How to Make Concrete Slabs
Instructions for making a garden path, describe the laying of concrete in situ. An alternative method of using concrete for a path is by making slabs. These concrete slabs may also be used for making garden walls, terraces, edgings to rock gardens, garden pools, etc. One of the easiest methods of making slabs of varying dimensions is between formwork. A site for manufacture of the slabs should be levelled off, and the formwork set up to enclose an area convenient for working. A reasonable area which can be easily handled at one time, is one measuring 3 ft. by 4 ft.
Care should be taken to level the edges of the formwork all round, and to ensure that the corners are square. The inside of the formwork should be coated with lime-wash to prevent the concerte from sticking to the wood. The ground inside the formwork should be levelled with a rake, and the depth of the formwork need only be 2 in. For making small slabs the depth reduced by a half, with a sand bed which should be laid inside the formwork and raked level to a depth of 1 in. With this preparation done, the concrete may be mixed and placed in the formwork. A suitable mixture for making small slabs is one part Portland cement to three parts sand, and the concrete may be tinted by the addition of a colouring powder as described above. Each batch of slabs that is made at one time in the formwork may be differently coloured.
The concrete should be thoroughly mixed and only sufficient water should be used to make the mass pliable. Lay the mixed concrete inside the formwork, level it roughly with a rake, and finish the surface with a screed. The screed used to make concrete slabs should have a straight edge and should not be hollowed to provide a camber. The screed should be worked twice over the surface of the concrete, once with a chopping action to level the mixture inside the formwork, and the second time with a see-sawing movement with the ends of the screed resting on the top edges of the formwork. The surface may then be smoothed with a wooden or metal float. The large slab of concrete is then cut into small slabs.
This is done as illustrated (right) which shows how a plank with straight edges is placed on the concrete, and the thickness of the concrete is then cut through with a trowel. The cutting should not be done until the concrete commences to stiffen and set off. If it is done too quickly after the concrete is laid, the cut edges will rejoin and the slabs will not separate.
With the small slabs neatly cut to shape, the concrete should be covered with wet sacks and left three or four days to harden. The slabs are then lifted with a shovel. When they have been separated the small slabs should be stacked on edge and left for as long as possible before using them. In this way batches of slabs may be made at weekends, until sufficient slabs have been accumulated for the job for which they are intended. If the pieces are intended for use in a path, the edges may be irregularly cut to form crazy pavement instead of sliced into rectangular slabs. When used for paths the slabs or pieces of crazy paving should be laid on a foundation of freshly mixed concrete — a suitable depth is 1 in. The cracks between the pieces may be filled with mortar, or left open.