How to calculate cover for slab, beam and column using eurocode and BS code

1. COVER
Moofaj Buhari
CSEC

2. Contents
 Minimum cover
 Pros and cons

3. Cover blocks
 The cover is the distance between the surface of the reinforcement closest to
the nearest concrete surface.
 It should be sufficient in order to guarantee :
 the protection of the steel against corrosion;
 the safe transmission of bond forces;
 an adequate fire resistance.
Cover

4. Concrete cover
 On drawings the nominal cover should be specified. In EC 02, it is defined as a
minimum cover cmin plus an allowance in design for deviation Δcdev, so
4.4.1.1EC 02-1-1 Clause 1

5. Cmin
0 0
Note: The value of Δcdur,γ for use in a Country may be found in its National Annex. The
recommended value is
0 mm. (4.4.1.2 (6))
0
1 2 3
EC 02-1-1, 4.2

6. Minimum cover Cmin,b (with regard to bond)
(EC 02-1-1- Table 4.2)

7. Procedure to determine Cmin,dur
 EC-2 leaves the choice of cmin,dur to the countries,
 The value cmin,dur depends on the
 Structural class
 Exposure class

8. 1. Exposure classes
The exposure classes are defined in EN206-1. The main classes are:
 XO – no risk of corrosion or attack
 XC – risk of carbonation induced corrosion
 XD – risk of chloride-induced corrosion (other than sea water)
 XS – risk of chloride induced corrosion (sea water)
 XF – risk of freeze thaw attack
 XA – Chemical attack

9. Further specification of main exposure classes in
subclasses EC 02-1-1: Table 4.1

11. Difference between the classifications
carbonation induced corrosion
-Long term water contact

12. 2. Structural class
 If the specified service life is 50 years, the structural class is defined as 4. The
“structural class”can be modified in case of the following conditions:
 -The service life is 100 years in stead of 50 years
 -The concrete strength is higher than necessary
 -Slabs (position of reinforcement not affected by construction process
 -Special quality control measures apply
The finally applying service class can be calculated with Table 4.3N

13. Table for determining final Structural
Class – EC 02-1-1: Table 4.3N

15. Final determination of cmin,dur
 The value cmin,dur is finally determined as a function of the structural class and the
exposure class: EC 02-1-1: Table 4.4N
Slab
Beam and column

16. Allowance in design for deviation, Δcdev
EC 02-1-1: 4.4.1.3
 The determination of Δcdev is up to the countries to decide, but:
 Recommended value 10mm
 Reduction allowed if:

17. For an example
 For 16 mm bar column,
 Cmin,b = 16, Cmin,dur = 25
=max(16, 25, 10)
Cmin= 25 mm
= 25+10
= 35 mm
10 mm
So, cover for column is 35 mm

18. Fire resistance - slabs
Retrieved from IStructE EC02

19. Fire resistance- Beam
If the width of the beam is
more than the minimum in
Table 5.10 the cover may be
decreased as follows:
Retrieved from IStructE EC02

20. Fire resistance- Column
Retrieved from IStructE EC02

21. Fire resistance for minimum 2 hours
So, cover is = 35 mm
Hence the cover for column should be max(35,35)
Cover for column= 35 mm

22. Concrete cover - BS 8500
 The guidance in this annex applies to ordinary carbon steel reinforcement and
prestressing steel.
 Guidance on cover to stainless steel is not given
 Limited to XC, XD and XS exposure classes
 Compressive strength is included as an indirect control on these parameters.

23.  Table A.3 gives designated concretes that are suitable to resist carbonation-
induced corrosion in normal building structures (intended working life at least 50
years)
Designated concretes are not recommended for resisting chloride-induced
corrosion (XD and XS exposure classes).

24. Designated concretes cannot be used in foundations in exposure class AC-2
or higher
Ex: Column
Concrete C 40/50

26. Cement and combination type

27. Fire Resistance BS 8110-1, Table 3.4
 Generally, we consider exposure condition and fire rating when selecting the cover
to the reinforcement.
 Highest of the above will be selected as cover to the reinforcement.
Cover for fire resistance = 25 mm

28. Tolerance for cover
BS 8110-1: 1997

29. Strength of cover block
Ref- BS 7973 – 1 clause 6.3

30. Spacing distance

31. Slab Bottom Reinforcement Reference: BS 7973-2

32. Spacers for Slab Edges Reference: BS 7973-2
8.1.1.3.1 Vertical reinforcement
Vertical reinforcement, i.e. reinforcement at right angles
to the top and bottom surfaces of the slab
(including bent bars), shall be tied at every intersection
and have spacers on every other vertical bar
[see Figure 5a)].
8.1.1.3.2 Horizontal reinforcement parallel to the edge of
the slab
Horizontal reinforcement, parallel to the edge of the slab,
shall be tied at every other intersection and shall
have spacers at centres not exceeding 50d and not
exceeding 1 000 mm centres on each bar [see Figure
5b)].

33. Spacers for Beams Reference: BS 7973-2
8.2.1 General
The links or fabric to which the spacers are attached shall
be at the ends of the beam and at centres not
exceeding 1 000 mm along the beam [see Figure 7a)]

34. Spacers for Columns Reference: BS 7973-2
8.3 Spacers within columns
Links to which the spacers are attached shall be at the top,
middle and bottom of each lift of concrete, and
at centres not exceeding 100D [see Figure 8a)]

35. Carbonation of Concrete
 Carbonation is the formation of calcium carbonate (CaCO3) by a chemical
reaction in the concrete.
 It starts as soon as the concrete is exposed to air. Carbon dioxide begins to
penetrate the surface and react with calcium hydroxide within the concrete to
form calcium carbonate.
 The creation of calcium carbonate requires three equally important substances:
 carbon dioxide (CO2),
 calcium phases (Ca), and Ca(OH)2
 water (H2O).

36.  Soon the carbon dioxide reaches the passivating layer and begins to break it (the
passivating layer is the protective layer surrounding the reinforcing steel as a result
of the concrete’s alkalinity).
 Once the passivating layer is broken, the reinforcing steel is exposed to the effects
of air and water.
 The steel then begins to rust and expand putting pressure on the concrete and
causing cracks and spalls.
 Once carbonation has begun to affect the steel, the chance of failure in the
reinforced concrete member rises dramatically.

38. Deterioration of concrete
 Reinforcement corroding due to lack of cover

39. Penetration of corrosion stimulating
components in concrete

42. Insufficient cover to reinforcement
cage in forms
Correct cover blocks in place

43. Maximum cover to steel reinforcement
 No reference to maximum reinforcement cover is given
 BS EN 13670 Execution of concrete structures, has a maximum tolerance or
ΔC(plus). The minimum deviation is termed ΔC(minus).
 ΔC(plus) varies depending on the depth of the structure h and the Tolerance Class
of the structure. The National Structural Concrete Specification (NSCS) 4th edition
follows this standard, although

44. Potential problems of excessive concrete covers
 An increase in concrete cover the crack width will increase.
 The weight of the concrete structure is increased by an increase in concrete
cover.
Failure of member due to too
much cover

45. PVC Cover Blocks
 Does not bond with concrete
 Permit development of hair cracks
 Melts in heat
 Release toxic carbon monoxide upon burning
 PVC cover made of recycled material deform during concreting

46. Concrete Cover Blocks The Proper Solution
 Concrete to concrete bond perfect hence no hair cracks
 Low water absorption
 Sustains extreme heat
 No deformation during concreting, ensure proper cover and fixity to rebars
Cover block compressive strength should not less than the surrounding
concrete

47. Draw Back of Site Made Cover Blocks
 Porous
 Allow ingress of water vapour and gases
 Strength below 10 MPa