Just before or during the process of mixing concrete add mixtures are often added. Admixtures are added as means to improve quality, acceleration, manageability, or retardation. Most of concrete mixes currently have one or more concrete admixtures. Concrete admixtures serve different purposes which may include the following;
Concrete admixtures used to set-retarding- the set retarding concrete admixtures are often used to inhibit the chemical reaction which occurs when the concrete starts the setting process. The main use of these this type of concrete admixture is often to bring down the effects of high temperatures which may bring about a speedier setting of concrete. This type of admixtures is most commonly used in making pavements. They are used in pavement construction to give more time to finish concrete pavements. This reduces anymore costs that would have been incurred to put up a new concrete batch plant on the construction site; it also eliminates cold joints in concrete.
Retarders also can be used to help resist cracking caused by form deflection which occurs when horizontal slaps are put in sections. Retarders can also be used as water reducers; they also help to entrain some air in the concrete.
Concrete admixtures for air entrainment- the fresh-thaw durability of concrete can be increased by air entrained concrete. Air entrained admixture often produces an easy to use concrete compared to concrete that has no air entrained admixture added to. Air-entrained admixture also helps to reduce bleeding and segregation of fresh concrete. Another advantage of using air-entrained admixture on concrete is the fact that it helps improves the resistance of concrete on severe frost action or freeze thaw cycles. Air-entrained admixture also brings about high resistance to cycles of wetting and drying, high degree of workability, and high degree of durability.
Concrete admixtures for water reducing purposes-water reducing admixtures are often used to serve the purpose of lowering the water content in a cement ration. They are often used to bring about desired concrete strength by use of lower cement content. This type of admixture helps improve concrete properties by enhancing the placement of concrete under difficult conditions. Some of the places where water reducing admixtures have been used is in the construction of bridge decks, low –slump concrete overlays, and in patching concrete.
Accelerating concrete admixtures- they are used to serve the purpose of speeding the rate of concrete strength development. They also serve the purpose of reducing concrete setting time. Calcium chloride is one notable ingredient of this admixture although it could also increase the corrosion of steel reinforcement. This can be avoided by doing proper consolidation, using enough cover, and also practicing proper concrete mix design. Accelerating admixtures often come in handy when modifying the properties of concrete in cold weather.
Shrink reducing concrete admixtures– they are often added to concrete during the early stages of concrete mixing. The main purpose of this admixture is to reduce early and long term drying shrinkage. These admixtures serve the purpose of alleviating problems that often result from shrinkage cracking thus ensuring the durability of structures. Shrinkage reducing admixtures could also reduce strength development.
Super plasticizers concrete admixtures-these admixtures are majorly used for the production of flowing concrete with very high slump. They are often used in heavily reinforced structures and also in placement s where enough consolidation cannot be gotten easily by vibration. They are also used in the production of high strength concrete at W/C’S that ranges from .03 to 0.4. Super plasticizer when used with most types of cements improves the workability of concretes.
Corrosion inhibiting concrete admixtures-they are classified into the specialty admixture ( Corrosion Inhabiting ) category. They are used to slow the corrosion of reinforcing steel in concrete. Using them in construction always helps to reduce the maintenance costs of reinforced concrete structures.