Concrete being the most popular material nowadays, it is used for various purposes under different site conditions. In some conditions, ordinary concrete fails to give the required quality performance or durability. In such situations, admixtures are used to modify the properties of ordinary concrete so as to match the requirement, thereby making it more suitable for the situation.
Admixtures are ingredients that are added to the concrete batch immediately before or during mixing. They confer certain beneficial effects to concrete, including water resistance, sulfate resistance, controlled setting and hardening, improved workability, increased strength, etc. There are several types of admixtures based on their functions. For example,
i) Plasticizers or Water reducing admixtures
ii) Superplasticizers
iii) Retarding Admixtures
iv) Accelerating Admixtures
v) Damp-proofing and Waterproofing Admixtures
vi) Corrosion Inhibiting Admixtures
Plasticizers or Water Reducing Admixture
Plasticizers are the admixtures used in the concrete mix to improve its qualities and make it more suitable for construction.
It’s effects on concrete:
1)Reduce the water/cement ratio without any effect on workability.
2)To increase the compressive strength
3)To avoid cracks and shrinkage in concrete.
The function of plasticizers is essentially to fluidify the mix, decrease the friction between particles and to improve the workability of concrete, grout or mortar. Water-reducing admixtures are recommended both for avoiding excessive cement content for a given free water/cement ratio at the required consistency class and to reduce any tendency towards excessive bleeding by minimizing the free water content. Plasticizers can reduce up to 10-15% water.
Superplasticizers
Superplasticizers (SPs), also known as high range water reducers. It is used in making high strength concrete. Superplasticizers are more effective than plasticizers and allow reduction in water content by 30% or more. As a result, superplasticizers are used to get higher strength concrete in construction projects all over the world.
Retarding Admixture
Retarder admixtures slow down the setting time of the concrete, thus, it remains workable for a longer time than normal. It helps to carry or transport the concrete for long distance sites.
Many times retardation of setting time offered by admixtures will not be sufficient. Thus, retarders are mixed with plasticizers or superplasticizers at the time of production instead of adding them separately. Such admixture is known as retarding plasticizers or retarding superplasticizers. Retarding plasticizers or superplasticizers are an important category of admixtures which are frequently used in the Ready mix concrete industry for the various purposes like – retaining the slump loss, during high temperature, long transportation, to avoid construction or cold joints, slip form construction and for regulation of heat of hydration. Using of retarders is very common in the construction industry in Bangladesh.
Accelerating Admixture
Accelerating admixtures are added in the concrete to increase the rate of early strength development in concrete so as to:
1) Permit earlier removal of shuttering
2) Reduce the required period of curing
3) Fasten the time of building structure
Accelerators help to work in the cold weather. Normally in winter, the curing and setting time of the concrete slows down. A contractor can use it anytime when the curing process needs speed. The admixture may allow a concrete worker to remove shuttering earlier, get onto a concrete surface earlier for finishing, and sometimes for precast production.
Water Proofing and Damp proofing Admixture
Waterproofing admixture is a type of construction chemical that is added to concrete mixtures to improve the water resistance of the finished structure. This is an important feature in construction, as water damage can cause a number of problems, such as cracking, spalling, and corrosion of steel reinforcement. Waterproofing admixtures can also help to reduce the amount of water that is absorbed by the concrete, which can improve the overall durability of the structure.
Waterproofing admixtures are used to fill the pores of concrete and reduce the water permeability of concrete. It is one of the most used construction chemicals. Waterproofing admixtures can be obtained in powder, paste or liquid form. It reduces the water ingress in concrete structure by integral waterproofing process and makes it more durable and strong. Waterproofing admixture is also known as damp proofing or permeability reducing admixture. Overall, waterproofing admixtures are an essential part of construction, as they help to protect buildings and other structures from water damage. They also play an important role in improving the overall durability of the structure and prolonging its lifespan.
Corrosion Inhibiting Admixture
Corrosion of steel in concrete structures plays a significant role in affecting the service life of the concrete structures. It is hard and costly to repair and replace as well. Various methods have been developed with the intent of preventing corrosion and to enhance the service life. The concrete admixture industry has developed new techniques that are claimed to prevent the steel from corrosion. These techniques are the introduction of corrosion inhibitors into the carbonated or chloride contaminated concrete. Inhibitors are mixed to the concrete to improve its quality and integrity. In recent years the use of inhibitors in producing high-performance concrete has increased significantly as a result of the harsh environment.
