Minimum concrete cover

Last Updated on Tue, 18 Oct 2016 | Concrete Structures

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

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

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	<<<	<<<	<<<	<<<	<<<	<<<
XC4		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
	XD3f	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	XF1	IIB-V, IIIA	-	-	-	BS 8500	C32/40, 0.45, 360	C28/35, 0.50, 340	C25/37, 0.55, 320	<<<
	XS1f	-	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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osman
What should be the minimum concrete cover?

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claudia
What should be the adquete concrete cover?

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Cerdic Took
How to calculate concrete structural column dimensions?

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dahlak
What is minimum concrete cover corrosion after eurocode?

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Annett
What is nominal cover to concrete?

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