Friday, March 16, 2012

Use of Admixtures (chemicals) in Mixing Cement concrete


Chemicals or Admixtures when concreting is widely used all over the world today. admixtures are chemicals which are added to cement while or after concrete has been mixed with Aggregates and Sand. there are various types admixtures, and each admixture has its own use, the amount of admixture added to concrete is dependent to the type of chemicals used in it. 

Most of the admixtures are ready to use, no need of doing your own experiments on it. 

ASTM C 494, “Standard Specification for Chemical Admixtures for Concrete,” classifies admixtures into seven types as follows;

Type A - Water-reducing admixtures;
Type B - Retarding admixtures;
Type C - Accelerating admixtures;
Type D -  Water-reducing and retarding admixture;
Type E -  Water-reducing and accelerating admixtures;
Type F - Water-reducing, high-range, admixtures; and
Type G - Water-reducing, high-range, and retarding admixtures

Type A - Water-reducing admixtures
this type will reduce the amount of water needed for the mix, there for it increases the concrete strength,  its useful when concrete is been pumped by pump or tremi. Type A reduces water content by 5 to 15%, Typically the dosage rate of Type A water-reducers range from 130 to 390 mL per 100 kg (2 to 6 fl oz. per 100 lb) of cementitious materials. Higher dosages may result in excessive retardation of the concrete setting time.

Type B - Retarding admixtures and Type D -  Water-reducing and retarding admixture

Both of this is to offset the unwanted effect of temperature, since cement is exothermic and weather temperature place a roll too, this formula is needed on hotter weather conditions,  factors depending are; admixture concentration, dosage rate, concrete proportions, and ambient and concrete temperatures.

Type C - Accelerating admixtures and Type E -  Water-reducing and accelerating admixtures

both this shortens the setting time and accelerates the early strength of concrete. mostly used in freezing temperatures.