The nominal cover refers to the minimum thickness of concrete cover provided to the reinforcement in structural elements. The values can vary based on factors like structural requirements, exposure conditions, and local building codes. Here are typical nominal cover values:
1. Slab: Usually, the nominal cover for slabs is around 20-30 mm.
2. Beam: For beams, the nominal cover is often in the range of 25-50 mm.
3. Column: Columns generally have a nominal cover of about 40-50 mm.
4. Staircase: Similar to slabs, staircases often have a nominal cover of around 20-30 mm.
5. Footing: The nominal cover for footings is typically in the range of 50-75 mm.
Hi guys in this article we know about nominal cover for foundation, footing, raft Foundation top,Raft Foundation bottom, strap beam, grade slab, column, beam, slab, flat slab, staircase,shear wall, retaining wall and lintel.
Nominal cover is followed by concrete cover is provided in all structure member like different types of footing, column, beam and slab. Nominal cover is provided for long lasting life of concrete and keep RCC structure free from corrosion and rust formation.
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What is nominal cover? Nominal cover is the least and minimum distance between outer face of concrete fibre and the outer face of main reinforcement with link excluding thickness of Plaster and finishing of concrete. Mostly cover is described as clear cover or nominal cover generally speaking at construction site.
According to IS code 456 -2000 clause 26.4.1 nominal cover can be defined as is the design depth of concrete cover to all Steel including link. So nominal concrete cover is also known as clear cover.
What is importance of nominal cover of concrete? function and importance of nominal cover of concrete are following:- 1) nominal cover of concrete provide and maintain long lasting life of concrete, 2) it keep reinforcement free from corrosion and rust formation and prevent from damaging due to moisture condition in environment and in soil, 3) outer face of concrete fibre is thermal insulator and protect reinforcement to sheild it, 4) it provides adequate embedding to reinforcement bar to fasilitate them to be stressed without losing grip, 5) it prevent reinforcement from environmental condition moisture and soil and 6) thickness or size of clear cover depends on the environmental circumstances and type of a structure member.
Nominal cover for slab, beam, column, staircase and footing
The size of nominal cover for slab, beam, column, staircase and footing should be depending on various factors like design of RCC structure, weather condition, wet and dry condition, heavy load structure, residential or commercial building,etc.
Generally, minimum size of nominal cover 40mm is apply for column, 25mm for beam,15 to 20 mm for slab, 15mm for staircase and 40 to 50mm for footing/Foundation.
What is nominal cover in RCC? According to IS code 456 -2000 clause 26.4.1 nominal cover can be defined as is the design depth of concrete cover to all Steel including link. Generally, 20mm to 75mm size of nominal cover is used in RCC work. Nominal cover size 40mm is apply for column, 25mm for beam,15 to 20 mm for slab, 15mm for staircase and 40 to 50mm for footing/Foundation.
Nominal cover for Beam :- size of nominal cover of beam should be ranging between 25 mm to 40 mm. Nominal cover of size 25 mm is apply in presence of dry climatic environment and 35 mm to 40 mm size should be taken in presence of wet climatic environment or sea side location.
As per IS 456:2000 clause 26.4.3 suggest that 40mm size of nominal cover used for simply supported beam and 30 mm for continuous beam with fire resistance 2 hours.
Clear cover for column :– the size of nominal cover for column should be ranging between 40mm to 50mm. Nominal cover of size 40 mm is good for dry weather condition and 50 mm for wet moist weather condition.
As per IS 456:2000 clause 26.4.2.1 suggest that column nominal cover size should not be less than 40mm using longitudinal reinforcement.
Nominal cover for slab :- the size of nominal cover of slab should be ranging between 20mm to 30 mm. Nominal cover of size 20mm apply for 4 to 5 inch thick rcc slab, 20mm for flat slab and 30mm for 6 inch thick slab.
As per IS 456:2000 clause 26.4.3 suggest that 20mm size of nominal cover used for simply supported beam and continuous RCC slab with fire resistance 1 hours. If fire resistance is 1.5 hours then nominal cover size should be 25mm.
Nominal cover for footing/foundation :- the size of nominal cover of footing/foundation should be 40 mm to 75 mm. Nominal cover size 40mm is used for general footing, 50mm is apply for Raft foundation top and 75mm for Raft foundation bottom/side.
As per IS 456:2000 clause 26.4.2.2 suggest that for Foundation footing nominal cover size should not be less than 50mm using longitudinal reinforcement with mesh bar.
Nominal cover for retaining wall:– the size of nominal cover for retaining wall should be ranging between 20 to 30 mm.
Nominal cover for strap beam:- the size of nominal cover for a strap beam should be ranging between 40mm to 50mm.
Nominal cover for shear wall:- size of nominal cover for shear wall should be a ranging between 20 to 25mm.
Nominal cover for staircase:- the size of nominal cover for staircase should be ranging between 15mm to 20mm.
Nominal cover for lintel :- the size of nominal cover for lintel should be ranging between 15mm to 20mm.
Nominal cover for mild exposure condition:- As per IS 456:2000 clause 26.4.2 suggest that for mild exposure condition nominal cover size should not be less than 20mm using longitudinal reinforcement with minimum M20 grade of rcc concrete.
Nominal cover for moderate exposure condition:- As per IS 456:2000 clause 26.4.2 suggest that for moderate exposure condition nominal cover size should not be less than 30mm using longitudinal reinforcement with minimum M20 grade of rcc concrete.
Nominal cover for severe climatic condition:- As per IS 456:2000 clause 26.4.2 suggest that for severe climatic condition nominal cover size should not be less than 45mm using longitudinal reinforcement with minimum M25 grade of rcc concrete.
Nominal cover for very severe climatic condition:- As per IS 456:2000 clause 26.4.2 suggest that for very severe climatic condition nominal cover size should not be less than 50mm using longitudinal reinforcement with minimum M35 grade of rcc concrete.
Nominal cover for extreme climatic condition:- As per IS 456:2000 clause 26.4.2 suggest that for extreme climatic condition nominal cover size should not be less than 75mm using longitudinal reinforcement with above M35 grade of rcc concrete will be used.
Why the size of nominal cover for footing is greater than other structure member?
As we know that Earth excavation done for footing beneath the soil, in foundation footing there is a sand filling ,brick soling layer, PCC layer rcc layer which is coming direct contact with moisture or water present in soil, moisture present in soil wants to damaging concrete fibre and rusting of reinforcement which is inserted inside concrete fibre.
To prevent from rusting and corrosion of reinforcement in soil footing size of nominal cover is provided is greater than other structural member like beam column and slab.
Conclusions:-
The nominal concrete cover for a reinforced concrete slab is approx. 20-30 mm, for a beam approx. 25-40 mm, for a column approx. 40-50 mm, for stairs approx. 20-30 mm and for footing approx. 50-75 mm .