slab-on grade stress due to shrinkage

[ajk1]

Can anyone suggest how to calculate the stress in a slab-on-grade due to shrinkage?

The common method of relating frictional drag to stress in the slab does not seem to account for the difference caused by concrete mix with low vs. high shrinkage strain.

I am interested in comparing the shrinkage stress in a slab-on-grade made with 0.07% shrinkage concrete mix with that made with a 0.04% shrinkage starin concrete.

 

[Teguci]

Depends where you are looking at, but for the critical locations the shrinkage stress is 0 because the concrete has cracked.

I've always looked at it more as a volume change problem then a stress problem. Once you get to concrete rupture, you get a crack regardless of rebar.

As for volume change, I've used the PCI manual for rough numbers and considering the top surface to volume ratio only.

 

[ajk1]

That is is not helpful. In order to design the slab to resist the loads, I need to be able to calculate the stress due to shrinkage and diferntial temperature effects and add it to the flexural stresses due to the concentrated post loads.

 

[Teguci]

If you are looking at the slab directly under the post, the shrinkage stress will be 0 because the slab will shrink to that location (think about normal loads and friction factors). At approximately halfway to the next post or 20'-0" away, the shrinkage stress will be zero because the slab will have cracked. If you are counting on the slab as a spread footing, your critical section is directly under the post

 

[rowingengineer]

Ajk1,
Normally I design my slabs ignoring stress from thermal and shrinkage for flexural stress (unless it is un-jointed). I then detail the slab to release the stress build up due thermal and shrinkage stresses.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[ajk1]

interesting thoughts. i will have to review the posy and joint locations to see if that is how it works out. Thanks

 

[Ron]

ajk1...attached is an academic article on drying shrinkage.

For your issue, I would take a more practical approach.

The stress/strain curve of the two concretes you are comparing are likely similar. I would equate the shrinkage percentage to a linear strain, neglecting volumetric strain (or you could equate with Poisson's ratio, 0.15). Then you only have the differential strain to deal with. Bracket the stresses with your different stress/strain curves and you'll get a reasonable estimate.

 

[ajk1]

Ron - not sure that I am understanding your advice, but I will think about it. Can you give me an simple example of how you would calculate the stress due to shrinkage, in say a 225 mm thick slab on grade, assuming a 0.04% shrinkage concrete?