Polished concrete  explained

How to get a polished (or honed) finish on concrete floors

See a photo gallery of  Polished Concrete

There are ten to fifteen steps required to produce polished concrete floors with up to eight grinding passes although there are other ways to get a similar polished concrete appearance as explained below.

The general rule of grinding is to double the diamond grit size for each subsequent pass, so you might start with very coarse, metal bond (see below) 16 or 32 grit diamonds, followed by 60 grit diamonds followed by 120, then start again with resin pads (see below).

 

Polished concrete floor for Puma showroom

Metal Bond

This refers to the segments found on most diamond grinding wheels or plates which are metal blocks containing the diamonds which are welded to a steel backing plate or wheel. These segments are strong and flat so they level the concrete aggressively and can contain diamonds from big number 10 grit to as fine as 250 grit. The higher the number the smaller the size of the diamonds.

Resin Bond

To polish concrete it is necessary to change to resin bond pads which are softer and less aggressive. They consist of a resin matrix which contain the diamonds  and these range from 50 grit to extremely fine 10,000 grit. Polished concrete is usually achieved at around 3,000 grit resin pads.

Changing from Metals to Resins

Most operators grind the floor up to 120 grit using metal bond segments and then change to resin pads starting with 50 grit and going all the way to 3,000 or above.

Edge Grinding

While their are many machines available to grind and polish concrete floors a separate edging task is usually required after each pass. Often the edges are polished first and the rest of the floor blended into the edges.

Edge grinding into corners requires the Corner Shroud

For more detailed information  on concrete polishing methods, composition, laying and finishing download this PDF file from CCAA.

 Two more important steps along the way:

1.   Hardening the surface

Polished concrete floors usually have the surface hardened with a chemical before the second, third or fourth grinding pass. The chemical soaks into the floor  and causes a chemical reaction that hardens (densifies) the floor to produce a high shine when the polishing is finished and to increase the wear resistance of the floor.

Quickly do the first cut of edges with Situp N Grind

Read about the best edging dust shrouds

2. Filling holes

After the first concrete grinding pass which removes the top layer of concrete paste millions of tiny air holes will be exposed. If these are not filled before the hardening process then the final polished concrete floor will show these unsightly imperfections.

The holes can be filled with an acrylic tile adhesive type of product mixed with either the grinding dust or cement powder which is hand scraped across the floor using a trowel.

Other rubberized adhesives can also be sprayed ahead of the grinder, or the floor can be wetted with water and the adhesive splashed across the floor so that the diamonds mix it into the holes with the grinding dust on the third or fourth pass at around 120 grit. See your supplier for more details.

Polished concrete stained floor

The floor can either be ground flat to fully expose the aggregate like terrazzo, or the aggregate can be partially exposed for a "salt and pepper" look, or the concrete grinding can expose no more than the fine sands at the surface.

For fast preparation of edges see Edge Grinding video(.9mb)

It takes skill and experience to control the process of polished concrete floors and a contributing factor is the quality of the concreter's original laying work.

* Original boot prints can show through.

One trap is when a concreter has walked over the concrete when it is too soft which pushes down the aggregate so that when the floor is ground down to expose the aggregate there are distinct bootmarks left showing.

There are polished concrete floors that look wavy and polished concrete floors that are very flat.

Flatness and polishing are not necessarily the same thing. Some machines can produce both by using three sets of diamonds per plate under planetary grinders for a wavy look or they can use six sets of diamonds for each plate for a flatter finish. The main difference is that only high spots are removed for a flatter floor while high and lower areas are both ground for slightly wavy floors.

Polish into corners with the Corner Shroud

If the surface is to be polished without necessarily exposing the aggregate then only the finer resin pads need to be used on a machine that allows for movement of the pads so they can follow the contours of the surface.

This will result in a polished floor that does not show the pattern and texture of the exposed aggregates within the concrete which is faster and less expensive and common in large warehouses or trade depots. Hardening is still important to help the durability of the surface and to produce a final gloss.

Problems with exposed aggregate

Sometimes the finish of exposed aggregate can be uneven if the mix of concrete was poured unevenly or finished off poorly. Boot marks or kneeling board marks can appear suddenly because they have pushed the aggregate down further which might require grinding down another two or three millimetres (quarter of an inch) which will cost more than was quoted. Grinding this far may not be what the customer wanted either so it can be a risk.

Exposed aggregate

Single head or multiple head machines

Original terrazzo grinding was done with single head grinders until the production of three-head planetary machines. Planetary means that each head turns one direction while the turntable that houses the heads turns independently in either the same direction or the opposite direction. Some grinders can vary the direction of both the turntable and the heads and some can vary the speed of each and some have counter rotating heads as well as the turntable.

The planetary heads can follow the contours better than single or twin head grinders and are faster to use with less effort due to eliminating the requirement to physically move the grinder from side to side or in a circular motion. Single head terrazzo grinders should be moved in a circular motion to avoid grinding lips or shoulders.

Situp N Grind concrete grinder

Situp N Grind is a favorite tool of polished concrete floor contractors, although it is not used for any of the finer diamond grinding or polishing work.

It is an excellent tool for first-cut and second cut work on small  floors and all the edges for small and large areas. The nine inch angle grinder that is used with Situp N Grind is powerful and spins at 6,000 rpm which restricts it to the first stages of preparation because polishing pads will tear loose at these speeds.

Cornerhood dust shrouds

These special dust shrouds for angle grinders were primarily developed for the floor polishing industry where there was a problem polishing into the corners without creating airborne dust. They have a cutting edge at the front to follow along the walls and another at the side for when a corner is reached. The Cornerhood shrouds enable polishing or grinding into a corner without restriction and without dust escaping. They fit all five inch grinders and most seven inch polishers.

Simple look-a-like polished concrete floors using urethanes

The number of processes for polishing concrete can be reduced by up to 60% and still achieve a similar appearance by grinding and then coating with a clear sealer. The first step is to grind with 30/40 grit diamonds to expose the aggregate, then fill all the small air holes as described above (in 10 to 15 steps....) before a second grind with 60/80 grit or 80/100 grit diamonds.

This will produce a smooth enough surface to coat with a sealer. Polyurethanes are very hard sealers with a high gloss and can be purchased as UV stable to stop yellowing (usually double the price of non UV stable urethanes), clear epoxy sealers are not as smooth and will chalk and deteriorate if exposed to sunlight and acrylic sealers have a much shorter lifespan due to their poor resistance to wear. Two coats are necessary to provide a high gloss level.

A quite new way to achieve a polished look is with lithium hardeners and dyes

The floor can be ground with 50, 100 and 200 resin diamond pads to get a clean and smooth surface followed by a water based stain, another fine grind, more stain, a spray on hardener, a burnish with a polishing pad, the application of a polymer sealer and more burnishing. This will result in a high gloss floor that will need resealing and polishing every 18 mths and will reveal the character of the concrete underneath.

Different ways to achieve "polished concrete"

The first method is described above. This is the true polishing or "honing" system.

A variation of the above is to cut perhaps half of the height of the higher peaks by starting with a medium coarse diamond grit and proceed through to polishing using a machine that can ride up and over small rises.

A third alternative way of producing a honed surface is to start with relatively fine diamonds and simply polish to top of the surface after hardening without removing much of the top cement paste. It is easier to do this when the floor has been laid flat and smooth.

Next, a "polished look" can be obtained by grinding with coarse diamonds and then fine diamonds before coating over with a clear sealer and

Lastly, you can use grinding to 200 grit followed by dyes, hardener and sealer and burnished with a high speed polisher.

Next page "Avoiding Grinder Burnout"