How to prepare for a concrete pour
There are a few things you need to do in preparation for the concrete arriving on site. What follows is a step by step guide with a few tips to ensure your concrete pour is hassle free.
Step one - prepare the ground
It doesn’t matter how big or small your concrete pour is, the first step is to prepare the ground. This involves excavating to the required depth: for strip footings this is generally to undisturbed ground, while for pad foundations and floor slabs you will need to allow enough depth for your sub base and damp-proof membrane (dpm) as well as the concrete. Make sure all debris, stones and plant material are removed before levelling and compacting the soil to create a level base.
Step two - add your sub baseand install a dpm
Now add your sub base – MOT type 1 hardcore is suitable for most domestic builds – and compact. A dpm should then be installed to protect the underside of the finished slab from rising damp and against any chemicals ground water might bring into contact with the concrete. A dpm also prevents the fresh concrete from drying out too quickly (due to water being drawn into the sub base), which will help improve the final strength and reduce the chance of the concrete cracking.
Step three - build the formwork
The next step is to build your formwork, which is usually made from well supported 25mm thick timber planks, to keep the concrete in place until it has had a chance to set. The formwork needs to be as deep as the concrete slab will be and this will depend on use. Concrete paths, patios and concrete pad foundations for shed bases, for example, need to be 75mm-100mm thick, but domestic drives and garage bases need to be at least 100mm thick. If you are unsure about which type/strength of concrete, or how much you will need, take a look at our or try using our . You can also call us on telephone: .
Use a spirit level to check the formwork is even and then you are ready to pour your concrete. Please remember that when you are working with wet concrete, 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 .
Step four - unload, level and cover the concrete
At this stage time is not on your side as concrete typically starts to go off within two hours of being mixed. The actual time will depend on the type of concrete and the ambient temperature: in cold weather it can take twice as long for the concrete to set; in hot weather the setting time could be reduced to 30 mins. The concrete will need to be unloaded and levelled as quickly as possible, so make sure the site has been prepared in readiness and you have all the tools you will require to hand:
- Rake and shovel to move the concrete around and roughly level it
- Waterproof membrane
- Steel mesh if required
- Spirit level
- Straight edge to strike the concrete surface
- A straight-edged piece of timber to tamp down the concrete and eliminate any air pockets.
It’s important to make sure there’s enough room on site for the delivery truck: they are around 9.5 metres long, three metres wide and four metres high with a turning circle of about 17.5 metres and weigh up to 32 tonnes. If you are using wheelbarrows, make sure you have the manpower available, to speed up the job as 1m3 of concrete will fill around 40 wheelbarrows! Make sure that paths have been cleared and planks laid to deal with any slopes or uneven ground. If your site can’t accommodate a truck, you may need to hire a pump to pump the concrete from a suitable access point. Our sales staff will be able to recommend where you can hire one locally.
It is not advisable to lay concrete in any adverse weather conditions but, if it is unavoidable, there are a few things to consider. The minimum temperature to pour concrete (air temperature) is 3°C to make sure that ice doesn’t form within the mix, which could compromise the strength of the concrete. You should also never pour on top of ice or frost so, if this is looking likely, protect the sub base with insulating blankets or thaw it with heaters before the pour. Once poured, if temperatures are likely to dip below 4°C, a frost blanket or similar should be used to insulate the slab and protect the surface from frost. For more information on Cold Weather concrete please .
You can pour concrete in the rain as long as the ground drains well and there aren’t any rainwater pools. Once it is poured, cover using tarpaulin or sheeting while it cures. Heavy rain will damage the surface of the concrete so, if appearance is important, make sure there is cover to keep the rain away until you are ready to apply the final finish.
Concrete also mustn’t dry out too quickly as it hardens as this can cause cracking and a weak/dusty surface. The easiest way to prevent this from happening is to keep the slab damp by covering it with plastic sheeting and this is particularly important in temperatures over 20°C or if there is a stiff breeze that could dry the surface.
Step 5 - Truck chute cleaning
The truck mixer will need to make sure the chute is clean of concrete before it leaves the site to prevent any spillage on the road. They will need to wash off away from any carriageways i.e. into a skip or a large wheelbarrow before leaving site.