Blistering Is the irregular raising of a thin layer at surface of concrete during or right after finishing. Blisters form when bleed water or entrapped air is kept from escaping from the concrete. Some causes for blistering might be: subgrade is cooler than the concrete causing non uniform setting, insufficient or excessive vibration, the top surface of the concrete is sealed prematurely during finishing, rapid surface evaporation causing finisher to mistakenly assume bleeding period is complete, entrained air added to non air entrained concrete (or when air entrainment is abnormally high), excessive concrete thickness resulting in longer times for bleed water and entrapped air to rise to surface, dry shake prematurely applied to the surface, or concrete is placed over a vapor barrier. Below are some ways to reduce the likelihood of such problems:

-Use proper concrete consolidation practices

-Use proper finishing tools and keep the troweling blades as flat as possible to avoid sealing the surface

-Place concrete during periods of lower surface evaporation

-Avoid placing concrete on cold subgrades if possible, heat subgrades prior to placement

-Avoid use of chemical retarders, they will only intensify the problem

-Consider the use of chemical accelerators

-Avoid placing over a vapor barrier if possible