precast quality guide

7/31/2019 Precast Quality Guide

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Fundamentalsof Quality Precast

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Fundamentals of Quality Precasttoc-2

CONTENTSCopyright 2000Revised 2002, 2004, 2005National Precast Concrete Association

All rights reserved. No part of this publication may be reproducedor used in any form or by any means, electronic, or mechanical,including, photocopying, recording, taping, or information storageand retrieval systems withour the prior written permission of thecopyright owners.

INTRODUCTIONREFERENCE MATERIAL

esirable Properties -1

esirable Properties ofardened Concrete -2

ssentials for Producing

Quality Concrete -3

Summary -5

Section 1

CONCRETE MATERIALS

REFERENCE MATERIAL

Aggregates 1-1

ightweight Aggregates 1-2

Aggregate Testing 1-3

Cement 1-5

Types of Cements 1-6

lended Cement 1-8

Admixtures 1-9

Water 1-12

aterials Certificates

requency Report 1-13

Samples of Material Certificates 1-15

Summary 1-27

andouts

Contributors

Carl Buchman, P.E.Dean Frank, P.E.Mel C. Marshall, P.ENGBrian MillerAlex Morales

NPCA | 10333 N. Meridian St., Suite 272 | Indianapolis, IN 46290(800) 366-7731 | (317) 571-0041 fax | www.precast.org


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Section 2EINFORCEMENT

REFERENCE MATERIAL

Introduction 2-2

on-Reinforced Concreteehavior 2-2

einforced Concrete

ehavior 2-3

ffectiveness of Placement 2-3

einforcement Types 2-3

einforcing Bars 2-4

.S. Customary and

etric Bars 2-4

ebar 2-4

einforcing Wire 2-4

ar Mars and WeldedWire Reinforcement 2-4

lan Welded Wire

einforcement 2-4

eformed Welded Wire

einforcement 2-5

Zinc or Epoxy-Coated

einforcement 2-5

Concrete Cover foreinforcement 2-5

Structural Integrity 2-5

inimum Reinforcementend Diameters 2-5

Carbon Equivalence 2-6

ASTM A615 Reinforcement 2-6

ASTM A706 Reinforcement 2-6

Steel Area and Bar Sizes 2-7

Summary 2-9

Section 3MISC. MATERIALS AND COMPONENTS

REFERENCE MATERIAL

Lifting Devices

and Apparatus 3-1

Embedded Steel Shapes,

Plates, and Hardware 3-2

Accessories 3-3

Fiber Reinforcement 3-3

Summary 3-5

Section 4

CONCRETE MIXES

REFEREN E MATERIAL

Concrete Mixes 4-1

Compressive Strength 4-1

Wet Cast Concrete 4-2

Dry Cast Concrete 4-2

Air In Concrete 4-2

Alkali-Silica Reaction (ASR) 4-3

Delayed Ettringite

Formation (DEF) 4-3

Summary 4-5


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Fundamentals of Quality Precasttoc-4

Section 5

BATCHING AND MIXING

REFERENCE MATERIAL

PCA Requirements 5-1

onsiderations for

izing Batch Plants 5-1

electing Mixer 5-1

ommon Types of Mixers 5-2

onsiderations forAggregate Storage 5-2

roper Aggregate Storage 5-2

illing Aggregate Bins 5-3

ischarge of Rawaterials into Mixer 5-3

ischarge of Concrete

rom Mixer 5-3

oncrete Dischargerom Mixer 5-3

andling Concrete 5-3

ouring Concrete 5-3

onveyor Belts 5-3

amples of Batch Plant

chematics 5-8

ummary 5-17

Section 6

PRE-POUR OPERATIONS

REFERENCE MATERIAL

easoning 6-1

re-pour Inspectionnd Checklist 6-1

orm Preparation 6-1

re-pour Checklist 6-2

leaning 6-3

orm Release Agents 6-3

lockouts 6-4

lockout Securing Methods 6-4

ummary 6-5

Section 7

PRODUCTION PRACTICES

REFERENCE MATERIAL

Consolidation 7-1

Why Vibrate? 7-1

Amplitude & Frequencyof Vibration 7-2

Factors to Consider

When Selecting aConsolidation Method 7-2

Vibration Frequency 7-2

Interal Vibration

Procedure 7-2

Overlapping Field of

Actions 7-3

Form (External) Vibration 7-3

Form Vibrator Sizing 7-4

Form Vibration Procedure 7-4

Table Vibration 7-4

Unidirectional Vibration 7-4

Effects of Under-Vibration 7-4

Effects of Over-Vibration 7-5

Curing 7-5

Why is Curing Essential? 7-6

Essentails of

Proper Curing 7-6

Concrete Strength vs.

Moisture Condition 7-6

Concrete Setting Times 7-7

Low Temperature

vs. Strength 7-7

Maintaining Moistureby Wetting 7-7

Spraying/Misting 7-7

Accelerated Curing 7-8

Wet-Cast Product Curing 7-8

Dry-Cast Product Curing 7-8

Special Conditions 7-8

Considerations forAccelerated Curing 7-8

Typical Accelerated

Curing Cycle 7-8

Target Temperatures 7-9


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els Method for

etecting Flueing 7-9

Summary 7-11

Section 8

POST-POUR OPERATIONS

REFEREN E MATERIAL

Stripping 8-1

roduct Checking 8-2

epair 8-2

Storage 8-3

oading Out 8-3

Shipping 8-5

Summary 8-7

Section 9

QUALITY CONTROL AND OPERATIONS

REFERENCE MATERIAL

The Big Picture 10-1

PCA Plant Certification Program 10-2

Safety 10-3

Specs, Standards and References 10-4

rawings 10-4

Concrete Testing 10-4

Aggregate Testing 10-7

ecord Keeping 10-9

Summary 10-10


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Fundamentals of Quality Precast i-1

Introduction

INTRODUC

Desirable Properties

Concrete is basically a mixture of twocomponents: aggregates and paste. Toachieve good concrete we, of course, mustuse good materials and good practices.Further, fresh concrete needs to have thefollowing desirable properties:

Consistency Loosely defined,consistency is the relative mobility ofthe concrete mixture. It is measured interms of slump the higher the slump

the more mobile the mixture and itaffects the ease with which the concretewill flow during placement. It is related to,but not synonymous with workability. Inproperly proportioned concrete, the unitwater content required to produce a givenslump will depend on several factors.Water requirement increases as aggregatesbecome more angular and rough textured(but this disadvantage may be offset byimprovements in other characteristicssuch as bond to cement paste). Required

mixing water decreases as the maximumsize of well-graded aggregate is increased.It also decreases with the entrainment ofair. Mixing water requirements usually arereduced significantly by certain chemicalwater-reducing admixtures.

Workability r Placeability Workabilityis considered to be that property of

concrete that determines its capacity to beplaced and consolidated properly and tobe finished without harmful segregation.It embodies such concepts as mold-ability, cohesiveness, and compactability.Workability is affected by: the grading,particle shape, and proportions ofaggregate; the amount and qualities ofcement and other cementitious materials;the presence of entrained air and chemicaladmixture; and the consistency of themixture.

Uniformity We must be confident thatall portions of a batch or load of concretecontains the same proportions of the sameingredients, will behave the same way, andwill give essentially similar results aftercuring. Uniformity results from procuringraw materials that do not vary in eitherproportion or makeup. Additionally, properuniformity can be achieved by performingmixing and transport according to properprocedures, and consistently using good

placing, vibration, finishing and curingtechnique.

Finishability This is an important part ofworkability, since it strongly relates to thelook and quality of the exposed surfacesof the hardened concrete. A mix that


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i-2

ECTION ntroduction

requires over-troweling or the addition ofsurface water to bring up an acceptablenish, or develops excessive bleed water,oes not have good finishability (since

ach of these will effect the exposed layerf concrete).

Bleeding rate Bleeding is the migrationf water to the top surface of freshly

placed concrete caused by the settlementf the solid materials within the mass.

Excessive bleeding increases the water-ement ratio near the top surface, and aeak top layer with poor durability may

result (particularly if finishing operationsake place while bleed water is present).

Therefore, a minimum amount of bleedater is desirable, barely sufficient tonhance finish troweling or screeding.

ontrolled setting time The bindingquality of portland cement paste is dueo the chemical reaction between theement and water (called hydration). The

rate of reaction is important because therate determines the time of setting andhardening. The initial reaction must beslow enough to allow time for the concrete

o be transported, placed, and finished, butnot so slow that setting and strength gaino not begin within a reasonable time after

being finished.

Temperature Ideal placing temperatureis between 50 and 60 degrees F. Cooleremperatures simply slow down the timef initial set and the rate of strength gain,hile hotter temperatures do the opposite.

Excessively hot temperatures, whetheraused by over-mixing, hot ingredients,

hot forms or ambient temperatures,an create difficulties in fresh concrete.

These include increased water demand,accelerated slump loss, faster setting rate,more plastic cracking, and reduction inntrained air.

Desirable Properties ofHardened Concrete

Durability Ideally wed like concrete tolast thousands of years (and some has).To achieve long-term durability it must beresistant to: exterior chemical attack, aswell as that which could occur internally,like alkali-silica reaction; carbonation;ultra-violet ray deterioration; and freeze-haw deterioration.

Water-tightness Whereas it may beacceptable to have water pass easilyhrough a sidewalk, it isnt acceptable

o have underground structures leakingontained fluids or allowing water

infiltration. Good concrete is watertight,ven without applied membranes.

(However, membranes have their use andheir place.)

Strength Compressive strength is thegeneral measure of concrete quality,although tensile, flexural, shear, andorsional strengths are also importantstrengths. Compressive strength is also

he easiest to evaluate, and is thereforehe normal specification measure; we countn concrete to be strong in compression.

Abrasion resistance Floors, pavements,and hydraulic structures are subject toabrasion, and therefore require a highabrasion resistance. Test results indicateabrasion resistance is closely related tohe compressive strength of concrete.Also, in these cases, an abrasion resistantaggregate is also required.

Appearance Appearance is importanto the buyer. Bug holes, bleeding,surface rusting, form joints, dimensionaliscrepancies, finish or color variations,

and pour lines are unacceptable.


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Fundamentals of Quality Precast i-3

Introduction

Dimensional stability Concrete isnot silly putty, and it is counted on tobe dimensionally stable. Nevertheless,concrete does change in volume due to

temperature change, moisture change(drying or the reverse), load application(both immediate deformation and a timedependent change, called creep), and evensome chemical reactions. Fortunately, withproper manufacture, reinforcing, designand application, good concrete staysrelatively stable dimensionally.

Economy Concrete is probably the mostwidely used building material in the world,generally available locally, and at a very

competitive price. We must always keepit that way, without sacrificing any of itsother important properties.

Essentials for ProducingQuality Concrete

Suitable materials he quality of theconcrete depends primarily upon thequality of the paste (cement and water);and since aggregates make up 60% to70% of the total volume of concrete, their

characteristics are also vitally important.All the ingredients of a mix must beselected to achieve the design objectives,for compatibility with each other, and for aconsistent level of quality.

Design Without it, nothing will workvery well. This is true of the mix design,the product and shape design, thereinforcing design, and the applicationdesign. Poor workmanship can ruin agood design, but good workmanship wontimprove bad design.

Proportioning, mixing, transportingof batch As was said about design,each step in the process must be donecorrectly, over and over again. If theproper mix design isnt accuratelyproportioned, thoroughly mixed, and

properly and swiftly delivered, all thesteps that follow will not produce qualityconcrete.

Piece and form setup Here is where itgets down and dirty for the employees on

the shop floor (who usually dont haveanything to do with the original concretebatch). From here on, just remember theold question: If we have time to do itright the second time around, why didntwe take the time to do it right the firsttime?

Placing and consolidation Concreteresponds well to patient handling and

good technique. When we start gettinglazy, sloppy, rushed like dumpingloads from too high up or trying to moveconcrete with vibrators rather than handtools, skipping a corner with the vibratoror over vibrating to be safe and drivingout too much air results surely suffer.Proper technique must be taught and thenexpected.

Finishing Over-finishing can be asharmful as not properly finishing at all.

And who hasnt failed to see that littlenote on the drawing calling for a differentor special finish on some part of thattypical piece until after it was made?

Curing Rather than an after thought,or just the last step, curing is the desertfollowing a perfect meal. Curing is themaintenance of a satisfactory moisturecontent and temperature in concreteduring a definite time period immediatelyfollowing placing and finishing so the

desired properties may develop. The needfor adequate curing of concrete cannotbe overemphasized. Curing has a stronginfluence on durability, strength, water-tightness, abrasion resistance, volumestability, and resistance to freezing andthawing and deicer salts.


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ECTION ntroduction

SUMMARY

Essentials for ProducingQuality Concrete

l Suitable materialsl Designl Proportioning, mixing, transporting of

batchl Piece and form setupl Placing and consolidationl Finishing

l Curingl Quality Control procedures

Desirable Propertiesof Fresh Concrete

l Consistency

l Workability

l Uniformity

l Finishability

l Bleeding ratel Controlled setting

l Temperature

l Economically

Desirable Properties ofHardened Concrete

l Durability

l Water-tightness

l Strength

l Abrasion resistance

l Appearance

l Dimensional stability.

NotesNotes


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Fundamentals of Quality Precast 1-1

Concrete Materials

CONCRETEMATERIALS

Grading & ASTMSpecification C33

grading is thedistribution of an aggregate as determinedby a sieve analysis. Look at these twoscreens for an example of the C 33 limitsfor fine aggregate and for one typicallyused grading (size number) of coarseaggregate. Reference Design and Controlof Concrete Mixtures, PCA, 14th edition

fig. 5-6, Page 82 Fine Aggregate Grading

Limits, Table 5-3, Page 83. There areseveral reasons for specifying gradinglimits and maximum aggregate size. The

grading and maximum size of aggregateaffect relative aggregate proportions aswell as cement and water requirements,workability, pumpability, economy,porosity, shrinkage, and durability ofconcrete. Variations in grading canseriously affect the uniformity of concretefrom batch to batch. Very fine sands areoften uneconomical; very coarse sandsand coarse aggregate can produce harsh,unworkable mixes. In general, aggregatesthat give a smooth grading curve and do

not have a large definciency or excess ofany size will produce the most satisfactoryresults.

Coarse and fine Since the cement is themost expensive ingredient in a mix, andin many ways the most troublesome, wewant to proportion the mix to require the

Aggregates

Manufacture Natural gravel and sandare usually dug or dredged from a pit,river, lake, or seabed. Crushed aggregateis produced by crushing quarry rock,boulders, cobbles, or large-sized gravel.Aggregates are usually graded andwashed at the pit or plant. Some variationin the type, quality, cleanliness, grading,moisture content, and other properties isto be expected, as we are dealing with amaterial made by nature.

Classification We have two tablesto look at and discuss. The first isRock and Mineral Constituents inAggregates; it is important to notenot all naturally occurring aggregatesare good constituents for concrete.The second table, Characteristics andTests of Aggregates, pulls together allthe important aspects we need in goodaggregates, and the reasons for them,as well as giving you the ASTM Test

Designation which is used to evaluatethem. Reference Design and Control ofConcrete Mixtures, PCA, 14th edition Table

5-1, Page 80 and Table 5-2,

Page 81


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Fundamentals of Quality Precast1-2

1SECT ONEleast amount of cement paste necessary tooat all the aggregate particles and floathem in suspension, while giving us therequired strength, workability, durability,

tc.Reference Design and Control of

oncrete Mixtures, PCA, 14th edition Fig.

5-7, Page 82.

Moisture condition Reference Designand Control of Concrete Mixtures, PCA,

14th edition Fig. 5-12, Page 88. This pictureillustrates the moisture condition ofaggregates; they are designated:

Oven dry fully absorbent

Air dry dry at the particle surface butcontaining some interior moisture, thusstill somewhat absorbent

Saturated surface dry (SSD) neitherabsorbing water from nor contributingwater to the concrete mixture

Damp or wet containing an excessof moisture on the surface, called freewater

The amount of water used for the concretemixture must be adjusted for the moistureonditions of the aggregates in order to

meet the designated water requirement. Ifhe water content of the concrete mixtureis not kept constant, the compressivestrength, workability, and other properties

ill vary from batch to batch.

Deleterious substances This referso any material in the aggregate that isetrimental to the concrete; these would

include clay lumps and friable particles(examples include sandstone and shale),hert (due to cracking associated withrost susceptibility or the formation ofsurface pop outs), material less than theNo. 200 sieve (75-mm), coal and lignite,and such debris as plant roots or twigs.

Size determination & limitation Theaggregate sizes are usually chosen byhe mix designer from experience, orsometimes as required by specification.

Use of the largest size aggregate generallyleads to the most economical mix, butsmaller size aggregates are preferred overlarger size aggregates for high strengthmixes. Specifications limit the maximumsize of the coarse aggregate to 1/5 thenarrowest dimension between sides oforms, 1/3 the depth of slabs, or theminimum clear spacing between individualreinforcing bars or wires, bundles or bars,r prestressing tendons or ducts.

ap-Graded aggregates Gap-gradedmixes are used to obtain uniform texturesin exposed-aggregate concrete by omittingertain particle sizes. They are also used

in normal structural concrete becausef possible improvements in density,

permeability, shrinkage, creep, strength,onsolidation, and to permit use of local

aggregate gradations. There are manyther adjustments the mix designer must

make when using gag-graded mixes torender them useable.

Lightweight Aggregates

Lightweight aggregates are used primarilyo reduce weight, and that can be toreduce weight of a structure because ofload bearing foundation problems, orbecause of shipping or handling problems for example, to keep a piece weightlegal on a truck, or so it can be liftedby available cranes. You should know

hat lightweight aggregates have manyther uses as well, such as insulatingoncrete units, cellular or foam concretes,

and no-fines concretes. Since the usef lightweight aggregates is generally

minimal compared to normal weightaggregates, this section will be somewhatabbreviated.


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Concrete Materials

Manufacture Structural gradelightweight aggregates are produced inmanufacturing plants from raw materialsincluding suitable shales, clays, slates,

fly ashes, or blast furnace slags. Thereare also naturally occurring lightweightaggregates, which are mined from volcanicdeposits, such as pumice and scoria types.Pyroprocessing methods include:lRotary kiln process a long, slowly

rotating, nearly horizontal cylinder linedwith refractory materials similar toement kilns

l intering process a bed of rawmaterials including fuel is carried by a

raveling grate under ignition hoods

lRapid agitation of molten slag withontrolled amounts of air or water

Gradations Lightweight aggregatescome in various gradations, much likenormal weight aggregates. But becausethey are at least twice as expensive asnormal weights, a mix will sometimesbe composed of only lightweight largeaggregate plus normal weight sand. The

determining factor is what weight percubic foot is required, which is directlyrelated to what compressive strength isdesired. In other words, structural mixesof all lightweight normally run between90 to 115 pounds per cubic foot (pcf).Combinations can run anywhere betweenthis and normal weight concretes.

ASTM C130 Lightweight aggregateshave their own ASTM designation,recommending material weights,

compressive strength, and gradations.For example, the maximum unit weightfor fine aggregate is 70 pcf, for coarseaggregate 55 pcf, and for combined fineand coarse aggregate 65 pcf. There areno minimums given. Compared to normalweight aggregates, which range from 75 to110 ppf.

Moisture condition Due to their cellularstructure, lightweight aggregates absorbmore water than their normal-weightcounterparts (2.5 to 12 times as much).

The important difference in measurementsof stockpile moisture contents is thatwith lightweight aggregates the moistureis largely absorbed into the interior ofthe particles whereas in normal weightaggregates it is primarily surfaceadsorption. Recognition of this isimportant: keep the lightweight stockpileappropriately wetted to prevent thematerial from sucking water out of thebatch, resulting in a dry and unworkablemix.

Other differences Lightweight concretes,which have many advantages, also havemany differences besides weight. Forexample, tensile strength, shrinkage andcreep factors, and modulus of elasticityare different. But if you do all the rightthings, just like in normal weight concrete,it is just as durable and serviceable.

Aggregate Testing

Gradation This is one you arerequired to do in-plant, regardless ofwhat information the supplier gives you.Reference Design and Control of Concrete

Mixtures, PCA, 14th edition Table 5-5, Page

84. The most desirable fine-aggregategrading depends on the type of work, therichness of mix, and the maximum size ofcoarse aggregate. In leaner mixes, or whensmall-size coarse aggregates are used,a grading that approaches the maximum

recommended percentage passing eachsieve is desirable for workability. Thegrading for a given maximum size coarseaggregate can be varied over a moderaterange without appreciable effect on cementand water requirements. The test here isASTM C13 .


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1SECT ONEIn general, if the ratio of water-cementis kept constant and the ratio of fine tooarse aggregate is chosen correctly,

a wide range in grading can be used

ithout measurable effect on strength.The greatest economy will sometimesbe achieved by adjusting the concretemixture to suit the gradation of the localaggregates. The more uniform the grading,he greater the economy.

Soundness Soundness embodies manyproperties which are listed in Design andontrol of Concrete Mixtures, PCA, 14th

edition Table 5-2, Page 81. Most aggregatesuppliers will have performed the ASTM

ests listed, and have the results availableor you. An aggregate should have asatisfactory score in the ASTM tests. If youhave any doubts or specific requirements,it will pay to have your own tests run atlabs that specialize in these procedures.

leanliness All aggregates should beashed at the manufacturing site, butlean is a relative term. For instance,

if wash water is immediately recycledithout having time for the fine silt to

settle out, it is very likely that a finelm of dust or dirt will be left on the

aggregate. If the washing process doesntadequately remove twigs and decomposedegetation, the washed material is

still contaminated. The easiest test forleanliness is visual inspection and feel of

a handful of material.

A simple lab test is to take a clear masonjar and fill it 20% full of aggregate andhe rest clean water. Shake up the jar

horoughly, and let it stand on a shelf tosettle out. Do this test regularly, using anew clean jar (at least once per week),and compare the layer of silt that formsn top of the aggregate. You can readily

see any increase in dirt coming in on theaggregate. (A series of four jars is enougho keep recycling to give you a clear

picture of any trends or variations.) Thestandard ASTM Test for this is C117.

Moisture content Since the water to

ementitious materials (w/c ratio) is anxtremely important indicator of concretequality, care must be taken to know themoisture content of the aggregates used,and to adjust the batch weights and waterweight accordingly. This is probably thebiggest variable in putting together eachonsecutive batch, as the moisture usually

varies from the top of an aggregate bin orstockpile to the bottom. If it has recentlyrained, or is raining, there can be bigswings in moisture content of the material.

Here is another description of theaggregate moisture conditions:

It is very important to measure the sandmoisture content each day, and possiblymore than once if noticeable variations areccurring, so that proper adjustments inhe sand weight and water can be made.In all new automated plants today, this canbe integrated into the system, or set up toautomatically compensate. But a physical

measure is still important to verify theautomated readings, or to set the manualadjustment factors.

Sulfate reactivity Sulfate attack tooncrete normally occurs to the cement

paste portions, not to the aggregate.However, there is another standardest, ASTM C88, Standard Test Methodor Soundness of Aggregates by use ofSodium Sulfate or Magnesium Sulfate;it covers the testing of aggregates

o estimate their soundness whensubjected to weathering action inoncrete applications. This test furnishes

information helpful in judging thesoundness of aggregates when adequateinformation is not available from servicerecords of the material exposed to actualweathering conditions.


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Concrete Materials

Alkali-Silica Reactivity (ASR) Symptomsof ASR are typified by the expansion ofconcrete, usually accompanied by cracking.Undesirable aggregates are potentially

present in every state in the Union; ASRwill only occur in concretes with highmoisture contents. It is bad for concrete,and every producer needs to avoidaggregates which are reactive, particularlyif the cement is high in alkali. These tests,primarily ASTM C 227, 289, and 295 arelaboratory tests. You need your aggregatesupplier to have them professionallyperformed, and you need to have someonein your company read them carefully tobe sure you do not have aggregates high

in certain forms of undesirable mineralconstituents, such as certain forms ofsilica or carbonates. Generally, statesmaintain lists of acceptable supplierswith aggregates suitable for low or highalkalinity cement.

Cement

Portland cements are hydraulic cementscomposed primarily of hydraulic calcium

silicates. Hydraulic cements set andharden by reacting chemically with water.During this reaction, called hydration,cement combines with water to form astonelike mass. When the paste (cementand water) is added to aggregates, it actsas an adhesive and binds the aggregatestogether to form concrete.

Portland Cement Manufacture Theessential materials from which portlandcement is made are hydraulic calciumsilicates along with some calciumaluminates and calcium aluminoferrites.These materials must contain appropriateproportions of calcium oxide, silica,alumina, and iron oxide components. Theraw materials, which are finely groundand intimately mixed are heated to thebeginning of fusion (about 2700 F), in

great rotary kilns. These kilns may bein excess of 700 ft in length and 18 ft indiameter. The partially fused materialwhich emerges from the kiln is called

clinker. The clinker is cooled and groundto a very fine powder. During grinding asmall amount of calcium sulfate (gypsum),about 3 to 6 percent, is added to controlthe setting properties. When first madeand used in the early nineteenth centuryin England, it was termed portland cementbecause its product resembled a buildingstone from the Isle of Portland off theBritish coast.

Basic cement components Reference

Design and Control of Concrete Mixtures,PCA, 14th edition Table 2-6, Page 42.

Notice that the chemical makeup of TypeI and Type III cements, the kinds usedalmost exclusively in precast plants,are nearly identical. It is the fineness towhich they are ground which makes thedifference, the finer grind of the Type IIImaking it the higher early strength cementthat it is.

l S (Tricalcium silicate) hydratesand hardens rapidly and is largelyresponsible for initial set and earlystrength. In general, the early strength ofportland cement concrete is higher withincreased percentages of C S.

l S (Dicalcium silicate) hydratesand hardens slowly and contributes largelyto strength increase at ages beyond oneweek.

l A (Tricalcium aluminate) liberatesa large amount of heat during the firstfew days of hydration and hardening. Italso contributes slightly to early strengthdevelopment. Gypsum, which is addedto cement during final grinding, slowsdown the hydration rate of C

3A. Without

gypsum, a cement with C3A present would


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1SECT ONEset rapidly. Cements with low percentagesf C

3A are especially resistant to soils and

ater containing sulfates.

C4AF (Tetracalcium aluminoferrite)

reduces the clinkering temperature,hereby assisting in the manufacture ofement. It hydrates rather rapidly butontributes very little to strength. Mostolor effects are due to C

4AF and its

hydrates.

Types of cements

Type This is the general purposeement suitable for all uses where the

special properties of other types are notrequired. It is used in concrete that is notsubject to aggressive exposures, such assulfate attack from soil or water, or to anbjectionable temperature rise due to heat

generated by hydration. Its uses, besidesin precast products, are pavements, floors,reinforced concrete buildings, bridges,railways structures, tanks and reservoirs,pipe, and masonry units.

Type Used where precaution

against moderate sulfate attack isimportant, as in drainage structures wheresulfate concentrations in groundwatersare higher than normal but not unusuallysevere. Type II cement will usuallygenerate less heat at a slower rate thanType I. The requirement of moderate heatf hydration can be specified at the optionf the purchaser. If heat of hydration

maximums are specified, this cement canbe used in structures of considerablemass, such as large piers, and heavy

abutments and retaining walls. Its useill reduce temperature rise, which isspecially important when concrete is

placed in warm weather.

Type Provides high strengths atan early period, usually a week or less.It is chemically and physically similar toType I cement, except that its particles

have been ground finer. It is used whenorms need to be removed as soon aspossible, or when the structure must beput into service quickly. In cold weather its

use permits a reduction in the controlleduring period. Although richer mixes ofType I cement can be used to gain higharly strength, Type III may provide it

more satisfactorily and more economically.In prestress plants mixes are routinelyesigned with Type III cement to give

strengths of 4000 to 6000 psi in 10 to 14hours.

l Type Used where the rate andamount of heat generated from hydration

must be minimized. It develops strengthat a slower rate than other cement types.Type IV cement is intended for use inmassive concrete structures, such as largegravity dams, where the temperaturerise resulting from heat generated duringhardening must be keep down.

l Type V Used only in concretexposed to severe sulfate action

principally where soils or groundwatershave a high sulfate content. It gains

strength more slowly than Type I cement.The high sulfate resistance of Type Vis attributed to a low C

3A, tricalcium

aluminate. Sulfate resistance also increaseswith air entrainment and increasing cementontents (low water-cementitious material

ratios). Type V cement, like other portlandements, is not resistant to acids and other

highly corrosive substances.

A TM 150 This is the specificationunder which portland cement is made. We

wont go into its detail; as long as yoursupplier states that it is in compliance withC150, it is safe to assume that it is.

Mill Certificates The cement suppliershould give you a mill certificate for everyload, or at least indicate that it is from theidentical batch as the last load. This is his/her certification that the cement complies


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Concrete Materials

with ASTM C 150. It also gives morespecific data on the chemical ingredients,fineness, etc. (See samples starting onpage 1-18.)

Color consistency There is very little auser can do regarding color consistency ofcement. A simple test for color is to takea pinch of cement regularly, say weekly,and press it between two clear glassplates. Keep the assembly undisturbedon a shelf in your QC lab. If the colorsvary unacceptably, there is a problem. Itis good business to talk to the supplieras soon as you feel there is a problem,and if it isnt corrected satisfactorily, then

change suppliers.

Care We dont give cement muchthought since it comes in without ourhandling it, and it goes into the mixerwithout handling it. But it requiresexceptional care, primarily in the storagebin. Not only must it be kept dry, it mustbe fluffed to overcome any bulking effectwhich would impact smooth and constantflow from the storage bin into the weighbatcher, and then into the mixer. Fluffing

is usually done by the introduction ofcompressed air. If there isnt a moistureseparator on the air line, moisture will beblown into the bin along with the air. Theeffect of this moisture hydrating somecement is not terribly important, exceptfor the lumps it causes, and some of thebulking within the bin. Another cause ofmoisture problems in bins is cracks in

joints, or penetrations of bolts or auxiliaryequipment which arent tight.

Correlating deliveries to products Asa matter of interest, the PrestressedConcrete Institute has new requirementsfor its plants that records be kept so thatit can determine which cement deliverywent into which products produced. Ifproblems develop later on, investigationof quality and reasons for the problemscan be conducted. Whereas NPCA doesnt

require this as part of its Certificationprogram, it is worthwhile to think throughyour system to help you in this mattershould the occasion arise.

Blended Cement

What it is Originally, concern withenergy conservation prompted the use ofby-product materials in portland cementconcrete. Blended hydraulic cements areproduced by intimately and uniformlyblending two or more types of finematerials. The primary blending materialsare portland cement, ground granulatedblast-furnace slag, fly ash and other

pozzolans, hydrated lime, and preblendedcement combinations of these materials.Now that time has proven the benefits ofthese additional materials, their productionis not solely the result of a need to saveenergy.

Why use one Most concrete plantsdont have enough available silo storagespace to devote to fly ash or a pozzolanicmaterial to enable them to introducecement alternatives themselves. Blendedcements present options for getting theadvantages of the combined materialswithout having to add silos and conveyingmechanisms, particularly if the need is fora short period of time. In addition, thereare unique requirements which can only besatisfied by a blended hydraulic cement.

Material choices Blended cements mustconform to the requirements of ASTM C595, which recognizes five classes:

l Portland blast-furnace slag cement,Type IS Granulated blast-furnaceslag of selected quality is eitherinterground with portland cementlinker, separately ground and blended

with portland cement, or producedwith a combination of intergrindingand blending.


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1SECT ONEPortland-pozzolan cement, TypeIP and P Made by intergrindingportland cement clinker with a suitablepozzolan, by blending portland cement

or Portland blast-furnace slag cementand a pozzolan, or by a combinationof intergrinding and blending. Thepozzolan content of these cements isbetween 15% and 40% by weight.

Pozzolan-modified portland cement,Type I(PM) Manufactured bycombining portland cement orportland blast-furnace slag cementand a fine pozzolan, in the same waysas outlined above.

Slag cement, Type S This is usedwith portland cement in makingconcrete or with lime in makingmortar, but is not used alone instructural concrete. Manufacturingis similar to what has already beenoutlined. The minimum slag content is70% by weight of the slag cement.

Slag-modified portland cement, TypeI(SM) This type is manufactured

similar to above; slag is less than25% of the finished cement.

There are also masonry cements,xpansive cements, special cements,

such as oil-well cements, waterproofedportland cements, plastic cements,regulated-set cements, and even more.Some types of cement may not bevailable in certain areas.

Advantages/disadvantages The

advantages of using blended cementsare those gained from the introductionf the other materials into the portlandement. Slags, fly-ash, and pozzolans are

generally less expensive than portlandement. Manufacturing space andomputer programs may not be able to

handle separate additional materials. Most

blended cement admixtures will developstrength at a slower rate compared tohe mix without them, and possibly alower 28 day strength. These effects

an become even more noticeable ifemperatures are low.

Admixtures

The major reasons for using admixtures:1. To reduce the cost of concrete

construction2. To achieve certain properties in

concrete more effectively than byother means

3. To ensure the quality of concreteduring the stages of mixing,transporting, placing, and curing inadverse weather conditions

4. To overcome certain emergenciesduring concreting operations.Even though an admixture mayproduce concrete with the desiredproperties, the same results can oftenbe obtained just as economically bychanging the mix proportions or byselecting other concrete ingredients.Whenever possible, a comparison

should be made between the cost ofchanging the basic concrete mixtureand the additional cost of using anadmixture.

Chemical Admixtures

l Air-entraining admixtures Entrainmeans to suspend in a medium or carrier.Microscopic means so small as to be

invisible except through a microscope.Thus, air-entraining admixtures are usedo purposely manufacture and add verymicroscopic air bubbles in concrete.Lets also make clear that there are twoypes of air in concrete: entrained andntrapped. We make the entrained air

bubbles by mixing in the admixture; theseare the beneficial bubbles. The larger,

QUICK

NOTE


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Concrete Materials

visibly noticeable, entrapped air bubblesare not desirable; they are formed by themixing action, or placing procedures.

Entrained air greatly improves concretesresistance to surface scaling caused bychemical deicers; it dramatically improvesthe durability of concrete exposed tomoisture during cycles of freezing andthawing. The workability of fresh concreteis also improved significantly, andsegregation and bleeding are reduced oreliminated.

l ater-reducing admixtures Theseare used to reduce the quantity of mixing

water required to produce concrete of acertain slump, reduce water-cement ratio,or increase slump. Typical water reducersreduce the water content by approximately5% to 10%. High range water reducers(superplasticizers) reduce water contentby 12% to 30%. An increase in strengthis generally obtained with water-reducingadmixtures as the water-cement ratiois reduced. Despite reduction in watercontent, water reducing admixtures cancause significant increases in drying

shrinkage. Water reducers can decrease,increase, or have no effect on bleeding,depending on the chemical composition.

l Superplasticizers are added toconcrete with a low-to-normal slump andwater-cement ratio to make high-slumpflowing concrete. Flowing concrete ishighly fluid, but workable concrete, thatcan be placed with little or no vibrationor compaction and can still be freeof excessive bleeding or segregation.

Flowing concrete is used (1) in thin sectionplacements, (2) in areas of closely spacedand congested reinforcing steel, (3) intremie pipe (underwater) placements,(4) in pumped concrete to reduce pumppressure, thereby increasing lift anddistance capacity. The addition of asuperplasticizer to a 3-in. slump concrete

can easily produce a slump of 9-in.Flowing concrete is defined in ASTM C1017 as having a slump greater than 7 1/2in. yet maintaining cohesive properties.

(Excessively high slumps, 10 in. or more,may cause concrete to segregate.)Superplasticizers can also be used to makelow water-cement ratio, high-strengthconcrete with workability in the rangesgenerally specified for consolidation byinternal vibration. The reduced watercontent, usually in the range of 12% to30%, and the resultant reduced water-cement ratio can produce concrete with(1) ultimate compressive strengths inexcess of 10,000 psi, (2) increase early

strength gain, and (3) reduce chloride ionpenetration as well as other beneficialproperties.

High-range water reducers are generallymore effective, but more expensive, thanregular water-reducing admixtures inproducing workable concrete. The effectof most superplasticizers in increasingworkability or making flowing concreteis short-lived, 20 to 60 minutes, and isfollowed by a rapid loss in workability

(slump loss).

l Retarding admixtures Hightemperatures of fresh concrete (85 to90 F) are often the cause of an increasedrate of hardening that makes placing andfinishing difficult. Retarders are sometimesused to (1) offset the accelerating effectof hot weather on the setting of concrete,(2) delay the initial set of concrete whendifficult or unusual conditions of placementoccur, or (3) delay the set for special

finishing processes such as an exposedaggregate surface. Most retarders also actas water reducers, and may entrain someair in concrete. Some reduction in earlystrength (1 to 3 days) accompanies the useof retarders.


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1SECT ONECorrosion inhibitors Concrete

protects embedded steel from corrosionhrough its highly alkaline nature. Thehigh PH environment (usually greater than

12.5) causes a passive and noncorrodingprotective oxide film to form on steel.However, carbonation or the presence ofhloride ions from deicers or seawater canestroy or penetrate the film. Once this

happens, the electro-chemical process oforrosion begins. Rusting is an expansive

process rust expands up to four timesits original volume which inducesinternal stress and eventually cracking andspalling around reinforcing steel.

Corrosion-inhibiting admixtures chemicallyarrest the corrosion reaction. Calciumnitrite, the most commonly used liquidorrosion inhibitor, blocks the corrosion

reaction of the chloride ions by chemicallyreinforcing and stabilizing the passive film.The nitrite ion causes the ferric oxides tobecome insoluble. In effect, the chlorideions are prevented from penetrating thepassive film and making contact withhe steel. A certain amount of calciumnitrite can stop corrosion up to a certain

hreshold level of chloride ion. More isbetter; however, calcium nitrite is also anaccelerator, so too much is not good.

Lubricants & densifiers Primarilyused in the block industry, but alsoommon in the dry-cast concrete products

industry, these additives enhance themovement of fresh concrete through orpast steel forms, and give the formedsurface a smoother finish. In so doing,he wear on the steel form is reduced, as

is the friction between the form and theoncrete, thus reducing the drag or force

required in the manufacturing process.Since zero slump mixes tend to bulk,making them difficult to compact to thesame cross sectional density as wet mixes,here are admixtures which help achieve a

enser product. With many suppliers, thesame admixture aids in both lubricatingand densifying.

l Metakaolin This is an examplef the new admixtures that keep being

eveloped. It is a pozzolanic materialhat reacts with free lime (calciumhydroxide, CaOH

) produced during the

hydration process of portland cement. Thepozzolanic reaction forms additional andbeneficial cementitious products. These arelaimed to be:

u Increased strength compressive, flexural, and tensile

u Increased chemical resistenceu Reduced permeability and

increased durability

u Reduced drying shrinkage

u Prevention of alkali silica reactions(ASR)

u Control of efflorescence

ASTM Specifications Reference Designand Control of Concrete Mixtures, PCA,14th edition Table 6-1, Page 106.

Mineral Admixtures

Finely divided mineral admixtures arepowdered or pulverized materials addedo concrete before or during mixing toimprove or change some of the plastic orhardened properties of portland cementoncrete. Based on their chemical or

physical properties, they are classified as(1) cementitious materials, (2) pozzolans,

(3) pozzolans and cementitious materials,and (4) nominally inert materials.

l Pozzolans Fly ash Silica fume Apozzolan is a siliceous or aluminosiliciousmaterial that in itself possesses little orno cementitious value but will, in finelyivided form and in the presence of


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Concrete Materials

water, chemically react with the calciumhydroxide released by the hydrationof portland cement to form compoundspossessing cementitious properties. To

varying degrees, they are used to: improveworkability, plasticity, sulfate resistance;reduce alkali reactivity, permeability(increase density), heat of hydration;partially replace cement.

Fly ash is a finely divided residue (powderresembling cement) that results fromthe combustion of pulverized coal inelectric power generating plants. Fly ashis commonly used to replace some of theportland cement in a batch for economy. It

can increase strength and workability, andreduce sulfate attack and alkali aggregatereaction. It reduces water demand in thesame manner as liquid chemical waterreducers.

Silica fume, also referred to as microsilicaor condensed silica fume, is a resultof the reduction of high-purity quartzwith coal in an electric arc furnace in themanufacture of silicon or ferrosilicon alloy.It reduces the permeability of a concrete

mixture (often used to lower coulombvalues below 1000), and produces a verydense concrete with excellent late agestrength. Since it is a very fine material, ithas high water demand, which can causeearly set. It requires careful and thoroughmoist curing to prevent shrinkage cracks.It generally darkens the color of thefinished concrete.

l ranulated blast furnace slag Ground granulated blast furnace slag

is a cementitious material with hydrauliccementing properties (they set andharden in the presence of water). Aircooled slag does not have the samehydraulic properties of water-cooledslag. Slag generally improves workability,may help finishability or pumpability,improves compressive strength, reduces

permeability, reduces alkali-aggregatereactivity, increases resistance to sulfateattack. But then, the same can generally besaid of fly ash and silica fume.

l oloring agents These aregenerally made of finely ground minerals,some are natural and some are syntheticmaterials. They are used primarilyin architectural or exposed concreteapplications. They have no cementitiousproperties, and therefore may reducecompressive strength; as a finely ground,inert material, they probably will reduceair content. They should be experimentedwith in trial mixes and samples to

determine the final color achieved, and itsability to last in sunlight and weathering.Variations in color of a finished productare more the result of variations in thewater content than they are in variations inthe pigment.

Water

Potable Almost any natural water that isdrinkable (potable) and has no pronounced

taste or odor can be used as mixingwater for making concrete. However,some waters that are not fit for drinkingmay still be suitable for concrete. Watercan be tested and trial mixes performedto evaluate the effects of the water inquestion. In general, if mortar cubes madewith such water have 7 day strengthsequal to at least 90% of companion cubesmade with drinkable or distilled water, itcan be used. Excessive impurities in mixingwater may effect setting time and concrete

strength, cause efflorescence, staining,corrosion of reinforcement, volumeinstability, and reduced durability.

Recycled In many European concreteplants, everything is recycled; there isessentially no waste leaving the property.We may be forced closer and closer to


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1SECT ONEhat situation in time. Wash water can beused as mix water. It is generally pondedrst to allow suspended fines to settle, butven a small percentage of the fines can

be regularly introduced in the mix waterithout undue detriment. With the helpf chemists, other waste water supplies

(such as from industrial plants or sewagereatment plants) could conceivably beused as mix water. For acceptabilityriteria, see ASTM C94.


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Concrete Materials

Material Certification Frequency Report


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1SECT ONE

Samp esof Materials

Certifications


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Concrete Materials

Aggregate Reports


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1SECT ONE

Aggregate Reports


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Concrete Materials

Cement Mill Certifi cate


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1SECT ONECement Mill Certifi cate


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Concrete Materials

Reinforcement Supplier Reports


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1SECT ONEAdmixture Certifi cation


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Concrete Materials

Admixture Certifi cation


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1SECT ONE

Admixture Certifi cation


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Concrete Materials

Cementitious Materials Certifi cation


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1SECT ONEReinforcement Supplier Report


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Concrete Materials

SUMMARY

NotesGenerall he quality of raw materials is proved

by certificates of conformance

l Certificates of importance toprecasters are:

Cement and mineral admixturemill certificates,

Aggregate supplier and testreports,

Mix water potability test

certification (annually) unlessusing municipal water supply,

Chemical admixture certifications(annually).

l est Report- states that material hasbeen tested and lists results of theests performed.

l Material Certification-certifies thatmaterial meets appropriate ASTM (orther) standards.

Aggregates

l he quality of aggregates is importantbecause they make up between 60 75% of the concrete volume

l Properties of aggregates needed toproduce quality concrete

Conform to ASTM C33

Non-reactive (properties do notaffect hardened concrete)

Clean

Free of deleterious substances

Durable

Hard

Properly sized and graded

l t is very important to know themoisture content of the aggregates toproduce quality mix designs


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1SECT ONE

Notesl Lightweight aggregates

s Reduce weight ands Have different properties than

normal aggregates

AggregateTesting

Important Tests

l Aggregate sampling

l Aggregate gradation

l Moisture content in aggregates

l Organic impurities

Reasons or t ese tests:

Aggregate Sampling

l Required to achieve representativeresults

Sieve Analysis of Fine and

Coarse Aggregates

l To determine aggregate gradations (orsizes) which affect mix designs andoverall concrete quality

Organic Impurities in Sand

l Determine any foreign substances insand that may harm your concrete

Moisture Content inAggregates

l Determine the amount of water inaggregates that will add to your mixwater

l Determine mix water compensation


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Concrete Materials

Cement

l Portland cement is hydraulic meaning

it reacts chemically with water to formhe hardened paste of concrete

l Five types we use are

Type I General all purpose

Type II Provides protectionagainst moderate sulfate attack

Type III Provides high earlystrengths

Type IV Sets slower than othertypes

Type V- Provides protection

against severe sulfate attack

Admixtures

l Enhance properties of plastic andhardened concrete

Reduce cost

Increase durability

Reduce set time

Increase workability

l Five common types of admixtures andheir purposes

1. Air Entrainment

s Protection againstreeze/thaw cycles

s ncreased durability

2. Water reducing admixtures

s Reduce mixing water content

s Reduce water-to-cement ratio

s ncrease concrete durabilitys ncrease strength

Notes


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1SECT ONE

Notes3. Superplaticizers

s Are high range waterreducers

s Transform low slump to

high slump-concrete withoutadding H

s Same properties as waterreducers

4. Retarding Admixtures

s Slow hydration in hot weather

s Delay initial concrete set

5. Corrosion Inhibitors

s Protect embedded steel fromcorrosion

Water

l Must be potable

l If you can drink it, you can useit for concrete

l Non-municipal water needs tobe tested annually


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Reinforcement

Comparison of U.S. Customaryand Metric Rebars

U.S. Customary Bar Metric Bar Metric Bar IsBar Size Area in 2 Area mm Bar Size Area in 2 Area mm 2

#3 .11 71 10M .16 100 45% larger

#4 .20 129 10M .16 100 20% larger

#4 .20 129 15M .31 200 55% larger

#5 .31 200 15M .31 200 Same#6 .44 284 20M .47 300 6.8% larger

#7 .60 387 20M .47 300 22% larger

#7 60 387 25M .78 500 30% larger

#8 .79 510 25M .78 500 1.3% smaller

#9 1.00 645 30M 1.09 700 9% larger

#10 1.27 819 30M 1.09 700 14% smaller

#10 1.27 819 35M 1.55 1000 22% larger

#11 1.56 1006 35M 1.55 1000 0.6% smaller

#14 2.25 1452 45M 2.33 1500 3.5% larger

#18 4.00 2581 55M 3.88 2500 3.0% smaller


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2SECTION O

I. Introduction

Concrete is very strong in compression,but very weak in tension. Consequently,

hen we speak of concrete strength, werefer to the concrete compressive strength.When testing a concrete cylinder, a loadis applied that compresses the cylinder.The testing machine indicates the valuef the applied load, usually in pounds.

Standard concrete strength, however, doesnot directly refer to the amount of forcehe concrete can carry, but the amountf stress it can carry expressed in units

known as PSI (pounds per square inch).This means the applied force (in pounds)

is divided by the area (in square inches) ofhe concrete cylinder. The maximum loadapplied at the time when the test cylinderails is used to compute the compressivestrength of the concrete.

If we pull the concrete cylinder at bothnds, we apply a tensile force. Instead ofompressing it, all the material within theoncrete is pulled apart. In this situation,he concrete only has about 10 percentf the strength it has under compression.

For example, 4,000 PSI concrete canithstand 4,000 pounds per square inch

in compression, but only 400 pounds persquare inch in tension. It is important tonote that concrete cylinders do not containreinforcement, which would likely increasehe load-carrying capacity of the concrete.

II. Non-reinforcedConcrete Behavior

When a vertical load is applied to the

op of a concrete beam, the beam bendsslightly downward to absorb the force.The concrete along the top edge of thebeam (often referred to as the top fibers)xperiences compression, while theoncrete along the bottom of the beamxperiences tension. In the absence of

reinforcement, cracks will form in the

bottom fibers of the beam and progressup through its cross-section and finallyause the beam to fail. Since concreteannot generally withstand much tension,

reinforcement is used (because steel isvery strong in tension) to increase theload-carrying capacity the beam. Thelogical place to put steel is in the bottomareas of the beam, where the tensionorces are.

As an example, consider grade 60reinforcement, which has a tensile strengthf 60,000 PSI. This reinforcement isapable of supporting tension of at least

60,000 pounds per square inch compared

o the 400 pounds per square inch theoncrete can withstand. Therefore, youan see how reinforcement increaseshe structural load-carrying potential ofoncrete considerably.

III. Reinforced ConcreteBehavior

Believe it or not, reinforced concrete isesigned to crack. The amount, location

and size of reinforcement ultimately

etermines the size of these cracks theyare usually designed to be so small thathey do not create a problem. But whyoes reinforced concrete crack? Recallur example of concrete with a 4,000

PSI design compressive strength has theability to carry only about 400 PSI inension.

The truth about most precast concreteproducts is that they experience tensionmore often than you might think. It doesnot take much of a load to exceed 400PSI, and the concrete begins to crackvery soon after a load is applied. Ofourse, reinforced concrete is designedo transfer the tensile forces in theoncrete to the steel reinforcement, but it

is impractical to place the reinforcementalong the outside edge of the concrete,


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Reinforcement

where the tensile forces are the greatest.This is because the steel will be exposedto the elements and eventually corrode.Hence, the reinforcement is placed within

the concrete, usually an inch or twoaway from the surface. This leaves theamount of concrete between the surfaceand the reinforcement to absorb tensileforces by itself. This small portion of theconcrete will begin to crack before thereinforcement begins to absorb the tensilestress.

IV. Effectiveness ofPlacement

Mistakenly placing the reinforcing steeljust one inch from its intended designlocation can lower the strength of theproduct by as much as 25%. Often times,when reinforcing steel is not placed inthe correct location, it is because theappropriate spacer or bar chair is notused. Take the time to purchase and usethe correct spacers to ensure that thereinforcing steel is placed in the exactdesign location.

V. Reinforcement Types

There are many different types ofreinforcement, including:

w Reinforcing barsw Reinforcing wirew Bar mats and welded wire fabricw Zinc or epoxy-coated reinforcementw Pre-stressing and post-tensioning

strand

It is important to have the most recentcopies of the specifications for the typeof reinforcement being used. If you donthave a specification, then it is advisable topurchase it.

VI. Reinforcing Bars

The most common specifications governingconcrete reinforcing bars are:

w ASTM A615/A615M StandardSpecification for Deformed andPlain Billet-Steel Bars for ConcreteReinforcement

w ASTM A996/A996M StandardSpecification for Rail-Steel andAxle-Steel Deformed Bars forConcrete Reinforcement

w ASTM A706/A706M StandardSpecification for Low-Alloy SteelDeformed and Plain Bars forConcrete Reinforcement

Other bars may be used if permitted bydesign. Mill certificates should be obtainedfor each shipment regardless of thetype of bar since steels manufactured inEurope and elsewhere may not meet NorthAmerican specifications.

VII. U.S. Customary andMetric Rebars

In English Units, the numbers on the bar

refer to the number of eighths of an inchin the diameter of the bar. For instance:

w No. 3 Bar is 3/8 diameterw No. 7 bar is 7/8 diameter

In SI Units (metric) the numbers refer tothe number of millimeters in the diameterof the bar. For instance,

w No. 10 bar is 10mm diameterw No. 22 bar is 22mm in diameter

By comparison, a:w No. 3 bar (English) is a

No. 10 bar (SI)w No. 7 bar (English) is a

No. 22 bar (SI)


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2SECTION O

VIII. Rebar

The reinforcing bars purchased can beidentified by their physical appearance.The different rib patterns that are used onrebars are used to signify the strength,r grade. Rebars are available in manyifferent grades (i.e. Grade 40, Grade 60).

IX. Reinforcing Wire

Reinforcing wires must conform to ASTMA82 Standard Specification for Steel

ire, Plain, for Concrete Reinforcementor ASTM A496 Standard Specificationor Steel Wire, Deformed, for Concrete

Reinforcement. Other wire may be usedif permitted by design. Mill certificatesshould be obtained for each shipment.

X. Bar Mats and WeldedWire Reinforcement

These reinforcing types must conformo ASTM A184 Standard Specificationor Fabricated Deformed Steel Bar Matsor Concrete Reinforcement, ASTM A185Standard Specification for Steel Welded

ire Reinforcement, Plain, for Concreteor ASTM A497 Standard Specificationor Steel Welded Wire Reinforcement,Deformed, for Concrete. Mill certificatesshould be obtained for each shipment.

XI. Plain Welded WireReinforcement

Again, plain welded wire reinforcement(WWF) must conform to ASTM A185Specification for Steel Welded Wire

Fabric, Plain for Concrete Reinforcement.These wires can be identified by theirsmooth surface. With plain welded wirereinforcement, the only place that wherebond is developed is at each weldedintersections.

XII. Deformed WeldedWire Reinforcement

Deformed WWR must conform to ASTM

A467 Specification for Steel WeldedWire Fabric, Deformed, for ConcreteReinforcement. The maximum spacingor welded intersections is 12 inches.Deformed wires have small ribs or othereformations along the surface to enhancehe bond with the concrete along thentire length of the wire.

XIII. Zinc or Epoxy-CoatedReinforcement

Galvanized reinforcement must conformo ASTM A767 Standard Specification forZinc-Coated (Galvanized) Steel Bars forConcrete Reinforcement. Epoxy-coatedreinforcing bars must conform to ASTMA775 Standard Specification for Epoxy-Coated Reinforcing Steel Bars or ASTMA934 Standard Specification for Epoxy-Coated Prefabricated Steel ReinforcingBars. WWR must conform to ASTM A884Standard Specification for Epoxy-CoatedSteel Wire and Welded Wire Fabric for

Reinforcement. There is no ASTM standardor zinc-coated reinforcement.

XIV. Concrete Cover forReinforcement

Proper concrete cover (generallyreferred to as the number of inches thereinforcement should be buried intohe concrete) is necessary to provideprotection against reinforcing steelorrosion. There are many varyingoverage depths that can be used,epending on the environment and the

size of the reinforcing bars. The amountf concrete cover should be specified in

project specifications or drawings.


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Reinforcement

XV. Structural Integrity

Structural integrity of a product dependson:

w Size of the reinforcing steelw Spacing of the reinforcing steelw Positioning of the reinforcing

steel

XVI. MinimumReinforcement BendDiameters

All rebar bends are described in terms ofinside diameter of the bend, which is thesame size as the pin used to bend the

reinforcement. Certain factors affect theminimum required bend diameters, such asthe feasibility of bending without breakingand avoiding concrete crushing inside thebend.

Specific formulas for bending welded wirereinforcement include:

w 4d (4 x the diameter of the wire)minimum for deformed wire largerthan D6

w 2db minimum for all other wiresIf you bend reinforcing wire,welded wire reinforcement orreinforcing bars around too smalla pin, it can cause problems.Many times, plants have oneset of pins that is used foreverything which is contrary tospecifications, as listed in thefollowing table.

Minimum Bar Bend Diameters MinimumBar ize DiameterNo. 3- No. 8 6d

b

No. 9, No. 10, No. 11 8dNo. 14 and No. 18 10d

XVII. Carbon Equivalence

There is a technical term called carbonequivalent(CE) that should be checkedfor rebar if it is going to be welded.In addition, there are certain limits tochemical components in the steel thatshould be checked. One is the actualcarbon content of the steel. If reinforcingsteel mill certificates are not obtainedon a regular basis, then it is not likelythat these values will be known by plantpersonnel. Carbon content should belimited to less than 0.30% if the steel willbe welded. Also, the carbon equivalent,which is often listed on the mill certificate

or can be easily calculated at the plant,should be checked. The American WeldingSociety (AWS) has set requirements forwelding reinforcement in their StructuralWelding Code Reinforcing Steel. Inaddition, the American Concrete Institute(ACI) refers to these same requirements intheir codes.

In general, for bars #6 and smaller, whichis most common in precast concreteplants, the carbon equivalent should be

less that 0.55%, otherwise additionalsteps such as preheating the steel priorto welding must be done. For bars #7and bigger, the limit is 0.45%. The overallrule of thumb is the lower the carbonequivalent, the better the weldability.

XVIII. ASTM A615Reinforcement

So how do you calculate carbon

equivalencies? When using standard ASTMA615 rebar, the carbon equivalent shoulddefinitely be checked if you are welding.It is a very simple calculation, but again,it cant be done unless mill certificatesare obtained for the steel being checked.The calculation is performed by adding


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2SECTION Ohe carbon content with the manganeseontent divided by six, that is:

XIX. ASTM A706Reinforcement

When using ASTM A706 rebar, which isonsidered a weldable grade rebar, there

is another formula. The formula is a littlemore complicated it is necessary to alsoknow how much copper, nickel, chromiumare in the reinforcing steel. Again, these

alues should be shown on the millertificate. The calculation is performedith the following formula:

If the results are .45% or less for bars#7 and larger, or .55% for bars #6 andsmaller, then preheating is not necessary.


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Reinforcement

Combinations of Bar Sizesnd Spacing

acin # # #6 # #8 # #10 #11

2 1.20 1.86 2.64

0.80 1.24 1.76 2.40 3.16 4.003

0.60 0.93 1.32 1.80 2.37 3.00 3.81 4.684

0.48 0.74 1.06 1.44 1.90 2.40 3.05 3.745

0.40 0.62 0.88 1.20 1.58 2.00 2.54 3.126

0.34 0.53 0.75 1.03 1.35 1.71 2.18 2.67

0.30 0.47 0.66 0.90 1.19 1.50 1.91 2.348

0.27 0.41 0.59 0.80 1.05 1.33 1.69 2.089

0.24 0.37 0.53 0.72 0.95 1.20 1.52 1.8710

0.22 0.34 0.48 0.65 0.86 1.09 1.39 1.7011

0.20 0.31 0.44 0.60 0.79 1.00 1.27 1.5612

0.18 0.29 0.41 0.55 0.73 0.92 1.17 1.4413

0.17 0.27 0.38 0.51 0.68 0.86 1.09 1.3414

0.16 0.25 0.35 0.48 0.63 0.80 1.02 1.2515

0.15 0.23 0.33 0.45 0.59 0.75 0.95 1.1716

0.14 0.22 0.31 0.42 0.56 0.71 0.90 1.1017

0.13 0.21 0.29 0.40 0.53 0.67 0.85 1.0418


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2SECTION O

SUMMARY

Purposel Needed to supply concrete with

strength to resist tensile and shearforces

l Tensile stresses sometimes greatestduring handling and transporting

Conformance as a material

l

Mill certificates prove reinforcementsconformance to specifications

Fabrication

l Reinforcing bar bending must conformto ACI and CRSI

l Welding

Maintain product integrity

Two weldability limitsMax. carbon content (0.30%)

Carbon Equivalent (CE) values

ACI 318 And AWS D1.4, CE values are:

For #6 and smaller - 0.55%

For #7 and larger - 0.45%

The lower the CE, the better theweldability

For larger CE values, the rebar mustbe preheated

Notes


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Reinforcement

Positioning

l dequate concrete cover is necessaryo protect steel against corrosion

l Concrete cover must conform to ACIlimits

l Not touching formwork

l Supported by adequate spacers

l Positioning of reinforcement shouldbe checked and authorized by QCIpersonnel prior to casting

Condition prior to casting

l Properly tied and spliced

l Do not get form release agent on thereinforcement

Notes


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Fundamentals of Quality Precast -1

Miscellaneous Materi-als and Components

MiscellaneousMa erials andCompenentsLifting Devices and

Apparatus

Verified design OSHA requires that alllifting devices and apparatus be designedfor its intended use, and in most cases,be tested. Some of these requirementsare very specific, and others are rathervague. Purchased, standard items of liftinghardware, such as slings, hooks, shackles,etc. come with published capacity ratingsand manufacturers backing of havingtested the items, including the load factor(factor of safety) used. It is recommended

that all special lifting devices andapparatus, such as lifting beams, strongbacks, turning trundles, etc. be designedby an engineer. Furthermore, a file shouldbe kept containing all designs, andinformation such as date of manufacture,manufacturers name (if not self made),and a unique identification number whichties it to the correct piece of equipment.

Factor of safety Safety factor, or Factorof Safety is a number used in designing

manufactured items that compensates forstresses that exceed the items intended,or designed stress limit. The factor ofsafety therefore provides a safety cushionagainst unknown variables.Concrete is not always consistent andwhen it is handled there is always thechance that something could go wrong,

meaning the structuremay encounter stresses that

exceed its design strength.

Factors of Safety in the precaindustry range from 3 to 6 anthere often is judgment requirconcerning what number, or fOSHA requires specific numbers n caseslike testing personnel safety lanyards.But OSHA is vague on design factors forlifting devices. Items designed for limiteduse in unique, repetitive applications,where experienced users work in

desirable conditions like in an enclosedprecast plant using an overhead crane a minimum factor of safety of four isrecommended. For variable use items,exposed to unknown conditions of user,handling, and weather a factor of safety of6 is recommended. For more informationsee Precasters Notebook, January 1997,Ch 4.

Rated capacity displayed Itemsnormally purchased as standard or stock

items come with their rated capacitypermanently marked on the item, or incases of standard manufactured slingsand chokers, with a rating tag attached.It is important that all these identifiersremain on the item. For items such aslifting beams, you need to mark them withsome identifying number (which ties them


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-2

3SECTION REE


o the items design), plus have the ratedapacity clearly and legibly displayedn the equipment. This is another OSHA

requirement.

Hooks, slings, shackles Properly sizedand rated equipment displaying thesenumbers are critical for lifting loads safely.Purchase documents, which include themanufacturer and load ratings, should bekept on file for the life of the item. It isery important to not overlook the effectsf wear, age, and sometimes weather or

misuse, on small miscellaneous equipment.Therefore, every plant should have annualraining programs, as well as annual

inspections.

Training programs should include care ofquipment, proper equipment use, andaily equipment inspection before use. The

purpose here is to remove equipment fromuse if unduly worn or damaged. Annualinspections should be performed on alllifting equipment by an outside company.The reason for having an independentinspection is that users get comfortable

ith what they are doing, and after a

hile, they just dont see the changes thathave evolved with use (or abuse). Youneed a fresh set of eyes now and then.

Embedded Steel Shapes,Plates, and Hardware

ASTM A36 steel This is the standardgrade of steel plates and shapes used forstructural applications, like beams andolumns, plus all the connecting plates

and hardware in precast concrete. It isreadily available, easily welded, and hasgood strength and design characteristics.If a design, as noted on piece fabricationrawings, calls for special steel, the

inspector should have an easy timestablishing that the correct steel is being

used. This is not generally visible, so a

paper trail is required, as well as clearlymarked identification on the item.

Special exposure requirements If

project specifications dont require specialrequirements for exposure conditions,good plant practice should. Stainless steelis an obvious choice for exposed steelhardware, but it is also expensive, andnot always practical for normal exposureo weather, nor obtainable in the itemneeded.

Many items can be purchased with a nickeloating at very little extra cost. This is a

good idea, as it not only protects the item

rom weathering in the product, it alsokeeps the item rust free while in storage inyour plant. Galvanizing is the next choice,but it also has drawbacks: you cantgalvanize some items, like interior threads,and some areas of the country dont havegalvanizers economically accessible. Analternative is to apply paint (or spray)with significant zinc contents, called zincrich coatings. Epoxy painting is effective,but not as good as purchasing itemswith factory applied epoxy coating. At a

minimum, every steel item cast into yourproduct which will be exposed should beoated with a metal primer.

Proper design Every item cast into apiece should be identified on the castingrawing, and uniquely tied to a specificesign or plant standard. For example,

WP-1 might be a plants standard 4 x 41/4 weld plate with one 1/2 diameter 4inch long headed stud anchor. But if thatesignation has been used previously

n a particular job as the identifier fora special, different, custom weld plate,here is a chance for error ever after. As aurther example, simply calling for a 4 inchinsert allows the use of any insert in theplant which receives a 4 inch bolt. Goodesign will specify exactly which insert

is to be used, which will have considered


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its capacity in tension or shear loading,its proximity to an edge, the length ofthe bolt to be received, the exposurecondition, etc.

Thorough inspection QCI personnelshould consider the following:

s it in the right place?

s it the right item?

s it properly made

(per plans, if it is a custom item)?

s it secured to the forms

adequately?

These are the items often overlooked,

and while some can be improvised andput in after a product is made, most cannot. Thorough inspection can save a piecefrom being rejected.

Purchased items It is recommendedthat purchase orders clearly containspecifications for the item in additionto quantity and delivery requirements.These should include types of material,capacities, finishes, shapes, sizes, andspecial identifying criteria (such as

assembly number or job identifier), andany references to ASTM or industrystandards required (such as for weldingprotocol). If the item isnt purchasedcorrectly, you cant be sure it is correctfor the application.

Acceptance, identification Upondelivery, it is recommended that embeddedsteel shapes, plates, and hardware beclearly marked or kept individually isolatedin clearly marked containers. The receiving

slips containing product identification andspecification, manufacturer, supplier, datereceived, and purchase order numbershould be kept on file (usually by the QCdepartment or the production office) insuch a manner as to be easily tied intothe product on hand. This will help avoidincorrect use by distinguishing betweendifferent grades of otherwise similar items.

Accessories

Spacers, ties, chairs, etc This sectionis added to remind you that even the littlethings are important. There are chairs ofevery size, spacers of every type, and soon. See that the correct one is used forthe job, not an improvised jury-riggedcombination. These seemingly insignificantitems can cause serious fault or damageif incorrectly sized or carelessly used.Question anything that appears incorrect.

Exposure Exposure is one of the keyshere. Chairs touching exposed surfacesneed plastic feet or stainless or nickel

composition to avoid future rust spotsappearing. Even tie wire can causeproblems if not carefully bent so looseends point into the product and away fromoutside surfaces.

Acceptance, verification, purchase orders This is a repeat of what was said aboveunder embedded steel shapes, plates,and hardware. It is here to remind youthat it is also important for these items,which are sometimes considered as not

significant.

Fiber Reinforcement

Fiber-reinforced concrete is conventionalconcrete to which discontinuous discretefibers are added during mixing. In a broadsense, fibers used for reinforcing concreteare small versions of conventional steelreinforcement, and they provide a similarservice. For fibers to be useful andeffective, they must enhance the physical

attributes of concrete, and be durable.If fibers are not added to the mix in theproper batching sequence or if the volumepercentage of fibers is too high, fibersmay clump together or ball up duringmixing. Fiber contents up to 4% or 5% byvolume of concrete or mortar can be used;however, 1% to 2% is the practical upperlimit for field placement of most fibers.


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-4

3SECTION REE


Why use Among the desirableharacteristics that fibers can impart tooncrete are flexural strength, resistanceo fatigue and impact, and increased

racture resistance. Many small precastoncrete items are too thin or awkwardlyshaped to easily reinforce with steel barsr mesh, and are ideal for this method of

reinforcing.

Types used Steel, glass, polyethylene,polypropylene, nylon, and carbon. Steelhas been used crimped and straight;glass and carbon are usually used in fiberormat as in roving, or chopped lengths,r as a deformed bar of some type.

Precautions Steel fibers are subject tohe same type of corrosion as reinforcingsteel, and thus the durability of concretemade with steel fibers and used innvironments chemically aggressive tohe steel may be poor. Thus, exposure tohlorides, presence of cracks, etc. will leado problems and possible failures. If glassbers are not properly treated duringheir manufacture, they can be sensitive toalkalis in portland cement paste, causing

eterioration or embrittlement. Differentbers offer different advantages so their

selection for use must be done carefully.For example, polymer fiber such as nylonand polypropylene, improve impactstrength of concrete, but not tensile orexural strengths, because they have

a low modulus of elasticity. In general,bers do not replace steel reinforcing

in any structural application. Muchxperimentation and development work is

being done with carbon fibers; this is an

area to watch, as the properties carbonreinforcement potentially can bring tooncrete is amazing.

ost versus benefit Compared to steelreinforcing, fibers are an expensive wayo reinforce concrete, considering thematerial cost, pound for pound, and the

xtra handling involved to get it intohe mix. However, they can enhance theproperties of a product, or reduce thelikelihood of cracks, or their severity,reduce handling damage, and sometimesgo into a product which would otherwisebe unreinforced. The comparison needs tobe evaluated for each product.


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SUMMARY

Lifting Devices andApparatus

l Need verified designl Rated capacity must be clearly labeledl nspect equipment at least yearlyl fficial equipment inspection at least

yearlyl Users should do daily visual

inspections

Embedded Steel Shapes,Plates, and Hardware

l horough inspection to check Correct item Placement

Properly secured Non-corrosive material

Accessories

l mportant to use correct typel Non-corrosive materialsl Use proper quantity

Fibers

l Used to increase durabilityl ncreased resistance to crackingl Do not replace structural

reinforcement with fibers

Notes


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CONCRETE MIXES

Concrete Mixes

dry-cast and alsofor wet-cast. This

is an importantconcept sincesuperplastercizerscan make an ordinarilystiff mix as workable aif it were produced witwater. Many people mistakenly assumethat a concrete mix with a low w/c is awet-cast mix if a superplasticizer has beenadded, but the flowability/worakabilityof a mix tells you very little about itswater content. The terms wet-cast or

dry-cast refer to the natural slump ofthe concrete that is, the slump in theabsence of any water reducing admixtures.Therefore, a mix with a w/c of 0.40 orless is generally considered a dry-castmix even if it contains superplasticizingadmixtures that make it flow.

NPCA does not recommend w/c ratioshigher than 0.48. If the concrete is goingto be exposed to freeze/thaw, it should notbe higher than 0.45; and if it is exposed to

harsh environments, it should not be anymore than 0.40.

II. Air Content

Many people new to the industry ask whywe put air inconcrete only to later vibrateair out. The answer is because these aretwo different types of air. The air that is

I. Concrete Mixes

Although this section does not coverspecific mix designs, it discusses theimportant concepts any mix designerneeds to know. For instance, manypeople in the concrete industry arenot familiar with the importance of thewater/cementitious (w/cm) ratio. Thisratio compares the amount of water ina mix to the amount of cementitiousmaterial, including any pozzolan such ascement, fly-ash, slag and silica. For easeof discussion, we will refer to this ratio

as the water/cement ratio (w/c), althoughin reality it includes all cementitiousand pozzolanic materials, since theyare all included when doing mix designcalculations.

Water/cement (w/c) ratio is simply theweight of the water (in pounds) divided bythe combined weight of the cementitiousmaterials (also in pounds). The goal inproducing high-quality concrete is todecrease the amount of water in a mix in

order to lower th

precast quality guide

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