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Stripping Times

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Raúl,

I think that there are a couple of things that you can require so that forms 
can be stripped early.  

One is to use reshores and is probably the one that the contractor will 
prefer.  However, with reshores, the contractor will need to have panels in 
the forms that can be dropped out so that the reshore can be installed 
*before* the forms are stripped.  It also precludes the use of flying forms.

The second is to use field cured concrete cylinders.  The cylinders should be 
cured in the same manner as the concrete in the forms and they should also be 
cured *with* the concrete in the forms.  If side forms are stripped at 24 
hours, the cylinder containers should also be removed at the same time.  A 
sufficient number of cylinders need to be field cured so that adequate sets 
of cylinders will be available for test in the event the first set tested 
does not indicate the strength required for stripping.  Since I am a 
"3-cylinder" person, (a strength test consists of the average strength of a 
set of 3-cylinders from the same batch tested at the same time) with a 
minimum of 9-cylinders cast, a lot of cylinders need to be field cured and 
protected from construction damage.

With Type I or Type II cements, I have generally found that the 7-day 
laboratory cured cylinder strengths are typically in the range of 70 percent 
of the 28-day laboratory cured cylinder strengths.  With Type III cement 
(high early strength), you will get your 7-day strengths earlier, but the 
strength gain tapers off more sharply than for Type I/II cements.  Unless the 
concrete producer has a history of using Type III cements and has mixes 
available, I would think that time savings anticipated by the contractor 
would be eaten up in testing mixes.

HTH

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Raúl Labbé wrote:

. > Colleagues :

. > I would like some opinions regarding the timing for stripping forms
. > and props in RC structures, when using special, early strength concrete.
. > The code states these times, for normal conditions and pending on the
. > span and the ratio ( self weight / total final load ). More than once we 
. > have had the question from contractors to diminish this timing, with the 
. > use of some special cements, or additives  ( accelerators ) reaching 
. > higher strength in early days.

. > What I know, is that diminishing the stripping times may result in 
. > premature capilar cracking, which is hardly ( if not impossible ) to 
. > repair later. Right now we are facing the following case : an RC 
. > industrial building with one story frame structure and the main heavy
. > equipment ( Turbogenerator ) supported at the same level but in an 
. > independent structure ( big slab-pad, 4.5' thick, spanning 5 and 7 mts.
. > between columns ). According to ACI ( ACI code number . . . I don't have 
. > it right now at home ), we stated 14 days for stripping the building 
. > frames, which have not more than 5 mts.span and a low self weight, and 21 
. > days for the Turbogenerator pad. In order to rush-up the works ( which are
. > already late ), the contractor is raising the special cement solution ( 
. > I've not seen, as yet, specifically which one ) declaring that with this, 
. > he can improve the timing in some 10 days ! Regardless an early high 
. > strength, a lower bound is to be used, which ? Thanks, in advance, for 
. > your opinions.

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