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Reinforcement

Cover is required for the reinforcement in concrete structures for protection against corrosion. Though several codes of practices have specified minimum cover for various climatic conditions, it is often not maintained in practice. For durable concrete structures, it is imperative that the steel embedded in concrete is protected adequately against corrosion. It is also necessary that concrete should be dense, uniform and free from deleterious components. For this, adequate cover to the steel should be provided on all faces of the concrete element. All reinforcement should be adequately tied together and properly supported to ensure that the reinforcement does not displace and move in position during the concreting process as this will affect the cover to bars and the effective functioning of the concrete member.

Functions of Concrete Cover

  • For prevention of corrosion in rebars
  • For providing proper bond to rebars
  • Resisting fire

The Problem

  • Concrete Cover – Critical and often neglected aspect of construction
  • Stone pieces, mosaic, marble or cudappah used as covering blocks which serve as improper cover

Draw Back of Site Made Cover Blocks

  • Allow ingress of water vapour and gases
  • Strength below 10 MPa

PVC Cover Blocks

  • Does not bond with concrete
  • Permit development of hair cracks
  • Water vapour seeps to the reinforcement
  • Melts in heat
  • Release toxic carbon monoxide upon burning
  • PVC cover made of recycled material deform during concreting

Concrete Cover Blocks:

  • Concrete to concrete bond perfect hence no hair cracks
  • Compressive strength not less than the surrounding concrete
  • Low water absorption
  • Sustains extreme heat
  • No deformation during concreting, ensure proper cover and fixity to rebars

Provisions as per IS 456-2000 (Relevant clauses are reproduced here)

8.2.3 Requirement of Concrete Cover

8.2.3.1 - The protection of the steel in concrete against corrosion depends upon an adequate thickness of good quality concrete.

8.2.3.2 - The nominal cover to the reinforcement shall be provided as per cl.26.4.

26.4 Nominal Cover to Reinforcement

26.4.1 Nominal Cover: Nominal cover is the design depth of concrete cover to all steel reinforcements, including links. It is the dimension used in design and indicated in the drawings. It shall be not less than the diameter of the bar.

26.4.2 Nominal Cover to Meet Durability Requirement: Minimum values for the nominal cover of normal weight aggregate concrete which should be provided to all reinforcement, including links depending on the condition of exposure described in 8.2.3 shall be as given in Table 16.


Exposure Nominal Concrete Cover in mm not less than
Mild 20
Moderate 30
Severe 45
Very severe 50
Extreme 75

Table 16, Nominal Cover to Meet Durability Requirements (Clause 26.4.2 IS 456)


NOTES

  • For main reinforcement up to 12 mm diameter bar for mild exposure the nominal cover may be reduced by 5 mm.
  • Unless specified otherwise, actual concrete cover should not deviate from the required nominal cover by +I0 mm
  • For exposure condition ‘severe’ and ‘very severe’, reduction of 5 mm may be made, where concrete grade is M35 and above.

26.4.2.1 - However for a longitudinal reinforcing bar in a column nominal cover shall in any case not be less than 40 mm, or less than the diameter of such bar. In the case of columns of minimum dimension of 200 mm or under, whose reinforcing bars do not exceed 12 mm, a nominal cover of 25 mm may be used.

26.4.2.2 - For footings minimum cover shall be 50 mm.


21 FIRE RESISTANCE

21.1 A structure or structural element required to have fire resistance should be designed to possess an appropriate degree of resistance to flame penetration; heat transmission and failure. The fire resistance of a structural element is expressed in terms of time in hours in accordance with IS 1641. Fire resistance of concrete elements depends upon details of member size, cover to steel reinforcement detailing and type of aggregate (normal weight or light weight) used in concrete. General requirements for fire protection are given in IS 1642.

21.2 Minimum requirements of concrete cover and member dimensions for normal-weight aggregate concrete members so as to have the required fire resistance shall be in accordance with 26.4.3 and Fig. 1 respectively of IS 456-2000.

21.3 The reinforcement detailing should reflect the changing pattern of the structural section and ensure that both individual elements and the structure as a whole contain adequate support, ties, bonds and anchorages for the required fire resistance.

21.3.1 - Additional measures such as application of fire resistant finishes, provision of fire resistant false ceilings and sacrificial steel in tensile zone, should be adopted in case the nominal cover required exceeds 40 mm for beams and 35 mm for slabs, to give protection against spalling.

21.4 Specialist literature may be referred to for determining fire resistance of the structures which have not been covered in Fig. 1 or Table 16A.


26.4.3 Nominal Cover to Meet Specified Period of Fire Resistance

Minimum values of nominal cover of normal-weight aggregate concrete to be provided to all reinforcement including links to meet specified period of fire resistance shall be given in Table 16A.

Fire Resistance Nominal Cover
Beams Slabs
 
Ribs Columns
Simply Supported Continuous
 
Simply Supported Continuous
 
Simply Supported Continuous
 
 
h mm mm mm mm mm mm mm
0.5 20 20 20 20 20 20 40
1 20 20 20 20 20 20 40
1.5 20 20 25 20 35 20 40
2 40 30 35 25 45 35 40
3 60 40 45 35 55 45 40
4 70 50 55 45 65 55 40

Table 16A Nominal Cover to Meet Specified Period of Fire Resistance (Clauses 21.4-and 26.4.3 and Fig. 1)  IS 456: 2000

NOTES

  • The nominal covers given relate specifically to the minimum member dimensions given in Fig.1
  • Cases that lie below the bold line require attention to the additional measures necessary to reduce the risks of spalling (see 21.3.1)

Acknowledgements: The content is sourced from IS-456, Astra Concrete Products and other open sources for knowledge dissemination.

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