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MILITARY COLLEGE OF ENGINEERING
RISALPUR
CE 308 – PRC I - LECTURE 8
ADMIXTURES_2
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Chemical admixtures have formulated chemical
composition and special chemical action, and are used tomodify certain properties of concrete.
Primarily used to reduce the cost of concreteconstruction; to modify the performance of hardenedconcrete; to ensure the quality of concrete during mixing,transportation, placing, compacting and curing; and toovercome certain emergencies during concretingoperations.Properties commonly modified are the rate of hydration or
setting times, workability, dispersion and air entrainment.Generally added in a relatively small quantity.nsure proper quantity of an admixture, as an excessquantity may be detrimental to the properties of concrete.!ost admixtures are supplied in ready"to"use liquid form and are added to the concrete.
CHEMICAL ADMIXTURES
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#he benefits of reduced time of setting may include$
arly finishing of surface; and
%eduction of pressure on forms or of period of time
during which the forms are sub&ected to hydraulic
pressure.
'ith the availability of powerful accelerators, the underwater concreting, the basement waterproofing operations,
the repair work of the waterfront structures in the tidal
(ones have become easy.
'ith proper proportion these admixtures partlycompensate for the retardation of strength development
due to low temperatures in cold weather concreting.
ACCELERATING ADMIXTURES
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)igh temperatures, low humidity, and wind cause rapid
evaporation of water from the mix during summer. #hisdrying of concrete leads to the cracking of the surface.
%etarders delay setting of cement either by forming a thin
coating on the cement particles and thus slowing down
their dissolution in and reaction with water or by increasing
the intra"molecular distance of reacting silicates and
aluminates from water molecules.
RETARDING ADMIXTURES
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IMPACT ON HEAT EVOLUTION
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*ir"entraining admixtures help to incorporate a controlled
amount of air , in the form of millions of minute bubbles distributed throughout the body of concrete, during mixing,
without significantly altering the setting or the rate of
hardening of concrete.
* proper amount of entrained air results in improved
properties of plastic concrete like workability, easier placing
and finishing, increased durability, better resistance to frost
action and reduction in bleeding and segregation.
#he entrained air bubbles, ranging approximately from
+.+ to +.- mm diameter and spaced +.++ mm apart
reduce the capillary connectivity in concrete.
AIR-ENTRAINING ADMIXTURES
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AIR-ENTRAINING ADMIXTURES
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MECHANISM
At the air-water interface the polar groups are oriented
toward the water phase lowering the surface tension, promoting bubble formation, and counteracting the
tendency for the dispersed bubbles to coalesce (combine).
At the solid-water interface where directive forces exist in
the cement surface, the polar groups become bound to thesolid with the nonpolar groups oriented toward the water,
making the cement surface hydrophobic so that air can
displace water and remain attached to the solid particles as
bubbles.
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!ost fine ingredients of concrete tend to flocculate or
clump together ./locculated fines cause an increase in viscosity by
entrapping a part of the water and by physically resisting
the flow 0 in turn effect workability.
!ore water is required to achieve desired slump " 'atercan be added up to a certain point, beyond which the
intended plastic and hardened physical properties of
concrete are compromised.
'ater reducers are often used the solution1
#he organic or combinations of organic and inorganic
substances to achieve these ob&ectives are termed as
water reducers or plastici(ing admixtures.
PLASTICIZING ADMIXTURES
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Mechanism of Water
Reduction
Green is
admixture
MECHANISM
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2uperplastici(ers " a relatively new category and improved
version of plastici(ers, the use of which was developed in3apan and Germany during 456+ and 457+ respectively.
#hey are chemically different from normal plastici(ers.
8se of superplastici(ers permit the reduction of water tothe extent up to + 9 without reducing workability.#he use of superplastici(er is practiced for production offlowing, self leveling, self compacting and for the productionof high strength and high performance concrete.#he mechanism of action of superplastici(ers is more or
less same as in case of ordinary plastici(er . :nly thing isthat the superplastici(ers are more powerful as dispersingagents.#hey are called )igh %ange 'ater %educers in *mericanliterature.
SUPERPLASTICIZERS
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2uperplastici(er made it possible to have a wc ratio as low
as +.- or even lower and yet make flowing concrete toobtain strength of 4-+ !Pa or more.
2uperplastici(er made it possible to use fly ash, slag and
particularly silica fume to make high performance concrete.
2uperplastici(ers can produce$ *t the same wc ratio much more workable concrete;
/or same workability, it permits the use of lower wc ratio;
/or increased strength with lower wc ratio, it permits a
reduction of cement content.
#he superplastici(ers also produce a homogeneous,
cohesive concrete generally without any tendency for
segregation and bleeding.
SUPERPLASTICIZERS
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BENEFITS
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< 2eries *, had a ++ kgm cement content and a +.6-
water"cement ratio; the fresh concrete showed +"mm
slump, and the hardened concrete gave -" and 7"!Pa
compressive strength at 7 and -= days, respectively.
< 'ith #est 2eries >, the purpose was to increase the
consistency of the reference concrete mixture without
adding more cement and water. #his was easily achievedby incorporating a small dosage of the water"reducing
admixture. 2uch an approach is useful when concrete is to
be placed in heavily reinforced sections by pumping. ?only
slump increased as an effect of plastici(er, by dispersingthe flocculated cement particles@
SUPERPLASTICIZERS
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< 'ith #est 2eries C, the ob&ect was to achieve higher
compressive strengths without increasing the cement
content or reducing the consistency of the reference
concrete mixture. Ancorporating the same amount of the
water"reducing admixture as used with 2eries > made it
possible to reduce the water content by 4+ percent ?from
4=6 to 46= kgm@ while maintaining the +"mm slump.< *s a result of reduction in the water"cement ratio, the 7"
day compressive strength increased from - to B !Pa
and the -="day strength from 7 to B6 !Pa. #his approach
may be needed when &ob specifications limit the maximumwater cement ratio but require high early strength to
develop.
SUPERPLASTICIZERS
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TYPES OF PLASTICIZERS
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MINERAL ADMIXTURES
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