Duff Abrams published data that showed that for a given set of concreting materials, the strength of the concrete depends solely on the relative quantity of water compared with the cement. In other words, the strength is a function of the water to cement ratio (w/c) where w represents the mass of water and c represents the mass of cement. This became known as Abrams law and it remains valid today as it was in 1918. However, more often, w/cm is used and cm represents the mass of cementing materials, which includes the portland cement plus any supplementary cementing materials such as fly ash, slag cement, or silica fume.

Unnecessarily high water content dilutes the cement paste (the glue of concrete) and increases the volume of the concrete produced. Some advantages of reducing water content include:

• Increased compressive and flexural strength
• Lower permeability and increased watertightness
• Increased durability and resistance to weathering
• Better bond between concrete and reinforcement
• Reduced drying shrinkage and cracking
• Less volume change from wetting and drying

The less water used, the better the quality of the concrete provided themixture can still be consolidated properly.
Smaller amounts of mixing water result in stiffer mixtures; with vibration, stiffer mixtures can be easily placed. Thus, consolidation by vibration permits improvement in the quality of concrete.

Reducing the water content of concrete, and thereby reducing the w/cm, leads to increased strength and stiffness, and reduced creep. The drying shrinkage and associated risk of cracking will also be reduced. The concrete will have a lower permeability or increased water tightness that will render it more resistant to weathering and the action of aggressive chemicals. The lower water to cementitious materials ratio also improves the bond between the concrete and embedded steel reinforcement.

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