What Is Admixture?
Admixtures are the special ingredients added during concrete mixing to enhance the properties of fresh concrete. Admixtures are materials other than the aggregate, water, and cement added to the concrete.
Different Types of Admixtures are added to the concrete mix are used to upgrade the behavior of concrete under different weather conditions.
Admixtures minimize the construction cost by altering the properties of hardened concrete, ensure the quality of concrete during mixing, transporting, placing, curing, and overcome certain emergencies during concrete operations.
Functions of Admixtures in Concrete
- Followings are the main purposes for which admixtures should be added in the concrete mix:
- To increase or decrease the setting time of the fresh concrete mix.
- To make better or enhance the workability or flowability of concrete mix which is the main property of the concrete.
- To maximize the strength and durability of the concrete.
- To reduce the heat of hydration.
- To lowers the segregation and bleeding which may occur during the placing of concrete.
- To reduce the permeability of concrete.
- To achieve other desirable properties.
Types of Admixtures
Concrete admixtures are classified as follows,
- Water-reducing admixture
- Retarding admixture
- Accelerating admixture
- Air entraining admixture
- Pozzolanic admixture
- Damp-proofing admixture
- Gas forming admixture
- Air detraining admixture
- Alkali-Aggregate expansion inhibiting admixture
- Anti-washout admixture
- Grouting admixture
- Corrosion inhibiting admixture
- Bonding admixture
- Fungicidal or germicidal admixture
- Coloring admixture
1. Water-reducing Admixture
Water-reducing admixture, the name itself suggests that water reducers are added to a concrete mixture, mortar, or grout to increase the workability or flowability without increasing the water content.
Workability is the important property of concrete which is improved with the addition of water but if more water is added than required the strength and durability properties of concrete get affected.
These concrete admixtures in addition to improving the workability also enhance the strength of concrete, a good bond between concrete and steel. Prevents cracking, segregation, honeycombing, bleeding, etc.
Normal plasticizer reduces the water demand by up to 10%, mid-range plasticizers reduce up to 15% and superplasticizers reduce water demand by up to 30%.
Calcium, sodium, and ammonium lignosulfonates are commonly used as water-reducing admixtures.
2. Retarding Admixture
Retarding (retarders) admixtures slow down the initial rate of hydration of cement and extend the setting time of cement paste. These types of concrete admixtures can be used where the area is susceptible to more temperature changes and where the concrete has to be transported to a long distance.
The quick setting in some conditions may lead to discontinuities in structure, the poor bond between the surfaces creates unnecessary voids in concrete, etc. Retarders are best suited to eliminate this type of problem.
Commonly used retarding admixtures are calcium sulphate or gypsum. Some other materials used as retarders are starch, cellulose products, common sugar, salts of acid, etc.
3. Accelerating Admixture
These admixtures are used to lower the initial setting time of the fresh concrete mix. They speed up the process of the initial stage of the hardening of concrete hence they are also called accelerators. These accelerators also increase the strength of concrete in its early stage by increasing the rate of hydration.
Speedy hardening of concrete is useful in many situations such as early removal of formwork, less period of curing, emergency repair works, for constructions in low-temperature regions, etc.
In these concrete admixtures, the only used accelerators are silica fume, calcium chloride, finely divided silica gel, etc. Calcium chloride is cost-effective and commonly used accelerating admixture.
4. Air Entraining Admixture
The primary function of air-entraining admixture is to increase the durability of concrete under freezing and dewatering and thawing conditions. When added to the concrete mix, these admixtures will form millions of air bubbles throughout the mix and improves the properties of concrete.
Air entrainment in concrete will also improve the workability of concrete, prevents segregation and bleeding, lower the unit weight and elasticity of concrete improves the chemical resistance of concrete against temperature and other weathering effects, and decrease cement or sand or water content in concrete, etc.
Commonly used air-entrainment admixtures are vinsol resin, darex, teepol, cheecol etc. These admixtures are actually made of natural wood resins, alkali salts, animal and vegetable fats, and oils, etc.
5. Pozzolanic Admixture
Pozzolanic admixtures are used to prepare dense concrete mix which is best suitable for water retaining structures like dams, reservoirs, etc. They also reduce the heat of hydration and thermal shrinkage.
Best Pozzolanic material in minimal quantity gives the best results and prevents or reduces many risks such as alkali-aggregate reaction, leaching, sulfate attack, etc.
In admixtures, the naturally available pozzolanic materials are the clay, shale, volcanic tuffs, pumices, etc. and artificial available pozzolanic materials are fly ash, silica fume, blast furnace slag, rice husk ash, surkhi, etc.
6. Damp-proofing Admixture
Damp-proofing or waterproofing admixtures are used to make the concrete structure impermeable against water and to prevent dampness on the concrete surface. In addition to the waterproof property, they also act as accelerators in the early stage of concrete hardening.
Damp proofing admixtures are available in the market in liquid form, powder form, paste form etc. The main constituents of these admixtures are aluminum sulphate, zinc sulphate, aluminum chloride, calcium chloride etc.
7. Gas Forming Admixture
When these admixtures are added, it reacts with hydroxide obtained by the hydration of cement and forms minute bubbles of hydrogen gas in the concrete.
The range of formation of bubbles in concrete depends upon many factors such as the amount of admixture, the chemical composition of cement, temperature, fitness, etc. the formed bubbles help the concrete to counteract the settlement and bleeding problems.
Gas forming admixtures are also used to produce lightweight concrete. For settlement and bleeding resistance purposes, a small quantity of gas-forming admixture which is generally 0.5 to 2% by weight of cement is used.
Aluminum powder, activated carbon, hydrogen peroxide, etc. are generally used gas-forming chemical admixtures.
8. Air Detraining Admixture
Air-detraining admixtures are used to remove the excess air from the concrete voids. Sometimes, the aggregates may release the gas into concrete, and air-entrained is more than required then this admixture is useful.
Some of the mostly used air-entrained admixtures are tributyl phosphate, silicones, water insoluble alcohols etc.
9. Alkali Aggregate Expansion Preventing Admixture:
Alkali aggregate expansion in concrete is happened by the reaction of alkali of cement with the silica present in the aggregates. It forms a gel-like substance and causes volumetric expansion of concrete which may lead to cracking and disintegration.
Use of pozzolanic admixtures will prevent the alkali-aggregate reaction and in some cases, air-entraining admixtures are also useful. Commonly used admixtures to minimize the risk of alkali-aggregate reaction are aluminum powder and lithium salts.
10. Anti-washing Admixtures
Anti-washing admixtures are used in concrete especially for an underwater concrete structure. It protects the concrete mix from being washed out under underwater pressure. It improves the cohesiveness of concrete.
This types of concrete admixtures is prepared from natural or synthetic rubbers, cellulose based thickeners etc.
11. 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 of quick set grout and sometimes there is a need of slow set grout to spread into deep cracks or fissures.
In these admixtures, the accelerators like calcium chloride, triethanolamine, etc. are used 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.
So, different admixtures are used as grouting admixtures based on situation and structure. Gas-producing admixtures like aluminum powder are mixed with grout material to counteract the settling of foundations.
12. Corrosion Preventing Admixture
Corrosion in steel in reinforced concrete structures is general and it is severe when the structure is exposed to saline water, industrial fumes, chlorides, etc. to prevent or to slow down the process of corrosion preventing admixtures are used.
Generally used corrosion preventing admixtures used in reinforced concrete are sodium benzoate, sodium nitrate, sodium nitrite, etc.
13. Bonding Admixtures:
Bonding admixtures are used to constitute a strong bond between old and fresh concrete surfaces. In general, if fresh concrete is poured over a hardened concrete surface, there is a chance of failure of the fresh concrete surface due to weak bonds with the old surface.
To build the bond stronger, bonding admixtures are added to cement or mortar grout which is applied on the concrete surface just before placing fresh concrete. This type of admixtures is used for pavements overlays, screed over roof provision, repair works, etc.
Bonding admixtures are water emulsions and they are made from natural rubber, synthetic rubbers, polymers like vinyl chloride, polyvinyl acetate, etc.
14. Fungicidal or Germicidal Admixture
To prevent the growth of bacteria, germs, fungus on hardened concrete structures, it is recommended that the mix should have fungicidal, germicidal, and insecticidal properties.
These properties can be developed by adding admixtures like polyhalogenated phenols, copper compounds, and dieldrin emulsions, etc.
15. Coloring Admixtures
Coloring admixtures are the pigments (watercolor) that form color in the finished concrete. The admixtures used to produce color should not affect the strength of concrete.
General coloring admixtures are added to cement in a ball mill, then colored cement can be obtained which can be used for making colored concrete. Some of the coloring admixtures and their resultant colors are tabulated below.
Advantages of Admixture
Admixture Benefits are given below,
- Admixtures in concrete can accelerate the setting time.
- Some admixtures have enzymes that work as an anti-bacterial agent.
- Admixtures added in concrete can reduce the initial strength but increase the strength of concrete.
- They help in lowering the heat of hydration and reduce the chances of thermal cracking in concrete.
- It improves concrete resistance against the freeze-melting effect on concrete.
- It cut off cement quantity requirement in concrete and which makes concrete economical.
- Improve practicality of concrete.
- Admixtures provide early initial strength in concrete.
Disadvantages of Admixture
disadvantages of admixtures as given below,
- CaCl2 high added in concrete can increase the risk of corrosion of steel – not allowed in reinforced concrete.
- Some admixtures are more expensive and less effective.
- It can increase drying shrinkage in concrete.
- They provide less resistance to sulfate attacks.
- Concrete Cube Fails In Test After 28 Days
- Concrete – Its Materials, Properties, Tests & Cost
- How To Make Concrete – Batching, Mixing, Transporting & Placing
- How To Check Compressive Strength Of Concrete