What is required when trial mixes are prepared for compressive strength testing?

When preparing trial mixes for compressive strength testing, it is essential to utilize a set of three cylinders for each of the specified break times, which typically include 3, 7, and 28 days. This approach allows for a comprehensive evaluation of the concrete's strength development over time.

Using three cylinders provides a reliable average for the compressive strength since it helps mitigate the impact of variability in material properties, mixing, and curing conditions. By testing at different ages, one can observe how the concrete increases in strength with time, which is critical for determining if the mix design meets the necessary performance criteria for structural applications.

Choosing a single cylinder or two cylinders does not offer sufficient data to represent the reliability of the mix. Testing five random cylinders is also not effective for establishing a standard time progression for strength evaluation. Thus, using the measured sets at designated intervals allows for an accurate assessment of the concrete's performance according to specified standards.