Aggregates having a smooth grading curve and neither a deficiency nor excess of any one particle size will generally produce mixtures with fewer voids between particles.

Since cement costs more than aggregate and the cement paste requirement for concrete increases with increasing void content of the combined aggregates, it is desirable to keep the void content as low as possible.

Not enough fines: If there is not enough sand to fill the voids between coarse aggregate particles, the space must be filled with cement paste. These under sanded mixes also tend to be harsh and difficult to finish.

Excess fines: There are three options before us when the aggregate surface area increases due to increased fineness of aggregate particles:

Option 1	Cement paste content is kept as such (i.e without any change)	Thinner layers of paste surrounding the aggregate particles result in a stiffer concrete that is harder to place and compact
Option 2	Make the paste more fluid by adding water	Concrete strength and durability will suffer
Option 3	More cement and water are added	Cost of the concrete increases

It can be seen that the first option will lead to placement & compaction difficulties while the second option may not be technically acceptable.

Thus the only acceptable option in almost all the important works is option 3 which would mean a much higher cost of concrete. This is because of the larger surface area of finer particles: which requires more cement paste to coat the additional surface.(Please refer: Why the surface area of aggregate increases as the particle size decreases?)

Thus the presence of either “Not enough fines” or “Excess fines” in the mix is undesirable. Therein lies the significance of aggregate grading.