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