Cover: Concrete's Steel Secret

In everyday language, a cover is something placed over an object to conceal or protect it, whether a blanket, a lid, or a roof. In construction, however, cover refers to a specific measurement: the thickness of concrete between the outermost surface of a reinforced concrete member and the surface of the reinforcing steel bars or mesh embedded within it. This distance is measured in millimetres or inches and is a critical design parameter.

Cover exists for two essential reasons. First, concrete protects steel from corrosion by maintaining an alkaline environment around the bars. Second, cover must be sufficient to ensure that steel reaches the concrete's surface only after the concrete itself has failed structurally. The required cover thickness depends on the concrete's exposure class, the design life of the structure, and the quality of the concrete itself. A bridge deck exposed to saltwater may require 50 to 75 millimetres of cover, while an interior floor slab might require only 20 millimetres. If cover is too thin, water and chlorides penetrate to the steel, which begins to rust and expand, cracking the concrete. If cover is excessive, the reinforcing steel becomes ineffective and the concrete uneconomical.

Cover is maintained on site through spacers, chairs, and distance blocks attached to the reinforcing steel cage before concrete is poured. Inspectors measure cover with special cover meters that use electromagnetic induction to locate the steel and calculate the distance. The tolerance for cover is typically plus or minus 5 millimetres, though this can vary by standard and specification.

Other construction trades use cover in related but distinct senses. In roofing, double coverage means applying shingles or slate so that the overlap is at least two inches wider than the exposed portion, creating a two-layer barrier. In tunnelling, cut and cover