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Bleeding results is reduction of Water-Cement ratio. Then why is it considered harmful?

Bleeding is basically a form of segregation in which some of the water in the concrete mix comes out to the surface of concrete.

Bleeding is not necessarily harmful. In fact, removal of the water (after it has performed its role in providing workability) from the mix will result in increase in strength of the concrete due to lowering of effective water cement ratio.

However, if the rising water carries with it a considerable amount of finer cement particles a layer of laitance will get formed. This laitance layer has a higher content of water & is devoid of aggregates. As such, higher shrinkage cracks develop in it. The presence of laitance at top surface of slab & pavements will result in a porous surface having poor wearing quality. Laitance layer formed on concrete road surfaces will produce dust in summer & mud in rains.

Presence of laitance over the top of a lift where a subsequent lift is to be cast would result in a weaker plane & poor bond at the junction. Laitance should always be removed by brushing & washing before casting of next lift.

If the bleeding water is remixed during finishing of top surface a weak wearing surface will be formed. This can be avoided by delaying the finishing operations until the bleeding water has evaporated.

As the water moves up the concrete during process of bleeding, it forms continuous channels. For concrete with high water-cement ratio (say 0.7 or so) these channels will not get segmented by development of gel, thus resulting in high permeability of concrete.

Also, when the rising water is intercepted by aggregate/reinforcement it gets accumulated below them. This results in water voids & reduction of aggregate-paste bond & reinforcement-concrete bond. The poor bond caused due to bleeding may be remedied by revibration of concrete.

When concrete is in fully plastic state, the early bleeding will not cause much harm. This is due to the fact that the concrete being in fully plastic stage will get subsided & compacted. However, the delayed bleeding (i.e. when the concrete has lost its plasticity) will be harmful to concrete.