Determining the hardness of concrete
All concrete may feel hard (particularly if you fall over
on it), so what do we mean when we talk about hard,
medium and soft concrete?
All concretes are measured by their compressive
strength and depending on which part of the world you
are from, different compressive strength indices (e.g.
PSi & MPa). Generally speaking, the higher the com-
pressive strength rating, the harder the concrete and
therefore, the harder it will be to grind.
However, other factors beside compressive strength
ratings determine how hard the ﬂ oor will be, and
therefore, the correct diamond selection. Since grind-
ing normally only deals with the surface of the concrete
(top 5mm or 1/4 inch), often the way the concrete ﬂ oor
has been ﬁ nished or the condition of the surface will
have a greater bearing on what type of diamond to
select, rather than the compressive strength rating of
SURFACE FACTORS TO CONSIDER WHEN MAK-
ING A DIAMOND SELECTION
Generally speaking, if a concrete surface is very
smooth (i.e. Most likely it has been heavily trowelled/
helicoptered), the concrete will behave as if it has a
high compressive strength and therefore, require a soft
Accordingly, if a concrete surface is coarse/aggressive
(e.g. rain-damaged, shot blasted, scariﬁ ed, exposed
aggregate e.t.c.), the concrete will behave as if it has a
low compressive strength and therefore, require a hard
Surface coatings/contaminants (e.g. Epoxy coatings,
ceramic tile adhesives, levelling compounds/screeds)
will often have a larger bearing on what diamond to se-
lect, than will the compressive strength of the concrete.
As a general rule, when grinding a concrete slab for
the ﬁ rst time and you are unsure about its hardness,
always begin with harder bond diamonds under the
machine. This will ensure the least amount of wear on
the diamond segments. If a hard diamond segment
is not suited to the application, all it has cost is a little
amount of time without wearing out the diamonds.
If performed the other way around (i.e. a soft segment
is used to begin with) and the concrete is soft or pos-
sesses an abrasive surface or surface contaminant, it
is quite possible to wear-out a considerable amount of
diamond in very short period of time.