CONCRETE AND ADMIXTURES
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Transcript of CONCRETE AND ADMIXTURES
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CONCRETE AND
ADMIXTURES
y 071618y 071619
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Concrete Strength & Behaviory The compressive strength of concrete is high and istaken as an important index of its general quality.
y The tensile strength of concrete is roughly 10%of thecompressive strength and is often neglected by
structural engineers.y The strength is influenced by the age, size, shape of the specimen and loading rate.y Nonlinear stress-strain relationship.
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Factors Affecting Strengthy (1)Water/cement (w/c ) ratioy (2) Agey (3) Curing conditionsy (4) Other Factors Affecting Strength
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(1) Water/Cement Ratioy Optimum is generally 0.45 < w/c < 0.55y The lo wer w/c ratio, the higher strength isy Need sufficient water in mix for hydration to occur.y Excess water must be drained off after hydration,y leaving capillary voids that reduce strengthy Strength increases with decreasing w/cy Approximate 28 day y Strength = 24000 / 18 w/c (psi)
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(2) Agey Concrete strength continues to increase with time.y Design values are generally based on the strength at
28 days.y 90%+ strength can be achieved in 28 days.
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(3) Curing Conditionsy Time, Moisture, Temperature (TMT)y Concrete hardens by hydration which is chemical surface
reaction bet ween the cement and water.y Hydration releases heat which can drive off moisture,
hence concrete must be kept moist until hydration iscomplete.
y
The reaction takes time.y Increased temperature increases the reaction rate.y Allo wing the mix to freeze during hydration will destroy
the strength and durability of the concrete
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(4) Other Factors Affecting Strengthy Character of Cement.
Since hydration is a surface reaction, the finer the particles,
the more surface area and the more complete the reaction,resulting in increased strength.y Moisture content at the time of testing.
A moist specimen develops large pore pressures whichcause premature failure of compression specimens.y Cement Content and Aggregate Condition.
Angular aggregates require more paste to be workableMore cement content requires more water
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Design Procedure in MakingConcrete
y Decide the maximum size of aggregate; choose themaximum possible size using the follo wing guidelines:
y
Maximum size > 1/ 5 narro wer dimension bet ween formsy > 1/ 3 depth of slaby > 1/ 4 of clear spacing bet ween reinforcing barsy If slump is not given, decide the slump, using Table 1y
Decide the amount of water and air (Table 2)y Select the W/C ratio (Table 3)
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y Calculate cement content = C/W x wt of watery Choose the amount of coarse aggregate (Table 4)y Calculate the amount of fine aggregate using the
estimated weight of fresh concrete (Table 5) and the kno wn weights of water, cement, and coarse aggregate.
y Adjust for moisture content in coarse and fineaggregate
y Trial mix
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Important Mix DesignConsiderations
y (1)Strengthy (2) Workability y (3) Durability y (4) Economy y (5) Volume changes
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(1) StrengthThe lo wer w/c ratio, the higher strength isCharacter of Cement
Cement Content and Aggregate Condition
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or a y - roper es oFreshConcrete
y G enerally measured by the slump test Increases withincreased water content
y
Increases with increased air contenty Decreases with increase in coarse aggregatey Decreases with increase in angularity of aggregate
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(3) Durabilityy Freeze-Tha w Durability
Can be increased with propery entrained air content.
Use an air entraining admixturey Abrasion Durability
Affected by aggregate strength.Durability of cement paste is typically
y lo wer than aggregate strength
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(4) Economyy Minimize cementy Must have enough paste to coat aggregatesy Maximize the use of aggregates
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(5) Volume Changesy Shrinkage and creep increase with cementcontent.y Chemical hydration generally reduces
volume.y Moisture fluctuations cause expansion and
contraction. This is more pronounced in concrete withhigh w/c ratios and/or high cement contents
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Common Concrete Testsy Slump Test, ASTM C 143. Used to determine the workability of fresh concrete. Sometimes used as an
indication of water content.y Air Test, ASTM C 231. Used to the determine the air
content in fresh concrete.y Compression Test, ASTM C 39. Used to test the 7
and 28 day strength of 6 dia. by 12 long cast cylinders.
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Comprehensive concrete
Compression Testy Extensometer and chain kitfor lateral strain measurementConcrete specimen
Extensometers for axial strainmeasurement
Soft ware for test control, data
collection and reporting
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Concrete Splitting Tensile TestPlatens for longitudinalloading of specimensSafety chainsSoft ware for test control,
data collection and reporting
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Concrete Direct Tension TestHigh strength, rapid setting epoxy toglue specimens to end caps
Chain kit to minimize bending
moments during testSpherically seated ends to minimizenon-concentricity
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Confined Concrete Behavior
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Concrete structure failure
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ADMIXTURESy Materials added to the concrete besides cement, water
and aggregate.y
To improve the properties of the concrete required.y Admixtures can be divided in 2 groups that is:
a) Chemical admixturesb) Mineral admixtures
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FUNCTION OF ADMIXTUREy To improve workability of fresh concretey To improve durability by entrainment of airy To reduce the water requiredy To accelerate setting & hardening & thus to produce
high early strengthy To aid curingy To impart water repellent / water proofing property
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To cause dispersion of the cement particles whenmixed with waterTo retard settingTo improve wear resistance (hardness )To offset / reduce shrinkage during setting &hardeningTo cause expansion of concrete and automaticprestressing of steelTo aerate mortar / concrete to produce a light- weight
product
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To impart colour to concreteTo offset or reduce some chemical reaction
To reduce bleedingTo reduce the evolution of heat
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Among the type of chemical admixture used are:a) Acceleratorb) Water reducing Admixturec) Superplasticizerd) Air Entraining Admixturese) Retarding Admixturesf ) Corrosion Inhibitorsg) Alkali-Aggregate Reaction Inhibiting
Admixturesh) Shrinkage Reducing Admixtures
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ACCELARATORy Added to increase the rate of hydration of concrete
mix which then lead to the increases in the rate of
development of strength and greater heat evolution.y And to shorten the setting timey More rapid gain of strength & rapid settingy Disadvantages is possible cracking due to heat
evolution & possibility of corrosion of embeddedreinforcement
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WATER REDUCING ADMIXTURE /
PLASTICISERSy Used to reduce the amount of water necessary to
produce a concrete of a given consistency y To increase the slump for a given water contenty To obtain specified strength at lo wer cement contenty Increases workability with faster gain of strengthy Increase the slump, reduce water content, save cementy
Disadvantages, it has the risk of corrosion
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SUPERPLASTICIZERy B y adding to a hydraulic binder, gives very high
workability and allo ws a large decrease in watercontent for a given workability
y Allo ws the particles to be more workable where itenable working with lo w w/c ratio
y Enhances hydration process, increases strengthy Eliminate concrete segregation & allo w good
dispersion of cement particles in water, acceleratingthe rate of hydration.
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AIR ENTRAINING ADMIXTURESy An entraining concrete containing air in a rather
special form of a bubble that trapped in concretey Workability improved, increase in slump, easier
placing, increased durability, better resistance to frostaction
y Able to reduce segregation tendency and control of bleeding
y It is necessary to have a careful control of air contentand mixing time
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Wetter mixes tend to result in higher air content whilemixes rich in sand entrain less air as fine materialcompetes for available waterIf air entrainment is to beused, sand content need to be reducedB ubbles produced by air entrainment are quitedifferent from entrapped air because:They are sealed and wouldn't be filled with waterduring normal saturation of the concreteThey are very small and well distributed.
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y Used to increase concrete durability by protecting itagainst freeze-tha w cycle damage.
y B y entraining air in concrete to form a microscopic air- void system , the expansion is provided a relief valvesystem.
y The air void system in the hardened concrete pasteallo ws water to freeze, with the empty air voidsproviding room for the expansion that occurs as waterchanges to ice.
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CORROSION INHIBITORSy Able to reduce the rate of corrosion to a level that
major damage to concrete will be avoided or at leastgreatly reduced.
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ALKALI AGGREGATE REACTION
INHIBITING ADMIXTURESy L ithium and B arium salts can reduce the expansion
and cracking associated with alkali-silica reaction
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SHRINKAGE-REDUCING
ADMIXTURESy Able to reduce the amount of shrinkage that occurs ashardened concrete dries.
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MINERAL ADMIXTURESy Are natural pozzolanic materials or industrial by-
products that are commonly used in concrete toreplace part of the cement or sand.
y Types of mineral admixtures are:a) Fly ashb) Silica Fumec) B lended hydraulic cement
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