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Affect of Water/cement Ratio (w/c ratio)

Water/cementratio(w/cratio) theorystatesthatforagivencombinationofmaterialsandaslonga

sworkable

consistency

is

obtained,

the

strength

ofconcreteatagivenagedependsonthew/cratio.

In1918,DuffAbramsestablishedawater/cementratiolawforthestrengthof

concrete

:

Ac

B1.5(

w

/

c)

c compressive

strength

at

some

fixed

age,

A

=

empirical

constant

9!"#

$%a&,

'=

constant

thatdepends

mostl(

on

the

cement

properties

about

)&

,and

w/c

water/cement

ratio

b(

weight&"

Advantagesoflowwater/cementratio:

Increasedstrength"

*ower

permeabilit("

Increasedresistancetoweathering"

'etterbondbetweenconcreteandreinforcement"

+educeddr(ingshrinageandcracing"

*essvolumechangefromwettinganddr(ing"Methods of Mix Proportioning

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Absolute

volume

method

Mostcommonlyusedmethod(A!mixdsign)

"ther

methods

A!

#$$%$

&tandard

practice

for

selecting

'ormaleavyweightandMassoncrete%

A!#$$%#&tandardpracticeforselecting&tructurallightweightconcrete%

A!#$$%*&tandardpracticeforselectingProportions

for

no+slump

concrete%

A!#$$%,R &tandardpracticeforselectinghighstrengthconcretewithPortlandcementandflyash%

DesigningConcreteMixtures

Concrete

mixture

proportions

are

usuallyexpressedonthebasisofthemassofingredientsperunitvolume.

Theunitofvolumeusediseitheracubicyardor

a

cubic

meter

of

concrete.

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A-I $ix Design

he most common method used which isstablishedbyAC!ecommended"ractice#$$.$

ny mi& design procedure will provide a first

ppro&imation of the proportions and must behecked by trial batches.

ocal characteristics of materials should beonsidered.

he following seuence of steps should be

followed($)determinethefollowing

the*obparametersaggregateproperties

ma&imumaggregatesi+e

slump

w/cratio

admi&tures,

(#)calculationofbatchwei ht,and

)ad*ustmentstobatchweightsbasedontrialmi&.

heaimofthedesignershouldalwa(sbetoget

oncrete mixtures of optimum strength atminimum cement content and acceptable

orabilit("

nce

the

w/c

ratio

is

establishedand

the

orabilit(or consistenc( needed for thepecificdesignischosen,therestshouldbe

mple

manipulation

with

diagrams

and

tablesasedonlargenumbersoftrialmixes"

A! M-." "0 PR"P"R.!"'!'1

"'R-.- M!2-&

thetextbookratherthantheACIstandard).

Type

of

Construction Slump

(mm) (inches)

Reinforcedfoundation

wallsandfootings25-75 1-3

lainfootings!caissons

andsu"structurewalls25-75 1-3

#eamsandreinforced

walls

25-1$$ 1-%

#uildingcolumns 25-1$$ 1-%

a&ementsandsla"s 25-75 1-3

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heACIStandard211.1isaRecommendedPracticefor

electingProportionsforConcrete.Theprocedureisas

o!s"

Step1.Choiceofslump

Step2.Choiceofmaximumsizeofaggregate

Step.!stimationofmixing"aterandair

contentStep#.Selectionof"ater$cementratio

Step%.Calculationofcementcontent

Step&.!stimationofcoarseaggregatecontent

Step'.calculationof(ine)ggregateContent

Step*.)d+ustmentsfor)ggregateMoisture

Step

,.

Trial

-atch

)d+ustments

tep $% hoice of slump

su#pisnotspecified$a%aueappropriateforthe!orkcanbe

ectedfro#thebeo!Tabe!hichisreproducedfro#the

xtbookbeo!&$(notethatthetabenu#bersare'i%enfr