Segregation of concrete is the separation of cement paste and aggregates of the concrete from each other during handling and placement.
Segregation also occurs due to over-vibration or compaction of concrete. During over-vibration, cement paste comes to the top and aggregates settle at the bottom, due to gravity.
The strength and durability of concrete structures are greatly affected by segregation. In high-quality concrete, all aggregates are uniformly coated with cement paste, forming a homogeneous mass.
Due to jerks and vibrations during handling, transporting, and placing of concrete, the cement paste gets separated from the coarse aggregate. If concrete segregates during transit, it should be remixed properly before depositing.
Causes of Segregation of Concrete
- Generally, the main reason for segregation is the use of a high water-cement ratio in concrete mixed at the site by unskilled workers.
- Excessive vibration of concrete with vibrators makes heavier particles settle at the bottom and lighter cement sand paste comes on top.
- When concreting is done from height, in the case of underground foundations and rafts, it causes concrete to segregate.
How to Prevent Concrete Segregation?
Wherever the drop of concreting is more than 1.5 meters it should be placed using a temporary inclined chute. Keeping the angle of inclination of the chute between 1:3 and 1:2, concrete from the top of the chutes travels smoothly to the bottom. Using a small quantity of free water from the top periodically helps lubricate the path of flow of concrete to the bottom smoothly. The delivery end of the chute should be kept close to the point of deposit.
When the thickness of deep foundations and rafts exceeds 1 meter, there is every possibility of the presence of segregated concrete near the bottom or in the center, if not properly supervised. In case of doubt, random ultrasonic testing should be conducted. If segregation is present, it can be rectified by pressure grouting with special chemical compounds. After rectification, a core test has to be performed to ensure that the strength of the concrete has reached the desired level.