The concrete mix designated by compressive strength has a 42-day strength greater than how many psi?

• A. 3200
• B. 2800
• C. 3400
• D. 3600

Answer: (D)

Concrete continues to gain strength as it hydrates, so the 42-day strength is a key check on long‑term performance. For a mix designated by its compressive strength, the specification expects that the 42-day compressive strength will exceed a fixed minimum to ensure durability under typical conditions. That minimum is 3,600 psi, so the 42-day strength should be greater than 3,600 psi. The lower thresholds wouldn’t provide the same level of assurance for long‑term capacity, which is why 3,600 psi is the standard target.