Minimum concrete cover

The nominal cover can be assessed as follows: Cnom = Cmin+ D Cdev Exp. (4.1)

Where cmin should be set to satisfy the requirements below:

■ safe transmission of bond forces

■ durability

■ fire resistance and D cdev is an allowance which should be made in the design for deviations from the minimum cover. It should be taken as 10 mm, unless fabrication (i.e. construction) is subjected to a quality assurance system, in which case it is permitted to reduce D cdev to 5 mm.

Table 7

Exposure classes

Class

Description

No risk of corrosion or attack

X0

For concrete without reinforcement or embedded metal where there is no significant freeze/thaw, abrasion or chemical attack.

Corrosion induced by carbonation

XC1

Dry or permanently wet

XC2

Wet, rarely dry

XC3/4

Moderate humidity or cyclic wet and dry

Corrosion induced by chlorides other than from seawater

XD1

Moderate humidity

XD2

Wet, rarely dry

XD3

Cyclic wet and dry

Corrosion induced by chlorides from seawater

XS1

Exposed to airborne salt but not in direct contact with sea water

XS2

Permanently submerged

XS3

Tidal, splash and spray zones

Freeze/thaw with or without de-icing agents

XF1

Moderate water saturation without de-icing agent

XF2

Moderate water saturation with de-icing agent

XF3

High water saturation without de-icing agent

XF4

High water saturation with de-icing agent or sea water

Chemical attack (ACEC classes)

Refer to BS 8500-1 and Special Digest 111

Table 5

Characteristic tensile properties of reinforcement

Table 5

Characteristic tensile properties of reinforcement

Class (BS 4449) and designation (BS 8666)

A

B

C

Characteristic yield strength /yk orf0.2k (MPa)

500

500

500

Minimum value of k = ( ft/fy)k

> 1.05

> 1.08

> 1.15 < 1.35

Characteristic strain at maximum force euk (%)

> 2.5

> 5.0

> 7.5

Notes

1 Table derived from BS EN 1992-1-1 Annex C, BS 4449: 2005 and BS EN 1008010

2 The nomenclature used in BS 4449: 2005 differs from that used in BS EN 1992-1-1 Annex C and used here.

3 In accordance with BS 8666, class H may be specified, in which case class A, B or C may be supplied.

Table 6

Bending moment and shear co-efficients for beams

Moment

Shear

Outer support

25% of span moment

0.45 (G + Q)

Near middle of end span

0.090 Gl + 0.100 Ql

At first interior support

-0.094 (G + Q) l

0.63 (G + Q)a

At middle of interior spans

0.066 Gl + 0.086 Ql

At interior supports

-0.075 (G + Q) l

0.50 (G + Q)

Key a 0.55 (G + Q) may be used adjacent to the interior span.

1 Redistribution of support moments by 15% has been included.

2 Applicable to 3 or more spans only and where Qk < Gk.

3 Minimum span > 0.85 longest span.

4 l is the effective length, G is the total of the ULS permanent actions, Q is the total of the ULS variable actions.

National Annex (Table 4.3 (N) (BS)) gives durability requirements that comply with BS 8500, but which significantly modify the approach taken in Eurocode 2. To determine the minimum cover for durability (and also the strength class and minimum water cement ratio) either the UK National Annex or BS 8500 can be used.

The various exposure classes from BS 8500 are given in Table 7. Selected recommendations are given in Table 8 (on page 6) for the concrete strength, minimum cement ratio, minimum concrete cover and maximum cement content for various elements in a structure based on the exposure of that element.This is taken from Chapter 11, originally published as How to use BS 8500 with BS 811013.

Design for fire resistance

Eurocode 2 Part 1-2: Structural fire design", gives several methods for determining the fire resistance of concrete elements; further guidance can be obtained from specialist literature. Design for fire resistance may still be carried out by referring to tables to determine the minimum cover and dimensions for various elements, as set out below.

Rather than giving the minimum cover, the tabular method is based on nominal axis distance, a (see Figure 4). This is the distance from the centre of the main reinforcing bar to the surface of the member. It is a nominal (not minimum) dimension.The designer should ensure that a a cnom + f link + f bar /2.

There are three standard fire exposure conditions that may be satisfied: R Mechanical resistance for load bearing E Integrity of separation I Insulation

Tables 9 and 10 give the minimum dimensions for columns and slabs to meet the above conditions. The tables offer more flexibility than BS 8110 in that there are options available to the designer e.g. section sizes can be reduced by increasing the axis distance. Further information is given in Eurocode 2 and subsequent chapters, including design limitations and data for walls and beams.

Table 10

Minimum dimensions and axis distances for reinforced concrete slabs

Standard

Minimum dimensions (mm)

fire

One-way

Two-way spanning slab

Flat slab

Ribs in a two-v

ay spanning ribbed slab

resistance

spanning slab

ly/lx< 1.5

1.5 < ly/lx<2

(bmin is the width of the rib)

a =

80 20

80 10

80 15

a =

25

120 15

>200 10

a=

120 40

20

a=

160 45

190 40

>300 30

a=

175 65

175 40

175 50

a=

450 70

700 60

---

1 Refer to BS EN 1992-1-2 for design limitations.

2 a is the axis distance (see Figure 4).

3 hs is the slab thickness, including any non-combustible flooring.

Figure 4

Sections through structural members, showing nominal axis distance, a

Figure 4

Sections through structural members, showing nominal axis distance, a

Structur Concrete Column

Table 9

Minimum column dimensions and axis distances for columns with rectangular or circular section - method A

Table 9

Minimum column dimensions and axis distances for columns with rectangular or circular section - method A

Standard fire resistance

Minimum dimensions (mm] Column width ( famin)/axis <

istance (a) of the main bars

Column exposed on more than one side (m f i = 0.7)

Exposed on one side ( m fi = 0.7)

R 60

250/46 350/40

155/25

R 120

350/57* 450/51*

175/35

R 240

t

295/70

Notes

1 Refer to BS EN 1992-1-2 for design limitations.

2 m fi is the ratio of the design axial load under fire conditions to the design resistance of the column at normal temperature conditions. Conservatively m fi may be taken as 0.7

* Minimum 8 bars t Method B indicates 600/70 for R 240 and mfi = 0.7 and may be used. See EN 1992-1-2 Table 5.2b

Minimum cover for bond

The minimum cover to ensure adequate bond should not be less than the bar diameter, or equivalent bar diameter for bundled bars, unless the aggregate size is over 32 mm.

Minimum cover for durability

The recommendations for durability in Eurocode 2 are based on BS EN 206-112. In the UK the requirements of BS EN 206 -1 are applied through the complementary standard BS 8500. The UK

Table 10

Minimum dimensions and axis distances for reinforced concrete slabs

Table 8

Selected8 recommendations for normal-weight reinforced concrete quality for combined exposure classes and cover to reinforcement for at least a 50-year intended working life and 20 mm maximum aggregate size

Table 8

Selected8 recommendations for normal-weight reinforced concrete quality for combined exposure classes and cover to reinforcement for at least a 50-year intended working life and 20 mm maximum aggregate size

combination designations'1

Strength classc, maximum w/c ratio, minimum cement or combination content (kg/m3), and equivalent designated concrete (where applicable)

Typical example

Primary

Secondary

Nominal cover to reinforcementd

15 + D Cdev

20 + D Cdev

25 + D Cdev

30 + D Cdev

35 + D Cdev

40 + D Cdev

45 + D Cdev

50 + D Cdev

Internal mass concrete

X0

-

All

Recommended that this exposure is not applied to reinforced concrete

Internal elements (except humid locations)

XC1

-

All

C20/25, 0.70,240 or RC20/25

<<<

<<<

<<<

<<<

<<<

<<<

<<<

Buried concrete in AC-1 ground conditionse

XC2

AC-1

All

-

-

C25/30, 0.65,260 or RC25/30

<<<

<<<

<<<

<<<

<<<

Vertical surface protected from direct rainfall

-

All except IVB-V

-

C40/50, 0.45,340 or RC40/50

C30/37, 0.55,300 or RC30/37

C28/35, 0.60,280 or RC28/35

C25/30, 0.65, 260 or RC25/30

<<<

<<<

<<<

Exposed vertical surfaces

XC3

XF1

All except IVB-V

-

C40/50, 0.45, 340 or RC40/50

C30/37, 0.55,300 or RC30/37

C28/35, 0.60, 280 or RC28/35

<<<

<<<

<<<

<<<

XC4

XF3

All except IVB-V

-

C40/50,0.45, 340g or RC40/50XFg

<<<

<<<

<<<

<<<

<<<

<<<

XF3 (air entrained)

All except IVB-V

-

-

C30/37, 0.515, 370 plus air&h

C28/35, 0.60, 280 plus airg,h or PAV2

C25/30, 0.60, 280 plus air g,h,j or PAV1

<<<

<<<

<<<

Elements subject to airborne chlorides

XD1f

-

All

-

-

C40/50, 0.45, 360

C32/40, 0.55, 320

C28/35, 0.60, 300

<<<

<<<

<<<

Car park decks and areas subject to de-icing spray

IIB-V, IIIA

-

-

-

-

-

C35/45, 0.40, 380

C32/40, 0.45, 360

C28/35, 0.50, 340

-

CEM I, IIA, IIB-S, SRPC

-

-

-

-

-

BS 8500

C40/50, 0.40, 380

C35/45, 0.45, 360

IIIB, IVB-V

-

-

-

-

-

C32/40, 0.40, 380

C28/35, 0.45, 360

C25/30, 0.50, 340

Vertical elements subject to de-icing spray and freezing

XD3f

IIB-V, IIIA

-

-

-

-

-

C35/45, 0.40, 380

C32/40, 0.45, 360

C32/40, 0.50, 340

XF2

CEM I, IIA, IIB-S, SRPC

-

-

-

-

-

BS 8500

C40/50, 0.40, 380

C35/45, 0.45, 360

IIIB, IVB-V

-

-

-

-

-

C32/40, 0.40, 380

C32/40 0.45, 360

C32/40, 0.50, 340

Car park decks, ramps and external areas subject to freezing and de-icing salts

XF4

CEM I, IIA, IIB-S, SRPC

-

-

-

-

-

BS 8500

C40/50, 0.40, 380g

<<<

XF4 (air entrained)

IIB-V, IIIA, IIIB

-

-

-

-

-

C28/35, 0.40,380g,h

C28/35 0.45,360g,h

C28/35, 0.50,340g,h

XF1

CEM I, IIA, IIB-S, SRPC

-

-

-

BS 8500

C35/45, 0.45, 360

C32/40, 0.50, 340

<<<

<<<

Exposed vertical surfaces near coast

XS1f

IIB-V, IIIA

-

-

-

BS 8500

C32/40, 0.45, 360

C28/35, 0.50, 340

C25/37, 0.55, 320

<<<

-

IIIB

-

-

-

C32/40, 0.40, 380

C25/30, 0.50, 340

C25/30, 0.50, 340

C25/30, 0.55, 320

<<<

Exposed horizontal surfaces near coast

XF3 or XF4

CEM I, IIA, IIB-S, SRPC

-

-

-

BS 8500

C40/50, 0.45, 360g

<<<

<<<

<<<

Key a This table comprises a selection of common exposure class combinations. Requirements for other sets of exposure classes, e.g. XD2, XS2 and XS3 should be derived from BS 8500-1: 2006, Annex A. b See BS 8500-2,Table 1. (CEM I is Portland cement, IIA to IVB are cement combinations.) c For prestressed concrete the minimum strength class should be C28/35.

d D cjev is an allowance for deviations. _

e For sections less than 140 mm thick refer to BS 8500. f Also adequate for exposure class XC3/4. g Freeze/thaw resisting aggregates should be specified. h Air entrained concrete is required. j This option may not be suitable for areas subject to severe abrasion.

Not recommended

Indicates that concrete quality in cell to the left should not be reduced

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