In the designation of concrete mix M refer to the mix and the number to the specified compressive strength of 150 mm size cube at 28 days, expressed in (N/mm²).

Characteristic Strength of Concrete

The characteristic strength of concrete is an important parameter used in the design of structures. It represents the strength at which the material can withstand a certain amount of load without failure. The characteristic strength is determined through testing and is typically measured at a specific age of the concrete.

The correct answer to the given question is option 'B', which states that the characteristic strength of concrete is measured at 28 days. Let's understand why this is the case.

Importance of Concrete Strength
Concrete is widely used in construction due to its ability to provide strength and durability to structures. The strength of concrete determines its load-bearing capacity and its ability to withstand various forces and stresses. Therefore, it is crucial to accurately measure the strength of concrete to ensure the safety and reliability of structures.

Time Dependent Strength Gain
The strength of concrete is time-dependent, meaning it increases over time as the hydration process takes place. Hydration is the chemical reaction between cement and water, which results in the formation of a solid matrix that gives concrete its strength. Initially, when the concrete is fresh, its strength is low, but it gradually increases as the hydration process progresses.

Factors Affecting Strength Gain
Several factors influence the rate of strength gain in concrete, including the type and composition of cement, water-cement ratio, curing conditions, and temperature. These factors determine the rate at which hydration occurs and the final strength of the concrete.

Determination of Characteristic Strength
To determine the characteristic strength of concrete, samples are taken from the structure or batch and tested in a laboratory. The most common test used for this purpose is the compressive strength test, where concrete cylinders are subjected to a gradually increasing load until failure occurs.

28 Days as the Standard
The characteristic strength of concrete is typically measured at 28 days because it is considered the standard age for testing. By this time, most of the hydration process is complete, and the concrete has achieved a significant portion of its final strength. Testing at 28 days provides a reliable indication of the concrete's long-term strength and its ability to withstand the design loads.

Alternative Testing Durations
While the characteristic strength is commonly measured at 28 days, there are cases where other testing durations may be used. For example, in certain situations where early strength is critical, such as in precast concrete elements, the characteristic strength may be measured at 7 or 14 days. Similarly, for special concrete mixtures or specific project requirements, testing may be extended to 56 or 91 days.

Conclusion
In conclusion, the characteristic strength of concrete is an essential parameter in structural design. It is typically measured at 28 days because this duration allows for significant hydration and provides a reliable indication of the concrete's long-term strength. However, alternative testing durations may be used in specific circumstances.