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ContentAdmixtures
Definition
Types Of Admixtures
Purpose of Admixture
Chemical Admixture
Mineral Admixtures
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DMIXTURES
A material which , except in special cases, is added toconcrete mix in quantity not more
than 5% by mass of cement during mixing or anadditional mixing operation prior to the
placing of concrete for the purpose of achieving specific
modifications .
Admixtures maybe used in solid or liquid form . Liquids
are more used due to the fact thatliquid can be dispersed rapidly in uniform manner during
the mixing of concrete.
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Types of Admixtures
Mineral
Admixture
Mineral admixtures (fly ash, silica fume
[SF], and slug) are usually added to
concrete in larger amounts to enhance
the workability of fresh concrete; to
improve resistance of concrete tothermal cracking, alkali-aggregate
expansion, and sulfate attack; and to
enable a reduction in cement content.
Chemical
Admixture
Chemical admixtures are added to
concrete in very small amounts mainly
for the entrainment of air, reduction
of water or cement content,
plasticization of fresh concretemixtures, or control of setting time.
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Purposes of AdmixtureS
To accelerate the initial setting of concrete.
To retard the initial setting .
To improve the strength of concrete .
To reduce the heat of evolution.
To improve the workability.To inhibit the corrosion of concrete.
To increase the durability of concrete.
To increase the bond of concrete to the steelreinforcement.
To increase the resistance to chemical attack.
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ChemicalAdmixtures
AirEntrained
Accelerating PlasticizersSuper
PlasticizersRetarding
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Air Entrainment Admixture
They are traditionally based on Abeitic acidsalts(Vinsol resin) and fatty acids.
Now a days synthetic surfactants based on olefinsulphonates, Alkyl sulphates,
Diethanaol amines, Alcohol ethoxylates betainsare used.
Lowers the surface tension of the water and facilitatein bubble formation.
Decreases bleeding and segregation thus the provideresistance against freezing and thawing.
It improves workability but for every 1% ofadmixture strength decreases by 5%.
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Accelerating Admixture
The most commonly used accelerator is CaCl2but it causes corrosion in reinforcement.other accelerators are Ca(NO3)2, NaNO3,NaCl and sea water.
It reduces initial setting time and curing time.
It is used in cold weather concreting.
Accelerators are used in large projects and fasttrack construction.
The amount should not exceed by 2%.
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Retarding Admixtures
This admixture delay initial setting time from 1 hour to3 hours.
Organic: refined calcium, sodium, NH4, salts oflignosulfonic acids, hydrocarboxylic acids and
carbohydrates Inorganic: oxides of leads and zinc, phosphate,
magnesium salts, flourates and borates.
It is used when temperature is higher than 85oF (30o c)
It increase workability, prevent the formation of coldjoints and increase ultimate strength.
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Plasticizers
They are usually based on lignosulphnates.Hydroxylated polymers are also widely used as
plasticizers.
It reduces water cement ratio thus increase
strength without reducing workability. It is used in hot weather concreting.
They are added from 130 to 390 ml per 100 kg.
It improves durability, provide easier placement,control the setting time and reduce
segregation.
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Super Plasticizers
There are 4 main families of super pasticizers:1. Sulfonated melamine formaldehyde
condensates (SMF)
2. Sulfonated napthalene formaldehyde
condensates (SNF)3. Modified lignosulfonates (MLS)
4. Polycarbxylate derivatives.
Super Plasticizers are used in hot weather forlarge projects.
It reduce water requirement from 12% to 25 %.
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Super Plasticizers..
It produce high strength cement .
Water cement ratio ranges from o.3 to 0.4 and
compressive strength is upto 14ooo psi (96.5
MPa)
It produce flowing cement with very high
slump in range of 7-9 inches (175 to 225 mm)
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MineralAdmixtures
Cementitious Pozzolanic
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Cementitious
These have cementing properties themselves.
For example: Ground granulated blast furnace slag (GGBFS)
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Pozzolanic
A pozzolan is a material which, when combinedwith calcium hydroxide (lime), exhibitscementitious properties. Pozzolans are commonlyused as an addition (the technical term is
"cement extender") to Portland cement concretemixtures to increase the long-term strength andother material properties of Portland cementconcrete and in some cases reduce the material
cost of concrete.Examples are
Fly ashy Silica Fume
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Bibliography
http://www.admixtures.org.ukhttp://www.basf-admixtures.com
http://www.nrmca.org/aboutconcrete/cips/15p.
pdfhttp://www.fhwa.dot.gov/infrastructure/materi
alsgrp/admixture.html
http://www.toolbase.org/Technology-Inventory/Foundations/concrete-admixtures
http://www.concrete.org/general/fE4-03.pdf
http://www.admixtures.org.uk/http://www.basf-admixtures.com/http://www.nrmca.org/aboutconcrete/cips/15p.pdfhttp://www.nrmca.org/aboutconcrete/cips/15p.pdfhttp://www.fhwa.dot.gov/infrastructure/materialsgrp/admixture.htmlhttp://www.fhwa.dot.gov/infrastructure/materialsgrp/admixture.htmlhttp://www.toolbase.org/Technology-Inventory/Foundations/concrete-admixtureshttp://www.toolbase.org/Technology-Inventory/Foundations/concrete-admixtureshttp://www.concrete.org/general/fE4-03.pdfhttp://www.concrete.org/general/fE4-03.pdfhttp://www.concrete.org/general/fE4-03.pdfhttp://www.concrete.org/general/fE4-03.pdfhttp://www.toolbase.org/Technology-Inventory/Foundations/concrete-admixtureshttp://www.toolbase.org/Technology-Inventory/Foundations/concrete-admixtureshttp://www.toolbase.org/Technology-Inventory/Foundations/concrete-admixtureshttp://www.toolbase.org/Technology-Inventory/Foundations/concrete-admixtureshttp://www.toolbase.org/Technology-Inventory/Foundations/concrete-admixtureshttp://www.fhwa.dot.gov/infrastructure/materialsgrp/admixture.htmlhttp://www.fhwa.dot.gov/infrastructure/materialsgrp/admixture.htmlhttp://www.nrmca.org/aboutconcrete/cips/15p.pdfhttp://www.nrmca.org/aboutconcrete/cips/15p.pdfhttp://www.basf-admixtures.com/http://www.basf-admixtures.com/http://www.basf-admixtures.com/http://www.admixtures.org.uk/