the water-cement ratio is the ratio of water weight to the weight of cement used in the concrete mix. The lower ratios lead to higher strength and durability, but can make mixes difficult to work with and shape. Working ability can be overcome with the use of plasticizer or super-plasticizer.
Often, the ratio refers to the ratio of water to semen plus pozzolan ratio, w/(c p). Pozzolan is usually a fly ash, or a blast furnace slag. This can include a number of other ingredients, such as silica fume, rice husk ash or natural pozzolans. Pozzolans can be added to strengthen the concrete.
The concept of water-cement ratio was first developed by Duff A. Abrams and published in 1918. Refer to the concrete slump test.
Concrete hardened as a result of chemical reactions between cement and water (known as hydration, this produces heat and is called the heat of hydration). For each pound (or kilogram or weight unit) of cement, about 0.35 pounds (or 0.35 kg or equivalent unit) of water is required to complete a complete hydration reaction.
However, the mixture with a ratio of 0.35 may not be mixed thoroughly, and may not flow well enough to be placed. Then more water is used than it is technically necessary to react with cement. Water-cement ratio 0.45-0.60 is more commonly used. For higher strength concrete, lower ratio is used, along with plasticizer to improve flowability.
Too much water will result in segregation of sand and aggregate components of cement paste. Also, water that is not consumed by the hydration reaction can leave the concrete as it hardens, producing microscopic pores (bleeding) that will reduce the final strength of the concrete. Mixtures with too much water will experience more shrinkage due to excess water leaf, resulting in internal cracks and visible fractures (especially around corners), which again will reduce the final strength.
The Uniform Building Code of 1997 establishes a maximum ratio of 0.5 when the concrete is exposed to freezing and liquefaction under humid conditions or freezing chemicals, and a maximum of 0.45 ratios for concrete in severe or very severe sulfate conditions.
Video Water-cement ratio
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Source of the article : Wikipedia
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