Mixing concrete
All preparatory work on the sub-base should be completed before the concrete is mixed. If you are mixing by hand, rather than using a concrete mixer, mix on a clean, smooth surface. An old sheet of plywood or hardboard is excellent. Alternatively, you can work on a section of path or patio, protected if need be with a heavy layer of polythene sheet. Blend the cement/aggregate until the pile is a uniform colour with no patches of sand or cement. Make a well in the middle of the pile and pour in a little water.
With a shovel, work the inside walls of the well into the water until the water has been absorbed; then add more water and continue until the mix is just wet enough to place and compact. An easy way of checking this is to pat the surface a few times with the bottom flat of the shovel; after this compacting, the surface should be smooth and close-knit. Avoid using too much water, as this will weaken the concrete and cause shrinkage as it hardens. But the mix must be workable enough to be put in the moulds and compacted without leaving air-holes, which will result in honeycombing and loss of strength.
Concrete mixes
Although an average concrete mix could be used for most purposes, certain types are more suitable for particular projects, and the proportions of sand and cement must be adjusted accordingly. The mixes shown here are suitable for different kinds of concrete work outdoors. Where strength or resistance to wear is important, it is best to use mix ‘A’. Where a lower grade will do, mix ‘B’ could be used. Mix ‘C’ is a fine concrete suitable for very thin sections or for bedding mortars. Mix ‘D’ is for bedding paving stones. All proportions are by volume.
A 1 part cement, 2 of sand, 4 of coarse aggregate. Suitable for paths, pools, steps, fencing and edging.
B 1 part of cement,2,1/2 of sand, 4 of coarse aggregate. Suitable for foundations, garage floors, drives, filling for garden rollers, and thick walls.
C 1 part of cement, 3 of sand. For formal or crazy paving less than 50mm thick. This is also the mix for brick laying mortar if soft sand is used.
D 1 part of cement, 5 of sand. A stiff mortar mix for bedding paving.
Pre-mixed concretes
Many London builders make up their own concrete mix, although for small jobs it is more usual to buy a bag of dry mixed ingredients that requires only the addition of water. Dry mix can be bought from a builders’ merchant or DIY shop and is usually sold in 50kg bags, although some places sell bags as small as 3kg. As the cement in the mix deteriorates with storage, buy only enough for the job in hand. The dry mix is sold in a variety of proportions to suit different work, so when you order make sure you specify exactly what you are using the mix for.
For large jobs, such as for the foundation of a house extension, garage or for a long driveway, ready mixed concrete is the answer. This has the correct amounts of cement aggregate and water mixed in a central plant, and is delivered, ready to lay, in special agitator lorries. Ready-mix can be ordered in most places through a builders’ merchant and, in theory, can be obtained in any quantity. But in practice, quantities of less than 4 cubic metres (4 cubic yards) is uneconomical for the supplier.
A quantity of ready-mixed concrete can be delivered when it is most convenient, so that a large concreting job can be done in stages. And if there is access to the site, the agitator lorry can place the concrete directly into the formwork or trench, thereby saving a lot of back-breaking labour. If you do order ready-mixed, have your site ready before the concrete is due to arrive. And make sure that you have sufficient labour to handle the job quickly, 1 cubic meters of concrete weighs about 2 metric tonnes (2 tons) and in warm weather it may become unworkable in as little as an hour.
Setting and curing
Fresh concrete should not be allowed to dry out too quickly. If this happens, its strength will be reduced and cracking and ‘dusting’ will occur. Keep concrete damp after laying by covering it with a polythene sheet for 24 hours as soon as setting has completed sufficiently to prevent marking. Concrete goes through two stages before reaching its maximum strength. The first stage is ‘setting’, which is the initial reaction caused by the activation of the cement by water. In normal weather, setting takes about seven days, but hot weather could shorten the time to four days, and cold could extend the period to ten days.
It is easy to see when setting has completed because the concrete turns ‘green’, and literally takes on a greenish tinge. When this happens the concrete is starting to ‘cure’. When curing is complete, the concrete loses its green tinge and is said to have reached maximum strength, although it will actually go on getting stronger for up to 20 years. The complete cycle for setting and curing takes about 28 days. The formwork could be removed once the concrete has set, but if possible leave it in place until curing has completed. If it is unavoidable, you could start building on, or using, the concrete before it has cured. But it might crack badly if the maximum weight it is intended to carry is used during this time. Avoid laying concrete during frosty weather. Water in the mix will expand on freezing, and this is likely to make the concrete break up.
