Blisters are hollow; low profile bumps on the concrete surface typically ranging from the size of a dime up to an inch, but occasionally 2 or 3 inches in diameter.

Common Causes
The most common cause is a dense troweled skin of mortar about 1/8 inch thick covers an underlying void which moves around under the surface during troweling. Some causes of blisters are:

• Troweling began too soon; resulting in the surface being sealed too early while the underlying concrete is plastic and bleeding or able to release entrapped air..
• Rapid evaporation from the surface is increased by wind, low relative humidity or a warm concrete surface.
• A cool subgrade will delay set in the bottom and make the top set first.
• Entrained air is often involved since it reduces the rate of bleeding.
• The concrete is sticky from higher cement content or excessive fine sand.
• The slab is on a vapor barrier and the slump is more than 4 inch.
• Excessive use of a jitterbug or a vibrating screed which works up a thick mortar layer on top.

Prevention
The following practices will help to minimize the potential of curling:
• Float the surface immediately behind the screed and prior to the appearance of bleed water.
• Do not seal surface by steel troweling before entrapped air or bleed water from below has escaped. The surface of the slab must be firm enough to leave only ¼ inch deep footprints. Early finishing can cause crazing, dusting, blistering and or scaling.
• Avoid dry shakes on air entrained concrete.
• Use heated or accelerated concrete to promote even setting through the depth of the slab.
• Do not place slabs directly on vapor barrier. Cover vapor barrier with a damp compacted layer of sand 2 inch thick.
• Use low-slump concrete not to exceed 4 inch, or use water reducer to adjust slump.
• Avoid conditions that force early finishing.