Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

394 Chapter Thirteen
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Decentering and Form Removal
Decentering and removal of formwork for thin-shell roof slabs is
more important than removal of shoring and formwork for walls and
floor slabs in the interior of a building. Upon removal of centering
and forms, the roof slab may be subjected to the permanent live load
of wind, whereas the permanent live load on a floor slab will not
likely occur until the full 28-day strength of the concrete is attained.
Due to high deflections and high dead-load to live-load ratios
that are common in thin-shell roof slabs, decentering and removal of
formwork should not be permitted until appropriate tests have been
performed to demonstrate that the specified concrete strength and
modulus of elasticity have been achieved. It is possible that adequate
compressive strength may be attained, but the modulus of elasticity
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may not be adequate. ACI 318 requires the use of field-cured beam
test specimens if form removal time is based on development of a
specified modulus of elasticity. The value of modulus of elasticity
must be obtained from flexural testing of field-cured beam specimens,
not as a proportion of the compressive strength from the beam
test specimen.
Decentering methods should be planned to prevent any concentrated
reaction on any part of the permanent structure. Decentering
should begin at locations of maximum deflection and progress to
locations of minimum deflection, with decentering of edge members
proceeding simultaneously with the adjoining shell.
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