Segregation
Separation of coarse aggregate from the paste, leaving rocky areas and soupy areas within the same placement. It is caused by over-wet mixes, free-fall, or excessive vibration.
What does segregation mean?
Segregated concrete is non-uniform, so its strength, appearance, and durability vary within a single element. Common causes are dropping concrete from height, dragging it long distances with a rake or vibrator, and mixes that are too wet or poorly graded to hold their stone in suspension. Once segregation happens it cannot be mixed back out in place.
In practice
Pushing concrete across a slab with a vibrator instead of placing it where it belongs is the fastest way to create segregated, rocky spots.