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Admixtures Meaning
Concrete admixtures are substances that modify the properties of concrete during the mixing process. They can improve the strength, workability, durability, and other characteristics of concrete, making it more suitable for specific applications or environments.
Water-reducing agents are examples of concrete admixtures that reduce the amount of water needed in the mix, improving the strength and workability of the concrete.
Set retarders are used to slow down the setting time of concrete, while accelerators speed it up.
Air-entraining agents introduce tiny air bubbles into the concrete, which can improve its durability and freeze-thaw resistance.
Superplasticizers increase the flowability of concrete without the need for additional water.
Coloring agents can add aesthetic value to the concrete.
What Are Concrete Admixtures?
Admixtures are manufactured or natural chemicals that are added to the concrete during or before mixing. They are used to give some special properties to fresh or hardened concrete.
Admixtures may enhance the workability, durability, or strength characteristics of a given concrete mixture, they are also used to overcome tough construction situations, such as cold or hot water placement, early strength requirements, pumping requirements, or low water-cement ratio properties.
Types Of Concrete Admixtures
The following are the most common chemical admixtures which can reduce the cost of construction, modify characteristics of hardened concrete, ensure the quality of concrete during mixing/placing/curing/transporting and overcome certain emergencies during the operation of concrete.
- Water Reducing Admixtures.
- Air Entering Agents.
- Set Retarding admixtures.
- Set Accelerators.
- Superplasticizers.
1. Water Reducing Admixtures:
Water-reducing admixtures are chemical admixtures designed to reduce the water: cement ratio of concrete without adversely affecting the natural philosophical properties. Water reducing admixtures can reduce the amount of mixing water required to reach a given slump. This can result in a reduction of the w/c ratio which leads to more durable concrete and increased strengths.
To make concrete durable and high quality, reducing the w/c ratio has been identified as one of the most important factors. Water-reducing admixtures also reduce segregation and the flowability of the concrete improves. This is why it can be used for concrete pumping applications as well.
2. Air Entering Agents:
Air Entering admixtures smoothen the development of a stable air-void system within the concrete that increases the durability of concrete. These admixtures transmit the air entrainment that results in,
- Increased resistance to decline from cycling freezing and defrosting.
- Improve workability and cohesiveness of concrete placement.
- And can reduce segregation and bleeding.
3. Set Retarding Admixtures:
Set retarding admixtures are used to slow down the speed of the reaction in between cement and water by slowing down the rate of the hydration products and reducing the rate of water penetration to the particles of cement. With this, concrete stays workable for a longer period than it is in actuality.
Retarding Admixtures can be used to,
- Prevent early stiffening in hot weather (20 degrees)
- When a large pour of concrete needs long hours to place static.
- To place concrete in several layers with no joints
- To reduce temperature for mass concrete
- In case of delay or damage, it can prevent the setting of the concrete in the truck.
4. Set Accelerating Admixtures:
Set accelerating admixture affects the rates of reaction between cement and water from which the overall hydration rate increases. The use of set accelerators in concrete provides a shortening of setting time and can increase the development of early strength. It can be used to increase either the rate of stiffening or setting of the concrete or the rate of hardening and early strength gain to allow earlier demoulding or formwork striking.
In cold countries, the prime use of set accelerators is to control the setting time of floor slabs in cold conditions as extending setting time delays finishing and power trowelling. They are also used to reduce the damage to concrete by freezing when concreting in cold weather and to allow the earlier removal of formwork, the reduction in setting time is typically 1 to 2 hours and it is affected by accelerator type, temperature, and dose.
5. Superplasticizers:
Superplasticizer is a high range water reducer and is a synthetic water-soluble organic substance that reduces the amount of water required to achieve concrete stability, reduces water-cement ratio, increases slump, and reduces cement content.
Without reducing the workability use of plasticizers allows the water reduction in concrete up to 30 percent. The use of these admixtures is practiced for the production of flowing, self-compacting, self-leveling, and the production of high-performance concrete and high strength.