Concrete-based, the gravelled slabs are ideal for constituting a garden path, Japanese steps or to delimit the surface of a terrace, non-skid and robust. It is possible to manufacture them yourself from a mold made to measure...
The price of concrete slabs, usually proposed in 40 x 40 cm and 3.5 cm thick is not excessive, since we find in the trade for € 1.60 the slab, a little over 10 € the m². But at around 15 kg the unit, their transport can pose problems of logistics... And as they are most often square, it is necessary to manufacture them oneself if one wishes to obtain specific forms.
A time saving mold for making slabs
The formwork made here allows to shape four slabs at a time.
- The bottom is a plywood panel 1 m square and four cleats 35 x 35 mm section form the outline.
- To assemble the cleats correctly, they are sawn at 45° at the ends.
- The miter box is the most practical solution for this operation.
- They are then simply nailed to the panel.
To be able to remove the window during demolding, do not forget to nail in its center a cleat acting handle! Given the small amount of pourable concrete, a packaged product in a bag, ready to ruin, does the trick. For a larger quantity, a concrete mixer is very useful.
Make the gravel surface
For slab topcoat, 20/80 gauge rolled river gravel is used in material trades (to be bagged or loose depending on the quantity required). Before you start, you have to grease the mold with formwork oil and preparing a sufficient quantity of concrete to fill the four "cells" of the mold. The filling is done with the shovel, making sure to house two reinforcing bars diagonally to half-work to stiffen the slabs.
It will then be sharpened with a ruler, float, and sprinkle the surface of the slabs of gravel. To facilitate the reappearance of the latter, it can spray deactivating immediately after the float (it delays the setting of concrete on the surface). A shot of water jet will flush the gravel.
Several days are necessary before final hardening (ideally three weeks!).
The laying of the slabs is not very constraining and for regular maintenance, a high pressure cleaner will be enough to restore them to their original appearance... or almost.
- Make a plywood mold and battens, depending on the shape and size of the slabs.
- Coat the bottom of the mold and the inside of the clodding oil cleats.
- Mix the concrete sufficiently liquid to allow molding.
- Hammer the cleats from the outside with a mallet to distribute the concrete evenly.
- Before trowelling, cut concrete bars with a length equal to the diagonal of a slab.
- Place two cross pieces in each one.
- Hammer the sides of the formwork again.
- Immediately after bottling, spread the chippings into the still fresh concrete.
- Ensure that they are spread evenly over the entire surface of the slabs.
- Allow the concrete to settle and then give a strong jet of the stream to highlight the gravel.
- If you are using a surface deactivator, check the intervention time on the instructions.
- Unmould several days later (at least one week and ideally three), when the setting of the concrete is completed.
- Remove the crossover from the mold to facilitate drying.
- For an alley in a lawn, remove the grass, dig about twenty centimeters and establish a bed of sand and spread it to the rake and compact it to the lady.
- Lay the slabs, compact them by leveling them, then sprinkle with sand to make joints, or potting soil to grow grass.
Tips for DIYers
- The use of a colored deactivator makes it possible to control the regularity of the application of the product. Use a fairly wide spray nozzle.
- The laying of the slabs can be done on a concrete screed, by gluing or on studs, or on a bed of sand (0 / 2-0 / 5 mm) compacted about twenty centimeters thick.