Many important characteristics of concrete are influenced by the ratio (by weight) of water to cementitious materials (w/cm) used in the mixture. By reducing the amount of water, the cement paste will have higher density, which results in higher paste quality. An increase in paste quality will yield higher compressive and flexural strength, lower permeability, increase resistance to weathering, improve the bond of concrete and reinforcement, reduce the volume change from drying and wetting, and reduce shrinkage cracking tendencies (PCA, 1988).
Reducing the water content in a concrete mixture should be done in such a way so that complete cement hydration process may take place and sufficient workability of concrete is maintained for placement and consolidation during construction. The w/cm needed for cement to complete its hydration process ranges from 0.22 to 0.25. The existence of additional water in the mixture is needed for ease of concrete placing and finishing (workability of concrete). Reducing the water content in a mixture may result in a stiffer mixture, which reduces the workability and increases potential placement problems.
Water reducers, retarders, and superplasticizers are admixtures for concrete, which are added in order to reduce the water content in a mixture or to slow the setting rate of the concrete while retaining the flowing properties of a concrete mixture. Admixtures are used to modify the properties of concrete or mortar to make them more suitable to work by hand or for other purposes such as saving mechanical energy.
Water reducing admixtures (WRA)
The use of WRA is defined as Type A in ASTM C 494. WRA affects mainly the fresh properties of concrete by reducing the amount of water used by 5% to 12% while maintaining a certain level of consistency, measured by the slump as prescribed in ASTM C 143-90. The use of WRA may accelerate or retard the initial setting time of concrete. The WRA that retards the initial setting time more than three hours later is classified as WRA with retarding effect (Type D). Commonly used WRA is lignosulfonates and hydrocarboxylic (HC) acids. The use of HC acids as WRA requires higher water content compared to the lignosulfonates. Rapid bleeding is a problem for concrete treated with HC acids.
Increase of slump is different according to its type and dosage. Typical dosage rate is based upon the cementitious material content (milliliters per hundred of kilograms). The figure below illustrates the influence of dosage of Lignosulfonates and HC acid on slump. It is shown in the figure that HC acids give a higher slump compared to lignosulfonates with the same dosage.