The reinforcement covering is required for the following reasons:
1- Protection of reinforcement from corrosion (rusting).
2- The need for adequate adhesion between steel and concrete.
3- Fire protection: the concrete that covers reinforcement protects it from deformations caused by the development of high temperatures due to fire in the building.
4- The need to create cable and pipe channels without harming the reinforcement
Slabs lower reinforcement cover
The minimum required cover thickness for slab reinforcement usually ranges between 20 and 30 mm depending on the environmental conditions present throughout the building’s service life. The 20 mm would apply to a dry climate and the 30 mm to a seaside location. The required cover is maintained only with the use of special stands called spacers. These must not be affected by corrosion and should be placed approximately every 1.00 m.
The simplest solution for providing the necessary cover thickness of the reinforcement is special plastic underlayers like the ones shown in the above figure. Usage of steel rebar spacers is forbidden as they are highly susceptible to corrosion.
When rebars corrode the resulting volume expansion leads to concrete spalling and consequently to the cracking of the plaster. Extensive deterioration affects not only the residents’ safety but also the structure’s service life.
The position of the upper (negative) slab reinforcement, either placed over the support between two slabs or over the support between a slab and a balcony (cantilever slab), can be secured only with the use of special rebar chairs.
Prefabricated element, made out of thin steel rebar with plastic-tipped legs in order to prevent corrosion of the support area between the rebar chair and the formwork.
Indirect, S-shaped mesh spacer
Prefabricated, comes in packages of straight lengths. It is formed in an S shape during the placement.
Indirect, folded mesh spacer
It is easily formed by folding of a standard wire mesh e.g. Ø8/200, to the desired height. In cases of cantilevers apart from spacer it can be used as “J-pin” reinforcement, necessary for the cohesion of the free edges.
Indirect “J-pin” rebar chair
For sheer use in slabs’ free edges
In cases where the mesh is used as upper reinforcement at the support area of a slab, its position can be secured with the use of an S-shaped mesh spacer placed on the lower reinforcement grate along the length of the plastic spacer.
Bended rebars as upper reinforcement at the support area of a slab
In cases where support upper reinforcement is formed using bend up span rebars, its proper placement is ensured by the reinforcement bending itself and therefore bar chairs might not be necessary
Support of the slab’s negative reinforcement with indirect S-shaped mesh spacers
Two indirect S-shaped mesh spacers are placed to the support on the left of the continuous slab. These are fitted on the lower reinforcement grate along the length of the linear plastic spacers.
Support of the negative slab’s reinforcement with rebar chairs and folded mesh spacers
It is mandatory to prevent the folded mesh spacer from lateral slipping and this can be achieved with the use of local spacers. They must be placed right after the placement of the folded mesh spacers and prior to its ties with the slabs’ reinforcement. When using wheel spacers extra attention should be paid to their vertical placement so as to avoid drifting during concreting. However, when they are used in slab “foreheads” (as shown in the above figure) they can be horizontally placed since concrete does not fall directly upon these areas.
Support of the negative slab’s reinforcement with indirect S-shaped mesh spacers and folded mesh spacers
Alternatively, when having a light-weight steel mesh as the lower reinforcement of a cantilever it is recommended to use indirect S-shaped mesh spacers instead of direct rebar chairs. In that case, it is more practical to place a “J-pin” mesh spacer allowing the mesh to be properly placed inside it.
Four-legged rebar chairs and “J-pin” rebar chair in a cantilever
Another solution, even when there is a wire mesh reinforcement on the lower side of the slab or the cantilever, would be to use direct four-legged rebar chairs rather than indirect S-shaped mesh spacers.
Steel rebar chairs
Use of steel rebar chairs directly placed upon the formwork is strictly forbidden and furthermore, the use of old type improvised spacers is more expensive
Beam reinforcement cover
Beam stirrups should be supported at the base of the beam, by a uniform inactive bar since all reinforcement loadings are transferred to these areas.
The lateral cover thickness should be ensured with the use of special plastic spacers. When using stirrup cage it is wiser to place the wheel spacers at the connecting rebars in order to secure their position during concrete casting.
Column reinforcement cover
The minimum required cover thickness for column rebars usually ranges between 25 and 35 mm depending on the environmental conditions present throughout the building’s service life. The 25 mm apply to a dry climate and the 35 mm to a seaside location. Ensuring the desirable cover of column reinforcement is quite a simple task. For example, four (4) individual spacers placed at the column’s upper part, are enough since the column’s base rebars are tied to the lap-splice bars.
As far as the columns integrated at the composite wall edges are concerned, the required cover thickness is ensured as described in above referring to the columns’ reinforcement covering. As far as the wall body reinforcement is concerned, its cover thickness is ensured as follows: After forming the back of the wall, plastic rods are nailed to the formwork. These rods have a usual length around 2.0 m and they can be used as one single piece or separate smaller pieces.
Shear wall reinforcement cover: 2nd phase
Next, the edge columns and the wall rebars are placed and then another set of spacers are fitted to the internal reinforcement grate. In this way, following the placement of the formwork’s last piece the required cover thickness and the proper centring of the reinforcement will be secured.
Foundation reinforcement cover
The minimum required a cover thickness of the foundation reinforcement is around 40 mm for foundation lying on a lean concrete layer and around 70 mm for the foundation in direct contact with the soil. The required covering may be created by a point or even better by linear spacers. Because of the weight, they bear and due to their required height, it is recommended to use heavy duty spacers.
Ensuring the proper position of upper reinforcement in foundation slabs
Just like in superstructure slabs, in the areas around the slab edges, “J-pin” rebars may be combined with open or closed reinforcement mesh.
In the central area, the required cover thickness can be created with the use of special steel rebar chairs placed on top of the lower reinforcement grate.