CE 308 - Lec 6 - Admixtures

8/18/2019 CE 308 - Lec 6 - Admixtures

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MILITARY COLLEGE OF ENGINEERING

RISALPUR

CE 308 – PRC I - LECTURE 6

ADMIXTURES





ADMIXTURES

Often, instead of using a special cement, it is

possible to change some of the properties of themore commonly used cements by incorporating a

suitable additive or an admixture Additives and admixtures are used synonymously,

though strictly speaking, additive refers to a material

added at the manufacturing stage while admixtures

implies addition at the mixing stage



TYPES OF ADMIXTURES

Chemical admixtures

Mineral admixtures

Chemical admixtures are essentially water

reducers (plasticizers, set!retarders and

accelerators" A#$M C %&%!&' classified them as

$ype A, and C respectivelyMineral Admixtures are supplementary cementing

materials" Main use is to reduce cost as it isreplacing cement, improve workability and durability

and to reduce heat of hydrationClassified as pozzollanic materials (fly ash and

silica fume and cementations materials (slag



ADMIXTURES



TYPES OF ADMIXTURES

Use of fly ash

$he most extensively uses and widely availablematerialy product of coal after burning in power plants)conomical

Use of silica fumesy product in the manufacture of silicon metal

and alloys

*ery fine grains



Pozzolanic material

+lassy silica reacts with C formed from cement

hydrationComposition of C!#! produced by -A pozzolanic

reaction is similar to that formed in regular hydration .n terms of reaction heat, addition of -A leads to

lowering the amount of early heat evolution and

reduced early strength but not long term strength Addition of -A leads to an overall increase in solid

volume and eventually a decrease in porosity of thepaste, resulting in higher strength and durability

compared with plain paste





Pozzolanic material



hemical A!mi"tures



Precautions in use



Air entrainment



.ncreasing the amount of air in concrete

.ncorporated in concrete during mixing$he volume of air re/uired to produce optimum

frost action is &0 by volume of the mortar fraction

and ' to 1 0 by volume of concrete depending on

the size of coarse aggregates used$he natural air content is about 2 to 3 0 by volume

of concrete with naturally entrapped air (bigger pore

size, irregularly distributed$he air content increased by 3 to % 0 by entrained

air (smaller pore size, uniformly distributed

Air entrainment



Affects of air entrainment





Affects of air entrainment

Fresh concrete

)nhanced workability Air entraining agents help to produce more uniform

and well compacted concrete4educed amount of sand and water re/uired at a

desired slump#ar!ene! oncrete4eduction in compressive strength (2 0 of

entrained air, 5 0 reduction in strengthOptimum durability (weathering resistance at air

entrainment of 6 to 1 0







$ater re!ucin% a!mi"tures are ASTM &'&(

)*)+Chemical admixtures used for lowering the water

re/uired to attain a given slump" 7owering the water

content means the reduction in cement content for

same w8c ratio (at a given strength)conomical concrete by cutting the amount of

cement used (expensive ingredient

.ncrease in cracking index, i"e" lowering potentialearly age hydration heat , shrinkage etc" (low w8c

ratio-acilitates difficult placements by increasing slump

without increasing w8c ratio

$ater re!ucin% a!mi"tures





Set,acceleratin% a!mi"tures



Set,retar!in% a!mi"tures



Im-act on com-ressi.e stren%th



ThankYou

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CE 308 - Lec 6 - Admixtures