National Workshop on Is_456-2000

8/10/2019 National Workshop on Is_456-2000

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NATIONAL WORKSHOPO N

IS:456- 2000

16-17 August,2000

Organisedby

National Councilfor Cement and BuildingMaterials

NewDelhi

Jointlyw ith


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REVISION O F IS : 45 6 CODE O F PRACTICEFOR PLAIN AND REINFORCED CONCRETE

- OVERVIEW O F MODIFICATIONS

Centre for HumanResource and Continuing EducationNATIONAL COUNCIL FOR CEMENT AND BUILDING MATERIALS


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READING MATERIALon

IS:456-2000

02 - 03,tU(;UST, 2000

LIST OF CONTENTS

SN o Topic Page

NosI Revision ofIS:456 Code ofPracticefor Plain and

Reinforced con crete OverviewofM odifications01-25

2 M ix Propo rtioning andQualityAssurance 26-29

3 AcceptanceC riteria 30-34

4 Durability Requirements 35-44

5 a) Shearcapacity enhancementnear supp orts 45-50

b) Slabs Spanning in Two Directions a t Right AngIe 51-52

C)Control of Deflection 53-54

d) Lap Length ofRe inforcing bars 55-56

e) Coverto re in forcem ents 57-63

flDesign of Slabs 64-67

6 General Design considerations and design ofwalls 69-83


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REVISION OF IS 45 6 Code OF PRACTICE FOR PLAIN AND REiNFORCEDCONCRETE- OVERVIEW OF MODIFICATIONS

1. INTRODUCTION

Concrete is one ofth e most versatile amongmodern building materials. It ismost widely used, but unfortunately often most misusedmaterials. Properly appliedtechnology for concrete startswith a good knowledge of the concrete materials andthe main operationsof concreting i.e. selection of materials, it s proportioning, mixing,placing, compaction, curing and finishing. Thishas tobe supported~oless measureby efficient structural design, detai l ing, appropriate construction methods, qualitycontrol, site management and level of maintenance.

In order to have un iform guidelines tobe followedb y a ll concerned in thismostimportant and basic area of construction, IS 456w as brought out in 1953 by the then

Indian Standard Institution. As the knowledge grew the Code was revised numberoft imes tomake itcompetitive with similar Codes elsewhere in the world.

It is one of the most important basic standards widely used and accepted byengineers, technical institutions, professional bodies and the construction industry. TheCode is expected to be used as one package for the design ofconcrete structures ingeneral building construction. It does not advocate the use of different provision fromdifferentCodes in the design of concretest ructures. However, for the designof specialstructures, such as shell structures, folded p lates, arches, bridges, chimneys, blastresistant structures, hydraulic structures and liquid retaining structures specificrequirementsas specified in the respective Codesshall be adopted in conjunctionwith

the provisions of the Code as far as they are applicable.

2. HISTORY OF IS 45 6

2.1 First published in 1953 underthe titleIndianStandard Code of Practice forPlain and Reinforced Concrete forGeneral Building Construction.

Highlights: covered design basedo n working stress method. Stresses were based

on British Standard Code. unit usedwas FPS.

Printed copies of 1 953 version sold out rapidly indicating the need for suchCode in the country.

In orderto incorporate provision of additional materials and clarification onsome of the points raised while applying the Code to practical use, it w as revised in1957.


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Revision was limited to it s scope. However, new sections on composite

column, concentrated toads and staircaseswere introduced.

2.2 Th e second revisionw as in 1964.

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Scopew as enlarged to cover other types ofstructures. Thetitle was a lsomodified as Code of Practice for Plain and Reinforced Concrete.

Other highlights of the revisions are:

Inclusion of Ultimate Load (Load Factor) Theoryof design toobviate the

short comings of Working Stress Method.

Complete revision ofsection dealing with quality of concrete. Gradation

of concretewas done on the basis of it s strength.

Rationalisation of permissible stresses in concrete in metric system,

increase in permissible bond stress when deformed bars are used,increase in permissiblestresses in steel reinforcement and revision of theperiods for striking from Work were othermajor changes.

2.3 Third revision was done in 1978

Highlights of the revision:

Introduction of LimitState Design

Use of S I unit

Symbolsalignedwith IS O 3989-1976. Bases fo r Design of Structures

Notation.

Revision of sampling and acceptance criteria for concreteelaborating theconcept of statistical quality control and introduction of characteristicstrength of concrete.

Inclusion of moretypes ofcements, pozzolana,lightweight aggregate andcoldtwisted deformed bars.

Introduction of durability aspectscovering minimum cement contentandmaximum w/c ratio for different environment exposure conditionsincluding types ofcementto be used fo r resisting sulphateattack.

Other important changes included recommendation regarding substitutesframe, m inimum eccentricity for design of compression members,recommendationregardingside face reinforcementi n beam, detailingrule

for crack control, recommendation regarding design of deep beams,guidancefor the design of r ibbed and voided slabs.


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Since the publication of 1978 version ofthe Code there has been rapid

development worldover in the field ofconcrete technology,design and constructionpractices. Thereis general:eeling that in theCode, though the design requirements

areadequately covered, provsions onmaterials, workmanship, durability requirements

inspection and testing arerequired to be dealt in more details. Further, increasingnumber of failures ofcorcrete structures in recent past haddrawn attention towardsthe need to codify durability requirements for concrete and concrete structures.

The present fourth revision ofthe Code has given greater emphasis on theabove aspectsbesides bringing out otherimportant necessary changes.

3. NEED FOR FOURTH REVISION

Users of the Code-designers,constructionengineers and academicians fromeducational institution have been sending suggestionsfor modification in Code. The

major concernsshown are: To unify Codes on Plain Concrete, R CC and prestressedconcrete.

Adequate emphasis to be given to durability aspects of the structures.

The approach suggested demanded modificationsin the following areas.

- Exposure cond ition- Selection ofsuitableconstituent materials- Selection of proper m ix design including use of

admixtu res/additives- Specifying proper cement content and w/c ratio for different

exposureconditions.- Protection of reinforcementfrom corrosion by creating corrosion

inhibiting surrounding ie creating of dense cover concrete andproper limitingofchloride and sulphate in concrete.

- Selectionof appropriate structuralform and de tai ling.- Limitingcrack formation and development- Proper construction method

Use of new m aterials like f lyash, silicafume, rice huskash, metakaoline,blast furnace slag, super plasticisers etc. in concrete and the need forincreaseddura bility demanded modifications in the material clauseof the

Code.

Simplifyingthe acceptance criteria of concretewhich in the present Code,

is consideredto be cumbersome

Bringing service life approach in design.

To bring quality assurance concept to give due emphasis on goodpractices of construction.

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WhetherWorkingStressMethodshould be given an independent identity.

Provisicns fo r fire resistance structures

Modifications regradingtorsion and enhanced shear strength

Estimation of loads on beams supportingslabs.

Provision for design of walls, corb~Is.

4. DRAFT REVISION OF IS 456

While taking up the revision of the code, due considerations have been givenby the concerned committee to a ll the majorissuespointed out by the users.

Whereas it is desirable to have a unified code covering a ll types ofconcreteconstruction i.e. plain, reinforced and prestressed concrete as suggested by the users,it

was fell that revision of IS 456 in it s presentform was more important. Unificationcould be done subsequently which would require moretime.

4.1 Scope ofRevision

Every revision of the code has distinct landmark; 1964 version introducedultimate load theory as a method of design, 1978 version introduc~ LimitStateConcept of design and the proposed revision has brought out, in addition to otherchanges, durability aspect in concrete making and construction.

Changes in the analysis and design clauses are meant to improve the safetyand serviceability of the structures which may not bring substantial overall effect oncost, but changes in concrete technology aspect like minimum cement content,minimum grade of concrete, cover thickness etc. m ay affect cost of construction. Theimpact of such changes, however, has to be considered taking into account theresultant enhancementin durability and changes in the overall life cycle cost of thestructure.

Studies of distresses structure in recent past have clearly indicated thatthefailures have been more due to lack of proper durability considerations duringconstruction stage of a structure, Durability of a structure is affected due to variousphysicalchemical and biological factors. These factors can be taken care of by properassessment ofenvi ronment, select ion of right materiala nd mixes, adequate structuraldesign, good placement. provision of protective coating and preventive maintenance.This calls for systemapproach in the designcode. Accordingly, durability clause hasbeenenlarged to covera ll the factors in details so as to bring in-built protectionfromsuch factors keepingin view the overall life cycle costof the structure.

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IMPORTAN T MO DIFICATIONS5.

5 .1 Materials

5.1.1 Cement

Cement is one of the main ingredient ofconcrete. Originally concrete wasmade us ing a mixture of onlythree materials, cement, aggregate and water, almostinvariably, the cementwas Port land cement. Latero n in order to improve some of theproperties of concrete either in fresh or hardened state very small quantities ofchemical products as chemical admixture were added intothe mix.

Later still othermaterials, inorganic in nature, were introduced mainly to bringeconomy. Yet further encouragement for the use of some of the supplementary;material were provided by the ecologicalconcerns. Materials such as blast furnaceslag, flyash, silica fume considered as waste materials were found tobe very useful notonly in bringing economy but to help conservation of environment and getting some

properties helpfulfor durability ofconcrete.

Indian cement industryhas also grown rapidly in recentpast and at presenti~ranked4th in production next only to China, Japan and USA. Different varieties ofcements are covered by different Indian Standards, which are to be used dependingupon the intended use.

The revised standard accordingly statesthat:

The cement used shall be any of the following and type selected beappropriatefor the intendeduse:

The list contains standards for three gradesof OPC (i.e.3 3 grade~ (IS 269),43 grade(IS 8112))53 grade ( IS 12269)) rapid hardening Portland cement (IS 8041),Portland slag cement (IS 544), Portland pozzolana cement [IS 1489 (Part 1&2)},hydrophobiccem ent(~S8043)10w heat Port land cement (IS1 2 6 0 o ) , sulphate resistingcement (IS 12330).

Usual cautionary note given for the use of low heat cement, high aluminacement and supersulphate cement continuesin the revised version.

It has now been established that use of proper quality of flyash, groundgranulated blastfurnace slagand silicafume in certain proportion in concrete not onlysavesenergy and conserve resources, it brings technical benefits l ike influence on therate of development of heat, strength and on resistanceto chemical attack.

In order to encourage use offlyashand ground granulated blast furnace slagin concrete following have been added in the clause on cement

Othercombination of Portlandcementwith ground granulated blast furnaceslaga nd f lyash of qualityconformingto relevant Indian Standard m ay alsobe

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used in the manufacture of concrete provided there are satisfactorydata ontheir suitability, such as performanceteston concrete containing them.

TheCode alsogives acautionaryclauseemphasisingthat consumersshouldfollow the performance characteristics given in the respective Indian Standardspecification for the cement. Anyotherclaim by the manufacturerseithero n bags or

in advertisement are required to be dealt with caution toavoid any problem in concretemaking andconstruction.

5.1.2 Mineral Admixtures

Flyash, silicafume, rice huskash andmetakaoline, which have go t pozzolanicproperties and ground granulated blastfurnaceslag are being used with advantagesfo r concrete makingbyconcrete technologist. The revised Code has, therefore, givenprovision for such materials.

Pozzolana is defined as siliceousor siliceous and aluminous materials whichin itself possesses little orn o cementitiousvalue but will, in finely divided form and inthe presence of moisture, chemically react with calcium hydroxide at ordinarytemperatures to form compounds possessing cementitiousproperties. It is essentialthat pozzolana be in a finely divided state as i t isonlythen thatsilica can combine withcalcium hydroxide (produced by the hydrating Portland cement) in the presence ofwater toform stablecalcium silicateswhich have cem entitiousproperties. Furthersilicahas tobe amorphous, that is , glassy because crystallinesilicahas very lo w reactivity.

5.1.2.1 Flyash (Pu lverised FuelAsh)

Flyash is the ash precipitated electrostatically or mechanically from theexhaust gases of coal- fired power station. Flyash conforming toGradel of IS 3812hasbeen permitted as par t replacement ofordinaryPortland cement provided uniformblending with cementis ensured.

5.1.2.2 Silica Fume

Silica fume (very fine non crystalline silicon dioxide) is a by product ofthemanufacture of silicon, ferrosilicon or the l ike, from quartz andcarbon in electric a rc ..furnaces. Silica in the form ofglass (amorphous) is highlyreactive, and the smallnessof the particles (0.03 - .3 micron) speeds up the reaction with calcium hydroxideproduced bythe hydration of Portland cement. The verysmall particleof silica fumecan enters the space between the particlesof cement and thus improve packing.

Although no Indian Standardsspecification is available on silica fume and itis mostly imported , it has been foundto be very useful fo r achievinghighergrade ofconcrete. The Code therefore, has made adequate provision for use ofsilicafume.

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5.1.3.2 Chloride

Limit of chloride content in waterhas been broughtdown from 1 000 mg/I to50 0 mg/ I for reinforced concrete work. This is in line with the prov!sion of British

Standard.

Chloridesareamongthe more abundant materialso n earth and are presentin variable amounts in a ll of the ingredients of concrete. In orderto keep the limit ofchloride below the threshold level which can initiatecorrosionof reinforcement, limit of50 0 mg/I in water has been recommended. Potable water has been found to begenerallysuitable forconcrete work.

5.1.4 Size ofAggregates

The existing Code has a suggestive clause which states that fo r reinforced

concrete work aggregates having nominalsize of20 m m are generallysatisfactory. Ithas been observed thatfor a ll practical purposes this provision takes precedenceoverother provision regarding size of aggregates, though use of larger size whereverpossible are technicallymore appropriate and economically desirable. The revision,thereforeincludes followingadditionalsentencewhich can take care of this aspect.

For most work 20 m m aggregate is suitable. Where there is norestriction to th e flow of concrete Into section, 40 mm or larger size m a y b epermitted. In concrete elements with thin section. closely spaced reinforcementor small cover, consideration should be given to the use of 10 mm nominalmaximumsize.

5.1.5 Admixtures

With the availabilityand successful use of superplasticisers inimprovingthe workabilitywithout increasingthe w/c ratio,thus furthergainingin strengthof concrete, it w as felt that provision regarding admixturesrequired revision. IS 9103which covers the requirements of admixture has been revised . It now covers therequirements ofsuperplasticisersin line with ASTM and Brit ish Standards~Referenceof latestversion of IS 9103 in IS 45 6 willpave the way for use ofsuperplasticisers. Inaddition, some guidelinesforeffective use of admixtures in fieldhave also been made.The most notable guidelines is the verification of suitability and effectiveness of

admixtures bytrialmixes, using the same materialsofconcrete intended to be usedin the works.

5.2 Concrete

5.2.1 Grade of concrete

Grade of concrete denotei ts 28 daysstrengthwhich is commonlyconsideredit s most valuable properly,although in many practical cases, otherchaacteristics. suchas durability and permeability, may infact be more important. Nevertheless strength

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usually gives an overall picture of the quality of concrete since strength is directly

related to the structure of the hydrated cement paste.

5.2.1 .1 Minimum Grade ofConcrete

There has been sugge st ions to upgrade both minimum and maximum grade

of concrete in the Code. There has been tw o schools of thought regarding theminimum grade of concrete to be used fo rreinforced concrete. One sect ion feelsthatM 15 grade of concrete, keeping other factors affecting durability in control, is sufficient;while other section feels that it should be increased for better durability. ln thedeveloped world, even for ordinary structures the minimum grade of concrete is theequivalent of M25 or M30. It is now realised thatthese gradesare easily realised in thefield b y proper m ix design, particularlywith the availabilityof4 3 and 5 3 grade ofcementin the country. Further, in case ofnominal mix, the same proportion (1:2:4) used forMiS grade now give M 20 grade without any problem. In the revision of the Code, theminimum grade of concrete hasbeenrelated to exposureconditions.

For mild exposure condition, i.e. fo r concrete surfaces protected againstweather oraggressive conditionsexcept those situated in coastal areas, the minimumgrade shall be M 20 for R CC structures.

5.2.1.2 Maximum Grade of Concrete

In so far as high strength concrete is concerned, the present Code givesprovision upto M 40 . In the absence ofprovision of concrete more than M 4 0, it is feltthat even though higher grade concrete could be produced in the country with theavai lable resources and technology, it could not be used s ince provision does not existin the code.

Most national standards, rules and regulations for concrete structures areapplicable to concrete strength upto about 50 - 60 N/mm2. CEB-FIP Model Coderecommended 80 N/mm2and Norwegian Standard NS 3473:89 recommends upt 105N/mm2.

Realising the need, the revised Code has g iven provision upto 80 N/mm2. Itis , however, expected that users of high strength concretewill have sufficient data andtechnologywith them and will use high strength with proper care. Followingnoteemphasises this point.

For concrete of comprehensive strength greater than M55 design parameters

given in the Code may not be applicable and the values m ay be obtained fromspecialist literatures and experimental results.

Although minimum grade of concrete hasbeen kept as M20, provision havebeen given for use of concrete of lesser strength for plainconcrete construction, leanconcrete,simplefoundation,foundation fo r m asonry wallsorother simple or temporaryconstruction

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Thetable giving grades of concreteis as follows:

Grade Designation

M10M15M20M25M30M35M40M45M50M55M60

M65M70M75M80

Specified Characteristic Compressive Strengthof 150 m cubeat 28 days in N/mm2

101520253035404550556065707580

5.2.2 Modulus of Elas ticity of Concrete

Themodulus of elasticity of concrete(Ec is required for computation ofdeflectionof reinforcedconcreteflexural member. It has been observed that the value obtainedby the existingformula

Ec = 5700 ifck, where, fc k is characteristic strengthof concrete

is quite conservative comparedto experimentalva lues. Further, comparedtoother international Codes i.e BS 8110 AO l 318, A S 3600 and DIN 1045, the valuesobtained have been found to be mo re.Cbmparison of E~as given in IS 456:1978 andotherCodes is given below:

Ratio of Ec compared to value based on IS 45 6

Code fck (MPa)

20 30 40 50 60

IS 456 1.00 1.00 1.00 1.00 1.00

BS411O 0.94 0.83 0.78 0.74 0.72

ACI 318 0.76 0.76 0.76 0.76 0.75

BS 3600 0.92 0.88 0 .88 0.87 0.85

DIN 1045 1.08 0.98 0.97 0.94 0.89

Considering the above the Equation has beenchanged as follows:Ec = 5000~fck

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5.2.3 Age Factor of Concrete

Designers often express that the age factors given in the existing code istheoretically alright, bu t in practice it does not help the designers. The gainof strengthdepends upon the grade of concrete, type of cement, w/c ratio, curing regime etc.These age factors may help to asce rtain the actualbehaviour ofa distress structure but

should not be used fo r design. Accordingly the values of age factors have beendeleted

5.2.4 Workability

Existing code classifies various degrees of workability based on placingconditions on the basis of vee-beetime, compacting factorand slump test in a Note.Many users of the code felt that the values given in the note indicates certaininterconvertibitity of values are implied, in realitysuch a relationshipis however, noteasy to establish. It is , therefore, proposed to use in general the slump test formeasurement of w orkabi l i ty. The provision ofw orkability in the proposed revision is in

line with the provision of BS 5328. Guide to specifying concrete (in

four parts).However for very low workability, where strict control is necessary, measurement ofworkability b y determination of compactingfactory with a value of 0.75 to 0.80 and forveryhigh workability, measurement of w orkability by determination of flow have beensuggested. The proposed table is given below:

Workability of Concrete

Placing Condition Degree of Workability Slum(mm)

Blinding Concrete; Very low see Note

Shallow Section:Pavement using Payers,

Mass concrete, lightlyreinforced s~ctionin slabsbeams, w alls, column; FloorHand Placed pavementsCanal liningStrip footing

Low

25 . 75

Heavily reinforcedSection in slabsbeams,w all, columns;

Medium 50-100

slip formwork:Pumped Concrete 75-100

Trench fill:In-situ piling

High 100- 150

Tremie Concrete Very High see Note

Note:F ormost of th eplacingcon ditions, internal vibrators (needlevibrators) aresuitable. th ediameterofth eneedleshallbe determinedbasedo n th edensityandspacingofreinforcementbarsand thicknesssections. For tremieconcrete vibratorsare notrequired to be used

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Exposure ConditionsEnvironment Exposure Con dition

Mild Concrete surfaces protected againstweather or aggressiveconditionsexcept those situated in coastal area.

Moderate Concrete surfacessheltered f rom severe rain or freezing whilst wet.Concrete exposed to condensation and rainConcrete continuously underwaterConcrete i n contact orburied under non-aggressive soil/ground water.

Severe Concrete surfaces exposed tosevere rain, a lternate wett ing and dryingor occasional freezing whilstw et or severe condensationConcrete completely immersed in seawater

VerySevere Concrete surfaces exposed to sea water spray, corrosive fumes orseverfreezingconditionswhilstw et

Concrete exposed to aggressive subsoil ground water or coastalenvironment.

Extreme Surface of members in tidal zone. Members in direct contactwithliquid/solid aggressive chemicals.

While defining theexposure condition, provisions, ofothercodes likeA S 3600,ACI 318, Pr E NV 2 06 , CEB FIP Model Code were also discussed. It was discussedwhether location in relation to seacould be defined as in AS 3600. However, it was feltthat such details would be difficult to define. Provisions of Pr E N V 206 Concrete-Performance, production, placing and compliance cr i teria, was also not found suitingour requirements.

Abrasive environment has been kept separately since it requires differenttreatment,

b) Exposureto aggressive chemicals

Deterioration of concrete bychemical at tack may occurby contactwi th gases,liquid and solids of aggressive chemicals.

Naturally occurringsulphates of sodium, potassium, calcium or magnesium aresometimes found in soil or dissolved in ground water adjacent to concrete structures,

and theycan attack concrete. When evaporation cantake place from anexposed face,the dissolved sulphates (salts) m ay accumulate at the face, thus increasing theirconcentration and potential fo r deterioration.

There are apparently two chemical reactions involved in sulphate attack onconcrete.

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Formulationof a standard on Method of test for determination of total chloridecontentwhich may b e based on A S T M C 1152-90or BS 1881 : Part 124:1988 hasalso been initiated.

c) Alkali-aggregates reaction

Aggregates containing particular varietiesof silica may b e susceptible toattackby alkalis (a s Na20 and K20) originatingfrom the cement orother sources producingan expansive reaction which can cause cracking and disruption of concrete. RevisedCode suggests necessary precaution to be taken in case of use of unfamiliar materials.

d) Concrete Mix Proportion

British Code BS 8110, American Code ACI 318, Australian Code 3600 and C E B-FIP model code a ll identify permeability as being the key to durable concrete withoutputting any limit on this; instead prescription are given for m ix constituents andproportions, coverand curing.

On m ix constituent there is close agreementon limit of chloride-sulphatecontentand on specification foraggregate cement, admixture and the l ike. Additionally w/cratio, cement contentand concrete grades (in order of importance), are identified asbeing key factors.Actual values m ay be differing fo r different exposure condition, butthe broad trends are virtually identical.

Keepingth e provisions of,the above standards in mind, the provision in IS 456is given in Annex II .

There is feeling in somesection ofthe users ofthe codethatwith the availability

of cementof high strength,minimum cement content could be reduced further.

However, this is not true since the minimum cement is required firstto ensuresufficient alkalinity to provide a passive environment against corrosion of steel;secondary minimum cement and w/c ratioare so chosen thatshould resulti n sufficientvolume of cement paste to overfill voids in the compacted aggregates.

Maximum cement content has been restricted to 475 kg/rn3 unless special

consideration has been given in design to the increased risk of cracking due to dryingshrinkage in thin section or to early thermal cracking and to the increased risk todamage due to alkali-silica reaction.

e) Design Mix

Preference has been given todesign mix. For design m ix constructorshall carryout the m ix design and the mix so designed (not the method of design) shall beapproved by the employerwithin the limitations of parameters and other stipulation laiddown by the Code.

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f) Cover Requirements

Protection againstpenetration of salt to reinforcingsteel and otherembeddedi tems is affected considerablyby the thickness of concrete coverover the steel. Thetraditional code ofpracticeapproach is to specify nominal coversfor different conditionsof exposure. A study of relevant code, ie BS 8110, ACI 318, CEB-FIP Model Code

reveals that there are very considerable differences in values for nominal covers fornotionally identical environment; this is because the four Csconstituentsof the mix,cover, compaction and curing are integrated as a package in each codes (with theeffectivenessof the total package being the overriding concern).

In the existing IS Code coverrequiremen t is not given on the basis of exposurecondition. However,in the revised Code, it is given on thebasis ofexposure conditions.The provisions are reproduced below:

Exposure Nominal Coverin not less than(mm)

Mild 20Moderate 30Severe 45VerySevere 50Extreme 75

Notes:1) Formain reinforcementupto 12mm diameterbarformildexposure the nominal

covermaybereduced by 5m m.2) Unless specified otherwise, actual covershallnot deviate from the required

nominallOmor-O.

However, for a longitudinal reinforcing barin a column nominal covershall in anycase not be less than the diameter of such bar. In the case of column of minimumdimensions of 200 mm or underwhose reinforcing bars do not exceed 12 m m , a coverof 25 mm may b e used.

Forfootingsminimum covershallbe 50 mm. ~

It is felt that impropercoverhasbeenone ofthe main causes of deterioration ofstructures. Earlier provision in the code was for minimum clear cover. There istendency to attempt the minimum specified in construction which results in concretecovermuch belowthe requirements in actual practice. W ith the presentprovision of

nominal cover it has been attempted to achieve, in practice at least 20 mm covertoreinforcement including l inks.

5.2.6 Compaction and Finishing

The basic objective of compaction is to produce a solid void free mass. Dueimportance hasbeen g iven, in the revisionof the code, to this important aspect.

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5.2.7 Curing

Curing is usually specified to i) moisture retention, ii) permit proper strengthdevelopment, iii) prevent steep temperature gradient and iv ) maintain satisfactorytemperature regimes.

Adequate provision has been given in the Code and the concernedcomm itteehas initiated a separate code of practice fo r curing of concrete covering a ll aspectswhich could subsequently be referred in the Code.

5.3 Service Life of Structure

There has been proposal from some section of engineers that service life ofstructures should bedefined in ourcodes. It is , however, fel t that at this stage it wouldnot b e possible todefine service life of structures.

5.4 Acceptance Criteria

There is a general feeling thatthe acceptance criteria ofconcrete given in theexisting code which was based on Australian C ode A S 1480-1974, is quitecumbersome. The acceptance criteria has now been modified keeping internationalpractice and Indiancondition in mind. It lays down appropriate limits for flexuralandcompressive strength of individual test sample and also group of four consecutivesamples. It is feltthat this simplified acceptance criteria will be welcomed by all fieldengineers.

5.5 Quality Assurance

Quality in construction, in it s broadest sense, implies fulfilment of technical,technological, financial and othersocial needs that the constructed facility is expectedto ~atisfy. The requirement of the quality is the satisfaction of both the stated andimplied needs ofthe ownerand the user. Theconstructionshould resultin satisfactorystrength, serviceability and long term durabi li ty, so as tolower the overall life-cyclecostof the structure.

It is increasingly being recognised that adequate quality assurancemeasuresshould be taken in order thatthe propertiesofcompletedstructures are consistentwiththe requirements and the assumption madeduring the planning and design. To givespecial emphasis to the quality assurance aspect, a new clause covering quality

assurance and quality aspect during the planning, design and execution of anyconstruction has been proposed.

5.6 Batching

With the emergence of Ready M ix concrete plants in the country, it was feltnecessary to encouragethem to bring quality in concrete construction. It is , therefore,mentioned that Readymix concrete supplied by RMC p lant shallbe preferred.

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5.7 Inspection and Testing of Structures

The clause on inspection and testing of structures has been enlarged to givegeneral guidance on a ll aspects which should be considered for safety andserviceabilityof a given structure.

Other importantmodifications in Section 2 are:

a ) Proper limits have beenintroduced on the accuracy of measuringequipments

to ensure accurate batching ofconcrete.

b) Clause oftrealment of construction joint hasbeenmodified.

c) Stripping time for formwork has been modif ied. Now for vertical formwork tocolumn, walls, large beams minimum stripping time is 16 hrs.

d) A new clause o n placing of reinforcement has been introduced toemphasis that

rough handling, shock loading of reinforcementfrom a height should be avoided.It also givestolerances on placementand use of properspace chairsand othersupports so asto maintain propercover.

5.8 Design Requirements

Somesection of designersfeet that during the last revision ofthe Code WorkingStress Method (WSM) was retained alongwith Limit State Method (LSM) consideringthat it was a transition phase and slowlydesigners would change to limit state designcompletely. After 2 1 years of publication of that revision, the need to continue withworkingstress method.when code of othercountes have changed completelyto Limit

State Design, required rev iew . BS 8110:1985, CEB-FIP, Model Code 1990 havecompletely changed over to the limit state method, retaining the use of service loads(without load factor) only for carrying out serviceabilitylimit state calculation of crackWidth, deflection and vibration. The AmericanCode ACI 318:1989 allows the use ofworking stress method asan alternatemethod tolimitstate method and is included asa Appendixin that code. The GermanCode DIN 1045/1968 makes selective useof theWSM for shear and Torsion, for ensuring that the structure behaves properly underworking loads.

The feedback received during the workshop arranged by the Institution ofEngineers(India)atBombay few years a go indicated that eithera majorityofdesignersuseth e LSM orthatthe use of the WSM does not poseany undue difficulties. In therevised Code working stress methodhas been included in Annex.

5.8.1 ImportantModification in Section 3 General Design Requirement

1 ) Fire resistance - Clause on fire resistance have now been enlarged based on BS

8110. Thisnow includes apart from otherrequirements, minimum dimensionalrequirementofwall, column, slaba nd beamwith the minimum nominalcover fordifferentfire rat ings.

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ii ) Effective length of Cantilever has beenadded. As perthe new clause:The effectivelength of a Cantilevershall be takenas its length to the face of th e

support plus half the effective depth except where it forms the end of acontinuous beam where the length to the centre of supportshallb e taken.

iii) For substitute frame, it hasbeen recommended that rigorous analysis may be

required where side way consideration becomes critical.

iv ) Based on Comm ents/suggestion, bending moment coefficient at middle ofinterior span in Table7 of existing code hasbeen changed f rom 1/24 to1/16.It was felt that redistribution is assumed while giving these coefficient. Hence, ahigher moment value at mid span may occur than required for equilibriumcondition.

v) In addition to strength and s tability requirementscertain serviceability have tobeconsidered in designing fo r lateral load. These requirementsare intended toensure the satisfactory performanceof the structure underservice condition.

The main drift deflection criteria for high risebuilding is lateral drift. This is therelative magnitude of the lateral displacement at the top of a building withrespect to the height.

Under prominentwind load, the laterals sway at the top should not exceedH/500, where H is the total height of the bui lding. For seismic loading,referenceshould be made toIS 1893:1984.

Fig 3 of the existing code for modification factorfor tensionreinforcement hasnow been modif ied. Thecurves are now based on actualsteelstressat service

loads as comparedto the existing clauses which are based on allowable stress.(See AnnexIll).

vi) Clause of 23.2 of the existing code has been modified as follows, in view ofbetter clarity so that the two way slabs may not be designed as continuousbeam.

Slabsspanning in one direction and continuousover supportshall be designedaccordingtothe provision applicable tocontinuous beam.

via) Considering thatthe support momentsforadjacent panels calculated from Table22 would vary significantly, redistribution ofmoment should be allowed. Theprovision given in BS 8110(Part 1 ) was considered moreappropriateand a newclause regarding adlustment of support moment in resultant slab have beenadded.

viii) Recommendation regardingminimum eccentricity (Cl. 24.4 of IS 456:1978) hasbeen modified. It has been now addition that where biaxial bending isconsidered, it is onlynecessary to ensure that eccentricity exceeds the minimumabout one axis ata time. (Based on BS 8110 Part 1 )

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5.7.2 Requirements Governing Reinforcement and Detailing (Existingclause 25)

There has been proposals to bring some details regardingreinforcement anddetailing of reinforcementsfrom SP 24 . It is felt thatsince details are av ai lable in S P 24and SP 43:1981 Handbook on concrete reinforcementand detailing, one can alwaysreferto thosepublications, wheneverneeded.

However, provisionso f otherrequirementsin the code have beenreviewed in thel ight of comments received. Important Modification in this section are:

I ) Considering the bars in flexural or direct tension both have the same bondingcharacteristics with concrete. It was felt that therewas no need totreat the twocases separately. Accordingly the clause 25.2.5.1 has been modified. Themodified clause is in line with 3.12.8.13of BS 8110 (Part 1 ) and cl.12.2.3 ofA CI318:89.

ii) Recommendation regarding strength of welds have been modified to bring it in

line with the provision of c I. 3.12.8.18 of BS 8110 (Part 1). With thismodification, for joint in tension,value of 100% will be taken ifwelding is strictlysupervised a nd ifat anycross sect ion ofthe member not more than 20% ofth etensile reinforcement is welded.

5,7.3 Special Design Requirementsfor Structural Members and System (Section4, Clause 28 of IS 456:1978)

Important changes in this section are:

I) A new chaptero n design of concretecorbel has been added.A detailedchapter on walls based on Australian Code AS 3600 have beenincorporated.

ii) In additiontoth e requirementsof minimum tensile reinforcementbasedon depth of the section, provision of nominal reinforcementfor Concretesection ofthicknessgreater than 1 m also been given as 3 60 mm 2/meterlength in each direction on each face.

5.7.4 Structural Design(Limit State Method) (Section 5, Clause 34 of IS456:1978)

Majorchange brought in are:

i) Considering that the valuesof Design shearstrength?~fov 100 As/bd .~0.15 are used frequently in design, thesevalues have now been addedin Table 13 of Is 456. In addition values fo r 100 ,As/bd 13.00 has alsobeen added.

ii) A new clausehasbeen added for calculationof enhanced shearstrengthof sections close to supports (based on BS 8110).

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iii) Some modification in the torsion has also been made to make specificmention for equilibrium torsion and need fo r design fo r it . Thisis basedon Cl. 40.1 ofSF 24 and Cl.8.6 ofA O l 318:1989.

5.7.5 WorKing Stress Method (Section 6, Clause 43 of Is 456:1978)

In this section modifications regarding torsion and enhanced shearstrength, on

thesame linesas in section 5 (Limit State Method), have been made.

Other modificationbrought in is the existing clause46.3 on memberssubjected

to combineddirect load an flexure.

As per the existing clause, Members subjected to combined direct load andflexure and design by the methods basedo n elastic theory should be further checkedfor their strength under ultimate load con dit ion to ensureth e desired margin of safety.

In the proposed modification it is recommended that members subjected tocombined direct load and flexure shall be designed by LimitState Method.

CONCLUSION

Th e revision brings durability criteria as a major criteria keeping in view thechanging scenario world over in the field of concrete. Though it is required to bringchanges in the code keeping in viewthe presenttrend and knowledge, atth e sametimeit is also required to ensure that the revision is easily adaptable in the countryand itdoes not prevent the technological advancement in the country. Someofthe changesin the revised Code reflect that trend. Standards are always open for review. Any

proposals for modification at any s tage, can be referredto Bureau of IndianStandardsfor the consideration of the concerned Committee.

Ref: Dr. J.K.Prasad, FormerDeputy Director,Bureau ofIndianStandards.

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Annex 1

CONCRETE EXPOSEDT O SULPHATE ATTACK(Clause 8.2.2.4)

Class Concentration of Suiphates Express Dense fullycompactedas S O3 concretemade with 20

mm nominal maximumsizeaggregatescomplying with IS 38 3

In soil S O 3 in2:1 In groundTotal water:soil waterS O 3 extract

Cementcontentnotless thankg / rn

3

Traces Less than Less than Ordinary Port land(


27/96

Notes:

1 ) Cement contentgiven in Table for ordinaryPortlandCementis irrespectiveofgradesofcement.

2) Use ofsuper-suiphated cement is generally restricted where the prevailing

temperature isbelow40C.

3) Supersulphatedcementgives anacceptable lifeprovidedthattheconcreteisdense andpreparedwith a water/cementratio of0 .4 orless, in mineralacids,down topH3.5.

4) The cement contentsgiven inClass2aretheminimum recommended. For S O . ,contentsnearth e upperlimitofClass2, cementcontentsabove thesem inimumareadvised.

5) Forsevereconditionssuchas thin sectionsunderhydro-staticpressureonone

side onlyandsectionspartlyimm ersed, considerations shouldbe given to afurtherreductionofwater/cementratio.

6) Portland slag cement conforming to 1S:455 with slag content more than 50percentexhibitsbettersulphate resistingproperties.

7 ) Wherechlorideisalso encounteredalongwithsulphate insoilorgroundwater,ordinaryPortlandcementwithCA contentfrom5 to8percentshallbedesirabletobe used in concrete, insteadofsulphate- resisting cement. AlternativelyablendofordinaryPortlandCementandslagm ayalsobeusedprovidedsufficientinformation is available on performance ofsuch blended cements in these

conditions.


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Annex2

MINIMUM CEMENTITIOUS CONTENTS1MAXIMUM W/C RATIO AND MINIMUMGRADE OF CONCRETE FOR DIFFERENT EXPOSURE WITH NORMAL WEIGHT

AGGREGATES OF 20 MM NOMINAL MAXIMUM SIZE

Exposure Plain Concrete Reinforced Concrete Minimum GradeofConcrete

Minimum Maximum Minimum Maximum Plain ReinforcedCementkg/rn

3

Freew/c

Cementkg/rn3

Freew/c

C on cre te C on cre te

Mild 220 0.60 30 0 0.55 . M20

Moderate 250 0.60 30 0 0 .50 M15 M25

Severe 260 0.50 350 0 .45 M20 M30

Very 28 0 0.45 37 5 0 .45 M20 M35Severe

Extreme 30 0 0.40 375 0 .40 M25 M40

Notes:Cement contentprescribedin th e Table is irrespective ofthegradesofcementand it

is inclusive of addition mentioned in 5.2. Theadditions such as flyash orgroundgranulatedblastfurnaceslag mayb e taken intoaccountin th e concretecompositionwith respect to th ecement contentandw/cratio ifth esuitabilityisestablishedandaslong ast hemaximumamountstaken intoaccountd ono texceedth elimit ofpozzolanaandslagspecified in IS 1489(Part 1 ) and IS 455 respectively.

2 . Mirumum grade fo rPlain concrete undermildexposure conditionnot specified.

ADJUSTMENTS TO MINIMUM CEMENT CONTENTS FO R AGGREGATES OTHERTHAN 20 MM NOMINAL MAXIMUM SIZE

Nominal MaximumAggregatesSize

102040

Adjustments to Minimum Cement Contentsin above Table

kg/rn3

+400-30

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~

PERCENT AGE TENSION REINFORCEMENT

As requiredts = 0.58.fy.

A s provided

MODIFIC ATION F ACT OR FO R TENSION REINFORCEMENT

Annex3

D 4. 2 2.0 2.4 ~ 24 30

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MIX PROPORTIONING AND

QUALITY ASSURANCE

Centre forHuman Resourceand ContinuingE ducationNATIONAL COUNCIL FOR CEMENT AN D BUILDINGMATERIALS


31/96

CONCRETE MIX P ROPOR TIONING

The modification in the revision is marginal. In specifying a particulargradeof concrete, the following additional informationsare required.

Exposureconditions;

ii) Maximum temperatureofconcrete atthe time of placing;iii) Method ofplacing; andiv ) Degreeofsupervision.

Design Mix Concrete

The stipulations in the revision are asfollows:

As the guarantor o f quality o f concrete used in the construction, theconstructor shall carry out the m ix design and the mix so designed (not themethod of design) shall be approved by the employer within the limitations ofparameters and other stipulations laid down by thecode.

The m ix shall be designed to produce the grade of concrete having therequired workabilityand a characteristicstrength.

Mix design done earlier no t prior to on e year m ay be considered adequatefor later work provided there is no change in source an d the quality of thematerials.

Standard Deviation

The standard deviation for each grade of concrete shall be calculated,separately.

Standard deviation based on test strength of samples

a) Number of test results of samples The total number of teststrength of samples required to constitute an acceptable record forcalculation of standard deviation shall not be less than 30.

Attempts should be made to obtain the 30 samples, as early aspossible, when a mix is used forthe first time.

b) Whensignificant changes are made in the production ofconcretebatches (for example changes in the materials used, mix design,equipment or technical control), the standard deviation value shallbe separately calculated forsuch batches of concrete.

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c) Standarddeviation to b e broughtu p todate Th e calculationof thestandard d eviation shall be broughtup to date after every change of

mix design.

Assumed Standard Deviation

Where sufficient test results for a particular grade of concrete are notavailable, the value of standard deviation given in Table 1 m ay be assumed fordesign of m ix in the first instance. As soon a s the results of samples areavailable, actual calculated standard deviation shall be used and the mixdesigned properly.

Table 1: Assumed Standard Deviation

Grade of Concrete Assumed Standard Deviation, N/mm2

M 10M 15 3.5M 20M25M 30M 35M 45M45M 50

4.0

5.0

.

Note : Th e above values correspond to the s i te control having proper

storage of cement, weigh batching ofa ll materials, controlled addition ofwater, regular checking of a ll materials, aggregate gradings and moisturecontent, periodical checking of workability and strength. Where there isdeviation from the above, the values given in the above Table shall beincreased by 1 N/mm2.

However, when adequate past records for a similar grade exist and justifyto the designer a value of standard deviation different from thatshown in Table 1,it shall be permissible to use that value.

Nominal Mix Concrete

Nominal mix concrete m ay be used for concrete of M20 or lower. Theproportions of materials for nominal m ix concrete shall be in accordance withTable 2.

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Table2: Proportionsfor Nominal Mix Concrete

Grade of

Concrete

Total Quality of Dry Aggregates

by Mass per 5 0 k g of Cement , tobe taken as the sum of theIndividual Masses of Fine &Coarse Aggregates, I g , Max.

Proportion of

Fine Aggregateto CoarseAggregate (ByMass)

Quantity of

Water per 50kg of Cement ,Max.(I)~_________

M 5 800 Generally 1: 2bu t subject toan upper limitof1 :1 1/z anda lower limit of

1:21/2

6045M7.5 625

M 10 480 3432

MiS 330M 20 25 0 30

Note : The proportion of the fineto coarse aggregatesshould be adjustedfrom upper limit to lower l imi t progressively as the grading of fineaggregates becomes fine and the maximum size of coarse aggregatebecomes longer. Gradedcoarse aggregate shall be used.

Example : Fo r an average grading offine aggregate (that is Zone II ofTable 4 of IS:383), the proportions shall be 1: 1 % , 1: 2 and 1: 2 1/2 formaximum size ofaggregates 10mm, 20 m m and 4 0 m m respectively.

The cement content of the m ix specified in Table 2 for any nominal m ix

shall be proportionately increased if the quantity of water in a mix has to beincreased to overcome the difficulties of placement and compaction, so that thewater-cement ratio asspecified is not exceeded.

QualityAssurance Measures

In order that the properties of the completed structure be consistentwiththe requirements and the assumptions made during the planning and design,adequate quality assurance measures shall be taken. The construction shouldresult in satisfactory strength, serviceability and long term durability so as tolower the overall life-cycle cost. Quality assurance in construction activitiesrelates to proper design, use of adequate materials and components to besupplied by the procedures, proper w orkmanship in the execution ofworks by thecontractorand ultimately proper care during the use ofstructure including timelymaintenance and repair by the owner.

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Quahty assurance measures areboth technical and organizational. Somecommon cases should be specified in a general Quality Assurance Plan whichshall identi fy the key elements necessary to provide fitness of the strUcture andthe means by which they are to be provided and measured with the overallpurpose to provide confidence that the realized project will work satisfactorily inservice fulfilling the intended needs. The job of quality assurancewould involve

quality audit of both the inputs as well as the outputs; inputs in the form ofmaterials fo r concrete; workmanship in all stages of batching, mixing,transportation, placing compaction and curing and the related plant, machineryand equipments; resulting in the output in the form of concrete in place. Toensure proper performance, it is necessary that each step in concreting whichwill be covered bythe nextstep, is inspected as the workproceeds.

Each party involved in the realization of a project should establish andimplementa qualityassurance plan, for it s participation in the project. Suppliersand subcontractors activitiesshall be covered. Th e individual Quality AssurancePlan shall fit into the general QualityAssurance Plan. A quality Assurance Plan

shall define the tasks and responsibilities of all persons involved, adequatecontrol and checking procedures and the organization, and filling ofan adequatedocumentation of the building process and its results. Such documentationshould generallyinclude:

test reports and manufacturers certificate for materials, concretemix design detai ls;

ii) pour cards for site organization and clearance for concreteplacement;

iii) record of site inspectionof workmanship, fieldtests;iv) non-conformance reports, change orders;

v) control charts;vi) Statistical analysis.

Note Quali ty control charts are recommended wherever the concrete isin continuousproduction over considerableperiod.

1) r . S.(. Maiti, (icucral Managcr NCI3.

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ACCEPTANCE CRITERIA

Centre for Human Resource and Continuing EducationNATiONAL COUNCIL FOR CEMENT AND BUILDING MATERIALS


36/96

ACCEPTANCE CRITERIA

In L ! i e semiprobabilistic approach in IS 4561978,

loads a r i d strengths are treated as variable quantities and

the part:ial safety factors are meant to take into account

the variability, uncertainty and ignorance associated withthem, so as to result in an acceptable (low) probability ofattaining the limit states.

The random variation in the strength is assumed tofollow a normal distribution. The characteristics of thenormal distribution are such that in any set ofmeasurements, 68.27 per cent of results would lie withinone standard deviation from the mean, 94.45 per centresults within two standard deviations and 99.74 per cent

results within three standard deviations from the mean.

Accordingly, the characteristic strength is definedas that, below which not more than 5 per cent of resultsare expected to fall. This corresponds to a strength of

1.645 time the standard deviation (~) less than the meanstrength *

Characteristic strength = mean 1.645 i~.

IS 456 stipulates that for acceptance testing, random

samples from fresh concrete shall be made, cured and tested

at 28 days in accordance with Is 516. A random samplingprocedure shall be adopted to ensure that each concrete

batch shall have a reasonable chance of being tested; thatis, the sampling should be spread over the entire period ofconcreting and cover all mixing units.

In order to get relatively quicker idea of the quality

of concrete, e.g. to estimate the strength at the time ofremoval of forrnwork, tests may be carried out at early agesbut the acceptance is always on the basis of 28 daysstrength.

For judging the conformity of concrete strength, thequantity of concrete used for a structure, structuralcomponents, etc. is to be subdivided into lots on whichconformity is judged. The total volume of concrete for one

lot shall be produced under conditions, which are deemed tobe uniform (same family) . Concrete may be regarded as beinguniform (same family) if they are made with cement of the

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same type and strength class and from a single s~urce and

aggregate of the same geological origin and type (crushed

or uncrushed) . If admixtures or additions are used thesemay form separate lots.

The size of a lot may be:

The concrete produced (supplied) for each storey of abuilding or group of beams/slabs or columns/walls of astorey of a building.

But in no case more than 60 m3 or more than the production

of one weeks casting, which ever is less.

Is 456 recommendations on the number of samples required

for conformity control relates to sampling plan for sites

using site mixed concrete. When concrete Is being purchased

from a continuous production unit such as a RMC unit, atleast one sample shall be taken from each shift. Frequency

of sampling may be agreed upon mutually by suppliers and

purchasers.

The test result of a sample shall be the average of the

test results of three specimens made from one sample.

The main statistical features of the Acceptance Criteria

are as under:

i . Any individual test result is allowed to fall below

the characteristic strength. Such low results, whichare inevitable to occur, a re not regarded as

failures.ii. An absolute minimum value is also specified which is

taken as fck 3 N/mm2 for MiS and f~k 4 N/mm2 for M20and above. For purpose of compliance each sample testresult is expected to equal or exceed this value.

iii. Because of the random nature of strength of concrete,

a sample may have strength lower than thecharacteristic strength, but by the s am e lo gi c ther eshould be. other samples whose strength should have

exceeded the characteristic strength. The mean of all

such samples should then be not less than f~k+ k~(k is a constant and z~ is established standarddeviation).

iv. IS 456 Draft recommendation is based on the meanstrength determined from any group of four non-

overlapping consecutive test results. The mean of thegroup should equal or exceed the characteristic

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strengLh by 0.825 times established standarddeviation or 3N/rnm2 (4N/mm2 for M20 and above)whichever is greater.

v. The concrete is deemed to comply with strength

requirement when both the conditions are met.

(Acceptance criteria given in Is 456-1978 is appended for

the purpose of comparison).The acceptance is thus on the basis of average of fournon-overlapping consecutive samples tested from one lot.

The quantity of concrete represented by a group of

four consecutive test results shall include the batchesfrom which the first and last samples were taken togetherwith all intervening batches.

For the individual test results requirements, only theparticular batch from which the sample was taken shall be

at risk.

If the results of tests on moulded specimens do notfulfill the conformity requirements or not available or ifdefects of workmanship give rise to doubt as to strengthand the safety of the structure, supplementary testing oncores taken from the finished structure m ay be required ora combination of tests on cores and nondestructive testson the finished structure.

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Lclausc 1 5 JIJ!456-19781

15 . ACCEPTANCE CRJTERT..A

15.1 The c o l 1 c r c t c shall be deemed to comply with the strength require.ments if:

a) every sample hasa teststrength not less than the characteristic value;or

b) the strength df one or more samples though less than the cbarac.-terist.ic value, is in eachcase not less thao the greater of:

1) the characteristic strength minus 135 times the standard devia-tion; and

2) 080 times the characteristic strength;

and the average streog-th ol all the samples is not less than the

characteristic strength plus

1 1 6 5 1 6 5 ] times thestandard deviation.L ~/ number of samples j

15.2 Tbe concrete shall be deemed not to comply with the strength require-

m~tsi f :

~.) the strength olauy samplei s lees than the greater~f:I) the characteristic strength minus l~35tirne~the standard dcvia.

lion; and

2) 010 times the characteristic strength; or

b) theaverage strength olall thesamples is less than the cliaraclc~c~icstrength plus ~ ..

r1~65 __________________1 times the standard devhilion.L -.,~/ number ofsamples J

15.3 Concrete, which does not meet tbe strength requirements n~spcciIicdin 15J.. but has a .ctrebgth greater than that requiredby t5.2 may, at the dis~cretiori ofthe designer, be accepted as being structurally adu~quatc w h h c u u j ~further testing. .. ~1 - ~

15.4 Ifthe concrete is deemed not to compiy persuant to 15.2, the structuraladequacy oftheparts affected,sbaU be investigaI~d( cu~16 ) and an> :once.qucotial actionas neededshall be taken.

15.5 Concrete ofeach grade shall bea~sessed s e p a r a t e l y .

15.6 Concrete shall be assessed daily for compliance.

15.7 Concrete is liable to be rejectedif it is porous or boae~-combea; i i .placing has been interrupted without providing a proper cocsthictioojoint;the reinforcement has been displaced beyond the tolerances speciIicd; orconstruction tolerances have not been met. However, the bardenedconcrctcmay be accepted after carrying out suitable remedial measures o the s~~kfaction of ibe engineer-in-charge.

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ACCEPAThNCE CRITERIA

The concrete shall deemed to comply with the strength requirements if:

IS 456 1978 1S456 DRAFT

Any Individual

sample

Not less than

greater of:

i. f~1.35A

and

i i . . 0.8 f~

Not less than

- 3 N/mm2 (M15)

f0 k - 4 N/mm

2(M20 or more)

Average strength

( 4 samples)f~k+ 0.825 A Greater of

i. fa~ + 0.825 A

ii fo~ + 3 N/mm2(M15)

~ + 4 N/mm2(M20 or more)

p

2

-t

0~1

z

(A)

Notes:

For n number of samples

= f~+ 1. 65 A; favQ = fm,an 1. 65 A;

for n=4; fav. ~ + 1.65 A - 1.65 A = f~+ 0.825 A2

3A = f~, + 1. 65 A 3A = f~ - 1.35 A


41/96

DURABILITY REQUIREMENTS

Centre forHuman Resource and Continuing EducationNATIONAL COUNCIL FOR CEMENT AND BUILDING MATERiALS


42/96

DURABILITY OF CONCRETE

IS:456 1978 provides the following two paragraphs ondurabulit~~of concrete.

The durability of concrete depends on its resistance todeterioration and environment in which it is placed. ThereSiStaflCf? of concrete to weathering, chemical attack, abrasion,frost and fire depends largely upon its quality and constituentmaterials. Susceptibility to corrosion of the steel is governedby the cover provided and the permeability of concrete. The cubecrushing strength alone is not a reliable guide to the qualityand durability of concrete; it must also have an adequate cementcontent and low watercement ratio.

One of the main characteristics influencing the

durability of any concrete is its permeability. With strong,dense aggregates, a suitably low permeability is achieved byhaving a sufficiently low w/c ratio, by ensuring as thoroughcompaction of the concrete as possible and by ensuring sufficienthydration of cement through proper curing methods. Therefore,for given aggregates, the cement content should be sufficient toprovide adequate workability with low w/c ratio, so that concretecan be completely compacted with the means available.

In the revised version of 19:4546, the above two

paragraphs have been replaced by the following

GENERAL

A durable concrete is one that performs satisfactorilyin the working environment during its anticipated exposureconditions during service. The materials and mix proportionsspecified and used should be such as to maintain its integrityand, if applicable, to protect embedded metal from corrosion.

One o f the main characteristics influencing thedurability of concrete is its permeability to the ingress ofwater, oxygen, carbon dioxide, chloride, sulphate and other

potentially deleterious substances. Impermeability is governedby the constituents and workmanship used in making the concrete.With normalweight aggregates a suitably low permeability is

achieved by having an adequate cement content, sufficiently lowfree water/cement ratio, by ensuring complete compaction of the

concrete, and by adequate curing.

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The factors influencing durability include

a! the environment

b/ the cover to embedded steel

c/ the type and quality of constituent material

d/ the cement content and water/cement ratio of theconcrete

e/ workmanship, to obtain full compaction andefficient curing

f/ the shape and size of the member

The degree of exposure anticipated for the concrete during its

service life together with other relevant factors relating to mixcomposition, workmanship,design and detailing should beconsidered. The concrete mix to provide adequate durability underthese conditions should be chosen taking account of the accuracyof current testing regimes for control and compliance asdescribed in this code.

REQUIREMENTS FOR DURABILITY

ShaDe and Size of Member

The shape or design details of exposed structuresshould be such as to promote good drainage of water and to avoid

standing pools and rundown of water. Care should also be takento minimize any cracks that may collect or transmit water.

Adequate curing is essential to avoid the harmful effects ofearly loss of moisture. Member profiles and their intersectionswith other members shall be designed and detailed in a way toensure easy flow of concrete and proper compaction duringconcreting.

Concrete is more vulnerable to deterioration and due tochemical or climatic attack when it is in thin sections, insections under hydrostatic pressure from one side only, in partly

immersed sections and at corners and edges of elements. The hfof the structure can be lengthened by providing extra cover tosteel, by chamfering the corners or by using circular crosssections or by using surface coatings which prevent or reduce theingress of water, carbon dioxide or aggressive chemicals.

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Exposure Conditions

Appendix A of IS 4561978 provides guidance regarding

minin iu rn cement content and maximum W/C ratio required for plainas well as reinforced concrete to ensure durability under threeexposure conditions, ie mild, moderate and severe. Twomore exposure conditions i.e. very severe and extreme havenow been added. These have been detailed in Table 1 .

Abrasive i5 an another exposure condition. Concretemay sometimes be subjected to such condition e.g. action of metaltyred vehicles or say, water carrying solids in hydraulicstructures. For the durability requirements of concrete

subjected to such abrasive condition, specialist literature isto be referred.

Minimum Concrete Quality

The quality of concrete required to give satisfactory

performance depends on the severity of exposure and otherfactors, particularly the cover to steel reinforcements. The

quality of concrete, and specially that of cover concrete has tobe very good in order to resist the aggressive environments, ifany. The revised version of 18:456 includes minimum grade ofconcrete in addition to minimum cement content and maximumw/c ratio for different exposures. Table 1 9 of IS:4561978 hasthus been modified as given in Table 2.

Exposure to Sulphate Attack

Table 2~of IS:4561978 gives requirements for concrete

exposed to sulphate attack. This table has now been expanded forhigher concentrations of sulphates in soil and ground water. Thetable (Table 3 ) gives recommendations for the type of cement,maximum free W/C ratio and minimum cement -content, which arerequired at different sulphate concentrations in nearneutralground water of pH 6 to pH 9. For very high sulphateconcentrations in class 5 condition C 2 7 . SO3 in soil), some formof protection such as sheet polyethylene or polyehloroprene orsurface coating based on asphalt, chlorinated rubber, epoxy orpolyurethane materials should be used to prevent access by the

sulphate solution.

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Chlorides and Sulphates in Concrete

Whenever there is chloride in concrete, there is anincreased risk o - f corrosion of reinforcements in R.C.C. ThEhigher the chloride content or if subsequently exposed to warrr

moist conditions, the greater is the risk of corrosion. Some ofthe constituents of concrete may contain chlorides or concrete

may be contaminated by chlorides from the external environment.

IS:456-1978 stipulates that to minimize the chances ofdeterioration of concrete from harmful chemical salts, the levelsof such harmful salts in concrete coming from the concrete makingmaterials, that is cement, aggregates, water and admixtures, aswell as by diffusion from the environments should be limited.Generally, the total amount of chlorides (as CI) and the totalamount of soluble sulphates (as 903) in the concrete at the timeof placing should be limited to 0.157. by weight of cement and 4 / .by weight of cement respectively.

Sulphates are present in most cements and in someaggregates. Excessive amounts of watersoluble sulphate from

these or other mix constituents can cause expansion anddisruption of hardened concrete. In the revision of IS:456, theabove limit of 4 7 . sulphate (by weight of cement) is not changed.The 47 . limit, however does not apply to concrete made with supersulphated cement complying with IS 6909.

Regarding limit of chloride, a,.change has been made, byexpressing the chloride content as kg/m~of concrete, as shown inTable 4 .

Alkaliaggregate reaction

Some aggregates containing particular varieties ofsilica may be susceptible to attack by alkalis (Na

20 and K20)originating from the cement or other sources, producing anexpansive reaction which can cause cracking and disruption of

concrete. Damage to concrete from this reaction will normallyonly occur when all the following are present together:

a) a high moisture level, within the concrete;

b) a cement with high alkali content, or another source

of alkali;

c) aggregate containing an alkali reactive constituent.

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Where the service records of particular cement/aggregatecombination are well established, and do not include anyinstances of cracking due to alkaliaggregate reaction, nofurther precautions should be necessary. When the materials areunfamiliar, precautions should take one or more of the following

forms:

a) Use of nonreactive aggregate from alternate sources.

b) Use of low alkali ordinary Portland cement (OPC)having total alkali content not more than 0.6

percent (as Na20 equivalent)

C) Use of flyash conforming to IS 3812:1981 orgranulated blast furnace slag conforming to IS 12089as part replacement of ordinary Portland cement oruse of Portland Pozzoland Cement conforming toIS 1489 (Part I ) or Portland slag cement conformingto IS 455 provided pozzolana content is at least20 percent and in case of slag at least 50 percent.

d) Measures to reduce the degree of saturation of theconcrete during service such as use of impermeablemembranes.

e) Limiting the cement content in the concrete mix andthereby limiting, total alkali content in theconcrete mix. For more guidance specialist may bereferred.

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Environment

Mild

Moderate

Severe

Very severe

Extreme

TABLE I EXPOSURE CONDITIONS

Exposure Conditions

Concrete surfaces protected against

weather of aggressive conditions

Concrete surfaces sheltered from

rain or freezing whilst wet

Concrete exposed to condensation and

rain

Concrete continuously under water

Concrete in contact or buried under nonaggressive soil/ground water

Concrete surfaces exposed to severe rain,alternate wetting and drying oroccasional freezing whilst wet or severecondensation.

Concrete completely immersed in seawater

Concrete surfaces exposed to sea waterspray, corrosive fumes or severefreezing conditions whilst wet.

Concrete in contact or buried underaggressive subsoil/ground water

Concrete exposed to coastal environment

Surface of members in tidal zone, Mem-bers in direct contact with liquid/solidaggressive chemicals

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TABLE 2 MINIMUM CONTENTS OF CEMENTITIOUS MATERIALS, MAXIMUMW/C RATIO AND MINIMUM GRADE OF CONCRETE FOR DIFFERENTEXPOSURE WITH NORMAL WEIGHT AGGREGATES OF 2~MM NOMINALMAXiMUM SIZE

Plain Reinforced Minimum GradeConcrete concrete of concrete

Minimum Maximum Minimum Maximum Plain Reinforced

Cements Free Cement Free concrete concretekg/m3 w/c kg/m3 w/c

Severe 260

Very Severe 280

Extreme 300

NOTES

1 ) Minimum cement content prescribed in the Tableis irrespective of grades o f cement and it isinclusive of mineral admixtures such as flyash,

ground granulated blast furnace slag or silica

fume, The additions of flyash (conforming toGrade I of IS 3812) or ground granulated blastfurnace slag may be taken into account in theconcrete composition with respect to thecementjtjous materials content and water

cementitous materials ratio, if the suitabilityis established and as long as the maximumamounts taken into account do not exceed thelimit of pozzolana and slag specified inIS 1489(Part I ) and 19 455 respectively.

2) Minimum grade for plain concrete under mild

exposure condition not specified.

Exposure

Mild

Moderate

220 0.60 300 0.55 M20

250 0.60 300 0.50 MiS M25

0.50 350 0.4S M20 M30

0.45 375 0.45 M20 M35

0.40 375 0.40 M25 M40

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TABLE 3 CONCRETE EXPOSED TO SULPHATE ATTACK

1 Less than Less than Less than0.2 1.0 0.3

Ordinary Portlandcement or Portlandslag cement orPortland PozzolanaCement

280 0.55

0.3 to Ordinary Portland1.2 cement or Portland

slag cement orPortland PozzolanaCement

Supersul phatedcement or sulphateresisting Portland

Cemen t

310 0.50

3 0.5 to1.0

1.9 to3 .1

1.2 to Supersuiphated2.5 cement or Sulphate

resisting Portland

cementPortland Pozzolanacement or Portland

slag cement

330 0.50

350 0.45

5 Over2

2.5 to Supersuiphated or5.0 or sulphate

resisting Portland

Cement

Over 5.0 Over 5.0 Sulphate resistingPortland Cement orsupersulphated cement

with protective coatings

In soilTotal S03

Class Concentration of Sulphates Types of Cement Dense, fullyExpressed as 603 compacted

concrete made

with 20 mm.nominal maximumsize aggregates

complying withIS 383

In groundwater

603in 2:1water:

soil

extract

1 .g/L g/L

Cementcontentnot lessthan

kg/m3

Freewater

cementrationot

2 0.2 to0/5

1.0 to1.9

330 0.50

4 1.Oto

2.03.1 to5,0 370 0.45

400 0.40

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NOTES:

1 Cement content given in Table 3 for ordinary PortlandCement is irrespective of grades of cement.

2) Use of supersulphated cement is generally restricted wherethe prevailing temperature is below 40C.

3) Supersuiphated cement gives an acceptable life provided that

the concrete is dense and prepared with a water/cementratio of 0.4 or less, in mineral acids, down to pH 3.5.

4) The cement contents given in Class . 2 are the minimum recommended

For SO

3 contents near the upper limit of Class 2, cementcontents above these minimum are advised.

5) For severe conditions such as thin sections under

hydrostatic pressure on one side only and sectiDns partly

immersed, considerations should be given to a further

reduction o - f water/cement ratio.

6) Portland slag cement conforming to IS:455-19B9 with slag

content more than 50 percent exhibits better sulphate resisting

properties.

7) Where chloride is also encountered along with sulphate in soil

or ground water, ordinary Portland cement with 03A Content from5 to 8 percent shall be desirable to be used in concrete,instead of sulphate resisting cement. Alternatively, a blend

or ordinary Portland Cement and slag may be used provided

sufficientinformation is available on performance ofsuch blended cements in these conditions.

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TABLE 4 LIMITS OF CHLORIDE CONTENT OF CONCRETE

Type or use of concrete Maximum Total acid solublechlorid~ Content Expressedas kg/M of concrete

Concrete containing metal andsteam cured a elevcated

temperature and prestressedconcrete

Reinforced concrete or plain

concrete containing embedded

metal

Concrete not containing embeddedmetal or any material requiringprotection from chloride

0.4

0.6

3.0

R e f: I)r. S.(.. N1~uiti, GeiictaI Mana~cr, NCR.

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SHEAR CAPACITY ENHANCEMENTNEAR SUPPORTS

Centre forHuman Resource and Continuing EducationNATIONAL COUNCIL FOR CEMENT AND BUILDING MATERIALS


53/96

SHEAR CAPACITY ENHANCEMENT NEAR SUPPORTS

Shear failure at sections of beams without shearreinforcement will normally occur on plane inclined at an

angie 3 0 to the horizontal,. If the angle of failure planeis forced to be inclined more steeply than this(because the

section considered x-x in Fig.l is close to the supportor for other reasons) the shear force required to produce

failure is increased. The reason for this is that, for any

sections closer to the support than the critical section, a

substantial proportion of the load will be carried throughto the support directly by the strut and not by way of thenormal actions o f shear and bending. The closer the load to

the support, the greater is the proportion of the load thatwill be transmitted to the support in this way. The

enhancement of shear strength may be taken into account inthe design of sections in short members such as corbels or

in beams where the load is applied close to the support.Any such enhancement should be ignored when checking theshear resistance of the notional concrete compressivestruts.

A plot o f tes t results illustrating the relationship

between a/d and v/va , for beams without stirrups isreproduced in Fig. 2 . The line shown on the graph is

straight for all values of a~/dgreater than 2,when v/va IS1 . The results shown in Fig. 2 derive from tests on short-

span, point-loaded beams but the results are applicable to

any short member where the failure plan~is constrained toform at an angle greater than tan1 (1/2) to the horizontal.The enhancement in strength can therefore be applied forany section closer to a support than 2d.

The enhancement of shear strength may be takeninto account in the design of sections near a support by

increasing the design concrete shear stress, v t . . to v~2d/a~

provided that v at the face o f the support remains lesserthan the value given for maximum shear stress.

As outlined above, concentrated loads close to direct

supports lead to an enhancement of the design shear

resistance.

The strength of short beams depends to a great extent

upon the detailing of the reinforcement. Adequate anchoragemust be provided to the main tensile reinforcement.Vertical stirrups are not very effective in beams in which

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a~/d is less than 0.6. In such cases horizontal stirrupsparallel to the main tension reinforcement are recommended.

This enhancement is particularly useful for corbels orpile caps or where concentrated loads are applied close tothe support o f a beam.

Bottom loaded beams:

A further point to note is that this ~nhancement can

be applied only where the load is applied to the top faceof the beam and the support is at the bottom. Where load is

applied near the bottom of a section, sufficient vertical

reinforcement to carry the lo ad s ho ul d be provided in

addition to any reinforcement required to resist shear.

Shear reinforcement for sections close to supports:If shear reinforcement is required, the total area of thisis given by:

EA5~ = a~b {v2dv~/a~}/(0.87f~ )> 0.4 a~b/0.87 f~

wherev = nominal shear stress

= design shear strength of concreteb = breadth of the memberd = effective depth

a~=shear span

The above equation considers that the effect of enhancement

is only on v~and

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