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.
