Polished concrete is strong, durable and low maintenance while having that
earthy texture of stone with a polish equal to granite. Here is how it is
done.
It is similar to traditional polished terrazzo which is very flat
and poured as a special mix to make the finish more successful. Polished
concrete floors can also look almost bumpy and have little or no aggregate
showing. Flatness and polishing are not necessarily the same thing which is
a good point to clear up before agreeing to your project.
It costs extra time and diamond wear to cut floors with a concrete
grinder until they are flat. They can either be ground flat to fully expose
the aggregate like terrazzo, or the aggregate can be partially exposed, or
the concrete grinding can expose no more than the fine sands at the surface.
It often takes skill and experience to control the process of polished
concrete floors and the quality of the concreter's original laying work is
also a contributing factor in the quality of the finish. Where the aggregate
is a feature special materials can be added to the wet concrete mix such as
coloured pebbles, metals and glass to enhance the final appearance.
Most people are unaware that there are ten to fifteen steps for polished
concrete floors which takes a long time to complete and can be quite
expensive.
Typically it takes many grinding passes to finish a highly polished floor
although there are other ways as explained later. The general rule is to
double the diamond grit size under the concrete grinder for each pass so a
contractor might start with very coarse, 16 or 32 grit size diamonds, then
use 60 grit diamonds followed by 120, then start again with a 50 grit
diamond resin pad instead of a metal segment. Using the resin pads the steps
may be 100, then 200, 400, 800, 1500 and finally 3000 grit. That would be
ten separate grinding passes which does not count the two other essential
steps.
Two more essential steps:
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 to a few millimeters (up to half an inch) and causes a
chemical reaction to take place that makes the floor harder and easier to
polish to a high finish. These floors are very strong and durable without
having a surface coating.
2. Filling holes
After the first concrete grinder pass removes the top layer of concrete
paste it will expose millions of tiny air holes. If these are not filled
before the hardening process then the final polished concrete floor will
show these unsightly imperfections. The holes are usually 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. The
preferred way to fill the holes is to spray the acrylic adhesive ahead of
the grinder so that the diamonds mix it into the holes with the grinding
dust on the third or fourth pass at around 120 grit. This method is faster
and the dust matches the colour of the floor to hide the holes better than
when using cement powder.
Very flat or slightly bumpy polished?
Some floor grinder machines are designed to produce a very flat floor by
cutting off the high spots while some others are designed to follow the
contours of the floor more. If the surface is to be polished without
necessarily exposing the aggregate then only the finer resin pads need to be
used on a concrete grinder that allows for movement of the pads so they can
follow the contours of the surface. This will result in a polished floor
without showing the pattern and texture of the exposed aggregates within the
concrete which is faster and less expensive. Hardening can still be an
advantage 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 millimeters
(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.
Single head or multiple head machines
Original terrazzo grinding was done with single head floor 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. There are also multiple head planetary machines
with four or more heads.
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.
Edging
As with wood floors the edges must be finished separately to the main
floor area because the large machines may bump and damage the walls if they
are used too close. For the first grinding passes a nine inch angle grinder
is used having a diamond wheel attached with a dust extraction shroud fitted
to remove the dust. The sit-down-to-use edge grinders are more controllable
than stand up grinders while kneel-to-use grinders have good control, but
are exhausting.
After the first two or three cuts a different dust extraction shroud with
a corner feature is used with a seven inch polisher or a five inch, multi
speed grinder to polish with resin pads and get tight into the corners.
Simple polished concrete floors "look"
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.
Different ways to achieve "polished concrete"
The first is to cut the floor with coarse diamonds until all the high
spots have been removed to produce a flat floor and then use all of the 12
to 15 steps to eventually polish it or as it is sometimes called, hone it.
This is the true polishing 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.
Lastly, a "polished look" can be obtained by grinding with coarse
diamonds and then fine diamonds before coating over with a clear sealer.
Only truly polished floors will retain their gloss with little
maintenance because all coatings scratch with wear and lose their high
shine. Special buffing pads on a standard floor polisher that contain very
fine diamonds can be used to maintain cleanliness and the high gloss levels
of true polished concrete. Janitors can be provided with these and keep the
floors in great condition without special treatment.
Some hire companies provide the grinding equipment for do-it-yourself
concrete polishing and have all the pads for the purpose. This may not be so
economical though because the pads may be only half worn when you finish
depending on the area involved.
