Formwork is a temporary structural system, which provides the casing and supporting to the wet concrete to get the required shape. It should be stabled with the weight of the concrete, reinforcement, and other live loads during the construction and compacting of concreting.
After constructing the formwork for columns it is necessary to check for correct vertiality before concreting.
For small scale formworks a simple center plumb can be used to check the vertiality.
Two center plumbs and a steel tape is required for this process. Center plumbs were hanged at two positions in the same plane with a known distance of 150 mm measured from the inner surface of the plywood sheet(or steel formwork). The distance from the plywood sheet to the thread of the center plumb is measured using the steel tape at both top and the bottom of the form work.
calculations for a plywood formwork is as follows..
Offset distance from the column edge = 150 mm
Plywood thickness = 12 mm
Distance between the plywood and the plumb = 150-12
= 138 mm
If the verticality of the column is accurate the distance between the plywood and the center plumb thread should be 138 mm everywhere.Jack supports or locking systems of the formwork can be adjusted to obtain the correct vertiality in case of a mismatch.
A Thedolite and a steel tape can also be used to obtain better accracy. This method is perticulary usefull when checking a large number of formworks along a one grid line.
The process is described below.
Set up the digital theodolite centering a peg which was set up with a 500 mm offset to the column grid. Take the direction by focusing the telescope to a distant peg which was set up with a 90 º angle to column grid line. After theodolite was set up accurately turn on the laser beam and focused it to the steel tape which is held to the formwork. Take the reading of the steel tape through the telescope. Take the readings of two positions at the same level on both top and bottom levels of the formwork. By taking two readings at the same level any curvature on the surface can be identified.
Offset distance from the column grid = 500mm
Plywood thickness = 12mm
Size of the column = 300mm
Accurate reading through the telescope = 500-150-12
Accordingly for correct verticality 338mm reading should be read through the telescope.
Acro jacks and locks can be adjusted to obtain the correct verticality