Difficulty: Easy
Correct Answer: 28 days
Explanation:
Introduction / Context:
Concrete gains strength with time as cement hydrates. Codes define a standard test age for acceptance so results are comparable across sites and mixes. In Indian practice, quality control cubes are commonly tested at a specified age that represents the “full” characteristic strength for compliance and design assumptions.
Given Data / Assumptions:
Concept / Approach:
While concrete gains appreciable strength in the first week, strength development continues beyond 7 and 14 days. The conventional benchmark age for characteristic compressive strength (used for compliance) is 28 days. This standardization allows consistent specification, mix design, and acceptance criteria. Earlier tests (7-day or 14-day) are often used only as indicators or for early decision-making, not for final acceptance unless otherwise specified.
Step-by-Step Solution:
Verification / Alternative check:
Site QC logs and laboratory forms schedule cube tests at 7 and 28 days, with 28-day results used for contractual acceptance.
Why Other Options Are Wrong:
7, 14, and 21 days reflect partial hydration; they are useful indicators but not the standard acceptance age for characteristic strength.
Common Pitfalls:
Assuming slow strength gain implies failure; many mixes gain strength beyond 28 days, but acceptance remains tied to the 28-day benchmark unless otherwise agreed.
Final Answer:
28 days
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