What causes fresh concrete floor surfaces to blister?
Blistering occurs when a fresh concrete surface is sealed airtight at a time when air voids are migrating to the surface. The air collects under the airtight surface and creates blisters. Several conditions may contribute to their formation. One is wind blowing over the concrete surface, reducing its moisture content. Another is a subgrade that is cooler than the concrete so that the top surface sets faster than the bottom. A sticky mixture containing excessive fines also contributes to sealing of the surface and blistering. Lean mixes can add to the problem because they may have to be worked excessively to produce the desired finish and the extra work may seal the surface. Finishing too early and the misuse of tools also can lead to blistering. Examples of such misuse are making an excessive number of passes with a vibrating screed, excessive use of the jitterbug, excessive use of the bull float or using a power float with the blade not flat.