Usage of water reducing admixture is classified as Type A in ASTM C 494. WRA reduces the water usage by 5% to 12% but still maintains the consistency of concrete at the required levels. So it affects the fresh properties of concrete and consistency as measured by the slump. The use of WRA may accelerate or retard the initial setting time of concrete. If it retards the setting time it is classified as “retarding effect WRA”.
Two common chemicals used as WRA are lignosulfonates and hydrocarboxylic (HC) acids. Lignosulfonates have a little edge as they require less water compared to HC acids. Also the problem of rapid bleeding is an issues faced while using hydrocarboxylic acids as water reducing admixture in concrete.
Depending on the type of WRA used the slump in concrete varies. Typical dosage rate is based upon the cementitious material content (mm/quintal). Refer to the graph below to determine the influence of HRA dosage on slump. below illustrates the influence of dosage of Lignosulfonates and HC acid on slump. Apparently HC acids generate more slump compared to lignosulfonates when dosage is constant.
Benefits of water reducing admixtures in Concrete.
- Primarily used in hot weather to enable better concrete pumping, placing and tremie.
- Higher initial concrete compressive strength
- Makes concrete less permeable
- Increases durability