Free Moisture Content or Surface Moisture:

Free moisture content is the quantity of water, present in an aggregate sample, in excess of the saturated surface dry (SSD) condition. This is also termed as free moisture that is readily available for hydration process. Necessary correction in measuring the quantity of water must be done during batching so as to maintain the desired water cement ratio.

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It is necessary to know the free moisture content of aggregate at site. In the calculation of mix design the weights of aggregates are based on the SSD condition, but in practice this SSD condition is extremely rare. In fact, the actual moisture content of aggregates is mostly either more than or less than that required for SSD condition. So, it is necessary to make the necessary correction in the weight of aggregates and also in the quantity of water, so that the desired proportion of all the ingredients and required water cement ratio could be maintained as per the mix-design. Any variation in the quantity of water will lead to weak and poor quality concrete.