Concrete Polishing

Polished concrete is a multi-step process where a concrete floor is mechanically ground, honed and polished with bonded abrasives in order to cut a concrete floor's surface. It is then refined with each cut in order to achieve a specified level of appearance.

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1. How to Polish Concrete Floors?

Polished concrete is rapidly becoming a popular floor finish both in commercial applications such as restaurants,  shopping centers as well as residential homes. Polished concrete looks fantastic and has outstanding durability as well as provides an easy-to-maintain surface.

Polishing concrete is not a complicated process, and it’s one that any contractor given the right tools could easily perform. The most important thing to remember is that polishing takes time – a lot of time. Times can easily blow out to in excess of 10hrs/sqm depending on the level of gloss required.

Getting a great result is somewhat more difficult. Polishing concrete, just like painting relies on the suitable preparation of the surface, as well as the general quality of the formwork which will determine the eventual outcome and finish. Cracks and repairs will be more visible after polishing so bear this in mind when determining the suitability of a slab prior to proceeding with this style of finish.

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2. What is Concrete Polishing?

Polished concrete is a term given to describe a concrete finish, and needs to be defined whether it is ‘mechanically honed’ or ‘cut and coated’.

Polishing is the process used to create a smooth surface with a specific gloss level on a product.

The gloss is a result of light bouncing off the surface (reflecting). A perfect reflection is created by all the light bouncing back off the surface in exactly the right direction. This can only be achieved by having a perfectly smooth flat surface with no imperfections. Mirrors usually provide an accurate reflection because the glass and reflective film are perfectly flat and smooth.

One other thing to consider is color. White light is created by all the spectrum of colors (the colors of the rainbow). Colour is viewed by our eyes as the part which is reflected. When a source of light (such as the sun) reflects off a surface, almost always some of the colors of the spectrum (rainbow) are absorbed and some are reflected. This is why we see green grass, for example, only the green is reflected and the other colors are absorbed by the grass. A mirror contains a reflective sheet that reflects ALL COLOURS, making a mirror create an almost perfect color reflection. When a surface is bumpy, light is reflected in all different directions which affects the reflection and the ‘gloss’ we speak of.

Generally, there are two ways we can make that surface glossy. One way is to add some new surface material that is smooth, like a clear gloss paint. The liquid paint will fill up all the little bumps and gravity will cause it the surface to spread smoothly. You can test this principle by putting some water on your surface – it becomes instantly glossy because the water fills all the little bumps on the surface and is flat. This is called ‘Coating’ the surface and is a much less labour intensive way of achieving a ‘polished look’. The gloss level needs to be decided on whether it is full gloss (wet look), satin (medium gloss) or matt (dull shine).

Another way is to physically smooth the bumps out of the surface. This can be done by cutting away the surface layer (with a fine gritty product/cream) and buffing it smooth with a wax or filling material. This is known as ‘mechanical honing’. You can test this by getting some polish and applying it most surfaces. Notice after you’ve rubbed and buffed it that the area is more glossy that the surrounding area? This is the ‘true’ process required to polish any surface,  no sealer required , and extremely hardened and densified surface.

The finishes for concrete are endless and every slab has its own uniqueness, that can bring individual charm and style. To make it even more interesting and unique, concrete can be ‘seeded’ with colored aggregate, stainless steel, brass, colored glass, steel bolts coins, and chips of natural stone like marble and granite. Needless to say it is important that any additives like these are placed in such a way that they can be ground to the desired effect without being detrimental to both concrete integrity and the equipment being used to achieve your finish.

You also need to determine what your Clients’ expectations for the amount of aggregate exposure are – (if any). When selecting a polished concrete floor you can choose ,

 

  • Burnished top – where the creamy laitance has been heavily mechanically trowelled at time of placement and then densified and burnished with a floor grinder – nil aggreage exposure

  • Salt and Pepper Concrete effective – where ther eis only a slight grind to remove a micron of laitance and achieve very minimal exposure

  • Quarter Cut – where the aggregate is ground back to expose a light aggregate

  • Half Cut – this is when you grind the aggregate through to 50% of the stone being fully exposed

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3. What is required to Polish Concrete?

Concrete is not glossy in standard form because it is full of little bumps and imperfections causing light to be reflected in random ways. The key is to create a new surface that reflects all the light, or as much of it as possible, back to the viewer.

Preparation is the key to a brilliant polished surface, both if you are planning on coating the surface, or doing the ‘complete’ polish process. This generally comprises of removal of any old surface material, mechanical smoothing (usually grinding), and the process of additional smoothing to create the reflective surface (if you are proceeding with the ‘complete’ polish process).

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4. Preparing the slab.

In the case of a concrete slab being placed with the intention to polish, it is important to consider and discuss with both the concrete contractor and concrete supplier the following:

  • What color, size, and type of aggregate? Generally, 10mm is minimum for structural concrete

  • What strength? Generally, 32 Mpa is a good choice

  • With all polished finishes, you require a class 1 finisher that will not leave footprints, trowel marks, screed bar marks. Often with new slabs, it is good practice to trowel once only for a flat finish and to perform the initial grind within 5-10 days of placement.

  • Tip – it is a lot easier to grind a lightly broomed finish as this makes the laitance stand up instead of being smooth. Ensure your slab has been mechanically flattened with the use of the trowelling machine.

The preparation of the slab is an entire process in itself and will require a good quality stand-up grinder with the ability to accept polishing pads, a stand-up edger with the ability to accept polishing pads, OR another polishing machine with edging ability, proper dust control, and the right consumables. All Preparation Equipment can advise you based on industry knowledge, regularly dealing with thousands of contractors each year who rely on expert advice. Generally, the process is as follows:

  1. Remove any existing coatings.

  2. Seal up cracks and joints with a semi-rigid or epoxy type filler. Remember that these will be visible once the surface is polished.

  3. Use a 30/40 grit consumable in the correct bond on your large stand-up grinder to remove the surface imperfections. View the All Preparation Equipment diamond guide to learn about selecting the right diamonds for the slab (VERY IMPORTANT) or call Key Construction

  4. Use a 30/40 grit consumable with the correct bond on the edger to flatten and smooth all edges to match the finish on the main floor.

  5. Ensure the surface is nice and flat – all uneven surfaces will reflect light and be very obvious when the job is complete. Generally, the bigger the grinder the better for flatness.

  6. Use a 60/80 grit consumable in the correct bond on your large stand-up grinder to remove scratches and imperfections made with the 30/40 wheel previously.

  7. Use a 60/80 grit consumable with the correct bond on the edger to flatten and smooth all edges to match the finish on the main floor.

  8. Use a 100/120 grit consumable in the correct bond on your large stand-up grinder to remove scratches and imperfections made with the 30/40 wheel previously.

  9. Use a 100/120 grit consumable with the correct bond on the edger to flatten and smooth all edges to match the finish on the main floor.

  10. If available, consider using a 150 grit consumable in the correct bond on your large stand-up grinder to remove scratches and imperfections made with the 30/40 wheel previously.

  11. If you’ve ground the surface with the stand-up grinder with a 150 grit consumable, you must use a 150 grit consumable with the correct bond on the edger to flatten and smooth all edges to match the finish on the main floor.

  12. Note: Using a 150 grit consumable will provide less work when using the resin-backed polishing pads later in the process and reduce the overall time required to polish the surface.

At this point, you’ve completed the majority of the preparation work. The surface should be quite smooth but will lack any real sheen. It can be coated at this stage with epoxy or water-based epoxy product, or painted/sealed.

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5. Polishing the slab

In order to properly polish the slab, not only does it need to be properly prepared, but it also needs to be hard enough that the cement and slurry does not rub away when smooth polishing pads are applied to it.

Sometimes, it might even be necessary to harden the surface prior to the 150 grit diamond cutting phase if the surface will not further smoothen.

  1. Apply hardener or densifier. These products can be purchased through All Preparation Equipment. Ensure that you’ve allowed enough time for the product to work. Hardeners or densifiers chemically alter the concrete at the surface to ensure it is not powdery or flaky and that the abrasion of the polishing pads finely cuts away the surface.

  2. Fit polishing pad holders to your stand up and edge grinder, or stand up grinder and a polishing tool you will be used for edges. If you are using a single-speed machine to do your polishing, a neat little trick to compensate for the speed is to add more polishing pads to your machine. This will have the effect of reducing the speed of the machine.

  3. Begin with a 50grit Diamond Impregnated Resin pad. Ensure that you cover the entire area. Reduce speed or add more pads to improve the quality of finish.

  4. Use the same grade pads on your polishing tool or edger to complete the process with the 50 grit.

  5. Repeat Step 3 and 4 with 100 grit pads.

  6. Repeat Step 3 and 4 with 200 grit pads.

  7. Repeat Step 3 and 4 with 400 grit pads.

  8. Repeat Step 3 and 4 with 800 grit pads.

  9. Repeat Step 3 and 4 with 1500 grit pads.

  10. Repeat Step 3 and 4 with 3000 grit pads.

  11. Repeat Step 3 and 4 with Buff pads.

 

Once step 11 is completed, the surface will be a durable and easy-to-maintain polished concrete finish. Optional stain-resistant coatings may be applied to the surface if desired but generally, all that is required to keep the finish in great condition is the occasional buff with 3000/buff resin pads.

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