Q: When there has been no known change in concrete mix design, what are some of the other things that might account for a sudden drop in concrete strength below the specification limit?

A: Assuming that the concrete supplier is exercising good control, the following possibilities should be investigated:

  • excessive air content caused by changes in mixing time or temperature during transit
  • long waiting times in the trucks at the jobsite, especially in hot weather
  • water added to the mix at the jobsite
  • test cylinders not being properly made
  • improper handling of concrete cylinders enroute to the laboratory