Download - CHEMICAL ADMIXTURES FOR CONCRETE
CHEMICAL ADMIXTURES FOR
CONCRETE
K.Prashanth kumar
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.
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
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.
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.
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:
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
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.
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
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
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
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
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.
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.
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
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
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
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
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
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
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.