Building Construction formwork design

Slab Form Work

Slab form work

When the   slab form work was considered, it was necessary to consider the all the weight that can be applied to the form work such as total concrete weight, all the live load like labours and equipments, which were used for the concreting work.

Normally for the slab form work 12mm thickness plywood was used (coated plywood).

Method for fixing form for slab

Step 1: First step of the slab form work was the fixing of the collar to the columns, which were in the slab area. When the collar was prepared, it was necessary to minimize the off cut of the plywood board.(Most of the column 600*600, therefore ply wood board was cut the as describe column form work.)

Step 2: Bottom of the beam form works were fixed to each collar of the columns by considering the level of each beam bottom and all bottom of the beam form work

Step 3: Sideboards of the beams form work were fixed to the bottom of the beam form work by considering the level, height of the beams form work.

Levelling of the slab before concreting

Level instrument was used to check the level of the slab. It was necessary to calculate the level of the staff, before levelling the slab by considering the thickness of the board and size of the GI pipe. This has to ensure the slab was in the correct level before concrete. By adjusting the joint of the GI pipe support slab form work was levelled.

Some of the factor considered, when the leveling as follows

I.     Numbers of points were taken for level checking was depending on the size of the panel.

II.    Normally six or seven points were taken to ensure the level and if ever suspected level was noted there also level readings were taken.

III.  Especially in the middle of the slab noticeable sagging was noted.

Normally +5mm allowable was added to the sagging area of the slab.

Stair case form work

Specially, three methods were considered.

  1. Calculating method
  2. Used AutoCAD for finding the required soffit length
  3. practical method
  4. Graphical method

The staircase form work was not simple. Here the frame structure should be erected properly such that it can withstand all the loads that were affected such as dead load of concrete, reinforcement, and loads imposing while concreting. And also it should be properly levelled. To erect the structure we used 2”x 4” timber and to join them steel nails were used.

Inspections of form work

When erection of the form work had been completed and all the reinforcement was laid an inspection was carried out by consultancy.

Form work was inspected to check whether they were in accordance with the drawing and, whether they were in correct positions. In the same time as checking of the form work line and level was checked. The vertical of the wall and column form work was checked, by using a plumb bob. Slab and beam form work levels were checked by using an inverted staff and dumpy level.

Also joints were checked to ensure that grouts could not escape through them when the concrete was begun. Initially before the concreting a final check was made to that there were no wood shavings and dust left inside the forms.

Removing of form work

The striking or removal of form work should only take place on instruction from the engineer or agent. The appropriate time at which it was safe to remove form work can be assessed by rest on cubes taken from a similar batch mixed at the time the concrete was poured and cured under similar condition. The normal stripping times that we allowed in this site was as follows.

Time period of the form work removal,

  1. Sides of beams, walls, columns and piles                           03 day
  2. Soffit of secondary slab (Props left in)                                03 days
  3. Soffit of main slab (Props left in)                                           07 days
  4. Soffit of beams (Props left in)                                                  07 days
  5. Removal of props (Secondary slab)                                     07 days
  6. Removal of props (Beams & main slab)                               16 days

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