4 inches
General Gravel Driveway Information-Gravel layers should be at least 3-8 thick. -Gravel driveway surface should be crowned or sloped to a bank. Ideal Grade Percentage 2-5% Minimum 1% Maximum 11%. Calculate Grade Percentage: Difference in Elevation in feet divided by horizontal length in feet x 100.  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 adequate. 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. Once the site is ready, paving with asphalt follows. 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. For example, a total of 3 inches may be placed in two layers each 1.5 inches thick when compacted. A good way to pave a 4-inch pavement is to first place about 3 to 4 inches of loose asphalt, and compact it to a thickness of 2.25 to 3 inches, and then tack the surface and pave the remainder and compact again. 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.  The reputable contractors in your area will know what the proper thickness the asphalt should be for your driveway. And as the saying goes… your asphalt driveway is only as good as the subgrade beneath it! Paving a driveway to thin is actually a common rip off practice by the fly by night unethical contractors! Now the only way to ensure the right thickness of base and asphalt for your soil type, is to hire a soils engineer to evaluate your soil and come up with a pavement design for you. However that can multiply the price of an asphalt driveway by three times! Even still there’s no guarantee that it wont crack! Residential Driveways-should have a 4 - 5 inch gravel base prior to paving. Asphalt is only as good as what is underneath it! The asphalt when rolled should be 2.5 inches thick. Commercial Parking Lots-Parking lots and subdivision roads should have a minimum 6 inch gravel base due to heavy traffic & large machinery that will drive on it. The asphalt should be placed in 2 layers at 1.5 inches of asphalt per layer (referred to as 2 course) resulting in 3 inch thick asphalt. Asphalt is placed in 2 layers for this type of project for better compaction. Commercial jobs vary depending on what is required by management, state, etc Really, the reputable contractors in your area will know what the proper thickness the concrete should be for your driveway. Installing a concrete driveway to thin is actually a common rip off practice by the fly by night unethical contractors! 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!
http://www.thegravelexpert.com/Gravel_Information.html http://www.jacksonconcrete.com/testimonials.html http://www.asphaltpavement.org/index.php?option=com_content&view=article&id=120&Itemid=267 http://www.concretenetwork.com/concrete/concrete_driveways/basics.html http://www.allaboutdriveways.com/asphalt-driveways/faqs/asphalt-installation/asphalt-driveway-thickness/ http://getpavedmidmichigan.com/didyouknow.htm http://www.allaboutdriveways.com/concrete-driveways/faqs/concrete-installation/concrete-thickness/
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