
How to use concrete
Concrete is strong, versatile and inexpensive. It’s also easy to mix and use. This step-by-step guide is all you need to plan and complete simple concreting projects around your home.
Concrete is strong, versatile and inexpensive. It’s also easy to mix and use. This step-by-step guide is all you need to plan and complete simple concreting projects around your home.
Concrete is strong, versatile and inexpensive. It’s also easy to mix and use. This step by step guide is all you need to plan and complete simple concreting projects around your home.
The more preparation you put into your project, the better you’ll feel about the result. Small jobs don’t need a lot of planning but bigger projects, like driveways and patios, require a bit more thought. It’s a good idea to make a sketch plan of your project before you start.
For major paths, be generous about width - aim for 1.25 metres or more. 0.5 metres is wide enough for secondary paths. Your path should be at least 75mm thick. If the path is to be against the house, slope it away from your home so that water can drain away. A fall of 1 in 25 is an effective slope.
A large area of concrete can be laid in the same manner as a path, simply by laying strips about 2 metres wide. It's best to lay strips alternately, removing forms and filling in the remaining sections when the first strips are hard.
The minimum width for a drive is 2.4 metres, or two strips 750mm wide with 900mm space between. To withstand the weight of vehicles, a driveway must have a well compacted sub-base. You should plan to make the concrete at least 100mm thick. If you’re laying the drive on clay or other poor soil, increase the thickness to 150mm.
While reinforcement won’t stop a path or driveway from cracking, it will prevent the breaking up of concrete once a crack has formed. Reinforcing is recommended if the path or drive is to be used by heavy vehicles. It’s also a good idea if the foundation is likely to settle.
To estimate the amount of concrete you need, simply measure the area to be concreted and multiply by the thickness you want to achieve. For example, if you’re laying a path that’s 10 metres long, 1.2 metres wide and 100mm (0.1metres) thick, your calculation will be 10 x 1.2 x 0.1. The result is the number of cubic metres of set concrete required, i.e. 1.2 m³.
When you know how much concrete you want to make, establish the quantities of cement and aggregate you’ll need. The you'll need to add 10% for wastage. Sand, aggregates and builder’s mix can be stored indefinitely in separate covered piles on a hard surface, but don’t try to store cement for more than a week as it may harden in the bag.
REMEMBER: outside drains only drain RAIN. Remember to wash your tools on the grass (away from stormwater drains). Use sandbags to divert concrete wash water away from stormwater drains to an unsealed area.
Begin by removing all vegetation and topsoil and levelling the ground surface. On most soils concrete can be laid directly on well-compacted ground. For drives and hard stands or when working on clay or peaty soils, you will need a 100mm sub-base of free-draining hardfill. Clear the site at least 150mm beyond the edges of the area. Dig out the required depth, allowing a slight slope so that water will run off easily, and compact the soil thoroughly with a mechanical compactor, heavy garden roller or rammer.
The formwork needs to be rigid in order to withstand the pressure of the wet concrete. Use old fence or floor boards to provide a form to hold the wet concrete. Nail them to pegs driven into the ground at 0.6 to 0.9 metre intervals. Level the forms with a spirit level.
For curves, make a series of sawcuts in the side of the forms on the inside of the curve; you’ll then be able to bend the form without breaking it. Plywood, hardboard and fibreboard can also be used to form curves. It’s a good idea to brush the inside of your side forms with grease or oil before you pour the concrete to ensure the formwork is easy to remove later.
Large areas of concrete should have a ‘fall’ to one side, so that water will run off easily. Aim for a fall or slope of about 1 in 40 (25mm drop per metre run). To get an accurate slope set levels on the high side first. To set the lower level set the low side pegs to match the level of the high side pegs and measure down the required amount.
Over short distances levels can be set using a spirit level on a straight edge. Over greater distances a water level will be more accurate than a series of measurements made with a spirit level. If you choose to use a spirit level over long distances ‘end for end’ it after each transfer so that if it is not reading exactly level, the error will be corrected at each alternate mark.
A water level is simply a long polythene tube filled with water. If there are no air bubbles in the tube the water will always be at the same level at both ends. Put food colouring or dye in the water to make it more visible and stoppers at each end when not in use. Remove these stoppers when using the water level.
Concrete is safer to work with than most other common materials. There are two main safety points to remember:
Machine mixing is better for larger quantities:
It’s easy! Hand mixing is appropriate for small volumes.
Placing & Finishing
The Art Of Jointing
Concrete shrinks as it dries, causing small cracks. By putting joints in your concrete path or slab, you’ll avoid unsightly irregular cracks. The maximum distance between joints should be no more than twice the path width.
For unreinforced concrete slabs, joints should be no more than 3 metres apart. Here are two ways to create joints:
Curing
Proper curing of concrete is essential to ensure that it gains strength and durability. As soon as your concrete is hard enough not to be marked, curing should begin. For seven days the concrete must be kept moist, either by spraying water onto the surface regularly or by covering the new concrete with polythene or damp sacks. For driveways, wait at least 21 days before you allow heavy traffic to drive on it.
Finishing
By using a steel trowel to smooth your concrete you will achieve a very even surface. Be aware that this type of finish can be very slippery when wet and isn’t really suited to paths, patios and driveways.
Pulling a broom or brush over the concrete will give you an attractive non-slip surface. Start “brooming” after the bleed water has evaporated and work quickly for good results. For driveways, brushing across the width will assist drainage.
The back of your shovel can be used to create a non-skid pattern on your new concrete. Wait until the bleed water has gone and experiment on a small area first.
While the concrete is still soft you can sprinkle the surface with coloured aggregate. The aggregate should be pressed in with a wooden float. Once the concrete has hardened enough for the aggregate to stay in place, use the hose and broom technique to expose the aggregate.
A sponge can be used to create an attractive finish which is easy to achieve. Simply press the sponge lightly into the concrete after the bleed water has evaporated. Other finishes include stamp patterning, where you can achieve the look of cobblestones or slate. Using a jointer, imprinting by hand, can create a crazy-paving look. Use your imagination and decorate with shells, tiles or glass chips.
Cleaning Up
It is important to ensure that your concreting project does not damage the environment. Ensure that you have located all storm water drains around the work area and that these are suitably protected to ensure that concrete wash water and slurry does not go down these drains.
Failure to protect the storm water drains from contamination may result in action from the local authority and may result in a substantial fine.
Ensure that tools are thoroughly washed to remove all concrete before it sets. Do this on grass or unsealed ground. Remember to protect storm water drains from contamination.