Question

Briefly State the influence of maximum aggregate size (i.e 19 mm vs. 38 mm) on the mixing water content and the cement content of a concrete mix with a given water-cement ratio of 0.5

Answer 1

For a given water-cement ratio of 0.5, we have to fluctuate the amount of water and cement content in the same proportion.

The grading and size of aggregate is an important characteristic because it determines the cement paste requirement of workable concrete. The required amount of cement paste is dependent upon the amount of void space that must be filled and the total surface area that must be covered.

Note: While comparing the maximum size of aggregate we have to make the grading of aggregates to be constant i.e. amount of void space between the aggregates is the same for both the maximum size of aggregates. (Then only we can comment on the influence of the maximum size of aggregate on the water and cement content of the concrete mix).

1. If we use the maximum aggregate size of 19mm, then the total surface area, i.e. the specific surface area (surface area/volume of aggregate) required to cover for the cement paste will be more, hence we have to increase the amount of cement paste, i.e. we have to increase the water and cement content of a concrete mix to make the same workable concrete.
2. If we use the maximum aggregate size of 38mm, then the total surface area, i.e. the specific surface area (surface area/volume of aggregate) required to cover for the cement paste will be less, hence we have to decrease the amount of cement paste, i.e. we have to decrease the water and cement content of a concrete mix to make the same workable concrete.