What is Admixture in Concrete | Its type and Uses in Concrete

What do you mean by “Admixture in Concrete”?

Generally, we all know that concrete is prepared by a mixture of cement, sand, aggregate, and water. By mixing these 4 ingredients in a proper proportion concrete is prepared. But this is the normal concrete. It is not having any special characteristics involved in it or else we can say that this concrete is not prepared to satisfy any special purpose. Admixture in Concrete is used to develop special properties in concrete like to increase the workability of concrete at the same water-cement ratio, boosting initial setting time, delay the initial setting time, to increase the density of concrete, to develop small air bubbles in concrete or to increase workability by reducing water content in concrete.

So to develop the above-mentioned properties in the concrete mix the Admixtures are used in the concrete. To understand and learn more about Admixtures we can refer Indian Standard Code ( IS CODE: 9103 – 1999 ).

Different types of Admixtures used in concrete are as follows:

1. Plasticizer (Water Reducing Agent):

Plasticizers are organic or a combination of organic and inorganic substances. First of all, we will try to understand why plasticizers are required to be used in the concrete mix.

Initially, mixing all the required ingredients to prepare the concrete is mixed. So now we have the basic concrete mix ready to use. But as I explained earlier that these admixtures are used in the concrete mix to boost the properties of the concrete. So as per the requirement, these admixtures are used.

As the name suggests plasticizers are the water reducing agents. So when we have prepared a fresh concrete, so in this freshly prepared concrete, a constant water-cement ratio is used which gives us constant workability.

But when we have to increase the workability of the concrete by keeping the water-cement ratio constant i.e. by not making any changes in the water-cement ratio. Plasticizers are taken into use to achieve the desired workability.

Read Also: What is Workability of Concrete?

Work of Plasticizers and Super Plasticizers:

• 1. Plasticizers allow water reduction for a given workability.
• 2. Plasticizers give higher workability at the same water-cement ratio.

Working of Plasticizers Admixture in Concrete:

Initially, we will understand the cement particle. A cement particle consists of a face that has a positive charge (+) and an edge that has a negative charge (-). When we add water in this, these particles get charged and it gets attracted to each other to form a lump.

The positive charge gets attracted to the negative charge i.e. face gets attracted to the edge. By this attraction process, the cement forms a flocculent structure.

For better understanding please refer to the diagram. Due to the formation of this flocculent structure water gets entrapped in this. So in terms of increasing the workability of the concrete this entrapped water is to be removed out. This can be done by adding plasticizers.

When plasticizers are added in the concrete it creates repulsion force and it changes this flocculent structure into the dispersed structure. It is also known as dispersed orientation or parallel orientation.

So by adding plasticizer in the concrete, it forms the dispersed structure. That means it forms face to face structure i.e. face gets attracted to the face. Due to this, the entrapped water comes out and the water level increases in the concrete.

We can also call it the water-cement ratio is increased. So as the water-cement ratio is increased in the concrete the workability of the concrete is also increased. Because of this, it is said that at constant water-cement ratio if the plasticizers are mixed in the concrete then the workability of the concrete is increased.

Effect Of Plasticizers In Concrete:

• 1. The workability of the concrete is increased.
• 2. The slump of the concrete is increased.
• 3. We can reduce the water-cement ratio if the workability of the concrete is to be kept constant.
• 4. We can reduce up to 10% of water for constant workability.
• 5. Pumping ability of the concrete is increased as the workability is increased.

Dose Of Plasticizer Admixture in Concrete:

1. 0.1% to 0.4% by the weight of the cement.

2. If there is an increase in the finer particles then we have to increase the dose of plasticizer.

Examples: Ligno Sulphonic Acid, Polyglycolester, Hydroxylated Carbonic Acid.

2. Superplasticizer (High Range Water Reducer):

Plasticizers and Super Plasticizers are more or less one and the same thing. The working of the superplasticizers is also the same as plasticizers. The only thing differentiates superplasticizers from plasticizers is that they are high range water reducers. Superplasticizers are the improved version of plasticizers. It imparts high workability by reducing friction between the particle and reducing the amount of water to be added. We can reduce up to 30% of water for constant workability.

Effect Of Super Plasticizers In Concrete:

1. The workability of the concrete is increased at a very low water-cement ratio.

2. The slump of the concrete is increased.

3. We can reduce the water-cement ratio more than the plasticizers if the workability of the concrete is to be kept constant.

4. We can reduce up to 30% of water for constant workability.

5. Pumping ability of the concrete is increased as the workability is increased.

6. It removes air bubbles from the concrete.

7. Retards setting time of concrete.

Dose Of Super Plasticizer:

1. 0.1% to 0.4% by the weight of the cement.

2. If there is an increase in the finer particles then we have to increase the dose of plasticizer.

Examples: Sulphonated Melamine Formeldihide, Modified  Ligno Sulphonated Acid, Naphthalene.

3. Accelerator:

As the name suggests accelerators are used to accelerate the strength gaining process in the concrete. Let’s understand it in detail, as the concrete is prepared but if we want to boost some special properties in the concrete like increasing the setting time of the concrete then accelerators are used. By adding accelerators in the concrete it increases i.e. it accelerates the rate of chemical reaction or hydration process in the concrete and the setting time of the cement is decreased.

Uses Of Accelerators:

1. When concreting is done in cold places.

For example, If we have to do concreting in the cold places or at the places were snowfall takes place. So due to cold weather, it is very difficult for concrete to set. At that point, accelerators are used in the concrete.

2. When we require early removal of formwork.

For example, if the construction is to be completed early or on a given time frame so at that time it becomes very important that the initial setting time of the concrete is high. After concreting if the formwork is removed early due to the use of accelerators, so the work gets completed on time.

3. When we require the early strength of concrete.

4. When the rapid setting is required.

Examples: NaCl (Sodium Chloride), Na2So4 (Sodium Salphate), NaOH (Sodium Hydroxide), Na2Co3 (Sodium Carbonate), K2So4 & KoH, CaCl2 (Calcium Chloride)

Note:

• CaCl2 (Calcium Chloride) – When added up to 2% by the weight of the cement it acts like accelerator.
• CaCl2 (Calcium Chloride) & NaCl (Sodium Chloride) permits concreting in very cold weather (-23 Degree Celcius).
• All chloride-based accelerators promote corrosion in steel. So we should not use in reinforced concrete, steam cured concrete and water retaining structures.

4. Retarder:

As the name suggests retarders are used to slow down the strength gaining process in the concrete. Let’s understand it in detail, as the concrete is prepared but if we want to boost some special properties in the concrete like decreasing the setting time of the concrete then retarders are used. By adding retarders in the concrete it decreases i.e. it slows down the rate of a chemical reaction or hydration process in the concrete and the setting time of the cement is increased.

Uses Of Retarders:

1. When concreting is done in hot weather.

For example, when concrete is to be done in the Gulf countries or where the weather is very hot. At such locations use of retarders becomes very important. If we don’t use retarders then due to heat of hydration and addition to it the hot weather cracks develop in the concrete.

2. When we require a delay in the strength of concrete.

3. When a slow setting of concrete is required.

4. Concrete has to be transported for a long time.

For example, if suppose the RMC plant is far from the construction site were concreting is to be done, so the distance between these two points is very high. So to prevent the concrete from setting before reaching the destination it becomes very important to use retarders in the concrete.

Examples: CaSo4 (Calcium Sulphate), Sugar, Starch, FeCl3 (Feric Chloride), Cellulose, CaCl2 (Calcium Chloride).

Note:

• If 0.2% of sugar by mass of cement is added then the final setting of concrete may exceed till 72 hours.
• CaCl2 (Calcium Chloride) – When added more than 2% by the weight of the cement it acts like retarders.

5. Air Entraining Agent:

As the name suggests air-entraining agents are the admixtures used to boost the properties of concrete by developing very small air bubbles, let’s understand it in detail. Initially, when we mix the concrete we all know that it contains air bubbles in it.

So these air bubbles has to be removed out of the concrete mix by compaction because it is not good for the further strengthening of the concrete. But by adding air-entraining agents in the concrete mix it introduces air in the form of minute bubbles or uses the air present in the concrete and turns them into minute bubbles and distributed uniformly in concrete.

Due to this the workability of the concrete is increased and by reducing the bleeding and segregation in the concrete. Let’s have a look at the benefits in concrete.

Benefits Of Air Entraining Agents In Concrete:

• It reduces bleeding and segregation in the concrete.
• Improves the workability of concrete.
• Admixture in Concrete also Reduces unit weight and reduces density.
• Improves resistance of hardened concrete to damage from freezing and thawing.
• Decreases the permeability.

Demerits In Concrete: It reduces the strength up to 5% of concrete due to the air bubbles which are 1% of the total mass.

6. Water Proofer: As the name suggests waterproofers are used in the concrete mix to improve the waterproofing quality of the concrete. Basically it makes concrete water-resistant.

Work:

1. Makes cement mortar or concrete impervious to water.

2. Developed resistance against absorption of water in concrete.

Examples: Soda and Potash Soap, Resin and Vegetable Oil, Coal Tar.

7. Bonding Admixture:

It is used in concrete as an admixture to develop the bonding properties in concrete. Let’s understand it in detail when we have to use the concrete in big structures like dams. Where concreting is done in parts because it cannot be concreted in one take. So at that time, it becomes very important to use bonding admixtures in the concrete mix because it helps to develop a proper bond between the old concrete and new concrete surface properly.

Examples: Synthetic Latex Emulsions made by Natural Rubber or PVC (Polyvinyl Chloride).

8. Colouring Agent:

Colouring agents are used in concrete to provide colour in the concrete.

Examples:

• Chromium Oxide – Green colour.
• Ferrous Oxide – Black colour.
• Red Oxide – Red colour.
• Raw Umber – Brown colour.

The mixing process of admixture in concrete:

Admixtures have to be mixed before mixing the concrete i.e. when we are adding all the required ingredients for concrete or admixtures has to be mixed during the mixing process of concrete i.e. when we start the mixer. It has to be mixed slowly when the mixing process is going on.

Different Works Of Admixture in Concrete:

1. Workability increases without changing the water-cement ratio.
2. Reduce water-cement ratio without change of workability.
3. To reduce segregation and bleeding.
4. Increase the durability of concrete by reducing freezing and thawing, controls expansion and by providing strength between old and new concrete.
5. Decreases density of concrete.
6. Improves the pumping ability of concrete.
7. Accelerates the initial setting time of concrete.
8. Retards the setting time of concrete.
9. Reduces permeability in concrete.
10. Increases the resistance to the chemical attack.