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Flat plate sagging

Flat plate sagging

Flat plate sagging

I have a problem which occured on site. There is post tensioned flat plate which is broken up into 2 pours. Props supporting the slab in Pour 1 were removed along the column line separating the 2 pours after the slab had been fully stressed. The problem is that there is one column which does not line up with the rest of the columns and this column is actually locating in Pour 2. This means that for that particular span, the slab is hanging without support until the second pour is finished and the tendons are coupled together. The props in that area should have been left in place until the adjacent pour has been poured and the whole length of tendons in those spans fully stressed.

Here are the problems.

After the props had been removed, the slab exhibited excessive amount of deflection (approx 50mm-60mm) for a 8m span slab. Some cracks were also observed underneath. The props were then put back underneath that zone in the effort to reduce the deflection.

My question is:

Is it possible to use a hydraulic jack to push the slab back up without damaging the slab itself? (to reduce the visual impact and also to couple the tendons in Pour 2)

Would the deflection be reduced after the tendons are coupled to the one in the second pour?

I would also welcome other suggestions.


RE: Flat plate sagging


The slab is already damaged.

If the slab has been designed by normal ACI/PTI banded/distributed tendon approach, the basic assumption for this design method is that the slab remains uncracked at service. Your slab has already cracked so this is no longer possible. No amount of lifting of the slab is going to change this.

All you will achieve is to lift the slab back to level to allow the second pour to connect properly and for the continuous stressing to be done. This will not repair the damage already done by the cracking and its effects on the way the slab acts compared to the design assumptions.

Now that the slab is cracked, even when properly connected to the second pour, it will no longer perform as it was designed to do. Because the elastic load path is a two-way action and the cracked load path provided is a one-way action provided by the tendon and reinforcemnent layout, significant redistribution will occur when the slab cracks to transition between the elastic and the cracked load paths. This will result in significant increases in deflection and uncontrolled cracking.

If the slab had remained uncracked then the service condition would have remained as a two-way load path and the extra cracking and deflection would not occur. This load path is actually provided by the tensile capacity of the concrete (a real no-no in practice for concrete structures) but is the basis of the banded/distributed tendon design method under service conditions.

Because the slab has already cracked in this area, the only load path now available, now that the two-way path has been destroyed in this area, is the one-way path which was only meant to be used in the post-cracking ultimate strength range. Using this load path at service will result in increased deflection and cracking over and above the design amounts as redistribution and rotations need to occur to transfer to the cracked one-way load path from the elastic path.

RE: Flat plate sagging


Yep, "rapt" is right, the slab is already damaged.

But, what you describe has happened before, often a lot worse like partial collapse at the CJ. That is the good news, the bad news is that it the mistake is going to cost some $. Who pays is another topic!

Now that this mistake has happened someone qualified needs to do a visual and tactile inspection and document the crack patterns and widths in all immediate and adjacent areas. This is a cover-your-arse excercise, but necessary.

Since this is a flat plate, is there any cracking around the column indicative of punching shear cracks ? These are an area of concern too.

"rapt" has stated the effect if you do nothing pertaining to cracked sections, so I suggest that you do something to repair and re-instate to its condition prior to shoring removal.

Although it can be problematic, i would suggest de-stressing the tendons that are within the cracked area/s. This will mean you have to re-shore for the full tendon lengths of those tendons de-stressed. Since this is a coupled CJ i doubt any strand tails have been cut off, but if you do not have enough tail to de-stress you may have to relocate a new CJ.

Next, lift the slab. You can use small stroke jacks (<3") to lift the slab panel/edge without too much trouble. I would use a load calibrated jack and also a survey method to record the lifting dsisplacements.

Then pressure inject the cracks with a low viscosity epoxy resin. For QA/QC i would suggest coring and maybe ultrasonic testing to determie 90% + penetration.

Then re-stress the tendons.

Keep the shoring in this area for the balance of this floor's construction.

Proceed with the adjacent pour.

Failure to re-instate the cracks will result in excessive deflections, as stated by "rapt".

Is this is a grouted system, you will want to delay epoxy injection until the ducts have been grouted to save filling up the ducts with a low viscosity and expensive resin, and also not enable you to re-stress!!!. But after re-stressing some fine cracks will close up, and the these may be harder to inject unless a very low viscocity resin is used.    

This type of operation is not a General Contractor fix - if your P-T company is experienced with heavy lifting, concrete repairs and similar then they should be fine. Ask for documented experience of past projects for whom ever does the repairs.

Also, ask for a Method Statement before commnecing any work.


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