Uniformly Loaded Slab on Grade

[jjeng2]

I came across a situation which seemed simple but apparantley I have no references on the proper way to do this. I have a slab on grade that needs to be designed for a 460psf uniform load. Im specifying 6" porous fill under on controlled fill or original soil. I will spec contraction joints 20' max. It is a small area. It is a pit for a prefab "pool treadmill". It will not be exposed or walked on so minor cracking is not a huge concern.

I need advice or references on thickness and reinforcement.

Thanks

 

[jheidt2543]

One of the best books for slab-on-grade design is "Designing Floor Slabs on Grade" by Ringo and Anderson. One of their major concerns, in addition to the applied loading, is the subgrade soil conditions. Is your subgrade a granular soil, a plastic clay or a compressible clay?

A loading of 460 psf on a dense, granular subgrade is not too bad, but on a compressible clay it could lead to continual settlement problems.

So, first find out from your geotechnical engineer the subsoil charactoristics and capacities, then design your slab.

Differential settlement under your pool treadmill could ruin your pool!

 

[Lutfi]

I recommend that your contraction joints be placed at 2.5 times the slab thickness in feet (i.e 2.5X6 = 15.0 feet). This is a good rule of thumb for joint spacing.

I ditto Jheidt's input regarding the reference.

 

[Slowzuki]

I'm not a structural so correct me if I'm wrong, isn't the spacing of contraction joints dependand on your mesh strength, material the slab is placed on and the tolerance for displacment at the crack?

Ken

 

[SlideRuleEra]

Slowzuki - For unreinforced concrete (which this seems to be), I concur with Lutfi. If joints are spaced too far apart curling becomes excessive. This causes unpredictable cracking.

jjeng2 - As always,be sure to cure the slab thoroughly (preferably wet curing to minimize curling).

 

[PEStructural]

You might want to consider having thicker edge footings (turned down footings). This will help the curling issue because if you have a slab of uniform thickness, the edges dry first and begin to curl. If you thicken the edge, then it dries more uniformly and it won't (or shouldn't) curl.

 

[jheidt2543]

I'm not sure I subscribe to mjflorio's theory regarding a thicker edge footing (turned down footing) at a slab edge as a "cure" for slab curling.

First of all, if this were a cure, then each construction joint in a large floor slab would need a thickened edge. This would add a significant cost to floor construction.

Secondly, not all floors curl. From my experiance, floors that are wet cured and allowed to slowly loose their excess moisture, curl less. The key seems to be a more even drying of the slab through it's thickness. If a vapor barrier is used under the slab (which I am in favor of) the process takes longer, but it can be done. One thing that seems to help even out the rate of moisture loss is the use of a high solids curing agent on the floor slab.

 

[PEStructural]

It also depends on the pour size. I reread the original post, and I believe my comment doesn't apply because of the relatively close spacing of joints. But for large pours like that at Home Depot or a warehouse, this does present a solution.