Closing the surface too early, especially on air-entrained, sticky or slow-bleeding mixes, or over a cold subgrade with sun heating the top, seals a skin of mortar over air and water still rising from below. The result is blisters about 1/4 in to 4 in across that pop under a knife and break out later under traffic. Waiting longer before troweling and keeping the blades flat on early passes prevents it.
Got blistering on the north half, quit closing it up and give it twenty minutes.
Also heard as
- blisters
Related terms
Delamination
A surface layer that has split away from the concrete below it, found by the hollow sound it makes when tapped.
Bleed water
Water that rises to the surface of fresh concrete as the heavier solids settle.
Air entrainment
Deliberately building billions of microscopic air bubbles into concrete so it survives freezing and thawing.
Fresno
A long-handled steel trowel finishers swing across a slab from its edge to smooth it without walking on it.
Burn
To steel-trowel a slab again and again at high blade pitch until it turns hard, dark and glossy.