Blisters
These bumps, of varying size, appear at a time when bubbles of entrapped air or water rising through the plastic concrete get trapped under an already sealed, airtight surface.
Causes of Blisters:

1. An excess amount of entrapped air held within the concrete by a high percentage of material passing the number 30, 50, and 100 sieves, resulting in a sticky or tacky concrete that can become more easily sealed when floating or finishing at an early age.
2. Insufficient vibration during compaction that does not adequately release entrapped air; or overuse of vibration that leaves the surface with excessive fines, inviting crusting and early finishing.
3. Finishing concrete too early. Tools used to finish concrete will tend to force entrapped air toward the surface. Blisters, which will be full of water and air when picked, also can appear at any time and without apparent cause.

Ways to avoid Blisters:

1. Do not use concrete with a high slump, excessively high air content, or excess fines.
2. Use appropriate cement contents in the range of 515 lbs. to 565 lbs. per yard.
3. Warm the subgrade before placing concrete on it during cold weather.
4. Avoid overworking the concrete, especially with vibrating screeds, jitterbugs, or bullfloats. Overworking causes aggregate to settle and bleed water and excess fines to rise. Properly vibrate to release entrapped air.
5. Do not attempt to finish the surface too soon. Use a wood bullfloat on non-air-entrained concrete to avoid early sealing. Magnesium or aluminum tools should be used on air-entrained concrete.
6. Reduce evaporation over the slab by using a fog spray or slab cover.
7. Avoid using air contents over 3% for interior slabs.
   

Concrete Links

National Ready Mixed Concrete Association

Tilt-Up Concrete Association

The Portland Cement Association

American Concrete Institute

American Society of Testing Materials

Concrete.com

Effingham Area Home Builders Association