When we calculate quantity of materials for cement mortar, the voids are considered for sand. So, my question is why these voids are considered while calculating quantities of materials in cement mortar?
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Have you heard about bulking of sand. The free moisture content in fine aggregates results in bulking of volume. The moisture in the sand forms a thin film around each particle which exerts surface tension and keeps the nearby particles away from it. Thus due to surface tension, every particles keeps away from each other.
This bulking of sand increases with increase in moisture content upto a certain limit and decreases with further increase in moisture. This can be seen in the figure.
As you know that, we while calculating quantities of materials required for mortar, we consider the dry volume of the sand. But in practical cases, the sand has some free moisture due to which the volume of the sand is more than its dry volume.
Thus, while calculating the quantities of materials for mortar, the voids are considered in the sand and increased wet volume is used in the calculation. When the sand is dried up, the moisture in sand is replaced by the voids.