slab on grade for tile floor

[dmhb]

I am pouring a large slab on grade for a retreat center/conference center. The floor is independant of foundations for walls and other structural loads and so is very lightly loaded. (Walls and columns are already in place and supported by deep continuous strip foundations, I'll be pouring floors inside the rooms and in several exterior passageways.) The sizes of the rooms is about 22'x32' up to 35'x35' though I'll probably pour segments only about 12'x20' at a time and use rebar dowels at these construction joints (all conc. is site-mixed, no trucks or batch plant avail.). At least 10-12" of sandy clay fill has been compacted over the native clay soil and seems very stable.
I am looking for recommendations on a minimum thickness for the slab and recommended reinforcement. I was hoping to get away with no reinf., but I am concerned about future tile cracking.
A secondary question is the time necessary for curing (will wet cure, not a fan of curing agents and no admix avail.) for ordinary concrete before installation of ceramic tile.
Any recommendations or warnings based on your experience are greatly appreciated. (I am working pro bono for the church in Honduras and have few professional resources here; most construction here is by superstition, not science)

 

[Ron]

dmhb...absent movement of the subgrade from shrinking or swelling, you do not need to structurally reinforce the slab. If you have no welded wire fabric available, see if you can get some polypropylene fiber and mix it in. While you don't need structural reinforcement, you do need a bit of reinforcing for crack mitigation. If the subgrade is not active, I wouldn't worry about it. A 4-inch thick slab is probably sufficient, but make sure it is uniform in thickness (+/- 1/4").

Place your control joints a bit closer together...you mentioned a 12x20 area....put a control joint in the 20-foot side in addition to your construction joints. You don't really need dowels, but I would suggest a keyway at least.

Wet cure for at least 7 days, then keep a bit of moisture on it until it has likely gained most of its strength. Probably not necessary to keep until 28 days, particularly since it is inside.

Make the subgrade as flat as possible and prevent abrupt changes in grade...this promotes cracking. Use plastic sheeting if available, otherwise be sure the subgrade is well dampened prior to placement.

Coincide the tile joints with the control and construction joints. Make this tile joint "soft" by putting sealant in the joint rather than grout. This will allow movement of the slab and the tile together so that you reduce the chances of the tile being compressed and popping up as the concrete shrinks.

Good luck.