Refer to these two IS codes to better understand admixtures for concrete
Imagine your site is located in a remote place. The terrain is harsh and you cannot set up an RMC plant. So, you install the plant 10 km away. Now, considering the time taken to reach the tremix truck to the site and then pour the concrete, you need 1 hour. What if the concrete starts hardening in the truck?
How will you ensure that the concrete does not set?
Yes, you will use Admixtures.
What are admixtures?
Concrete admixtures are materials that are added to the concrete mixture before or during the mixing process to customize the concrete's properties.
Why are admixtures used?
Admixtures are used to improve the workability of the concrete, increase its strength, reduce the amount of water and cement needed, or achieve some other desired property.
There are many different types of admixtures, including air-entraining admixtures, water-reducing admixtures, accelerating admixtures, and retarders, among others.
Admixtures are used in a wide variety of construction projects, including buildings, bridges, roads, and other structures, in order to improve the quality and performance of the concrete.
Some common applications for admixtures in concrete include:
- Improving the workability of the concrete: Admixtures such as water reducers and plasticizers can be used to make the concrete more workable, which makes it easier to place and finish.
- Increasing the strength of the concrete: Admixtures such as super-plasticizers and pozzolans can be used to increase the strength of the concrete, which makes it more resistant to cracking and other forms of damage.
- Reducing the amount of water and cement needed: Admixtures such as water reducers and super-plasticizers can be used to reduce the amount of water and cement needed in the concrete mix, which can save resources and reduce the cost of the project.
- Controlling the setting time of the concrete: Admixtures such as accelerators and retarders can be used to control the setting time of the concrete, which can be important in situations where the concrete needs to be placed or finished within a specific time frame.
- Improving the durability of the concrete: Admixtures such as air-entraining agents and water repellents can be used to improve the durability of the concrete, making it more resistant to freeze-thaw cycles and other forms of weathering.
How to determine the right type of admixture?
When numerous options are available, and you have no idea where to start, you can refer to the IS codes related to admixtures.
The Indian Standard codes are ready reference benchmark guides published by the Bureau of Indian Standards to ensure quality construction work across the length and breadth of the country. These codes are available for free and can be downloaded from the internet with a simple search. Let's look at a few of the IS codes on admixtures.
The IS code defines admixtures as, "Admixtures are materials added to the concrete before or during its mixing, to change one or more of the properties of concrete in the plastic or hardened state."
The performance of an admixture is evaluated by comparing the properties of concrete with the admixture under test with those of concrete without any admixture or with a reference admixture.
IS 9103: 1999
Specification for admixtures for concrete (first revision) lays down the procedure for such a relative evaluation of admixtures for concrete.
This code also gives uniformity tests demonstrating that a particular consignment is similar to the material that has previously been submitted to the acceptance test. This code is also provided for the evaluation of admixtures for specific use with the materials and mix proportions to be used on the work.
The code covers five types of admixtures. They are:
• Accelerators
• Retarders
• Water-reducing admixtures
• Air entraining admixtures
• Superplasticizers
The definition of these terms is mentioned in clause 3 of the code.
The code highlights that the manufacturer should mention the amount of chloride. The chloride content should be minimum. While super-plasticizers are expected to be chloride free. Moreover, as chloride may accelerate the rate of corrosion of steel, admixtures to be used for pre-stressed concrete should be chloride free. In the case of RCC, the maximum chloride content should be as per the limits mentioned in IS 456.
The code provides details on tests on fresh and hardened concrete with and without admixtures to cross-examine the properties of concrete when used with admixtures. These tests help ensure that exact doses of admixtures are used while actual concreting operations.
In addition to these, the code also provides various tests on concrete in the Annexures.
- Annex B - Test to determine the resistance of concrete against freezing and thawing.
- Annex C - Method of determination of flow of concrete of high workability - also known as the Flow Table Test.
- Annex D - Method of test for bleeding of concrete
- Annex E - Characteristic properties of admixtures
Another admixture that is used as a partial replacement for cement is GGBS. Let’s get a brief overview on the IS code on GGBS.
IS 16714
Ground granulated blast furnace slag (GGBS) is a byproduct of the iron-making industry that is produced when iron ore is reduced with coke in a blast furnace. The molten slag that is generated in the process is rapidly cooled and ground into a fine powder, which is known as GGBS.
GGBS is a cementitious material, which means that it can be used in concrete as a partial replacement for Portland cement. It is often used in this way because it can improve the performance of the concrete, including its strength, durability, and resistance to chemical attack.
GGBS is also used in other construction materials, such as grout and mortar, and as a soil stabilizer in road construction.
IS 16714: 2018 provides specifications for Ground granulated blast furnace slag for use in cement, mortar, and concrete.
The specific instructions for using admixtures in concrete will depend on the type of admixture and the manufacturer's recommendations. In general, however, the following steps can be followed to use admixtures in concrete:
- Determine the desired properties of the finished concrete and choose the appropriate admixture or admixtures to achieve those properties.
- Read and follow the manufacturer's instructions for the admixture, including any safety precautions.
- Measure the required amount of admixture according to the manufacturer's recommendations and the volume of concrete being produced.
- Add the admixture to the mixing water before adding the water to the dry ingredients, or add the admixture directly to the dry ingredients before mixing.
- Mix the concrete according to the manufacturer's recommendations and the desired consistency.
- Place and finish the concrete according to the project specifications.
- It is important to follow the manufacturer's instructions for using admixtures in concrete, as using too much or too little of an admixture can negatively affect the properties of the finished concrete.
What next?
Rigorously follow the instructions shared in the IS codes.
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