Post-tension slabs in buildings 7

[derim01]

Hi...

If we have 10 floor building with in-situ post-tension slabs with bonding tendons.
I am wondering about typical schedule of construction for these floors.
I am clear for steps in case of one slab, but I am not sure about passing to the next floor.
Could we do concreting of the 2nd floor if the first floor isn't yet fully stressed.
Thx for the tips.

 

[ishvaaag]

Most likely you can postpone full stressing (and bonding) as long evertthing under construction and finished keeps working well. It is a matter of controlling the wanted stresses to useful feasible ranges, that you can do through careful investigation of the staged construction scheme.

It can even help when the loads to be added later are quite significant and fully stressing the slabs would be contrary to proper control of the stresses.

 

[ron9876]

PT slabs are typically stressed when the concrete reaches 2/3 or 3/4 of the design strength. If you wait too long you will get shrinkage cracking in the slab.

If the upper shores are placed directly over the lower shores there shouldn't be any problem with pouring the next level before stressing the lower level. However the normal approach is to pour, stress, strip, reshore and then pour the next level.

 

[rapt]

Even if the props are properly aligned there will still be stress induced in this floor from the pouring of the floor above. Stressing should e done before the floor above is poured.

Normally with bonded PT, 25% of the stressing is done at 1 day after the pour. The remainder when the concrete strength reaches the nominated value fot the anchorages normally 22MPa (clyinder) for mononstrand tendons and 25MPa for multistrand. (ignore suggestions such as 2/3 or 3/4 of concrete strength). The first stage can be ignored if sufficient unstressed reinforcement is provided on all tension faces to provide shrinkage crack control. But this is not preferred.

 

[pwht1]

If you were to pour a deck above a slab that hasn't been fully stressed then you need to double the number of levels of back-propping. I'd be a bit concerned about the prop capacity and wouldn't recommend it.

 

[rowingengineer]

For transfer slabs you can stage the stressing if required (aka weeks, months), to ensure over stressing of the beams doesn’t occur when dead loads are not present. IF you were to stress a transfer slab completely without dead loads present it could result in cracking of the concrete.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[pwht1]

Why didn't I get a star?

 

[dik]

Happy?

 

[pwht1]

Yay, thanks dik