Good site for this sort of thing.
http://www.pavingexpert.com/concrete.htm Quote:Concrete paths should be 75-100mm thick, whereas drives, garage bases or hardstandings should be at least 100mm thick. For heavier use, such as large vans, use a 150-200mm thick concrete slab. Concrete slabs intended for exceptional loads, such as commercial yards, lorry parks etc., will be at least 200mm thick and should be specifically designed as they will probably require a sub-base of at least 100mm thickness, and steel reinforcing mesh or a fibre-reinforced concrete.