66 Damage to concrete structures
3.8.2 Plastic settlement and bleeding
Freshly cast concrete can show the tendency to bleed. Due to gravity, coarse
aggregate particles can ‘sink’ somewhat downward as in a sedimentation
process. This is called plastic settlement. As a consequence, water and fine
particles (cement, powders) will go upward and a water film can be formed
on the concrete surface, the so-called bleed water. The intensity of the plastic settlement and bleeding process is mainly dependent on the concrete
composition (consistency and water content), but also on the height of the
concrete element.
Bleeding occurs before setting and can counteract somewhat the effect
of plastic shrinkage as long as the evaporation rate on the concrete surface is limited. However, bleeding leads to higher water content in the
upper concrete, thus locally increasing the water/cement ratio and consequently porosity. Near the surface, a water rich paste layer is formed,
called laitance. As a result, the concrete surface will be less resistant to
frost, de-icing salts, and mechanical loading. After evaporation of the
bleeding water, a dust rich surface is obtained. This dust can hinder the
accurate operation of high tech machines such as automatic forklifts in
storage buildings. The dust layer can also impair adherence of a new concrete layer cast in a later phase. Before casting new concrete on top of a
previously cast hardened concrete, the laitance and dust have to be carefully removed.
In thicker concrete elements, plastic settlement and bleeding can lead
to surface cracks following the upper reinforcement bars as illustrated in
Figure 3.19. This is due to the fact that the reinforcement bars are fixed
Plastic settlement crack
Rebar
Void
Bleed water
going upwards
Plastic settlement
Figure 3.19 Plastic settlement and bleeding.
3.8.2 Plastic settlement and bleeding
Freshly cast concrete can show the tendency to bleed. Due to gravity, coarse
aggregate particles can ‘sink’ somewhat downward as in a sedimentation
process. This is called plastic settlement. As a consequence, water and fine
particles (cement, powders) will go upward and a water film can be formed
on the concrete surface, the so-called bleed water. The intensity of the plastic settlement and bleeding process is mainly dependent on the concrete
composition (consistency and water content), but also on the height of the
concrete element.
Bleeding occurs before setting and can counteract somewhat the effect
of plastic shrinkage as long as the evaporation rate on the concrete surface is limited. However, bleeding leads to higher water content in the
upper concrete, thus locally increasing the water/cement ratio and consequently porosity. Near the surface, a water rich paste layer is formed,
called laitance. As a result, the concrete surface will be less resistant to
frost, de-icing salts, and mechanical loading. After evaporation of the
bleeding water, a dust rich surface is obtained. This dust can hinder the
accurate operation of high tech machines such as automatic forklifts in
storage buildings. The dust layer can also impair adherence of a new concrete layer cast in a later phase. Before casting new concrete on top of a
previously cast hardened concrete, the laitance and dust have to be carefully removed.
In thicker concrete elements, plastic settlement and bleeding can lead
to surface cracks following the upper reinforcement bars as illustrated in
Figure 3.19. This is due to the fact that the reinforcement bars are fixed
Plastic settlement crack
Rebar
Void
Bleed water
going upwards
Plastic settlement
Figure 3.19 Plastic settlement and bleeding.