What Is An Admixture in Construction?
What Is An Admixture in Construction?
An admixture is a substance added to concrete or mortar to modify its properties. These modifications can improve the suitability of the material for a specific purpose, reduce production costs, or conserve energy.
Admixtures can be either natural or manufactured chemicals that are added to the mixture during mixing.
The American Concrete Institute, ASTM International, and other organizations have established standards for chemical admixtures in construction materials.
Admixtures can affect the workability, durability, or strength of fresh or hardened concrete.
Concrete admixtures are substances that are added to concrete to alter its properties in some way. There are several different types of concrete admixtures:
- Water reducing admixtures are used to reduce the amount of water needed in a concrete mix, while maintaining its workability.
These admixtures can also improve the strength and durability of the concrete, and prevent cracking, segregation, and bleeding.
- Retarding admixtures slow down the rate of cement hydration and increase the initial setting time of the concrete.
These admixtures are often used in hot climates to prevent the concrete from setting too quickly.
- Accelerating admixtures are used to reduce the initial setting time of concrete and speed up the hardening process.
These admixtures are useful for early removal of formwork, emergency repairs, and construction in cold climates.
- Air entraining admixtures improve the durability of concrete in freezing and thawing conditions by creating millions of non-coalescing air bubbles throughout the mix. They also improve the workability and chemical resistance of concrete, and reduce the weight and modulus of elasticity.
- Pozzolanic admixtures are used to prepare dense concrete mixes suitable for water retaining structures like dams and reservoirs.
They reduce the heat of hydration and thermal shrinkage, and can prevent or reduce risks such as alkali aggregate reaction and sulfate attack. Pozzolanic admixtures can be natural or artificial, and include materials like clay, fly ash, and blast furnace slag.
- Grinding aids are used to improve the efficiency of cement grinding, and reduce energy consumption. These admixtures are often based on amino acids or glycols.
- Corrosion inhibitors are used to protect steel reinforcement in concrete from corrosion.
These admixtures can be either organic or inorganic, and may include compounds like calcium nitrite or sodium benzoate.
What Is The Ratio Of Admixture In Concrete?
To determine the correct amount of admixture to use in concrete to achieve the desired results, the Marsh cone test can be performed.
This test involves mixing cement, water, and a specific amount of admixture, and then measuring the time it takes for the mixture to flow out of a conical brass vessel with a smooth aperture at the bottom.
The mixture used in the test should consist of 2 kilograms of cement and 1 liter of water, with the admixture added at a rate of 0.1% by weight of the cement.
\The test can be repeated with different proportions of water to cement (w/c) ratios, different brands of cement or admixture, or different amounts of admixture, with one variable held constant.
The mix that results in the lowest Marsh cone time is considered the optimal dosage for that cement, admixture brand, and w/c ratio.
What Is Water Reducing Admixture In Concrete?
Water-reducing admixtures are used in concrete to improve its strength and workability while also reducing the amount of water needed in the mixture.
This can lead to lower cement content, which can save costs, or increased slump for the same water-to-cement ratio and cement content.
These admixtures are recommended to avoid using too much cement for a given water-to-cement ratio at the desired consistency and to reduce bleeding by minimizing the amount of free water. The choice of water-reducing admixture depends on the fresh concrete’s cohesion.
Admixtures that enhance cohesion and may entrain a small amount of air should be used in mixes that bleed excessively or segregate, while cement-rich concretes with a sticky mortar phase benefit from admixtures that lower water viscosity and disperse cement particles through deflocculation.
Which Accelerator Is Used In Concrete As Admixture?
An accelerating admixture is a chemical substance that is added to concrete to either shorten the time it takes for the concrete to set or to increase the rate at which it gains strength, or both.
Some common inorganic accelerators include soluble chlorides, carbonates, silicates, and fluosilicates, while common organic accelerators include triethanolamine.
Calcium chloride is a commonly used accelerator, but it must meet certain requirements and should not be used in large amounts or in cold climates as it can cause rapid stiffening, drying shrinkage, and reinforcement corrosion.
Accelerators are often used in cold weather to counteract the slowing of the curing and setting process.
Contractors may use accelerators to allow for earlier form removal, earlier concrete surface finishing, and even earlier loading in certain cases.
Other suitable accelerators for reinforced concrete include sodium thiocyanates and other thiocyanate salts, triethanolamine and other alkanolamines, and products based on sulfates, nitrates, and formates.