Slab-on-ground: EDGE THICKENING VS SHRINKAGE RESTRAINT

[BryanTheEng]

Hi,

For slab on ground illustration below, edge thickenings will partially(?) restraint slab shrinkage in X-direction (assume thickening is 1.5x slab thickness & sloped at 1:1). I suppose it won't be fully restrained as it isn't fixed or anchored to any type of footing, but it still providing some degree of restraint.



ACI 360R-10 Guide to Design Slab-on-Ground says that with 1:10 gradual slope and low friction of subgrade, shrinkage restraint effect shouldn't be a concern (considered no restraint at all). AS3600 & CCAA T48 don't cover this issue at all.


Any solid reference on shrinkage rebar design based on partial(?) restraint of edge thickening?

Much appreciated.

 

[KootK]

This has been an educational read for me as I was not aware of the ACI360 recommendation. So thanks for that.

Any solid reference on shrinkage rebar design based on partial(?) restraint of edge thickening?

I feel that the difficulty with that is in the quantification of "partial". If you are willing to design for full end restraint, then common thinking has been to provide 0.006 reinforcing rather than 0.0018. That can be perceived as an objectionable amount of reinforcing in many cases however.

If a control joint would be palatable, you could modify your detailing as I've shown below. I've seen plenty of this in residential and light commercial construction. Granted, not many architects will be enthused about seeing that joint in occupied space.

A 1:10 slope is pretty shallow really. Most of what I've seen of other people's designs are in the 1:1 to 1:2 range.

 

[GJoeRMK]

There are some thoughts I want say:

-For slabs on the ground, in general the major restriction for lateral movements is the soil friction.
-The edge thickening in the slab may provide some degree of lateral restriction but shouldn't be taken into account unless the friction is overloaded, which is a rare case scenario, and if that case is meet, you can estimate the restriction force with the theory of retaining walls and passive K. If you want a better model, you may try a beam with springs simulating the soil-structure interactions
-You usually thick the edges in slab on the ground because you want to reduce the curling as ACI 360R-10 says
-Personally, I will rebar the slab considering the thin section because it is what most will cause the bending effect rather than the edges that are far more rigid and probably wont contribute much to the curling.