Pour strip on post tensioned slab

[Sonbatii]

i have one post tensioned slab with single tendon ( anchors on one side and the other side has a dead point )
so i ask of i need to make pour strip on the side which has dead point or not .and there is a water tank wall on this edge .
Also , what is the usefl of pour strip except p.t service .
Thanks

 

[BAretired]

The pour strip is intended to reduce shrinkage cracking prior to stressing.

BA

 

[Sonbatii]

Ok but the slab has a pour strip from anchors side
Should it has from two sides ?!

 

[BAretired]

Normally, the pour strip is in the middle of a prestressed slab, not at an anchorage. After initial shrinkage, the pour strip is filled with concrete and stressing begins after the pour strip has cured. Both dead and live anchorage are remote from the pour strip.

Edit: The above is incorrect. See the post by Hokie66 for a better description.

A sketch of your slab showing anchorages and pour strip locations might be helpful.

BA

 

[Retrograde]

the pour strip is filled with concrete and stressing begins after the pour strip has cured. Both dead and live anchorage are remote from the pour strip

Pour strips that I have seen have the anchorages at the pour strip and they are filled after stressing.

 

[Sonbatii]

I think that we do not make pour strip when we have anchore box , but on my case there is no anchorages box .
So why we make a pour strip when we have a dead end of tendon .
And how can we cast the pour strip before stressing ?!

 

[rapt]

I agree with BA, this is impossible to answer logically without a drawing!

 

[hokie66]

Pour strips are not cast before stressing. The two parts of the slab which abut a pour strip are stressed, then the delayed pour strip is normally reinforced with deformed bars.

The purpose of delayed pour strips, whether in conventionally reinforced or post-tensioned slabs, is to reduce direct tension cracking due to restraint.

It would be an unusual situation to have a pour strip at the end of a stressed slab. Sometimes, the ends of slabs are supported on corbels with slide bearings, and the joint locked up later, but I wouldn't call that a "pour strip".

 

[rapt]

In a basement, it is common to have the pour strip around the edge to release the slab from diaphragm walls etc before stressing. This allows both movement plus stressing access at the edges. And then the infill slab provides the connection to the walls plus allows the slab to brace the walls.

We still need a drawing!

 

[hokie66]

OK, I defer to rapt on the edge pour strips. Just because I haven't seen it done doesn't mean it is unusual. I don't like the concept, as I like joints where the shear is minimal, thus my preferred option of supporting the edge of slabs on corbels or keys.

 

[Sonbatii]

At this water tank walls the tendon is dead so i ask why we need to make a pour strip at this area

 

[hokie66]

What does a section through the pour strip look like? Including both the tensioned and deformed reinforcement.

 

[KootK]

It appears that the designer intended to alleviate slab restraint in the left to right direction that would otherwise have been introduced by the tank wall. Like the basement wall scenario that rapt mentioned, a long wall like this will have a tendency to divert axial prestress away from the slab in the absence of measures such as this.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rapt]

Hokie,

I wold normally agree but in this case, the moments are normally small, the support is continuous so no shear concentrations and often you can organise a shorter span into the walls so shear is much reduced as well due to the longer internal span effect on the shorter end span.

The thing people often forget with pour strips is that most of the shrinkage and all of the temperature movement restraint effects still exist as the pour strip is filled in after stressing, so the concrete is only a couple of weeks old and even if it is left for 2 months, there is only a small reduction in shrinkage and none in temperature (unless you can time the pour to late at night on a cold night in winter!!). I often think you are better off connecting it all up and providing crack control reinforcing for the restraint.

 

[hokie66]

rapt,
Fair enough. Folks seem to get hung up on the shortening due to PT, not acknowledging that is only a small part of the axial problem.

 

[rapt]

Hokie,

Yes, normally only 10-15% of the total shortening!