The purpose of chemical admixtures is to give concrete additional properties in order to better handle a range of variables such as pumping requirements or placing concrete in extreme weather conditions. According to the National Ready Mix Concrete Association (NRMCA), admixtures are added to concrete to increase “durability workability or strength characteristics of a given concrete mixture.”
The Portland Cement Association further adds that, “Admixtures are classed according to function. There are five distinct classes of chemical admixtures: air-entraining, water-reducing, retarding, accelerating, and plasticizers (superplasticizers).”
Air-entraining is the process of intentionally inserting microscopic air bubbles into the concrete. It is done to improve the concrete’s workability as well as to increase its durability in extreme freezing and thawing temperature cycles. The only downside to air-entraining is that the concrete experiences a 5% reduction in strength for each 1% of air added; however, the segregation and bleeding is diminished or completely eliminated.
Accelerators are another type of chemical admixtures and are used to increase the early strength of concrete by causing it to harden faster. They are also used to protect concrete from extremely cold weather or if a project has a tight deadline. Chemical accelerators can be either chloride based or non-chloride based and one common inexpensive example is calcium chloride.
For situations where you need to slow down or delay the hardening process such as requiring a slower pour due to the scope of the project or in very hot temperatures, a retardant may be used. This gives crews more time to effectively place and finish the concrete. Sugars, lignin, and citric and tartaric acids are all common retardants.
Chemical admixtures can be in a liquid or powder form and are added to the concrete mix along with portland cement, aggregate and water and typically make up less than 5% of the total concrete mix. Regardless of whether chemical admixtures are derived from natural or manufactured chemicals, they are added to the concrete either immediately before or during the mixing process.
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