Concret lec 3

What is Concrete?What is Concrete?
 Concrete iConcrete iss one ofone of the most commonlythe most commonly
used building materialused building materials.s.
 Concrete is aConcrete is a composite materialcomposite material
made from several readily availablemade from several readily available
constituentsconstituents (aggregates, sand, cement,(aggregates, sand, cement,
water)water)..
 Concrete is a versatile material that canConcrete is a versatile material that can
easily be mixedeasily be mixed to meet a variety ofto meet a variety of
special needs and formed to virtuallyspecial needs and formed to virtually anyany

AdvantagesAdvantages
 Ability to be castAbility to be cast
 EconomicalEconomical
 DurableDurable
 Fire resistantFire resistant
 Energy efficientEnergy efficient
 On-site fabricationOn-site fabrication

DisadvantagesDisadvantages
 Low tensile strengthLow tensile strength
 Low ductilityLow ductility
 Volume instabilityVolume instability
 Low strength to weight ratioLow strength to weight ratio

CementCement
WaterWater
Fine Agg.Fine Agg.
Coarse Agg.Coarse Agg.
AdmixturesAdmixtures
ConstituentsConstituents

PROPERTIES OF FRESHPROPERTIES OF FRESH
CONCRETECONCRETE
 WorkabilityWorkability
 ConsistencyConsistency
 SegregationSegregation
 BleedingBleeding
 Setting TimeSetting Time
 Unit WeightUnit Weight
 UniformityUniformity

WORKABILITYWORKABILITY
It is desirable that freshly mixed concreteIt is desirable that freshly mixed concrete
be relatively easy to transport, place,be relatively easy to transport, place,
compact and finish without harmfulcompact and finish without harmful
segregation.segregation.
A concrete mix satisfying theseA concrete mix satisfying these
conditions is said to beconditions is said to be workableworkable..

Factors Affecting WorkabilityFactors Affecting Workability
 Method and duration of transportationMethod and duration of transportation
 Quantity and characteristics of cementingQuantity and characteristics of cementing
materialsmaterials
 Aggregate grading, shapeAggregate grading, shape andand surfacesurface
texturetexture
 Quantity and characteristics of chemicalQuantity and characteristics of chemical
admixturesadmixtures
 Amount of waterAmount of water
 Amount ofAmount of entrained airentrained air
 Concrete & normal air temperatureConcrete & normal air temperature

WORKABILITYWORKABILITY
 Workability is the mostWorkability is the most
important property of freshlyimportant property of freshly
mixed concrete.mixed concrete.
 There is no single testThere is no single test
method that canmethod that can
simultaneously measure allsimultaneously measure all
the properties involved inthe properties involved in
workability.workability.
 It is determined to a largeIt is determined to a large
extent by measuring theextent by measuring the
“consistency”“consistency” of the mix.of the mix.

 Consistency is the fluidity or degree ofConsistency is the fluidity or degree of
wetness of concrete.wetness of concrete.
 It is generally dependent on the shearIt is generally dependent on the shear
resistance of the mass.resistance of the mass.
 It is a major factor in indicating theIt is a major factor in indicating the
workability of freshly mixed concrete.workability of freshly mixed concrete.
CONSISTENCYCONSISTENCY

Test methods for measuring consistencyTest methods for measuring consistency
are:are:
 Flow testFlow test → measures the amount of flow→ measures the amount of flow
 Kelly-Ball test → measures the amount ofKelly-Ball test → measures the amount of
penetrationpenetration
 Slump test (Most widely used test!)Slump test (Most widely used test!)
CONSISTENCYCONSISTENCY

 Slump TestSlump Test is related with the ease withis related with the ease with
which concrete flows during placementwhich concrete flows during placement
(TS 2871, ASTM C 143)(TS 2871, ASTM C 143)

10 cm
20 cm
30 cm
The slump cone is filled in 3 layers. Every
layer is evenly rodded 25 times.
Measure the slump by determining the vertical difference
between the top of the mold and the displaced original center
of the top surface of the specimen.

 SegregationSegregation refers to a separation of the componentsrefers to a separation of the components
of fresh concrete, resulting in a non-uniform mixof fresh concrete, resulting in a non-uniform mix
Sp.Gr. Size
Cement 3-3.15 5-80 µm
C.Agg. 2.4-2.8 5-40 mm
F.Agg. 2.4-2.8 < 5 mm
SEGREGATIONSEGREGATION
 The primary causes ofThe primary causes of
segregation are differencessegregation are differences
in specific gravity and sizein specific gravity and size
of constituents of concrete.of constituents of concrete.
Moreover, improper mixing,Moreover, improper mixing,
improper placing andimproper placing and
improper consolidation alsoimproper consolidation also
lead to segregation.lead to segregation.

Some of the factors affecting segregation:Some of the factors affecting segregation:
– Larger maximum particle size (25mm) andLarger maximum particle size (25mm) and
proportion of the larger particles.proportion of the larger particles.
– High specific gravity of coarse aggregate.High specific gravity of coarse aggregate.
– Decrease in the amount of fine particles.Decrease in the amount of fine particles.
– Particle shape and texture.Particle shape and texture.
– Water/cement ratio.Water/cement ratio.
SEGREGATIONSEGREGATION

 Bleeding is the tendency of water to rise toBleeding is the tendency of water to rise to
the surface of freshly placed concrete.the surface of freshly placed concrete.
BLEEDINGBLEEDING
 It is caused by theIt is caused by the
inability of solidinability of solid
constituents of theconstituents of the
mix to hold all of themix to hold all of the
mixing water asmixing water as
they settle down.they settle down.
 A special case ofA special case of
segregation.segregation.

Undesirable effects of bleeding are:Undesirable effects of bleeding are:
• With the movement of water towards the top, the topWith the movement of water towards the top, the top
portion becomes weak & porous (high w/c). Thus theportion becomes weak & porous (high w/c). Thus the
resistance of concrete to freezing-thawing decreases.resistance of concrete to freezing-thawing decreases.
• Water rising to the surface carry fine particles ofWater rising to the surface carry fine particles of
cement which weaken the top portion and formcement which weaken the top portion and form
laitance. This portion is not resistant to abrasion.laitance. This portion is not resistant to abrasion.
• Water may accumulate under the coarse agg. andWater may accumulate under the coarse agg. and
reinforcement. These large voids under the particlesreinforcement. These large voids under the particles
may lead to weak zones and reduce the bondmay lead to weak zones and reduce the bond
between paste and agg. or paste and reinforcement.between paste and agg. or paste and reinforcement.
BLEEDINGBLEEDING

The tendency of concrete to bleedingThe tendency of concrete to bleeding
depends largely on properties of cement.depends largely on properties of cement.
It is decreased by:It is decreased by:
 Increasing the fineness of cementIncreasing the fineness of cement
 Increasing the rate of hydration (CIncreasing the rate of hydration (C33S, CS, C33A andA and
alkalies)alkalies)
 Adding pozzolansAdding pozzolans
 Reducing water contentReducing water content
BLEEDINGBLEEDING

MIXING OF CONCRETEMIXING OF CONCRETE
 The aim of mixing is to blend all of theThe aim of mixing is to blend all of the
ingredients of the concrete to form aingredients of the concrete to form a
uniform mass and to coat the surface ofuniform mass and to coat the surface of
aggregates with cement paste.aggregates with cement paste.

 Ready-Mix concrete: In this typeReady-Mix concrete: In this type
ingredients are introduced into a mixeringredients are introduced into a mixer
truck and mixed during transportation totruck and mixed during transportation to
the site.the site.
• Wet – Water added before transportationWet – Water added before transportation
• Dry – Water added at siteDry – Water added at site
 Mixing at the siteMixing at the site
• Hand mixedHand mixed
• Mixer mixedMixer mixed

Ready MixReady Mix ConcreteConcrete

Mixing time should be sufficient to produceMixing time should be sufficient to produce
a uniform concrete. The time of mixinga uniform concrete. The time of mixing
depends on the type of mixer and also todepends on the type of mixer and also to
some properties of fresh concrete.some properties of fresh concrete.
 UndermixingUndermixing → non-homogeneity→ non-homogeneity
 Overmixing → danger of water loss,Overmixing → danger of water loss,
brekage of aggregate particlesbrekage of aggregate particles

CONSOLIDATINGCONSOLIDATING
CONCRETECONCRETE
Inadequate consolidationInadequate consolidation
can result in:can result in:
– HoneycombHoneycomb
– Excessive amount ofExcessive amount of
entrapped air voidsentrapped air voids
(bugholes)(bugholes)
– Sand streaksSand streaks
– Placement linesPlacement lines ((Cold jointsCold joints))

VIBRATION OFVIBRATION OF
CONCRETECONCRETE
 The process of compacting concreteThe process of compacting concrete
consists essentially of the elimination ofconsists essentially of the elimination of
entrapped air. This can be achieved by:entrapped air. This can be achieved by:
– Tamping or rodding the concreteTamping or rodding the concrete
– Use of vibratorsUse of vibrators

VIBRATORSVIBRATORS
 Internal vibratorInternal vibrator:: The poker is immersedThe poker is immersed
into concrete to compact it. The poker isinto concrete to compact it. The poker is
easily removed from point to point.easily removed from point to point.
 External vibrators:External vibrators: External vibratorsExternal vibrators
clamp direct to the formwork requiringclamp direct to the formwork requiring
strong, rigid formsstrong, rigid forms..

Internal VibrationInternal Vibration
d
R
1½ R
Vibrator
Radius of Action

Internal VibratorsInternal Vibrators
DiameterDiameter
of head,of head,
(m(mmm))
RecommendedRecommended
frequency,frequency,
((vibvib././minmin.).)
ApproximateApproximate
radius ofradius of
action,action, ((mmmm))
Rate ofRate of
placement,placement,
((mm33
/h/h))
ApplicationApplication
20-4020-40 9000-15,0009000-15,000 80-15080-150 0.8-40.8-4
Plastic and flowingPlastic and flowing
concrete in thinconcrete in thin
members. Also used formembers. Also used for
lab test specimens.lab test specimens.
30-6030-60 8500-12,5008500-12,500 130-250130-250 2.3-82.3-8
Plastic concrete in thinPlastic concrete in thin
walls, columns, beams,walls, columns, beams,
precast piles, thin slabs,precast piles, thin slabs,
and along constructionand along construction
joints.joints.
50-9050-90 8000-12,0008000-12,000 180-360180-360 4.6-154.6-15
Stiff plastic concreteStiff plastic concrete
(less than 80-mm(less than 80-mm
slump) in generalslump) in general
construction .construction .
Adapted from ACI 309

Systematic VibrationSystematic Vibration
CORRECTCORRECT
Vertical penetration a few inches
into previous lift (which should not
yet be rigid) of systematic regular
intervals will give adequate
consolidation
INCORRECTINCORRECT
Haphazard random penetration of
the vibrator at all angles and
spacings without sufficient depth will
not assure intimate combination of
the two layers

 To aid in the removal of trapped air the vibratorTo aid in the removal of trapped air the vibrator
head should be rapidly plunged into the mixhead should be rapidly plunged into the mix
and slowly moved up and downand slowly moved up and down..
Internal VibratorsInternal Vibrators
 The actual completionThe actual completion
of vibration is judged byof vibration is judged by
the appearance of thethe appearance of the
concrete surface whichconcrete surface which
must be neither roughmust be neither rough
nor contain excessnor contain excess
cement paste.cement paste.

External VibratoExternal Vibrato rsrs
 Form vibratorsForm vibrators
 Vibrating tablesVibrating tables (Lab)(Lab)
 Surface vibratorsSurface vibrators
– Vibratory screedsVibratory screeds
– Plate vibratorsPlate vibrators
– Vibratory rollerVibratory roller
screedsscreeds
– Vibratory hand floatsVibratory hand floats
or trowelsor trowels

 External vibrators are rigidly clamped to theExternal vibrators are rigidly clamped to the
formwork so that both the form & concrete areformwork so that both the form & concrete are
subjected to vibration.subjected to vibration.
 A considerable amount of work is needed toA considerable amount of work is needed to
vibrate forms.vibrate forms.
 Forms must be strong and tied enough to preventForms must be strong and tied enough to prevent
distortion and leakage of the grout.distortion and leakage of the grout.

 Vibrating Table:Vibrating Table:
used for smallused for small
amounts ofamounts of
concreteconcrete
(laboratory and(laboratory and
some precastsome precast
elements)elements)

CURING OF CONCRETECURING OF CONCRETE
 Properties of concrete can improve with age asProperties of concrete can improve with age as
long as conditions are favorable for the continuedlong as conditions are favorable for the continued
hydration of cement. These improvements arehydration of cement. These improvements are
rapid at early ages and continues slowly for anrapid at early ages and continues slowly for an
indefinite period of time.indefinite period of time.
 Curing is the procedures used for promoting theCuring is the procedures used for promoting the
hydration of cement and consists of a control ofhydration of cement and consists of a control of
temperature and the moisture movement fromtemperature and the moisture movement from
and into the concrete.and into the concrete.

 Hydration reactions canHydration reactions can
take place in onlytake place in only
saturated water filledsaturated water filled
capillaries.capillaries.
CURING OF CONCRETECURING OF CONCRETE
 The primary objective of curing is to keepThe primary objective of curing is to keep
concrete saturated or as nearly saturated asconcrete saturated or as nearly saturated as
possible.possible.

Curing MethodsCuring Methods
1.1. Methods which supply additional water toMethods which supply additional water to
the surface of concrete during earlythe surface of concrete during early
hardening stages.hardening stages.
– Using wet coversUsing wet covers
– SprinklingSprinkling
– PondingPonding

2.2. Methods that prevent loss of moisture fromMethods that prevent loss of moisture from
concrete by sealing the surface.concrete by sealing the surface.
– Water proof plasticsWater proof plastics
– Use liquid membrane-forming compoundsUse liquid membrane-forming compounds
– Forms left in placeForms left in place

3.3. Methods that accelerate strength gain byMethods that accelerate strength gain by
supplying heat & moisture to the concrete.supplying heat & moisture to the concrete.
– By using live steam (steam curing)By using live steam (steam curing)
– Heating coils.Heating coils.

Hot Weather ConcreteHot Weather Concrete
 Rapid hydration  early setting  rapid loss of
workability
 Extra problems due to
– Low humidity
– Wind, excessive evaporation
– Direct sunlight
Solutions
– Windbreaks
– Cooled Concrete Ingredients
– Water ponding (cooling due to evaporation)
– Reflective coatings/coverings

Cold Weather ConcreteCold Weather Concrete
 Keep concrete temperature above 5 °C to
minimize danger of freezing
Solutions
– Heated enclosures, insulation
– Rely on heat of hydration for larger sections
– Heated ingredients --- concrete hot when placed
– High early strength cement

UNIFORMITY OF CONCRETEUNIFORMITY OF CONCRETE
 Concrete uniformity isConcrete uniformity is
checked by conductingchecked by conducting
tests on fresh andtests on fresh and
hardened concretes.hardened concretes.
Slump, unit weight, airSlump, unit weight, air
content testscontent tests
Strength testsStrength tests

UNIFORMITY OF CONCRETEUNIFORMITY OF CONCRETE
 Due to heteregeneous nature of concrete,Due to heteregeneous nature of concrete,
there will always be some variations. Thesethere will always be some variations. These
variations are grouped as:variations are grouped as:
– Within-Batch Variations : inadequate mixing,Within-Batch Variations : inadequate mixing,
non-homogeneous naturenon-homogeneous nature
– Batch-to-Batch Variations : type of materialsBatch-to-Batch Variations : type of materials
used, changes in gradation of aggregates,used, changes in gradation of aggregates,
changes in moisture content of aggregateschanges in moisture content of aggregates

PROPERTIES OFPROPERTIES OF
HARDENED CONCRETEHARDENED CONCRETE
 The principal properties of hardenedThe principal properties of hardened
concrete which are of practical importanceconcrete which are of practical importance
can be listed as:can be listed as:
1.1. StrengthStrength
2.2. Permeability & durabilityPermeability & durability
3.3. Shrinkage & creep deformationsShrinkage & creep deformations
4.4. Response to temperature variationsResponse to temperature variations
Of these compressive strength is the mostOf these compressive strength is the most
important property of concrete. Because;important property of concrete. Because;

PROPERTIES OFPROPERTIES OF
HARDENED CONCRETEHARDENED CONCRETE
Of the abovementioned hardenedOf the abovementioned hardened
properties compressive strength is one ofproperties compressive strength is one of
the most important property that is oftenthe most important property that is often
required, simply because;required, simply because;
1.1. Concrete is used for compressive loadsConcrete is used for compressive loads
2.2. Compressive strength is easily obtainedCompressive strength is easily obtained
3.3. It is a good measure of all the other properties.It is a good measure of all the other properties.

What AffectsWhat Affects
Concrete StrengthConcrete Strength
What
Doesn’t?

Factors Affecting StrengthFactors Affecting Strength
 Effect of materials and mix proportions
 Production methods
 Testing parameters

STRENGTH OFSTRENGTH OF
CONCRETECONCRETE
 The strength of a concrete specimenThe strength of a concrete specimen
prepared, cured and tested under specifiedprepared, cured and tested under specified
conditions at a given age depends on:conditions at a given age depends on:
1.1. w/c ratiow/c ratio
2.2. Degree of compactionDegree of compaction

COMPRESSIVE STRENGTHCOMPRESSIVE STRENGTH
 Compressive StrengthCompressive Strength is determined byis determined by
loading properly prepared and cured cubic,loading properly prepared and cured cubic,
cylindrical or prismatic specimens undercylindrical or prismatic specimens under
compression.compression.

COMPRESSIVE STRENGTHCOMPRESSIVE STRENGTH
 Cubic: 15x15x15 cmCubic: 15x15x15 cm
Cubic specimens are crushed after rotatingCubic specimens are crushed after rotating
them 90them 90°° to decrease the amount of frictionto decrease the amount of friction
caused by the rough finishing.caused by the rough finishing.
 Cylinder: h/D=2 with h=15Cylinder: h/D=2 with h=15
To decrease the amount of friction, cappingTo decrease the amount of friction, capping
of the rough casting surface is performed.of the rough casting surface is performed.

Cubic specimens
without capping
Cylindrical
specimens
with capping
COMPRESSIVECOMPRESSIVE
STRENGTHSTRENGTH

Bonded sulphur capping Unbonded neoprene pads
COMPRESSIVECOMPRESSIVE
STRENGTHSTRENGTH

STRENGTH CLASSESSTRENGTH CLASSES
(TS EN 206-1)(TS EN 206-1)
 The compressive strength value depends onThe compressive strength value depends on
the shape and size of the specimen.the shape and size of the specimen.

TENSILE STRENGTHTENSILE STRENGTH
 Tensile StrengthTensile Strength can be obtained either bycan be obtained either by
direct methods or indirect methods.direct methods or indirect methods.
Direct methods suffer from a number ofDirect methods suffer from a number of
difficulties related to holding the specimendifficulties related to holding the specimen
properly in the testing machine withoutproperly in the testing machine without
introducing stress concentration and to theintroducing stress concentration and to the
application of load without eccentricity.application of load without eccentricity.

DIRECT TENSILEDIRECT TENSILE
STRENGTHSTRENGTH

SPLIT TENSILESPLIT TENSILE
STRENGTHSTRENGTH
Due to applied compression load a fairly uniform
tensile stress is induced over nearly 2/3 of the
diameter of the cylinder perpendicular to the
direction of load application.

 The advantage of the splitting test over theThe advantage of the splitting test over the
direct tensile test is the same molds aredirect tensile test is the same molds are
used for compressive & tensile strengthused for compressive & tensile strength
determination.determination.
 The test is simple to perform and givesThe test is simple to perform and gives
uniform results than other tension tests.uniform results than other tension tests.
σst =
2P
πDl
P: applied compressive load
D: diameter of specimen
l: length of specimen
Splitting Tensile
Strength

The flexural tensile strength at failure or theThe flexural tensile strength at failure or the
modulus of rupture is determined by loading amodulus of rupture is determined by loading a
prismatic concrete beam specimen.prismatic concrete beam specimen.
FLEXURAL STRENGTHFLEXURAL STRENGTH
The resultsThe results
obtained are usefulobtained are useful
because concretebecause concrete
is subjected tois subjected to
flexural loads moreflexural loads more
often than it isoften than it is
subjected to tensilesubjected to tensile
loads.loads.

P
M=Pl/4
d
b
c
I =
bd3
12
2
3
σ =
M
cI
=
(Pl/4) (d/2)
bd3
/12
=
Pl
bd2
M=Pl/6
P/2 P/2
σ =
(Pl/6) (d/2)
bd3
/12
=
Pl
bd2

Factors Affecting the StrengthFactors Affecting the Strength
of Concreteof Concrete
1)1) Factors depended on theFactors depended on the
test type:test type:
– Size of specimenSize of specimen
– Size of specimen inSize of specimen in
relation with size of agg.relation with size of agg.
– Support condition afSupport condition af
specimenspecimen
– Moisture condition ofMoisture condition of
specimenspecimen
– Type of loading adoptedType of loading adopted
– Rate of loadingRate of loading
– Type of test machineType of test machine
2.2. Factors independent ofFactors independent of
test type:test type:
– Type of cementType of cement
– Type of agg.Type of agg.
– Degree of compactionDegree of compaction
– Mix proportionsMix proportions
– Type of curingType of curing
– Type of stress situationType of stress situation

STRESS-STRAIN RELATIONSSTRESS-STRAIN RELATIONS
IN CONCRETEIN CONCRETE
σult
(40-50%)
σult
εult
σ-ε relationship for
concrete is
nonlinear. However,
specially for
cylindrical
specimens with
h/D=2, it can be
assumed as linear
upto 40-50% of σult

MODULUS OF ELASTICITY OFMODULUS OF ELASTICITY OF
CONCRETECONCRETE
Due to the nonlinearityDue to the nonlinearity
of theof the σσ--εε diagram, Ediagram, E
is the defined by:is the defined by:
1.1. Initial Tangent MethodInitial Tangent Method
2.2. Tangent MethodTangent Method
3.3. Secant MethodSecant Method
ACI → E=15200 σult
½
→ 28-D cylindrical comp.str.
(kgf/cm2
)
½ 2

PERMEABILITY OFPERMEABILITY OF
CONCRETECONCRETE
 Permeability is important because:Permeability is important because:
1.1. The penetration of some aggresive solution may resultThe penetration of some aggresive solution may result
in leaching out of Ca(OH)in leaching out of Ca(OH)22 which adversely affects thewhich adversely affects the
durability of concrete.durability of concrete.
2.2. In R/C ingress of moisture of air into concrete causesIn R/C ingress of moisture of air into concrete causes
corrosion of reinforcement and results in the volumecorrosion of reinforcement and results in the volume
expansion of steel bars, consequently causing cracksexpansion of steel bars, consequently causing cracks
& spalling of concrete cover.& spalling of concrete cover.
3.3. The moisture penetration depends on permeability & ifThe moisture penetration depends on permeability & if
concrete becomes saturated it is more liable to frost-concrete becomes saturated it is more liable to frost-
action.action.
4.4. In some structural members permeability itself is ofIn some structural members permeability itself is of
importance, such as, dams, water retaining tanks.importance, such as, dams, water retaining tanks.

 The permeability of concrete is controlledThe permeability of concrete is controlled
by capillary pores. The permeabilityby capillary pores. The permeability
depends mostly on w/c, age, degree ofdepends mostly on w/c, age, degree of
hydration.hydration.
 In general the higher the strength ofIn general the higher the strength of
cement paste, the higher is the durability &cement paste, the higher is the durability &
the lower is the permeability.the lower is the permeability.
PERMEABILITY OFPERMEABILITY OF
CONCRETECONCRETE

DURABILITYDURABILITY
A durable concrete is the one which willA durable concrete is the one which will
withstand in a satisfactory degree, thewithstand in a satisfactory degree, the
effects of service conditions to which it willeffects of service conditions to which it will
be subjected.be subjected.
Factors Affecting Durability:Factors Affecting Durability:
 ExternalExternal → Environmental→ Environmental
 Internal →Internal → Permeability, Characteristics ofPermeability, Characteristics of
ingredients, Air-Void System...ingredients, Air-Void System...

Structure of “un-damaged”
Concrete
 Macrostructure
– Aggregates (CA, FA)
– Hydrated cement paste (hcp)
– Entrapped air voids
 Microstructure
– Hydrated cement paste (Hydration products: C-S-H,
ettringite, monosulfate; porosity: gel, capillary pores
entrained/ entrapped air voids)
– Transition zone (TZ)

Structure of “un-damaged”
Concrete
Macrostructure Microstructure

Structure of “damaged”
Concrete
Macrostructure
Visible cracks in hcp
and aggregates due
to volume changes
(to understand
cause of cracks,
microstructure
should be
examined)
Microstructure
 Alkali-silica reaction:
Reaction product forms
at TZ and expands
 Frost action: Water
freezes in capillary
pores and expands
 Sulfate attack: reaction
products form in hcp
and expand

Leaching & EfflorescenceLeaching & Efflorescence
 When water penetrates into concrete, itWhen water penetrates into concrete, it
dissolves the non-hydraulic CH (anddissolves the non-hydraulic CH (and
various salts, sulfates and carbonates ofvarious salts, sulfates and carbonates of
Na, K, Ca)Na, K, Ca)
 RememberRemember C-S-HC-S-H andand CHCH is producedis produced
upon hydration ofupon hydration of CC33SS andand CC22SS
 These salts are taken outside of concreteThese salts are taken outside of concrete
by water and leave a salt deposit.by water and leave a salt deposit.

Sulfate AttackSulfate Attack
 Ground water in clayey soils containing alkaliGround water in clayey soils containing alkali
sulfates may affect concrete.sulfates may affect concrete.
 These solutions attack CH to produceThese solutions attack CH to produce
gypsum. Later, gypsum and calcium aluminagypsum. Later, gypsum and calcium alumina
sulfates together with water react to formsulfates together with water react to form
“ettringite”.“ettringite”.
 Formation of ettringite is hardened cementFormation of ettringite is hardened cement
paste or concrete leads to volume expansionpaste or concrete leads to volume expansion
thus cracking.thus cracking.
 Moreover, Magnesium sulfate may lead to theMoreover, Magnesium sulfate may lead to the

 Seawater contains some amount of Na and MgSeawater contains some amount of Na and Mg
Sulfates. However, these sulfates do not causeSulfates. However, these sulfates do not cause
severe deleterious expansion/cracking becausesevere deleterious expansion/cracking because
both gypsum and ettringite are soluble in solutionsboth gypsum and ettringite are soluble in solutions
containing the Cl ion. However, problem withcontaining the Cl ion. However, problem with
seawater is the frequent wetting/drying andseawater is the frequent wetting/drying and
corrosion of reinforcing steel in concrete.corrosion of reinforcing steel in concrete.
 To reduce the sulfate attackTo reduce the sulfate attack
1.1. Use low w/c ratioUse low w/c ratio→ reduced permeability & porosity→ reduced permeability & porosity
2.2. Use proper cement → reduced CUse proper cement → reduced C33A and CA and C33SS
3.3. Use pozzolans → they use up some of the CH to produceUse pozzolans → they use up some of the CH to produce
C-S-HC-S-H
Sulfate AttackSulfate Attack

Acid AttackAcid Attack
 Concrete is pretty resistant to acids. But inConcrete is pretty resistant to acids. But in
high concentrations:high concentrations:
 Causes leaching of the CHCauses leaching of the CH
 Causes disintegration of the C-S-H gel.Causes disintegration of the C-S-H gel.

CarbonationCarbonation
 Ca(OH)Ca(OH)22 + CO+ CO22 → CaCO→ CaCO33 + H+ H22OO
 Accompanied by shrinkage → carbonationAccompanied by shrinkage → carbonation
shrinkageshrinkage
 Makes the steel vulnerable to corrosionMakes the steel vulnerable to corrosion
(due to reduced alkalinity)(due to reduced alkalinity)

Alkali-Agg. ReactionsAlkali-Agg. Reactions
 Alkalies of cement + Reactive Silica of AggsAlkalies of cement + Reactive Silica of Aggs
→ Alkali-Silica Gel→ Alkali-Silica Gel
 Expansions in volumeExpansions in volume
 Slow processSlow process
 Don’t use aggs with reactive silica or useDon’t use aggs with reactive silica or use
cements with less alkalies.cements with less alkalies.

CorrosionCorrosion
 Electrochemical reactions in the steel rebarsElectrochemical reactions in the steel rebars
of a R/C structure results in corrosionof a R/C structure results in corrosion
products which have larger volumes thanproducts which have larger volumes than
original steel.original steel.
 Thus this volume expansion causes cracksThus this volume expansion causes cracks
in R/C. In fact, steel is protected by a thinin R/C. In fact, steel is protected by a thin
film provided by concrete against corrosion.film provided by concrete against corrosion.
However, that shield is broken by COHowever, that shield is broken by CO22 of airof air
or the Clor the Cl--
ions.ions.

Freezing and ThawingFreezing and Thawing
 Water when freezes expands in volume.Water when freezes expands in volume.
This will cause internal hydraulic pressureThis will cause internal hydraulic pressure
and cracks the concrete.and cracks the concrete.
 To prevent theTo prevent the
concrete from thisconcrete from this
distress air-entrainingdistress air-entraining
admixtures are usedadmixtures are used
to produce air-to produce air-
entrained concrete.entrained concrete.

AbrasionAbrasion
 Aggregates have to be hard & resistant toAggregates have to be hard & resistant to
wear.wear.
 Bleeding & finishing practices are alsoBleeding & finishing practices are also
important.important.

 W+C+C.Agg.+F.Agg.+AdmixturesW+C+C.Agg.+F.Agg.+Admixtures → Weights / Volumes?→ Weights / Volumes?
 There are two sets of requirements which enableThere are two sets of requirements which enable
the engineer to design a concrete mix.the engineer to design a concrete mix.
1.1. The requirements of concrete in hardened state.The requirements of concrete in hardened state.
These are specified by the structural engineer.These are specified by the structural engineer.
2.2. The requirements of fresh concrete such asThe requirements of fresh concrete such as
workability, setting time. These are specified by theworkability, setting time. These are specified by the
construction engineer (type of construction, placingconstruction engineer (type of construction, placing
methods, compacting techniques andmethods, compacting techniques and
transportation)transportation)
PROPORTIONINGPROPORTIONING
CONCRETE MIXTURESCONCRETE MIXTURES

 Mix design is the process of selectingMix design is the process of selecting
suitable ingredients of concrete &suitable ingredients of concrete &
determining their relative quantities with thedetermining their relative quantities with the
objective of producing as economically asobjective of producing as economically as
possible concrete of certain minimumpossible concrete of certain minimum
properties such as workability, strength &properties such as workability, strength &
durability.durability.
 So, basic considerations in a mix design isSo, basic considerations in a mix design is
cost & min. properties.cost & min. properties.
PROPORTIONINGPROPORTIONING
CONCRETE MIXTURESCONCRETE MIXTURES

 Cost → Material + Labor
Water+Cement+Aggregate+Admixtures
Most expensive (optimize)
Using less cement causes a decrease in
shrinkage and increase in volume stability.
Min.Properties →Strength has to be more
than..
Durability→Permeability has to be
Workability→Slump has to be...

 In the past specifications for concrete mixIn the past specifications for concrete mix
design prescribed the proportions ofdesign prescribed the proportions of
cement, fine agg. & coarse agg.cement, fine agg. & coarse agg.
 1 : 2 : 41 : 2 : 4
Weight of
cement
Fine
Agg.
Coars
e Agg.
 However, modern specifications do not use
these fixed ratios.

 Modern specifications specify min compressiveModern specifications specify min compressive
strength, grading of agg, max w/c ratio, min/maxstrength, grading of agg, max w/c ratio, min/max
cement content, min entrained air & etc.cement content, min entrained air & etc.
 Most of the time job specifications dictate theMost of the time job specifications dictate the
following data:following data:
– Max w/cMax w/c
– Min cement contentMin cement content
– Min air contentMin air content
– SlumpSlump
– StrengthStrength
– DurabilityDurability
– Type of cementType of cement
– AdmixturesAdmixtures
– Max agg. sizeMax agg. size

PROCEDURE FOR MIXPROCEDURE FOR MIX
DESIGNDESIGN
1.1. Choice of slump (Table 14.5)Choice of slump (Table 14.5)

DESIGNDESIGN
2.2. Choice of max agg. sizeChoice of max agg. size
• 1/5 of the narrowest dimension of the mold
• 1/3 of the depth of the slab
• ¾ of the clear spacing between reinforcement
• Dmax < 40mm

DESIGNDESIGN
3.3. Estimation of mixing water & air contentEstimation of mixing water & air content
(Table 14.6 and 14.7)(Table 14.6 and 14.7)

DESIGNDESIGN
4.4. Selection of w/c ratio (Table 14.8 or 14.9)Selection of w/c ratio (Table 14.8 or 14.9)

DESIGNDESIGN
5.5. Calculation of cement content with selected waterCalculation of cement content with selected water
amount (step 3) and w/c (step 4)amount (step 3) and w/c (step 4)
6.6. Estimation of coarse agg. content (Table 14.10)Estimation of coarse agg. content (Table 14.10)

DESIGNDESIGN
7.7. Calculation of fine aggregate content withCalculation of fine aggregate content with
known volumes of coarse aggregate, water,known volumes of coarse aggregate, water,
cement and aircement and air
8.8. Adjustions for aggregate field moistureAdjustions for aggregate field moisture

DESIGNDESIGN
9.9. Trial batch adjustmentsTrial batch adjustments
 The properties of the mixes in trial batches areThe properties of the mixes in trial batches are
checked and necessary adjustments are made tochecked and necessary adjustments are made to
end up with the minimum required properties ofend up with the minimum required properties of
concrete.concrete.
 Moreover, a lab trial batch may not always provideMoreover, a lab trial batch may not always provide
the final answer. Only the mix made and used inthe final answer. Only the mix made and used in
the job can guarantee that all properties of concretethe job can guarantee that all properties of concrete
are satisfactory in every detail for the particular jobare satisfactory in every detail for the particular job
at hand. That’s why we get samples from the fieldat hand. That’s why we get samples from the field
mixes for testing the properties.mixes for testing the properties.

Example:Example:
 SlumpSlump → 75-100 mm→ 75-100 mm
 DDmaxmax → 25 mm→ 25 mm
 f’f’c,28c,28 = 25 MPa= 25 MPa
 Specific Gravity of cement = 3.15Specific Gravity of cement = 3.15
 Non-air entrained concreteNon-air entrained concrete
Coarse Agg. Fine Agg.
SSD Bulk Sp.Gravity 2.68 2.62
Absorption 0.5% 1.0%
Total Moist.Content 2.0% 5.0%
Dry rodded Unit Weight 1600 kg/m3
–
Fineness Modulus – 2.6

1.1. Slump is given as 75-100 mmSlump is given as 75-100 mm
2.2. DDmaxmax is given as 25 mmis given as 25 mm
3.3. Estimate the water and air contentEstimate the water and air content (Table 14.6)(Table 14.6)
Slump and DSlump and Dmaxmax → W=193 kg/m→ W=193 kg/m33
Entrapped Air → 1.5%

4.4. Estimate w/c ratio (Table 14.8)Estimate w/c ratio (Table 14.8)
f’f’cc & non-air entrained → w/c=0.61 (by wt)& non-air entrained → w/c=0.61 (by wt)

5.5. Calculation of cement contentCalculation of cement content
W = 193 kg/mW = 193 kg/m33
and w/c=0.61and w/c=0.61
C=193 / 0.61 = 316 kg/mC=193 / 0.61 = 316 kg/m33

6.6. Coarse Agg. from Table 14.10Coarse Agg. from Table 14.10
DDmaxmax and F.M. → Vand F.M. → VC.AC.A=0.69 m=0.69 m33
Dry WDry WC.A.C.A. = 1600*0.69 = 1104 kg/m= 1600*0.69 = 1104 kg/m33
SSD WSSD WC.A.C.A. = 1104*(1+0.005) = 1110 kg/m= 1104*(1+0.005) = 1110 kg/m33

7.7. To calculate the F.Agg. content theTo calculate the F.Agg. content the
volumes of other ingredients have to bevolumes of other ingredients have to be
determined.determined. V = M
Sp.Gr.*ρwVwater = 193
1.0*1000
= 0.193 m3
Vcement = 316
3.15*1000
= 0.100 m3
VC.Agg. = 1110
2.68*1000
= 0.414 m3
Vair = 0.015 m3
(1.5%*1)
ΣV = 0.722 m3
→ VF.Agg = 1-0.722 = 0.278 m3
WF.Agg = 0.278*2.62*1000 = 728 kg/m3

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