Concrete cover
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Concrete cover is the region between the exposed concrete surface and the nearest surface of the reinforcing bar, in reinforced concrete members.
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[edit] Purpose of Provision of Concrete Cover
- to protect the reinforcement bars from environmental effects to prevent corrosion
- to protect the reinforcement bars from fire
- to give reinforcing bars sufficient embedment to enable them to be stressed without slipping
[edit] Guidelines
The amount of cover depth that needs to be provided is usually based on the exposure conditions that the structure is likely to be subjected. The Indian Concrete Code mandates the following:
| Exposure Condition | Nominal Cover (mm) |
|---|---|
| Mild | 20 |
| Moderate | 30 |
| Severe | 45 |
| Very Severe | 50 |
| Extreme | 75 |
Other national codes also specify minimum cover requirements based on their respective local exposure conditions.
| Country | Concrete Code | Range of Concrete Cover (mm) |
|---|---|---|
| UK | BS:8110 | 25-50 |
| USA | ACI:318 | 40-50 |
| Australia | AS:3600 | 15-30 |
[edit] Paradox
Large cover depths (50-75 mm) are required to protect reinforcement against corrosion in aggressive environments, but thick cover leads to increased crack widths in flexural reinforced concrete members. Large crack-widths (greater than 0.3 mm) permit ingress of moisture and chemical attack to the concrete, resulting in possible corrosion of reinforcement and deterioration of concrete. Therefore, thick covers defeat the very purpose for which it is provided. There is a need for judicious balance of cover depth and crack width requirements.
[edit] Methods of maintaining cover
 Metal plastic tipped bar chairs supporting rebar to give correct cover on a suspended slab with reinforced concrete masonry walls. |
 Plastic strip bar chairs supporting heavy rebar on suspended slab. |
 Effects of too little cover - poor concrete. |
