How to build concrete slab foundations

Raft foundations are the large slabs of concrete on which rest garden sheds, greenhouses and outhouses. Their function is to spread the load of the structure above, together with its contents, over a wide area.

Laying a stretch of concrete—for a patio, path or driveway—is a good introduction to the concreting techniques used in making rafts. It also leaves some margin for error if you have never worked with concrete.

Planning the project

Make a scale drawing of your proposed project before you start work. If you are planning a patio, path or driveway it will be much easier to run this right up to one wall of the house. Multiplying the final area of the project by the required thickness of concrete gives you a guide to how much concrete you need. For a drive, aim for a minimum thickness of about 100mm concrete over 100mm well-tamped hardcore. This can be reduced to 75mm concrete for paths and small patios or a minimum 50mm concrete over 75mm well-tamped hardcore. But if your soil is particularly soft, consult the local building inspector.

Ordering the concrete

For work of this kind use a mix of one part cement to 21 parts sand to four parts aggregate or gravel, plus enough water to bind the ingredients together. If your estimates call for more than 1m3 of concrete. Consider having it delivered ready-mixed by a local concrete firm. The extra cost may be more than offset by the savings in time and effort.

Remember that if you are concreting in large ‘bays’, you need two loads of concrete delivered on separate occasions. Allow for the cost of this in your estimates.

When ordering ready-mixed concrete, bear in mind that the company’s truck will need access to the site. Be sure both of the mix and quantity of concrete you want and if there is more than one firm in your area, shop around for a competitive quotation.

If you are mixing the concrete yourself, the volume of concrete you will get will be roughly equal to the volume of the aggregate. Do your mixing on level ground as near as possible to where the drive or path is to be laid.

Setting out

You will find it helpful to choose a site which is more or less level: ground which is heavily sloping calls for more involved concreting techniques, covered further on in the masonry course.

If you are working from an existing house wall, take this as your base setting-out line and take other measurements from it. If not, drive in two wooden pegs and stretch a length of masons’ line or twine between them. Ensure that the pegs are well outside either side of the work area.

Your second setting-out line will run at right-angles to the baseline and mark one side of the project. Having measured and marked where the line will cross the baseline , set it up with pegs and line. Use a builder’s square to ensure that the angle between the two lines is exactly 90 degrees.

The third line—marking another side of the project—should run parallel to the baseline. Measure out from both sides to the baseline to ensure that this is so then set up the pegs and line. Again, use the builder’s square to check that the corner with the second line is 90 degrees.

The fourth line—marking the final side of the project—runs parallel with the second. Measure and set it out in the same way as the others.

As a final check to ensure that the corners of the project will be square, measure across the diagonals. If the measurements are not exactly the same, adjust the positions of the pegs accordingly.

Preparing the site

Start by removing all obstructions and traces of vegetation—such as weeds and roots—from within the boundaries of the site. If there are some areas of existing concrete on-site, these must be broken up into rubble with a pick.

To get the site level, you need a straight piece of timber 2-3m long and a supply of wooden pegs, about half as long again as your proposed depth of concrete.

Decide where you want the final level of the concrete to be—normally against a lawn or base of the house— then drive one of the pegs into the ground on the edge of the site. The top of the peg will mark the final level of the concrete and is known as the datum. Ensure that the finished level of the concrete is a minimum of 150mm below any adjacent damp plroof course—also that the concrete sl6pes away from the house in about a 1:60 fall to deal with surface water.

Use the straight piece of timber and a spirit level to check the height of the peg against the point you are taking your level from.

The site can now be excavated to the required depth, working away from the datum peg. As you progress, more pegs must be driven in to help you keep an eye on both the level and depth of the site. The pegs should be no further than the length of your straight edge away from each other— or from the datum peg.

You can then check each peg for level with a straight edge and spirit level.

Once the whole site is pegged out in this way, you can measure down the pegs to the required depth of concrete and level off the ground at this point. Keep the edges of the site sharp and in line with the setting out lines.

Foundations such as driveways which are going to bear the weight of a car need to be strengthened. This is done with wire mesh of about 3mm diameter which can be cut to size with bolt cutters.

At this stage, any obstructions on the site, such as drains, should be boxed off to prevent them being concreted over. Remove the box and concrete round the edges when all the foundations are down.

Lay the mesh down on lengths of timber—which are level with your setting-out lines—prior to concreting. Nail another length of timber on top of the first to hold the mesh in place while concreting the various sections. These lengths of timber act as the sectioning formwork necessary for concreting large areas. They should be removed after the adjacent section has been concreted.

Constructing the formwork

The purpose of formwork is to stop the wet concrete from spilling haphazardly over the boundaries of the site. It can also be used to split large sites into smaller areas, which can then be concreted in stages.

Make the formwork from lengths of timber, as wide as the depth of your concrete and about 25mm thick. Place them around the boundaries of the site, butted end to end so that there are no gaps.

To hold the timber in place, drive in more wooden pegs against the outside faces then nail all the pieces together.

If a large area is to be concreted or if the concrete is to run up to an existing wall, more formwork must be used to divide the site into sections of about 2m2. This will enable you to lay and level one section before starting on the next.

Where a path is being laid along a wall, the site should be divided into ‘bays’. Alternate bays can be filled, levelled and left to harden: you can then stand on them to fill and level the intervening bays.

Construct the sectioning formwork in the same way as that for the boundaries of the site. Remember to drive in the support pegs on the opposite side of the sectioning formwork to that being filled.

When all the formwork is in place , use the straight edge and spirit level to check that the top edges of the boards are level with the tops of your marker pegs.

Concreting

The actual job of concreting will be much easier if you have an assistant to help you. If the concrete is being delivered ready-mixed, make sure that the formwork is ready and the ground fully levelled off.

Shovel in the concrete by hand and level it off roughly 15mm above the height of the formwork.

When you have filled a section, the concrete must be compressed, or tamped.

The tool for doing this is called a tamping beam. You can make one from a length of 100mm x 50mm timber with wood offcut handles at either end. The tamping beam should be at least 150mm longer than the width of the section.

With the aid of an assistant, run the beam over the freshly concreted section in a sawing, chopping motion. The weight of the beam should be enough to tamp the concrete down to the height of the formwork, while the ‘chops’ will give the surface a rippled effect.

The distance you leave between ‘chops’ depends on how rippled a surface you want: the normal allowance is half the thickness of the beam.

If the tamping shows up any low spots, fill them immediately and re-tamp. When you are happy with the surface, two passes of the beam should be sufficient to complete the tamping process.

The final surface

This can be left as it is—rippled— roughened, or smoothed. For a rough surface, brush over the concrete with a stiff broom. To smooth the surface skim it over with a piece of timber or a plasterer’s wooden float.

Concreting subsequent sections

If you must complete the concreting in one go, you will need to provide expansion joints so that the concrete does not crack as it dries out. To do this, make up your sectioning form-work in the usual way. With the first section poured, place a thin board against the next length of sectioning formwork and check it for level. When you have poured the second section, carefully remove the formwork, but leave the thinner board permanently in place. Repeat this procedure for subsequent sections.

If you are concreting in ‘bays’ fill in alternate sections with the sectioning formwork left in place. After the concrete has hardened , remove the formwork and fill in the intervening sections.

Use a steel edging trowel to flatten down the edges of the concrete for a really neat finish.

Drying

The freshly-laid concrete should be given at least a week to ‘cure’—longer if the weather is especially cold or damp. Heavy loads should be kept off it for seven to ten days.

In warm weather, the concrete must be prevented from drying too quickly. Do this by covering the whole site with dampened sacking or a similar material. Every day, sprinkle the covering with water to keep it moist.

In cold, wet weather, the concrete will be more in danger of frost than of drying too quickly. To guard against this, cover it with sheets of heavy-duty polythene or waterproof building paper.

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