Question: In a floor made with a mix containing 4 to 6 percent air with a slump of 4 to 5 inches, we have had the experience a number of times of small pimples developing. These have been entrapped air, not entrained air, and extend down into the concrete about 1/2 inch. They commonly loosen, not on every job or in every load, but only once in a while. The mixers are clean, the fins in the mixer are good and the concrete has been well mixed. What causes the trouble?

Answer: Perhaps you are speaking of blisters, though blisters are usually 1/4 inch to 4 inches in diameter and only about 1/8 inch deep. Blisters occur when entrapped air (some workmen argue that it is bleed water) gets trapped in the concrete.

Preventive measures include:
  • If the subbase is cold, heat it before placing concrete.
  • Do not use sticky mixtures. Adjust proportions to eliminate stickiness.
  • If slump is over 3 inches do not use vibrating straightedge.If vibrating straightedge is used, move it forward as rapidly as proper consolidation allows.
  • Avoid using grate tampers.Use a wood, not metal, bull float and minimize the use of a vibrating straightedge.
  • Use the lightest weight power floats available.
  • Do not finish too early.

    The flat troweling; pitch the trowel slightly on the final pass.