Concrete is prepared by mixing aggregate, cement and water in such a proportion that it becomes workable in its plastic state and durable in its hardened state.
Our aim is to satisfy both of these requirements i.e. workability and durability.
Importance in terms of Workability
A workable concrete can be achieved by adding more water, but as we know the more water present in concrete the poor strength it gives. The most efficient way to achieve workable of concrete is by adopting a good gradation of aggregate.
Also aggregate comprises 85% of the volume of concrete. So we can definitely say that, most of the requirement of concrete in its plastic state is affected by aggregate properties and, “gradation of aggregate” is one of them.
Importance in terms of Durability
We use cement paste to bind all the aggregates (i.e. fine and coarse) so as to give a compact mass, and this is why concrete is considered as a two phase material, namely, paste phase and aggregate phase.
The paste has following disadvantages as compared to aggregate and, which affects the concrete in its durability aspect.
- Paste is more permeable than aggregate
- Paste has less strength than aggregate
- Paste is more susceptible to chemical attack as compared to aggregate.
The only way to minimize above factors is to use less amount of cement paste to produce concrete. This is achieved if we use a good gradation of aggregate which gives minimum voids. A sample containing minimum voids will require minimum paste to fill up the voids in the aggregates. Minimum paste will mean less quantity of cement and less quantity of water, which will further means increased economy, higher strength, lower shrinkage and greater durability.