Introduction:
When testing concrete specimens, the rate of loading refers to the speed at which the load is applied during the testing process. The compressive strength of concrete is a key property that determines its ability to withstand compressive loads without failure. This property is influenced by various factors, including the rate of loading.

Effect of increased rate of loading on the compressive strength of concrete:
When the rate of loading is increased during the testing of concrete specimens, the compressive strength of concrete tends to increase. This means that the concrete can withstand higher compressive loads without failure compared to when it is tested at a slower rate of loading.

Explanation:
When a concrete specimen is subjected to a load, the stress is applied gradually over time. The rate at which the load is applied determines the time available for the concrete to adjust and redistribute the stress internally. At a slower rate of loading, the concrete has more time to undergo microstructural adjustments, leading to a higher compressive strength.

On the other hand, when the rate of loading is increased, the concrete does not have enough time to undergo these microstructural adjustments. As a result, the stress within the concrete is concentrated in localized areas, leading to the formation of cracks and a reduction in the overall compressive strength.

However, when the rate of loading is increased beyond a certain limit, the effect of strain rate may dominate over the microstructural adjustments. This can lead to an increase in the compressive strength of concrete. This phenomenon is known as the strain rate effect and is more pronounced in high-strength concrete.

Conclusion:
In conclusion, the compressive strength of concrete tends to increase when the rate of loading during testing is increased. However, it is important to note that the exact effect of the rate of loading on the compressive strength can vary depending on several factors, including the type of concrete, its age, and the specific testing conditions.