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CHEMICAL ADMIXTURES FOR CONCRETE K.Prashanth kumar

CHEMICAL ADMIXTURES FOR CONCRETE

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Page 1: CHEMICAL ADMIXTURES FOR CONCRETE

CHEMICAL ADMIXTURES FOR

CONCRETE

K.Prashanth kumar

Page 2: CHEMICAL ADMIXTURES FOR CONCRETE

INTRODUCTION

Concrete has become established superb building material over the years, the quality and consistency of

which can be improved to meet the desired requirements by using appropriate chemical admixtures in the mix.

Making quality concrete is a precise technology, and in many conditions when ordinary concrete would not be

suitable, the technologist can ensure that the finished product meets this requirement by using the right admixtures

in the right proportion. It is important that the operator has an intelligent appreciation of concrete admixtures and

employs the correct admixture in the exact quantity specified in order to achieve the desired results. Broadly

speaking, admixtures may be divided into various categories, namely, accelerators, retarders, water reducers, air-

entraining agents, water proofers, pumping aids, plasticizers, superplasticizers etc.

While admixtures, unlike cement, aggregate and water, are not an essential component of the concrete

mix, they are an important and increasingly wide spread component in many countries, a mix that contains no

admixture is now a days an exception.

Page 3: CHEMICAL ADMIXTURES FOR CONCRETE

What is Admixture?

It is an optional ingredient of concrete

which is added to modify the properties of fresh

and hard concrete and grout materials as per

some specific requirements. Addition of admixture

may alter workability, pumping qualities, strength

development, appearance etc. in fresh concrete

and permeability, strength, durability etc. in

hardened concrete. But use of chemical admixture

is a must for producing high grade concrete.

Chemical Admixtures

Page 4: CHEMICAL ADMIXTURES FOR CONCRETE

USES OF ADMIXTURE:

Effect of Admixtures on the Properties of Fresh Concrete:

To increase workability without increasing the water

cement ratio.

To retard or accelerate time of initial or final setting.

To modify the rate of bleeding.

To retard segregation or increase cohesion.

To improve pumpability.

To reduce the rate of loss of workability.

Page 5: CHEMICAL ADMIXTURES FOR CONCRETE

Effect of Admixtures on the Properties of Hardened Concrete:

To accelerate the rate of strength development

especially at early stage.

To increase the strength

To increase the durability

To decrease the permeability

To control expansion by alkali – aggregate reaction.

To improve bond with reinforcement.

To improve bond with old /new concrete.

To improve impact or abrasion resistance.

To inhibit corrosion of embedded reinforcement.

Page 6: CHEMICAL ADMIXTURES FOR CONCRETE

TYPES OF CHEMICAL ADMIXTURES:

Water Reducing Admixtures

i. Plasticizers.

ii. Super Plasticizers.

Retarders and retarding admixtures.

Accelerators and Accelerating Plasticizers

Air-entraining admixtures.

Water or damp proofing and permeability reducing

admixtures.

According to characteristics effects produced by them chemical admixtures used are as follows:

Page 7: CHEMICAL ADMIXTURES FOR CONCRETE

PLASTICIZERS(Water- reducing):

Plasticizers are the organic or combination of organic and

inorganic substances which reduces water content for certain degree of

workability, when added in mix. The basic products consisting

plasticizers are anionic surfactants (such as lignosulphonates, salts of

sulphonates hydrocarbon) non-ionic surfactants (such as polyglycol

esters, hydroxylated carboxylic acid products) and others such as

carbohydrates, etc. among them calcium, sodium and ammonium

lignosulphonates are commonly used. Plasticizers are mixed from 0.1%

to 0.4% by weight of cement used and it reduces 5% to 15% of water

with the increment of workability from 3 to 8cm slump. In the mix, the

cement grains absorbs the plasticizers molecules and results change in

the surface charge of the same sign which causes repulsive forces and

makes the dispersion which increases plasticity and workability. Some

Plasticizer also entrails the air but a good plasticizer is that which

entrains air less than 2% only. The plasticizers are available in market in

various brands with specifications for composition, dosages etc.Plasticizer

Page 8: CHEMICAL ADMIXTURES FOR CONCRETE

Uses of plasticizers:

To achieve a higher strength by decreasing the water cement ratio at

the same Workability as an admixture free mix.

To achieve the same workability by decreasing the cement content so

as to reduce the heat of hydration in mass concrete.

To increase the workability so as to ease placing in accessible

locations.

Water reduction more than 5% but less than 12%

The commonly used admixtures are Ligno-sulphonates and

hydrocarbolic acid salts.

Plasticizers are usually based on lignosulphonate, which is a natural

polymer, derived from wood processing in the paper industry.

Retarding Effect:

Plasticizers get adsorbed on the surface of the cement

particles and form a thin sheath.

This sheath inhibits the surface hydration reaction

between water and cement as long as sufficient

plasticizers molecules are available.

become entrapped in hydration products.

Page 9: CHEMICAL ADMIXTURES FOR CONCRETE

SUPERPLASTICIZERS (High Range Water reducer):

Normal water reducers are well established

admixtures called plasticizers in concrete technology. A normal

water reducer is capable of reducing water requirements by 10

to 15%. Higher water reductions, by incorporating larger

amounts of these admixtures, result in undesirable effects on

concrete like bleeding, segregation and hardening. So, a new

class of water reducers, chemically different from the normal

water reducer and capable of reducing water content by about

30% has been developed. The admixtures belonging to this

class are known as super plasticizers.

SUPERPLASTICIZERS

No plasticizers With 0.3% superplasticizer

Page 10: CHEMICAL ADMIXTURES FOR CONCRETE

There are four types of super plasticizers which are generally used for concrete as given below.

Sulphonated melamine – It is suitable in low

temperature areas

Sulphonated naphthalene – It is more suitable in high

temperature areas

Ligno sulphates – It is suitable for Indian conditions

where temperature variation is high

Carboxylated admixture – It is suitable where

workability is required to be retained for large duration.

Advantages of Superplasticizers:

Significant water reduction

Reduced cement contents

Reduce water requirement by 12-30%

Increased workability

Reduced effort required for placement

More effective use of cement

More rapid rate of early strength development

Increased long-term strength

Reduced permeability

Page 11: CHEMICAL ADMIXTURES FOR CONCRETE

Disadvantages of Superplasticizers:

Additional admixture cost (the concrete in-place cost

may be reduced)

Slump loss greater than conventional concrete

Modification of air-entraining admixture dosage

Less responsive with some cement

Mild discoloration of light-coloured concrete

Page 12: CHEMICAL ADMIXTURES FOR CONCRETE

RETARDERS:

These admixtures are used where setting time of

concrete need to be delayed. Retarder delays the

hydration process but doesn’t affect the eventual process.

Initial setting time can be delayed by more than 3 hours.

The main application of retarding admixtures is in

eliminating the cold joints and controlling the setting time

of concrete.

RETARDERS

Page 13: CHEMICAL ADMIXTURES FOR CONCRETE

Advantages of Retarders:

Retarding admixtures delay hydration of cement.

To delay or extend the setting time of cement paste in

concrete.

These are helpful for concrete that has to be

transported to long distance, and helpful in placing the

concrete at high temperatures.

Used in casting and consolidating large number of

pours without the formation of cold joints

Used in grouting oil wells.

Page 14: CHEMICAL ADMIXTURES FOR CONCRETE

These are used to accelerate the setting time and hardening process of cement in concrete.

These are used in concreting under flowing water and in road repair works so that work can be put to

use as earliest as possible.

ACCELERATORS:

Accelerating admixtures can be divided into groups based on their performance and application:

1. Set Accelerating Admixtures:

Reduce the time for the mix to change from the plastic to the hardened state. Set accelerators

have relatively limited use, mainly to produce an early set.

2. Hardening Accelerators:

Which increase the strength at 24 hours by at least 120% at 20ºC and at 5ºC by at least 130% at

48 hours. Hardening accelerators find use where early stripping of shuttering or very early access to

pavements is required. They are often used in combination with a high range water reducer, especially in

cold conditions.

Page 15: CHEMICAL ADMIXTURES FOR CONCRETE

Advantages:

Reduced bleeding,

Earlier finishing,

Improved protection against early exposure to freezing

and thawing,

Earlier use of structure,

Reduction of protection time to achieve a given quality,

Early removal of form, and

In some case, early load application.

Shortens the setting time of cement and therefore

increases the rate of gain of strength.

Enables earlier release from precast moulds thus

speeding production.

Reduces segregation and increase density and

compressive strength.

Cures concrete faster and therefore uniform curing in

winter and summer can be achieved.

Early use of concrete floors by accelerating the setting

of concrete.

Reduces water requirements, bleeding, shrinkage and

time required for initial set.

ACCELERATORS

Page 16: CHEMICAL ADMIXTURES FOR CONCRETE

AIR-ENTRAINING ADMIXTURES:

Air-entraining admixtures cause small stable

bubbles of air to form uniformly through a concrete mix.

The bubbles are mostly below 1 mm diameter with a high

proportion below 0.3 mm. The benefits of entraining air in

the concrete include increased resistance to freeze-thaw

degradation, increased cohesion (resulting in less bleed

and segregation) and improved compaction in low-

workability mixes

AIR-ENTRAINING ADMIXTURES

Page 17: CHEMICAL ADMIXTURES FOR CONCRETE

Air entraining agents:

Natural wood resins

Animal and vegetable fats and oils such as tallow, olive oil and their fatty acids such as stearic and

oleic acids

Various wetting agents such as alkali salts or sulphonated organic compounds

Water soluble soaps of resins acid

Miscellaneous materials such as sodium salts of petroleum sulphonic acids, hydrogen peroxide

and aluminium powder, etc.

Factors affecting air entrainment:

Type and quantity of air entraining agents used

Water cement ratio of mix

Type and grading of aggregates

Mixing time

Temperature

Type of cement

Influence of compaction

Admixtures other than air entraining agents used

Page 18: CHEMICAL ADMIXTURES FOR CONCRETE

Effect of air entrainment on the properties of concrete:

Air entrainment will effect directly the following three properties of concrete

Increased resistance to freezing and thawing

Improvement in workability

Reduction in strength

Incidentally Air entrainment will affect the properties in following ways:

Reduces the tendencies of segregation

Reduces the bleeding and laitance

Decreases the permeability

Increases the resistance to chemical attack

Permits reduction in sand content, water content, cost, & heat of hydration

Reduces unit weight, alkali aggregate reaction, the modulus of elasticity

Page 19: CHEMICAL ADMIXTURES FOR CONCRETE

WATERPROOFING ADMIXTURES:

It is essential to water proof a structure to prevent the seepage of water for its durability. provide water

proofing solutions for basements, sunken portions, roofs, terrace gardens and expansion joints etc.

several ways of water proofing depending on a structure, site and conditions viz.:

• Polymer Modified Cementitious Membranes

• Polyurethane Membranes

• Box Type Method using Kota/Agra Stone

• Injection Grouting

• APP modified bituminous membranes / Trafelt

• EPD(ethylene-propylene-diene) Membrane

• PVC Membrane

Damp proofing treatment in building:

Treatment to foundation.

Treatment to floors.

Treatment to walls.

Treatment to flat roof. Treatment to parapet wall.

Treatment to pitched roof.

WATERPROOFING ADMIXTURE

Page 20: CHEMICAL ADMIXTURES FOR CONCRETE

Function:

The main function is to reduce either the surface adsorption into the concrete and the

passage of water through the hardened concrete. To achieve this, most products function in one or

more of the following ways:

Reducing the size, number and continuity of the capillary pore structure

Blocking the capillary pore structure

Lining the capillaries with a hydrophobic material to prevent water being drawn in by

absorption / capillary suction

Page 21: CHEMICAL ADMIXTURES FOR CONCRETE

CONCLUSION:

• Plasticizers, superplasticizers, & Air-entraining admixtures is

mostly used.

• Superplasticizers is widely used all over the world.

• India is catching up with the use of superplasticizer in the

construction of high rise buildings, long span brides Because it

reduced cement content, increased workability, & increased

strength.

• Superplasticizers is reduce the water content and increase the

strength.