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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.
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