Q: I have run into a new twist on an old routine detail (see drawing, below) in our area near the seacoast. The local code, the American Concrete Institute (ACI), and most construction plans all require a minimum of 3 inches of concrete cover over rebar in grade beams poured in earth trenches and not formed. I am inspecting the work of a contractor who is arguing that the engineered fill will settle an inch a year but that the beams themselves will not settle because they are supported by piles. His theory is that there will soon be a separation between earth and concrete; consequently 1 1/2 inches of concrete cover is all that is necessary. The building is only about 200 feet from the waterfront. That means that the beam soffit, at 4 feet above mean sea level, is in a corrosive environment. I say the contractor is wrong and 3 inches minimum cover is required.
A.: Our inquiry to the ACI reveals that the 3-inch cover requirement is intended to compensate for the possibility of low-quality concrete in this kind of construction. When concrete is placed directly on soil there is a greater likelihood that the soil will absorb water from the mix and that the concrete in direct contact with the earth won't be of as high a quality as the rest of the structural element. The fact that the earth may later settle away from the underside of the grade beam has little bearing on the requirement for a 3-inch cover. We note that Section 7.7.1 of the ACI Building Code, which requires a minimum cover of 3 inches, indicates that this is for "concrete cast against and permanently exposed to (italics ours) earth." Perhaps that's the contractor's basis for his reasoning. ACI tells us that code interpretations can be made by the ACI committee that wrote the code but that this is a lengthy and expensive process. But 3 inches of concrete cover is required.