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Last edited 04 Feb 2021
Concrete to cover
Concrete cover, in reinforced concrete, is the least distance between the surface of embedded reinforcement and the outer surface of the concrete (ACI 130). The concrete cover depth can be measured with a cover meter.
The concrete cover must have a minimum thickness for three main reasons:
- To protect the steel reinforcement bars (rebars) from environmental effects to prevent their corrosion.
- To provide thermal insulation, which protects the reinforcement bars from fire.
- To give reinforcing bars sufficient embedding to enable them to be stressed without slipping.
The carbon steel of rebars is protected from oxidation by atmospheric oxygen by the high pH of concrete interstitial water. Iron bar surface is passivated as long as the pH value is higher than 10.5. Fresh cement water has a pH of about 13.5 while evolved cement water pH ~ 12.5 is controlled by the dissolution of calcium hydroxide (portlandite).
Carbon dioxide present in the air slowly diffuses through the concrete cover over the rebar and progressively reacts with the alkaline hydroxides (KOH, NaOH) and with calcium hydroxide leading to the carbonatation of the hydrated cement paste. As a result, the pH of the cement drops and when its value is below 10.5 – 9.5, steel surface is no longer passivated and starts to corrode.
A sufficient thickness of concrete cover is thus required in order to slow down the carbonatation process towards the rebar. The minimum concrete cover will depend on the environmental conditions encountered and must be thicker when the concrete is also exposed to moisture and chloride (proximity to the sea, use of de-icing salt for bridges or roads, and so on).
A high quality concrete made with a low water-to-cement (w/c) ratio will have a lower porosity and will be less permeable to water and to the ingress of corrosive species (dissolved oxygen, chloride, and so on). A thicker cover or a more compact concrete will also reduce the diffusion of CO2 in the concrete, protecting it better from carbonatation and maintaining a higher pH for a longer time period, increasing so the rebar service life.
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