The most durable building material would definitely be concrete. It is easily available and materials made from it require low maintenance. To add to these two merits, concrete can be molded to form a number of different types of outdoor furniture, including bench.
First, choose a design of the bench. You may get several designs to choose from with the help of a professional in this field. If you wish, you have the option of making one of your choice. To do it, take a butcher's paper and a pencil, and then design the desired shape of your dream concrete bench on it.
You would require the sitting surface and two legs. Draw a pattern of the desired design to involve a rectangle for the top of the bench.
Draw two squares for the legs. Consider using a measuring tape and a T-squarer for accuracy while working on the actual process. Cut out a mock of the bench with the use of plywood. Testing the design will make you aware of many problems that may occur and hinder your fast completion of the task.
In case the design made by you is not giving you the expected size and shape of the bench, make another one. Make corrections wherever needed to make the design which can give rise to desired shape and size of the concrete bench.
Now, cut out the template on the butcher's paper. Tape it to the plywood. Use a temporary spray adhesive for bonding the template. The next step in making the mold is to cut out the pieces from the plywood. Sand down the inside of the pieces of the mold. This will allow better release during the de-molding process.
Form the mold sides by using the plywood. Keep the height of all the sides around 3-4 inches long. Correspond the side length with the size of the concrete bench. When you do it, the mold side is generally 4-5 inches tall.
In case you wish to have curved edges, it is good to work with a dense foam. Now, attach the sides to the molds at the base. Make use of wood glue and nails to do it. You may use a clamp to secure the joints. Clamping will also provide additional support when the glue dries.
Apply silicone around the inside edges of the mold for rounding it off. Make sure that the silicone lining is thin. Take a grouting tool to make the silicone edges smooth and take out the excess of debris. Allow the silicone to dry completely before you pour cement into the mold.
If you find the process tedious, you may opt for a ready-made one. You may easily find concrete bench molds for sale in the market. Search for them in your town or city and select one which matches with your choice and budget the best. You need to take a couple of points into consideration. See if the mold size is right for your intended purpose.
Do check in the market if the bench pieces, like legs and top, would be available and would get securely fastened together when the bench is made. Do check if the design on the top of the bench allows water collection. If it does, do not go for it. The reason is that the water may freeze and crack the top of the bench if frozen.