Shape of Coarse Aggregate

          Shape of three dimensional irregular bodies as coarse aggregate is difficult to describe; but is the vital property affecting the workability of fresh concrete and also its strength and durability. The characteristic of parent rock from which coarse aggregates have been produced and also the type of crusher used for crushing influence the shape of coarse aggregates.
According to shape the aggregate is classified as
*Rounded aggregate
*Irregular or partly rounded aggregate
*Angular aggregate
*Flaky aggregate
*Elongated aggregate
*Flaky and elongated aggregate
           Rounded particles are fully water-worn or completely shaped by attrition. Angular particles possess well-defined edges formed at the intersection of roughly planar faces. Flaky particles have thickness small relative to the other two dimensions. Elongated particles are usually angular of which the length is considerably larger than the other two dimensions. Irregular particles are naturally irregular, partly shaped by attrition and having rounded edges.

At a given water-cement ratio, rounded aggregates demands lower cement content because of its least surface area for a given volume which is called “specific surface area”. The higher cement demand in case of angular aggregates is offset to some extent by the higher strength and durability because of better interlocking. Flat surfaced particles such as “flaky” & “elongated “ones, having a larger specific surface area makes very poor concrete. Water demand and cement requirement for a given strength is higher for such particles. Presence of such particles is highly objectionable in a good concrete.