Florida DOT Concrete Batchman/Operator Practice Exam

If the temperature is over 100°F, aggregates and water must be mixed together prior to the addition of cement. What condition should be met regarding the mixing duration after discharging into the drum?

1. At least one minute

2. At least two minutes

3. At least five minutes

4. At least ten minutes

The correct answer is: At least two minutes

When the temperature exceeds 100°F, it is crucial to ensure proper mixing of the aggregates and water prior to adding cement. This procedure allows for better hydration of the cement and avoids the onset of undesirable properties in the concrete caused by rapid drying and heat effects. The mixing duration after discharging into the drum is vital for achieving a homogeneous mixture and ensuring the concrete achieves its desired strength and durability. Mixing for at least two minutes provides sufficient time for the aggregates to become thoroughly coated with water, leading to more effective hydration of the cement once it is added. This duration strikes a balance; it is long enough to ensure adequate mixing without unnecessarily prolonging the process, which could lead to material segregation or other issues. Mixing for shorter durations may not adequately prepare the mix, leading to inconsistent concrete quality, while longer durations, such as five or ten minutes, can cause other complications, including increased wear on equipment and a higher risk of the mix starting to set. Therefore, the specified minimum mixing duration of at least two minutes is a standard practice designed to enhance the quality of the final concrete product in hot weather conditions.