What Are Blisters?
Blisters are hollow, low-profile bumps on the concrete surface typically from the size of a dime up to an inch, but occasionally even 2 or 3 inches in diameter. A dense troweled skin of mortar about 1/8 inch thick covers and underlying void which moves around under the surface during troweling.
The void forms under a dense surface skin by one of two phenomenons. Some believe that incidental air voids rise in sticky concretes and are trapped under a dense surface skin produced by troweling. Others believe that bleed water rises and collects to 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 troweling to form a round blister which moves during subsequent troweling. Frequently, the blister is lined with a faint layer of “washed” sand.
In poorly lighted areas, small blisters may be difficult to see during finishing and may not be detected until they break under traffic.
Why Do Blisters Form?
Blisters form when the fresh concrete surface is sealed by troweling 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 troweling.
Moderately rapid evaporation of bleed water makes the surface ready to be troweled 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 prevented – but plastic shrinkage cracks may develop!
Entrained air is often involved since it reduces the 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:
- The subgrade is cool and the concrete in the bottom sets slowly.
- Entrained air is used or is higher than normal so that the surface is ready to finish earlier.
- A dry shake is used, particularly over air entrained concrete.
- 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.
- The slab is thick.
- The slab is on polyethylene and the slump is less than 3 or 4 inches.
- Excessive use of a jitterbug or a vibrating screed which works up a thick mortar layer on top.
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, straight edging 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 polyethylene or otherwise protected from evaporation. In initial floating the float blades should be flat to avoid densifying the surface too early. Use of an accelerator or heated concrete often prevents blisters in cool weather.
If blister 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 than can be taken to slow evaporation should help.
Follow These Rules to Avoid Blisters
- Do not seal surface before air or bleed water from below have escaped.
- Avoid dry shakes on air-entrained concrete.
- Use heated or accelerated concrete to promote even setting throughout the depth of the slab.
- Do not place slabs directly on polyethylene sheeting.
- “Guide for Concrete Floor and Slab Construction,” ACI 302.1R, ACI Manual of Concrete Practice, American Concrete Institute, P.O. Box 19150 Redford Station, Detroit, MI 48219.
- Carl O. Peterson, “Concrete Surface Blistering-Causes and Cures,” Concrete Construction, September 1970, p. 317. Concrete Construction publications, Inc., 426 S. Westgate, Addison, IL 60101.
- “Finishing,” Concrete Construction, August 1976, p. 369.
- J.C. Yeager, “Finishing Problems and Surface Defects in Flatwork,” Concrete Construction, April 1979, p. 247-258.
- Problems and Practices, ACI Journal, December 1995, p. 492.