For exposed concrete surfaces, what is the minimum duration of moist curing typically specified to achieve adequate early hydration and durability?

Introduction / Context:
Proper curing maintains moisture and temperature to support cement hydration, reduce plastic shrinkage cracking, and improve durability. For exposed surfaces, inadequate curing leads to a weak, porous skin and higher permeability, undermining long-term performance.

Given Data / Assumptions:

• Surface type: exposed concrete (not permanently protected).
• Objective: select the minimum moist curing period commonly required.
• Assume ordinary Portland cement without special accelerators or hot-weather adjustments.

Concept / Approach:

Baseline prescriptions for OPC concretes specify not less than about 7 days of continuous moist curing for exposed surfaces. Longer periods (10–14 days) are recommended for blended cements or harsh climates, but the minimum benchmark remains 7 days for standard conditions.

Step-by-Step Solution:

Verification / Alternative check:

Field QA often checks curing records and surface temperature/humidity; achieving 7 continuous days significantly improves near-surface strength and permeability versus shorter durations.

Why Other Options Are Wrong:

• 3 or 5 days: Too short for reliable hydration and durability gains.
• 14 days: Beneficial but beyond the asked minimum.
• 10 days: Typical for blended cements; not the baseline minimum for OPC.

Common Pitfalls:

• Stopping curing early due to schedule pressure, leading to dusting or map cracking.
• Counting intermittent wetting as continuous curing; continuity matters.

Final Answer:

7 days