concrete and admixtures

8/7/2019 CONCRETE AND ADMIXTURES

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CONCRETE AND

ADMIXTURES

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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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