicrobial agent that does not allow Thiobacillus to live on (or in) the concrete. By eliminating the bacteria, you can eliminate the acid and the corrosion.
The other issue is the high sulfate content of sewage. This could be addressed by following Portland Cement Association’s “Design and Control of Concrete Mixtures” requirements for concrete exposed to sulfate-containing solutions, (i.e. low water cement ratio, use of a pozzolan, higher compressive strengths, low alkali cements). By designing for a “moderate” sulfate environment and using an antibacterial agent as an admixture, you could eliminate the need for a liner.
What other issues should be addressed in this type of application?
A. Liners, coatings, and materials designed to form a barrier on a concrete surface have been popular choices for years in attempting to preserve the integrity of concrete structures. Unfortunately, water and vapor transmission still allows chemical reactions to take place on the surface or in the concrete matrix. In addition to reactions with alkali, calcium and lime, a lowered pH causes the loss of the passivating layer on steel, setting up an environment for corrosion.
Instead of modifying a portland cement product, some have suggested neutralizing the alkali by chemical reaction and forming a solid in the gel pore/capillary system which prevents water/vapor transmission and any further chemical reaction. Once you seal the concrete against water and vapor, you also shut out the bacteria.
One product that takes this approach is StableCrete, from a Florida company named Conselcor (www.conselcor.com). An application of this product also leaves a paintable surface, which does not allow alkali to come to the surface "burning" a coating off.
If you prefer modifying the concrete mix to sealing the concrete surface, you can consider using an antimicrobial admixture. One source of information on this approach is Conshield Technologies Inc. (www.conshield.com), which also sells such an admixture.
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Aggregate Research Industries’ Web site (www.aggregateresearch.com) includes topic-specific forums where forum members can pose questions and respond, creating an interactive discussion group. These two questions and answers were based on ARI concrete construction forum postings.