Generally, pouring concrete in narrow columns needs great caution to ensure the achievement of designated strength and durability because of its narrowness. Place the concrete in layers, and compact each layer properly to obtain a consolidated concrete mass that encircles steel bars and has low permeability. Use vibrations to consolidate fresh concrete, but avoid over-vibration as it may cause segregation.
Here are some steps or points which are helpful for pouring the concrete in narrow/sleak column :-
1. If the height of the column form is not considerably long, pour concrete from the top of the forms.
2. Place concrete at or close to its final position.
3. Pour concrete in layers with a thickness ranging from 30cm to 50cm if the height of the RC column is large.
4. The period between successive concrete layers should not be longer than half an hour for normal concreting conditions and 20 minutes in case of hot weather conditions.
5. Freefall of concrete from the trunk end should be done from 0.9 to 1.5m (occasionally restricted to 0.6m); otherwise, segregation would occur.
6. The concrete freefall should be continuous. Prevent separation because of free falling of concrete over reinforcement or other embedded objects.
7. Discharge concrete into the formworks directly without the use of chutes, trunks, or hoppers, if possible, to decrease segregation possibilities.
8. Vibrate each layer to compact it properly. Lift the vibrator after the complete compaction of each concrete layer. Extend vibrator into the previous layer by 10-15cm.
9. If the supply of concrete interrupts during concrete pouring, try to avoid the formation of cold joints in the column.
10. Sometimes, set retarder materials might be used to the concrete surface to delay its setting and create a good bond with the next batch of concrete. Sugar can retard the concrete setting time by up to four hours. The next layer of concrete should be thinner than previous ones, and vibration should extend into the previous layers.
11. In RC columns, start concrete pouring with 5 to 10 cm grout to avoid collection of loose stones at the bottom that lead to honeycomb formation. The grout slump is the same as the concrete slump with the same or lower w/c ratio.
12. Alternatively, place the same concrete mix with half coarse aggregate at the bottom with a thickness range from 15-30cm.
13. Roughen concrete surfaces after initial set with brush, if, for any reason concreting stops for a day to create good bonding with the next concrete layer.
14. In summary, avoid problems arising from improper concreting practices by preventing full depth placement of concrete at one point, a lower slump for upper layers, preventing lateral movement of vibrators in concrete, and thoroughly vibrating concrete.