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Banishing Blisters in a
Stampable Overlay
Question:
When I apply stampable overlays I often get
blisters ranging in size from a dime to a half dollar.
What causes these flaws, and how do I get rid of
them after the surface has cured?
Answer:
When a dry cement-based mix of any kind is
combined with water, it begins to react as it
hydrates, giving off gases (CO2, H20, and others).
Most concrete surfaces are porous enough during
the finishing stages to allow for these gasses to work
their way to the surface and escape directly into
the atmosphere. Since stampable overlays are
polymer-modified, off-gassing can be more of
problem when compared to typical concrete. The
polymer (plastic) creates a tighter surface when
finished with a trowel. Finishing the surface too
early or adding extra water or a finishing aid to
help close up the surface can trap gas and create
blisters. A second factor that can contribute to
blister formation is failing to apply a primer to the
substrate before overly application. Not only does
the primer act as a glue to adhere the overlay to
the surface, it also closes the pores in the substrate
and prohibits gas transmission to the overlay.
To prevent blistering, wait a bit longer to finish the
surface or cut back on the amount of surface
moisture used to aid in finishing. Remember that
stampable overlays are porous by nature. Trying
to get a tight finish similar to what you can
achieve with concrete is not feasible and can create
problems, blisters being one of them. If you see
blisters forming while finishing, use a sharp-
pointed object to pierce the blister and then use a
trowel to rework the material back into the
surface. If you find blisters after the overlay has
cured, try injecting the hollow areas with epoxy or
remove the blister and fill the spot with the same
overlay material, blending the edges to create a
less noticeable patch.
Shrinkage Cracks in Overlay
Question:
Why did this integrally colored stampable overlay
crack so drastically? It was applied at 8 a.m. on a
hot July day. The air temperature was 85 F, but
the surface temperature was over 100 F.
Answer:
Even though stampable overlays are polymer-
modified, they are still concrete mixtures and
exhibit all the characteristics of concrete,
including shrinkage cracking if care is not taken
during installation. The dry, hot weather and high
surface temperature, in conjunction with the thin
cross section of the overlay, created a situation
where the little moisture present in the overlay
evaporated too quickly, resulting in shrinkage
cracks.
Here are the key factors to remember when
working with overlays in hot conditions:
The temperature of the surface to receive the
overlay is as important as the air temperature.
Only apply overlays when [air and surface?]
temperatures range from 50 F to 90 F. If in doubt,
hold off until temperatures moderate.
Avoid application in direct sunlight or excessive
wind.
To slow hydration, add approved set-retarding
agents to the overlay mix or apply evaporative-
control agents to the surface.
Cure the overlay with plastic or an approved
curing compound.
Not all overlays are the same. Some products
contain water reducers or hydration control
admixtures to help control shrinkage cracking.
Check with the manufacturer of the product you’re
using
Stampable Overlay
Question:
When I apply stampable overlays I often get
blisters ranging in size from a dime to a half dollar.
What causes these flaws, and how do I get rid of
them after the surface has cured?
Answer:
When a dry cement-based mix of any kind is
combined with water, it begins to react as it
hydrates, giving off gases (CO2, H20, and others).
Most concrete surfaces are porous enough during
the finishing stages to allow for these gasses to work
their way to the surface and escape directly into
the atmosphere. Since stampable overlays are
polymer-modified, off-gassing can be more of
problem when compared to typical concrete. The
polymer (plastic) creates a tighter surface when
finished with a trowel. Finishing the surface too
early or adding extra water or a finishing aid to
help close up the surface can trap gas and create
blisters. A second factor that can contribute to
blister formation is failing to apply a primer to the
substrate before overly application. Not only does
the primer act as a glue to adhere the overlay to
the surface, it also closes the pores in the substrate
and prohibits gas transmission to the overlay.
To prevent blistering, wait a bit longer to finish the
surface or cut back on the amount of surface
moisture used to aid in finishing. Remember that
stampable overlays are porous by nature. Trying
to get a tight finish similar to what you can
achieve with concrete is not feasible and can create
problems, blisters being one of them. If you see
blisters forming while finishing, use a sharp-
pointed object to pierce the blister and then use a
trowel to rework the material back into the
surface. If you find blisters after the overlay has
cured, try injecting the hollow areas with epoxy or
remove the blister and fill the spot with the same
overlay material, blending the edges to create a
less noticeable patch.
Shrinkage Cracks in Overlay
Question:
Why did this integrally colored stampable overlay
crack so drastically? It was applied at 8 a.m. on a
hot July day. The air temperature was 85 F, but
the surface temperature was over 100 F.
Answer:
Even though stampable overlays are polymer-
modified, they are still concrete mixtures and
exhibit all the characteristics of concrete,
including shrinkage cracking if care is not taken
during installation. The dry, hot weather and high
surface temperature, in conjunction with the thin
cross section of the overlay, created a situation
where the little moisture present in the overlay
evaporated too quickly, resulting in shrinkage
cracks.
Here are the key factors to remember when
working with overlays in hot conditions:
The temperature of the surface to receive the
overlay is as important as the air temperature.
Only apply overlays when [air and surface?]
temperatures range from 50 F to 90 F. If in doubt,
hold off until temperatures moderate.
Avoid application in direct sunlight or excessive
wind.
To slow hydration, add approved set-retarding
agents to the overlay mix or apply evaporative-
control agents to the surface.
Cure the overlay with plastic or an approved
curing compound.
Not all overlays are the same. Some products
contain water reducers or hydration control
admixtures to help control shrinkage cracking.
Check with the manufacturer of the product you’re
using
