Concrete is made up of cement, aggregate (particles of stone) and sharp sand. When you mix these ingredients with water, they bind into a solid, hard material. In different proportions, they produce concrete of different strengths for different uses.

It’s important to get the proportions right. If there’s too much aggregate, you’ll struggle to get a satisfactory finish. And too much sand will make the mix weak, so that the finished surface could be damaged by the elements.

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Concrete begins to harden after about two hours, and then becomes unworkable. But it doesn’t become really strong for several days, and continues to harden as long as there’s some moisture in it.

Read our helpful new buying guide for cement, mortar and concrete

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To make sure that your concrete is strong enough, you’ll need to take a number of factors into account when designing your path, pad or drive.

- The type of mix will be decided by what the concrete will be used for (see the table for more details).
- The thickness of the concrete is determined by the weight it’ll be supporting. A pad for a lightweight wooden shed would only need to be 100mm deep. But for a stone, brick or block garage, you’d need a 150mm sub-base of compacted hardcore and a 125mm concrete pad. Also, the edges of this pad would need to be deeper (at least 200mm) to support the walls.
- The surface of the concrete must form a slight slope, so that water drains off. It should slope away from buildings or (in the case of a slab used to site a shed) away from the door.
- When it’s laid against the wall of your house, the surface must be at least 150mm below the damp-proof course.
- In an area that has to cope with heavy weights (such as a driveway), you should reinforce concrete with a steel grid at half its depth.

### Top tip – Easy mix

Concrete mix is an ideal option if you don’t want to combine the ingredients yourself, or you only need a small amount of concrete.

### Mixing concrete by hand

Mixing concrete by hand is only really an option for small amounts. You prepare it in the same way as mortar, mixing it thoroughly and methodically until it’s an even consistency – neither too dry nor too sloppy. When you pick up a shovelful it should stay on the shovel, and when you drop it onto the sub-base it should spread slightly but retain some shape.

### Using a mixer

Start by pouring about a quarter of a bucket of water into the mixer. Add half the aggregate and half the sand (or half the all-in aggregate), and all the cement. Then check the mix to see if you need to add more water. Add the rest of the aggregate (and sand, if you’re using it), and more water if necessary. If you find it difficult to assess how much is in a shovelful, try using buckets instead . If the final mix is too sloppy, add more aggregate and cement in proportion. An average-sized mixer will take 12-14 shovelfuls of aggregate and two to three shovelfuls of cement.

### Ready-mixed concrete

If you’re planning to lay a large area of concrete or you’re short on time, ready-mixed concrete is well worth thinking about. Depending on how easy it is to get to your site, the concrete will be delivered in lorries and either poured directly on to the area or wheelbarrowed from the lorry to the site. It’s a much more expensive way of buying concrete, but it does cut down on labour enormously – and you can get any quantity delivered. If you decide to use ready-mixed concrete, you’ll need to ensure that everything is organised before the lorry arrives and that help is on hand. You order ready-mixed concrete by the cubic metre: it’s sensible to add 10% for wastage and specify the job you’re doing, so the right mix can be prepared.

### Concrete mixes

Ingredients |
Amounts required for approx. 1 cubic metre of concrete |

Paving mix (strong mix suitable for exposure to all weathers, including frost), using separate aggregate1 part cement 1.5 parts sharp sand 2.5 parts 20mm coarse aggregate |
16x25kg bags 0.5 cubic metre 0.75 cubic metre |

Paving mix(strong mix suitable for exposure to all weathers, including frost), using all-in aggregate 1 part cement 3.5 parts all-in aggregate |
16x25kg bags 1 cubic metre |

Foundation or footing mix (and for concreting in fence posts), using separate aggregate1 part cement 2.5 parts sharp sand 3.5 parts 20mm coarse aggregate |
11.2x25kg bags 0.5 cubic metre 0.75 cubic metre |

Foundation or footing mix(and for concreting in fence posts), using all-in aggregate1 part cement 5 parts all-in aggregate |
11.2x25kg bags 1 cubic metre |

For garage or garden shed bases (not exposed to weather), using separate aggregate1 part cement 2 parts sharp sand 3 parts 20mm coarse aggregate |
12.8x25kg bags 0.5 cubic metre 0.75 cubic metre |

For garage or garden shed bases (not exposed to weather), using all-in aggregate1 part cement 4 parts all-in aggregate |
12.8x25kg bags 1 cubic metre |

Working out how much concrete you need calls for a bit of basic geometry. Measure the surface area of your site by multiplying the length by the width (in metres). Then multiply that figure by the depth or thickness you need (as a fraction of a metre) to get the volume in cubic metres.

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In practice, figures are rounded up and about 10% added for wastage. For example: (width) 2m x (length) 6m x (depth) 0.15m = 1.8 cubic metres, which you’d round up to 2 cubic metres of concrete.

If your site is circular, use the formula ?r2 (? = 3.14 and r2 = the radius of the circle squared). Multiply them together to get the surface area, then multiply that number by the depth of the concrete needed to get the volume. For example: (?) 3.14 x (radius) 2m x (radius) 2m x depth 0.15m = 1.884 cubic metres of concrete. Rounded up, you would need 2 cubic metres of concrete.

### Irregular shapes

To work out the volume of concrete you need for an irregularly-shaped site, draw it accurately and to scale on graph paper. Each square should represent one square metre of the site. Count all the squares you’re covering, and roughly add up the part squares. Then multiply that total by the depth of the concrete, and round the figure up by 10%.

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