Concret lec 3

CONCRETECONCRETE

What is Concrete?What is Concrete? Concrete iConcrete is s one of one of the most commonly the most commonly

used building materialused building materials.s. Concrete is a Concrete is a composite materialcomposite material

made from several readily available made from several readily available constituentsconstituents (aggregates, sand, cement, (aggregates, sand, cement, water)water)..

Concrete is a versatile material that can Concrete is a versatile material that can easily be mixedeasily be mixed to meet a variety of to meet a variety of special needs and formed to virtually special needs and formed to virtually any shapeany shape..

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

CementCementWater Water

Fine Agg.Fine Agg.Coarse Agg.Coarse Agg.AdmixturesAdmixtures

ConstituentsConstituents

PROPERTIES OF FRESH PROPERTIES OF FRESH CONCRETECONCRETE

WorkabilityWorkability ConsistencyConsistency SegregationSegregation BleedingBleeding Setting TimeSetting Time Unit WeightUnit Weight UniformityUniformity

WORKABILITYWORKABILITY

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

Factors Affecting Factors Affecting WorkabilityWorkability

Method and duration of transportationMethod and duration of transportation Quantity and characteristics of Quantity and characteristics of

cementing materialscementing materials Aggregate grading, shape Aggregate grading, shape andand surface surface

texturetexture Quantity and characteristics of Quantity and characteristics of

chemical chemical admixturesadmixtures Amount of waterAmount of water Amount ofAmount of entrained air entrained air Concrete & normal air temperatureConcrete & normal air temperature

WORKABILITYWORKABILITY Workability is the most Workability is the most

important property of important property of freshly mixed concrete. freshly mixed concrete.

There is no single test There is no single test method that can method that can simultaneously measure simultaneously measure all the properties involved all the properties involved in workability. in workability.

It is determined to a large It is determined to a large extent by measuring the extent by measuring the “consistency”“consistency” of the mix. of the mix.

Consistency is the fluidity or degree Consistency is the fluidity or degree of wetness of concrete. of wetness of concrete.

It is generally dependent on the shear It is generally dependent on the shear resistance of the mass. resistance of the mass.

It is a major factor in indicating the It is a major factor in indicating the workability of freshly mixed concrete.workability of freshly mixed concrete.

CONSISTENCYCONSISTENCY

Test methods for measuring Test methods for measuring consistency are:consistency are:

Flow test Flow test → measures the amount of flow→ measures the amount of flow Kelly-Ball test → measures the amount of Kelly-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 is related with the ease with which concrete flows during with which concrete flows during placement (TS 2871, ASTM C 143)placement (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.

Segregation Segregation refers to a separation of the refers to a separation of the components of fresh concrete, resulting in a components of fresh concrete, resulting in a non-uniform mix non-uniform mix

Sp.Gr. Size

Cement 3-3.15 5-80 m

C.Agg. 2.4-2.8 5-40 mm

F.Agg. 2.4-2.8 < 5 mm

SEGREGATIONSEGREGATION

The primary causes of The primary causes of segregation are segregation are differences in specific differences in specific gravity and size of gravity and size of constituents of concrete. constituents of concrete. Moreover, improper Moreover, improper mixing, improper placing mixing, improper placing and improper and improper consolidation also lead consolidation also lead to segregation.to segregation.

Some of the factors affecting Some of the factors affecting segregation:segregation:– Larger maximum particle size (25mm) and Larger 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 Bleeding is the tendency of water to rise to the surface of freshly placed rise to the surface of freshly placed concrete. concrete.

BLEEDINGBLEEDING

It is caused by It is caused by the inability of the inability of solid constituents solid constituents of the mix to hold of the mix to hold all of the mixing all of the mixing water as they water as they settle down.settle down.

A special case of A special case of segregation.segregation.

Undesirable effects of bleeding are:Undesirable effects of bleeding are:• With the movement of water towards the top, the With the movement of water towards the top, the

top portion becomes weak & porous (high w/c). Thus top portion becomes weak & porous (high w/c). Thus the resistance of concrete to freezing-thawing the resistance of concrete to freezing-thawing decreases.decreases.

• Water rising to the surface carry fine particles of Water rising to the surface carry fine particles of cement which weaken the top portion and form cement 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. and Water may accumulate under the coarse agg. and reinforcement. These large voids under the particles reinforcement. These large voids under the particles may lead to weak zones and reduce the bond may 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 bleeding The tendency of concrete to bleeding depends largely on properties of depends largely on properties of cement. It is decreased by:cement. 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 A

and alkalies)and alkalies) Adding pozzolansAdding pozzolans Reducing water contentReducing water content

BLEEDINGBLEEDING

MIXING OF CONCRETEMIXING OF CONCRETE

The aim of mixing is to blend all of The aim of mixing is to blend all of the ingredients of the concrete to the ingredients of the concrete to form a uniform mass and to coat the form a uniform mass and to coat the surface of aggregates with cement surface of aggregates with cement paste.paste.


Ready-Mix concrete: In this type Ready-Mix concrete: In this type ingredients are introduced into a ingredients are introduced into a mixer truck and mixed during mixer truck and mixed during transportation to the site. transportation to 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 Mix Ready Mix ConcreteConcrete

MixMixing at Siteing at Site

Mixing time should be sufficient to Mixing time should be sufficient to produce a uniform concrete. The time produce a uniform concrete. The time of mixing depends on the type of of mixing depends on the type of mixer and also to some properties of mixer and also to some properties of fresh concrete.fresh concrete.

Undermixing Undermixing → non-homogeneity→ non-homogeneity Overmixing → danger of water loss, Overmixing → danger of water loss,

brekage of aggregate particlesbrekage of aggregate particles


CONSOLIDATING CONSOLIDATING CONCRETECONCRETE

Inadequate Inadequate consolidation can result consolidation can result in:in:– HoneycombHoneycomb– Excessive amount of Excessive amount of

entrapped air voids entrapped air voids (bugholes)(bugholes)

– Sand streaksSand streaks– Placement linesPlacement lines ( (Cold jointsCold joints))

VIBRATION OF CONCRETEVIBRATION OF CONCRETE

The process of compacting concrete The process of compacting concrete consists essentially of the elimination consists essentially of the elimination of entrapped air. This can be of entrapped air. This can be achieved by:achieved by:

– Tamping or rodding the concreteTamping or rodding the concrete– Use of vibrators Use of vibrators

VIBRATORSVIBRATORS

Internal vibratorInternal vibrator:: The poker is The poker is immersed into concrete to compact immersed into concrete to compact it. The poker is easily removed from it. The poker is easily removed from point to point.point to point.

External vibrators:External vibrators: External vibrators External vibrators clamp direct to the formwork clamp direct to the formwork requiring strong, rigid formsrequiring strong, rigid forms..

Internal VibrationInternal Vibration

d

R

1½ R

Vibrator

Radius of Action

Internal VibratorsInternal VibratorsDiameterDiameterof head,of head,

(m(mmm))

Recommended Recommended frequency, frequency, ((vibvib././minmin.).)

Approximate Approximate radius of radius of

action, action, ((mmmm))

Rate of Rate of placement,placement,

((mm33/h/h))ApplicationApplication

20-4020-40 9000-15,0009000-15,000 80-15080-150 0.8-40.8-4Plastic and flowing Plastic and flowing concrete in thin concrete in thin members. Also used for members. 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 thin Plastic concrete in thin walls, columns, beams, walls, columns, beams, precast piles, thin slabs, precast piles, thin slabs, and along construction and along construction joints. joints.

50-9050-90 8000-12,0008000-12,000 180-360180-360 4.6-154.6-15Stiff plastic concrete Stiff plastic concrete (less than 80-mm (less than 80-mm slump) in general slump) in general construction . construction .

Adapted from ACI 309

Systematic VibrationSystematic VibrationCORRECTCORRECTVertical penetration a few inches into previous lift (which should not yet be rigid) of systematic regular intervals will give adequate consolidationINCORRECTINCORRECTHaphazard 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 To aid in the removal of trapped air the vibrator head should be rapidly plunged vibrator head should be rapidly plunged into the mix and slowly moved up and into the mix and slowly moved up and downdown..

Internal VibratorsInternal Vibrators

The actual The actual completion of completion of vibration is judged by vibration is judged by the appearance of the appearance of the concrete surface the concrete surface which must be which must be neither rough nor neither rough nor contain excess contain excess cement paste.cement paste.

External VibratoExternal Vibratorsrs

Form vibratorsForm vibrators Vibrating tablesVibrating tables (Lab) (Lab) Surface vibratorsSurface vibrators

– Vibratory screedsVibratory screeds– Plate vibratorsPlate vibrators– Vibratory roller Vibratory roller

screedsscreeds– Vibratory hand floats Vibratory hand floats

or trowelsor trowels

External vibrators are rigidly clamped to the External vibrators are rigidly clamped to the formwork so that both the form & concrete formwork so that both the form & concrete are subjected to vibration. are subjected to vibration.

A considerable amount of work is needed to A considerable amount of work is needed to vibrate forms. vibrate forms.

Forms must be strong and tied enough to Forms must be strong and tied enough to prevent distortion and leakage of the grout.prevent distortion and leakage of the grout.


Vibrating Table: Vibrating Table: used for small used for small amounts of amounts of concrete concrete (laboratory and (laboratory and some precast some precast elements)elements)


CURING OF CONCRETECURING OF CONCRETE Properties of concrete can improve with age Properties of concrete can improve with age

as long as conditions are favorable for the as long as conditions are favorable for the continued hydration of cement. These continued hydration of cement. These improvements are rapid at early ages and improvements are rapid at early ages and continues slowly for an indefinite period of continues slowly for an indefinite period of time.time.

Curing is the procedures used for promoting Curing is the procedures used for promoting the hydration of cement and consists of a the hydration of cement and consists of a control of temperature and the moisture control of temperature and the moisture movement from and into the concrete.movement from and into the concrete.

Hydration reactions Hydration reactions can take place in can take place in only saturated water only saturated water filled capillaries. filled capillaries.

CURING OF CONCRETECURING OF CONCRETE The primary objective of curing is to keep The primary objective of curing is to keep

concrete saturated or as nearly saturated concrete saturated or as nearly saturated as possible.as possible.

Curing MethodsCuring Methods

1.1. Methods which supply additional Methods which supply additional water to the surface of concrete water to the surface of concrete during early hardening stages.during early hardening stages.

– Using wet coversUsing wet covers– Sprinkling Sprinkling – PondingPonding


2.2. Methods that prevent loss of moisture Methods that prevent loss of moisture from concrete by sealing the surface.from concrete by sealing the surface.

– Water proof plasticsWater proof plastics– Use liquid membrane-forming Use liquid membrane-forming

compoundscompounds– Forms left in placeForms left in place

3.3. Methods that accelerate strength gain Methods that accelerate strength gain by supplying heat & moisture to the by supplying heat & moisture to the concrete.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 is Concrete uniformity is checked by conducting checked by conducting tests on fresh and tests on fresh and hardened concretes.hardened concretes.Slump, unit weight, air Slump, unit weight, air

content testscontent testsStrength testsStrength tests

UNIFORMITY OF CONCRETEUNIFORMITY OF CONCRETE

Due to heteregeneous nature of Due to heteregeneous nature of concrete, there will always be some concrete, there will always be some variations. These variations are variations. These variations are grouped as:grouped as:– Within-Batch Variations : inadequate Within-Batch Variations : inadequate

mixing, non-homogeneous naturemixing, non-homogeneous nature– Batch-to-Batch Variations : type of Batch-to-Batch Variations : type of

materials used, changes in gradation of materials used, changes in gradation of aggregates, changes in moisture content aggregates, changes in moisture content of aggregatesof aggregates

PROPERTIES OF PROPERTIES OF HARDENED CONCRETEHARDENED CONCRETE

The principal properties of hardened The principal properties of hardened concrete which are of practical concrete which are of practical importance can be listed as:importance can be listed as:

1.1. StrengthStrength2.2. Permeability & durabilityPermeability & durability3.3. Shrinkage & creep deformationsShrinkage & creep deformations4.4. Response to temperature variationsResponse to temperature variationsOf these compressive strength is the Of these compressive strength is the most important property of concrete. most important property of concrete. Because;Because;

PROPERTIES OF PROPERTIES OF HARDENED CONCRETEHARDENED CONCRETE

Of the abovementioned hardened Of the abovementioned hardened properties compressive strength is properties compressive strength is one of the most important property one of the most important property that is often required, simply that is often required, simply because;because;

1.1. Concrete is used for compressive loadsConcrete is used for compressive loads2.2. Compressive strength is easily obtainedCompressive strength is easily obtained3.3. It is a good measure of all the other It is a good measure of all the other

properties.properties.

What AffectsWhat AffectsConcrete StrengthConcrete Strength

What Doesn’t?

Factors Affecting Factors Affecting StrengthStrength

Effect of materials and mix proportions

Production methods Testing parameters

STRENGTH OF STRENGTH OF CONCRETECONCRETE

The strength of a concrete specimen The strength of a concrete specimen prepared, cured and tested under prepared, cured and tested under specified conditions at a given age specified conditions at a given age depends on:depends on:

1.1. w/c ratiow/c ratio2.2. Degree of compactionDegree of compaction

COMPRESSIVE STRENGTH COMPRESSIVE STRENGTH

Compressive StrengthCompressive Strength is determined is determined by loading properly prepared and by loading properly prepared and cured cubic, cylindrical or prismatic cured cubic, cylindrical or prismatic specimens under compression.specimens under compression.

COMPRESSIVE STRENGTH COMPRESSIVE STRENGTH

Cubic: 15x15x15 cmCubic: 15x15x15 cmCubic specimens are crushed after Cubic specimens are crushed after rotating them 90rotating them 90°° to decrease the amount to decrease the amount of friction caused by the rough finishing.of friction caused by the rough finishing.

Cylinder: h/D=2 with h=15Cylinder: h/D=2 with h=15To decrease the amount of friction, To decrease the amount of friction, capping of the rough casting surface is capping of the rough casting surface is performed.performed.

Cubic specimens without capping

Cylindrical specimens

with capping

COMPRESSIVE COMPRESSIVE STRENGTHSTRENGTH

Bonded sulphur capping Unbonded neoprene pads

COMPRESSIVE COMPRESSIVE STRENGTHSTRENGTH

STRENGTH CLASSESSTRENGTH CLASSES(TS EN 206-1)(TS EN 206-1)

The compressive strength value The compressive strength value depends on the shape and size of the depends on the shape and size of the specimen. specimen.

TENSILE STRENGTHTENSILE STRENGTH Tensile StrengthTensile Strength can be obtained can be obtained

either by direct methods or indirect either by direct methods or indirect methods. methods.

Direct methods suffer from a number of Direct methods suffer from a number of difficulties related to holding the difficulties related to holding the specimen properly in the testing machine specimen properly in the testing machine without introducing stress concentration without introducing stress concentration and to the application of load without and to the application of load without eccentricity.eccentricity.

DIRECT TENSILE DIRECT TENSILE STRENGTHSTRENGTH

SPLIT TENSILE SPLIT 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 The advantage of the splitting test over the direct tensile test is the same molds the direct tensile test is the same molds are used for compressive & tensile are used for compressive & tensile strength determination.strength determination.

The test is simple to perform and gives The test is simple to perform and gives uniform results than other tension uniform results than other tension tests.tests.

σst = 2PπDl

P: applied compressive loadD: diameter of specimenl: length of specimen

Splitting Tensile

Strength

The flexural tensile strength at failure or the The flexural tensile strength at failure or the modulus of rupture is determined by loading modulus of rupture is determined by loading a prismatic concrete beam specimen. a prismatic concrete beam specimen.

FLEXURAL STRENGTHFLEXURAL STRENGTH

The results The results obtained are obtained are useful because useful because concrete is concrete is subjected to subjected to flexural loads flexural loads more often than more often than it is subjected to it is subjected to tensile loads.tensile loads.

P

M=Pl/4

d

b

c I = bd3

12

23

σ =M cI = (Pl/4) (d/2)

bd3/12 = Plbd2

M=Pl/6

P/2 P/2

σ =(Pl/6) (d/2)

bd3/12

= Plbd2

Factors Affecting the Factors Affecting the Strength of ConcreteStrength of Concrete

1)1) Factors depended on Factors depended on the test type:the test type:

– Size of specimenSize of specimen– Size of specimen in Size of specimen in

relation with size of agg.relation with size of agg.– Support condition af Support condition af

specimenspecimen– Moisture condition of Moisture 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 of Factors 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 RELATIONS STRESS-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 OF MODULUS OF ELASTICITY OF CONCRETECONCRETE

Due to the Due to the nonlinearity of the nonlinearity of the σσ--εε diagram, E is the diagram, E is the defined by:defined by:1.1. Initial Tangent MethodInitial Tangent Method2.2. Tangent MethodTangent Method3.3. Secant MethodSecant Method

ACI → E=15200 σult½ → 28-D cylindrical comp.str.

(kgf/cm2)

TS → E=15500 W ½→ 28-D cubic comp.str. (kgf/cm2)

PERMEABILITY OF PERMEABILITY OF CONCRETECONCRETE

Permeability is important because:Permeability is important because:1.1. The penetration of some aggresive solution may The penetration of some aggresive solution may

result in leaching out of Ca(OH)result in leaching out of Ca(OH)22 which adversely which adversely affects the durability of concrete.affects the durability of concrete.

2.2. In R/C ingress of moisture of air into concrete In R/C ingress of moisture of air into concrete causes corrosion of reinforcement and results in causes corrosion of reinforcement and results in the volume expansion of steel bars, consequently the volume expansion of steel bars, consequently causing cracks & spalling of concrete cover.causing cracks & spalling of concrete cover.

3.3. The moisture penetration depends on The moisture penetration depends on permeability & if concrete becomes saturated it permeability & if concrete becomes saturated it is more liable to frost-action.is more liable to frost-action.

4.4. In some structural members permeability itself is In some structural members permeability itself is of importance, such as, dams, water retaining of importance, such as, dams, water retaining tanks.tanks.

The permeability of concrete is controlled The permeability of concrete is controlled by capillary pores. The permeability by capillary pores. The permeability depends mostly on w/c, age, degree of depends mostly on w/c, age, degree of hydration.hydration.

In general the higher the strength of In 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 OF PERMEABILITY OF CONCRETECONCRETE

DURABILITYDURABILITY

A durable concrete is the one which A durable concrete is the one which will withstand in a satisfactory degree, will withstand in a satisfactory degree, the effects of service conditions to the effects of service conditions to which it will be subjected.which it will be subjected.Factors Affecting Durability:Factors Affecting Durability:

External External → Environmental→ Environmental Internal → Internal → Permeability, Characteristics Permeability, Characteristics

of ingredients, Air-Void System...of 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

MacrostructureVisible 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, it When water penetrates into concrete, it dissolves the non-hydraulic CH (and dissolves the non-hydraulic CH (and various salts, sulfates and carbonates of various salts, sulfates and carbonates of Na, K, Ca)Na, K, Ca)

Remember Remember C-S-HC-S-H and and CHCH is produced is produced upon hydration of upon hydration of CC33SS and and CC22SS

These salts are taken outside of concrete These 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 Ground water in clayey soils containing alkali sulfates may affect concrete.alkali sulfates may affect concrete.

These solutions attack CH to produce These solutions attack CH to produce gypsum. Later, gypsum and calcium gypsum. Later, gypsum and calcium alumina sulfates together with water react alumina sulfates together with water react to form “ettringite”. to form “ettringite”.

Formation of ettringite is hardened cement Formation of ettringite is hardened cement paste or concrete leads to volume paste or concrete leads to volume expansion thus cracking.expansion thus cracking.

Moreover, Magnesium sulfate may lead to Moreover, Magnesium sulfate may lead to the decomposition of the C-S-H gel. the decomposition of the C-S-H gel.

Seawater contains some amount of Na and Seawater contains some amount of Na and Mg Sulfates. However, these sulfates do not Mg Sulfates. However, these sulfates do not cause severe deleterious expansion/cracking cause severe deleterious expansion/cracking because both gypsum and ettringite are because both gypsum and ettringite are soluble in solutions containing the Cl ion. soluble in solutions containing the Cl ion. However, problem with seawater is the However, problem with seawater is the frequent wetting/drying and corrosion of frequent wetting/drying and corrosion of reinforcing steel in concrete.reinforcing steel in concrete.

To reduce the sulfate attackTo reduce the sulfate attack1.1. Use low w/c ratioUse low w/c ratio→ reduced permeability & porosity→ reduced permeability & porosity2.2. Use proper cement → reduced CUse proper cement → reduced C33A and CA and C33S S 3.3. Use pozzolans → they use up some of the CH to produce Use 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. Concrete is pretty resistant to acids. But in high concentrations:But in 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 → carbonation Accompanied by shrinkage → carbonation shrinkageshrinkage

Makes the steel vulnerable to corrosion Makes the steel vulnerable to corrosion (due to reduced alkalinity)(due to reduced alkalinity)

Alkali-Agg. ReactionsAlkali-Agg. Reactions

Alkalies of cement + Reactive Silica of Alkalies of cement + Reactive Silica of Aggs Aggs → Alkali-Silica Gel→ Alkali-Silica Gel

Expansions in volumeExpansions in volume

Slow processSlow process

Don’t use aggs with reactive silica or use Don’t use aggs with reactive silica or use cements with less alkalies. cements with less alkalies.

CorrosionCorrosion

Electrochemical reactions in the steel Electrochemical reactions in the steel rebars of a R/C structure results in rebars of a R/C structure results in corrosion products which have larger corrosion products which have larger volumes than original steel.volumes than original steel.

Thus this volume expansion causes Thus this volume expansion causes cracks in R/C. In fact, steel is protected cracks in R/C. In fact, steel is protected by a thin film provided by concrete by a thin film provided by concrete against corrosion. However, that shield against corrosion. However, that shield is broken by COis broken by CO22 of air or the Cl of air or 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 pressure This will cause internal hydraulic pressure and cracks the concrete.and cracks the concrete.

To prevent the To prevent the concrete from this concrete from this distress air-distress air-entraining entraining admixtures are admixtures are used to produce used to produce air-entrained air-entrained concrete.concrete.

AbrasionAbrasion

Aggregates have to be hard & Aggregates have to be hard & resistant to wear.resistant to wear.

Bleeding & finishing practices are Bleeding & finishing practices are also important.also important.

W+C+C.Agg.+F.Agg.+Admixtures W+C+C.Agg.+F.Agg.+Admixtures → Weights / → Weights / Volumes?Volumes?

There are two sets of requirements which enable There 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 as The requirements of fresh concrete such as workability, setting time. These are specified by the workability, setting time. These are specified by the construction engineer (type of construction, placing construction engineer (type of construction, placing methods, compacting techniques and methods, compacting techniques and transportation)transportation)

PROPORTIONING PROPORTIONING CONCRETE MIXTURESCONCRETE MIXTURES

Mix design is the process of selecting Mix design is the process of selecting suitable ingredients of concrete & suitable ingredients of concrete & determining their relative quantities determining their relative quantities with the objective of producing as with the objective of producing as economically as possible concrete of economically as possible concrete of certain minimum properties such as certain minimum properties such as workability, strength & durability.workability, strength & durability.

So, basic considerations in a mix So, basic considerations in a mix design is cost & min. properties. design is cost & min. properties.

PROPORTIONING PROPORTIONING CONCRETE MIXTURESCONCRETE MIXTURES

Cost → Material + Labor

Water+Cement+Aggregate+AdmixturesMost 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 In the past specifications for concrete mix design prescribed the proportions mix design prescribed the proportions of cement, fine agg. & coarse agg.of cement, fine agg. & coarse agg.

1 : 2 : 41 : 2 : 4

Weight of

cement

Fine Agg

.

Coarse

Agg.

However, modern specifications do not use these fixed ratios.

Modern specifications specify min Modern specifications specify min compressive strength, grading of agg, max compressive strength, grading of agg, max w/c ratio, min/max cement content, min w/c ratio, min/max cement content, min entrained air & etc.entrained air & etc.

Most of the time job specifications dictate the Most 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– Strength Strength – DurabilityDurability– Type of cementType of cement– AdmixturesAdmixtures– Max agg. size Max agg. size

PROCEDURE FOR MIX PROCEDURE FOR MIX DESIGNDESIGN

1.1. Choice of slump (Table 14.5)Choice of slump (Table 14.5)


2.2. Choice of max agg. size Choice 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


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


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


5.5. Calculation of cement content with selected Calculation of cement content with selected water amount (step 3) and w/c (step 4)water 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)


7.7. Calculation of fine aggregate content Calculation of fine aggregate content with known volumes of coarse with known volumes of coarse aggregate, water, cement and airaggregate, water, cement and air

8.8. Adjustions for aggregate field moistureAdjustions for aggregate field moisture


9.9. Trial batch adjustmentsTrial batch adjustments The properties of the mixes in trial batches The properties of the mixes in trial batches

are checked and necessary adjustments are are checked and necessary adjustments are made to end up with the minimum required made to end up with the minimum required properties of concrete.properties of concrete.

Moreover, a lab trial batch may not always Moreover, a lab trial batch may not always provide the final answer. Only the mix made provide the final answer. Only the mix made and used in the job can guarantee that all and used in the job can guarantee that all properties of concrete are satisfactory in properties of concrete are satisfactory in every detail for the particular job at hand. every detail for the particular job at hand. That’s why we get samples from the field That’s why we get samples from the field mixes for testing the properties.mixes for testing the properties.

Example:Example: Slump Slump → 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 concrete Non-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 mm2.2. DDmaxmax is given as 25 mm is given as 25 mm3.3. 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%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 content Calculation of cement content

W = 193 kg/mW = 193 kg/m33 and w/c=0.61 and 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 14.10DDmaxmax and F.M. → V and 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 = 1104*(1+0.005) = 1110 kg/mkg/m33

7.7. To calculate the F.Agg. content the To calculate the F.Agg. content the volumes of other ingredients have volumes of other ingredients have to be determined.to be determined. V = M

Sp.Gr.*w

Vwater =

1931.0*1000

= 0.193 m3

Vcement =

3163.15*1000

= 0.100 m3

VC.Agg. = 11102.68*100

0= 0.414 m3

Vair = 0.015 m3 (1.5%*1)V = 0.722 m3 → VF.Agg = 1-0.722 = 0.278 m3

WF.Agg = 0.278*2.62*1000 = 728 kg/m3

Concret lec 3

Engineering

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