How to lay a concrete base
For all you need to know about laying a concrete slab, follow this step-by-step guide:
Preparing the ground
First you need to mark out where you are going to cast your slab. You can use wooden pegs and string to mark out the area where the concrete is going to be poured, allowing an extra 75mm for the formwork. Dig the area deep enough to accommodate the sub base (100mm) and damp-proof membrane (dpm) as well as the thickness of the concrete slab itself. As a rule of thumb, concrete slabs supporting lighter weights such as paths, patios and shed bases will need to be 75-100mm thick, while driveways and garages need to be at least 100mm thick. Slabs intended for commercial use should be at least 175mm thick and reinforced. Make sure you remove all plant matter, rocks and debris and compact the area before adding the hardcore sub base and compacting again. Then lay a dpm – ensuring the edges are turned up to form a tray and any joints are overlapped and taped – to protect the concrete slab and help prevent it drying out too quickly, which can cause it to crack.
How to frame a concrete slab
Now create the formwork using 25mm-thick timber planks to support the wet concrete as it hardens. It is often easier if you use the top of the formwork to determine the height of the finished slab. This can then act as a guide when striking the surface. It is vital that you take your time with this stage, checking and double checking the height, width and depth of the formwork, remembering to allow for run off: a fall of 50mm for every three metres should do it.
How much concrete do I need?
Once this preparation is complete, you are ready to lay your concrete slab. To help you determine how much concrete you need we have developed an easy-to-use online concrete calculator. It asks for the basic shape of the area you are working with – square/rectangle, right-angled triangles, parts of a circle – and dimensions to provide you with an estimate of the volume you require. If you've got a complicated area to estimate, you can build up the calculation by adding the different shapes together.
Pouring concrete
Concrete typically starts to go off within two hours of being mixed, so it will need to be unloaded and levelled as quickly as possible. Make sure the site has been prepared for the delivery truck – ensuring there is enough room and a suitable surface to support its weight – and you have all the tools you need at hand. If your site can’t accommodate a truck, you may need to hire a pump and, if you are using wheelbarrows, make sure you have sufficient manpower available as 1m3 of concrete will fill around 40 wheelbarrows!
Please remember that when you are working with wet concrete, cement or mortar suitable protective clothing must be worn and care must be taken to avoid contact with skin or eyes. More information can be found here. The concrete will be reasonably wet when it’s poured so a shovel or rake can be used to spread it between the forms and roughly level it. A straight-edged piece of timber can then be used to tamp down the concrete, eliminating any air pockets and ensuring the concrete gets into the corners of the forms. Repeated tamping will create a reasonably smooth surface but a steel float trowel can be used as the concrete starts to harden for a finer finish. It is worth noting that smooth concrete can be quite slippery so often a tamped finish is preferable.
Curing concrete
It is important that the concrete doesn’t dry out too quickly as it hardens. The easiest way to achieve this is to keep the slab damp by covering it with plastic sheeting. The drying time of concrete is 48 hours for light foot traffic but, ideally leave it for at least five days. Formwork can be removed after 24 hours, although it is best to leave it for 72 hours to avoid any possible damage to the edges of the new slab. The concrete will take 28 days to reach its full strength.
