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CONCRETE ADMIXTURES
1.
2. Admixtures are those ingredients in concrete other
than cement, water, and aggregates that are added to
the mixture immediately before or during mixing as
to obtain the required properties.
4. Air-entraining admixtures are used to introduce and stabilize
microscopic air bubbles in concrete.
These bubbles are introduced by an air entraining agent,
a type of chemical that includes detergents.
6. In cold weather, water in concrete converts into ice .
As ice occupies more volume than liquid water so it
exerts stress on surrounding concrete as a result
concrete might crack . But air bubbles inside the
concrete provide space for the enhanced volume of
ice and prevents cracks in the concrete.
7. Plasticizers or water reducers, and superplasticizer
or high range water reducers, are chemical
admixtures that can be added to concrete mixtures
to improve its workability.
8. In order to produce stronger concrete, less water
is added which makes the concrete mixture less
workable and difficult to mix, necessitating the
use of plasticizers.
9. The action of plasticizers is mainly to fludify the
mix and improve the workability of concrete ,
mortar or grout .
10. Plasticizers are also often used when pozzolanic
ash is added to concrete to improve strength.
Adding 1-2% plasticizer per unit weight of cement is
usually sufficient. Adding an excessive amount of
plasticizer will result in excessive segregation of
concrete which is not advisable.
11.
12.
13. Plasticizers are commonly manufactured from
pop lignosulfonates which is a by-product from the
paper industry.
In ancient times,the Romans used animal fat, milk
and blood to improve workability of concrete mixes.
14. Retarding admixtures are used to slow down the
speed of the reaction between cement and water by
affecting the growth of the hydration products or
reducing the rate of water penetration to the cement
particles. The use of a retarder will increase the
setting time and may delay strength development of
the concrete.
15. Retarders can be used:
in hot weather to prevent early stiffening.
to allow the placing of a large pour of concrete over
several hours.
to place concrete in layers without cold joints.
to extend the time between mixing and placing.
to prevent setting of the concrete in the truck in case
of delay.
17. The admixture that causes
an increase in the rate of
hydration of the
hydraulic cement and
thus shortens the time of
setting, increases the
rate of strength
development.
18. The admixture may allow a concrete worker to
remove forms, get onto a concrete surface for
finishing, and sometimes even put loads on it
earlier, such as when diverting foot traffic to do
patching.
Accelerators counteract the influence of cold
weather, which slows down the curing and setting
process.
19. Calcium chloride is a common accelerator, used
to accelerate the time of set and the rate of
strength gain.
20. The function of this type of
admixture is to enhance the long
term durability of reinforced
concrete by enhancing passivation
to the cathodic and/or anodic areas
of embedded steel.
21. Corrosion-inhibiting admixtures can significantly
reduce maintenance costs of reinforced concrete
structures throughout a typical service life of 30 to
40 years.
22. Structures built in the 1970s with calcium nitrite
still contain the original level of nitrite and show
no signs of deterioration.
23. Water is essential to concrete production, placement,
and curing. But once it fulfills its role in those
processes, water is no longer concrete’s friend.
Depending on its function and the nature of its
exposure, concrete can of course perform well in wet
environments. As a naturally porous material, though,
and one that is prone to cracking, concrete is
vulnerable to water infiltration. The unfortunate
results can be freeze/thaw damage and deterioration
due to corrosion of embedded steel reinforcement.
24. One method that can simplify the protective process
is to make concrete with admixtures that reduce its
permeability—in effect to make the concrete itself
waterproof.
These consists of hydrophobic, or water-repellent,
chemicals derived from soaps or fatty acids, vegetable
oils, and petroleum.
25. It is a powder admixture which can be used for
making neat cementitious grouts. It comprises a
water reducing / plasticising agent and a gas
producing expansion medium.
ADVANTAGES
Higher Strength●
Higher fluidity●
Lower permeability●
Reduced bleeding●
26. APPLICATIONS
Grouting joints of precast-concrete members●
Non-shrink infilling and jointing●
Grouting of post-tensioned cable ducts●
Repair of honeycombed and void-filled concrete●
structural
Bed grouting●
27. Mineral admixtures are “inorganic” materials that
also have pozzolanic properties. These very fine-
grained materials are added to the concrete mix to
improve the properties of concrete, or even as a
replacement for portland cement.
29. Fly ash is a by-product of the combustion of
pulverized coal in thermal power plants.
Concrete using fly ash is generally reported to
show reduced segregation and bleeding and to be
more satisfactory than plain concrete when placed
by pumping.
30. Blast furnace slag is a by-
product of pig iron
manufacture.
Concrete containing slag as a
mineral admixtures generally
offers better chemical resistance
due to improved watertightness
31. Silica fume is an industrial by-product consisting
of ultrafine particle. It is recovered from electric
furnace by means of dust collectors from the waste
gas emitted during the production of ferro-silicon
metal.
It can be used as an water reducing admixture.
32. Mineral admixtures are generally used in concrete
to help make it stronger as well as to make it more
economical. Another very important benefit is
that it is an environmentally responsible and
effective way to recycle industrial and other waste
by-products.
