Q: An engineer has requested concrete with a continuously high pH to help protect underground conduit from corrosion due to magnetic-field induction. I have not heard of this, and I am looking for advice. All data I have found indicates that concrete has a pH of 12 to 13 in its plastic state, but after cement hydration the pH drops to 8 to 9. How can I maintain a continuously high-pH environment in the concrete?
A: Fresh concrete does have a pH of about 13. In hardened concrete, the pH at the level of the reinforcing steel may decrease for two reasons:
- Carbonation. The reaction with atmospheric carbon dioxide can drop the pH to 9 or slightly lower over a long time period.
- Reduction in alkalinity-generally by leaching of soluble alkaline materials as water passes through the concrete. Calcium hydroxide is the largest-volume alkali since it's a by-product of hydration. A well-hydrated portland cement contains 15% to 30% calcium hydroxide by weight of cement, and this is usually enough to maintain the pH at 13.
Also, when pozzolans such as fly ash are added to concrete, they combine with calcium hydroxide and lower the pH. Though one study showed that adding a pozzolan caused corrosion in a salt-water environment, another study showed that a pozzolan helped to prevent corrosion (see reference), probably because pozzolans create more hydrated cementitious product, thus lowering the permeability to air and chlorides.
Since the conduit and concrete are underground and not exposed to atmospheric carbon dioxide, carbonation isn't likely to be a problem. Leaching of soluble alkalis from the concrete also is unlikely unless the water-table level fluctuates at the elevation of the concrete housing the conduit. Thus, the underground concrete should maintain a high pH naturally without any effort by the designer.
Good concreting practices that reduce corrosion potential include a low water/cementitious-materials ratio and as large a depth of cover over the steel as can be justified by structural design considerations. The low w/cm helps to prevent leaching of alkalis, carbonation, and chloride intrusion. Check the surrounding soils or groundwater for the presence of chlorides. If chlorides are present, they will be the largest single environmental contributor to any corrosion problems. Then a corrosion inhibitor or other measures may be called for. The reference describes several strategies for rebar corrosion prevention.
ACI 222R-96, "Corrosion of Metals in Concrete," American Concrete Institute, Farmington Hills, Mich., 1996.