Crack control in PT Slab

[Robert216]

I have a PT slab over a parking level. Half is in heated space while the other half is exposed. I really would like to avoid expansion joints. What other options besides typical T & S reinforcing would help prevent temperature and shrinkage cracking (due to differential temp. changes)? Any help is greatly appreciated.

 

[csd72]

Have you looked into increasing your level of post-tensioning.

 

[Robert216]

Our firm isn't designing the actual tendon placement. This will be done by the engineer for the pt supplier. I just need to determine thickness of slab and columns, foundations, etc. Would a note on the drawing to the pt supplier be sufficient? Where could I find guidance on this matter? It is just hard to address now since the contractor hasn't selected a pt supplier and the information is needed for design/development. Thanks again.

 

[hokie66]

Direct tension cracking is usually due primarily to restraint. That is where you can control the design. You can limit the restraint by preventing stiff elements at the perimeter, or by delaying connection to these elements. You can also specify that the PT has to be at a certain level in the finished structure. At least 2 MPa, preferably 2.5, is required for a watertight slab, and that is assuming you do not get restraint cracking.

 

[rapt]

Rpbert216,

How big is your slab?

 

[tngolfer]

I agree with hokie66. Really the only way to prevent cracking is by limiting restraining elements. Extra mild reinf. or specifying a minimum P/A will help keep cracks widths small but will not prevent the cracking. My principal is on the ACI 318 committee and has me use a minimum P/A = 200 psi for parking structures.

 

[Teguci]

Keep the exposed side out of the sun (probably not an option).

Provide additional tendons at the perimeter beams that go between the conditioned to unconditioned space. They will experience double bending as they transition from the conditioned to unconditioned space. Additional tendons will be required at the interface between spaces as well.

Increase slab thickness - takes longer for daily temperature changes to affect the slab and reduces long term shrinkage and creep (V/S is larger).

If you want to model it, find the longterm volume change differences from your conditioned space to unconditioned space. Determine the required axial force needed to transition from one length to the other (F = PL/EA) and then, after careful deliberation, install the expansion joint (smile).

 

[hokie66]

tngolfer,

I would vote for 300 psi(about 2.1 MPa) minimum.