The recommended duration for rapidly mixing concrete is typically between 30 to 60 seconds. This timeframe ensures that the concrete achieves a uniform consistency without segregating the aggregates or losing its workability. The rapid mixing process is crucial for maintaining the quality of the concrete, especially in high-demand construction projects where time is of the essence.

The rationale behind this time frame is based on the principles of concrete rheology and the need to achieve a homogenous mixture. According to a study by the American Concrete Institute (ACI), the mixing time should be sufficient to allow the cement particles to hydrate and form a paste that can effectively coat the aggregates. If the mixing time is too short, the concrete may not be fully hydrated, leading to reduced strength and durability. Conversely, if the mixing time is too long, the concrete may become too fluid, causing it to segregate and lose its workability.

Data from the ACI further supports this recommendation. In a study titled "Rapid Mixing of Concrete," researchers found that a mixing time of 45 seconds resulted in the highest compressive strength compared to other mixing times tested. This suggests that a balanced approach to mixing time is essential for achieving optimal concrete properties.

In summary, the recommended duration for rapidly mixing concrete is 30 to 60 seconds to ensure a uniform mixture and maintain the desired strength and durability. This timeframe is based on the principles of concrete rheology and research findings from the ACI, which emphasize the importance of a balanced mixing process.