An overview of the most popular types of concrete admixtures.
Concrete admixtures are chemicals added to concrete mixtures during mixing to create properties that increase the quality of concrete, its usability, acceleration, or deceleration of setting time, among other specific results.
Here are the most popular types of concrete admixtures:
Accelerating concrete admixtures are chemicals used to escalate the rate of concrete strength development or to shorten concrete setting time.
By stepping up the process, a contractor is able to remove forms much earlier, or to get onto a concrete surface ahead of time for finishing.
Accelerating concrete admixtures are commonly used during cold weather. This is due to their ability to counter the effects of low temperatures, which slow down the curing and settling process.
However, these types of chemicals can be used even during warm weather to hasten the curing process.
Calcium chloride is arguably one of the best concrete accelerator components ever. This chemical is, however, no longer a popular option as it causes severe corrosion in steel reinforced concrete.
Most admixtures in the market today are made using nitrate and thiocyanate, although the two salts are not as effective as calcium chloride. Nitrate aids settling of concrete while thiocyanate hardens the material.
A set retarding admixture is a chemical that delays the compound reaction that occurs when the settling process begins. It works by increasing the time of settling of portland cement paste and subsequently that of its mixtures.
Set retarding concrete admixtures are used during hot weather to counter the effects of high temperature, which quicken the settling process.
These admixtures are commonly used in construction of concrete pavement as they allow more time for finishing while eliminating cold joints in concrete.
Set retarding concrete admixtures can also be used to prevent cracking due to form bending that can occur when horizontal slabs are positioned in sections.
A water-reducing admixture is defined as a chemical product that helps create a desired concrete slump at a lower water-cement ratio than what is the norm.
The admixtures are used to gain specific concrete strength using lower cement content. By using lower cement content, a contractor is able to cut carbon emissions as well as energy usage per volume of concrete generated.
Water-reducing admixtures improve the properties of concrete, making it ideal for placing under difficult situations. As such, the admixtures are commonly used in low-slump concrete, bridge decks, and reinforcing concrete.
As the name implies, shrinkage-reducing concrete admixtures are chemicals used to produce concrete with greatly reduced drying shrinkage and the potential for subsequent cracking.
These admixtures are commonly used in situations where shrinkage cracking could lead to durability problems or where large numbers of shrinkage joints are undesirable for economic or technical reasons.
Shrinkage reducing admixtures works by lowering capillary tension of pore water – which is the main cause of drying shrinking. They use calcium sulfo-aluminate or calcium aluminate and calcium oxide for shrinkage compensation.
Corrosion-inhibiting admixtures are used to alleviate corrosion of reinforced steel in concrete due to aggressive chloride exposure conditions. This results in a more durable reinforced concrete structure.
The admixtures work by forming a passivation layer on the steel surface to protect the material against chloride and other salt ion damage. The chemicals also reduce the sorptivity of concrete, thus minimizing the amount of water and dissolved salts that can infiltrate to corrode structural steel.
Air entraining admixtures are used to create ultra-stable, small and closely spaced air bubbles to provide concrete with extra protection.
These admixtures improve the durability of concrete when experiencing cyclic freezing and thawing weather. Air entraining admixtures work by preventing an increase in internal pressure, which forms cracks during low temperatures.
Air entraining admixtures can be used together with most types of admixtures.
Also known as high range water reducers, superplasticizers are synthetic water-soluble organic substances used to produce high strength and high performance concrete.
Superplasticizers reduce the amount of water required to achieve a certain stability of concrete without reducing workability. The admixtures reduce both the water-cement ratio and cement content while increasing slump.
They are mainly used to produce flowing concrete with 7 to 9-inch slump for use in heavily reinforced structures and in situations where sufficient consolidation by vibration is not easily attainable.
Admixtures are chemicals added to concrete to get new properties for the material, while additives are added to cement during manufacturing to get new properties for the cement.
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