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Concrete Strength for Stressing

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In a message dated 3/14/2003 5:53:55 AM Eastern Standard Time, kenmail234(--nospam--at)yahoo.com writes:


Does anyone know WHEN the prestressing force can be applied by the contractor after the concrete was placed?  


Required concrete strength is nominally based on equations in the ACI and PTI specifications and is a function of the anchor size and spacing.  However,  if one looks at the ICBO reports from different p-t suppliers,  they all specify different concrete strengths and anchor spacing, despite the fact they all have the exact same anchors - for 1/2 in. strand it is a 5 x 2-1/4 in. plate.  I am not sure how that happened ...

Required strength needs to be specified in the project specifications and tends to be a function of local practice rather than the equations mentioned above.  On the East Coast it tends to be 3000 psi,   on the West Coast, it tends to be 2750 psi.  ACI 318 requires 2500 psi although I am not sure where this came from, I don't think anyone uses this in practice.  Many of the ACI 318 requirements for post-tensioning seem to be arbitrary and non-sensical.

Many specs required the concrete to have a strength that is 75% of the 28 day strength, which incorrectly implies the required strength for stressing is somehow a function of the 28 day strength.  I think it is a hold-over from the days when 4000 psi concrete was standard and 75% of 4000 was 3000 psi.  Unfortunately, many engineers writing specfications for post-tensioned concrete construction don't seem to know very much about post-tensioned concrete construction.

In general, forms are often required to stay up until the concrete has reached 75% of its design strength.  With the higher strength concretes currently used,  you may not be able to strip the forms immediately after stressing as was often the practice with 4000 psi concrete.

Gail Kelley

p.s.  in DC, almost all houses are sold "as-is".  There is a further category "absolutely as-is"  which guarantees that the major mechanical systems don't work.