At what age are cylinder strengths commonly compared to the project target strength in FDOT projects?

• A. 7 days
• B. 56 days
• C. 90 days
• D. 28 days

Answer: (D)

The main idea is that the project target strength is the strength the concrete must reach at a standard testing age, and the actual cylinder strengths are compared to that target to decide acceptance. In FDOT projects, the 28-day strength is the baseline age used for this comparison because it best represents the long-term performance of the concrete and is the standard reference for most mix designs. Early-age results, like 7 days, are used for early strength checks or process monitoring but do not set the acceptance criterion. Ages like 56 or 90 days are not the typical basis for project target comparisons unless a special specification calls for them. So, comparing cylinder strengths to the project target at 28 days is the standard practice.