A chemical admixture can be added to concrete to achieve or modify its properties. Admixtures are added to the concrete, in addition to cement, water and aggregate, typically immediately before or during the mixing process. Admixtures can be used to reduce the cost of building with concrete, or to ensure certain required properties or quality of the cured concrete. If problems arise with the concrete during the construction process, admixtures can be used as an emergency measure to try and prevent failure. Concrete admixtures can improve concrete quality, manageability, acceleration, retardation of setting time etc.
Set retarding admixture
Set retarding admixtures are water soluble chemicals that delay the setting of the cement. They do not plasticise significantly and have little or no effect on the water demand or other properties of the concrete. Set retarding admixtures not only delay the setting of the cement but also increase initial workability by plasticising the concrete or reduce its water demand. The majority of commercially available retarding admixtures are of this type. here are two major classifications of admixtures used in concrete – Mineral admixtures and Chemical admixtures. Use of set retarders in concrete pavement construction enables farther hauling, thus eliminating the cost of relocating central mixing plants. It allows more time for texturing or plastic grooving of concrete pavements. It also allows more time for hand finishing around the headers at the start and end of the production day and helps eliminate cold joints in two-course paving and in the event of equipment breakdown. As with plasticisers, ultimate strength gain increases as water is reduced. Set retardation allows the slower formation of a more ordered, smaller, denser cementitious matrix. This increases ultimate strength relative to an unretarded mix with the same w/c ratio.
Air entrainment admixture
Air entraining admixture entrains a large number of uniform, stable and closed tiny bubbles in the process of mixing concrete to reduce the segregation of concrete mixture, improve the workability, and also enhance anti-freeze ability and durability of concrete. Air-entraining admixtures facilitate the development of a stable air-void system within concrete that increases the durability of the concrete. It can improve the workability of concrete mixtures. The closed bubbles are like balls that can reduce the friction among cement particles to improve mobility. It helps to incorporate a controlled amount of air, in the form of millions of minute non-coalescing bubbles distributed throughout the concrete, during mixing, without significantly altering the setting or rate of hardening of concrete. With the proper amount of entrained air, workability and durability of concrete is increased. The compounds which can be used are Natural wood resins, fats, lignosulfonates, alkyl sulphates. Sodium salts of petroleum and sulphonic acids. Air entrainment is the intentional creation of tiny air bubbles in concrete.
Water-reducing admixtures are used to increase the strength and workability of concrete and to reduce cost. They consist of organic compounds or mixtures of organic and inorganic compounds to reduce the water requirement of the mixture for a given slump. Reduction in water demand may result in a reduction in the w/cm for a given slump and cement content, an increased slump for the same w/cm and cement content, or a lower cement content Water-reducing admixtures also reduce segregation and improve the flowability of the concrete. Therefore, they are commonly used for concrete pumping applications as well. Water-reducing admixtures typically fall into three groups: low-, medium- and high-range. Water-reducing admixtures essentially neutralize surface charges on solid particles and cause all surfaces to carry like charges. Since particles with like charges repel each other, they reduce flocculation of the cement particles and allow for better dispersion. They also reduce the viscosity of the paste, resulting in a greater slump.
Accelerating admixtures can be used to increase either the rate of stiffening or setting of the concrete or the rate of hardening and early strength gain to allow earlier formwork striking and demoulding. Most accelerators achieve one rather than both of these functions. Accelerators can be used with superplasticizers where early age strength is required, especially at lower temperatures. Accelerating admixtures affect the rates of reactions between cement and water to give an overall increase in the hydration rate. Thus, the use of accelerators in concrete provides a shortening of setting time and/or an increase in early strength development. These kinds of chemical admixtures, added to a concrete batch either immediately before or during mixing. Accelerators make concrete set faster, also known as increasing the rate of hydration. At the same time, they promote strength development so it happens earlier in the set time of a slab.
Superplasticizers are high-range water reducers that are additively used in making high-strength concrete. Plasticizers are chemical compounds that enable the production of concrete with approximately 15% less water content. Superplasticizers allow a 30% or more reduction in water content. It is chemical admixtures that are added to the concrete to improve their flowing ability, they help to reduce the amount of water in the concrete and to improve the strength and durability of concrete. They achieve a reduction in water content without loss of workability. Superplasticizers are used for producing flowing concrete to be used in inaccessible locations, floors or where very quick placing is required. A self-leveling and self-compacting concrete is called flowing concrete. Superplasticizers are also used for the production of high-strength and high-performance concrete. With the use of superplasticizers, flowing concrete could be produced with the water/cement ratio as low as 0.25 or even less.
Corrosion inhibiting admixture
Corrosion inhibiting admixture increases the passivation of reinforcement and other embedded steel. This can inhibit corrosion when passivation would otherwise have been lost as a result of chloride ingress or carbonation. They are added to concrete during production and are referred to as ‘integral’ corrosion-inhibitors. So-called ‘migratory corrosion-inhibitors’ applied to hardened concrete are not admixtures. Corrosion-inhibiting admixtures have little effect on strength at either early or later ages. The function of this type of admixture is to enhance the long term durability of reinforced concrete by enhancing passivation to the cathodic and/or anodic areas of embedded steel.
Every type of chemical admixture has its one individualistic properties and material composition. It is very important to know the applicability of each one before using them. It is suggested to take expert opinion when selecting them.