Corrosion Induced By Chlorides Other Than From Seawater

Providing Essential Materials

Table 1

Exposure Classes

Table 1

Exposure Classes

Class

Class description

Informative example applicable to the United Kingdom

No risk of corrosion or attack (XO class)

X0

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

Unreinforced concrete surfaces inside structures. Unreinforced concrete completely buried in soil classed as AC-1 and with hydraulic gradiant not greater than 5. Unreinforced concrete permanently submerged in non-aggressive water. Unreinforced concrete in cyclic wet and dry conditions not subject to abrasion, freezing or chemical attack. NOTE: For reinforced concrete, use at least XC1.

Corrosion induced by carbonation (XC classes)9

(Where concrete containing reinforcement or other embedded metal is exposed to air and moisture.)

XC1

Dry or permanently wet.

Reinforced and prestressed concrete surfaces inside enclosed structures except areas of structures with high humidity. Reinforced and prestressed concrete surfaces permanently submerged in non-aggressive water.

XC2

Wet, rarely dry.

Reinforced and prestressed concrete completely buried in soil classed as AC-1 and with a hydraulic gradient not greater than 5. For other situations see 'chemical attack' section below.

XC3 & XC4

Moderate humidity or cyclic wet and dry.

External reinforced and prestressed concrete surfaces sheltered from, or exposed to, direct rain. Reinforced and prestressed concrete surfaces inside structures with high humidity (e.g. poorly ventilated, bathrooms, kitchens). Reinforced and prestressed concrete surfaces exposed to alternate wetting and drying.

Corrosion induced by chlorides other than from sea water (XD classes)9

(Where concrete containing reinforcement or other embedded metal is subject to contact with water containing chlorides, including de-icing salts, from sources other than from sea water.)

XD1

Moderate humidity.

Concrete surfaces exposed to airborne chlorides. Parts of structures exposed to occasional or slight chloride conditions.

XD2

Wet, rarely dry.

Reinforced and prestressed concrete surfaces totally immersed in water containing chloridesb.

XD3

Cyclic wet and dry.

Reinforced and prestressed concrete surfaces directly affected by de-icing salts or spray containing de-icing salts (e.g. walls; abutments and columns within 10 m of the carriageway; parapet edge beams and buried structures less than 1 m below carriageway level, pavements and car park slabs).

Corrosion induced by chlorides from sea water (XS classes)9

(Where concrete containing reinforcement or other embedded metal is subject to contact with chlorides from sea water or air carrying salt originating from sea water.)

XS1

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

External reinforced and prestressed concrete surfaces in coastal areas.

XS2

Permanently submerged.

Reinforced and prestressed concrete completely submerged and remaining saturated, e.g. concrete below mid-tide levelb.

XS3

Tidal, splash and spray zones.

Reinforced and prestressed concrete surfaces in the upper tidal zones and the splash and spray zones c.

Freeze/thaw attack (XF classes)

(Where concrete is exposed to significant attack from freeze/thaw cycles whilst wet.)

XF1

Moderate water saturation without de-icing agent.

Vertical concrete surfaces such as facades and columns exposed to rain and freezing. Non-vertical concrete surfaces not highly saturated, but exposed to freezing and to rain or water.

XF2

Moderate water saturation with de-icing agent

Elements such as parts of bridges, which would otherwise be classified as XF1 but which are exposed to de-icing salts either directly or as spray or run-off.

XF3

High water saturation without de-icing agent.

Horizontal concrete surfaces, such as parts of buildings, where water accumulates and which are exposed to freezing. Elements subjected to frequent splashing with water and exposed to freezing.

XF4

High water saturation with de-icing agent or sea waterd.

Horizontal concrete surfaces, such as roads and pavements, exposed to freezing and to de-icing salts either directly or as spray or run-off. Elements subjected to frequent splashing with water containing de-icing agents and exposed to freezing.

Chemical attack (ACEC classes)

(Where concrete is exposed to chemical attack.) Note: BS8500-1 refers to ACEC classes rather than XA classes used in BS EN 206-1

a The moisture condition relates to that in the concrete cover to reinforcement or other embedded develop a specification that is appropriate to the actual conditions likely to be encountered.

metal but, in many cases, conditions in the concrete cover can be taken as being that of the r , ... Tl ... . , Tl c Exposure XS3 covers a range of conditions.The most extreme conditions are in the spray zone.The surrounding environment.This might not be the case if there is a barrier between the concrete and , ..,.,, , ,. . . . . •

least extreme is in the tidal zone where conditions can be similar to those in XS2.The its environment.

recommendations given take into account the most extreme UK conditions within this class.

b Reinforced and prestressed concrete elements, where one surface is immersed in water containing . , ,, . , , , , , , d It is not normally necessary to classify in the XF4 exposure class those parts of structures located in chlorides and another is exposed to air, are potentially a more severe condition, especially where the , ,, . , . , . , , the United Kingdom which are in frequent contact with the sea.

dry side is at a high ambient temperature. Specialist advice should be sought where necessary, to

Table 2

Selected9 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 2

Selected9 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

-

C40/50,045, 340g or RC40/50XFg

<<<

<<<

<<<

<<<

<<<

<<<

XF3 (air entrained)

All except IVB-V

-

-

C30/37, 0.55, 300 plus air g,h

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

C25/30, 0.60, 280 plus airg,h, 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

C28/35, 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

Eurocode 24 recommends that 4cdev is taken as 10 mm, unless the fabrication is subjected to a quality assurance system where it is permitted to reduce 4cdev to 5 mm, or 0 mm if the element can be rejected if it is out of tolerance (e.g. precast elements).

Cement types and minimum cement content

Table 4 may be used to understand the cement/combination designations. It should be noted from Table 2 that the strength, water/cement ratio and minimum cement content may vary depending on the cement type used. In the UK, all cement/combinations are available (except SRPC), although in most concrete production plants either ground granulated blastfurnace slag or flyash (pfa) is available; not both. When using a designated concrete (see section below), it is not necessary to specify the types of cement/combinations.

Table 3

Compressive strength class for normal and heavyweight concrete

Explanation of the compressive strength class notation

C

40/

'50

'C' for normal

Cylinder

Cube

weight

strength6

strengths

concreteA

'LC' for

lightweight

concrete

A Includes heavyweight concrete

B Minimum characteristic 150 mm diameter by 300 mm cylinder strength, N/mm2

C Minimum characteristic cube strength, N/mm2

A Includes heavyweight concrete

B Minimum characteristic 150 mm diameter by 300 mm cylinder strength, N/mm2

C Minimum characteristic cube strength, N/mm2

Air content

Where air entrainment is required for exposure classes XF3 and XF4 the minimum air content by volume of 3.0%, 3.5% or 5.5% should be specified for 40 mm, 20 mm and 10 mm maximum aggregate size respectively.

Freeze/thaw aggregates

For exposure conditions XF3 and XF4 freeze/thaw resisting aggregates should be specified. The producer is then obliged to conform to the requirements given in BS 8500-2: 2006, Cl.4.3.

Aggressive ground

Where plain or reinforced concrete is in contact with the ground further checks are required to ensure durability. An aggressive chemical environment for concrete class (ACEC class) should be assessed for the site. BRE Special Digest 12 gives guidance on the assessment of the ACEC class and this is normally carried out as part of the interpretive reporting for a ground investigation. Knowing the ACEC class, a design chemical class (DC class) can be obtained from Table 5.

For designated concretes, an appropriate foundation concrete (FND designation) can be selected using Table 6; the cover should be determined from Table 2 for the applicable exposure classes. A FND concrete has the strength class of C25/30, therefore, where a higher strength is required a designed concrete should be specified. For designed concretes, the concrete producer should be advised of the DC-class (see section on specification).

Table 3

Compressive strength class for normal and heavyweight concrete

Example Compressive strength classes (BS 8500)

Designated concrete (BS 8500)

Previous Grade of concrete

(BS 53283 & BS 81105)

C20/25

RC20/25

C25

C25/30

RC25/30

C30

C28/35

RC28/35

C35

C30/37

RC30/37

-

C32/40

RC32/40

C40

C35/45

RC35/45

C45

C40/50

RC40/50

C50

C45/55

-

-

C50/60

-

C60

NOTE: Refer to BS 8500-1: 2006,Table A.20 for full list of Compressive strength classes.

Table 4

Cement and combination typea

Table 4

Cement and combination typea

Broad designationb

Composition

Cement/combination types (BS 8500)

CEM I

Portland cement

CEM I

SRPC

Sulfate-resisting Portland cement

SRPC

IIA

Portland cement with 6-20% of fly ash, ground granulated blastfurnace slag, limestone, or 6-10% silica fumec

CEM II/A-L, CEM II/A-LL, CIIA-L, CIIA-LL, CEM II/A-S, CIIA-S CEM II/A-V, CIIA-V CEM II/A-D

IIB-S

Portland cement with 21-35% ground granulated blastfurnace slag

CEM II/B-S, CIIB-S

IIB-V

Portland cement with 25-35% fly ash

CEM II/B-V, CIIB-V

IIB+SR

Portland cement with 25-35% fly ash

CEM II/B-V+SR, CIIB-V+SR

IIIAd,e

Portland cement with 36-65% ground granulated blastfurnace slag

CEM III/A,,CIIIA

IIIA+SRe

Portland cement with 36-65% ground granulated blastfurnace slag with additional requirements that enhance sulfate resistance

CEM III/A+SRf, CIII/A+SRf

IIIBe,g

Portland cement with 66-80% ground granulated blastfurnace slag

CEM III/B, CIIIB

IIIB+SRe

Portland cement with 66-80% ground granulated blastfurnace slag with additional requirements that enhance sulfate resistance

CEM III/B+SRf, CIIIB+SRf

IVB-V

Portland cement with 36-55% fly ash

CEM IV/B(V), CIVB

a There are a number of cements and combinations not listed in this table that may be specified for certain specialist applications. See BRE Special Digest 12. for the sulfate-resisting characteristics of other cements and combinations. b The use of these broad designations is sufficient for most applications.Where a more limited range of cement or combinations types is required, select from the notations given in BS 8500-2:2006,Table 1. c When IIA or IIA-D is specified, CEM I and silica fume may be combined in the concrete mixer using the tvalue concept; see BS EN 206-1:200, 5.2.5.2.3. d Where IIIA is specified, IIIA+SR may be used.

e Inclusive of low early strength option (see BS EN 197-4 and the "L" classes in BS 8500-2:2006,Table A.1.). f "+SR"" indicates additional restrictions related to sulfate resistance. See BS 8500-2:2006,Table 1, footnote D.

g Where IIIB is specified, IIIB+SR may be used.

Table 5

Selection of the DC-class and the number of Addition Protection Measures (APMs) where the hydrostatic head of groundwater is not more than five times the section width8, ^ ^ 4 e

Table 6

Guidance on selecting designated concrete for reinforced concrete foundations

Fire design

Having selected concrete cover and strength to meet the durability recommendations of BS 8500, the nominal cover should be checked in accordance with Eurocode 24, for fire cover.

Concrete cast against uneven surfaces

The nominal cover (i.e. minimum cover plus fixing tolerance) should be a minimum of 75 mm for concrete cast directly against the earth and 50 mm for concrete cast against blinding.

Abrasion

BS 8500 does not contain abrasion classes; instead reference should be made to BS 8204-26 or Concrete Society Technical Report 347 Tabie 7 summarises the factors that affect the abrasion resistance of floors.

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