Dear reader we have already learnt about conditions where dry aggregate is used in concrete mix. It is usually assumed that required water will be absorbed by aggregate to reach saturated condition. In dry weather, where there is no way to allow aggregate to lack moisture, actually aggregate cannot absorb sufficient water required for saturation.
This is due to rapid coating of aggregate with by cement paste. The coated aggregate losses its water supply from the concrete mix, required to reach saturated and our assumption becomes invalid.
This condition is prominent in coarse aggregate as it has option for water to travel from its surface to interior pores remain in aggregate. So the water-cement ratio that we assumed becomes higher than that of actual case of partial absorption of moisture into the aggregate.
The term ‘rapid coating’ is not always valid as it depends on cement or cementitious content in concrete mix i.e. coating rate becomes slower where cement content is low. So in case lean mix this partial saturation situation is not valid. So water-cement ratio is carefully set in case of rich mix.
The water absorption of desired aggregate for duration identical to the tentative coating time of aggregate is examined. Depending on the result (degree of saturation) water-cement ratio is designed. It is interesting to know that the behavior of mix is actually affected to some extent depend on sequence of feeding, in to the mixer, different ingredient of concrete.