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RE: CONC - Stripping of Deck Forms

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Most of the deflection problems I've seen with flat plates result from the 
mistaken belief that only strength counts.  The creep deflection is a 
function of the maturity of the concrete when it is loaded.

You need to be careful that in the process of stripping the forms on 
"green" concrete you keep the load out of the concrete in order to avoid 
creep effects.  If the contractor strips an entire bay and then reshores, 
it is too late.  Once the load is in the concrete, it will relax (unload) 
plastically and you will get the deflection you were trying to avoid by 
reshoring.  "Back shoring", i.e., stripping portions of the slab and 
reshoring as you go, will alleviate this problem.

-----Original Message-----
From:	Bill Sherman [SMTP:SHERMANWC(--nospam--at)cdm.com]
Sent:	Thursday, August 14, 1997 6:05 AM
To:	seaoc(--nospam--at)seaoc.org
Subject:	Re: CONC - Stripping of Deck Forms


> The contractor wants to know if he can strip the forms after 7 days,
assuming the concrete has reached full design strength. <

How has it been confirmed that the concrete has achieved its full design
strength?  ACI 301-2.3.4 addresses removal of forms based on design 
strength.
Some engineers allow removal of forms even before full design strength has
been reached if the element is protected from live loads.  Some responders
have mentioned creep/ modulus of elasticity - but I believe modulus of
elasticity is proportional to strength; thus if it has developed full 
design
strength, shouldn't it also have developed its modulus of elasticity?  I 
don't
see it as a problem if strength development has been documented.






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