Admixtures in concte

8/13/2019 Admixtures in concte

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C H I E F I N S T R U C T O R

E N G R . M O I N - U D - D I N R A T H O R E

S U P E R I N T E N D I N G E N G I N E E R

M E S S C H O O L

P R E S E N T E D B Y : -

A K A S H S H A H Z A D K H A N A / X E N ( B & R )S H E R A F G A N A / X E N ( B & R )

M . A B U - B A K A R A / X E N ( E & M )

USE OF ADMIXTURES

IN

CONCRETE STRUCTURES


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TABLE OF CONTENTS

Admixtures Types of Admixtures

Accelerators

Retarders

Water Reducers

Super Plasticizers

Special Admixtures

Air Entraining Admixtures

Shrinkage Reducing Corrosion-Inhibiting Special Admixtures


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ADMIXTURES

Chemicals added to concrete in order to change its

properties as required are called admixtures


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ADMIXTURES

Admixtures are added to concrete mixes not more than 5%

by weight of cement during mixing for the purpose of

achieving a specific modification to normal properties of

concrete.

These impart considerable physical and economicalbenefits to concrete.

While properly used admixtures are beneficial to concrete,

there is no remedy for poor quality mix ingredients, use of

incorrect mix proportions or for poor workmanship intransporting, placing and compaction.


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TYPES OF ADMIXTURES

Admixtures may be organic or inorganic in their composition. They are

normally classified by their function in concrete. The classification of

ASTM C 494-92 is as follows,

Type A Water Reducing

Type B Retarding

Type C Accelerating

Type D Water Reducing and Retarding

Type E Water Reducing and Accelerating

Type F High range water reducing or super- plasticizing

Type G Super plasticizing and retarding


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ACCELERATING ADMIXTURES

Function:

Their primary function as the name suggests is to increase

rate of early strength development.

These are used when concrete is to be placed at low

temperatures say 2 to 4oC, in the manufacture of precast

concrete or in rapid repair works.

Other benefits include early finishing of concrete surface,

putting the structure into service early and application of

insulation for protection.


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Limitation:

Although CaCl2 is the cheapest accelerator, but due to

presence of chloride ions in vicinity of reinforcement, it

promotes corrosion activity of steel reinforcement.

Accelerators cannot be used as anti-freezing agents; they

depress the freezing point of concrete but no more than

2oC.

At high temperatures use of accelerators may result in too

high rate of development of heat of hydration and inshrinkage cracking.


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Examples:

CaCl2, NaCl

Calcium nitrate and Calcium nitrite are possible

accelerators; the former also appears to be corrosion

inhibitor.

Calcium formate and sodium formate are also possibilities,

although the latter would induce sodium ions which may

influence hydration and have a potential to react with

aggregates.

Triethanolamine is a possible accelerator but it us very

sensitive to dosage.


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RETARDERS

Function:

A delay in setting time of the cement paste can be achieved by the

addition of retarding admixture.

Set Retarding concrete admixtures delay the chemical reaction that

takes place when the concrete starts the setting process. These types of concrete admixtures are commonly used to reduce the

effect of high temperatures that could produce a faster initial setting of

concrete and also prevent formation of cold joints.

The retarder is applied to inner side of formwork so that the adjacent

cement setting is delayed. This cement can be later on struck off. Continuous massive pouring can be done with controlled retardation.


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RETARDERS

Limitation:

Retarders tend to increase the plastic shrinkage because

the duration of plastic shrinkage is increased.


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RETARDERS

Examples:

Retardation action is exhibited by

Sugar

carbohydrate derivatives soluble zinc salts

soluble borates and some other salts

Methanol is also a possible retarder.


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WATER REDUCING

Function:

Water-reducing admixtures when added to concrete cancreate a desired slump at a lower water cement ratiothan what is normally designed.

The reduce water content from 5-10%. Thus they reducethe water cement ratio at a desired workability or theycan give high workability at low water cement ratios.

Water reducers have been used primarily in bridge

decks, low-slump concrete overlays, and patchingconcrete.

Concrete having water reducers exhibits low segregationand good flow ability.


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WATER REDUCING

Limitation:

Ligno-sulfonate based admixtures increase shrinkage

while others do not.


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WATER REDUCING

Examples:

Two main groups of type D admixtures are,

Lingo-sulfonic acids and their salts

Hydro-oxylated acids and their salts


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SUPER PLASTICIZERS

Function:

Super plasticizers are high range water reducing

admixtures.

The main purpose of using super plasticizers is to

produce flowing concrete with very high slump in the range

of 7-9 inches to be used in heavily reinforced structures

and in placements where adequate consolidation by

vibration cannot be readily achieved.

The other major application is the production of high-strength concrete at w/c ratios ranging from 0.3 to 0.4.


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At a given w/c and water content super plasticizersmay increase slump from 75mm to 200mm, resulting inflowing concrete which can be placed easily with littleor no compaction, without segregation and bleeding.

Low w/c ratio of 0.2 has been used to achieve 28 daycylinder strength of about 150MPa (22000psi).

Generally they can reduce water content by 25 to 35%and increase 24 hr strength by 50 to 75%.

They do not change the structure of hydrated cementand only increase the distribution of cement particlesand hence give better hydration.

SUPER PLASTICIZERS


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Pumping of Highly Workable Concrete by use of Super

plastisizers


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Examples:

There exist four main categories of super plasticizers:

Sulfonated melamine formaldehyde condensates.

Sulfonated naphthalene formaldehyde condensates. Ligno-sulfonates.

Other sulfonic acid esters and carbohydrate esters.

SUPER PLASTICIZERS


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SPECIAL ADMIXTURES

Air-Entraining:

Air entrained concrete can increase the freeze-

thaw durability of concrete.

This type of admixture produces a more workable concrete

than non-entrained concrete while reducing bleeding and

segregation of fresh concrete, improved resistance of

concrete to severe frost action or freeze/ thaw cycles.

Other benefits from this admixture are,

High resistance to cycles of wetting and drying.

High degree of workability.

High degree of durability.


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Concrete with Air-Entraining admixture


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SPECIAL ADMIXTURES:Shrinkage Reducing This type of admixture could reduce early and long term

drying shrinkage.

Shrinkage reducing admixtures are used in situations

where shrinkage cracking could lead to durability problems

or where large numbers of shrinkage joints are undesirable

for economic or technical reasons.


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SPECIAL ADMIXTURES:

Corrosion-Inhibiting:

Corrosion-inhibiting admixtures fall into the specialty

admixture category and are used to slow corrosion of

reinforcing steel in concrete.

Corrosion inhibitors can significantly reduce maintenance

costs of reinforced concrete structures throughout a typical

service life of 30 40 years.

Corrosion inhibiting admixtures have little effect on strength

at later ages but may accelerate early strengthdevelopment.


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REFERENCES

PROPERTIES OF CONCRETE (Fourth and Final Edition) by A.M.

Neville

American Association of State Highway and Transportation Officials,

Washington, D.C., 2001. Abrams, Duff A., Calcium Chloride as an

Admixture in Concrete, Bulletin 13 (PCA LS013),

Structural Materials Research Laboratory, Lewis Institute, Chicago,

http://www.portcement.org/pdf_files/LS013.pdf, 1924.

ACI Committee 212, Chemical Admixtures for Concrete, ACI 212.3R-

91, American Concrete Institute, Farmington Hills, Michigan, 1991.

ACI Committee 212, Guide for the Use of High-Range Water-Reducing

Admixtures (Superplasticizers) in Concrete, ACI 212.4R-93

(Reapproved 1998).

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Admixtures in concte