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CIP15-Chemical Admixtures for Concrete.pdf

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NRMCA What, Why & How? Chemical Admixtures for Concrete CONCRETE IN PRACTICE CIP 15 L to R: HRWR, Air-Entraining Agent, Retarder Relative quantities for one cu.yd. Chemical Admixtures for Concrete WHAT are Admixtures? Admixtures are natural or manufactured chemicals added to the concrete before or during mixing. The most often used admixtures are air-entraining agents, water reducers, water-reducing retarders and accel- erators. WHY Use Admixtures? Admixtures are used to give special properti
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  NRMCA What, Why & How? Chemical Admixturesfor Concrete CONCRETE IN PRACTICE CIP 15 L to R: HRWR, Air-Entraining Agent, RetarderRelative quantities for one cu.yd. Chemical Admixtures for Concrete WHAT  are Admixtures? Admixtures are natural or manufactured chemicalsadded to the concrete before or during mixing. Themost often used admixtures are air-entraining agents,water reducers, water-reducing retarders and accel-erators. WHY  Use Admixtures? Admixtures are used to give special properties to freshor hardened concrete. Admixtures may enhance the du-rability, workability or strength characteristics of agiven concrete mixture. Admixtures are used to over-come difficult construction situations, such as hot or cold weather placements, pumping requirements, earlystrength requirements, or very low water-cement ratiospecifications. HOW  to Use Admixtures Consult your ready mixed concrete supplier about whichadmixture(s) may be appropriate for your application.Admixtures are evaluated for compatibility withcementitious materials, construction practices, job speci-fications and economic benefits before being used. Follow This Guide to Use Admixtures 1. A IR -E NTRAINING  A DMIXTURES are liquid chemicals addedduring batching concrete to produce microscopic airbubbles, called entrained air, when concrete is mixed.These air bubbles improve the concrete’s resistanceto damage caused by freezing and thawing and deic-ing salt application. In plastic concrete entrained airimproves workability and may reduce bleeding andsegregation of concrete mixtures. For exterior flatwork(parking lots, driveways, sidewalks, pool decks, patios)that is subject to freezing and thawing weather cycles,or in areas where deicer salts are used, specify a nor-mal air content of 4% to 7% of the concrete volumedepending on the size of coarse aggregate (see Tableon the next page). Air entrainment is not necessary forinterior structural concrete since it is not subject tofreezing and thawing. It should be avoided for con-crete flatwork that will have a smooth troweled finish.In high cement content concretes, entrained air willreduce strength by about 5% for each 1% of air added;but in low cement content concretes, adding air hasless effect and may even cause a modest increasedstrength due to the reduced water demand for requiredslump. Air entraining admixtures for use in concreteshould meet the requirements of ASTM C 260, Speci- fication for Air-Entraining Admixtures for Concrete  .2. W ATER  R EDUCERS are used for two different purposes:(1) to lower the water content in plastic concrete andincrease its strength; (2) to obtain higher slump with-out adding water. Water-reducers will generally reducethe required water content of a concrete mixture for agiven slump. These admixtures disperse the cementparticles in concrete and make more efficient use ofcement. This increases strength or allows the cementcontent to be reduced while maintaining the same Copyrighted information from the National Ready Mixed Concrete Association. All rights reserved.Reproduction prohibited without permission. Print copies can be obtained from NRMCA (301) 587-1400 x 101   ©National Ready Mixed Concrete Association.  All rights reserved.  No part of this publication may be reproduced in any form, including photocopying or other electronic means, without permission in writingfrom the National Ready Mixed Concrete Association. Technical information prepared by National Ready Mixed Concrete Association 900 Spring StreetSilver Spring, Maryland 20910 References 1 .ASTM C 260 C 494, C 1017, D 98, American Society for Test-ing and Materials (ASTM), West Conshohocken, PA, www.astm.org. 2. Chemical and Air-Entraining Admixtures for Concrete , ACIEducational Bulletin, E4, American Concrete Institute,Farmington Hills, MI, www.aci-int.org. 3. Chemical Admixtures for Concrete , ACI 212.3R, AmericanConcrete Institute, Farmington Hills, MI.4.  Building Code Requirements for Structural Concrete , ACI 318,American Concrete Institute, Farmington Hills, MI.5. Understanding Chloride Percentages , NRMCA Publication No.173, NRMCA, Silver Spring, MD, www.nrmca.org.  ©National Ready Mixed Concrete Association.  All rights reserved.  No part of this publication may be reproduced in any form, including photocopying or other electronic means, without permission in writingfrom the National Ready Mixed Concrete Association. Technical information prepared by National Ready Mixed Concrete Association 900 Spring StreetSilver Spring, Maryland 20910 Printed in U.S.A.Copyright NATIONAL READY MIXED CONCRETE ASSOCIATION, 1987 AND 1989, 2001CIP 15/1090/10.0/DMSC strength. Water-reducers are used to increase slumpof concrete without adding water and are useful forpumping concrete and in hot weather to offset the in-creased water demand. Some water-reducers mayaggravate the rate of slump loss with time. Water-re-ducers should meet the requirements for Type A inASTM C 494 Specification for Chemical Admixtures for Concrete  .Mid-range water reducers are now commonly used andthey have a greater ability to reduce the water con-tent. These admixtures are popular as they improvethe finishability of concrete flatwork. Mid-range waterreducers must at least meet the requirements for TypeA in ASTM C 494 as they do not have a separate clas-sification in an admixture specification.3. R ETARDERS are chemicals that delay the initial settingtime of concrete by an hour or more. Retarders areoften used in hot weather to counter the rapid settingcaused by high temperatures. For large jobs, or in hotweather, specify concrete with retarder to allow moretime for placing and finishing. Most retarders also func-tion as water reducers. Retarders should meet the re-quirements for Type B or D in ASTM C 494.4. A CCELERATORS reduce the initial set time of concreteand give higher early strength. Accelerators do not actas an antifreeze; rather, they speed up the setting andrate of strength gain, thereby making concrete stron-ger to resist damage from freezing in cold weather.Accelerators are also used in fast track constructionrequiring early form removal, opening to traffic or loadapplication on structures. Liquid accelerators meetingthe requirements for ASTM C 494 Types C and E areadded to the concrete at the batch plant. There aretwo kinds of accelerating admixtures: chloride basedand non-chloride based. One of the more effective andeconomical accelerators is calcium chloride, which isavailable in liquid or flake form and must meet the re-quirements of ASTM D 98. For non-reinforced con-crete, calcium chloride can be used to a limit of 2% bythe weight of the cement. Because of concerns withcorrosion of reinforcing steel induced by chloride, lowerlimits on chlorides apply to reinforced concrete. Pre-stressed concrete and concrete with embedded alu-minum or galvanized metal should not contain anychloride-based materials because of the increased po-tential for corrosion of the embedded metal. Non-chlo-ride based accelerators are used where there is con-cern of corrosion of embedded metals or reinforce-ment in concrete.5. H IGH  R ANGE  W ATER  R EDUCERS  (HRWR)  is a special classof water-reducer. Often called superplasticizers, HRWRsreduce the water content of a given concrete mixturefrom 12 to 25%. HRWR are therefore used to increasestrength and reduce permeability of concrete by reduc-ing the water content in the mixture; or to greatly in-crease the slump to produce “flowing” concrete withoutadding water. These admixtures are essential for highstrength and high performance concrete mixtures thatcontain higher contents of cementitious materials andmixtures containing silica fume. For example, adding anormal dosage of HRWR to a concrete with a slump of3 to 4 inches (75 to 100 mm) will produce a concretewith a slump of about 8 inches (200 mm). Some HRWRsmay cause a higher rate of slump loss with time andconcrete may revert to its srcinal slump in 30 to 45minutes. In some cases, HRWRs may be added at the jobsite in a controlled manner. HRWRs are covered byASTM Specification C 494 Types F and G, and Types 1and 2 in ASTM C 1017. Specification for Chemical Ad- mixtures for Use in Producing Flowing Concrete. Besides these standard types of admixtures, there are products available for enhancing concrete properties for a wide variety of applications. Some of these productsinclude: Corrosion inhibitors, shrinkage reducing admix-tures, anti-washout admixtures, hydration stabilizing or extended set retarding admixtures, admixtures to reduce potential for alkali aggregate reactivity, pumping aids,damp-proofing admixtures and a variety of colors and  products that enhance the aesthetics of concrete. Con-tact your local ready mixed concrete producer for moreinformation on specialty admixture products and the benefits they provide to concrete properties. Recommended Air Content in Concrete 4  Nominal maxaggregate size,mm (in.)Air content, percentSevereModerateexposureexposure9.5(3/8)7.5612.5(1/2)75.519.0(3/4)6525.0(1)64.537.5(1 1  ⁄  2 )5.54.550(2)5475(3)4.53.5 Severe exposure — concrete in cold climate will becontinuously in contact with water prior to freezing or where deicing salts are used.Moderate exposure — concrete in a cold climate will beonly occasionally exposed to moisture prior to freezing and not exposed to deicing salt application.
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