Chemical Admixtures Used In Concrete

TYPES OF ADMIXTURES

Admixtures are categorized into two main groups – chemical admixtures and mineral admixtures. Chemical admixtures are manufactured chemicals which are added to the concrete during mixing to modify the properties of the fresh or hardened concrete. They are normally added to the concrete in very small quantities, in the range of 0.005% – 2% by mass of cement. Mineral admixtures occur naturally or as a by-product of an industrial activity, and are normally added to the concrete in large amounts. Examples of mineral admixtures include fly ash and ground granulated blastfurnace slag. This article will focus on chemical admixtures only. Some common types of chemical admixtures are discussed below:

Accelerators

These admixtures increase the rate of hydration of hydraulic cement. When added to concrete, they shorten the setting time and accelerate the hardening or strength development of the concrete. They enable earlier release of concrete from precast moulds thus speeding up construction. They also reduce segregation and increase density and compressive strength. Concrete cures faster and therefore uniform curing in winter and summer can be achieved. They allow for early use of concrete floors by accelerating the setting of concrete. They reduce water requirements, bleeding, shrinkage and time required for initial set.

Retarders

This type of chemical admixtures decreases the initial rate of reaction between cement and water and thereby retards the setting of concrete. Retarders act by forming a coating around certain components of the cement and they slow down the formation of reaction products in the concrete. Setting and hardening are therefore delayed, and early compressive strength is reduced. These admixtures are particularly useful in conditions where the rate of stiffening of concrete is too fast for the time required for concrete transportation and placement before it sets. Retarders can delay setting for up to more than 6 hours. Retarding admixtures improve the workability, cohesion and extend the setting time of concrete, and provide protection against delays and stoppages in construction work. They keep concrete workable for extended periods of time and in large constructions, they help to prevent cold joints. They also reduce bleeding and segregation of concrete where poor sand grading is unavoidable. Retarders also improve pumpability of concrete by extending the setting time and improving the workability.

Plasticizers

Plasticizers either increase the workability of freshly mixed concrete without increasing water cement ratio or maintain the workability with a reduced amount of water and are therefore referred to as water reducing admixtures. Their function is to reduce the water content of the mix, usually by 5 to 10%, and sometimes even up to 15%. Thus, the purpose of using a water reducing admixture in a concrete mix is to allow a reduction in the water cement ratio while retaining the desired workability or, alternatively, to improve its workability at a given water cement ratio. The actual reduction in water depends on dose of admixtures, cement content, type of aggregate used, ratio of cement, fine and coarse aggregate etc. Therefore, in order to achieve the optimum properties of the concrete, it is essential to do trial mixes with the actual material to be used on the job. With the use of plasticizers, it is possible to increase the workability of the concrete without changing water-cement ratio and thus without reducing the compressive strength of the concrete. This is particularly useful when concrete pours are restricted either due to congested reinforcement or due to thin sections.

Super-plasticizers
Super-plasticizers perform a similar function to plasticizers, but are able to reduce the water content by up to 30%. These admixtures are a different class of water-reducers which may be used without facing the problems associated with using plasticizers in larger quantities, such as bleeding, segregation and hardening. They are an extended version of plasticizers, and they increase the workability of concrete, with the ability to increase the slump from 75mm to 200mm, while maintaining the cohesiveness of the concrete. Super-plasticizers are particularly useful for self-compacting/flowing concrete for application in elements such as heavily reinforced sections and inaccessible areas. They don’t entrain air and therefore reduce permeability.

Air-entraining admixtures

Air entraining admixtures are used to form small (smaller than 1mm), stable, evenly distributed air bubbles in the concrete mix. Entrained air improves cohesion and reduces bleeding; it improves the compaction of low-workability concrete, it improves freeze/thaw resistance of hardened concrete and it improves handling properties of concrete.

Water-resisting admixtures

Water-resisting admixtures are materials that reduce the capillary absorption of water into hardened concrete. Their main function is to reduce the permeation of water into the concrete. They do this by reducing the size, number and the continuity of pores in the concrete; as well as by blocking the pore structure by adding fine-unreactive or reactive fillers. They also prevent permeation by lining the capillaries with a hydrophobic material to prevent water from being drawn in by capillary suction. They are particularly effective against rain, surface water, low pressure heads in structures, water ingress in tidal and splash zones as well as the build-up of absorbed chloride at the surface of the concrete. They are often used in combination with water-reducing plasticizers. Water-resisting admixtures assist with enhancing the durability of concrete structures by reducing the ingress of water and air into the concrete.

Bonding admixtures

Bonding admixtures are polymer emulsions which serve the purpose of improving the adherence of fresh concrete to hard concrete. They are particularly suited for repair work. The combination of the emulsion with concrete results in the formation of a latex-modified concrete, or a polymer Portland cement concrete. Bonding admixtures improve the tensile and flexural strength of concrete, the durability and the bonding properties of concrete.