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8/18/2019 CE 308 - Lec 6 - Admixtures
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MILITARY COLLEGE OF ENGINEERING
RISALPUR
CE 308 – PRC I - LECTURE 6
ADMIXTURES
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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
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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
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ADMIXTURES
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TYPES OF ADMIXTURES
Use of fly ash
$he most extensively uses and widely availablematerialy product of coal after burning in power plants)conomical
Use of silica fumesy product in the manufacture of silicon metal
and alloys
*ery fine grains
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Pozzolanic material
+lassy silica reacts with C formed from cement
hydrationComposition of C!#! produced by -A pozzolanic
reaction is similar to that formed in regular hydration .n terms of reaction heat, addition of -A leads to
lowering the amount of early heat evolution and
reduced early strength but not long term strength Addition of -A leads to an overall increase in solid
volume and eventually a decrease in porosity of thepaste, resulting in higher strength and durability
compared with plain paste
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Pozzolanic material
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hemical A!mi"tures
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Precautions in use
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Air entrainment
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.ncreasing the amount of air in concrete
.ncorporated in concrete during mixing$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
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Affects of air entrainment
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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
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$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
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Set,acceleratin% a!mi"tures
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Set,retar!in% a!mi"tures
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Im-act on com-ressi.e stren%th
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ThankYou
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