Exterior Concrete Repairs
Many outside repairs need concrete, mortar or cement. The important constituent is Portland cement, which is bought as a fine powder. It is cheapest when bought in large bags, but as it will deteriorate and harden if kept for long after opening, it may be more economical to buy only sufficient for a job. A mixture of sand and stones, called aggregate, is added to the Portland cement. You can mix sand and stones, or buy them already mixed. When sand and cement are mixed with water a chemical reaction causes hardening. For small repairs, it might be more sensible to buy ready-mixed sand and cement.
The proportions in the mixture have to be varied according to the job. The aggregate in foundations is very coarse, whereas only sand would be used in a mortar mix for pointing or filling a crack.
The maximum size of stone can be specified when ordering aggregate —20 mm (¾ in) may suit heavy work, 10 mm (2/5 in) would suit paths, while sand only would suit repairs. If you ask for all-in aggregate, sand and stones are already mixed.
For cracks and similar repairs use a proportion of one part cement to three parts coarse sand. For paths use one part cement to three or four parts all-in aggregate. For foundations use one part cement to four or five parts all-in aggregate. If sand and stones are separate, the second mixture is conveniently remembered as 1:2:3 (cement: sand: stones).
Use clean water to mix concrete. The amount of water should be kept to a minimum, as too much gives a weaker result. Put the aggregate on a flat hard surface in a broad pile, and lay the cement on top. Using a spade or shovel, mix the aggregate from the bottom with the cement until the
whole pile is an even colour. Make a hollow in the top and pour in a little water, mixing from the edge to the centre. Add more water and continue to mix, turning in dry material until the whole mixture is wet. Try flattening the heap and cut into it with the spade. The ridges formed should stay there. If they flow back into a smoother surface, the mixture is too wet.
There is no need to rush laying concrete. Allow time to do the job properly, but go straight into the work after mixing. If concrete dries too quickly it may eventually cause cracks. Polythene sheeting should be laid over concrete to allow for slow drying. Full drying and hardening will take from four to ten days, according to atmospheric conditions.
Concrete does not wear well in thin sections. In the long run it may be better to increase the area in need of repair, so as to allow for a thicker layer of concrete.
A mason uses several trowels, but most repairs can be done with a small diamond-shaped pointing trowel. For a crack, poke out all loose material with the trowel or a spike and use a stiff bristle brush. Prepare a mixture using one part cement and three parts coarse sand. Brush water into the crack just before you start to work. Using the point of the trowel, force the concrete as far into the crack as possible. Press down and work in more until full, then level with the trowel.
Repairs to cracks may be improved with a PVA adhesive. Put the adhesive directly in the crack and leave to dry. Mix a small amount of the adhesive with the cement and sand mixture.
In a simple crack with fairly sharp edges, filling alone should be sufficient, but if the edge is worn away, the feather edge of the new concrete will almost certainly crumble after a short time. To avoid this, use a cold chisel to chop into the old concrete, making a recess with clean angles. The new concrete will have thicker edges.
If concrete is to be laid to extend a path, use pieces of wood to mark the boundary and restrict the new concrete. The surface can be levelled with a straight-edged board moved up and down over it and the border. To achieve a smoother surface, use a float, which is a flat piece of wood or metal with a handle. This can be moved backwards and forwards over the concrete, but do not work it excessively. Leave the wooden boundaries in place until the concrete has set.