Concrete moulds are precast shapes that are used to make concrete bricks. They enable the construction of uniformly shaped bricks without having to individually shape them all the time. Different materials can be used to make moulds. Rubber is widely used for this purpose because of its flexibility in making irregularly shaped moulds. Materials for making such moulds are easily available in hardware stores.
Make a model shape of the concrete brick for which you want to make a mould. To make the model, cut pieces of 2-by-4 wood with a hand saw according to the measurements of the brick you want and attach them with nails. Put the model in a container that is leak proof and apply a release agent, Synlube 531, on the walls of the container to ensure that the mould-making rubber does not stick when it is poured.
Pour liquid rubber over the model, ensuring that the rubber follows the curves of the model as much as possible. It is important to use enough rubber. If in doubt about the quantity needed, fill the form with water from a measuring cup, and make note of how much water it takes. Mix a little more rubber than this so that it fills to the top of the model. Give it enough time to dry, preferably for a day.
Check your mould for strength, and reinforce if necessary using craft sticks glued to the opposite edges of the mould. Remove the rubber from the container when it becomes hard enough. It will come out easily because of the release agent you put on the walls. Clean it up by smoothing the interior edges of the mould using a hammer and chisel.
Test the success of your mould making exercise by actually making a concrete brick. Prepare the concrete mixture using cement, sand and ballast.
Place the mould on the ground where you want to put concrete. Ensure that you place it on a flat surface so as to make uniformly shaped bricks.
Apply the concrete release agent on the walls of the mould to ensure that the concrete does not stick and fail to come out of the mould when it dries.
Pour concrete in the mould. Repeatedly push a shovel into the concrete to get rid of air gaps that might make the concrete brick weak.
Use a concrete scrapper to remove excess concrete on the surface of the mould to give it a smooth shape.
Leave it to dry for a day or two, but pour water on it occasionally to avoid cracks during the drying process.
Tap the mould on the side and pull out the concrete brick when it has dried fully. If it comes out with the exact shape of your model, you have a perfect mould that you can use to make as many concrete bricks as you wish.
Make many moulds to ease the work of making the bricks. The more moulds you have, the easier and faster the work gets.