We have started having problems with variations in air content of our air-entrained mix designs. Air content stays uniform for a while, then goes out of the specification range. Usually, measured air content is too low, but sometimes we get exceptionally high values. I know that many factors affect air content. Is there a systematic method for tracking down the problem?

Assuming that your air content measuring methods are accurate, fluctuations in air content can be caused by changes in the materials used to make concrete, changes in mix proportions, and changes in production methods. When the dosage of air-entraining agent remains constant, expect changes in the following variables to have the effects notes.

Materials
Cement

  • Air content decreases as cement fineness increases.
  • Air content increases as cement alkali content increases.
Fly Ash
  • Air content decreases with increasing carbon content of the fly ash. Loss on ignition is a good indicator of fly ash carbon content.
Aggregate
  • Air content decreases with increases in the amount of minus 200 mesh fines in the aggregate.
  • Air content increases with increasing amounts of intermediate sand sizes.
Mixing water
  • Air content decreases when truck mixer wash water is used as mixing water.
Other admixtures
  • Air content increases when lignosulfonate water reducers and retarders are used.
  • Air content decreases when some coloring pigments are used.
  • Melamine-based superplasticizers may decrease air or have little effect. Napthalene and lignosulfonate superplasticizers increase air content.
  • Calcium chloride increases air content.

Mix proportions
Cement content

  • Air content decreases with an increase in cement content.
Sand content
  • Air content increases with increasing sand content.
Slump (water content)
  • Air content increases with increasing water content and slump up to a slump of about 6 or 7 inches. Then higher slumps decrease air content.

Production procedures
Batching sequence

  • Air content decreases when air-entraining agent is simultaneously batched with cement or with other admixtures.
  • Air content increases when air-entraining agent is added late in the batching sequence.
Mixer capacity
  • Air content increases if the mixer is loaded to less than rated capacity and decreases if the mixer is overloaded.