What Is Cover in Concrete?
Cover in Concrete
The clear cover is the distance between the exposed concrete surface (without plaster and other finishes) to the nearest surface of the reinforcing bar Look into the following image for more detail.
Mostly, it describes as “ clear cover” or “minimum concrete cover” in general speaking on construction site.
Also, read: Procedure for Rcc Concrete
Different Types of Concrete Cover Blocks
- Cement Mortar Cover Block
- PVC Cover Block
- Aluminum Cover Block
- Steel Cover Block
How to Define Nominal Cover
In simple words, Cover is defined as the small space remain between the outside of the concrete surface to the reinforcement inserted inside of that concrete structure.
Here all pieces of information about the specifications for reinforcement cover for different structural members in different conditions.
Nominal Cover as Per Is 456 :
As per IS 456(Clause 26.4. 1), the term clear cover is replaced by the term Nominal cover.
The nominal cover is the distance between the exposed concrete surface to nearest reinforcement bar ( it may be any bar main bar, longitudinal bar and even links or stirrups).
Also Read: Slab Constructions
What Is Clear Cover?
The clear cover is the distance between the exposed concrete surface (without plaster and other finishes) to the nearest surface of the reinforcing bar Look into the following image for more detail
What Is Effective Cover?
The effective cover is the distance measured from the face of the member to the centre of the area ( centroid ) of reinforcement, i.e., tension or compression reinforcement. Effective cover = overall depth – effective depth. Or. Effective cover = clear cover + (diameter of bar/2)
Importance of Clear Cover in Concrete:
This sort of thing requires to maintain long-lasting life of the concrete structure or say RCC, some given below to elaborates its importance in structure.
Cover keep reinforcement bars to corrode due to weather’s worse effect. In case of fire, the concrete cover works as a thermal protector of reinforcement bars to shield it.
It provides adequate embedding to reinforcement bars to facilitate them to be stressed without losing grip.
The thickness or size of cover depends on the environmental circumstances and type of various structural members. It shall be obtained from the relevant code of practice.
The Concrete Cover Used in Various Parts of the Structure
Clear Cover for Beams :
Clear Cover in Beam
Beam or column, The minimum essential cover thickness usually ranges between 25mm to 35 mm depending on the ecological conditions present during the building’s once-over life.
The 25 mm apply to a dry climate and the 35 mm to a seaside location.
A cover placed in the beam at the bottom and sometimes cover the tie with the side face of steel. Plastic spacers also adaptable instead of concrete cover, according to its size of use.
Spacers with circular shapes preferable, when using the stirrup cage to place the connecting rebar to maintain their position during concrete casting.
Clear Cover for Slabs:
Clear Cover in Slabs
When rebars corrode, the resulting volume spreading out leads to concrete spalling and thus the cracking of the plaster.
Extensive corrosion affects not only the structure’s service life but also the residents’ safety. The minimum cover thickness for slab reinforcement generally between 20 and 30 mm depending on the ecological circumstances present during the building’s service life.
The essential cover is maintained only with the help of a particular structure known as spacers.
These must be unaffected by corrosion and should be positioned approximately every 1.00 m.
Clear Cover for Column
Minimum clear cover to the reinforcement: 25 mm to bars in columns. In large columns, say 450 mm in thickness, the cover should be 40 mm.
Clear Cover for Footings:
Clear cover in footings
Mostly in the case of footing structure, it directly relies on ground or soil, or in some cases, the PCC layer is there to get hard and level surface for mesh made by reinforcing bars.
This mesh rests on concert cover to maintain space between soil or bar to filled concrete in it, hence typically covered used is 70mm and 50 mm for PCC.
The minimum necessary cover thickness in the foundation reinforcement is around 40 mm for the foundation set on a concrete layer and approximately 70 mm for the foundation, which comes in touch with the soil.
The essential covering may be formed by a point or by linear spacers.
Allowable Concrete Cover for Reinforcement in all kind of structure:
The terms for reinforcement cover for various types of structural members in different circumstances given underneath.
According to relevant code, the concrete cover must have a minimum size of 25 mm, or less than twofold of the diameter of the bar should be provided. It should be placed at the end of the reinforcing bar.
In a beam, longitudinal reinforcement bars size shall not less than 30 mm or lesser compared to the diameter of the bar.
In the case of a longitudinal reinforcement bar in a column, the size of concrete cover not less than 40 mm or minimum of bar diameter whichever provided.
In this kind of case like, the dimension of Colum is 20 cm or less than the size of columns.
In this case, the reinforcement bar’s diameter does not exceed 12 mm, where the concrete cover of 25 mm is applicable for reinforcement.
When structure experiences tension, compressive shear, or other stresses, reinforcements bars in a slab or wall should not less than 15 mm and not less the diameter of the bar.
Where concrete structures outer face comes in direct contact with the weather or the ground after removing formwork, likewise footing sides, grade beams, retaining walls, and top slab, etc. its cover should not be less than 50 mm.
In unusual cases like concrete cover for footing, and main structural members where concrete filling directly comes in contact with soil or ground, 75 mm size is minimum for the cover of the bottom reinforcements bars.
When concrete is poured on the PCC layer, the cover size may be decreased up to 50 mm.
The area of constructions, where concrete works always in touch with marine weather or experiences harmful chemical attacks on the outer body. The only way is to an Increased cover thickness to protect surfaces exposed to the action of toxic chemicals (or exposed to ground polluted through such chemicals)smoke alkali, acid, saline atmosphere, sulfur, etc.
Retaining structures like retaining wall at the seashore, swimming pool, structure nearby rivers where the minimum cover to all-steel bars shall be 40mm or equal to the diameter of the main bar, whichever is greater.
In the direct contact of seawater and waters with destructive content and oils, the covers shall be increased by 10 mm.
Sheltering the reinforcement bars in case of concrete exposed to the damaging atmosphere also be given by only if an intense impervious concrete with standard defending coatings.
The accurate cover shall be maintained by cement mortar cubes and other permitted resources.
EIC allocated precast concrete block structure for reinforcements for footings, grade beams, and slabs on a sub-grade shall be rest on it.
Some time unskilled labor used pebbles or stones instead of concrete cover; however, all those things shall not be acceptable.
The minimum clear cover between reinforcing bars shall be in match with IS:456 – 2000 code of practice.
Clear Cover and Effective Cover
The effective cover is the distance between the outermost compression face of RCC to the centre of the area of main reinforcement in tension.
Hence effective cover = Clear cover + Total Diameter of Stirrup (d)+ (Diameter of main reinforcement bar (D))/2