Besides cement, aggregate, sand, and water, concrete admixtures (also known as concrete additives) modify the permeability, durability, ease of handling, and strength of a typical concrete mixture. This article will discuss the types of concrete admixtures and how each type changes the usual properties of concrete and the particular conditions under which they’re used.
Types of Concrete Admixtures
Concrete admixtures fall into two broad categories depending on their nature; chemical admixtures and mineral admixtures. Chemical admixtures, as the name suggests, are manufactured substances that are added to concrete during the mixing process to improve its quality or quicken some processes such as curing.
Chemical admixtures are divided into various categories, which will be discussed below. Mineral admixtures, on the other hand, are finely divided mineral components used to influence the properties of concrete in various ways. They’re also available in multiple forms.
Categories of Chemical Admixtures
1.Air entrainers
These are admixtures used to increase concrete durability once it hardens, especially in places prone to freeze and thaw. Moreover, air entrainers make the workability of plastic state concrete easier and more manageable. They work by entrapping more air bubbles in the concrete.
2.Water reducers
As the name suggests, these admixtures reduce the amount of water needed to prepare the concrete mixture while maintaining given workability. Additionally, water reducers also boost the concrete’s strength once it hardens, thereby increasing its durability.
3.Set retarders
Set retarders admixtures ‘retard’ the process of concrete hardening they allowing more time for the transportation of concrete from its area of preparation to its area of use. You should note that prolonging the time the concrete takes to harden translates to more setting time for the concrete.
4.Set accelerators
Set accelerators are the opposite of set retarders. They speed up the time concrete takes to set and hence starting and completing the curing process early. They are used in frigid regions to reduce the risk of frost damage to the concrete. These admixtures also enhance early strength development for concrete, even when it is still curing.
5.Superplasticizers
Superplasticizers are an advanced form of water reducers. They transform low-slump concrete from being stiff to easy flowing concrete. While reducers reduce the amount of water needed by about 15%, superplasticizers reduce this amount by about 30%. They also increase concrete strength and increase its durability.
6.Specialty admixtures
These are the other forms of chemical admixtures that perform other specific functions in concrete. Such functions include inhibiting the concrete’s corrosion, controlling concrete shrinkage, inhibiting alkali-silica reactivity, or even changing concrete color.
Types of Mineral Admixtures
1.Pozzolans
A pozzolan is a substance that, when mixed with water, forms cement and contains siliceous and aluminous material. Pozzolan admixtures come in many forms. That is:
- fly ash-which increases concrete workability before it dries and boosts its strength once it dries
- silica fume– which improves concrete compressive and bond strength
- rice husk ash– which has a waterproofing effect in concrete.
2.Hydraulic admixtures
This type of mineral admixtures improves the drying capacity of concrete when it is in contact with water. They ensure that the concrete dries without compromising on its strength, thereby ensuring durability.
Conclusion
Concrete admixtures are substances that are added to concrete during its mixing process to improve its properties. They are divided into two categories; chemical and mineral additives. Thus, anytime your buying any concrete additive, make sure you well know its purpose and why you need it.
