Laying a concrete driveway over an existing gravel one is possible, but it is not generally recommended. The kind of gravel that is intended to go under cement is typically a smaller grade than what is used for gravel driveways. However, if the gravel in your driveway is small enough, it is possible to create a cement driveway over the existing one.
Rarely, it is appropriate to add cement mix over a gravel driveway. Many cement driveways use pea gravel under the cement to prevent the cement from cracking and breaking when the ground shifts. However, most gravel used for this purpose is specifically designed to be used under cement. Many forms of gravel used for driveways are too large to use with cement added on top later. Study your gravel driveway to determine the kinds of rocks used for the gravel drive. If most of your rocks are smaller than ½ inch, it is suitable to be used as a gravel underlay for cement mix.
The kind of cement used to lay over gravel is important as well. Typically, most cement driveway projects use a mixture of Portland cement, sand and water. Make sure to follow the specification "ASTM C-94" by the American Standards for Testing and Materials. This guide will show what kinds of cement are best in your area. Slump is also necessary for creating a driveway over gravel. Check with a cement professional for tips on choosing the right slump to use with your grade of cement and with the slope of your driveway. Generally, 4 inches of slump are recommended.
Sweep away any loose gravel rocks. The gravel should be firmly tamped into the ground. For a driveway cement addition, the use of forms is mandatory. Make sure the ground is completely level. Add firmly tamped dirt or sand to low areas. Add 4 inches of sump and 1 inch of aggregate before adding cement. The cement should be mixed and smoothed over the aggregate surface. Usually, 4 inches of cement will create the height you want for your driveway. Make sure the cement is as smooth as possible. This is usually done with a tool that looks somewhat like a squeegee. You must let it cure for three to seven days by applying water, plastic covering or curing compounds to the surface. Finally, joints are sawn into the surface of the concrete to prevent cracking.
If the idea of laying concrete in the professional manner is overwhelming to you, there is another way to add cement over gravel without so much work. The cement is laid over the gravel and then landscaping pavers are laid on top of the cement to add decoration and to prevent much of the smoothing process typically associated with making a driveway. The base system for this style of driveway is gravel, concrete, cement grout and paving stones.