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RE: Early form release for two-way slabs

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I'm not clear on how that form system works, but it seems that you are
talking about leaving the shoring in place and only removing formwork

ACI 301-99 (applicable only if referenced from the project specifications)
has a default of full "specified compressive strength" before removal of
formwork or shoring but also states "If a lower compressive strength is
proposed for removal of formwork and shoring, submit detailed plans for
review and acceptance." (Section Full compressive strength is the
"default" just to be safe, in lieu of detailed analysis. It also states that
"When shores and other vertical supports are arranged to allow the
form-facing material to be removed without loosening or disturbing the
shores and supports, the facing material may be removed at an earlier age
unless otherwise specified." This sounds like what you are describing? It
basically comes down to: it is acceptable if you "accept" the contractor's
analysis as being valid. 

Project specifications often define minimum requirements for formwork
removal. We typically specify 30% of compressive strength for form removal
and 60% of compressive strength for shoring removal, as well as some minimum
curing limits before removal. 

Perhaps you could allow the form removal with the stipulation that the
Contractor remains responsible for removal or repair if there is excessive
deflection, cracking, etc. 

William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)

-----Original Message-----
From: Keith Pabst [mailto:kpabst(--nospam--at)] 
Sent: Wednesday, April 07, 2004 6:45 AM
To: seaint(--nospam--at)
Subject: Early form release for two-way slabs

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

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)587-8164 fax
email: kpabst(--nospam--at)

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