Accelerators

Accelerating admixtures are added to concrete to increase the rate of early strength development in concrete to

• Permit earlier removal of formwork
• reduce the required period of curing
• Advance the time that a structure can be placed in service
• Partially compensate for the retarding effect of low temperature during cold weather concreting
• In the emergency repair works

 Calcium chloride is the material very commonly used as accelerator.

The action of calcium chloride is assisting concreting at low temperature is three fold as-

• It lowers the temperature at which freezing of the wet mixture takes place
• It helps to keep the mixture warm by accelerating the generation of heat by chemical action 
• It increases the ability of the concrete to resist frost action by speeding up the rate of gain of strength

Effects of calcium chloride on concrete:

• Increases volume changes in concrete 
• Increases the alkali-aggregate reaction 
• Reduces the resistance of concrete  to sulphate attack