If the concrete compressive strength specified is less than 3600 psi, it has the option of being tested at 28 days and is able to accept the concrete if it attains _________ percentage of the minimum required strength.

• A. 80
• B. 85
• C. 75
• D. 90

Answer: (B)

For concrete with a specified strength under 3600 psi, the 28‑day acceptance rule uses a clear threshold: the 28‑day strength must reach at least 85% of the required minimum strength. This provides an early, practical check of quality without waiting for higher-strength mixes to mature. For example, if the specified strength is 3200 psi, the 28‑day test must be at least 0.85 × 3200 = 2720 psi to be accepted. The 85% criterion helps balance the need for timely progress with ensuring adequate performance. If the specified strength were 3600 psi or more, a different, typically stricter criterion would apply.