Minimum thickness of 4 inches.
These heavy vehicles can cause severe damage over time regardless of the concrete thickness. Now the only way to guarantee the right concrete thickness for your soil type, is to hire a soils engineer to evaluate your soil and come up with a concrete pavement design for you. However, that can multiply the price of a concrete driveway by two to three times!  Thicker concrete is a good idea for driveways. I would like to see you install a minimum 5 inch thickness. Often contractors might try to tell you that a 4 inch thickness is sufficient. The cost to upgrade to a 5 inch thickness on an average driveway is chicken feed when you compare it to the total cost of the job. For example, if your driveway measures 720 square feet, a 4 inch thick pour will require 9 cubic yards of concrete. Increasing the thickness to 5 inches requires 11 cubic yards of concrete. The extra two yards of concrete will only cost about $130.00. (1999 pricing-adjust for inflation). The main priority in having a properly built driveway is the concrete thickness. You will want to consider the type of vehicles you will be parking upon that drive before you decide the thickness. If you will be parking normal cars, you can go with a four inch concrete thickness. If you have plans to park a heavy truck, camper, or a boat on your driveway, it is recommended to pour a five inch thick driveway. Place concrete at a minimum thickness of 4 inches. Increasing the thickness from 4 inches to 5 inches will add approximately 20% to your concrete cost, but will also boost your driveway's load-carrying capacity nearly 50% , according to the Tennessee Concrete Association.  In many cases a 4-inch thickness may be adequate, but 5 or even 6 inches of full-depth asphalt will assure you of a stronger, stable driveway under a wider range of climate and loads. As an option, some contractors use 6 to 8 inches of compacted aggregate, or gravel, as a base for 3 inches of asphalt pavement. Existing driveways of asphalt or concrete can be overlaid with asphalt with excellent results. Usually for driveways, a surface course 1.5 to 2 inches thick will suffice. Before paving, be sure the contractor patches any serious holes or cracks with asphalt and compacts it. In order for your concrete driveway to last for years and years, the first steps in preparing the subgrade are critical. Your subgrade should be evenly compacted. A standard driveway is 4” thick of concrete. It should be 4” thick throughout and should not vary 3-4”. A 1” deep joint should be created in the concrete in a 4” thick driveway. There is also proper spacing that should be followed for the joint placement. On a 4” thick driveway, joints should be spaced no farther than 8’-12’ apart. Concrete cures to required strengths in 28 days. Driveway concrete is recommended to have 4000 PSI. The concrete will achieve approximately 70% of its required strength in 7 days, which would be about 2800 to 3000 PSI. That is sufficient strength for you to drive a standard family vehicle on the concrete. In other words, thickening your concrete slab carries more load. Increasing your driveway's thickness from 4 inches to 5 inches will add 20% to your concrete cost, but the additional inch of concrete will add almost 50% to the load-carrying capacity of your driveway. If the driveway will be used by both cars and light trucks or SUVs, then a 4 thickness should be adequa Joints should also be spaced 2-3 times in feet the thickness of the concrete: so a 4 thick driveway should have joints no farther than 8- 12 feet, and e) A simple broom finish for sufficient traction We also found out that concrete can now be made to match any almost any color desired. The concrete thickness should be 4-5 Inches, we went with the 5 inches thick driveway, 4) When it came to reinforcing with rebar, most contractors balked at doing it. They preferred wire mess, because it is very easy to work with. Rebar will take time to install. 10. Grade the fill material or existing soil so that your slab of concrete will be the correct depth, or thickness. Usually driveways are at least four inches thick; for heavier vehicles or difficult to stabilize soil conditions, thicker concrete is suggested. 
http://www.allaboutdriveways.com/concrete-driveways/faqs/concrete-installation/concrete-thickness/ http://www.askthebuilder.com/concrete-crack-prevention/ http://www.planconcrete.com/how-thick-does-a-concrete-driveway-need-to-be/ http://www.concretenetwork.com/concrete/concrete_driveways/basics.html http://www.asphaltpavement.org/driveways http://www.concrete-driveway.org/construction.html http://www.jacksonconcrete.com/testimonials.html http://www.millennialliving.com/Activities/Renovation/concretedrivewaysdos-donts.htm http://www.wikihow.com/Build-a-Concrete-Driveway
how thick does concrete have to be for a driveway