|
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.
|
