Dos and Don’ts of Pouring Concrete in Columns and Walls
In architecture or civil engineering, a column, which is also called pillar, is a structural element that carries the weight of the structure above (from a ceiling, floor slab, roof slab, or a beam) to other structural elements such as a floor or foundations through compression. This means that a column is a compression member.
A column also carries bending moments about one or both of the cross-section axes. In mega structures and other tall buildings, columns support compressive forces during natural disasters like earthquake and fire. Failure of one or more columns may lead to the collapse of the structure. Hence, the concrete in the columns is reinforced with high quality iron rod to give it adequate strength and ductility.
Like columns, reinforced concrete wall is designed as a compression member in case beam is not provided and the load from the slab is heavy or when the masonry wall thickness is restricted. Due to their importance and to achieve great strength and durability, concrete must be poured in columns and walls with caution and appropriately.
To obtain a consolidated concrete mass that covers iron rods and has low permeability, concrete must be placed in layers and each layer must be properly compacted. If concrete is not properly poured in columns and walls, it may lead to sloping layer lines, leaking if water present, and honeycombs. This may cause the columns to deteriorate and fail.
Steps to Pouring Concrete in Columns and Walls
In order to avoid problems related to improper concreting practices, the following steps should be taken as a guide to pouring concrete in columns and walls.
- Concrete should be poured from the top of the forms if the height of the wall or column form is not considerably long.
- Then, the concrete should be placed at or close to its final position.
- Pour concrete in layers with a thickness ranging from 30cm to 50cm if the height of the RC column is large.
- Place concrete walls with a maximum 60cm layer in one pass to avoid segregation.
- 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.
- For walls deeper than 1.2m, pour concrete through vertical trunks or chutes positioned at an interval of 2.4m.
- Free fall of concrete from the trunk end should be done from 0.9 to 1.5m (occasionally restricted to 0.6m); otherwise, segregation would occur.
- The concrete free fall should be continuous. Prevent separation because of free falling of concrete over reinforcement or other embedded objects.
- Discharge concrete into the formworks directly without the use of chutes, trunks, or hoppers, if possible, to decrease segregation possibilities.
- 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.
- If the supply of concrete interrupts during concrete pouring, try to avoid the formation of cold joints in the wall or column.
- 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.
- Prevent strikes of concrete against the formwork wall during pouring; otherwise, concrete separation would occur that may create honeycomb at the bottom of the concrete element.
- In RC columns and narrow walls, 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.
- Alternatively, place the same concrete mix with half coarse aggregate at the bottom with a thickness range from 15-30cm.
- 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.
- Sloping layer lines (leaking if water is present) and honeycombs are signs of improper concrete pouring in walls.