Why is presence (in excess) of Flaky or Elongated particles in concrete considered undesirable?
Flaky & Elongated particles have a high ratio of surface area to volume. This increases the water requirement of the aggregate & hence results in lowering of workability of mix.
Further, Flaky & Elongated particles tend to be oriented in one plane, with water and air voids forming underneath. This can adversely affect durability of concrete.
Due to these reasons the presence of Flaky & Elongated particles in excess of 10-15% of the weight of coarse aggregate is generally considered undesirable.