Florida DOT Concrete Batchman/Operator Practice Exam

When silica fume or metakaolin is used in a concrete mix, what must also be included?

1. Fine aggregates

2. An approved High Range Water Reducer (HRWR)

3. Accelerating admixture

4. Standard water reducing admixture

The correct answer is: An approved High Range Water Reducer (HRWR)

Including an approved High Range Water Reducer (HRWR) in a concrete mix with silica fume or metakaolin is essential due to the properties of these supplementary cementitious materials. Silica fume and metakaolin can significantly improve the strength and durability of concrete but also tend to increase the water demand of the mix. The HRWR helps to reduce the water-to-cement ratio while maintaining the workability of the concrete. This is crucial because a lower water-to-cement ratio enhances the overall performance of the concrete, leading to improved mechanical properties and reduced permeability. Therefore, the combination of these materials with a suitable HRWR creates a balanced mix that achieves high performance and desired specifications. In contrast, the other options do not play the same role in maintaining the proper workability and performance when using silica fume or metakaolin. Fine aggregates might be necessary in general concrete mixes, but they do not specifically counteract the effects of these materials the way an HRWR does. Accelerating admixtures can alter the setting time, which may or may not be desired in the context of silica fume or metakaolin use. Standard water reducing admixtures may not provide the same level of effectiveness as HRWR when such materials are present