1. WHAT are Blisters?
Blisters are hollow, low-profile bumps on the concrete surface typically
from the size of a $2 coin up to an inch, but occasionally even
2 or 3 inches in diameter. A dense trowelled skin of mortar about
1/8 inch thick covers an underlying void which moves around under
the surface during trowelling.
The void forms under a dense surface skin by one of two phenomenon.
Some believe that incidental air voids rise in sticky concretes
and are trapped under a dense surface skin produced by
trowelling.
Other believe that bleed water rises and collects for form a void
under this skin. That water is reabsorbed into the underlying concrete
leaving a layer of irregular void space under the surface which
is then consolidated by trowelling to form a round blister which
moves during subsequent trowelling. Frequently, the blister is lined
with a faint layer of "washed" sand.
In poorly lit areas, small blisters may be difficult to sec during
finishing and may not be detected until they break under traffic.
2. WHY Do Blisters Form?
Blisters form when the fresh concrete surface is sealed by
trowelling
while the underlying concrete is plastic and bleeding or able to
release air. The small round blisters form fairly late in the finishing
process, after floating and after the first trowelling.
Moderately rapid evaporation of bleed water makes the surface ready
to be trowelled while the underlying concrete is still bleeding or
still plastic and releasing air. Evaporation from the surface is
increased by wind. low relative humidity or a warm concrete surface.
If evaporation is too rapid. the slab will be affected to a depth
of an inch or more and blisters will be prevent – but plastic
shrinkage cracks may develop!
Entrained air is often involved since it reduces that rate of bleeding
and supplies the fat necessary to produce the dense impermeable
surface layer. A cool subgrade will delay set in the bottom and
make the top set first.
Blisters are more likely to form if:
a. The subgrade is cool and the concrete in the bottom sets slowly.
b. Entrained air is used or is higher than normal so that the surface
is ready to finish earlier.
c. A dry shake is used. particularly over air entrained concrete.
d. The concrete is sticky from higher cement content or excessive
fine sand. Lean mixes bleed rapidly for a shorter period. have higher
total bleeding and tend to delay finishing.
e. The slab is thick.
f. The slab in on polythene and the slump is less than 3 or 4 inches.
g. Excessive use of a jitterbug or a vibrating screed which works
up a thick mortar layer on top
3. How to Prevent Blisters
The finisher should be wary of a concrete surface that appears to be ready to trowel before it would normally be expected to be. Emphasis in finishing should be on placing, straightening and floating the concrete as rapidly as possible and without working up an excessive layer of fat. After these operations are completed, further finishing should be delayed as long as possible and the surface covered with polythene or otherwise protected from evaporation. In initial floating the float blades should be flat to avoid densifing the surface too early. Use of an accelerator or heated concrete often prevents blisters in cool weather.
If blisters are forming, try to either flatten the trowel blades or tear the surface with a wood float and delay finishing as long as possible. Any steps that can be taken to slow evaporation should help.
Follow These Rules to Avoid Blisters
1. Do not seal surface before air or bleed water from below have
escaped.
2. Avoid dry shakes on air-entrained concrete.
3. Use heated or accelerated concrete to promote even setting throughout
the depth of the slab.
4. Do not place slabs directly on polythene sheeting.
References.
1. "Guide for Concrete Floor and Slab Construction" ACI
302 1R-8C, Sections 2.4.1, 8.4 and 11.7 Concrete International.
June 1980 pp. 51-96 and ACI Manual of Concrete Practice, Pan 2,
1983. American Concrete Institute.
2. "Carl O. Peterson "Concrete Surface Blistering –
Causes and Cures" Concrete Construction Publications, September
1970
3. "Finishing" Concrete Construction, August 1976 p. 369
4. "J.C. Yeager. "Finishing Problems and Surface Defects
in Flatwork. Concrete Construction, April 1979
5. Problems and Practices. ACI Journal. December 1955