The question asks about how the strength of concrete changes as it gets older. Concrete is a material that gains strength over time through a process called hydration.

Hydration is a chemical reaction between cement and water. This reaction produces a paste that binds the aggregates (like sand and gravel) together. This binding material hardens over time, leading to the development of concrete strength.

Understanding Concrete Strength Development

Concrete strength does not appear instantly. It develops gradually as the hydration process continues. The rate of strength gain is rapid in the early days and weeks but slows down considerably later on. If moisture is available and the temperature is suitable, the hydration process can continue for a very long time, potentially years, leading to continued strength gain.

Typical concrete mixes are designed to achieve a significant portion of their target strength relatively quickly, often specified as the strength after 28 days. However, the concrete continues to gain strength beyond this period.

Here's a general idea of how strength develops relative to the 28-day strength:

• After 1 day: Around 16% to 30% of 28-day strength
• After 3 days: Around 40% to 60% of 28-day strength
• After 7 days: Around 65% to 80% of 28-day strength
• After 28 days: Considered 100% of the specified design strength
• After 3 months: Can be 110% to 125% of 28-day strength
• After 1 year: Can be 115% to 135% of 28-day strength

These percentages can vary depending on the concrete mix design, type of cement, curing conditions (temperature and moisture), and admixtures used.

Analyzing the Concrete Age Options

Let's consider the given options based on our understanding of concrete strength development:

• Increases: This aligns with the process of hydration, where the cement-water reaction continues over time, leading to a stronger binding paste and thus increased concrete strength.
• Decreases: Concrete strength does not naturally decrease with age under normal conditions, especially if it is protected from harmful environments or structural damage.
• No effect: This is incorrect as the primary process of concrete hardening and strengthening is time-dependent hydration.
• Increases, then decreases: While strength gain slows down, it does not typically decrease with age unless external factors cause deterioration.

Therefore, the most accurate description of how concrete strength differs with age is that it increases.

Concrete Strength Development Factors

Several factors influence the rate and extent of concrete strength gain:

• Water-Cement Ratio: A lower water-cement ratio generally leads to higher potential strength, provided there is enough water for complete hydration.
• Curing Conditions: Maintaining adequate moisture and temperature (curing) is crucial for hydration to continue and strength to develop. Poor curing leads to lower strength.
• Type of Cement: Different types of cement hydrate at different rates, affecting early and later age strengths.
• Admixtures: Chemical admixtures can accelerate or retard the hydration process and affect the ultimate strength.
• Aggregates: The strength and properties of aggregates also play a role, but the binding paste's strength is often the limiting factor.

Revision Table: Concrete Properties and Age

Additional Information: Factors Affecting Concrete Strength

Beyond age and hydration, several factors can impact the final strength and durability of concrete. Proper mixing, placement, and compaction are essential to achieve the designed strength. Segregation of aggregates or excessive voids from poor compaction can reduce strength. Environmental exposure, such as freeze-thaw cycles or chemical attacks (like sulfates), can cause deterioration and a reduction in strength over long periods if the concrete is not designed and cured properly to resist these conditions. However, in the absence of such harmful factors, the natural tendency of concrete strength with age is to increase.