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Re: Concrete Strength required for stressing - Part 2

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I would agree.  While the application of the prestressing force earlier
rather than later may prevent some shrinkage cracks from drying from
occuring or at best opening up (they may still occur), you could
potentially be causing cracks due to the application of the prestressing
force (as I believe you pointed out in an ealier post in reference to the
need for nonprestressed steel on occasion when a PT slab is heavily
stressed).  Thus, to me, the time of application of prestressing should be
an engineering judgement based upon the individual circumstances of the
particular project.


Ypsilanti, MI

On Fri, 14 Mar 2003 GSKWY(--nospam--at) wrote:

> Some specifications require that stressing be done within a certain amount of
> time after the concrete is cast - typically 96 hours.  It is also in some
> local building codes.
> Application of this requirement requires "engineering judgment," however.
> The intent is to make sure stressing is done early enough that there is no
> (or minimal) shrinkage cracking and is particularly of concern in parking
> structures where the cracks might end up leaking.
> During cold weather,  depending on the mix used, the concrete might not gain
> the required strength within the specified time.  Some engineers require a
> two-stage stressing, with the intial stressing to half of the normal
> stressing force.  This represents an extra cost to all involved;  in some
> cases, it may be justified.  In most cases, however,  if the concrete has not
> gained the required strength due to cold weather,  there is also no danger of
> shrinkage cracking.
> Arbitrarily enforcing requirements, particularly when one does not understand
> the basis for the requirement,  is often a waste of money.  In some cases, it
> may also result in a structure with less satisfactory performance.
> Gail Kelley

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