When grinding hard concrete your
diamond segment bond needs to match the
concrete hardness.
To
make a diamond segment the diamonds are
mixed with metal powders in a mold and
put into a furnace where the metal is
melted into a block.
The "bond" is the metal matrix of the
block into which the diamonds are
embedded.
These
blocks, more commonly called diamond
segments are then welded onto a metal
plate to form a grinding wheel.

Diamond
segment showing exposed diamonds
Rocket trails
Exposed diamonds
on diamond segments form rocket trails
where the front of the rocket trail is
the exposed diamond and the trail behind
it is the metal bond that has not been
worn away. Polished or flattened
segments do not have rocket trails.
How diamonds cut
concrete
When
grinding concrete the diamonds
cut into it and scratch it away as long
as the diamonds are exposed like the
above image (see the rocket trails).
This breaks the concrete
down to sands and dust. During
this process the diamonds fracture and wear
down, but at the same time the bond
metal is worn away by the sands of the
concrete which exposes more of the
embedded diamonds so that cutting can continue.
Hard
concrete
When grinding hard concrete the
diamonds cut, blunt and break as
normal, but he bond is not worn away
easily so the diamond are not exposed as
much as with medium
hardness or soft concrete.
This is when the
diamond segment stops
working and rubs on the floor instead of
cutting into it because the diamonds
have worn down to the metal, but the
metal matrix is not wearing away to
expose new diamonds.
See our
range of
edging dust shrouds
The diamond block is then flattened,
polished and becomes very hot with
continued use. At this stage you need to press
very hard and stay in the same
spot longer for the cupwheel to grind
any concrete which causes the
temperature of the diamond segments to
rise rapidly until the metal wheel turns
blue.
The
confusing
thing is that soft bonds are for hard
concrete and hard bonds are for soft
concrete.

Diamond
segments after grinding hard
concrete. Note the absence of "rocket
trails"
To recognize all the signs of
polished (glazed) diamond segments go to
Glazed Diamond Segments
Dry Grinding hard concrete
For dry grinding hard concrete use a soft
bond diamond segment with larger diamonds
around 20 grit size if possible so that the bond can
be worn away at a similar rate to the
diamonds. Concrete grinding will still
be slow, but at least it will be
happening. When that fails to
work there is another trick that often does the job.
Grinding hard
concrete trick
Wetting the concrete before concrete grinding
helps to expose the diamonds a little
more. Try to scratch open the surface
first, then apply water and give it a
couple of minutes to soak in. Broom off the
puddles and remaining water to leave the
surface wet. This
process makes the dust a little heavier
which cuts the matrix more to expose
more diamonds.
Don't
be fooled
The
dust will
appear to be the same as if the water has done nothing
, but
grinding will be more productive. Repeat
this process for each new cut.
To find out how to renew your polished cupwheels
go to
Glazed Diamond Segments
What makes concrete
hard to grind?
This is the million dollar question,
but two main things contribute to
concrete hardness - the ratio of cement
& water
in the mix which determines the hardness
of the the concrete paste and the type of
aggregate
used.

Exposed
aggregate showing the stones cut down
level with the surrounding paste
Hard concrete means a hard paste (the
grey matter that we recognize as
concrete).
Hard aggregate means the hardness of
the stones used in the concrete mix of
which
there are very hard ones and quite soft
ones used depending on the local supply.
Grinding aggregate
The unexpected variation comes when
you have medium strength concrete (with
very hard aggregate) which you grind
easily at first, but as you get into the
aggregate your speed slows down
dramatically because there is possibly
only 35% concrete paste now and 65%
aggregate which may be considerably harder than the
concrete.
So then you may have to change to a
softer bond diamond segment than the
one you started with.
Diamond size
Generally speaking you use a larger
diamond grit (low number) for faster
removal of concrete and for harder
concrete.
Because larger diamond grits are more
aggressive they are not as smooth to
grind with which you will notice at
30/40 grit and below.
Polished concrete
For
polished concrete you start
concrete grinding with a
coarse diamond and approximately double
the grit number (halve the diamond size)
with each new pass until the floor is
polished which may take several steps. ie. 30/40, 50/60, 80/100, 200, 500,
1000, 2000 and then 5000.
For example, with Situp N Grind
concrete grinder use
30/40+40/50 Coarse for first cut then change
to 100/80 Fine for second cut using soft
bond segments for hard concrete.
The concrete will not be polished but
it will be very smooth, ready for resin
bond pads to complete the polishing work
using low speed hand grinders.

Polished concrete diamond resin
bond pads
Situp N
Grind concrete grinder is for first and
second cut only
Note that
Situp N Grind fitted to a
9 in (230mm) angle grinder runs too fast
for diamond resin bond pads, and grits above 80 mesh
will quickly heat up the segments if the
concrete is medium to hard strength. If
the concrete was hard you
would need to use a slow speed grinder or
grind it wet.
Polished concrete look
using a clear sealer
After using the fine cupwheel
you can get a polished concrete look by
applying a couple of coats of clear
urethane or clear epoxy without the need to
actually polish the surface. In between
the grinding it will be necessary to
fill in the thousands of tiny holes in
the concrete to stop bubbles and holes
appearing in you final coat.
Next page "Glazed
Diamond Segments"