Strength testing — age for specified compressive strength of concrete (cube test) At what standard age is the specified compressive strength of concrete determined from cube tests for acceptance and design purposes?

Introduction / Context:
The characteristic or specified strength of concrete is tied to a standard age of testing to enable consistent design and quality control. Codes standardize the acceptance age to harmonize mix design, curing, and structural timing decisions.

Given Data / Assumptions:

• Normal Portland cement and ordinary curing conditions.
• Standard cube specimens prepared and cured per code.
• Definition of specified/characteristic strength.

Concept / Approach:

While early-age tests (3, 7 days) are useful indicators of strength development, the specified strength for design is conventionally taken at 28 days. This correlates with maturity models and typical construction timelines, providing a balance between early assessment and near-ultimate strength for many mixes.

Step-by-Step Solution:

Verification / Alternative check:

Most national standards define characteristic strength at 28 days, with provisions for other ages only if explicitly specified by the designer.

Why Other Options Are Wrong:

• 3, 7, 14, and 21 days are interim ages and typically show lower strengths.

Common Pitfalls:

• Assuming 7-day strength guarantees 28-day strength without considering curing and cement type.

Final Answer:

28 days.