Monolithic Slab questions

[sprintcar]

For a steel truss building, 24' x 36' - normal garage/shop application. Specs are 4" slab, combined with 12" x 12" footing around perimeter, fiber fill 3000psi concrete, rebar in footing, anchor "J" bolts set in place, located in Georgia (minimum freeze potential).
QUESTIONS:
Does the slab need either inserts or saw cuts for crack paths, or is it best left intact?
Should it be poured on tamped clay/sand or a gravel base?
Polyethylene underlay to prevent ground moisture from getting to slab - good or bad idea?
Any other words of wisdom on this project?
Thanks guys!

 

[ishvaaag]

If the fiber fill has equivalent to 4 cm2 effective section /m width of slab in steel, cracks should be minor and maybe no worth making the cuts. On the other hand sand gravel bases are good almost anywhere. Adding an impermeable layer won't be worth if not all around, I think.

In general, I also say, in my practice that does not include fiber reinforced concrete, I would make the slab 15 cm thick, doubly reinforced, and any edge beam on the ground no less than 40x40 cm.

 

[Ron]

Slab should be sawcut at 12' centers, each way (convenient to your slab size). Sawcuts should be placed within 8 to 12 hours of concrete placement.

Concrete should be placed on compacted fill, preferably sand. Polyethylene sheeting not necessary, but may be required by local code. Leave it out if you can.

 

[sprintcar]

Thanks! I was thinking along the same lines. I did a 24'x40' in Wisconsin with wire mesh/rebar and only had one minor settling crack in 15 years.
I'll ask contractor to increase edge beam size. Your dimensions will let the surface of slab be higher above finished grade as well.
Most contractors prefer the fiber, since it eliminates the problem of wire mesh sagging down or not staying in center of section.
Any tips on best cure/seal material to use?