STAIN RESISTANCE AND DURABILITY OF POLISHED CONCRETE

Having spoken with hundreds of clients over the years, the general consensus is that they often believe concrete to be rock hard, unbreakable, difficult to damage and so resilient that it will either never need to be maintained or will simply last forever.

If only that was the truth of the matter! Concrete, like any other flooring option such as natural stone, epoxy resin flooring, timber and tiles can be damaged, stained and abused and still need to be treated with care.

Mechanically polished concrete flooring by way diamond polishing tools offers very good resistance to knocks and bangs where objects have been dropped. That is not to say that they are impervious to damage; they are not. However, given that the concrete was of good quality, mixed and laid well initially, it will have an inherent resistance to damage through regular, everyday use once polished.

Other benefits of polished concrete floors is the inherent resistance to footfall is high and it is typically only the damage from grit and stones trapped in the soles of shoes that creates small amounts of wear. The use of boundary mats is recommended and will significantly reduce the potential for wear and the amount of cleaning required in both residential and commercial environments.

Whilst concrete will not be damaged by falling objects as easily as resin flooring, timber or tiles, it is still possible to dent and chip a polished concrete floor by dropping hard and/or heavy objects. This often shocks clients who have let heavy items fall onto their floor, especially from height. Admittedly, any damage tends to be far less visible than the same damage on another seamless floors, but there may be damage nonetheless.

Concrete can also be etched by almost anything acidic coming into contact with the polished concrete areas. As concrete is a calcium based structure and inherently includes a source of free lime in the concrete’s makeup, there is always a chance the concrete could be damaged by things such as fruit juices, vinegars and fizzy drinks when these are on the surface of the concrete for an extended period of time.

Fats, oils, wines and dropped foods can also stain polished floors and leave darkened patches if they are given the opportunity to linger on and penetrate the surface. The use of a quality concrete hardener together with a stain resistor will lessen the penetrative aspects of these contaminates but it should always be the aim of the user to clear any accidental spills as quickly as possible.

We use a solvent based penetrating sealant as well as our professional grade silicate densifier on all of our polished concrete floors projects. The idea is that while the densifier adds strength and begins to seal the concrete from the inside out, the solvent soaks into the pores of the concrete, lining all the sand and aggregates with an ultra fine film of resin. This film of resin sets rock hard, actively increasing the surface tension of the concrete to resist and repel water and oil based contaminants.

We feel it is important to address the term of ‘stain resistance’ overall. There is not a sealant on the market that can 100% protect concrete from staining indefinitely. They all give you a window of resistance and opportunity to remove any spills from the surface. Failure to remove spills within this window will likely lead to either staining or etching of your polished concrete floors.

DOES POLISHED CONCRETE ALWAYS LOOK THE SAME?

When decorative concrete flooring is mentioned, friends and other contractors will often think of some kind of grey flooring usually with a type of lacquer applied. Thankfully, floor polishing has way more to it than just that, there is a huge amount of skill and knowledge required to refine the surface with diamond abrasives in a productive and aesthetically pleasing manner.

A few years back we created our own concrete polishing concept called InfinityFloor, which is our systematic process for polishing concrete. Like everything we develop, InfinityFloor is continually upgraded and improved as technology advances.

So, let’s go through the three finishes most commonly specified when polishing concrete floors.

HOW ARE COSTS CALCULATED FOR CONCRETE POLISHING?

The question we are asked most often - what does polished concrete cost Very briefly, polished concrete floor cost are determined by the location, size of project, number of rooms, level of gloss and amount of grinding and polishing required. As a rough ballpark figure, you should expect to pay between £40 - £80 per square metre for mechanical polishing. Concrete overlay products start at around £80+ per square metre.

Costs can be reduced greatly be installing the concrete flooring professionally to reduce any additional grinding charges, by erecting dividing walls after the polishing has been completed and ensuring all obstacles such as kitchens and bathrooms are not in place before polishing starts.

HOW TO INSTALL CONCRETE FOR A POLISHED FLOOR

It goes without saying, if you want a surface that is smooth, easy to maintain and suitable for polishing, you need to have your concrete installed by a professional contractor following a tried and tested concrete specification. We can of course give your installer our well respected specification to follow, unless they already have one they currently use.

We recommend that the mix for a new polished concrete flooring should be a minimum of C35 grade, which is a good hard concrete supplied to the building site via a concrete mixing truck. We would advise for this type of concrete floor that you do not use a volumetric mixing solution as every floor we’ve worked on that has used this method of delivery has been of substandard mix quality.

It is CARRcrete’s recommendation that you use major suppliers like Tarmac, Lafarge, Hanson etc as their quality control and documentation seems to be far superior to that of small companies.

100mm of slab thickness is a minimum over underfloor heating systems and insulation boards. Laying poured concrete to this depth reduces the risk of cracking through shrinkage and movement. Using suitable steel mesh is also helpful for reducing shrinkage of the slab.

The C35 fresh concrete should be placed with no added water (which would reduce the strength). The mix should ideally be screeded into place and magnesium floated in multiple directions to remove any tamp lines and to flatten the floor out ready for the power trowel.

It is really important that no low spots, lines or gouges are left in the surface after the magnesium float work has been done. If the concrete is not flat and smooth the power trowel will not remove any lack of preparation.

The finish the client requests determines how much power trowel work is needed. For example a Cream finish concrete floor needs multiple passes with a trowel to tighten and burnish the finished surface to produce the cloudy, swirly patterns we often see. This is a highly specialised technique which requires years of experience to master.

Salt and Pepper floors need plenty of power trowel work to compress and tighten the mix but not nearly as much as the cream finish. This is mainly because the surface of a Salt and Pepper floor receives a light grinding before the polishing process.

The Stone finish needs the least amount of work with a powered float, but it still needs to be done to ensure the floor is flat, smooth and compact. The act of trowelling the surface helps remove air, breaks down installation marks and lines and overall produces a much better polished concrete floor.

Once the concrete has been laid, is smooth and trowelled to our recommended specification for finished floors, you need to allow any surface bleed water to evaporate before curing the slab to ensure that it hardens correctly. Curing of the concrete can be done either by covering with plastic sheeting (ensuring that there are no creases in the sheets which could cause curing marks) or by ponding water on the surface.

It is our recommendation that newly installed concrete is left for at least 28 days before any grinding, polishing, densification or sealing works are carried out to the final surface. Special care should be taken during the colder months of the year as concrete will not cure properly in cold temperatures. Ideally the temperature should be at least 10degrees centigrade for best results. Concrete should never be left to cure on a non-weatherproof site or be left exposed to the elements.

HOW DO YOU POLISH CONCRETE FLOORS?

At CARRcrete we’re always being asked how to polish concrete and the truth is we employ a few different techniques in our polishing process.

We often use a combination of wet polishing and dry polishing techniques with our polishing equipment as we have found through years of experience that this allows us far greater flexibility when dealing with all the different concrete mixes we encounter across the country.

Most of our projects employ the use of a concrete grinder or a high-speed burnisher.

EXAMPLE POLISHING PROCESS

In brief, here is a quick overview of our InfinityFloor polishing system, we will at a later date give each finish far more detail in their own article.

For a polished concrete Cream finish we use diamond grit impregnated fibre polishing pads to gently smooth the power trowelled surface. This very delicate process removes almost nothing from the surface to ensure that the texture and colour of the floor is not erased, while still allowing the diamonds to polish and refine the surface.

As with all concrete there is a quality concrete densifier applied to convert the source of free lime within the concrete to a much harder, less acid sensitive and less porous substance. Once this chemical solution has dried we are then free to continue polishing through the fine grit fibre pads to mechanically polish the surface to a light reflective sheen or high gloss.

After a thorough cleaning down, the polished surface of the concrete is then treated with our impregnating solvent based sealer PROseal. We literally saturate the concrete with this solution so that all the pores of the concrete are fully coated with the stain resistant properties of this product.

The process differs for a Salt and Pepper concrete as we need to be more aggressive in our approach so that the surface laitance material is removed and the machines get down into the tougher sand layers of the concrete itself.

Normally we will start with a relatively fine metal bond diamond to remove 1-2mm millimetres of surface, this may be done wet or dry depending on the concrete at hand. Once we have attained a suitable level of sand exposure, we start removing the scratches created during the initial grinding phase with transition tools, which again can be a wet or dry process.

After the transition phase, resin bonded diamonds from 200 grit up to 1500 grit are used to hone and polish the surface to the desired level of light reflectivity and gloss. Silicate densifiers are used at multiple times during the polish process to ensure the highest level of robustness are achieved and the floor is finally sealed with PROseal to help protect against stains.

InfinityFloor Stone polished concrete is the same as the Salt and Pepper system but with a lot more coarse grinding to remove at least 2mm millimetres of concrete mass. This process gets down into the hardest regions of the concrete mix, where all the larger aggregates reside. The more material you remove the more uniform (in theory) the aggregate exposure is although this is not always the case.

Please bear in mind, lots of diamond grinding is expensive! A well mixed and well laid floor will always save you money in the long run.

WHAT COLOUR IS CONCRETE?

This is a very common question and most people when asked seem to think that concrete will always be a shade of grey. Whilst there is usually a grey cement added to the concrete, it is by proportion a very small percentage of the overall mix. So, more often than not, concrete is the colour of the sand used by your local concrete supplier, unless you request a colour pigment to be added to the mix.

If you require a specific tone or colour we would always recommend the use of a pigment or alternatively have a concrete overlay applied. Both of these options will get you closer to an actual colour palette than waiting for your concrete to be ground and polished only to find out it is a shade of beige as opposed to grey. We do not offer any stained concrete services purely because the results can be very hard to predict and vary across the country according to different mixes. Concrete is certainly not an exact science.

There is a wide range of colour tones available for concrete mixes and overlays, most are based around iron oxide powders but there are also liquid nano dispersions available; everything from reds, pinks, blue, green and yellow through to greys and blacks. Using white cement instead of grey can expand the colour range significantly, though availability is largely down to your supplier.

CAN AN EXISTING CONCRETE FLOOR BE POLISHED?

If you have an existing concrete slab for example in a garage, workshop, commercial or industrial premises it can usually be polished. The final outcome is large down to the quality of the installation, condition of the concrete and indeed the quality of the concrete used.

It is very rare to find an existing slab that is suitable for a cream polish as most floors do not have the amount of power trowelling required to create the cloud effect that is desired. It is also common to have to utilise the grinder to remove surface contaminants and built up dirt associated with past use therefore most existing floors end up with a Salt and Pepper or exposed Stone.

As mentioned previously, taking into account the condition of the concrete you want polishing is really quite important. A poor installation rarely lends itself to a premium polishing process or outcome and will often eat into your budget.

We design and manufacture our own repair products for concrete slabs which can be used to patch up holes, cracks and damage to concrete that is already in place. Please bear in mind that while repair products may get close to same colour as the surrounding material, it will never be exactly the same and variation should be expected.

USING UNDERFLOOR HEATING WITH CONCRETE?

Yes indeed! In fact we have found that most slabs laid in the last couple of years are over water fed underfloor heating systems. We would say that the most important factors to consider when installing concrete over UFH is that the concrete is laid to a depth of at least 100mm and perimeter movement joints and crack inducement joints are cut or placed when necessary.

It is also highly important to commission your underfloor heating system in the correct manner, after polishing has taken place, by raising the water temperature slowly and over a number of days to reduce the risk of cracking. This gives the huge rigid slab that are concrete floors time to slowly acclimatise with the heat running through it. From experience we have found that floors which have a surface temperature of 35 degrees Celsius or less experience the least amount of problems from cracking, shrinkage and signs of deterioration.