Admixtures are artificial or natural additional materials may be added to concrete mix, just before or during the mixing, to change one or more properties of the concrete in the plastic or hardened state as required in our structure.
Admixtures (concrete admixtures) can be used for the following purposes:
Functions of Admixtures
(a) To increase the rate of strength development at early ages – calcium chloride is the most widely used accelerator,
(b) To retard the initial setting time while pumping concrete over long distance,
(c To increase the workability without changing the water content – pozzolana such as fly ash is used,
(d) To increase the strength,
(e) To increase the resistance to freezing and thawing – vinsol resin is an aar entrainment admixture which is used for this purpose,
( f) To decrease heat evolution,
(g) To increase water tightness,
(h) To decrease capillary flow of water, and
(i) To decrease rate of bleeding and segregation.
Related post: Types of cement their properties and Uses.
Types of Admixtures
There are two types of admixtures:
- Mineral admixtures
- Chemical admixtures
(1) Mineral admixtures
(a) Fly ash
(b) Silica fume
(c) Ground granulated blast furnace slag
(d) Rice husk ash
(2) Chemical admixtures
(a) Accelerating admixture – A substance which increases the rate of hydration of a hydraulic cement, reduces the setting time, or in other words, increases the rate of strength development.
(b) Retarding admixture – A substance which delays the setting time of cement paste.
(c) Water – reducing admixture – A substance which either increases workability of freshly mixed mortar or concrete without increasing water – cement ratio, or maintains workability with reduced water- cement ratio.
(d) Air – entraining admixture – A substance which causes air to be entrap/Cd in the form of tiny bubbles in mortar or concrete during mixing to increase its workability and resistance to freezing and thawing.
(e) Super plasticizing admixture – A substance which has a very high workability with a large decrease in water content (at least) for a given workability. A high range water reducing admixture (HRWRA) is also referred to as a superplasticizer.
A water reducing admixture can be utilized in three ways:
(i) By the addition of the admixture with the reduction in water – cement ratio, a concrete having the same workability and greater compressive strength can be obtained than that without the admixture. A 0.5% concentration of admixture by weight of cement can increase, the 28 days compressive strength by 30% with 20% reduction in water-cement ratio.
(ii) By the addition of the admixture with no decrease in water-cement ratio, a concrete having same compressive strength but greater workability can be obtained. A 0.5% concentration of admixture by weight of cement can increase slump from 25mm to 100 mm.
(iii) BY the addition of the admixture, a concrete with same workability and compressive strength can be obtained at lower cement content. A 0.5% concentration of admixture by weight of cement may require 15% less cement.
Any of the following substances may be used as water-reducing admixtures:
(i) Lignosulphonic acid and its salts
(ii) Hydroxylated carboxylic acids
(iii) Formaldehyde derivatives such as melamine-formaldehyde and naphthalene sulphonate formaldehyde.
These materials essentially work as surface-active agents, reduce surface tension of water and disperse cement particles more effectively, thus lowering the water requirements.
It is essential to know the dose of a given admixture and its possible detrimental effects on concrete or on corrosion of reinforcement. Sufficient trials should be made on a given admixture to assess its effectiveness in terms of workability, setting time and compressive strength, etc. on the given concrete mix.
A similar exercise should be done on the various admixtures available in the market so as to identify the best admixture and its optimum dose.
Calcium chloride or admixtures based on chlorides must not be used. The manufacturer of admixture is expected to give detailed instructions on the correct use of the admixture for the purpose indicated.
When proportioning concrete containing a separately batched, and cementitious material such as fly ash, natural pozzolana, GGBF slag, or silica fume, some factors must be considered.
These factors are:
(a) Chemical activity of cementitious material and its effect on concrete strength at various ages
(b) Effect on the mixing – water demand required for workability and placeability
(c) The specific gravity of the material and its effect on the volume of concrete produced in the batch
(d) Effect on dosage rate of chemical admixtures and/or air-entraining admixtures used in the mixture
(e) Effect of combinations of ingredients on other critical properties of the concrete such as time of set under ambient temperature conditions, heat of hydration, rate of strength development, and durability
(f) Quantity of binding materials and cement needed to meet the requirements for the particular concrete
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