Water Reducing Admixtures

Water reducing agents, or plasticizers, are added to provide workability in the freshly mixed concrete matrix while using significantly lower amounts of mix water, thus achieving better strength and durability.

Water reducing agents provide the lubricity in coarse mixes that would normally require additional paste or more water in the paste. According to ASTM C 494, these admixtures are classified as Type A and must allow at least a 5% reduction in water without changing the consistency or reducing the strength of the control batch having the higher water content without this admixture. They often are able to reduce the water demand by 10% and as much as 15% for even greater strength and durability benefits derived from lowering the water/cement ratio.This class of admixtures is typically made from relatively inexpensive lignosulfonates, hydroxylated carboxylic acids, or carbohydrates.

Several theories exist as to what mechanisms are responsible for the reduction in water demand in  the plastic concrete matrix, but all agree that the improvement is mainly due to the chemical and physical effects of the water reducing admixtures (WRAs) on the surface of the hydrating cement particles. Deflocculating and dispersion of the cement particles are the net result and allow better use of the available water for more uniform lubrication and hydration. Also, because many water reducing admixtures entrain as much as 2% air, increased lubricity is partly due to the distribution of tiny added air bubbles.

Many WRAs retard set times and are sometimes used with an accelerator for compensation. WRAs that are combined with an accelerator into one admixture are classified as Type E under ASTM C 494. Of course, retardation may be desired for higher temperature concreting conditions. Whenever the natural tendency of WRAs to retard hydration is not adequate for the desired application, additional retarders are added. Commercially available single admixtures that combine a retarder with the water reducing admixtures are classified as Type D under ASTM C 494.

Typical relationship between 28-day compressive strength and water/cement ratio for a wide variety of air-entrained concretes using Type I cement.

Many water reducing admixtures are associated with higher shrinkage rates and faster slump loss even though the water/cement ratio is reduced. Bleeding properties, too, are sometimes affected by the choice of water reducing admixture. To overcome these tendencies WRAs are often added at the batch plant along with much less of the more expensive, but more efficient, high-range (or mid-range) water reducers described in the next section.

Considerations for usage of water reducing admixtures are economically based, and strategies fall into the following three main categories:

1. Reduce the water/cement ratio for higher strengths and improved durability while maintaining the same workability and cement content.
2. Reduce the paste portion of the matrix, water, and cement, for the purpose of reducing shrinkage and heat development in massive placements; workability, strength, and durability are maintained at a comparative level.
3. Keep water and cement the same and maintain the same strength and durability but improve flow and workability.

Efficient dosage rates vary with chemical composition of the water reducing admixture, individual batch designs, cement types, other admixtures, environmental conditions, flow and workability constraints in the job, and end-product needs. Manufacturers’ guidelines typically recommend 2 to 7 fluid ounces per 100 pounds cement added into the mix water. These recommendations should be used as a starting place for several trial batches closely monitored for critical properties under field conditions in both the fresh and hardened states.