cover

In reinforced concrete, cover is the thickness of concrete that sits between the outermost surface of the structure and the nearest point of any reinforcing steel, whether rebar, wire mesh, or strand. This seemingly simple distance is one of the most critical dimensions in concrete design, because it determines how well the steel is protected from corrosion, fire, and mechanical damage.

The required cover depth varies by exposure condition and structural element. A basement wall in a dry environment might specify 40 millimetres of cover, while a marine structure exposed to saltwater spray could require 50 to 75 millimetres or more. Slabs-on-grade typically use 40 to 50 millimetres; columns and beams in normal conditions, 40 millimetres. These minimums exist because concrete's alkalinity creates a protective oxide layer on steel, but once that layer breaks down (carbonation or chloride ingress), unprotected steel rusts rapidly and expands, cracking the concrete.

How cover is controlled and measured

Cover is established during design and enforced during placement. Rebar is held at the correct distance from formwork edges using plastic or concrete spacers (chairs, blocks, or clips) clipped directly to the bars. The reinforcement cage is positioned in the formwork, spacers engage the form face, and concrete is then placed around it. Poor placement practice, such as pushing rebar too close to the forms while vibrating, is the most common cause of insufficient cover. Inspectors may verify cover by measuring from the concrete surface with a cover meter (electromagnetic), tapping test, or (destructively) breaking out samples.

Excessive cover wastes material and adds cost and dead load; insufficient cover invites corrosion and failure. Most building codes and standards specify cover as a range with tolerances, typically allowing 5 to 10 millimetres variance. In aggressive environments such as coastal zones, underground structures, or splash zones, cover requirements are tighter and sometimes doubled.

The term "cover" also applies to protective coatings on steel bars themselves, such as epoxy-coated rebar or stainless steel reinforcement, used in particularly harsh settings. These add cost but reduce the dependence on concrete depth alone for protection.