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Practical problem in flat slab

 
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sanyojan
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Joined: 26 Jan 2003
Posts: 13

PostPosted: Mon Dec 22, 2008 6:05 am    Post subject: Practical problem in flat slab Reply with quote

Dear all,

I have a practical problem.
I have a PT flat slab which was to be cast in three pours. The first part of the slab has been cast more than a month ago. It appears that the second and third part are likely to be further delayed. The cables along the part which has to be cast further, has been stressed and anchored, but has not been grouted.

Q1) What should be done at this live end?
Q2) How to ensure that the cables along this direction have not started rusting?
Q3) What if they have started rusting?
Q4) How should this end of the slab, which was a part of a larger slab, be treated?/ propped?/ supported?

Regards

Anand Bhagwatwar

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biju
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Joined: 26 Jan 2003
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Location: Mumbai

PostPosted: Mon Dec 22, 2008 7:00 am    Post subject: Practical problem in flat slab Reply with quote

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sahi
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Joined: 02 Oct 2008
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PostPosted: Mon Dec 22, 2008 8:31 am    Post subject: Practical problem in flat slab Reply with quote

Grout the part already cast. Clean the bare part of slab (sand paper
etc) on day before casting. Cement sllury coat can be applied on bare
cables ater tensioning for fprtection.

On 12/22/08, sanyojan <forum@sefindia.org> wrote:
Quote:
Dear all,

I have a practical problem.
I have a PT flat slab which was to be cast in three pours. The first part of
the slab has been cast more than a month ago. It appears that the second and
third part are likely to be further delayed. The cables along the part which
has to be cast further, has been stressed and anchored, but has not been
grouted.

Q1) What should be done at this live end?
Q2) How to ensure that the cables along this direction have not started
rusting?
Q3) What if they have started rusting?
Q4) How should this end of the slab, which was a part of a larger slab, be
treated?/ propped?/ supported?

Regards

Anand Bhagwatwar









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mkalgal
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Joined: 26 Jan 2003
Posts: 54

PostPosted: Wed Dec 24, 2008 4:52 pm    Post subject: Practical problem in flat slab Reply with quote

Dear Anand
This has reference to your queries on parts of PT slabs being cast after time gap.
1. In PT slabs one needs to provide what are known as Closure Strips sometimes. These are also referred to as a pour strip. They are generally necessitated due to site specific requirements like inability for deployment of large areas of shuttering, very long cable length necessitating both ends stressing etc. This is a temporary separation of about 1m between two regions of slab which will be constructed and post-tensioned separately. The gap between the two slab regions is closed by placing and consolidating non-shrink concrete. The width of the strip is decided by net distance required to position the jacks. The preferred location is typically quarter span where moments are small. Proper corrosion protection is to be given to anchors during the time the anchors are kept exposed to atmosphere. The time to keep the gap open is dependent on the time taken for most of the shortening process to be over. If the pour strip concrete is poured earlier, cracks may develop since edges on either side of proposed would be moving away from each other during the process of shortening. The reinforcement across the closure strip has to take care of the stresses in that non-prestressed region since it would be a part of the continuum.  

The above is the recommended and accepted method.

2. What you seem to refer to is where a part of the slab is cast, a part of the cable is stressed and anchored at both ends (I wonder how) and some more portion of the cable is lying open, exposed to amosphere. This is not the usual practice. The cable needs to be protected at any cost and shall be kept covered (not easy I know!)

3. In both the cases above, the cable inside the sheathing is also likely to be rusted if not corroded (there is a difference between the two) if the sheathing is not grouted and the sheathing is not sealed. One simple method of prevention is to apply cutting oil to the strand before insertion and stressing and to properly seal the grout inlets. If there is no oxygen and moisture, there is no corrosion. If this has not been done and if some rusting is expected, one method followed is to send mild lime solution (not plain water) through the grout hole and watch the colour of the solution emerging. If there is brown tinge, continue the flow till the loose rust comes out and the brown tinge vanishes. Then grouting can be done. If corrosion is expected, it is not possible(as I understand) to test this. The cables will have to be replaced(I know it is difficult. Hence prevention is better than cure)

4. If slabs are cast in parts as in 1 or 2 above, the edge of the first part needs to be supported till the other part of the slab is cast, stressed, pour strip poured and allowed to gain strength to act as a continuum.

Sorry for a very long reply.
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mkalgal
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Joined: 26 Jan 2003
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PostPosted: Wed Dec 24, 2008 7:36 pm    Post subject: Reply with quote

Dear All

One of the aprehensions in choosing PT flat slab is about future flexibility to have openings if an when required. Following two articles throw some light on it

http://www.cement.org/buildings/Openings%20in%20Two%20Way%20Slabs.pdf
http://www.ptia.org.au/Photos/Penetrations%20in%20P-T%20slabs.pdf

I thought this topic would fit in this section

It is seen that lot of "views" are there but not many are participating.

Hoping for a larger participation

with warm regards

Kalgal
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