TYPES OF CONCRETE ADMIXTURE
Admixtures are defined as the material other than the aggregate, water and cement added to the concrete. The use of suitable admixture can result in the enhancement the performance of concrete.
Types of Admixture
Air Entraining Agents
Air entraining agents are used primarily to increase the resistance of freezing and thawing. They also supply greater resistance to deicing chemicals, improve workability, lower water demands for a particular slump, reduce the amount of fine aggregate needs in the mixture, reduce segregation and bleeding, increase durability, and improve pump ability. Even though air entrainments have no effect on drying shrinkage, they do create a slight loss in the concrete strength.
Water Reducing Agents
A certain amount of the water is added to concrete to make the concrete place able and to ignite the hydration reaction of Portland cement. Over 50% of the water have no useful effect and have direct results in drying shrinkage, durability, and the strength of concrete. Water reducing agents added to the concrete reduce the water demands of the mix, maintain the workability, increase the strength, reduce the cost, reduce bleeding, reduce segregation, reduce honeycombing, reduce cracking and permeability, increase bond strength of concrete to steel, and reduce drying shrinkage. An average of 12-15 % reduction in the total water content will attain this goal.
Set Retarding Agents
These agents are primarily used for delaying the setting time of concrete. Set retards are useful, but are not the solution to poorly designed mixes, inferior materials, or low cement factors. The set retarder must be uniformly dispersed throughout the whole mix. Failure to do so will result in the admixture remaining in voids, causing some areas to remain plastic. This leads to plastic shrinkage.
Accelerating Agents
Accelerating agents are used to shorten the setting time and increase the early strength of the concrete. These agents are mainly used for concrete needing to be used immediately.
Calcium chloride
Calcium chloride is generally used in cold weather to hasten the setting time and produces an early finish of the concrete. Calcium chloride can effect the characteristics of concrete causing temperature rise, increased internal stresses, corrosion of unprotected reinforcement, a decrease in the resistance to freeze, an increase in the attack of sulfates, and an increase in the amount of drying shrinkage between 10 to 50 %.
