DRYING SHRINKAGE OF CONCRETE
When finished concrete hardens and effectively has dried out for the first time, its linear dimensions will contract by approximately 0.4mm per metre. This reduction in volume is primarily due to the excess water in the concrete (that used for workability as opposed to hydration) being yielded up to the atmosphere by way of evaporation. On subsequent wetting from rain, for example, it will expand again but it will not quite achieve its original dimensions.
The issue of drying shrinkage becomes more significant on larger areas of concrete. It can be minimized by reducing the amount of excess water in the original mix, provided that full compaction of the concrete can still be achieved. Whilst a reduced water/cement ratio may mean that the concrete is more difficult to place and compact, there is the advantage that the finished concrete will be stronger and less porous, which will decrease moisture movement in terms of swelling and contracting in wet weather.
The more quickly concrete is allowed to release its excess moisture content, the more it will shrink, so drying shrinkage can also be reduced by way of effective curing. This will allow the concrete to retain its moisture content whilst it gains strength for the first few days after placing.