Concretework – Working With Concrete
Concretework – Working With Concrete
Working with concrete can be arduous, but the principles involved are not complicated. It is important to ensure that the proportions of cement, sand and crushed stone are correct and that you mix it properly with just the right quantity of water. Certain guidelines should also be followed when laying the concrete.
The proportions of cement, sand and stone (aggregate) used for concretework depend primarily on the use to which the concrete will be put. Generally there are three grades of concrete — low strength, medium strength and high strength — although the actual ratios of the dry materials used will sometimes vary depending on the quality of the materials available. While low-strength concrete is commonly used for foundations and footings, a medium strength is preferred for garden footpaths, domestic driveways and steps, and for patio slabs that will be exposed to weathering. High-strength concrete is really only necessary for watertight walls and industrial situations, or driveways that will take very heavy traffic.
Mixing concrete is a laborious task. Even if you have a mechanical concrete mixer, you will have to shovel the dry materials into the machine. Whether you are mixing by hand or by machine, you will also have to measure the materials in batches to ensure that the proportions used are accurate. It is advisable to use one strong, rigid container (a builder’s bucket or a clean 25 litre paint tin) for measuring.
If you have a machine, load the stone with a little water first to prevent the mortar from building up on the blades, then load the sand and finally the cement and more water. When mixing by hand, combine the cement and sand first, either in a wheelbarrow or on a flat, dry surface. Do not mix them directly on the ground as soil, dead leaves and small twigs will almost inevitably get mixed in, and water may be absorbed from the mixture. Make a crater in the centre, then add water, shovelling the dry mixture from the edges to the centre. Once it is smooth, add the stone and continue shovelling. Aim for a firm, consistent mixture which is neither too runny nor too dry.
Formwork (shuttering) is essential if concrete is to be laid above ground level or if steps are to be built from concrete cast in situ. Various materials are suitable for this framework, including timber. Old wood may be used, provided it has reasonably straight edges and is rigid enough to bear the weight of the amount of concrete to be poured. For a curved path, you will need formwork which can be bent to shape. Hardboard (masonite) is ideal as it is reasonably flexible but strong enough to support the concrete. You will need to hold the shuttering in place with loose pegs, or nail it to flat stakes which can be hammered into the ground. Whichever method is used, make sure that the pegs or stakes are on the outside of the formwork.
Laying or pouring concrete is much easier if you have help. Before you start, wet the ground to prevent the moisture in the concrete from being absorbed into the ground and possibly causing cracking. Either shovel the mixture into the trench or formwork or pour it directly from the wheelbarrow. Chop into the concrete with your spade to allow trapped air to escape, and compact it roughly with the back of the spade or shovel. Then use a straight-edged plank to compact the concrete more thoroughly, using a chopping movement. As water comes to the surface, use a smoother sawing motion to level the concrete to the height of the formwork. Make sure there are no gaps or hollows.
If you are planning to screed the surface of the concrete with mortar (for tiles, perhaps), the finish of your slab will not be important, but if the concrete itself forms the surface of the patio, path or treads of steps, you will want an even finish. Use a wooden float to smooth it or a stiff-bristled broom to create a rough texture. Alternatively, for a slip-proof finish, scatter fine crushed stone or pebbles on the wet surface and tamp lightly with or straightedge. When the concrete has almost set, spray with water and brush with a stiff broom to expose the aggregate.
When laying concrete over a fairly large area, you will need to create expansion joints to help prevent cracking. Do this by working in sections not larger than 3m x 3m, and laying alternate panels. When the first panels have set, fill in the remaining areas between them with more concrete.
Allow the concrete to set thoroughly before you remove any shuttering. Do not allow it to dry out too rapidly or it may well crack. In hot weather, cover it with sacking or plastic sheeting, or moisten it now and then with water.