concrete general guidance

8/10/2019 Concrete General Guidance

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E10 General guidance

1 New concrete standards 1.1 BS EN 206-1 and BS 8500 1.2 Concepts andterminology in BS EN 206-1 and BS 8500 1.2.1 Specifier 1.2.2 Exposure anddesign chemical classes (X and DC) 1.2.3 Intended working life 1.2.4 Cover toreinforcement 1.2.5 Constituent materials 1.2.6 Characteristics 1.2.7 Productionand conformity 1.2.8 Testing 2 Methods of specifying concrete 2.1 Designatedconcrete (clause 105) 2.2 Standardized prescribed concrete (clauses 125 and160) 2.3 Designed concrete (clause 132) 2.4 Prescribed concrete 2.5Responsibility for concrete specification 3 Factors affecting durability ofconcrete 4 Selecting concrete for durability 4.1 Exposure classes 4.2 Selectionof designated and designed concrete 4.2.1 Designated concrete 4.2.2

Designed concrete 4.3 Designated and designed concrete exposed tochemical attack (AC exposure class) 4.3.1 Additional protective measures(APMs) 4.3.2 Forms of chemical attack outside the scope of BS 8500 5Cements (CEM) and Combinations (C) 5.1 Selection for designated anddesigned concretes 5.2 Type II additions fly ash, pfa, ggbs, microsilica andmetakaolin 6 Consistence, compaction and curing 6.1 Consistence andcompaction 6.2 Curing 7 Environmental issues 7.1 Sources of information 7.2Recycled and secondary aggregates 7.3 Recycling fresh concrete 8 Health andsafety

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1 New concrete standards

1.1 BS EN 206-1 and BS 8500

BS EN 206-1 and complementary Standard BS 8500 deal with thespecification of in situ concrete (including lightweight) and also concrete forprecast structures and products. The standards are quite complex and have tobe used together. To aid the specifier, BSI have produced a single deriveddocument BIP 2001 Standards for fresh concrete . This document includesthe requirements of both BS EN 206-1 and BS 8500 , but organizes them in amore assimilable way and adds guidance and commentaries on the clauses,but note the revision of the standards to which the guidance applies.

BS 8500 covers a wider range of aggressive ground conditions than BS EN

206-1 , based upon the recommendations of BRE Special Digest 1 .


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For an introduction to the standards see Concrete Society Guide CS 149 , andBCA Specifying concrete to BS EN 206-1/ BS 8500 consisting of ten parts,but note the revision of the standards to which the guidance applies.

The 2006 revision of BS 8500 takes into account recent research andintroduces a number of changes in the way concrete quality is specified. Forconcrete in aggressive ground, these changes include:

The concept of aggregate carbonate range has been deleted, togetherwith the starred and double-starred Design Chemical Classes anddesignated concrete mixes have been deleted.

Amendments have been made in maximum water:cement ratio andminimum cement/combination content to resist sulfate attack.

It is no longer necessary to adopt more than one additional protectivemeasure (APM), and the number of APMs has been reduced at highersulfate levels.

The concept of structural performance level has been replaced byintended working life in harmony with Eurocodes and BS EN 206-1 .

DC class to cater for assessed ACEC conditions is only tabulated forsection widths of 140450 mm. Specifiers/ designers must makeadjustments for widths outside this range.

Relaxation for surface carbonation is restricted to precast concrete.

Strength class of FND designated concretes has been reduced to C25/30

Other changes affect:

Resistance to chloride-induced corrosion, and chloride class for post-tensioned prestressed concrete.

Concrete quality in long life structures.

Designations for cement/ combinations.

Use of recycled aggregate (RA) and recycled concrete aggregate (RCA),and test methods for demonstrating their compliance.

1.2 Concepts and terminology in BS EN 206-1 and BS 8500

1.2.1 Specifier

The term specifier refers to the organization in the specification chain thatpasses the specification for the type of concrete to the concrete producer. This

is usually the contractor or the purchaser of fresh concrete. Specifications


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produced by the designer should utilise the skills of the contractor andconcrete producer by not being unnecessarily prescriptive.

1.2.2 Exposure and design chemical classes (X and DC)

Conditions to which each concrete element and surface is exposed areclassified and also related to a specific deterioration process.

1.2.3 Intended working life

The concept of intended working life for structures is one of the criteria fedinto the design process for selecting the type of concrete. See 4.

1.2.4 Cover to reinforcement

Nominal cover is expressed as: Minimum cover (basis for design for durability)+c (a tolerance for fixing). c is typically 515 mm and is selected accordingto the type of construction and standard of quality control available on site.BS 8110-1 , clause 3.3.1 and BS EN 1992-1-1 , clause 4.4.1.3 recommend thatc be 10 mm unless special steps are taken to assure cover. For concrete inground containing chlorides, BS 8500 , table A.9 requires c of at least 25 mmfor concrete cast against blinding and 50 mm for concrete cast directly againstsoil.

1.2.5 Constituent materials

Established suitability:This concept allows materials to be used that are notcovered currently by European standards, but have a satisfactory history ofuse by concrete producers.

Cement additions:Type I additions are inert materials (e.g. pigment; filleraggregate). Type II additions have pozzolanic or latent hydraulic properties(e.g. pulverized-fuel ash, silica fume, ground granulated blastfurnace slag,

metakaolin).

Cements/ Combinations:See 5.

Recycled aggregates:See 7.

Normal and heavyweight aggregates: These have an oven dried densitygreater than 2000 kg/m and are required to conform to BS EN 12620 .

Lightweight aggregates: These are of mineral origin with particle densities

not exceeding 2000 kg/m, or loose bulk densities not exceeding 1200 kg/m,and are required to conform to BS EN 13055-1 .


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Additional aggregate requirements: These are requirements for specialcircumstances. Requirements may include:

Freeze thaw resistant aggregate is recommended for exposure classes

XF3 and XF4.

Rounded aggregate: May be preferred when concrete is to be pumped ortremied.

Aggregates for wearing surfaces: Use of special classes of coarseaggregates for wearing surfaces is rarely necessary, because the defaultset for normal and heavyweight aggregate by BS 8500-2 , clause 4.3, isregarded as suitable for most industrial floors (see Concrete SocietyReport 34 ).

Fire resistance of concrete: Can be improved by use of aggregates with

lower thermal expansion, e.g. limestone.

Aggregate drying shrinkage: The vast majority of UK aggregates produceconcrete with a drying shrinkage well below the default limit of 0.075%set in BS 8500-2 , clause 4.3. However, in a few areas of the UK (notablycentral Scotland), it is possible that aggregates conforming to this limitmay not be readily available.

1.2.6 Characteristics

Consistence:Formerly known as workability. A consistence class system isused.

Compressive strength class:This is a dual classification system using theminimum characteristic strengths derived from the cylinder strength (300 x150 mm diameter used in some European countries) and the cube strength(150 mm used in the UK). For example, in a C25/30 concrete, 25 is theminimum characteristic cylinder strength and 30 the minimum characteristiccube strength.

Chloride content class:This classification describes the maximum chloridecontent of fresh concrete. For example, Cl 0.40 means that the concrete has amaximum chloride ion content of 0.40% by mass of cement/ combination.

Density class:This applies to lightweight concrete. A target density is used forheavyweight concrete. For lightweight concrete there are six density classes.

1.2.7 Production and conformity

Detailed requirements for production control and conformity control areprovided in the Standards. Significantly, the concrete producer is required to


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determine conformity of all concretes produced during an assessment period,and must declare any nonconformity that is not obvious at the time ofdelivery these will include strength, maximum water:cement ratio andminimum cement content. See clause 215.

1.2.8 Testing

Identity testing is acceptance testing in all but name. It is carried out on siteeither on a regular basis when included in the specification or as spot checkswhere there is doubt about quality. Regular identity testing for quality controlis usually only necessary for designed concrete without third partycertification, but may be necessary to justify design assumptions when usingprescribed or proprietary concrete (see 2.4and 2.5). It is quite separate frominitial testing (conformity testing) which is part of the producers conformity

control procedure for designated and designed concrete.

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2 Methods of specifying concrete

2.1 Designated concrete (clause 105)

Specifying designated concrete is the most straightforward method ofprocuring concrete. Designated concretes are in essence a range of productsof certified quality, stated to be suitable for defined purposes, and specifierselects a product suitable for purpose under the known conditions ofexposure and chemical attack. Designated concretes are suitable for a widerange of applications see table 4. Situations where designated concretes arenot suitable include:

Reinforced concrete exposed to chlorides (exposure classes XD1, XD2 andXD3) or sea water (exposure classes XS1, XS2 and XS3), or de-icing saltscontaining chlorides (exposure classes XF2 and XF4). See 4.

Lightweight or heavyweight concrete.

Required strength classes not within the range for designated concrete see BS 8500-1 , table A.14. There is no option of selecting a higherstrength class.

Special cements/ combinations are required (e.g. to control heat ofhydration).

Required proportions/ attributes differ from those given in BS 8500-2 ,


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table 5 (e.g. maximum water:cement ratio and cement content for waterresistant concrete).

Designated concrete can only be produced by plants having accredited

production control and product conformity certification. Identity testing (see1.2) is not normally necessary. For selection procedures for designatedconcrete see 4.2.

2.2 Standardized prescribed concrete (clauses 125 and 160)

Standardized prescribed concretes are site mixed alternatives for ready-mixeddesignated concretes that can be:

Specified when a project contains very small quantities of concrete, e.g.for padstones (clause 160).

Used in lieu of specified designated concrete (clause 125).

Standardized prescribed concrete can be produced by plants withoutaccredited production control and product conformity certification, and hencecan be site mixed. Clause 218can be used to control maximum pour size forsite mixed concrete.

2.3 Designed concrete (clause 132)

Designed concrete can be used for all exposure classes. As with designatedconcrete, the concrete producer must determine conformity of the concrete tothe specification (see 1.2.7).

For selection procedures for designed concrete see 4.2.

2.4 Prescribed concrete

It is generally preferable to specify designated concrete or designed concrete.With prescribed concrete, it is the designer who is responsible for determiningthe mix proportions and ensuring that the concrete meets the intendedperformance. However, producers can be made responsible for ensuring thatthe specified mix proportions will not promote damaging alkali-silica reaction(ASR). Either trial mixes or experience based upon previous examples are usedto verify the finish and constituent materials and other mix characteristics (e.g.strength and water/ cement ratio).

2.5 Responsibility for concrete specification


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BS 8500-1 , clause 4.1 lists items that the specifier is required to take intoaccount. Some of these are the responsibility of, or should be readily apparentto the contractor, and unless the designer has special requirements in theserespects, it is preferable to leave them to be considered by the Contractor.

When types and classes of constituent materials and environmental conditionsare not detailed, BS 8500-2 , clause 4.1 states that producer will selectconstituent materials for specified requirements only.

For each method of specifying concrete there are:

Basic requirements that must be stated.

Additional requirements that may be deemed necessary.

These latter include some items that are again better left to the decision of thecontractor, unless the designer has special requirements.

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3 Factors affecting durability of

concreteDurability is determined by specification of appropriate quality concrete, coverto reinforcement and the implementation on site of correct compaction andcuring procedures. The appropriate quality of concrete for an elementdepends upon:

The intended working life of the structure and required structuralperformance. See 1.2.3.

The environment to which concrete will be exposed and the mechanismsof deterioration (e.g. carbonates, chlorides, freeze/ thaw). See 4.1.

The type of reinforcement material, e.g. carbon or corrosion resistantstainless steel and the minimum cover to reinforcement. The provisionsfor durability in BS 8500 are based on carbon steel reinforcement.

Limiting the quantity of reactive ingredients and chemicals in aggregates,cement and admixtures (e.g. chlorides).

Limiting the penetrability of concrete to carbon dioxide, chloride ions,

oxygen and water.


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The degree of penetrability, determined by the pore structure and the bindingcapacity of the concrete, has the greatest influence on the durability ofconcrete. Low penetrability of the surface zone of concrete is necessary toresist the entry of harmful liquids, vapours and gases into the body of the

concrete and to the reinforcing steel.

Alkali-silica reaction (ASR) between alkaline elements in the concrete andsome types of reactive silica contained in aggregate, is the only form of alkali-aggregate reaction known to have affected structures in the UK. The reactioncan cause cracking of concrete. Guidance on minimizing the risk is given inBRE Digest 330 and Special digest 1 . BS 8500-1 requires the producer tominimise risk for designated, designed, and standardized prescribed concrete.

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4 Selecting concrete for durability

4.1 Exposure classes

The environmental and ground conditions to which concrete is exposed aredescribed in terms of exposure classes see BS 8500-1 , table A1 for class and

sub class descriptions and examples, and table 1 in this section for a summaryof exposure classes.

Exposure classes are a critical part of the design procedure for selectingconcrete with a durability that is appropriate for its location and conditions ofexposure.

Table 1 Summary of exposure classes

Main class Exposure and risk

X0 No risk of corrosion or attack from aggressive conditionsor chemicals (unreinforced concrete in non-aggressiveconditions, reinforced concrete in very dry conditions).

XC Reinforced concrete exposed to air and moisture leadingto carbonation of concrete and subsequent corrosion ofreinforcement

XD Corrosion of reinforcement induced by chlorides otherthan from seawater. (This class cannot exist on its own andmust be combined with an XC class).

XS Corrosion of reinforcement induced by chlorides from


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seawater. This class cannot exist on its own and must becombined with an XC class).

XF Freeze-thaw attack, with or without de-icing agents.

Chemical attack(XA classes in BSEN 206-1 )

BS 8500-1 does not follow BS EN 206-1 recommendations for class XA but substitutes aclassification of aggressive chemical environments (ACEC) see BS 8500-1 , table A.2.

4.2 Selection of designated and designed concrete

The procedures for selecting designated concrete are more straightforwardthan for designed concrete. Where aggressive ground conditions apply,procedures for both designated and designed concrete are more involved,

particularly where more than one exposure class applies to a concreteelement. For detailed guidance on procedures see:

BSI document BIP 2001 .

BCA Specifying concrete to BS EN 206-1/ BS 8500 publications 45.313 and 45.314 these include worked examples.

BRE special digest 1 for concrete in aggressive ground.

4.2.1 Designated concrete

For surfaces not exposed to chemical attack, identify exposure classes from BS8500-1 , table A.1, and select designated concrete and nominal cover from BS8500-1 , table A.3, A.8 or A.14.

See 4.3for surfaces exposed to chemical attack.

For the characteristic values associated with each concrete designation(minimum strength class, minimum cement content, maximum water/ cement

ratio) and associated cement/ combination groups see BS 8500-1 , table A14generally, and BS 8500-2 , table 5 for variations with alternative aggregatesizes.

4.2.2 Designed concrete

BS 8500 does not permit concrete to be specified by exposure class butrequires limiting values or DC-class to be specified (see BS 8500-1 , clause4.3.2, note 1).

Combinations of exposure classes can make the procedure more involved (e.g.the different surfaces of a concrete element may be exposed to different


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conditions and exposure classes). The selection procedure may involve severalstages and iterations in order to arrive at an optimum solution (e.g. changesto initial design strength and, when acceptable, depth of cover toreinforcement to provide a more economic solution).

For surfaces not exposed to chemical attack, identify exposure classes from BS8500-1 , table A.1, and specify a suitable strength class, maximumwater:cement ratio and minimum cement/ combination content for from BS8500-1 , tables A.4, A.5 or A.8, taking into account the intended working life ofthe structure and the nominal cover to the reinforcement.

For unreinforced concrete containing no embedded metal, durability is onlyaffected by frosting, chemical attack and abrasion, but see BS 8500-1 , tableA.4, A.5 and A.11 for recommendations.

See 4.3for surfaces exposed to chemical attack.

4.3 Designated and designed concrete exposed to chemical attack (AC

exposure class)

Chemical attack on concrete can be in the form of gases or solutions.

In the ground, chemicals in soil or groundwater that can cause deteriorationof concrete are solutions of sulfate salts and acidic solutions. Sulfates cancause expansion and cracking of concrete. The commonest form of sulfateattack involves the formation of the reaction products ettringite (a calciumaluminate sulfate hydrate) and gypsum. Another less frequently found form ofsulfate attack which weakens concrete is that involving the formation ofthaumasite (a calcium silicate carbonate sulfate hydrate).

Acids in the ground are commonly derived from naturally occurring sulfides orsulfides from industrial wastes (e.g. iron sulfide), and acids found in moorlandwaters or peaty soils.

The exposure classes in BS 8500 for concrete exposed to aggressive groundconditions (acid and sulfate attack) are based upon BRE Special Digest 1 . Seealso BRE IP 11/01 and BRE IP 4/03 .

The suffix symbols to the ACEC class numbers have the following meanings:

s indicates that the ground water has been classified as static. Such aclassification will generally only be possible if the ground is permanentlydry and has a permeability less than 10-6m/s.

z indicates that concrete will primarily need to resist acid conditions.


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m indicates higher levels of magnesium in Design Sulfate Classes 4 and 5.

The aggressive chemical environment (ACEC) and design sulfate (DS) classesare identified from BS 8500-1 , table A.2, adjusting, as necessary for acid on

brownfield sites (see BRE special digest 1 ). Suitable values for the designchemical (DC) class and the additional protective measures (APMs) aredetermined from BS 8500-1 , table A.9, adjusting parameters as necessary forhydraulic gradient across element and thickness of element then:

For designated concrete:

Using the DC-class, select designation from BS 8500-1 , table A.9 or A.13,or from table 4 in this section or by replacing DC- with FND, e.g. DC-3s becomes FND3s.

Take account of other exposure classes that apply to the concrete, e.g.XC2 (see 4.2.1). For reinforced concrete, if XF2 , XF4 or classes XD or XSapply use designed concrete.

Check that strength class C25/30 is adequate for structural purpose (seestructural design codes, e.g. BS EN 1992-1-1 or BS 8110-1 ), otherwiseuse designed concrete.

For designed concrete: Preferably specify required DC-class, but when thisis not possible because of e.g. complex combined exposure conditions ortype of chemical attack outside the scope of BS 8500 , specify limitingvalues necessary to assure adequate concrete durability (i.e. maximumwater:cement ratio, minimum cement/ combination content and permittedtypes of cement/ cement combination, see BS 8500-1 , table A.11). Check that these values satisfy the requirements for other exposure

classes that apply to the concrete, e.g. XD2 (see BS 8500-1 , table A.4, A.5or A.8).

Check that strength class is adequate for structural purpose (see structuraldesign codes, e.g. BS EN 1992-1-1 or BS 8110-1 ).

4.3.1 Additional protective measures (APMs)

As some of the APMs have implications for the overall design (e.g. sitedrainage) the designer should normally make a choice but allow thecontractor to propose alternatives. For the types of APMs that may beselected to suit project requirements see table 2. For further guidance see BRESpecial Digest 1 , part D6.

Table 2 Additional protective measures

ReferenceDescription of APM

APM1 Enhance concrete quality by selecting next higher DC-class with


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the same suffix (if present), e.g. by increasing concrete quality fromDC-2z to DC-3z. This measure is not available when the identifiedDC-class from BS 8500-1 , table A.9 is DC-4, DC-4z or DC-4m.

APM2 Use controlled permeability formwork. Specify in NBS section E20 .

APM3 Use surface coatings or water resisting barriers, for example:

Liquid applied waterproof coatings (specify in NBS sectionJ30 ).

Mastic asphalt tanking (specify in NBS section J20 ).

Flexible waterproof membranes (specify in NBS section J40 ).

APM4 Provide sacrificial layer to element in addition to nominal coverfor reinforced concrete (see 1.2.4). Describe on drawings.

APM5 Use drainage system to, e.g. divert aggressive ground water awayfrom building. Specify in section R12 or R13 .

4.3.2 Forms of chemical attack outside the scope of BS 8500

For guidance see BRE Special Digest 1 .

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5 Cements (CEM) and Combinations

(C)

5.1 Selection for designated and designed concretes

See BS 8500-1 , table A.6 for broad designations and compositions ofavailable cements and combinations.

For suitable types of cements and combinations for different categories and ofexposure and concrete see:

Designated concrete: BS 8500-2 , table 5 and 7 for designated concreteslists permitted types for each designation of concrete, but the specifier ispermitted to further restrict cement type.

Standardized prescribed concrete: BS 8500-2 , clause 9.2 lists permittedtypes, but the specifier is permitted to further restrict cement type.

Designed concrete: BS 8500-1 , tables A.11 lists permitted types for eachDC-class concrete. Guidance on permitted types for other exposure


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classes are listed in BS 8500-1 , tables A.4 and A.5, but specification muststate the permitted types.

Cements are factory produced and preblended. The most commonly used

cement is Portland cement CEM I. Cements are also manufactured usingcementitious additions fly ash, pfa or ggbs. Combinations are manufacturedin the producers mixer from CEM I cement plus additions of fly ash or pfa orggbs or limestone fines.

Combinations are designed to achieve an equivalent performance to cements they count fully towards the cement content and water:cement ratio inconcrete. The concrete producer can choose to use either a cement or theequivalent combination. The full range of cements and combinations whichthe producer is permitted to use are specified in BS 8500-2 , table 1. In reality,the range of cements and combinations currently used by UK concreteproducers is relatively small see table 3.

Table 3 Cements (CEM) and combinations (C) commonly used in the uk

Designation Description

CEM I Portland cement

SRPC Sulfate resisting Portland cement

IIA Cement or combination with 6 to 20% of a second material suchas pfa, ggbs or limestone

IIB Cement or combination with 21 to 35% fly ash, pfa or ggbs

IIB+SR Cement or combination with 25 to 35% pfa

IIIA Cement or combination with 36 to 65% ggbs

IIIB Cement or combination with 66 to 80% ggbs

IIIB+SR Cement or combination with 66 to 80% ggbs suitable for sulfateresisting concrete

IVB Cement or combination with 36 to 55% fly ash or pfaIVB+SR Cement or combination with 36 to 40% pfa

Notes:

1. +SR denotes sulfate resisting proportions achieved by control ofadditions and other technical requirements specified in BS 8500-2 .

2. Special cements are also available, e.g. White Portland cement.

5.2 Type II additions fly ash, pfa, ggbs, microsilica and metakaolin


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The incorporation of pfa, fly ash or ggbs can benefit hardened concrete by, forexample:

Increasing resistance to sulfate attack and chloride induced corrosion.

Reducing risk of thermal cracking in large concrete sections.

For further guidance on the use of pfa, fly ash or ggbs additions andimplications for fresh and hardened concrete see BCA Publication 48.037 .

Microsilica (condensed silica fume), a by-product from the smelting processfor ferrosilicon alloy, and metakaolin obtained from kaolin (china clay) can beused to produce concrete with low permeability, high strength, chemicalresistance, sulfate resistance, high wear resistance and ease of finishing. These

properties mean that its greatest potential use is in the construction ofindustrial floors.

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6 Consistence, compaction and

curing

6.1 Consistence and compaction

Full compaction is generally taken to mean the virtual exclusion (typically lessthan 11.5%) of air voids from the concrete. For each 1% of entrapped airthere will be a 5 to 6% loss of strength; thus if there is 5% air left in theconcrete, there will be a loss of strength of 20% or more. The ability of suchconcrete to protect reinforcement and resist freeze-thaw and sulfate attackwill be severely reduced. It should be noted that concrete containing an air

entraining admixture is a quite separate consideration in this case the air isin the form of minute bubbles introduced to give resistance to freeze-thawdamage, and it will not be expelled by vibration. The effect of an air entrainingadmixture will be taken into account when designing the concrete to achievethe specified compressive strength class. Thorough vibration of the placedconcrete and adequate consistence are both necessary.

6.2 Curing

Low permeability depends, among other things, on effective hydration of thecement to fill interstices in the concrete originally occupied by water. This is


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particularly important for the surface layer of the concrete which providesprotection to the reinforcement and resistance to the penetration ofaggressive agents. Curing is the process of keeping the concrete moist and ata favourable temperature for several days after casting, to ensure proper

hydration.

Curing is also a vital operation in the production of self-finished concretefloors it increases wear resistance and reduces dusting. Effective early curingwill also significantly reduce the risk of plastic shrinkage cracking.

BS 8110-1 , table 6.1 gives recommended minimum curing periods related totype of cement, weather conditions and temperature, but makes no distinctionbetween the different end uses of the concrete. Many experts consider thecuring periods required by BS 8110-1 to be insufficient for surfaces exposed

to the elements and wear. NBS clause 820permits the specification of longerperiods for such surfaces.

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7 Environmental issues

7.1 Sources of information

For applications and sourcing of recycled and secondary aggregates see thesustainable aggregates information service AggRegain atwww.aggregain.org.uk. See also CIRIA Publication C513 , sections 10 and 11and BRE Information Papers 5/94 and 3/97 .

For a UK cement and concrete industry view of environmental issues seewww.ConCemSus.info.

7.2 Recycled and secondary aggregates

Recycled aggregates can be used as an alternative or partial replacement toprimary quarried aggregates. In addition to grading, the degree of processingdepends on the source of the material and the amount of contamination.Recycled concrete aggregate (RCA) consists of crushed concrete. For RCAmost reinforcement is removed before crushing; the scrap value of the steelalone can often justify the recycling of reinforced concrete.

Recycled aggregate (RA) is obtained from reprocessing inorganic materialspreviously used in construction (e.g. fired clay and concrete).


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Recycled secondary coarse aggregates (RSA) are by-products from otherindustrial processes and not previously used in construction, e.g. air-cooledblast furnace slag, china clay waste and, glass, or may be manufactured fromindustrial by-products, e.g. lightweight aggregates based on pfa and

blastfurnace slag.

BS 8500-2 allows the use of coarse recycled concrete aggregates for certainexposure and strength classes and specifies limitations on contaminants andforeign matter (see BS 8500-2 , clause 4.3 and tables 2 and 3), and restricts itsuse in designated concrete (see BS 8500-2 , clause 6.6.2). Subject to theselimitations, the concrete producer can use coarse recycled concrete aggregate(RCA) providing the specification does not prohibit its use.

BS 8500-2 permits the use of RA but requires the specifier to establish quality

requirements (see BS 8500-2 , clause 4.3 and tables 2 and 3) and to name themethods by which compliance will be determined. For this reason, the use ofRA is not covered in this section, but the use of coarse RCA should beencouraged.

The use of fine recycled aggregates is not prohibited in BS 8500-2 but its useis left to the project specification and a warning is given concerning the risk ofexcessively high levels of sulfates (from gypsum plaster) in RCA and some fineRA. The inclusion of gypsum plaster can lead to delayed ettringite formation.

Sourcing recycled aggregates of the right quality and quantity is a key issue.For applications and sourcing of recycled and secondary aggregates see thesustainable aggregates information service AggRegain atwww.aggregain.org.uk.

WRAP publication ASR Testing on recycled aggregates-guidance on alkalilimits and reactivity , reports tests of the alkali content and reactivity ofrecycled aggregates, and WRAP papers Conglasscrete I and ConglasscreteII report tests on glass used as pozzolan and aggregate.

7.3 Recycling fresh concrete

Increasingly returned concrete is being recycled. Aggregates are recoveredand returned to the stockpile after the cement is washed from the concrete.The wash water and fines (hydrated cement particles) are added in smallquantities to other batches of concrete so that there is no waste. Analternative technique is to add water and let the weak concrete harden. Whensufficient stock is available, a crusher is brought in and the stockpile turned

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8 Health and safety

See section E05 general guidance 2.

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Table 4 Designated and standardized

prescribed concrete in housing and

other applications intended

working life 50 years(1)

Application

(Concrete

containing any

form ofembedded

metal is

treated as

reinforced)

Minimum

designate

d concrete

Standardize

d prescribed

concrete(2)

Assume

d

strength

class

cylinder/ cube

(3)

Nomina

l cover

(mm)

Recommende

d

Consistence

class fordesignated

concrete (4)

Unreinforced foundations and associated works requiring DC-1

concrete(see 4.2.1)

Blinding andmass concrete

fill

GEN 1 ST2 C8/10 S3

Strip footings GEN 1 ST2 C8/10 S3

Mass concretefoundations

GEN 1 ST2 C8/10 S3

Trench fillfoundations

GEN 1 ST2 C8/10 S4(5)

Drainage worksto give

immediate

GEN 1 ST2 C8/10 S1(5)


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support

Other drainageworks

GEN 1 ST2 C8/10 S3

Oversite belowsuspendedslabs

GEN 1 ST2 C8/10 S3

Unreinforced foundations requiring DC-2 to DC-4 concrete (See 4.2.1and4.3)

DC2 FND2 C25/30 Default S3 isrecommendedformechanically

compactedconcrete.S4 isrecommendedfor trench filland other selfcompactinguses.

DC2z FND2Z C25/30

DC3 FND3 C25/30 DC3z FND3Z C25/30

DC4 FND4 C25/30

DC4z FND4Z C25/30

DC4m FND4M C25/30

Reinforced foundations (See 4.2.1and 4.3)

ACEC class AC-1 and hydraulicgradient notgreater than 5

RC25/30 C25/30(6) 50 whencastagainstblinding,75 whencastdirectlyagainstsoil

DC2 FND2 C25/30(7)

DC2z FND2Z C25/30


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(7)

DC3 FND3 C25/30(7)

DC3z FND3Z C25/30(7)

DC4 FND4 C25/30(7)

DC4z FND4Z C25/30(7)

DC4m FND4M C25/30(7)

General

applications

Kerb beddingand backing(see NBSsection Q10 )

GEN0 ST1 C6/8 S1(5)

Floors

House floorswith no

embeddedmetal

Permanent finish tobe added,e.g.screed orfloatingfloor

Nopermanent finish tobe addede.g.carpeted

Bondedormonolithi

c screed

GEN1(8)GEN2(8)

RC28/35

ST2ST3

C8/10C12/15

C28/35

S2recommended.

Default is S3.


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(see NBSsectionsM10 andE41 )

Garage floorswith noembeddedmetal

GEN3(8) ST4 C16/20

Wearingsurface: lightfoot and trolleytraffic

RC25/30 ST5 C25/30(9)

Wearingsurface: generalindustrial

RC32/40 C32/40

Wearingsurface: heavyindustrial

RC40/50 C40/50

Paving (SeeNBS sectionQ21 )(10)

House drivesand domesticparking

PAV1 C25/30 S2(12)

Heavy dutyexternal pavingwith rubbertyre vehicles(11)

PAV2 C28/35 S2(12)

Reinforced

concrete not

subject to

chemical

attack(13)

Exposure classXC1 (insideenclosedbuildingsexcept poorlyventilatedrooms with

RC20/25 C20/25 (15 +c)

Default S3 isrecommendedfor mechanicalcompaction(see BS 8500-1 , Table A.16).


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high humidity)

Exposure classXC3/XC4 + XF1(External

elements ofbuildingssheltered from,or exposed to,rain)

RC40/50 C40/50 (20 +c)(15)

RC32/40 C32/40 (25 +c)(15)

RC28/35 C28/35 (30 +c)(15)

Exposure classXC4 + XF3(Horizontalelements withhigh saturationwithout de-icing agent andsubject tofreezing whilewet)(14)

RC40/50XF C40/50 (20 +c)(15)

PAV2 C28/35 (30 +c)(15)

(16)

PAV1 C25/30 (35 +c)(15)

(16)

Concrete to resist freezing and thawing(17)

Exposure classXF1

RC28/35 C28/35 To suitapplication

(see BS 8500-1 , Table A.16).Default for RCseries is S3 andfor PAV seriesS2.

PAV1 C25/30

Exposure classXF2 unreinforced

RC32/40 C32/40


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concrete(18)

PAV1 C25/30

Exposure classXF3(14)

RC40/50XF C40/50

PAV1 C25/30

Exposure classXF4 unreinforcedconcrete(18) (14)

RC40/50XF C40/50

PAV2 C28/35

Notes:

1. See BS 8500-1 , tables A.3, A.8 and A.13 and associated clauses for fullrequirements. When not indicated otherwise, recommended concretequalities are for at least 50 years intended working life.

2. When strength or durability is important specify designated or designedconcrete. ST1 and ST2 concrete should not be specified for concretecontaining reinforcement or embedded metal, as chloride class for theseconcretes is Cl 1.0.

3. For strength class classification system see 1.2.6. For minimum strengthclass of designated concrete and strength class assumed for structural

design purposes for standardized prescribed concrete see BS 8500-1 ,tables A.14 and A.15 respectively. Except as otherwise indicated, minimumstrength class for designated concrete and assumed strength for thealternative standardized prescribed concrete are the same.

4. For default consistence class see BS 8500-1 , table A.14. Forrecommended consistence class suitable for different uses see BS 8500-1 ,tables A.13 and A.16. Specify consistence class when required value differsfrom default.

5. Default consistence is S3.

6. Use higher grade if necessary for structural purposes.

7. Use designed concrete when compressive strength class of C25/30 isinsufficient for structural purposes.

8. GEN concrete with relatively low cement contents may not be suitable forobtaining satisfactory cast and direct finished surfaces nor for methods ofplacing such as pumping.

9. Strength class for ST5 is C20/25, i.e. less than that for RC25/30

10. Cast in-situ concrete for house drives and similar external areas is liable toattack by freezing and thawing, which is made worse by the use of de-


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icing salts. PAV series concrete contains entrained air to counteract thiseffect. Designated concretes are not recommended for reinforcedconcrete to resist corrosion induced by chloride de-icing salts (XDexposure class).

11. For extreme applications, e.g. heavy industrial floors, seek specialist advice.

12. This is default consistence class for this concrete. Consistence class mayneed to be changed to suit method of placing.

13. If grade for application is higher than grade for exposure, use grade forapplication.

14. Freeze thaw resistant aggregate is recommended for exposure classes XF3and XF4

15. Increase the minimum cover by 5 mm if IVB-V cements and combinations

are to be permitted.16. Default consistence class is S2. See BS 8500-1 , table A.16 for

recommended consistence class.

17. Recommended concrete qualities are suitable for both at least 50 yearsand at least 100 years intended working life. see BS 8500-1 , clause A.4.3.

18. Class assumes application of de-icing agent. Hence, for reinforcedconcrete an XD class will also apply for which designated concrete is notsuitable, and a designed concrete should be specified.

concrete general guidance

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