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RE: Early form release for two-way slabs

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With regards to the High Early Strength Additive below, please be aware that, while they have their benefits, they can also have drawbacks. Yes, you will get early strength, but that comes about by accelerating hydration, which generates heat, which expands the concrete and can ultimately result in more cracking. If the project will be built this summer, you may need a set retarder and/or a different moisture retention and curing system.

For anyone conducting accelerated slab stripping, we recommend in-place testing. In-place tests made during the summer will often show strengths of as much as 1000 psi greater than cylinders. We have used pull-out tests from Germann Instruments in the past with great success. TxDOT uses maturity testing (I think the Intellirock system), but primarily for design strength verification. A combination of the two is a nearly foolproof way of determining in-place strength. Each test method will require calibration with the intended mix.

That being said, be aware that modulus of rupture at early ages may not relate to strength in the same way that later ages relate. Progressive reshoring is essential.

Jay Shilstone

At 08:40 AM 4/7/2004, you wrote:
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 (6.2.2.1(c)).

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 product.

500psi 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.

Even 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 deflection.

Craig


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From: Keith Pabst [mailto:kpabst(--nospam--at)ha-pa.com]
Sent: Wednesday, April 07, 2004 8:45 AM
To: seaint(--nospam--at)seaint.org
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 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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