Question

How does the angularity of coarse aggregate impact the water demand of a concrete mix, assuming all other factors remain constant?

Accepted Answer

The angularity of coarse aggregate significantly impacts the water demand of a concrete mix. Angular aggregates, characterized by their sharp edges and rough surfaces, require more water to achieve a given level of workability compared to rounded aggregates. Workability refers to the ease with which concrete can be mixed, placed, consolidated, and finished. The higher water demand stems from the increased surface area and interlocking nature of angular aggregates. First, angular aggregates have a greater surface area compared to rounded aggregates of the same volume. This increased surface area requires more water to coat each particle fully and provide adequate lubrication for the mix to flow. Second, the angular shape promotes interlocking between aggregate particles. This interlocking increases the internal friction within the concrete mix, making it stiffer and more difficult to work with. More water is then needed to overcome this friction and achieve the desired slump, which is a measure of concrete consistency. If the water content is not increased to compensate for the angularity, the concrete mix will be harsh, difficult to place, and prone to segregation, where the different components of the concrete separate. Conversely, if rounded aggregates are used, the water demand is lower due to their smaller surface area and reduced interlocking. This can lead to a more workable mix at a lower water-cement ratio, potentially resulting in higher strength and durability. Therefore, concrete mix designs must account for aggregate shape to achieve the desired workability and performance characteristics.

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How does the angularity of coarse aggregate impact the water demand of a concrete mix, assuming all other factors remain constant?