SEGREGATION
OF CONCRETE – CAUSES AND PREVENTIONS
Segregation of concrete is separation of
ingredients of concrete from each other. In 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:
1) Use of high
water-cement ratio in concrete. This general happens in case of concrete mixed
at site by unskilled workers.
2) Excessive vibration
of concrete with mech
3) When concreting is
done from height in case of underground foundations and rafts, which causes
concrete to segregate.
Prevention of Segregation of
Concrete:
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.
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