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Segregation of concrete is the separation of cement paste and aggregates of concrete from each other during handling and placement. Segregation also occurs due to over-vibration or compaction of concrete, in which cement paste comes to the top and aggregates settles at the bottom.
Segregation of concrete affects strength and durability in structures. In a good concrete, all concrete aggregates are evenly coated with sand and cement paste and forms a homogeneous mass.
During handling, transporting and placing, due to jerks and vibrations the paste of cement and sands gets separated from coarse aggregate. If concrete segregates during transit it should be remixed properly before depositing. However a concrete where initial setting time is over, should not be used.
Causes of Segregation of Concrete
- Use of high water-cement ratio in concrete. This general happens in case of concrete mixed at site by unskilled workers.
- Excessive vibration of concrete with mechanical needle vibrators makes heavier particles settle at bottom and lighter cement sand paste comes on top.
- When concreting is done from height in case of underground foundations and rafts, which causes concrete to segregate.
How to Prevent Concrete Segregation?
Wherever depth of concreting is more than 1.5 meters it should be placed through temporary inclined chutes. The angle of inclination may be kept between 1:3 and 1:2 so that concrete from top of chutes travels smoothly to bottom, use of small quantity of free water from top at intervals helps in lubricating the path of flow of concrete to bottom smoothly.
The delivery end of chute should be as close as possible to the point of deposit.
Segregation in deep foundations and rafts of thickness more than 1 meter, there is every possibility of presence of segregated concrete near bottom or in center if proper supervision is not there. Such segregation can be detected by advanced method of testing like ultrasonic testing.
In case of doubt random ultrasonic testing should be conducted and if it is present, designer’s opinion should be taken. This type of segregation can be rectified by pressure grounding with special chemical compounds.
After any defect rectified by pressure grouting core test has to be performed to ensure that the strength of concrete has reached to the desired level.