Introduction:
The characteristic strength of cement concrete refers to the compressive strength of a concrete cube or cylinder specimen at a specified age. It is an important parameter used in the design of structural elements. The characteristic strength is determined by conducting laboratory tests on concrete samples cured under specified conditions.

Increase with increase in age of concrete:
The correct answer is option 'B' - the characteristic strength of cement concrete increases with an increase in the age of concrete. This means that as the concrete gets older, its compressive strength increases. There are several reasons for this phenomenon:

1. Cement hydration: Cement is the binding agent in concrete and it undergoes a chemical reaction called hydration. During hydration, cement particles react with water, forming calcium silicate hydrate (C-S-H) gel. This gel contributes to the strength and durability of the concrete. The hydration process is time-dependent and continues for a long period of time. As the concrete ages, more cement particles undergo hydration, resulting in increased strength.

2. Pore structure development: When fresh concrete is placed, it consists of a mixture of cement, aggregates, and water. Over time, the cement particles hydrate and form a solid matrix. Additionally, the water in the concrete gradually evaporates or is consumed by the hydration reaction. As a result, the concrete undergoes a drying process, leading to shrinkage and the development of capillary pores. These capillary pores are gradually filled with C-S-H gel, which enhances the strength of the concrete.

3. Strength gain due to curing: Proper curing is essential for the development of concrete strength. Curing involves maintaining favorable moisture and temperature conditions for the hydration process. During the initial stages of curing (usually the first 28 days), the concrete gains strength rapidly. However, the strength gain continues even after this period, albeit at a slower rate. The longer the concrete is cured, the more time it has to develop its strength.

4. Evidence from testing: Experimental studies and field observations have shown that the characteristic strength of concrete increases with age. Concrete specimens are typically tested at different ages (e.g., 7 days, 28 days, 56 days) to determine their compressive strength. The results consistently demonstrate that the strength increases as the age of the concrete increases.

Conclusion:
In summary, the characteristic strength of cement concrete increases with an increase in the age of the concrete. This is due to cement hydration, pore structure development, strength gain through curing, and evidence from testing. Understanding the relationship between concrete age and strength is crucial for designing and constructing durable and reliable structures.