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"