Q:In designing concrete structures, normally maximum aggregate sizes are adopted with ranges from 10mm to 20mm. Does an increase of maximum aggregate size benefit the structures?

To answer this question, let’s consider an example of a cube. The surface area to volume

ratio of a cube is 6/b where b is the length of the cube. This implies that the surface area to

volume ratio decreases with an increase in volume. Therefore, when the 

size of maximum  aggregate is increased, the surface area to be wetted by water per unit volume is reduced.

Consequently, the water requirement of the concrete mixes is reduced accordingly so that

the water/cement ratio can be lowered, resulting in a rise in concrete strength.

However, an increase of aggregate size is also accompanied by the effect of reduced

contact areas and discontinuities created by these larger sized particles. In general, for

maximum aggregate sizes below 40mm, the effect of lower water requirement can offset

the disadvantages brought about by discontinuities as suggested by Longman Scientific and

Technical (1987).