Concrete cover
As a concrete cover , partly concrete cover , in is reinforced concrete , the distance between the concrete surface and the outer edge of the concrete coated reinforcing steel , respectively.
Sufficient reinforcement cover, usually referred to as concrete cover or minimum concrete cover in Germany , is required for reinforced concrete components in order to ensure the necessary bond between concrete and reinforcement, sufficient durability of the component and a corresponding fire resistance . To create the required bond, the reinforcement cover should at least correspond to the diameter of the reinforcing steel. According to Eurocode 2 , taking into account the national annexes for the required durability (see also carbonation (concrete) ), the required concrete cover above the reinforcing steel results from the environmental conditions ( exposure classes ) . The values described therein assume a useful life of at least 50 years with normal maintenance costs. It must not be smaller than 15 mm, 20 to 50 mm are common. The concrete cover is guaranteed by spacers and support frames or cages. Support frames are usually made of appropriately bent reinforcing steel and are mainly used for the upper reinforcement layer of slabs. Spacers, on the other hand, are used to ensure the distance between reinforcement and formwork . These are available in various variations. It can be, among other things, blocks or snakes made of concrete or fiber cement; blocks, strips or rings made of plastic are also common. Metal spacers are rather uncommon these days as they can lead to rust spots in the concrete surface.
Purpose of the concrete cover
- Protection of the reinforcing steel from environmental influences, in particular from oxygen and water , in order to prevent corrosion .
- Protection of the reinforcement from the effects of fire
- Ensuring the bond between concrete and reinforcement (fixed integration of the steel in the concrete)
Guidelines
The concrete cover depends on the environmental influences that are to be expected at the respective location. They are regulated throughout Europe in Eurocode 2 according to exposure classes and the following concrete coverings are required there in requirement class S3 for the different corrosion triggers:
| Type of corrosion | Minimum concrete cover c min (mm) for reinforcing steel |
|---|---|
| carbonation-induced corrosion (XC) | 10 (XC 1) to 25 (XC 4) |
| chloride-induced corrosion (excluding seawater) (XD) | 30 (XD 1) to 40 (XD 3) |
| chloride-induced corrosion from seawater (XS) | 30 (XS 1) to 40 (XS 3) |
Depending on whether the guarantee of the bond or the corrosion protection are decisive, the values in the table must be increased by 10 to 15 mm so that nominal dimensions (c nom ) of 20 to 55 mm result. According to the general building inspectorate approval (abZ) for hot-dip galvanized reinforcing steel, it is possible to reduce the concrete cover by 10 mm when using hot-dip galvanized reinforcing steel in exposure classes XC1 to XC4.
Other national building standards prescribe different minimum concrete coverings depending on the local climate:
| country | Concrete standard | Area of minimum concrete cover (mm) |
|---|---|---|
| UK | BS: 8110 | 25-50 |
| United States | ACI: 318 | 40-50 |
| Australia | AS: 3600 | 15-30 |
Paradox
Large concrete coverings (50 to 60 mm) are necessary to protect the reinforcement against corrosion in the event of aggressive environmental influences, but large coverings also lead to large crack widths. Crack widths greater than 0.3 mm can lead to corrosion of the reinforcement due to the penetration of moisture and oxygen, which in turn causes the concrete to flake off, exposing the reinforcement. These contradicting effects can, for example, through the use of non-corroding reinforcement with a lower concrete cover or by non-corroding, finely distributed reinforcement (skin reinforcement) with the best possible bond, for example. B. hot-dip galvanized steel, stainless steel , GRP reinforcement, textile or fiber composite within the concrete cover.
Methods of maintaining the coverage
- Downward free surface (e.g. for ceilings): spacers in the form of small feet made of plastic, fiber cement are placed on the level formwork.
- Vertical surface (e.g. walls): Elongated, partly wave-shaped spacers made of fiber cement or plastic are tied to the outside of the reinforcement.
- Surface free at the top (e.g. for ceilings): Since the formwork is open at the top, support cages ensure that the upper reinforcement is covered.
Web links
Individual evidence
- ↑ General building authority approval Z 1.4-165. Retrieved May 6, 2020 .