What is segregation in concrete and how is it prevented?

Segregation is the separation of concrete constituents - coarse aggregate settling to the bottom while cement paste rises, or water bleeding to the surface. It results in non-uniform concrete with poor strength and durability. Prevention methods include: proper mix design with adequate fines, avoiding excessive water, limiting free fall height during placement to 1.5m, using proper vibration (not over-vibrating), and maintaining appropriate slump.