bleeding of concrete.
Bleeding in concrete continues till the layer of water on the surface of concrete has set.
As long as the rate of evaporation is
lower than the rate of bleeding, there is a continuous layer of water at the
surface known as “water sheen”, and shrinkage does not occur.
When the concrete surface loses water
faster than the bleeding action bring it to the top, shrinkage of top layer
takes place, and since the concrete in plastic state can’t resist any tension,
cracks develops on the surface. These cracks are common in slabs.
The extent of
plastic shrinkage depends on:
Exposure to the heat from sun
radiation,
Relative humidity of ambient air and
velocity of wind
Rate of Initial heat hydration of
cement
Water absorption of aggregates
Prevention Methods
1. Laying Plastic sheet over the concrete
after finishing
2. Use Blended Cement (PPC/PSC/PCC)
to reduce initial heat of hydration
3. Make sure the fine aggregate silt
content is within the limit
4. Wet the Coarse aggregate if it is
in very dry condition
** Over-usage of cement or fine
aggregate will increase the chance of plastic shrinkage cracks as both will
increase the water demand and more cement increses the heat of hydration