At a project that has been on-going for over a year, the testing agency recently reported a reduction in 28-day strength of a 5,000-psi mix. This project uses multiple strength concrete from the same supplier ranging from 3,000-psi to 7,000-psi, but only the 5,000-psi mix has demonstrated this reduction in strength. There have been no changes in the testing agency, that is, the field technician casting the test specimens is the same, the initial and lab curing techniques have not changed, and the lab strength testing personnel and equipment are no different. Additionally, the concrete supplier is showing 28-day strengths for specimens cast at the batch plant from the same mix between 750 and 1200-psi higher than the specimens cast in the field. The slumps and air content at both the plant and the site are similar for comparison samples, and the concrete temperature is predictably 8 to 10 degrees F. higher on site. Any ideas why only the 5,000-psi concrete is showing this reduction in strength?