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Not all the construction water used in concrete to make it workable is used in the hydation process. Much evaporates with time as the concrete dries out. But how much is held in the concrete? The following is a simple calculation, but the actual amount will depend on the environment in which the concrete is situated.
A long concrete wall of 45 x 18 x 1.1m
Volume of concrete
45 x 18 x 1.1 = 891m3
Mass of concrete
Density assumed as 2,400kg/m3 thus mass is 2400 x 891 = 2,138,400kg
Assuming 360kg/m3 of Portland cement and a maximum w/c ratio of 0.50, the water available is 180 litres per m3. This equates to 180 x 891 = 160,380 litres of water and 360 x 891 = 320,760kg of cement. However, combined water in cement paste is between 18 and 23% w/w cement, say 20% (AM Neville, Properties of concrete). This equates to 320,760 x 20% = 64,152 litres of combined non-evaporable water.
Also, concrete does not completely dry out, some 1% by mass of concrete (CIBSE Guide A) is held within the capillaries. This equates to 2,138,400 x 1% = 21,384 litres. Note, it may be up to 2%.
Hence total held water is (160,380 – 64,152 – 21,384) = 74,844 litres (roughly 47% water held in the concrete).
For calculating slab drying times see Concrete Advice 67
Acknowledgement: Concrete Society
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