CE 308 - Lec 8 - Admixtures_2

8/19/2019 CE 308 - Lec 8 - Admixtures_2

1/22

MILITARY COLLEGE OF ENGINEERING

RISALPUR

CE 308 – PRC I - LECTURE 8

ADMIXTURES_2


2/22


3/22

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


4/22

#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


5/22


6/22

)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


7/22

IMPACT ON HEAT EVOLUTION


8/22

*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


9/22

AIR-ENTRAINING ADMIXTURES


10/22

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.


11/22

!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


12/22

Mechanism of Water

Reduction

Green is

admixture

MECHANISM


13/22


14/22

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


15/22

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


16/22

BENEFITS


17/22

< 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


18/22

< '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


19/22


20/22

TYPES OF PLASTICIZERS


21/22

MINERAL ADMIXTURES


22/22

ThankYou

Documents

CE 308 - Lec 8 - Admixtures_2