prof. m.s. shatty
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ANENGIMATIC
STRUCTURAL
BY
PROFM.S.SHETTY
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INTRODUCTION
Earliest reference about concrete wasrecorded in journal of scientific American
April 1848 issue. In that Journal it is statedthat concrete is a useful material for subs ra a or p an roa s.
Concretecomposition
:
70parts
of
limestone
10partsoflime
storynow.
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admixtures with four basic ingredients
sers nee : g s reng , g per ormance,
Durability, Volume stability, Impermeability,
em ca res s ance, g s ump a oo a
a very low w/c ratio Normal concrete itself is complex
But modern concrete, naturally should show
enigmatic (complex) behavior.
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Concrete is not an inert material like wood, steel,
Concrete is a living entity and a loyal friend. Itcan take considerable abuse and still serve ou
even in hostile situations It can be moulded when young
Gain strength with age
Shrinks and creeps with age as s own e ec s an au s
It is capable of self healing its defects
Ifbreathingisobstructeditissusceptiblefor
diseasesandde radation
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Ins ite of
eni matic
and
com lex
behavior
it
ismostwidelyusedversatileconstruction
material.
Onan
average
1
ton
of
concrete
is
used
in
.
In
terms
of
weight
it
is
only
next
to
water. Intermso cost,itaccounts or2530 o our
Unionbudget,peryear.
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It
is
a
com lex
material
because
it
is
made
from
naturallyavailablematerialsandcement
Cementitselfisaverycomplexmaterial
Cementchemistry,
in
its
simplified
form,
has
beenunderstoodtosomeextent
Complexityisprovidedbyimpurityincoal,
temperatureof
burning,
rate
of
cooling,
fineness
o gr n ng,e ec o gr n nga ,par c es zedistributionetc.
.
Processofmanufactureofconcrete,makes
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It is not the intention to deal with all the
complexities,involvedinConcreteTechnology.
Onl a few of the followin as ects are taken
fordiscussion.
Finenessofcement.
.
RoleofGypsum.
.
Implicationsarisingoutofcuring.
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Steamcuringandhighpressuresteamcuring.
Airentrainmnet andstrenght.
Sulphate resistanceandchlorideaction.
Maximumsize
of
aggreate and
strength.
Coverdepthandcrackwidth.
Covercrete.
Epoxycoating
and
corrosion.
Blendingmaterial.
Transitionzoneorinterface.
Strengthof
mass
concrete
at
eelevated
temperature
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Finenessofcement
se u
roper es:
Fasterdevelopment
of
strength
Fasterremovalofformwork
Usefulfor
cold
weather
concreting
Harmfulpropertiesoffinercementare
Rapidrateofevolutionofheat
Earlythermal
cracking
Necessitatesforearlycuring
RequiresmoreGypsum
equ res
more
osage
o
p ast c zers
Asmostcementparticlesgetshydratedearly,lessscope
for auto enous healin .
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Modulusofelasticity
resss ra n
e av our s
vary ng
Stressstrainbehaviour dependsupon1. Strengthofconcrete
2. Stress
level Sizeandextentofcoarseaggregatemoisture
.
Modulusofelasticity(Evalue)isapproximate.
Structuraldesign
based
on
E
value
is
likely
to
be
approximate.
EvenEvaluefoundoutfromnondestructivetestingis also not accurate.
Viscoelastic
behaviour,
heterogeneity,
and
the
mannerinwhichwaterheldingelporesarethe
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RoleofGypsum
ypsum
a o4. 2
wason y
use
ear er
o
controlflashsettingofC3A.
CrystallizedNatural
anhydrite:
CaSO4
Gypsum: CaSO4.2H2O Hemihydrate: CaSO4.1/2H2O
Synthetic
gypsum:
CaSO4
formsofgypsum.
Solubilit rate of sum chan es in the resenceof
superplasticizer.
Situationcancauseflashsetorfalseset.
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RoleofGypsum
Resultsinseriousproblemof incompatibility
betweencementandsuperplasticizer.
Atlow
W/C
ratio
there
is
further
site
problems
such
aslowerslumpandretentionofslump.
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Water/Cementratioandhydration
c encean
s ory
o
concre e
s ar s
w en
wa er
s
addedtocement
T.C.Powers showedwaterrequirementforfullhydrationas38%forthenprevailingC2SandC3S
OvertheyearsC3SisincreasedandC2Sisdecreased
, ,greaterrelevance
Presentlysupplementary
cementitious materials
are
use .
ThereforeW/Cisofter substitutedbyW/borW/(C+F)orW/(C+F+S)orevenbyW/(C+Fk1+Sk2)wherek1andk2
etcare
efficiency
factors.
W/Crelationshipwithstrengthholdsgood,atbest,for
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Water/Cementratioandhydration
. .
Waterbinderratio MaxCompStrengthRange
0.4to0.35 50to75Mpa
0.35to0.30 75to100Mpa
0.3to0.25 100to125Mpa
. .
UseofverylowW/Bratioresultsinhighearlystrength,
highmodulus
of
elasticity
at
an
early
age,
low
creep,
.
Onewonderswhetherthehighearlystrengthconcreteisastepforwardtowardsdurableconcrete?
Fieldexperiences
have
shown
that
W/C
ratio
of
order
of
0.30waspronetomicrocrackingaffectingthelongtermdurability.
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Water/Cementratioandhydration
,entirelyconvincing
Lowwatercontentinhighearlystrengthconcrete,only
The
existence
of
large
amount
of
unhydrated core
of
cement
grainshasstrengthinreserve
,throughmicrocracks,generatesproductsofhydrationtorestoretheintegrity ofconcrete
,ratioisofhighquality
AtlowW/CorW/Bratiothequalityoftransitionzoneisbetter
Atlow
W/C
ratio,
the
inter
connected
network
of
capillaries
aresofinethatwatercannotflowthroughthem.
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Water/Cementratioandhydration
rom
ea ove
scuss ons
s
cu
oconc u e
whethermicrocracks developedinhighearly
longterm
durability,
OR
the
delayed
hydration
of
unhydrated coreofcementgrainwouldhealupthe
microcracksandimprovethelongtermdurability,
aidedby
better
product
of
hydration,
reduced
permea ty,an ,strongertrans t onzone.
Itisrealy acomplexsubjectfitforresearch.
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Implicationsarisingoutofcuring
Normalconcrete
with
W C
ratio
0.4
and
above
can
be
curedinnormalmannerorbycoveringwithpolythenesheetorbymembraneformingcuringcompound
Concrete
with
low
W/C
ratio
necessitates
early
curing
onlymist
fogging
is
possible
Whenconcreteundergoesfullhydrationallcement
particlesgets
hydrated
and
therefore,
there
is
less
scope orau ogeneous ea ng
Whenconcreteattainsfullstrengthasitcanpossiblyattain,itwillhavehighermodulusofelasticity,bemore
brittle,and
crack
at
lower
strain
level.
Reducedstraincapacityisnotadesirablepropertyin
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Steamcuring
ofconcrete
Thisneedsunderstandingofwaitingperiod,rateofraising, , .
getintended
benefits.
Evenotherwisesteamcuredconcreteundergoescertain
subjectedearlyhighheatofHydration,asinMassconcrete.
Thequality
of
gel
formed
is
inferior
in
quality
just
like
Onthecontrarythehighpressuresteamcuredconcretedoesnotshowsretrogressionofstrength Aseemingly
Highpressuresteamcuredconcreteshrinksonlyabout1/3to1/6ofordinarycuredconcrete
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AirEntrainmentandStrength
Popularbelief
prior
to
1930
was
that
denser
and
strongerconcreteisalwaysadurableconcrete
eacc en a scoveryo a ren ra ne
concrete,it
was
found
that,
air
entrained
concrete
,
timesmoredurable,whensubjectedtofreezingand
thawing
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Sulphate resistanceandChlorideAction
Insu p ate attac
Ca cium
Su p oa uminates are
orme
wit in
t e
frameworkofhydratedhardenedcementpaste,causingexpansion
RemedyliesintheuseoflowC3Aincement.GenerallyC3Acontent
Chloride
does
not
attack
concrete,
but
induces
corrosion
of
steel
reinforcement
ChlorideionsareboundbyC3A
ItiscontradictorysituationthatlowC3Aisrequiredagainstsulphateattack
and
hi her
C3A
is
desirable
a ainst
chloride
action
AhigherC3Aalsoresultsinhigherrateofhydration,earlyevolutionofheat.
TheincreaseofC3Aalsocausescement superplasticizer
incompatibilityand
faster
slump
loss.
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MaximumSizeofaggregateandStrength
neo
e
popu ar
me o s
o
m x
es gn
s
ase
onsurfacearea
Biggersizeofaggregategavelessersurfacearea
It
was
thought,
bigger
the
MSA,
higher
the
strength Onthecontrary,theabovewastrueuptoalimit(For
downisused)
Concrete
does
not
behave
as
we
expect
it
to
behave
Thereasonsforcontradictorybehaviour are1. Heterogeneitycausedbybiggeraggregate
3. Lowersurface
area
gave
less
bond
4. Largerinterfacecausedweakertransitionzone
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Coverdepthandcrackwidth
a e,
c nesso
cover
s
ncrease
or
e er
durability
andcracks
Crackingisfurtherextendedtothesurface
widthatthesurface detrimentaltodurability
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Covercrete
Itwasseenfrom1000softrialsthatconcreteisnotuniformthroughoutitscrosssectionduetowall
Formedsurface
i.e.
External
skin
or
what
you
call
covercrete isinferiorinitsqualitywhencomparedwithn er orconcre e
Inskinportion,thereisanincreaseof0.1w/cratioand
decreaseof
100
kg
of
cement
per
cubic
meter
of
concrete
Suchweakcovercrete giveslessprotectiontorebarby
Permeableformwork
technique
is
sometimes
adoptedtoimprovequalityofcovercrete
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Epoxycoatingandcorrosion
Theearlierbeliefthatconcreteneedsnoprotectionisprovedtobeinvalid
Concretenee sprotectionespecia yinur anand
coastal
area.(CO2content
may
up
to
the
extent of 0.3 to 1
Epoxycoatingisoneofthematerialssuggested
Epoxyis
not
a
right
material
as
it
stops
breathing
inconcrete
EpoxyitselfisnotdurableagainstUVraysofsun
Acryliccoatingisfoundtobebettermaterialforthis ur ose
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BlendingMaterial
Todaysconcreteinvariablycontainblending
materialssuchasflyash,GGBS,Silicafumeetc.
The
addition
of
blending
materials
make
concrete
morecomplexmaterial,particularlywhenmore
thanonechemicaladmixturesaresimultaneously
added Thereactivityandtherateofreactivityofmineral
admixturesdependuponfinenessofcement,
percentreplacement,
type
of
blending
material
andcontinuedavailabilityofmoistureetc.
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Transition
zone Anothermostimportantparameterswhichexhibit
considerablecom lexitiesinthe ro ertiesandbehaviour ofconcreteisthetransitionzone
Concretehasbeenconsideredastwophasematerial
Now,concreteisconsideredasthreephasematerialTransitionzoneisthethirdphase
consideredasaCHAIN
Considerationandeffectoftransitionzoneorinterface
zonebetween
aggregate
and
paste
provide
full
of
complexities
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StrengthofMassConcreteatelevated
Tem erature YoungConcreteisrequiredtobelookedafterin
respectof foodandTemperature
ItsfoodisWater/Moisture buildsstrongstructure.
Concretegrowswellataparticulardesirableempera ure
Youngconcrete,ifsubjectedtohigherthandesirable
getsreducedatlaterage(28days)
Structuresaredesigned,generallyfor28daysstrength,
basedon
Cube
strength
cured
at
27
o
C IncaseofMassConcreteifappropriatestepsarenot
o
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StrengthofMassConcreteatelevated
Tem erature Ifyoungconcretesubjectedtohightemperaturethe
stren th etreducedconsiderabl sa 6to10M a
Strengthassumed
by
structural
designer
based
on
cubestren th,willbewron inrealit ACom lex
behaviour
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Conclusion Mo ern
concrete
is
a
mixture
o
a mixtures
wit
our
conventionalingredients
ThepropertiesdemandedbyArchitects,Structuralengineers,u ers, m xturemanu acturers, qu pmentmanu acturers
andcement
companies
are
so
divergent,
that
modern
concretehasbecomeanenigmaticstructuralmaterial
ngenera ,sop s ca on ncreasescomp ex es oncre e snoexception
Inmany
situations
concrete
shows
a
consistent
behaviour,
but
Oneshouldunderstandthecomplexbehaviour ofconcretetouseitasaversatile,efficientandusefulconstructionmaterial
or
oncre e
ec no og s s
oncre e
mus
e
s
LOVE
FIRSTBEING
HIS
WIFE
AS
COMPLICATED
AS
THAT
OF
CONCRETE
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ThankYou
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