How to lay a concrete base: a step-by-step guide
Mastering how to lay a concrete base is essential for creating a durable and reliable foundation for any project. With preparation, patience, and the right products, you can lay the perfect concrete base to ensure a firm and sturdy foundation for every project.
In this guide, we will explore the steps towards preparing and laying a concrete base, including framing, calculating, and curing the concrete, along with sizing and tamping recommendations.
What safety measures do I need to take around concrete?
Safety is paramount when working with wet concrete, cement or mortar as these materials can be caustic. Protective clothing must be worn, and care must be taken to avoid contact with skin or eyes. 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.
Learn more about safety measures when working with wet concrete or mortar in our working with wet concrete or mortar guide.
Step 1: Prepare the ground
Firstly, begin your concrete base preparation by marking 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.
To understand which concrete you need for your project, take a look at our guide to concrete types and, if in doubt, always seek advice from an expert.
Step 2: Dig to the correct depth
Dig the area deep enough to accommodate the 100 mm sub base 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 and patios 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. Remove all plant matter and debris and compact the area before adding the hardcore sub base and compacting again. Be as accurate as possible with your levels as a deviation of as little as 1cm over a 25m2 area will increase the volume of concrete you require by ¼ of a cubic metre (Over half a tonne). 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 strengthen it, preventing it from drying out too quickly.
Step 3: Frame the concrete slab
Now, create the formwork using 25mm-thick timber planks, to support the wet concrete as it hardens. You can use the top of the formwork to determine the height of the finished slab, and this can then act as a guide when striking the surface.
Take your time with this stage, checking and double-checking the formwork's height, width, and depth. Remember to allow for runoff: a fall of 50mm for every three metres.
Step 4: Work out how much concrete you need
After digging and framing your base location, you are ready to lay your concrete slab. You can determine how much concrete you need using our easy-to-use online concrete calculator.
Using the basic shape of the area you are working with – square/rectangle, right-angled triangles, parts of a circle – and the dimensions, the calculator provides you with an estimate of the volume of concrete you will require. If you've got a complicated area to estimate, you can build up the calculation by adding the different shapes together.
Step 5: Pour the concrete
Concrete typically starts to harden within two hours of being mixed, so it will need to be unloaded and levelled as quickly as possible. Prep your site for the delivery truck, ensuring there is enough room and a suitable surface to support its weight, and that you have all the tools you need at hand.
If your site can’t accommodate a truck, you may need to hire a pump. If you are using wheelbarrows, make sure you have sufficient manpower available as 1m3 of concrete will fill around 40 wheelbarrows.
Step 6: Tamp the concrete
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.
Step 7: Cure the concrete
To prevent the concrete from drying out too quickly as it hardens, keep the slab damp by covering it with plastic sheeting. Concrete typically gains sufficient strength after 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 and be able to withstand weight. Note gaining strength and drying out are different processes. Concrete typically takes one day per mm of thickness to dry fully, ie a 100mm thick slab can take three months to fully dry when protected from the weather.
For further information and expert advice, explore our ready-mixed concrete range or speak to our team today.
