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A concrete admixture is a material other than water, sand, aggregate, and cement and is added to the batch immediately before or during its mixing.
Types of Concrete Admixtures are used for various purposes.
It is used to improve or give special properties to concrete. It should not adversely affect any properties of the concrete. The admixture is generally added to a concrete mix in a relatively minute quantity.
The degree of control must be higher to ensure that overdosages that are unlikely to occur. Excess quantities of admixture may cause harm to the properties of concrete.
1. Functions of Admixtures
Some of the important purposes for which the admixtures could be used are:
1. To accelerate the initial setting of concrete.
2. To retard the initial set.
3. To increase the strength of concrete.
4. To reduce the heat of evolution.
5. To improve workability.
6. To inhibit the corrosion of concrete.
7. To increase the durability of concrete.
8. To increase the bond of concrete to the steel reinforcement.
9. To increase the resistance to chemical attacks.
10. To control the alkali-aggregate expansion.
11. To reduce the segregation in grout mixtures.
12. To decrease the weight of concrete per cubic meter.
13. To produce colored concrete or mortar for colored surfaces.
14. To produce concrete with fungicidal, germicidal, and insecticidal properties.
15. To strengthen the bond between new and old concrete surfaces.
2. Types of Concrete Admixtures
There are 12 types of concrete admixtures that are commonly used in the construction field as follows:
a. Air- entraining agents:
The main objects of entraining air in concrete are:
i. To increase the workability and,
ii. To improve its resistance to weathering and especially to the action of frost.
There are three main ways of incorporating air or gas cells in concrete:
1. By the use of gas-forming materials such as aluminum or zinc powder, hydrogen peroxide.
2. By the use of surface-active agents which reduce surface tension. These are known as air-entraining agents and are subdivided into:
i. Natural wood resins and their soaps, e.g. vinyl resin;
ii. Animal or vegetable fats or oils, their fatty acids, and their soaps;
iii. Alkali salts of sulfonated or sulfated organic compounds such as synthetic detergents, e.g. Darex.
3. By the use of cement dispersing agents.
Air-entraining should not be used unless site control is good, since the amount of air entrained may vary considerably with changes in sand grading, errors in proportioning, workability of the mix, and temperatures.
The retarders, as the name suggests delay the setting time of cement. Calcium sulphate, a form of gypsum is generally added during the manufacture of cement to retard the setting.
Some other materials which are effectively used to retard the rate of hydration are Ammonium chloride, ferrous and ferric chlorides, alkali bicarbonates, salts of carboxymethyl cellulose, etc.
The agents that are added to the cement to make it set and acquire strength more rapidly are called accelerators.
Calcium chloride is a commonly used accelerator.
Sodium chloride may also be used as an accelerating agent.
d. Water-proofers Adimixture
These may be obtained in powder, paste, or liquid form and can consist of pore-filling materials or water-repelling materials. They can be chemically active or inactive.
Alkaline silicates notably silica of soda, aluminum, and zinc sulphates, and aluminum and calcium chlorides are the chief pore-filling materials.
These are all chemically active and hence may accelerate the setting time of the concrete thus making it more impervious at an early age.
Chalk, Fuller’s earth, and talc in a very finely ground form are chemically inactive pore-filling materials.
They improve the workability and improve the density of concrete. Materials in the water-repellent class are soda and potash soap.
Pozzolana is a siliceous material which while itself having no cementitious properties will either be processed or unprocessed and in finely divided form, react in the presence of water with lime at normal temperatures to form compounds of low solubility having cementitious properties.
Pozzolanas may be natural or artificial. The examples are:
Natural: Clay sand shales (to be calcined to become active), diatomaceous earth and opaline cherts and shales and volcanic tuffs and policies, etc.
Artificial: Ground blast furnace slag and fly ash.
The following advantages are obtained through the use of pozzolanas:
1. The heat of hydration is reduced.
2. Workability with less amount of water is improved.
3. Attack from salts and sulfates are reduced.
4. Calcium hydroxide leaching is prevented.
Some of the disadvantages of pozzolanas are:
1. Strength development is slowed.
2. Drying shrinkage may be increased.
3. Durability may be impaired.
Pigments are substances added to produce colored cement. The chief pigments used in concrete are Brown, black, red, green, blue, and yellow.
To obtain a good coloring effect, the pigment should be ground with cement in a ball mill.
g. Workability agents
Workability agents and water-reducing retarders are sometimes grouped together because many materials appear to perform all these functions. These are mineral powder grounds at least as fine as cement and usually much finer.
They are generally classified into three chemical types:
Lignosulfonates, organic acids, and carbohydrates.
The excessive use of these agents may increase the shrinkage of concrete.
h. Bonding Admixture
Bonding admixtures are utilized to make a good bond between old and fresh concrete surfaces.
In general, if fresh concrete is applied over a hardened concrete surface, there is a case of failure of the fresh concrete surface due to a weak bond with the old surface.
To create the bond stronger, bonding admixtures are applied to cement or mortar grout which is used on the concrete surface just before putting fresh concrete.
This type of admixture is employed for pavement overlays, screed-over roof conditions, repair works, etc.
Bonding admixtures are water emulsions and they are produced from natural rubber, synthetic rubbers, and polymers like polyvinyl chloride, polyvinyl acetate, etc.
i. Corrosion Preventing Adimixture
Corrosion of steel in reinforced concrete structures is all-around and it is more damageable when the structure is uncovered to saline water, industrial fumes, chlorides, etc. To control or to slow down the process of corrosion controlling admixtures are employed.
A few of the corrosion-preventing admixtures employed in reinforced concrete are sodium benzoate, sodium nitrate, sodium nitrite, etc.
j. Coloring admixtures
Coloring admixtures are the pigments that deliver color to the finished concrete. The admixtures employed to make color should not affect the concrete strength.
Generally coloring admixtures are added to cement in a ball mill, then colored cement can be acquired which can be employed for making colored concrete. A few of the coloring admixtures and their resultant colors are noted below.
|Iron or Red Oxide||Red|
|Hydroxides of iron||Yellow|
|Barium manganite and Ultramarine||Blue|
|Chromium oxide and chromium hydroxide||Green|
|Manganese black, Raw umber||Brown|
k. Grouting Admixtures
Grouting admixtures are added to grout materials to improve the grout properties according to the requirement of grout.
Sometimes, there is a need for quick-set grout and sometimes there is a need for slow-set grout to spread into deep cracks or fissures. Hence, different admixtures are used as grout admixtures based on the situation.
Accelerators like calcium chloride, triethanolamine, etc. are used as grout admixtures when the grout is to be set rapidly.
Similarly, retarders like mucic acid, gypsum, etc. are used to slow down the setting time of grout. Gas-forming admixtures like aluminum powder are added to grout material to counteract the settling of foundations.
l. Fungicidal, Germicidal, Insecticidal Admixtures
To control the growth of bacteria, germs, and fungus on hardened concrete structures, it is suggested that the mix should have fungicidal, germicidal, and insecticidal properties.
These properties can be created by adding admixtures like polyhalogenated phenols, copper compounds, dieledren emulsions, etc.
This was for the Types of Concrete Admixtures used in Construction sites.
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