Civil Engineering - Concrete Technology - Discussion

According to SSC JE CE 2019,

Rounded aggregate : High workability and low strength.

Angular aggregate: Low workability and high strength.

Flaky & elongated aggregates provide higher surface area.

I think the angular shaped aggregates are more used construction & it gives more strength, w workability.

Workability decreases if the concrete aggregate contain above options bcoz, surface area is high.

Rounded and semi-rounded aggregates increases workability because for a given volume it has less surface and less void, so excess paste is available to give better lubricating effect and rounded aggregates has less friction resistance.

Important nature of workable concrete is its lubricating nature.

The aggregate with rounded shape has the minimum percentage of voids ranging from 32 to 33%. It gives the minimum ratio of surface area to given volume and hence requires minimum water for lubrication. It gives good workability for the given amount of water and hence needs less cement for a given water cement ratio. The only disadvantages are that the interlocking between its particles is less and hence the development of bond is poor. This is why rounded aggregate is not suitable for high strength concrete and for pavements subjected to tension.

The more the surface are the more cement paste required for enveloping them. So as flaky, elongated particles have more surface area less cement available, less cohesion, more segregation.

Smooth surface texture of aggregates give better workability of concrete.

Only on rounded aggregate workability is increased.

Rounded shape particles are the most suitable for the high work-ability, but angular shape of aggregate is best for high strength b/c of good bonding between cement paste and aggregates.

Water cement ratio indirect proportion to the size of practical and workability.