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RE: Early form release for two-way slabs
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Early form release for two-way slabs
- From: "Craig Leech" <cleech(--nospam--at)nyc.rr.com>
- Date: Wed, 7 Apr 2004 09:40:19 -0400
My understanding of Section 6.2 of ACI318-02 was that there
were no strict rules. Simply that someone must design the removal
system. Strength of the concrete at stripping must be validated by tests
If you are worried about the strength of concrete, get them
to use a High early sterngth additive eg master builders glenium3200.
We were partially involved in a project were they used this on a prestressed
flat plate building built on a 2day cycle using this
sounds very low. We have a 2day cycle in NY and require 2000-2500psi
before preshoring, partial stripping and then reshoring (about 8'
o.c.). I would encourage them strip the beams in segments and reshore,
never leaving the beam completely unshored.
if strength wise things look ok, I would think carefully about
deflections. One of the most influential factors in the deflection of a
slab is the stripping process. Do it too early or leave a portion
unsupported and you will get a large, irreversible
I am working on a
project which has a 12 " thick, two-way flat slab system. The contractor is
proposing to use a forming system that offers an early release of the beam and
girder forms. The way the system works is there are shores at 6'-0" on
center each way with a beam and girder system supporting the plywood at 16 or
24" o.c.. The forming manufacturer has made recommendations for an early release
of the beam and girder system but leaving the vertical supports in place until
the concrete has reach the required strength. They have a one day release time
specified for a 12" slab as long as the min. compressive strength is 500 psi.
Basically they consider the slab to be a 2-way slab supported at 6'-0" o.c.e.w.,
unreinforced. They have analyzed the slab based on the modulus of rupture to
determine the early release time. I have looked through the ACI Manual
and have only found the section which requires forms for horizontal surfaces to
remain in place until the concrete has reached 70% of the required
compressive strength. I guess I have the following
this type of early release system
allowed per code.
2. Is the method of
analysis described above acceptable per ACI utilizing the modulus of
3. Can anyone point me to a portion of the code which
would allow such a system to be used without achieving the 70%
I would appreciate
any information anyone could provide
me regarding this matter.
Keith E. Pabst, E.I.T.
Highland Associates, Ltd.
Architects Engineers Interior
102 Highland Avenue
Clarks Summit, PA 18411