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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