Cement Hydration – Pattern of hardening over time Which statement best describes how the hardening (strength gain) of ordinary Portland cement concrete progresses with age under normal curing?

Introduction / Context:
Strength development in concrete is a direct consequence of cement hydration, which is temperature and moisture dependent. Understanding the general trend of hardening helps in planning formwork removal, prestressing operations, and early loading decisions.

Given Data / Assumptions:

• Ordinary Portland cement concrete with continuous moist curing.
• Ambient temperatures within normal ranges.
• No special admixtures intended to drastically alter early-age kinetics.

Concept / Approach:

Hydration is fastest immediately after casting and during the first few days, leading to rapid early strength gain. Over time, as unhydrated cement becomes less accessible and products densify, the rate of strength gain decreases. Nevertheless, strength continues to increase beyond 28 days, albeit slowly, especially in mixes with supplementary cementitious materials.

Step-by-Step Solution:

Verification / Alternative check:

Typical strength–time curves show steep initial slope flattening with age. Field practice reflects this with 7-day indicator tests and 28-day acceptance tests.

Why Other Options Are Wrong:

Uniform rate is unrealistic; a delayed rapid increase contradicts hydration kinetics; “none” and “stops after 7 days” are incorrect.

Common Pitfalls:

Equating slow gain at low temperatures with failure; removing formwork too early; neglecting curing which strongly affects early and later strength.

Final Answer:

Rapid rate during the first few days, then continues to increase at a decreased rate