Laying concrete drives and paths

Preparation for laying drives and paths starts with clearing all the soil and garden loam from the site down to firm ground. The depth is perhaps not so important for paths as they have little weight to carry, but drives need to be firmly based because the weight of the car passes over a narrow strip at each side. If the drive is not firmly based this edge load will cause the concrete to break away from the unloaded central area.

A drive needs to be a minimum of 2.1 m wide and the concrete needs to be 75 mm to 100 mm thick. It is laid on a base of well compacted hardcore made of broken bricks and stones. This layer should not be less than 75 mm thick and it is covered with a layer of sand well rolled to compact it.

Wooden formwork is set up along the sides of the path and held in place by 50 mm square pegs driven into the ground on the outside of the boards. The formwork should not be less than 25 mm thick and the pegs should not be more than 760 mm apart. The formwork is set to the appropriate falls so that the water will run away quickly. A drive can be laid with a slight camber to run the water to both sides or it can be given a slight slope to one side. A fall of 12 mm to 25 mm is sufficient to get the water away.

Large areas of concrete need a proper arrangement of drains and gullies to get the water away without causing ponding in the garden at the sides, but water on drives can usually be allowed to run off the sides and soak into the ground.

Drainage work has to be carried out before any of the drive is laid and where gullies are provided they must be positioned before any concreting starts. A central gully is positioned about 25 mm below the level of the formwork so that the concrete can be given a fall towards it. Drainpipes with a diameter of 100 mm are given a fall of 1 : 40 and their joints are packed with hemp and then filled with cement mortar neatly bevelled off around the collar. Water which is being taken to a soakaway can be run through unglazed earthenware pipes that are not joined but simply laid end to end. The soakaway must be at least 3 m from any building.

The concrete is laid in sections of not more than 3 m long to minimise the risk of cracking due to thermal movement. A temporary board is set up across the drive at this point to provide an edge to finish the concrete against. The board is removed when the concrete has set and is replaced by a strip of thick bituminous felt when the next section of concrete is laid. This acts as a soft cushion which will take up the expansion of the concrete in hot weather.

When ready-mixed concrete is to be used it is essential that all the joint boards are positioned and that there is clear access for wheelbarrows. It is not often that the delivery truck can get any closer than one end of the drive and as the driver will want to unload and return to base as soon as possible, there will be no time for measuring and cutting timber.

In this case the section boards can be quite thin as concrete will be placed on both sides of them at once and they can be left in place instead of putting in strips of felt.

For drives the concrete mix is 1 : 2 ½: 4 and is made just wet enough to be placed and tamped without being too sloppy. The drive is filled to about 19 mm above the level of the formwork and the concrete is tamped down using a board that will reach from one side of the drive to the other. This board is used with a chopping action and needs two persons to operate it. When the concrete is almost level with the formwork they change to a sawing action dragging the board forward cutting the concrete off level with the top of the formwork and drawing the excess material in front of the board. Where a cambered surface is required, the underside of the tamping board must be hollowed to the required camber.

The surface of the drive can be left with the rough ridges across it produced by this method of laying. They will give extra grip if the drive is laid on a steep slope, but domestic drives are usually trowelled smooth after the initial set has started to take place. Trowelling with a steel float will give a fine surface, but it could be too smooth for safety in the winter when it would be frequently wet and often frozen. A wooden float gives a better surface as the finish is slightly roughened, though otherwise flat and even. A fine ribbed finish can be provided by brushing the concrete straight across as it sets. For an exposed aggregate surface the concrete is swept with a soft brush about an hour after it has been placed. The concrete is then allowed to harden until the stones cannot be dislodged. The surface is then brushed with a stiff brush and sprayed with water to clear the stones and leave them standing proud.

Imitation paving slabs can be marked on the surface by rubbing the rounded edge of a piece of 6 mm batten on the concrete to form shallow groves in the required pattern. Crazy paving patterns can be made in the same way.

If real paving slabs are to be used for drives they should be laid on a concrete base of the type described, but they can be laid on firmly compacted sand. The concrete base need be only 75 mm thick and the slabs are laid on a bed of mortar made of one part cement and three or four parts sand. A full bed of mortar should be used for each slab, the five pats of mortar generally used for paths would not be sufficient to support the car without the slabs cracking.

Small concrete sets, rather like bricks, can be laid on a base of hard-packed sand using no mortar. Some of these specially made sets are interlocking so that they will hold together under the movement of the wheels and will not spread at the corners. Edge restraints such as concrete curbs should be provided. A similar method is used for laying paving slabs for paths. When the garden soil has been removed, a hardcore base is laid and this is covered with sand, well-compacted to take the weight of whatever traffic is likely to pass over it. The paving slabs are levelled on to this base by laying a small trowel-full of sand at each corner position and one in the middle. When the slab is placed on these heaps of sand it can be knocked down to the required level using a heavy wooden mallet or the end of a length of timber.

A string-line down each side of the path will serve as a guide both for direction and for level. The slabs should be levelled individually and a straightedge is used to check three or four at a time to ensure that they are all level or have the desired fall.

In order to prevent the sand under the slabs being washed away in wet weather, an edging must be provided. This can be preservative-treated boarding which finishes flush with the surface of the path, or it can be purpose-made concrete edging bedded in weak mortar. The edging can be finished flush with the top of the path or it can have rounded edges and can stand up above the surface. If this method is used, then some of the joints must be left open to allow water to escape.

Mortar for bedding paving slabs is made of 1 part lime to 4 parts sand. In sheltered places where there will not be much water on the surface of the path, the sand can be increased to 6 parts to make an even softer mix. Where a lot of water is to be expected on the surface of the paving and where cars are expected to use the area, some cement is added to the mix, but it should not be stronger than 1 part cement : 3 parts lime : 12 parts sand. It can be used as a full bed for the paving slabs or as dots.

Whatever method is used to bed the slabs they are best laid with a joint of 6 mm to 12 mm. These joints are filled when the area is complete. The filling is a dry mix of the same type as the one used for laying the slabs. In sheltered areas, sand can be brushed into the joints and will bed down surprisingly hard. Filling the joints with a wet mix of mortar is not recommended because it is nearly impossible to keep the surface of the paving clean and free from cement stains which will not come off.

The less formal crazy paving is laid in the same way as the whole paving slabs. Pieces with straigh edges are picked out to fit neatly along the edging of the path. If the pieces of paving are laid in a cement : lime : sand mortar mix a formal straight edging need not be provided and the ragged edge of the path can be covered by the plants. The pieces of paving should appear to have been scattered haphazardly, with joints of various widths. If the pieces fit too carefully the path will look like a jigsaw puzzle. Because the joints of this type of paving are much wider than those of formal paving it is best to fill them with a mortar mix instead of just sand. These very wide joints are filled with a wet mix taking care not to get the mortar on the paving.

Paths laid in mortar or which have mortar joints should not be walked on for a day or two to give the mortar time to set. Paths and patios can also be laid in bricks, of special quality or frost resistant types.

Cobble stones make decorative features and, as they are uncomfortable to walk on, they can be used to discourage people from taking short cuts across lawns by placing a wide strip of cobbles alongside paths. The base for these stones is laid in the same way as the base for the path, but the surface of the concrete is kept down about 38 mm below the surface of the path. This allows for the mortar mix of 1 part cement : 3 parts sand in which the stones are set. The cobbles are about 75 mm or more in size and are each placed by hand in the wet mortar, bedding them to about half their depth.

Only small quantities of mortar should be made up at each time, the actual amount depending on the rate at which the stones can be bedded. The work should start as soon after the base concrete has set as possible as this will improve the bond between the concrete and the bedding mortar. Keep the tops of the stones as clean as possible during laying operations as they will be difficult to clean after the mortar has set.

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