7 concrete revised

7/27/2019 7 Concrete Revised

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CONCRETE


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What is Concrete?

Concrete is one of the most commonlyused building materials.

Concrete is a composite material madefrom several readily available constituents(aggregates, sand, cement, water).

Concrete is a versatile material that caneasily be mixed to meet a variety ofspecial needs and formed to virtually any

shape.


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Advantages

Ability to be cast

Economical

Durable

Fire resistant

Energy efficient

On-site fabrication


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Disadvantages

Low tensile strength

Low ductility

Volume instability

Low strength to weight ratio


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Cement

Water

Fine Agg.

Coarse Agg.

Admixtures

Constituents


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WORKABILITY

It is desirable that freshly mixed concretebe relatively easy to transport, place,

compact and finish without harmfulsegregation.

A concrete mix satisfying these

conditions is said to be workable.


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Factors Affecting Workability

Method and duration of transportation Quantity and characteristics of cementing

materialsAggregate grading, shape and surface

texture Quantity and characteristics of chemical

admixturesAmount of waterAmount of entrained air Concrete & ambient air temperature


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WORKABILITY

Workability is the mostimportant property of freshlymixed concrete.

There is no single testmethod that cansimultaneously measure allthe properties involved in

workability. It is determined to a large

extent by measuring theconsistencyof the mix.


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Consistency is the fluidity or degree ofwetness of concrete.

It is generally dependent on the shearresistance of the mass.

It is a major factor in indicating the

workability of freshly mixed concrete.

CONSISTENCY


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Test methods for measuring consistencyare:

Flow test measures the amount of flow

Kelly-Ball test measures the amount of

penetration Slump test (Most widely used test!)

CONSISTENCY


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Slump Test is related with the ease withwhich concrete flows during placement(TS 2871, ASTM C 143)


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10 cm

20 cm

30 cm

The slump cone is filled in 3 layers. Everylayer is evenly rodded 25 times.

Measure the slump by determining the vertical differencebetween the top of the mold and the displaced original centerof the top surface of the specimen.


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

Sp.Gr. Size

Cement 3-3.15 5-80 mm

C.Agg. 2.4-2.8 5-40 mm

F.Agg. 2.4-2.8 < 5 mm

SEGREGATION

The primary causes ofsegregation are differencesin specific gravity and sizeof constituents of concrete.

Moreover, improper mixing,improper placing andimproper consolidation alsolead to segregation.


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Some of the factors affecting segregation:

Larger maximum particle size (25mm) and

proportion of the larger particles. High specific gravity of coarse aggregate.

Decrease in the amount of fine particles.

Particle shape and texture.

Water/cement ratio.

SEGREGATION


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Bleeding is the tendency of water to rise tothe surface of freshly placed concrete.

BLEEDING

It is caused by theinability of solidconstituents of the

mix to hold all ofthe mixing water asthey settle down.

A special case of

segregation.


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Undesirable effects of bleeding are:

With the movement of water towards the top, the top

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

Water rising to the surface carry fine particles ofcement which weaken the top portion and form

laitance. This portion is not resistant to abrasion.

Water may accumulate under the coarse agg. andreinforcement. These large voids under the particlesmay lead to weak zones and reduce the bond

between paste and agg. or paste and reinforcement.

BLEEDING


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The tendency of concrete to bleedingdepends largely on properties of cement.

It is decreased by: Increasing the fineness of cement

Increasing the rate of hydration (C3S, C3A andalkalies)

Adding pozzolans

Reducing water content

BLEEDING


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MIXING OF CONCRETE

The aim of mixing is to blend all of theingredients of the concrete to form a

uniform mass and to coat the surface ofaggregates with cement paste.


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MIXING OF CONCRETE

Ready-Mix concrete: In this typeingredients are introduced into a mixer

truck and mixed during transportation tothe site.

Wet Water added before transportation

Dry Water added at site Mixing at the site

Hand mixed

Mixer mixed


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Ready Mix Concrete


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Mixing at Site


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Mixing time should be sufficient to producea uniform concrete. The time of mixing

depends on the type of mixer and also tosome properties of fresh concrete.

Undermixing non-homogeneity

Overmixing danger of water loss,brekage of aggregate particles

MIXING OF CONCRETE


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CONSOLIDATING CONCRETE

Inadequate consolidationcan result in:

Honeycomb Excessive amount of

entrapped air voids

(bugholes) Sand streaks

Placement lines (Cold joints)


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VIBRATION OF CONCRETE

The process of compacting concreteconsists essentially of the elimination of

entrapped air. This can be achieved by: Tamping or rodding the concrete

Use of vibrators


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VIBRATORS

Internal vibrator: The poker is immersedinto concrete to compact it. The poker is

easily removed from point to point. External vibrators: External vibrators

clamp direct to the formwork requiring

strong, rigid forms.


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Internal Vibration

d

1 R

Vibrator

Radius of Action


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Internal Vibrators

Diameterof head,

(mm)

Recommendedfrequency,

(vib./min.)

Approximateradius of

action, (mm)

Rate ofplacement,

(m3/h)

Application

20-40 9000-15,000 80-150 0.8-4

Plastic and flowingconcrete in thin

members. Also used forlab test specimens.

30-60 8500-12,500 130-250 2.3-8

Plastic concrete in thinwalls, columns, beams,

precast piles, thin slabs,and along construction

joints.

50-90 8000-12,000 180-360 4.6-15

Stiff plastic concrete(less than 80-mmslump) in generalconstruction .

Adapted from ACI 309


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Systematic Vibration

CORRECT

Vertical penetration a few inchesinto previous lift (which should not

yet be rigid) of systematic regularintervals will give adequateconsolidation

INCORRECT

Haphazard random penetration ofthe vibrator at all angles andspacings without sufficient depthwill not assure intimate combination

of the two layers


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To aid in the removal of trapped air thevibrator head should be rapidly plunged intothe mix and slowly moved up and down.

Internal Vibrators

The actual completionof vibration is judged by

the appearance of theconcrete surface whichmust be neither roughnor contain excess

cement paste.


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External Vibrators

Form vibrators

Vibrating tables (Lab)

Surface vibratorsVibratory screeds

Plate vibrators

Vibratory rollerscreeds

Vibratory hand floats

or trowels


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External vibrators are rigidly clamped to theformwork so that both the form & concrete aresubjected to vibration.

A considerable amount of work is needed tovibrate forms.

Forms must be strong and tied enough to prevent

distortion and leakage of the grout.

External Vibrators


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Vibrating Table:used for small

amounts ofconcrete(laboratory and

some precastelements)

External Vibrators


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CURING OF CONCRETE

Properties of concrete can improve with age aslong as conditions are favorable for the continuedhydration of cement. These improvements are

rapid at early ages and continues slowly for anindefinite period of time.

Curing is the procedures used for promoting the

hydration of cement and consists of a control oftemperature and the moisture movement fromand into the concrete.


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Hydration reactions cantake place in onlysaturated water filled

capillaries.

CURING OF CONCRETE

The primary objective of curing is to keepconcrete saturated or as nearly saturated aspossible.


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Curing Methods

1. Methods which supply additional water tothe surface of concrete during early

hardening stages. Using wet covers

Sprinkling

Ponding


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Curing Methods

2. Methods that prevent loss of moisture fromconcrete by sealing the surface.

Water proof plastics Use liquid membrane-forming compounds

Forms left in place


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3. Methods that accelerate strength gain by

supplying heat & moisture to the concrete.

By using live steam (steam curing) Heating coils.

Curing Methods


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Hot Weather Concrete

Rapid hydration early setting rapid loss ofworkability

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


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Cold Weather Concrete

Keep concrete temperature above 5 C tominimize 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


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UNIFORMITY OF CONCRETE

Concrete uniformity ischecked by conductingtests on fresh andhardened concretes.

Slump, unit weight, aircontent tests

Strength tests


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UNIFORMITY OF CONCRETE

Due to heteregeneous nature of concrete,there will always be some variations. These

variations are grouped as: Within-Batch Variations : inadequate mixing,

non-homogeneous nature

Batch-to-Batch Variations : type of materials

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


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PROPERTIES OFHARDENED CONCRETE

The principal properties of hardenedconcrete which are of practical importance

can be listed as:1. Strength

2. Permeability & durability

3. Shrinkage & creep deformations

4. Response to temperature variations

Of these compressive strength is the mostimportant property of concrete. Because;


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PROPERTIES OFHARDENED CONCRETE

Of the abovementioned hardenedproperties compressive strength is one of

the most important property that is oftenrequired, simply because;

1. Concrete is used for compressive loads

2. Compressive strength is easily obtained

3. It is a good measure of all the otherproperties.


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Wh t Aff t


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What AffectsConcrete Strength

WhatDoesnt?


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Factors Affecting Strength

Effect of materials and mix proportions

Production methods

Testing parameters


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STRENGTH OF CONCRETE

The strength of a concrete specimenprepared, cured and tested under specifiedconditions at a given age depends on:

1. w/c ratio

2. Degree of compaction


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COMPRESSIVE STRENGTH

Compressive Strength is determined byloading properly prepared and cured cubic,cylindrical or prismatic specimens undercompression.


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Cubic: 15x15x15 cm

Cubic specimens are crushed after rotating

them 90 to decrease the amount of frictioncaused by the rough finishing.

Cylinder: h/D=2 with h=15

To decrease the amount of friction, cappingof the rough casting surface is performed.



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Cubic specimenswithout capping

Cylindrical specimenswith capping




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Bonded sulphur capping Unbonded neoprene pads



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STRENGTH CLASSES(TS EN 206-1)

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


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TENSILE STRENGTH

Tensile Strength can be obtained either bydirect methods or indirect methods.

Direct methods suffer from a number ofdifficulties related to holding the specimenproperly in the testing machine withoutintroducing stress concentration and to the

application of load without eccentricity.


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DIRECT TENSILE STRENGTH


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SPLIT TENSILE STRENGTH

Due to applied compression load a fairly uniformtensile stress is induced over nearly 2/3 of thediameter of the cylinder perpendicular to thedirection of load application.

2P P li d i l d


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The advantage of the splitting test over

the direct tensile test is the same moldsare used for compressive & tensilestrength determination.

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

st =2P

DlP: applied compressive load

D: diameter of specimen

l: length of specimenSplitting Tensile

Strength

FLEXURAL STRENGTH


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The flexural tensile strength at failure or themodulus of rupture is determined by loading aprismatic concrete beam specimen.

FLEXURAL STRENGTH

The resultsobtained are usefulbecause concrete

is subjected toflexural loads moreoften than it issubjected to

tensile loads.


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P


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M=Pl/4

d

b

cI =

bd3

12

2

3 =

M cI =

(Pl/4) (d/2)

bd3/12 =Pl

bd2

M=Pl/6

P/2 P/2

=(Pl/6) (d/2)

bd3/12=

Pl

bd2

Factors Affecting the Strength


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g gof Concrete

1) Factors depended on thetest type:

Size of specimen

Size of specimen in relation

with size of agg. Support condition af

specimen

Moisture condition ofspecimen

Type of loading adopted Rate of loading

Type of test machine

2. Factors independent oftest type:

Type of cement

Type of agg.

Degree of compaction

Mix proportions

Type of curing

Type of stress situation


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STRESS-STRAIN RELATIONS INCONCRETE

ult

(40-50%)

ult

ult

- relationship for

concrete is

nonlinear. However,specially forcylindricalspecimens withh/D=2, it can beassumed as linearupto 40-50% ofult

MODULUS OF ELASTICITY OF


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CONCRETE

Due to thenonlinearity of the -diagram, E is the

defined by:1. Initial Tangent Method2. Tangent Method

3. Secant Method

ACI E=15200 ult 28-D cylindrical comp.str.

(kgf/cm2)

TS E=15500 W 28-D cubic com .str. k f/cm2

PERMEABILITY OF


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CONCRETE

Permeability is important because:1. The penetration of some aggresive solution may result

in leaching out of Ca(OH)2 which adversely affects thedurability of concrete.

2. In R/C ingress of moisture of air into concrete causescorrosion of reinforcement and results in the volumeexpansion of steel bars, consequently causing cracks &spalling of concrete cover.

3. The moisture penetration depends on permeability & ifconcrete becomes saturated it is more liable to frost-action.

4. In some structural members permeability itself is ofimportance, such as, dams, water retaining tanks.

PERMEABILITY OF


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The permeability of concrete is controlledby capillary pores. The permeability

depends mostly on w/c, age, degree ofhydration.

In general the higher the strength of

cement paste, the higher is the durability &the lower is the permeability.

PERMEABILITY OFCONCRETE


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DURABILITY

A durable concrete is the one which willwithstand in a satisfactory degree, the

effects of service conditions to which it willbe subjected.

Factors Affecting Durability:

External Environmental Internal Permeability, Characteristics of

ingredients, Air-Void System...

St t f d d


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Structure of un-damagedConcrete

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)

St t f d d


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Structure of un-damagedConcrete

Macrostructure Microstructure

St t f d d


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Structure of damagedConcrete

Macrostructure

Visible cracks in hcpand aggregates dueto volume changes

(to understandcause of cracks,

microstructureshould be examined)

Microstructure Alkali-silica reaction:

Reaction product forms

at TZ and expands Frost action: Water

freezes in capillarypores and expands

Sulfate attack: reactionproducts form in hcpand expand

hi & ffl


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Leaching & Efflorescence

When water penetrates into concrete, itdissolves the non-hydraulic CH (andvarious salts, sulfates and carbonates ofNa, K, Ca)

Remember C-S-H and CH is producedupon hydration ofC3S and C2S

These salts are taken outside of concreteby water and leave a salt deposit.


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Sulfate Attack

Ground water in clayey soils containing alkalisulfates may affect concrete.

These solutions attack CH to produce gypsum.Later, gypsum and calcium alumina sulfatestogether with water react to form ettringite.

Formation of ettringite is hardened cement

paste or concrete leads to volume expansionthus cracking.

Moreover, Magnesium sulfate may lead to the

decomposition of the C-S-H gel.


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Sulfate Attack


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Seawater contains some amount of Na and MgSulfates. However, these sulfates do not causesevere deleterious expansion/cracking because

both gypsum and ettringite are soluble insolutions containing the Cl ion. However, problemwith seawater is the frequent wetting/drying andcorrosion of reinforcing steel in concrete.

To reduce the sulfate attack1. Use low w/c ratio reduced permeability & porosity2. Use proper cement reduced C3A and C3S

3. Use pozzolans they use up some of the CH to

produce C-S-H

Sulfate Attack

Acid Attack


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Acid Attack

Concrete is pretty resistant to acids. But inhigh concentrations:

Causes leaching of the CH

Causes disintegration of the C-S-H gel.

Carbonation


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Carbonation

Ca(OH)2 + CO2 CaCO3 + H2O

Accompanied by shrinkage carbonationshrinkage

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


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Corrosion


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Electrochemical reactions in the steel rebarsof a R/C structure results in corrosionproducts which have larger volumes than

original steel.

Thus this volume expansion causes cracks in

R/C. In fact, steel is protected by a thin filmprovided by concrete against corrosion.However, that shield is broken by CO2 of airor the Cl- ions.


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Freezing and Thawing


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Freezing and Thawing

Water when freezes expands in volume.This will cause internal hydraulic pressureand cracks the concrete.

To prevent theconcrete from thisdistress air-entraining

admixtures are usedto produce air-entrained concrete.

Abrasion


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Abrasion

Aggregates have to be hard & resistant towear.

Bleeding & finishing practices are alsoimportant.


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PROPORTIONING


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

There are two sets of requirements which enablethe engineer to design a concrete mix.

1. The requirements of concrete in hardened state.These are specified by the structural engineer.

2. The requirements of fresh concrete such asworkability, setting time. These are specified by theconstruction engineer (type of construction, placingmethods, compacting techniques andtransportation)

CONCRETE MIXTURES

PROPORTIONINGCO C S


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Mix design is the process of selectingsuitable ingredients of concrete &determining their relative quantities with the

objective of producing as economically aspossible concrete of certain minimumproperties such as workability, strength &durability.

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

CONCRETE MIXTURES

Cost Material + Labor


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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..

DurabilityPermeability has to be

WorkabilitySlump has to be...

In the past specifications for concrete mix


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p pdesign prescribed the proportions ofcement, fine agg. & coarse agg.

1 : 2 : 4

Weight ofcement

FineAgg.

CoarseAgg.

However, modern specifications do not usethese fixed ratios.

Modern specifications specify min compressive


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strength, grading of agg, max w/c ratio, min/maxcement content, min entrained air & etc.

Most of the time job specifications dictate thefollowing data: Max w/c

Min cement content Min air content

Slump

Strength

Durability

Type of cement

Admixtures

Max agg. size

PROCEDURE FOR MIX DESIGN


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1. Choice of slump (Table 14.5)



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2. Choice of max agg. size

1/5 of the narrowest dimension of the mold

1/3 of the depth of the slab

of the clear spacing between reinforcement

Dmax < 40mm



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



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



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5. Calculation of cement content with selected wateramount (step 3) and w/c (step 4)

6. Estimation of coarse agg. content (Table 14.10)



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7. Calculation of fine aggregate content withknown volumes of coarse aggregate, water,cement and air

8. Adjustions for aggregate field moisture



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9. Trial batch adjustments The properties of the mixes in trial batches are

checked and necessary adjustments are made to

end up with the minimum required properties ofconcrete.

Moreover, a lab trial batch may not always providethe final answer. Only the mix made and used in

the job can guarantee that all properties ofconcrete are satisfactory in every detail for theparticular job at hand. Thats why we get samplesfrom the field mixes for testing the properties.

Example:

Sl 75 100


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Slump 75-100 mm

Dmax 25 mm

fc,28 = 25 MPa

Specific Gravity of cement = 3.15

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. Slump is given as 75-100 mm

2 D i i 25


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2. Dmax is given as 25 mm

3. Estimate the water and air content(Table 14.6)

Slump and Dmax W=193 kg/m3

Entrapped Air 1.5%

4. Estimate w/c ratio (Table 14.8)


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fc & non-air entrained w/c=0.61 (by wt)


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5. Calculation of cement content

W = 193 kg/m3 and w/c=0.61

C=193 / 0.61 = 316 kg/m3

6. Coarse Agg. from Table 14.10


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Dmaxand F.M. VC.A=0.69 m3

Dry WC.A. = 1600*0.69 = 1104 kg/m3

SSD WC.A. = 1104*(1+0.005) = 1110 kg/m3

7. To calculate the F.Agg. content thevolumes of other ingredients have to be


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volumes of other ingredients have to bedetermined. V = M

Sp.Gr.*rwVwater = 1931.0*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)

SV = 0.722 m3 VF.Agg = 1-0.722 = 0.278 m3

WF.Agg = 0 278*2 62*1000 = 728 kg/m3

S f Mi D i


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Summary of Mix Design

Based on SSD weight of aggregates

8. Adjustment for Field Moisture of Aggregates


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WSSD =WDry *(1+a) WField =WDry *(1+m)

Correction for waterFrom coarse aggregate: 1127-1110 = 17

From fine aggregate: 759-728 = 31

48 kg

extra

Corrected water amount : 193 48 = 145 kg

S mma of Mi Design


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Summary of Mix Design

Based on field weight of aggregates

9. Trial Batch


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Usually a 0.02 m3 of concrete is sufficient

to verify the slump and air content of themix. If the slump and air content aredifferent readjustments of the proportions

should be made.

7 concrete revised

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