Concrete slab thickness vs.footers

[747driver]

When i pour concrete that requires extra footing support i just dig deeper for that specific area and tie the rebar in. Just taper your sides of the deep footing to prevent 90 degree sharp edges and corners. Also i would go with a 8" thick x 6" wide thick outer footer all the way around and then taper up to 4" " thick. Otherwise i think your building corners will crack over time.

 

[Streetcar]

A 4" thick slab is very hard to properly reinforce except to keep cracks from growing. Fiber , which will give a slightly fuzzy surface,will give better crack control.
The lift will cause concrete slab to fail in punching shear. Anything you can do to spread the load will helps lab capacity. Thicker concrete footing or steel plate are two of the easiest methods to spread the load out.
If you do use rebar, the spacing should be closer than 24" to increase shear strength of slab

 

[USAFpj]

Thanks. I was visiting my Uncle's shop last night- 5in monolithic reinforced with fiber- which was a new(ish) offering when it was poured in 92. We were looking at the only two cracks he has in the entire 30x50. Each was about 3ft in length, but both were hairlines that you couldn't even sweep dust into. Hope I get as lucky.

 

[Cord]

You could use a thickened slab under the lift, but you must know exactly where you want the lift and won't be able to move it later. If you go with a thicker slab everywhere, you can put the slab where ever you'd want. Personally, I think a 4" slab is a bit too thin for #4 grid. The slab will be getting pretty thin at the intersections.

 

[SPIKER]

Some advice to slow/decrease cracks is to seal up the building and keep the concrete wet as long as possible. It needs to set but once the surface cure is set and troweled smooth (or brush finish) use light spray to keep it damp. after 10 or so hours a light spray to flood surface and keep it wet for several days. This is what i did, I used plastic nailed over window openings and kept all the doors closed. the morning after it was poured I sprayed it down 2 times and that evening ran hose to add almost an inch of water over top w old sheets as seals at doors. I did this for a week keeping it wet, the concrete is so hard it wont take any stain.

I did NOT cut any expansion cracks inside as I have radiant tubing in the floor, I DID cut them outside however this was a mistake. The cracks outside let water get under the slab and it heaved & cracked one triangle section. Inside there is a 2foot crack hairline no separation right at the roll up door by the 6x6. it is not noticeable no cuts to catch dirt much happier w inside than outside where cleaning cracks is a pain, they are sealed to for this winter so will see how it's 4th winter is.

Mark