Admixtures are defined as a material other than aggregate, water, of cement which is added to the concrete. The use of suitable admixtures can result in an enhancement of the performance of concrete.

Air Entraining Agents

Air entraining agents are used primarily to increase the freeze-thaw resistance of concrete.  They also supply greater resistance to deicing chemicals, improve workability, lower water demand for a particular slump, reduce the amount of fine aggregate needed in the mixture, reduce segregation and bleeding, increase durability, and improve pumpability. Even though air entrainments have no effect on drying shrinkage, they do create a slight loss in concrete strength.

Water Reducing Agents

A certain amount of water is added to concrete to make the concrete placeable and to ignite the hydration reaction of portland cement. Over 50 percent of the water added has no useful effect and has a direct impact on drying shrinkage, durability, and strength of concrete. Water reducing agents added to the concrete reduce the water demand 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 percent reduction in 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 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/thaw, an increase in the attack of sulfates, and an increase in the amount of drying shrinkage between 10 and 50 percent.

Other Agents

Other admixture agents have a varying result on the increase on drying shrinkage, but the percent of shrinkage is constant for a given admixture.