Prof. M.S. Shatty

8/3/2019 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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. .

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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,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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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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Prof. M.S. Shatty