If you’ve uncovered an existing concrete slab surface while renovating or restoring your home or business premises, you might be considering pouring a fresh slab of concrete directly on top of it. After all, removing the existing slab can be a time-consuming, labour-intensive and difficult process, so simply covering it up could be a simpler option.
There are a number of factors that need to be considered if you decide to go down this route, so it’s important to make sure you’ve carefully planned your project before you begin to avoid any complications.
Here’s everything you need to know about laying polished concrete floors over an existing concrete slab.
Your polished concrete floors will only be as strong as the surface beneath it
The key thing to consider when pouring a new concrete slab over an existing slab is that the quality of the new slab will only be as good as the slab underneath it. If the quality of the slab underneath is poor, the quality of your new surface will be poor too.
Your polished concrete floors may not last as long
If poured and polished correctly, polished concrete floors can last for decades (if not hundreds of years). Concrete is an extremely long-lasting and durable material. However, the quality, strength and lifespan of your new slab will be jeopardised if the existing slab is damaged or hasn’t been poured properly. If the foundation it sits on is not structurally sound, the new concrete could sink, form cracks or develop potholes.
Laying a new slab over an existing slab will affect the height and depth of your polished concrete floors
Pouring a new layer of concrete over an existing slab will raise the level of the surface by several inches. This could be a problem if the raised level means you can no longer open doors or floors are no longer properly aligned with steps and other structures. This misalignment could also create trip hazards when the floor is finished.
The new slab will also add considerable weight to the surface, so it’s important to consider this at the planning stage.
There may be complications if the new concrete floors bond to the existing slab
When laying new concrete over an existing slab, it’s important to separate the two layers. This is because if the new layer is poured directly onto the existing slab, the two slabs will bond together. If the slabs bond together, any cracks or damage in the old surface will spread to the new surface.
Concrete slabs will also shrink and expand when the temperature fluctuates. If one slab starts to expand, it may cause damage to the other slab. By ensuring your slabs aren’t bonded, each slab is given the flexibility to shrink and expand independently.
To achieve this, a layer of sand or plastic should be placed between the two slabs. It might also be necessary to install steel reinforcement bars to help control shrinkage and reduce the risk of cracks forming.
Your polished concrete floors may require more maintenance
One of the benefits of polished concrete is that it’s an extremely low-maintenance material. It simply requires regular cleaning to keep it in top condition and, if properly installed, should last for years to come.
However, a new slab poured over an existing slab is more vulnerable to cracks and other damage, such as water damage or frost heave (when freezing temperatures cause water under the surface to freeze, expand and push the concrete up - something to think about if you live in a colder climate).
If cracks do appear, it’s critical to patch them as soon as possible to stop them from spreading. These patches should be sealed with a penetrating sealer to prevent any water damage. If long, deep cracks begin appearing, it may be a sign of a structural problem with the surface underneath your slab.
Complications can occur if your existing slab is not properly prepared
One thing to consider is that concrete slabs, when poured, are never completely flat. Installing anything over a concrete slab when it’s not flat and clean is a risk.
As having a quality base is critical when installing another surface on top, it’s a good idea to prepare the underlying concrete slab as a first step.
Policrete can prepare the concrete slab by conducting a light surface grind to level the surface and remove any dirt, dust and debris that might affect the quality of the surface applied above it.
This is also a good move if the existing slab has been damaged by water or rain or if it has been poorly poured.
An alternative solution: polishing an existing slab to achieve beautiful polished concrete floors
While pouring concrete over an existing slab is possible, there are a number of challenges if you choose to go down that route.
An alternative solution is to polish the existing slab. You’d be happy to know that any concrete surface can be ground back and polished up. This may be a less risky option to pouring a new slab over an existing one.
The look of the finished floors will still depend on the condition of the slab, but it’s possible to achieve a natural concrete look. Alternatively, you can apply a coat of coloured concrete or aggregate concrete over the existing slab for a different look.
If pouring a new slab of concrete next to an old slab, both surfaces can be dyed to achieve a similar look; however, these slabs will absorb the dye at a different rate so the colour may not be an exact match.
Policrete uses specialist equipment and methods to remove unwanted surfaces, such as tiles, carpets and adhesives, to uncover the concrete slab below. If the slab is suitable, this can then be polished to achieve a stunning floor.
How to lay polished concrete floors on an existing slab
While pouring polished concrete over an existing slab is possible, there are a number of challenges involved.
That’s why it’s so important to work with an experienced polished concrete installer. You can discuss the specific requirements of your project at the planning stage to ensure you don’t run into any problems during installation.
Whether you decide to rip up your old existing slab, lay a new slab over the top, or grind and polish what’s there, it’s important to work closely with an expert concrete installer to ensure you’re happy with the quality and look of your finished floors.
Get in touch with one of Policrete’s concrete specialists for expert advice and recommendations on your next project. We can help you achieve high-quality, long-lasting floors with risk-free installation.
