Concrete Foundation Preparation Guide
What are concrete foundations?
When it comes to construction, the word Foundation is used to describe the lower part of a structure. Our concrete foundation preparation guide will help you understand what mix to use and what measurements to take to ensure your foundations stay standing and strong. For this we recommend using a C25 strength mix, find out more about concrete strengths here.
Concrete foundations are specifically designed to evenly distribute the weight of a building and provide it with a firm base to rest on. To ensure that your foundations are strong, it’s important that you use the right mix strength of concrete. Depending on the soil type that your structure is going on, this may vary. If you’re in doubt as to what strength to use, we always advise to speak to an expert i.e Structural Engineer or Building Inspector. So now let’s begin with the steps.
Prepare the ground for your foundations
The first and most important step of any concreting project is the ground preparation. A building is only as strong as the foundations that it rests on, and with the right ground preparation, your concrete foundations will stay strong. Once you have all the relevant building permissions in place, you can begin preparing the ground.
You will start off by using wooden pegs and string to start marking out the area in which your foundations will be dug and the concrete will be poured. It’s important to allow an additional 75mm of space in preparation for the formwork to be added. The formwork will then be used to hold the wet concrete in the correct place while it’s drying, or ‘curing’. After this is in place, you can begin to dig your foundations to the depth required.
Side Note: If you are using Strip Footings, the ground will as a general rule be undisturbed, solid ground. However, for individual footings and floor slabs, it is important that you allow enough additional depth for the sub base (100mm), and damp-proof membrane (DPM). If access is possible, this is a step where it’s easier to let the machines do the work for you by using a Mechanical Excavator. You must ensure that all plant material, stones and debris is removed prior to compacting and leveling the ground soil into a level base.
The next step is to add your sub base on top of the level base and then compact once more. For Domestic Concrete Footings, a base aggregate of 10mm in thickness is suffice. After the area has been compacted you will need to add your DPM. The correct way to do this is to ensure that the edges of the DPM are turned upwards, creating a tray effect. You must make sure that any joints in the DPM are overlapped and taped up properly.
This process is designed to protect the underside of the covered area from any form of rising damp or chemicals that water from the ground will potential mix in with your concrete. It also helps to prevent your concrete from drying out too quickly which ultimately improves the cured strength of the concrete and will lessen the risk of cracks forming.
Once the DPM is in place, you can begin to put your formwork together. Formwork is made up of thick timber planks (25mm), and these planks will keep your foundation concrete in place, allowing it to set properly without movement. The formwork will need to be the same depth as what you expect your concrete slab to be.
Important Note: None of the above can be left to guess work so it’s essential that either a laser marker or spirit level is used to check that the formwork is even. Without this you will end up compromising the integrity of the formwork and your foundations may crack.
Order your concrete
This is perhaps the easiest stage but still one of the most important. Finding the right concrete supplier in your area that provides concrete for foundations might not be as easy as it sounds, but you’re already reading the 365 Concrete guide so why not give us a call? We have over 40 years of concreting experience, covering Berkshire, Middlesex, Surrey and Greater London.
We use a fleet of Volumetric concrete mixers that allow us to mix your concrete onsite. By using these advanced concrete mixers, we are able to fully mix your concrete onsite, ensuring that the concrete is fresh and mixed to your exact requires. You can find out more about our volumetric mixers here.
Another thing to consider is whether or not you need a concrete pump for your concrete foundations project. We often find that for these kind of jobs, the surrounding area is hazardous to walk on or access is limited. Using a concrete pump is the perfect way to counter this and can also save you time and labour costs. 365 Concrete can also provide you with concrete pump hire, take a look here.
Finally, to ensure that you have the right measurements, we always advise to use a concrete calculator. If you’re having any difficulty getting your measurements correct, our team is always available to help.
Secure enough parking for delivery
To make sure things go smoothly, it’s always best to ensure that there is a parking area cleared for the concrete truck to set up in. Typically a concrete delivery truck will take up around 2 normal sized parking bays and need a little extra wiggle room for offloading, if you are also using a concrete pump then that amount of space will need to be doubled. Our volumetric mixers also carry extendable chutes which means that if we can get close enough, we may be able to directly pour the concrete for you.
Pouring your concrete
The time it takes concrete to begin to set is usually around the 2 hour mark after the concrete has began to mix. Having 365 Concrete deliver your concrete already puts you at an advantage as our concrete is only mixed on site, giving you the maximum time available. The actual setting time is also effected by a couple of different factors also: Type of Concrete and Ambient Temperature. In colder temperatures it can take twice as long for the concrete to begin to set whereas in a hot temperature the setting time can be reduce by up to 30 mins, so always pay attention to the weather on the day.
Because of these additional factors, it’s important that your concrete is mixed, poured and leveled quickly and efficiently. You can be a little bit more prepared for this by having the required tools needed ready in advance. We’d advice having the following at hand.
The concrete can be easily pulled around the trench when wet by using either your shovel or rake (easier with a rake), this also helps to remove trapped air. Another handy tool to use for removing trapped air in concrete is a Vibrating Concrete Poker. Once you’re happy with how your concrete has been distributed, it can be further flattened by using the tamping board and then finished off with a concrete hand float.
Concrete can be poured directly to the job site using either extended chutes or a concrete pump. If you are using wheelbarrows though, it’s important to make sure you have enough man power on site to get the job done in a timely fashion and that the pathways to the site are cleared and have any protective material laid in advance (planks, sheets etc). Bare in mind that 1m3 of concrete fills up roughly 30 – 40 wheelbarrows, that’s a lot of pushing!
Will the weather affect my concrete?
When it comes to adverse weather, we would generally advise not to lay concrete in those sort of conditions but being from the UK, we do understand that weather is not always on our side. With that said, when it comes to laying concrete in adverse weather, there are a few things to consider:
Outside temperature – To avoid ice forming in the concrete mix and weaken your concrete, you minimum temperature concrete can be mixed at is 3oC.
Ice or frost on the floor – This is a big no-no. You should never lay concrete on top of an ice or frost covered surface. If the forecasts are suggesting that frost or ice is due, you should protect your sub base using some sort of insulator such as blankets or a heater.
Heavy rain – If the ground area where your concrete is going to be poured has good drainage, as well as no water pools, you will be able to pour concrete in the rain. You should cover your concrete with tarpaulin or plastic sheeting once poured to avoid any rain damage on the surface of the concrete.
How to get a smooth concrete finish?
The concrete can be easily pulled around the trench when wet by using either your shovel or rake (easier with a rake), this also helps to remove trapped air. Another handy tool to use for removing trapped air in concrete is a Vibrating Concrete Poker. Once you’re happy with how your concrete has been distributed, it can be further flattened by using the tamping board. Repeatedly tamping the concrete will begin to create a reasonably smooth surface but for a flat finish, a steel float trowel should be used as the concrete begins to harden.
Concrete foundation curing & aftercare
Ensuring your concrete does not dry out too quickly is an important part of the process. If allowed to dry out too quickly, your concrete will be left with a weak and dusty surface that is prone to cracking. Using plastic sheeting is one of the easier ways to make sure it doesn’t dry out too quickly. You must make sure that the edges of the plastic sheeting are sealed though to prevent any wind from getting underneath and creating wind tunneling affects.
Hotter temperatures of over 20°C with a stiff breeze and colder temperatures of 4°C and below can have adverse effects on your concrete so it’s important to keep your concrete insulated for each situation. Keeping the concrete free from wind in hot temperatures and using a frost blanket or something like it in the colder temperatures.
You can begin work on your newly laid foundations after 48 hours but we always advise to speak with your builder or concrete supplier just to make sure. Each job is different so there may be additional factors. Formwork should be left in place for a minimum of 72 hours to prevent possible damage to the edges of the curing concrete.
A building inspector may insist on checking your new concrete foundations before any further work is allowed to continue so make sure you contact the right people before you carry on building. Your concrete will need a full 28 days of curing to be at full strength.
Still have questions that aren’t covered in our guide? Then get in touch with our team today.