Q.: Our pipe company has been asked to produce pipe to resist an acid solution with a pH of about 5.2. How can we calculate what the life expectancy of the pipe should be? What admixture, if any, would make the concrete resistant enough to prolong its life? Would it help to use sacrificial concrete?
A.: Calculation is an unlikely way to get an answer because there are too many unknowns of temperature, rate of flow and the rate at which reaction products are removed and new surface is exposed. We do not know of any admixture, other than a polymer used to make polymer concrete, that will significantly improve the acid resistance of portland cement concrete. Fly ash and other pozzolans will help, but will probably not give the amount of increased life needed. Switching from portland cement to calcium aluminate cement might also be helpful. Sacrificial concrete (made by using limestone as the aggregate) would prolong the life to some extent, but possibly not to the extent needed in a flowing system where there is an essentially inexhaustible supply of acid. Some pipes have survived for substantial periods in nonflowing, mildly acidic waters, as described in Hubert Woods' Durability of Concrete Construction, American Concrete Institute Monograph Number 4, pages 123-126.
The best way to ensure pipe longevity is to use a coating or liner. ACI 515R-79, "A Guide to the Use of Waterproofing, Dampproofing, Protective, and Decorative Barrier Systems," provides further information.