Importance of Concrete Admixtures
However,
note that admixture must not be used for the replacement of poor quality of
ingredient or poor workmanship. Concrete Admixtures are used to improve the
behaviour of concrete under various conditions and ensure the quality of
concrete during mixing/transporting/placing/curing, both as a fresh concrete
and hardened concrete.
Admixtures are primarily used for:
·
To reduce water content in concrete i.e.
keep water cement ratio as less as possible.
·
To increase compressive strength and
durability parameter of concrete.
·
To enhance the workability and give more
transportation and placing time.
·
To impart specific property of concrete or
to create favourable condition during placing.
·
To control the shrinkage cracks.
·
To achieve specific concrete properties
more effectively.
·
To ensure quality of concrete during
mixing, transporting, placing, and curing in adverse weather condition.
·
Modify the rate or capacity for bleeding.
·
Improve pumpability
·
Decrease permeability
·
Increase bond between existing and new
concrete.
Use
of admixtures in concrete successfully depends on the use of appropriate
methods of concreting and batching. Most admixtures are supplied in
ready-to-use liquid form and are added to the concrete at the plant or on the
project site.
Concrete Admixtures are of two main types:
1. Chemical Admixtures for Concrete:
Chemical
admixtures are water-soluble chemicals that are added in small quantity to
concrete in order to change certain properties. It reduces the cost of
construction and enhances the properties of hardened concrete.
Chemical Admixtures are classified as below.
(a) Air Entertainers:
Air
entertainers are used to distribute tiny air bubbles in the concrete. Adding
this admixture reduces the bleeding and
segregation of fresh concrete. It also improves the resistance of concrete to
severe frost action that reduce the damage during freeze-thaw cycles, there by
increasing the durability of concrete. They also help to reduce the density of
concrete.
(b) Water Reducers:
Water
reducer admixtures are used to reduce the quantity of mixing water required to
produce concrete of a desired slump. They reduce water-cement ratio, reduce
cement content and increase slump and we can obtain specific strength of
concrete.
(c) Set Retarders:
A
retarder is an admixture that slows down or to delay the chemical process of
hydration so that concrete remains plastic and workable for a longer time than
concrete without the retarder. These types of concrete admixtures are widely
used where concreting is carried out at high temperature. As high temperature
could produce a faster initial setting of concrete.
(d) Set Accelerators:
Accelerators
are added to concrete to increase the rate of early strength in concrete to
permit earlier removal of formwork and reduce the required period of curing.
Calcium chloride is the most common accelerator; however, it could promote
corrosion activity. But, best concrete practices like proper mix design, consolidation,
adequate cover prevent the corrosion issues. These types of concrete admixtures
are especially useful for modifying the concrete properties in cold weather.
(e) Corrosion Inhibitors:
Corrosion
inhibitors are used to reduce the corrosion in steel and steel bars in
concrete. We can reduce the maintenance cost of structure by using corrosion
inhibitors concrete admixtures. They can reduce maintenance cost of RCC
structures throughout a typical service life of 30-40 years.
2. Mineral Admixtures for Concrete:
Cement
is a most common and widely used as binding material in construction. For
concreting cement is mixed with crushed rock, sand and water in precise
proportion to produce concrete. For getting better results, better workability,
more strength, and finishing, cement or mortar mineral admixtures are used.
A
substance added to concrete both as a filler i.e. improving the physical
structure by occupying the spaces between the cement particles, and as a
“pozzolana,” reacting chemically to impart far greater strength and durability.
A
mineral admixture makes mixtures more economical, reduce permeability,
influence other concrete properties and increase the strength.
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