Laying concrete outdoors

The preparation required largely depends on the existing surface and what the concrete will be used for, but as an example we will look at the preparation needed for laying a concrete base for a shed. Try to avoid laying concrete in very cold weather to prevent frost damage.

• Measure and mark out the area to be concreted with a string line tied to wooded pegs that are hammered into the ground. Use a builders' square to ensure the corners have 90° angles. Most sheds are supplied with a base plan, detailing the size of base needed for the weight and size of the shed.
• Cut back any bushes and tree branches from the area to give you at least 300mm  clearance around the concrete base.
• Using the blade of a garden spade cut into the turf following the string lines. Once the area has been marked out in this way, remove the strings.
• Cut away the turf and excavate to the depth specified by the shed manufacturer's instructions. Once again this will depend on the size and weight of the shed, but generally the depth should be at least 100mm and deeper for soft ground.
• Level the bottom of the excavated area and compact the soil using a small garden roller or a rammer.

The formwork will support the concrete before it hardens and is made up of timber planks positioned on their edge that go around the perimeter of the base. The top edge of the formwork is also the surface from which you level the concrete.

• To assist you in erecting the formwork, reposition the string lines ensuring the angle at each corner is 90°.
• Cut lengths of 25mm thick softwood planks slightly longer than the dimensions of your concrete base.

• If you are intending to leave the timber frame in place after the concrete has dried, treat it with wood preservative.
• Position the first timber plank into the excavated area along one of its sides.
• To keep the timber planks firmly in place, use a club hammer to drive wooden stakes into the soil at the back of the timber at approximately 1m intervals.
• The timber plank should be nailed securely to the wooden stakes, ensuring the tops of the stakes are level with the top edge of the plank. Remember the top of the formwork corresponds to the surface of the concrete base.
• Now fix the second timber plank into position opposite the first. However, it is important to remember a concrete base should have a slight fall or drop to allow water to drain off the surface. As an example a concrete base that is 2 metres wide should have a fall of 25mm across that distance.
• To achieve the correct amount of fall, fix a shim to a straightedge and rest it across the excavated area from the fixed plank to the top of a stake on the other side. Hammer the stake further into the ground until the spirit level reads flush. Repeat for all the other stakes.
• Nail the second board to the stakes.
• Fix the end boards to complete your form checking that the corners are completely square. By using longer boards than necessary the ends will overlap at the corners making it easier to dismantle the form when the concrete has set.

If laying a circular concrete base or a curving concrete path the formwork should be constructed as follows:

• Use a garden hosepipe or a long length of rope to mark the outline of the curve you want to follow.
• An optical illusion can make the curved part of a path seem narrower than the straight stretches, so you will need to compensate for this by making the curved sections a little wider.
• When satisfied with the curve mark it out using pegs and string, remembering that the pegs will need to be closer together than when marking out a straight line.
• To reproduce the curve with timber it needs to be cut to about half its depth on the side that will form the inside of the curve. These cuts should be 125mm  apart or closer for a very sharp curve.
• Nail the timber edging to the stakes in the same as you would when constructing straight edging.

• A sub-base is needed on to which the concrete will be poured. A mix of gravel and sand - sometimes called hogging - is ideal, but a layer of hardcore (crushed brick) or scalpings can also be used as it is easy to compact.
• The depth of the sub-base depends on the size of the area you are concreting and what the concrete base is going to be used for, particularly in terms of the weight that will be resting upon it. Generally the minimum depth for a sub-base is 100mm.
• The sub-base material should be fairly even and compacted firm using a rammer.

• Mix the concrete as close to your formwork as possible.
• Wear protective gloves when working with concrete, as cement burns can occur when concrete comes in contact with the skin.
• The concrete mix for this type of base is generally:
  1 x part cement
  2 x parts sand
  3 x parts aggregate (particles of stone)
  Mixed with water into a firm but manageable consistency.

If you are laying concrete in warm weather, it may be a good idea to divide the formwork into sections to prevent the concrete drying too quickly. To do this, follow the same method as for concreting large areas below. 

• Dampen the surface of the sub-base with water.
• Pour the concrete from a wheelbarrow onto the sub-base, starting at one end of the form. Ensure the concrete fills the corners of the form.
• Continue mixing and pouring the concrete until the form is filled with the surface of the concrete standing about 12mm above the top of the form.
• The concrete now needs to be compacted and smoothed with a tamping beam with a straight edge. A timber plank longer than the width of your form is ideal for this purpose. This is an operation requiring two people.
• Start from one end of the form and tamp the concrete by raising the tamping beam and bringing its edge down on to the surface of the concrete. Gradually make your way across the surface of the concrete. This operation should then be repeated.
• To remove the excess concrete, move the tamping beam across the surface of the concrete in a backwards and forwards motion, progressing across the surface at the same time. 
• Fill in any low spots, then tamp and level the surface again.
• Create a non-slip textured surface by lifting and dropping the tamper to create waves. Alternatively, you could press a pipe into the surface or scatter a layer of stones.
• To create a neat finish along the edges of the concrete, run an arissing tool along the gap between the concrete edge and the formwork.
• To prevent the concrete drying out too quickly, cover the concrete with polythene sheeting weighted down at the edges of damp sacking.
• Remember to thoroughly clean your wheelbarrow and cement mixer if you've been using one.
• Leave the concrete for a minimum of three days to allow it to cure before removing the covering, and an additional day before removing the formwork.
• To remove the excess concrete, move the tamping beam across the surface of the concrete in a backwards and forwards motion, progressing across the surface at the same time. 
• Fill in any low spots, then tamp and level the surface again.
• Create a non-slip textured surface by lifting and dropping the tamper to create waves. Alternatively, you could press a pipe into the surface or scatter a layer of stones.
• To create a neat finish along the edges of the concrete, run an arissing tool along the gap between the concrete edge and the formwork.
• To prevent the concrete drying out too quickly, cover the concrete with polythene sheeting weighted down at the edges of damp sacking.
• Remember to thoroughly clean your wheelbarrow and cement mixer if you've been using one.
• Leave the concrete for a minimum of three days to allow it to cure before removing the covering, and an additional day before removing the formwork.

A control joint is a gap between sections of concrete fitted with strips of flexible board running across the width of the base.

The maximum area of concrete you can lay in one go before a control joint is required will vary depending on the thickness of the concrete and the width of the area. The narrower the concrete base (e.g. for paths) the closer the control joints should be. 

As a rough guide for calculating how far apart control joints should be, multiply the thickness of the concrete by 30:

• For concrete 150mm thick, control joints are needed every 4.5m.
• For concrete 100mm thick, control joints are needed every 3m.

• For concrete 75mm thick, control joints are needed every 2.25m.

If the area is narrow, you may need to reduce these intervals. For example if the area is less than 2m wide, reduce the distance between control joints by 1m. If the area is less than 1m wide, you will need a control joint every 1.8m.

• Cut a length of hardwood filler strip to fit between the sides of the formwork. The filler strip should be the same height as the formwork.
• Create the first bay by placing a hardboard filler strip inside the formwork at right angles to the formwork sides.
• If the formwork is curved, bend the hardboard filler strip to make it at right angles with the formwork sides.
• If you want to conceal the hardboard filler strip, cut it slightly lower than the height of the formwork and fix another temporary filler strip on top. When the concrete has set, you can then remove the temporary filler strip and insert a suitable sealant into the gap.
• Cut a timber batten to the height of the filler strip.
• Place the batten immediately behind the filler strip to support it.
• Secure the batten with stakes driven into the ground with a mallet.
• Spread the concrete inside the bay (as above) up to the height of the filler strip.
• Spread 300mm of concrete behind the batten supporting the filler strip i.e. into the next bay.
• Remove the timber batten and stakes, leaving just the filler strip, and push the concrete against it.
• Continue to concrete the second bay.
• Follow this procedure until all the bays have been concreted.
• When you have finished concreting use an arissing tool on both sides of each of the hardboard filler strips.