All about concrete

Most modern constructions stand on foundations made from concrete. Such foundations are necessary for three reasons. They spread a structure’s load over an area large enough to prevent undue settlement occurring. They bridge over any soft spots which may exist in the ground. And they provide a level base for the structure.

Simple foundations

Although foundations vary enormously in complexity, those used for garden projects – walls, barbecues and planters – can be of the simple, strip type. Also known as footings, these are made by digging a trench and then pouring in a measured amount of concrete.

Concrete paths and the slabs on which greenhouses and garden sheds rest come under the classification of raft foundations. These are made with the aid of pieces of timber – formwork – laid out before the concrete is poured to give the finished slab a uniform shape. Raft foundations are covered in detail further on in the course.

Concrete is a mixture of Portland cement, aggregate and water. Aggregate is made from sand, stones, or some other inactive material and is supplied in fine, coarse, or mixed varieties. Mixed aggregate is often called all-in ballast: since it is a mixture of sand and stones it is easily hand mixed and is suitable for all kinds of amateur projects.

When first mixed, concrete consists of particles of stone and sand bound together by a paste of cement and water. As the paste dries it hardens – bonding the stones into a strong, durable structure.

This process takes place in less than a couple of hours so time is always an important factor when you are working with concrete.

Which mix to use

The composition of a batch of concrete is expressed in the building trade as a ratio of cement to sand to shingle, in that order. So if a mix is described as 1:3:6, it means one volume of cement and three of sand to six volumes of shingle.

At the builders’ merchants, however, the sand and gravel is normally sold mixed as all-in ballast. For simple foundations, ask for ‘18mm 1:6 ballast’. The 18mm means there are no particles in the mix bigger than 18mm across and 1:6 means that you need six volumes of ballast to one of cement for each batch.

The strength of a batch of concrete also depends on its water content at the time of mixing and laying so it is important to make sure that the correct amount of water is used. For a 1:6 cement to ballast mix, use about two volumes of water.

Calculating quantities

For simple footings, there is no need to involve yourself in the complex calculations used in housebuilding. Find the volume of concrete required. Add on a little extra to the result – it is better to have too much concrete than to have to buy more half way through the job.

If you are planning to build on particularly soft soil, you may need to reinforce your foundation trench with a little hardcore before you pour in the concrete. Your local building inspector can advise on this.

Readymix or do-it-yourself?

The ingredients for concrete can either be bought unmixed from a builders’ merchant or delivered by truck, ready mixed. The latter saves a great deal of hard work but will probably be too expensive for small projects using less than 2.5m:! of concrete. You may also have difficulty persuading the concrete company to deliver such a small load.

If you decide on ready-mixed concrete, bear in mind that it must be laid and levelled within two hours of delivery and that the truck will need access to the site.

One of the advantages of mixing your own concrete is that you can do the job in stages, mixing just enough for your needs. If the concreting is extensive, consider hiring a small mixer. Make sure, though, that you have it fully demonstrated before leaving the hire shop.

Mixing your own concrete

To hand mix concrete, measure out bucketfuls of sand and gravel – or all-in ballast – and arrange them in a large heap on-site. Each bucketful should correspond to one ‘volume’ of your mix. Hollow out a crater in the top of the heap and add a measured volume of cement.

Turn over the mixture three or four times until it is uniformly coloured – with no cement streaks – then re-form it into a volcano shape.

Now pour in some of your measured volumes of water – not from the same bucket as the cement – and shovel up the dry ingredients from the bottom of the heap. Turn the mixture over again, adding more water as you go, until a firm, even paste is formed.

To check if a batch has been correctly mixed, press hard on it with the back of the trowel or the heel of your boot. The impression you make should have a firm, closed surface free of water patches, pits and lumps.

Setting out

For setting out simple footings, you need a couple of sets of bricklayers’ line and a supply of wooden pegs half as long again as the depth of the concrete. You can use nylon fishing line instead of bricklayers’ line – but not string, which will stretch and sag. Make the pegs from wood offcuts.

The lines are used to indicate the widths of both the foundation trench and the structure above. If you are building flush against a concrete drive or path, only two lines are needed: one to indicate the other boundary of the foundations and one to mark the main wall of your structure.

Having decided on your site, measure it out and score the boundaries with a spade. At either end, drive in stakes to mark the widths of the foundation trench and the structure to be built on it.

Connect the stakes with your line, tied taught. Keep the lines as far up the stakes as possible so that you have room to dig underneath them.

Where the ground is soft- enough, you may find it easier to score out the sides of the trench then remove the stakes and lines before you start digging. To do this, hold a spirit level on end, plumb against each line and mark off the ground at regular intervals. Join up the marks with the edge or your spade.

Otherwise, use the lines as a digging guide. Try not to disturb the ground any more than is necessary – simply dig down to the required depth of the foundations, keeping the floor of the trench as flat as possible.

With the trench dug, the next step is to drive your wooden pegs into the floor. These will stay in place when the concrete is poured, indicating the final level of the foundations.

Start at one end of the trench, driving your first peg in until it protrudes to the required depth of concrete. Drive in subsequent pegs at intervals of about lm. Using a spirit level, check each peg with the preceding one to ensure that they are all exactly horizontal.

Pouring the concrete

Concreting should begin at the furthest point from where the concrete is being mixed. Avoid forceably throwing the concrete in to the trench as this causes the larger particles of ballast to separate from the mix.

Fill the trench up to the level of your marker pegs. If the foundations are to be particularly deep, pour in concrete to a depth of about 200mm then flatten it out with a piece of timber before you add more.

When you reach the stage where the concrete looks more or less level with all the pegs, begin levelling off. Use the edge of a piece of timber to flatten the surface. Do this in a ‘chopping’ motion, moving the timber along about 50mm each time.

Finally, test the overall level of the concrete by resting a straight edge or spirit level on each marker peg in turn. Remove any excess concrete and re-level where necessary.

Drying

At this point, the footings are complete and ready to be left to dry. Although they should be ready to build on the following day, the more drying time you can allow the better. In hot weather, moisten the surface of the setting concrete regularly with a watering can. This will stop it setting prematurely and cracking. If conditions of extreme cold are expected, place some hay or old packaging material over the concrete to insulate it, then cover it with plastic sheet.

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