PT slab construction steps

[Akhilkp]

Hi all,

I would like to know more about PT flat slab construction steps / sequences .

As per the received method statement from contractor , following are the steps

1.Slab pouring with rebar and tendons as designed.
2.Initial stressing (25% of total stressing) when concrete reaches 15 Mpa ( approximately after 24 hours of pouring)
3.Final stressing (reaming 75% ),when concrete attain 30Mpa ( approximately after 3-4 days)
4.Cutting of strands
5.Grouting of tendons( bonded PT)
6.Removal of slab shuttering and reproping.

This is a 50Mpa grade concrete PT suspended flat slab designed as per ACI codes .

I have the following questions to asks:

1. Any problem if delays the initial stressing ( after 48hours ) ?
2. Has the initial stressing and final stressing any effect on shrinkage crack control ?
3. Any problem if delays the final stressing ?
4. Can we start de-shuttering just after the final stressing ( concrete reached only 70% of the design strength ) ?

Any help and comments really appreciated.

Thanks

 

[Ingenuity]

Akhilkp,

Delays to both initial and final stressing can result (or increase) shrinkage cracking. It is dependentant on your concrete shrinkage properties, the amount and location of mild steel rebar, concrete curing, and the environmental conditions.

If the project site only works 5 days per week, Friday's are usually a bad day to place concrete, as on Saturday or Sunday you have to get the formworkers to come in and strip the edge forms, and then get the PT sub in too to set up (remove plastic recesses, install anchor blocks and wedges) then do the initial stress.

For large slab areas, it is often necessary to have two or more crews to set up and do the initial stress - it is easy to underestimate the time required to set up and do the initial stress!

Depending on the design and the concrete properties etc, sometimes shrinkage cracks do develop, but if the axial P/A is large enough the shrinkage cracks may close after final stressing.

Some consultants permit formwork removal after final stressing (before grouting) if the contractor has an abrasive schedule (multi-story high-rise construction, for example), using controlled re-shoring. For large slab areas, grouting can take a bunch of time.

 

[Ingenuity]

Forgot to also mention, restraint-to-shrinkage from columns and walls will also effect shrinkage cracking.

 

[rapt]

Delay to the initial is definitely not good.

Also relieving restraint to shrinkage is very important.

Stripping timing is also dependant on curing time as it provides curing to the soffit.

Remember that the tendons are unbonded until grouted, so if they are being relied upon for slab capacity, when ungrouted the slab will only have about 70% of the design ultimate capacity. And the grout will take several days to provide full bond. You do not want to be subjecting the tendons to high change in stress for a few days after grouting.