Polished Concrete Flooring – What you need to consider
Midland Polished Concrete: the UK's Leading Nationwide Specialists
Polished Concrete Considerations:
There are a multitude of polished concretes, polished screeds, polished micro concretes and imitation type products like resins and vinyl’s on the market, the information following is intended to give some guidance on the options we install and the elements that need consideration to achieve a beautiful floor for the best value for money.
- Depth Available.
- Size of Area.
- Aesthetic look and colour.
- The process used to achieve the floor.
- The cost of a polished concrete floor.
- The compatibility of Underfloor Heating.
We supply and install at 3 different depths:
150mm Structural Concrete this is generally for new builds and has the advantage that the polished concrete floor can replace up to 3 layers of a traditional floor construction, please see section drawing below, in this drawing the concrete base slab sometimes called an oversight, the screed and the floor finish layer has been replaced with a 150mm structural polished concrete floor with underfloor heating suspended within the floor.
There are three frequent concerns that our customers have with using the polished concrete floor as the structural floor:
- The architect is unfamiliar with the construction of the floor.
- The builder is familiar with working off an oversight concrete, frequently installed before walls and roof go on.
- There is a concern that the under floor heating will be less responsive as it is encased in 150mm of concrete.
The first two concerns remain a challenge with some architects and builders being not keen to consider different ways of construction and some preferring to stick with traditional tried and tested methods.
The third concern is correct, the underfloor heating will be less responsive, however underfloor heating systems are designed to hold the temperature of a specific zone at the desired temperature so the only time the system heats up from cold is during the commissioning of the system, after that the heating is generally varying around 3 to 4 degrees if set back timers are being used to allow room temperatures to drop lower whilst rooms are not in use. The bigger challenge with underfloor heating in deeper concretes is cool down times because the concrete is acting as a storage heater.
For example: If the underfloor heating is installed in a 150mm concrete slab in a heavily glazed area and the room stat is set at 21 degrees during the spring and summer there is a risk that the system will call for heat at 3am to bring the room up to Temperature and then at 6am the sun rises and warms the room through the glazing, the mass of heat stored in the floor can cause the room to over heat, therefore we recommend that room temperatures are reduced by 2 or 3 degrees during the warmer months in heavily glazed areas, when installed in deeper polished concrete floors.
Apart from the benefit of the thermal store 150mm polished concrete floors require less contraction joints, normally at around 6m centres depending on the aspect ratio of the room.
Our polished concrete floors can be installed at 75mm, in a traditional flooring design our polished concrete floor would replace what would have traditionally been a 75mm screed floor. Section drawing below.
For projects where there is very little depth available to install we have a 7 to 10mm polished micro concrete.
Due to the different installer skill sets involved with installing polished concrete floors, our cost to mobilise the team is expensive, so the bigger the area we are given to install the lower the rate per m2. Areas under 30m2 are in excess of £8000.
Areas over 100m2 range from £120 to £200 /m2 depending on depth, finish and location.
To get more value for money from your polished concrete flooring consider if there is the possibility to install in more rooms or if the floor can be installed externally as a patio area. External polished concrete needs to be a minimum of 200mm onto hardcore and we recommend that the hardcore is installed falling at 1:40 in one direction to allow us to maintain a minimum depth of 200mm for the polished concrete. The concrete cannot be installed to multiple falls as the polishing machines are not able to polish multiple angles. A section of external polished concrete is below.
There are many different products and processes in the market place to create different aesthetic looks to the polished concrete floor we offer 4 different options:
Refined Polished Concrete;
This is the minimal amount of grinding, usually around 1mm so very little aggregate (stones) are visible within the surface of the polished concrete. With a refined floor we lay and powerfloat the concrete and then return to site around 30 days after the installation to grind and polish the floor, this results in most of the cream or fat of the concrete still being visible in the surface and produces the most amount of texturing within the concrete surface.
Fine Polished Concrete;
When we grind and polish the floor to a fine standard we grind around 3mm off the surface, this results in exposing part of the aggregate (stone) the stone exposure can be sporadic and change across the floor depending on where the stones are sitting with the matrix. These floors tend to be more consistent in backing colour but more interesting due to the patterning of the stone exposure.
Exposed Polished Concrete;
Exposed Polished Concrete is the deepest grind we do and exposes lots of the aggregate (stone) within the floor. Exposed concrete floors are durable, hard wearing and useful for all sorts of applications from warehouses to art galleries and garages to domestic living rooms – especially high traffic areas. These floors create a similar look to a terrazzo floor.
Micro Polished Concrete;
The micro polished concretes we install are produced in a factory and therefore are generally more consistent and predictable in terms of finish look and colour, some customers would argue that they are too consistent. Micro polished concretes have a smaller aggregate size and due to the composition of the product our micro concretes are installed in line with our fine process as due to the depth of the floor and the aggregate content the refined and exposed finishes are not possible to produce.
Aggregate / Stone within the polished concrete floor;
With the exception of the micro concretes which are factory produced our concrete supply will originate from a plant local to the project we are installing. Most concrete plants are either on Limestone, Granite or Gravel as their aggregate base, this can result in only certain types of aggregate being available in certain parts of the country and unless the size of the project is significant concrete plants are reluctant to change aggregate for one project. This does mean that in some areas the choice of aggregate may be limited to just one type of stone.
There are flooring options available in resins, vinyl’s and tiles that attempt to replicate polished concrete however we are unable to comment on these products as we do not install them.
There are two approaches generally available within the market place, a dry shake topping and an integral colour. Both Approaches have pros and cons.
Dry Shake Colours Pros & Cons
Pros: The colour is a powder sprinkled over the top of the concrete and therefore the finished colour of the floor is fully controllable and immediate.
The addition of the dry shake enables more patterning and changes in textures within floor as it is applied with the power floats.
Cons: The original concrete floor has a powder installed on top therefore the process creates a false floor rather than an authentic polished concrete.
dry shake floors are generally not diamond polished as the diamond polishing process could cut through the dry shake and expose the concrete below which may be a different colour.
When using a dry shake it is preferable not to grind the floor and therefore the stain protect or seal coat applied sits on the Surface of the floor and will need to be periodically re-applied.
Using a dry shake on the floor increases the risk of de-lamination where the dry shake has not bonded to the base floor. Should the floor become damaged or stained grinding out the stain is not an option as the machines will grind through the Dry shake to expose the base concrete below.
Integral Colours Pros & Cons
Integral Colours Pros: The Colour runs through the full depth of the concrete, if repairs or grinding work are required at a later date whilst this could will expose some stone the base colour of the floor remains the same.
- The floor looks more authentic
- There is no risk that the diamond polishing will grind away the colour.
- The surface can be ground deeper to expose some or lots of stone.
- Advanced stain protection can be used to penetrate into the pores of the concrete creating a longer lasting stain prevention.
Integral Colours Cons: The Colour runs through the full depth of the concrete, if repairs or grinding work are required at a later date whilst this could will expose some stone the base colour of the floor remains the same.
As the colour is full depth there can be more variations especially in edge work, however these do settle down over time as the moisture in the floor reaches equilibrium.
The Concrete Polishing Process
At Midland Flooring we install the concrete and then powerfloat until the floor has set, we then spray with a curing densifier, depending on the style of the floor required we then return after 10 or 30 days to polish the floor. Polishing the floor involves spraying with densifiers which react with the cement and the previously installing cure to make the surface of the concrete harder and start to block the pours of the concrete, we then work through grades of diamond tooling starting from very course diamond polishing pads through to very fine. The floor is generally shiny and polished without the application of seal coats which are then applied and burnished (melted) into the floor to create a long lasting stain protection coat.
In a similar vein to polishing the floor there are a variety stain protection and seal coats being applied in UK some are good, some are not so good and give limited (if any) stain protection and can wear very quickly. The best stain protection systems are generally not available to purchase in the UK and only available as imported products from Europe, America or Australia as these markets are further advanced with polished concrete technology. The best stain protection systems tend to be the types that soak into the concrete and seal or block the pours of the concrete floor this resulting in a floor with a much higher wear resistance. Protective coats that sit on the surface of the floor tend to wear down at a quicker rate under traffic.
At Midland Flooring our stain protection system are imported from market leading suppliers with the aim of providing a long lasting stain protection system.
Whilst polished concretes can have stain protection systems applied similar to granite and marble if harsh products are spilt on the floor there is only a limited time before they will begin to burn through the stain protects and mark the concrete floor. We therefore advise that all spills are cleaned up immediately.
When installing a concrete floor there is always a risk of cracking. Reducing the risk of cracking can be achieved by:
Installing saw cuts into the floor, usually the day after installation, the design and layout of the cuts is generally proposed in line with industry guidelines which are 40 times the depth of the floor and an aspect ratio of no more than 1 to 1.5.
Installation of reinforcement. In order to tie the floor together reinforcement mesh is used, the specification of the mesh is generally governed by what the floor is to be used for.
If the floor contains underfloor heating then when commissioning the system the temperature needs to be raised gradually over time to reduce the risk of thermally shocking the floor.
As has been stated earlier polished concrete in small areas can be expensive when reflected in a rate per square meter, however areas over 80m2 start to become cost effective when comparing with alternative floor finishes, especially if you are able to use the polished concrete floor to replace what would have been just a structural concrete floor as it allows you to remove the cost of the structural floor, the cost of the screed and the cost of the final floor finish from your project.
Polished Concrete & Underfloor Heating
Whilst some of the micro concretes on the market may not be compatible with the underfloor heating due to different rates of expansion and contraction within the different floor layers, a 150mm or 75mm deep concrete floor is ideal for underfloor heating as concrete is a natural conductor of heat and also acts as a storage heater once the building has reached the desired temperature.