Once all the moulds are ready, mix the concrete. You'll need to wear a mask for this as it's a dusty job. First pour the concrete mix into the tub, then make a well in the centre for adding the liquids. (It's rather like making cake batter.) For a stronger concrete, we replaced half the water called for in the mix instructions with acrylic bonding agent (which is usually sold in the same section as concrete mix). For example, our 30-kilogram bag of Sand Mix called for four litres of water. We mixed in two litres of water and two litres of acrylic bonding agent.
First fold the ingredients together, then thoroughly mix, making sure to turn the tub on its side to incorporate any dry ingredients remaining in the corners. Mixing concrete requires some muscle and should be done quickly so it doesn't have time to harden. We used a drill attachment to make
the job easier. The concrete-mixing attachment resembles a large hand-mixer blade. You'll need a drill with a large chuck to accommodate the attachment (we used a 4.5-volt drill). You shouldn't do any damage to the drill if you're only using it occasionally for this purpose. The resulting mix should have the consistency of thick oatmeal.
MAKING THE PAVERS
Next, pour the concrete into the moulds. Vibrate the side panels with a hammer (tapping all around the edges) or hold a palm sander without sandpaper along the sides. This will get rid of air bubbles and level out the surface. You may need to add more mix once air bubbles are removed. Then draw a flat piece of wood across the top of the paver to level it off (called screeding).
If there is any leftover concrete in the tub, you can pour it onto a grassy area to set, then dispose of it. Rinse the tub out with water immediately, before the concrete has a chance to set.
The pavers should be removed from the moulds while they're "green", which is the first level of curing (four to five hours after pouring if the pavers are in a warm, dry place, but drying time will vary if the air is cold or humid). They should feel solid to the touch.
Unscrew the corner bracket or screws attaching the side panels to the base and remove the panels. You can prop up the pavers on sticks to increase air circulation or just leave them on their bases. They must be left undisturbed for three to four days. Cover them with a damp cloth or spritz with water a couple of times a day so the concrete cures more slowly. If the water evaporates too quickly, the structure of the concrete is weakened.
Once the concrete is dry (though it keeps curing for a total of 30 days), you'll need to brush the top layer of sand away from your pattern. A stiff paint brush should do the trick. Then the pavers are ready to grace your garden.