In this stage of works the location of columns are determined practically in field. It is done by laying rope according to grids shown in the drawing and then mark the location of columns related to rope.
In drawing, column locations are shown related to grid-line with dimension. Practically, in field, ropes are our grid-line. So we place columns related to rope-line by measuring dimension shown in the drawing.
After marking the column locations, we then start to place reinforcement as instructed in the structural drawing.
This is normally described in the drawing like –
C1-12#16 mm⌀ and stirrup-10 mm⌀ @ 15 cm c/c.
That means column C1 will have 12 numbers of 16 mm diameter bar as vertical bar and 10 mm diameter steel should be placed 15 cm center to center as stirrup.
or
C2-8#20 mm⌀ + 10#16 mm⌀ and stirrup-10 mm⌀ @ (10 cm+15 cm) c/c.
This C2 column’s reinforcement specification means that it’ll have 8 numbers of 20 mm diameter bar as well as 10 numbers of 16 mm diameter bar as vertical reinforcement and (10 cm+15 cm) center to center of stirrups placement means middle-half portion of clear height of column will have 15 cm center to center spacing of stirrups and upper one-fourth as well as bottom one-fourth height of column’s clear height will hold stirrups at 10 cm center to center spacing.
There is a sheet in structural drawing which contains structural notes from structural designer. In that drawing sheet, you’ll find suggested lap length for column’s steel of different diameter bar and other important notes. You should read those before column reinforcement work.
In building, floor height is normally kept 3 m. If the slab has beam then we have to pour concrete up to beam bottom level. Suppose, beam height specified in drawing is 0.45 m. So, the casting height of our column will be 2.65 m. And our formwork height will be 2.65 m. But one thing should be considered here is that dropping concrete from above 1.5 m height isn’t suggested during pouring. Because it leads concrete segregation. So, we should make one-side of column formwork within 1.5 m height range. After casting 1.5 m of column, we just lift the short side up to full-casting height of column next day.
Another way to cast column without segregation is to keep a small window at 1.5 m level of full-height formwork. After casting up to that level, close the window and cast the rest of the column.
Casting column is easy. For small quantity of concrete volume, we normally depend on machine-mix concrete and for large concrete quantity we order ready-mix concrete. I would suggest machine-mix concrete. Because, if you use moving pump with ready-mix concrete and if you want not to exceed 1.5 m height range for dropping concrete that would be difficult.
If you don’t use moving pump, yet there are some problems. Suppose, you have decided to use ready-mix concrete without pump. In that case, you have to manually unload concrete on job site from ready-mix concrete truck and have to manually pour into column. That’ll take long time and you’ll exceed initial setting time of concrete. As a result, concrete will lose its quality. So it is better to cast column with machine-mix concrete.
| • For less quantity machine mix concrete is done and for larger quantity ready mix concrete (RMC) is ordered. |
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| • Approval of placing concrete whether by pump or manually has to be taken from client. |
| • Concrete should be poured up to slab bottom; the remaining column gets concreted during pouring of slab & beam. |
| • Mechanical vibrator to be compulsorily used in the column but excess vibration can cause segregation. Each layer should be thoroughly compacted. |
| • Target slump to be 160 mm. |
| • Construction joint should be avoided in the column. |
| • Proper cover as per structural drawings to be maintained. |
| • Temperature should be below 30 degree while pouring concrete. |
| • Height of pouring concrete should not exceed 1.5 m. |
| • After the pouring of concrete and vibrating it with the help of a vibrator the horizontality and verticality of column to be checked |