Standard reference age for full (characteristic) strength of concrete as per IS practice At what standard age is the characteristic compressive strength of concrete specified and accepted for most design and quality control purposes in IS practice?

Introduction / Context:
Concrete continues to gain strength with time, but codes adopt a standard reference age to define characteristic compressive strength for design and acceptance. In Indian Standards and most international practice, the 28-day age is the benchmark.

Given Data / Assumptions:

• Standard curing conditions in water at controlled temperature.
• Ordinary Portland cement or blended cements typical of structural concrete.
• Characteristic strength defined statistically (e.g., fck).

Concept / Approach:
Hydration proceeds over months and years, but early-age strengths are used for construction scheduling while 28-day strength provides a consistent, comparable basis for mix design and structural calculations. Acceptance testing commonly relies on average strengths at 28 days meeting or exceeding specified characteristic values with defined variability margins.

Step-by-Step Solution:
Identify the standard reference age adopted by IS codes → 28 days.Recognize that earlier ages (7 or 14 days) are sometimes used for preliminary checks but not as the final acceptance strength unless specified otherwise.Select “28 days”.

Verification / Alternative check:
IS 456 and related testing standards specify 28-day cube tests for acceptance criteria; rapid methods (maturity, accelerated curing) are correlated back to the 28-day benchmark.

Why Other Options Are Wrong:
7, 14, 21, or 35 days may be used for progress estimation or special cements, but the universally adopted design strength age is 28 days unless the specification states otherwise.

Common Pitfalls:
Confusing early stripping strength with design strength; formwork removal may occur earlier depending on cement type, curing, and ambient temperature.

Final Answer:
28 days