Admixtures in concrete

An admixture is a substance which can be added to concrete to achieve or modify its properties. Admixtures are added to the concrete, in addition to cement, water and aggregate, typically immediately before or during the mixing process.

Admixtures can be used to reduce the cost of building with concrete, or to ensure certain required properties or quality of the cured concrete. If problems arise with the concrete during the construction process, admixtures can be used as an emergency measure to try and prevent failure. In addition, some of the main functions of using admixtures include:

• Water-reducing: Can reduce the water content needed to reach a required slump by 5-10%.
• Retarding: Slow the setting rate of concrete, keeping it workable and are often used to counteract the accelerating effect of hot weather.
• Accelerating: Increase the rate of early-strength development and reduce the time required for curing.
• Superplasticizers/plasticizers: Can reduce water content by 12-30% to make a highly fluid but workable form of concrete known as flowing concrete.
• Corrosion-inhibiting: Used to slow the corrosion of reinforcing steel in the concrete. Often used in marine structures, bridges and others that will be exposed to chloride in high quantities.
• Air-entraining: Small bubbles of air formed uniformly through the concrete mix to increase cohesion and resistance to freeze-thaw degradation.
• Improving the curing of the concrete.
• Providing waterproofing properties.
• To improve hardness.
• Providing colour.
• Offsetting or reducing a chemical reaction.
• Aeration to reduce the weight.
• Offsetting or reducing shrinkage.
• Dispersing cement particles when mixed with water.
• Alkali-silica reactivity reduction.

Admixtures are usually provided in a liquid form. Some admixtures, such as pigments, pumping aids and expansive agents, are typically added manually from pre-measured containers as the amount used is very small.

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