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Harris Calnan Cracks Polished Concrete Floors

Updated: Jun 4, 2019



“You have to give this much to the Luftwaffe: When it knocked down our buildings, it didn’t replace them with anything more offensive than rubble.” Prince Charles has never been known as a fan of concrete as this quote, taken from an address to town planners in the late 1980’s, colourfully demonstrates. But whilst heroic architects such as Le Corbusier, Erno Goldfinger and the Smithsons were early champions of a material which was previously employed on bridges, water towers and submarine bases, exposed concrete has always had its detractors.

In talking up the concrete architecture that emerged after World War II, architects talked about truth to materials but the real truth was that many non-architects simply saw these buildings as nothing more than smelly old men in dirty raincoats who insisted on exposing far more than what the laws of decency should have allowed.

50 years down the line and despite of all the vitriol, advances in concrete chemistry and new design tools now mean that the material is being used in ways never before thought possible, let alone sensible. The language around concrete has also changed: "Brutalist" - the word used to describe the early application of concrete to buildings has given way to terms like “fair-faced” and "architectural" which inspire something altogether more pleasant. The combined result is that concrete has never been more popular.

But with more and more concrete being left naked and exposed for everyone to scrutinise, the quality of the final finish has become a source of great anxiety for every designer and contractor alike. When the formwork is pulled away, will it be a beauty or will it just be more “rubble”?

Unlike stone, which is a product of the forces of nature and so given to impurities that mark it out as “unique”, concrete is perceived to be a product of science and so comes with an expectation of predictability. But the truth is far more interesting. It turns out that concrete can be just as fickle and temperamental as stone, but also just as charming.

To master concrete, you need to be part psychotherapist and part alchemist, and Jonathan Reid from Grey Matter Concrete is just such a person. He specialises in the dark arts of concrete science (you can read his own blog here) and is often called in to ease the crises that arise when the formwork comes down and all is not as it was intended to be.

But in addition to rescuing concrete jobs gone wrong, Grey Matter also provides a consultancy service for clients, architects, and contractors to help them work out which concrete finish is best for their needs and then how to go about getting it.

This week Harris Calnan asks Jonathan about an application that was originally intended for industrial factories and foundries but which has now found a niche in upmarket residential conversions: the polished concrete floor.

HC: What is polished concrete?

JR: Even in the trade it is amazing how many people are not clear about what polished concrete is or isn't.

Polished concrete is not a resin, nor is it a cementitious polymer, micro-cement or self-levelling topping.

I would define polished concrete as a simple base material of sand, aggregate and cement which is then cured and mechanically polished to achieve a desired surface finish. Both the contents of the base material as well as the curing process (see here for an article all about curing) can be varied to achieve a huge range of potential finishes. In addition, polishing can be done by different means, either trowel or diamond, to further expand the range of possible outcomes.

Polished concrete floors can appear in two main forms: either as the polished top surface of a structural concrete slab, or as a screed, which is laid over the structural slab and then polished.

HC: Sometimes you see a great floor but you don’t know how it was done or what the spec was - what do you do?

JR: My advice would be to try to avoid reverse engineering the floor to produce a method statement or specification for a floor that you hope will be a match. Whilst the first part of a polished concrete floor, the base material or type of concrete, can be relatively simple to describe, the process of getting it laid and then polished to achieve the desired effect is quite complex. This is a specialist activity and a specification can be deceivingly simple and may give contractors false comfort in thinking that they can achieve what’s needed.

So if you have seen a great floor, the question I would ask is not HOW was it done, but WHO did it. The knowledge and experience needed to develop a proficiency in the making of these floors is huge and should never be underestimated. Rather like a delicious but very complicated cake, you can give someone the recipe, but it doesn’t mean you will get what you were expecting!

HC: When is a polished concrete floor a bad idea?

JR: Too often the problems we see have arisen because concrete was added as an afterthought rather than being treated as an integral part of the project’s design from the outset.

This is particularly true when a design calls for a concrete screed to be laid onto a suspended timber floor. In this situation, substantial strengthening work is needed to stabilize the timber structure so that it can support the screed, but because the decision has been taken so late in the day, often it would be too disruptive to do everything that’s needed and that’s when problems can arise.

How much room you have for the layer of concrete is also crucial. If you are using a concrete screed, then in order to allow for threshold details and the inevitable variations in the level of your structural slab or base, the zone needed for the polished concrete screed should never be less than 100mm but ideally you want about 120mm.

If you are going to have a polished concrete structural slab or if you are going to use the material in an external setting, then the thickness should never be less than 150mm.

If you don’t have this amount of room, then you need to adjust the design to get it, or think about other alternatives.

HC: What are the most common design mistakes that you see?

JR: Unnecessarily dividing up the screed into lots of small shapes is probably the most popular mistake that I come across. Internal partitions and non-loadbearing walls need to sit on top of the polished concrete screed, rather than dividing it up by resting on the slab beneath. The screed relies on its continuity for even shrinkage and if you break it up then it will shrink at different rates, adding to potential cracking. The larger the surface area and the squarer you can make it, the better.

When trying to achieve a uniform appearance for walls, soffits and floors, it is often tempting to specify the same concrete mix for all applications. This is not necessarily the case as the various applications will have unique requirements and these will cause the concrete to behave differently in the production processes, leading to variations in the look and feel of the material.

Even something simple like lighting can mean that two surfaces, say a wall and a floor, which are made of exactly the same material, will look entirely different. When you add the fact that the floor will get much more use than the walls, then you can very quickly see that for visual continuity, consistency of materials is not always the best way to go. Again, you might be better served talking it though with the concrete sub-contractor to discuss what it is that you are trying to achieve and then giving them the chance to guide you to the best overall solution.

It is worth remembering that a lot of concrete floor specifications have their origins in the industrial arena where floor flatness is critical, with the specification allowing for minimal tolerances to reflect this. Thus, a contractor for a residential project might increase his price or even turn away from the project when a less arduous specification would have been perfectly acceptable.

Finally, it helps to take some time to find out how the material is going to behave once it has been laid. A degree of cracking is inevitable with all concrete and whether or not you choose to try to control this through the use of screed rails or crack-inducing saw cuts may dramatically impact the visual aesthetic you are trying to create. Sometimes, it may be better to just let the concrete “be” and accept its impurities and imperfections, rather than trying to control it too much.

HC: What’s the most important thing to do to ensure a happy outcome for both the floor and the project?

JR: Don’t tie yourself down to samples and photos!

There is no substitute for seeing these floors in the flesh and clients should be dragged, if necessary, to see as many floors in as many different settings as is possible so that they can get an understanding of the nuances and subtle variations that can occur. For example, by looking at light and dark floors clients will see that with dark floors there is much greater disparity in light reflection which accentuates tonal differences, making darker floors look more “blotchy” than lighter ones.

Clients should also be encouraged to talk to people who live with these floors every day so that they know what to expect – what’s the maintenance regime, what’s it like with kids and pets, how is it effected by underfloor heating, etc.

Once clients gain an understanding of the peculiarities and idiosyncrasies that make this material so special, it is these very quirks that will add to their enjoyment, rather than lead to disappointment. Remember that with slabs of stone, if you don’t like them, you can always rip them out and get others put in. But with a concrete floor, once it’s down, it’s there for good and it’s very unlikely that anybody will agree to taking it out!

HC: Why should people be falling back in love with concrete floors and concrete in general?

JR: In terms of sustainability and energy efficiency, concrete is a terrific product and it is getting even better.

(See Concrete Centre or Sustainable Concrete for more info.)

Concrete is the second most used material in the world after water, and in the UK the average delivery distance of ready-mixed concrete to construction sites is only 12km which means that, considering the volumes of concrete used, CO2 emissions with respect to transport and production are amongst the lowest.

The new field of what we like to call Visual Concrete is also opening up myriad new applications, of which polished concrete floors are just one, so together with its environmental credentials, there has never been a better time to consider using concrete in your own building project.

So there you have it. Thanks to innovations and a drive towards environmental sustainability, concrete has become a focus of attention for all the right reasons. And with people like Jonathan Reid to guide us through the process, there is every reason to believe that even the staunchest of critics such as His Royal Highness could be converted to the cause.


Jonathan Reid from Grey Matter Concrete was talking to Harris Calnan's Ryan von Ruben.

#concrete #residential #refurbishment #technical #Linked

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