Admixture in Concrete Technology

Admixture in Concrete Technology

Definition: Admixtures are artificial or natural additional materials other than aggregates, cement, and water that can be added to the concrete immediately before or during mixing to change one or more properties of the concrete in the plastic or hardened state as required in our structure.

Types: There are two major types of Admixtures

• Chemical admixtures (Artificial Admixture)
• Mineral admixtures (Natural Admixture)

Types of Admixtures:

1. Plasticisers & superplasticisers
2. Retarders
3. Accelerators
4. Waterproofing Compounds
5. Air entraining agents
6. Pozzolanas
7. Alkali Aggregate Expansion Preventing Admixture
8. Anti-washing Admixtures
9. Grouting Admixtures
10. Air Detraining Admixture
11. Gas Forming Admixture
12. Corrosion Preventing Admixture
13. Bonding Admixtures
14. Fungicidal or Germicidal Admixture
15. Coloring Admixtures
16. Plasticisers & superplasticisers

Normally, if you want to increase the workability of concrete (Note 1) you will need to add more water, with the help of Plasticisers & superplasticisers, workability of concrete will increase without addition of water, it will help in achieving lower water/cement ratio, in fact, it helps to increase the concrete strength as well. 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%

The major difference between Plasticisers & Superplasticisers is that Plasticizers are used for moderate increase of workability while Superplasticizers are used where very large increase in workability is required. Plasticisers & Superplasticisers are also known as Water-reducing admixtures

Merits:

• It improves the workability of concrete
• It enhances the strength of concrete
• It helps to form a good bond between concrete and steel.
• It prevents cracking, segregation, honeycombing and bleeding

Example of Plasticizer: 

1. Lignosulphonated formaldehydes
2. Sulphonated melamine-formaldehyde condensate, sulphonated naphtaleneformaldehyde condensate

• Hydroxylated carboxylic acids

Note 1: Workability of concrete means the ease of placing the concrete, a workable concrete will be easily placed and compacted without any segregation.

2. Retarders

They are used for retarding (delaying) the initial setting time of concrete. This is particularly required when longer placing times are desired and to prevent formation of cold joints when casting large concrete or where the concrete has to be transported to a long distance. It is however important to note that retarders reduce early strength of concrete e.g., 1-day and 3-day strength. but 28 days strength is not affected.

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. Accelerators

They are used for accelerating the initial strength of concrete. Basically, accelerators increase the 1-day (up to 50 %) and 3-days (up to 30 %) strength of concrete; however, their ability to increase strengths decreases after 3-5 days. Therefore, accelerators are commonly used in precast concrete elements for early removal of moulds, though they are expensive. 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. Waterproofing Compounds

Waterproofing or Damp-proofing admixtures, make the concrete structure impermeable against water and to prevent dampness on the concrete surface. These are chemicals that have double actions of blocking capillary pores and reducing the size of pores that may allow percolation of water and therefore have plasticizing effect and help to reduce water/cement ratio. Examples are aluminum sulphate, zinc sulphate, aluminum chloride, calcium chloride etc

5. Air Entraining Agents

They are chemicals that introduce small air bubbles of size less than 45microns and disperse uniformly into concrete. These work like ball bearings and increase the workability of the concrete; however, the density and strength of concrete are reduced.

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.

The entrained air has following advantages:

1. It helps to lower the water/cement ratio and thus reduces permeability of concrete (Note 2).
2. It makes concrete mixes cohesive and increases compatibility of concrete.
3. It reduces bleeding in concrete (Note 3).
4. It is particularly useful in freeze-thaw (Note 4) conditions for increasing durability of concrete in cold climatic conditions

Note 2: Permeability of concrete, when something is permeable, it can allow liquid or gas to pass through it that means it is porous, so, Permeability of concrete is the amount of water or air that can enter into concrete matrix, so Air Entraining Agents reduces permeability of concrete, Understood?

Note 3: Bleeding in concrete, when you are casting (Concreting), you will sometimes observe that the surface of the concretes has free water, that water on the surface is regarded as Bleeding which is also called “Laitance”, “Bleeding occurs in concrete when coarse aggregates tend to settle down and free water rises to the surface of that concrete”

Note 4: Freeze-thaw condition, this usually occurs during the winter, where the water in the concrete freezes and this freezing causing damaging effect like cracking, it is known as freezing and thawing.

6. Pozzolanas

These are chemical materials which can react with lime in the presence of water to improve the durability of cement. The lime is liberated or produced during the hydration process of cement. In fresh concrete, pozzolanas improve the workability, reduces shrinkage, bleeding and segregation potential. It also reduces the setting and hardening time and hence cracking in hardened concrete since the amount of heat liberated is reduced. 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.

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.

7. 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.

8. 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 type of concrete admixtures is prepared from natural or synthetic rubbers, cellulose based thickeners etc.

9. 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

 

10. Air Detraining Admixture

This is 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.

11. Gas Forming Admixture

This admixture 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.

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

• 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

• 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