Some of the coolest skate parks are concrete jungles, undulating landscapes of ramps and bowls and unexpected curves. The very first skateboarders sought out empty swimming pools to practice their off-season surf moves. The rounded concrete pool bottoms and walls provided the perfect surface for nonstop "wave" action. A few bold amateurs have built concrete skate bowls in their backyards, but this is usually a task reserved for the professionals. Again, if you have a pool, drain it and your work is done.
Concrete is an unforgiving building material. It dries fast, which means that mistakes are made for eternity. If you're going to attempt a backyard concrete bowl, spend some time working with concrete on smaller projects first. Get used to working with rebar, gravel, wooden forms and the concrete itself. Once you know that you have a good feel for all of the elements of forming and casting concrete, then you're ready for the planning stage.
Unlike building a wooden skate park component, building a concrete bowl requires lots of planning. To create the bowl, will you be digging down or piling up? Digging down requires a backhoe or lots of energetic friends with shovels. Piling up requires a few truckloads of soil and gravel, a bulldozer, or lots of energetic friends with pickup trucks and shovels.
The wooden frames for the concrete bowl will be made in small sections, with each board cut to precision lengths and nailed together at exact angles. Rebar must be bent and welded together leaving no more than 12 inches (30.48 centimeters) between each bar [source: SkateParkGuide.com]. When it's time to pour the concrete, you can either rent a small mixer or buy a whole truckload, depending on the size of your project. If you think a small concrete bowl wouldn't be fun, take a look at what these guys made.
For more details, SkateParkGuide.com has an excellent and in-depth introduction to concrete bowl construction and working with concrete in general.
Visit the next page for safety tips when building a backyard skate park.