7 concrete revised
TRANSCRIPT
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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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COMPRESSIVE STRENGTH
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.
COMPRESSIVE STRENGTH
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Cubic specimenswithout capping
Cylindrical specimenswith capping
COMPRESSIVE STRENGTH
COMPRESSIVE STRENGTH
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Bonded sulphur capping Unbonded neoprene pads
COMPRESSIVE STRENGTH
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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)
PROCEDURE FOR MIX DESIGN
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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
PROCEDURE FOR MIX DESIGN
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3. Estimation of mixing water & air content(Table 14.6 and 14.7)
PROCEDURE FOR MIX DESIGN
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4. Selection of w/c ratio (Table 14.8 or 14.9)
PROCEDURE FOR MIX DESIGN
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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)
PROCEDURE FOR MIX DESIGN
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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
PROCEDURE FOR MIX DESIGN
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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.