Segregation of concrete means parting of components of concrete from each other. A superior quality concrete contains various aggregates which are uniformly coated with sand and cement paste and develops a uniform mass. While dealing, transporting and arranging concrete, the cement and sands are parted from coarse aggregate because of jolts and vibrations. If concrete is separated throughout transportation, it must be remixed correctly prior to deposit. It is recommended not to use a concrete if initial setting time of it is ended.
Segregation of Concrete occurs due to following reasons:
1) Applying high water-cement ratio in concrete. This may occur if the concrete is blended at jobsite by inexperienced workers.
2) Too much vibration of concrete with mechanical needle vibrators result in placing heavier particles at bottom and lighter cement sand paste arrives on top.
3) If concreting is performed from height for underground foundations and rafts, it may cause concrete to separate
How to get rid of Segregation of Concrete:
When depth of concreting is over 1.5 meters, it should be arranged through temporary inclined chutes. The angle of inclination should be maintained between 1:3 and 1:2 in oder that concrete from top of chutes moves easily to bottom. Applying small amount of free water from top at intermissions allows in lubricating the path of flow of concrete to bottom easily. The delivery end of chute must be closer to the point of deposit.
Segregation in deep foundations and rafts of thickness over 1 meter may cause segregation of concrete near bottom or in center. In this case, proper observation should be taken. To identify this type of segregation, ultrasonic testing process can be used. To clear any confusion, random ultrasonic testing method is applied. This type of segregation is corrected by pressure grouting with special chemical compounds. If any fault is corrected by pressure grouting, core test is done to make sure that the strength of concrete is achieved with optimal level.