Concrete blisters

Blisters form when the fresh concrete surface is sealed by troweling while the underlying concrete is still plastic and bleeding or able to release air. Moderately rapid evaporation of bleed water makes the surface ready to trowel while the underlying concrete is still plastic.

Blisters are more likely when:

• The subgrade is cool and the concrete bottom sets slowly.
• Entrained air is used or is higher than normal, so 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.
• The slab is thick or placed on polythene with slump less than 3–4 inches.
• Excessive use of a jitterbug or vibrating screed creates a thick mortar layer on top.

Emphasis in finishing should be on placing, straightening and floating the concrete as rapidly as possible without working up an excessive layer of fat. After these operations are completed, delay further finishing as long as possible and cover the surface with polythene or otherwise protect from evaporation.

In initial floating, keep float blades flat to avoid densifying the surface too early. Use an accelerator or heated concrete to promote even setting throughout the depth of the slab in cool weather.

Rules to avoid blisters:

• Do not seal the 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 slab depth.
• Do not place slabs directly on polythene sheeting.