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RE: Early form release for two-way slabs

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My two cents says absolutely not. A 12" beam-supported slab implies a lot of load will be carried by this system and you cannot afford to have its integrity compromised right off the bat. The early set and curing period is critical for proper long-term concrete performance. The last thing you want is for the just-barely-set concrete to be loaded, especially to anything even close to the modulus of rupture. 500 psi is nothing and is hardly worthy of structural performance. Actually, ACI 22.2.4 (Structural plain concrete, as per their argument) limits f'c to a minimum of 2500 psi.

I would hang my hat on that and say no.

Good luck.


From: "Keith Pabst" <kpabst(--nospam--at)ha-pa.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: Early form release for two-way slabs
Date: Wed, 7 Apr 2004 08:44:45 -0400


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 questions:

1. Is this type of early release system allowed per code.
2. Is the method of analysis described above acceptable per ACI
utilizing the modulus of rupture.
3. Can anyone point me to a portion of the code which would allow such a
system to be used without achieving the 70% f'c.

I would appreciate any information anyone could provide me regarding
this matter.

Thank You,
Keith E. Pabst, E.I.T.
Highland Associates, Ltd.
Architects Engineers Interior Designers
102 Highland Avenue
Highland Center
Clarks Summit, PA  18411
(570)586-4334
(570)587-8164 fax
email: kpabst(--nospam--at)ha-pa.com


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