Understanding Ordinary Cement Strength Development

The strength of ordinary cement mortar or concrete develops over time through a process called hydration. Hydration is a chemical reaction between cement and water. This reaction forms a paste that binds the aggregate particles together.

Strength Gain Over Time

The strength gain of cement paste is not instantaneous; it is a process that continues for a long time, potentially years. However, the rate of strength gain is highest in the early days and weeks after mixing. Different standards and tests are used to evaluate the strength of concrete at various ages to understand its development.

Common ages for testing concrete strength include:

• 1 day
• 3 days
• 7 days
• 14 days
• 28 days
• 56 days
• 90 days
• 1 year

The question asks about the point at which ordinary cement achieves about 99% of its final strength. While strength development continues, a significant portion of the ultimate strength is achieved relatively early.

Significance of the 28-Day Strength

In concrete construction, the 28-day compressive strength is a very common benchmark. Specifications and designs are typically based on the concrete achieving a specified strength at 28 days. By 28 days, the hydration process has progressed significantly, and the concrete has gained substantial strength. Although strength continues to increase beyond 28 days, the rate slows down considerably.

Different types of cement and curing conditions affect the rate of strength gain. However, for ordinary Portland cement under standard curing conditions, the 28-day strength is considered a critical indicator of concrete quality and its ability to meet design requirements.

Based on standard practices and tests, while the exact percentage of final strength achieved by 28 days can vary, 28 days is the period commonly associated with reaching a very high percentage of the strength that is considered for practical purposes, often used as the 'specified strength'. The question states that about 99% of final strength is achieved in this timeframe.

Considering the options and the standard understanding of cement strength development, 28 days is the most relevant period among the choices for achieving a very high percentage of the final strength.

The table above shows strength gain relative to the 28-day strength, which is the standard benchmark. If 'final strength' refers to strength after a very long period (e.g., 1 year or more), then achieving 99% by 28 days implies that the strength gain after 28 days is very minor. While the rate slows down after 28 days, strength does continue to increase. However, given the options, 28 days is the standard age for significant strength achievement in ordinary cement.

Conclusion on Ordinary Cement Strength

The development of strength in ordinary cement is a function of hydration. While strength increases continuously, 28 days is a critical age for evaluation, as a large proportion of the total strength is typically gained by this point. The question indicates that around 99% of the final strength is achieved by this age, making 28 days the appropriate answer among the given options for when ordinary cement reaches this level of strength.

Revision Table: Ordinary Cement Strength

Additional Information on Concrete Strength

Concrete strength is influenced by several factors beyond just the age of the concrete and the type of cement. These factors include:

• Water-cement ratio: A lower water-cement ratio generally leads to higher strength, provided there is enough water for hydration.
• Curing conditions: Proper curing (maintaining moisture and temperature) is essential for complete hydration and optimal strength development.
• Type of aggregate: The strength and quality of the aggregates (sand and gravel) also affect the overall concrete strength.
• Admixtures: Chemical admixtures can be used to modify the setting time, improve workability, or enhance strength.
• Temperature: Higher temperatures generally accelerate early strength gain but can reduce ultimate strength if not controlled. Lower temperatures slow down hydration and strength gain.

Understanding these factors is crucial for producing concrete that meets the required strength and durability for construction projects.