It depends on the loading condition, type of loading, and the soil type, and to a lesser extent on the climate. There are several SOG spreadsheets available to help.
I occasionally design outdoor slabs on grade for vehicular traffic, such as semis and single-unit trucks (usually assuming AASHTO standard design vehicles), telehandler fork lifts, etc. My primary design guide is by the American Concrete Institute: "ACI 360R-10, Guide to Design of Slabs-on-Ground" (April 2010).
For secondary references, I use the Portland Cement Association document, "Slab Thickness Design for Industrial Concrete Floors on Grade" (1996), and the US Army and Air Force technical manual, "Army TM 5-809-12/Air Force AFM 88-36, Chapter 15, Concrete Floor Slabs on Grade Subject to Heavy Loads."
In addition to slabs handling traffic, these design guides also deal with heavy fixed loads such as warehouse shelving.
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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill
fel... that's a good publication. I have an old program written by the PCA titled 'Airport' that I used to use for designing runways, taxiways, hanger slabs, industrial slabs, etc. Worked just fine... and still have a running copy of it... originally on a 5-1/4" disk... forgot to add... it only runs on Win7...
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates
A couple weeks ago I consolidated and updated my various older Mathcad calculations into a comprehensive worksheet that handles multiple traffic scenarios: two different AASHTO load conditions, rubber-tired telehandlers (non-AASHTO loadings figured out via tire pressure and a couple other tricks), synthetic wheel pallet jacks, and miscellaneous pickups and equipment storage, all of which are conditions I have dealt with in the past. I now have a project where at least four of these conditions will exist, hence the need to consolidate my various past calculations into one worksheet. For now, I convert R-value to Modulus of Subgrade Reaction and California Bearing Ratio using Figure 4.3 in ACI 360R-10, but I plan to develop equations for this purpose so that the worksheet will be fully automated.
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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill