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Water Cement Ratio in Concrete Mix

Water has a great role on the strength and workability of concrete. Water cement ratio can be defined as the ratio of the volume of water to the volume of cement used in a concrete mix.
For every pound ( kg or corresponding unit) of cement, about 0.42 pounds (or 0.42 kg or corresponding unit) of water is needed to fully complete hydration reactions.
However, a mix with a ratio of 0.42 may not mix thoroughly, and may not flow well enough to be placed. More water is therefore used than is technically necessary to react with cement. Water-cement ratios of 0.45 to 0.60 are more typically used.
 For higher-strength concrete, lower ratios are used, along with a plasticizer to increase flowability.
required water-cement ratio(british specifications):
 Proportion  Water-Cement Ratio
     1 : 2 : 4       0.58
     1 : 1.5 : 3       0.51
     1 : 1: 2       0.43

Too much water will separate the sand and aggregates from the cement paste. In addition, water that is not consumed by the hydration reaction may allow the concrete to harden, resulting in microscopic pores (bleeding) that will reduce final strength of concrete.
 A mixture containing too much water will shrink more as the excess water leaves, resulting in internal cracks and visible fractures (especially around the inner corners), which will reduce the final resistance again.
Hence, water cement ratio attends an important role in producing concrete of required strength. The lower the ratio, the greater is the strength of concrete.