One
would tend to think that everyone in the stone
sector obviously knows what the tests in natural
stone are, what they mean and how to interpret
them. But the most common situation is something
like this -the architect asks, pompously, for
the test results of the stone being evaluated.
The seller (he has no idea at all what they are
), replies: "Of course! Here they are!" , and
he fishes out a photocopy with a lot of official
looking stamps and numbers. The architect (he
too probably has no idea at all but cannot afford
to look foolish by asking what they mean), has
a look, does not understand a thing but is impressed
by all those impressive stamps and signatures,
says: "O.K., now let us talk of the delivery schedule"
This
is more or less the reality never mind the hundreds
of conferences, articles, technological centres,
laboratory centres that exist in relation to the
topic. The objective of this article is one more
attempt to clarify some basic points relating
to the testing of natural stone. These mentioned
tests have been approached from a non- academic
point of view assumes that the reader is totally
ignorant , and the aim is that after reading the
article one has at least a general idea on the
topic. However, it is for the reader to judge
up to what point this article achieves the objective.
THE
NORMS OF THE TESTS IN NATURAL STONE
The
first problem that one encounters on examining
the laboratory data on a determined natural stone
is the norm used. The norm used is no more than
the description of the test that has been done.
The Spanish norm, for example : the U.N.E. defines
all the parameters for the realisation of the
test (type of testing sample, the number of testing
samples, machines to be used, the temperature
and humidity conditions , etc.) that measure the
precise properties of the stone. In other words,
the ruler for measuring each property is defined.
A
test that does not reflect the concrete norm does
not indicate anything about the properties of
the stone, since we cannot know of a measurement
without knowing the ruler used.
Therefore,
if we analyse the property of absorption of water
of two different stones using the U.N.E. norm
22-182-85, which defines how to do the absorption
test, we can find out which of the two absorbs
less water.
But
the problem begins when we find that in every
country there is a different norm (the NF in France,
ATSM in United States, the IAPMO in Singapore
, etc.) because every country defines the tests
in a different way: each country defines its own
ruler of measurement of each property in a unilateral
way. If we were to have two tests of two stones
that use two different norms, different ruler,
the comparison of the properties oft he two is
impossible. It is not rare to see in a big project
where different stones are competing that presented
with a large number of different test results
of each one of the candidate stones those in charge
of the project are not able to compare any of
the properties since each stone has been analysed
using different norms.
The
actual situation of the norms sometimes leads
to each of the property of the same stone being
tested according to the norms of each country
where the stone is to be commercialised with all
the extra costs and confusion that this implies.
This is perhaps one of the reason why the world
of stone offers such a deficit of technical information
to the consumer about its own products. This lack
of information may have led several times to the
choosing of other materials of which more technical
documentation is offered. However, there is a
new element in Europe which hopefully will modify
the mentioned deficit in a radical way in a few
years: the policy of standardisation in the European
Union.
In
a general way in the European Union the development
of this policy has the following objectives :
Oblige
the companies to have the tests of the different
stones being commercialised so as to allow the
users the choice of the stones with physical characteristics
in accord to its use.
Unification
of the European norm with the aim of simplifying
the interpretation of the tests in the whole territory
of the Union and permitting the possibility of
comparison. This is to say, the adoption at a
European level of only one ruler.
The
importance of these measures should be taken into
account because it would imply a much greater
importance of technical characteristics as a sales
tool.
THE
TESTING OF NATURAL STONE
The
testing in natural stone have basically the following
objectives :
1
Description of the mechanical quality of natural
stone.
2
Establish the suitability of a determined stone
for a concrete application.
3
Petrographic description of natural stone: the
alterability.
I.-
Description of the mechanical quality of natural
stone.
Without
any doubt the tests most often used in the world
of stone are those that measure the mechanical
quality of the stone. Among the different tests
the common ones are :
Apparent
specific weight.
This
test determines the density of the material.
TESTING
OF LOADS
These
tests determine the aptitude of a stone for its
use in places subject to heavy traffic over the
floors .from cleaning machines to vehicles and
there are basically three :
-
Resistance
to compression
-
Resistance
of a testing sample of a material to be broken
using a compression load.
-
Resistance
to flexion.
-
Resistance
of a testing sample of a material to be broken
by means of flexion force.
-
Resistance
to blows.
The
height .from which a steel ball has to be fallen
so that a testing sample placed on the floor is
broken.
However,
there is a problem to these tests, and, in general,
to all the tests oft his type. The result of a
test without any reference does not indicate anything,
we need an evaluation of the same so as to determine
the quality of the material.
Some
organisations such as the commission that awards
the Avis Technique Frances, analyses these results
in the classification UPEC that is a classification
of four numbers, one for each letter, that indicates
the suitability of each material for each use.
Thus the number that corresponds to the letter
P of this classification identifies the suitability
of each material for each type of use in function
of the load characteristic, thus a product with
P4 would be ideal ] with respect to load resisted
for all kinds of floors, a product P 3 would be
ideal for not very heavy traffic, P2 for pedestrian
traffic, etc.
On
this point it must be mentioned that a deficient
installation of a material several times leads
to granite and marble tiles with sufficient properties
to resist a determined type of traffic ending
up breaking.
ABRASION
TESTS
The
abrasion tests measure the deterioration through
.friction that is suffered on the surface of the
material subject to traffic. There are different
forms of measuring the abrasion though in general
a machine for wear and tear is used evaluating
at the end of the test the quantity of material
lost or the height lost by the surface.
The
problem of evaluating the results is resolved
in many norms installing some minimum parameters
of wear and tear that the material must complete
as a function of the floor where it is destined.
For example, in the test of wear and tear by the
Disk method (PNE 127-020 ** ) the parameters that
a stone must complete for its use on different
type of floors are:
Floors
subject to normal traffic : Index of mini- mum
wear and tear 23 mm.
Floors
subject to intense traffic : Index of mini- mum
wear and tear 21 mm.
Floors
subject to industrial traffic : Index of minimum
wear and tear 28 mm.
In
the mentioned French classification UPEC the parameter
of the letter U that is found between 2 and 4
identifies its suitability for different type
of floors.
REACTION
TO HUMIDITY
The
reaction of a material to humidity is one of the
properties in which the tests of the different
norms present greatest difference of opinions
, and this is owing to the fact that a material
absorbs water as a function of the time of absorption
and in function of the temperature of the water.
The absorption of water of a material is important
to determine the easiness of cleaning,. the particles
of dirt, usually penetrate into the material through
water, less absorption means less penetration
of dirt.
However,
the measuring method is similar in the different
norms: a dry testing sample is weighed, and is
placed in a water container with water at a standard
temperature, one waits for a certain time, the
testing sample is taken out and weighed again,
the difference of measured weight in percentage
is the parameter that is used for measuring the
absorption.
It
is also an important parameter for identifying
the suitability of a material for claddings, a
high level of absorption along with a cold climate
can provoke that the water absorbed in the surface
of the stone .freezes and ends up breaking the
surface.
REACTION
TO CHEMICAL AGENTS
The
reaction of the materials to chemical pro- ducts,
generally to acids, has the objective of establishing
the suitability of the use of the material in
situations that are subject to attacks of aggressive
products : from kitchens to industrial areas.
One
can affirm that these characteristics form the
basis of mechanical behaviour of the stone, for
the prescription of stone in usual applications
these tests are sufficient.
II
ESTABLISHING SUITABILITY OF A STONE FOR CONCRETE
USE.
There
are a series of more specific tests the existence
of which is justified by the specific uses of
stone and therefore its use depends on the project
being developed, in general these tests are done
in the case of these projects, and never before
.
Apart
.from the tests mentioned below there are a large
number of more specific tests for even more special
cases, but since dealing with them in this article
would be tedious they have not been included.
Among
the tests we cite below :
Resistance
to thermal changes.
The
loss of weight with temperature is measured, at
the same time observations are made about the
reaction to heat and the effect on the shine with
these changes.
A
low resistance to the heat changes is a necessary
condition for the installation of stone in open
spaces.
Resistance
to ice.
The
loss of weight as a percentage of material is
measured at low temperatures .
As
in the previous test the low loss of weight is
necessary for the use of stone in open spaces
.
Expansion
Module.
This
type of test determines the expansion coefficient
of the material as a function of the measuring
plane and a function of the change of temperature.
Even
though nowadays the use of these tests is very
limited , however the importance of taking into
account this coefficient in the planning of installation
is beyond doubt, up to such a point that not using
the mentioned result leads many times 10 problems
in installation with serious economic damage.
Resistance
to cladding.
The
suitability of installation of a material in a
wall cladding should be judged in these type of
tests.
Electric
characterisation of product.
The
determination of electric resistance of a stone
material is done following the standard system
of including the material in an electric circuit
and then measuring the resistance .
This
parameter is increasingly important for the use
of stone in environments with a propensity to
the generation of static charge, computer centres,
banks, surgery theatres, highly computerised offices,
etc.
Sound
insulation.
These
tests measure the capacity of a stone to insulate
sound, the determination of insulation capacity
of a material is an expensive test so its use
is restricted to places where this parameter is
very important : hospitals, sound laboratories,
some hotels, etc.
Slippage
tests.
These
tests try to measure the resistance to slipping
of a floor, they are done with dry and a wet material.
for this several types of stimulators of shoe
floors are used. Usually there is a threshold
point which is necessary to cross. However, it
must be mentioned that the resistance to slipping
depends not only on the material but also the
surface treatment (polished, honed, bush hammered,
sanded, etc.)
These
type of tests are necessary in many countries
for the prescription of a material in public places
like ramps and pavements.
III
MINERAL DESCRIPTION OF NATURAL STONE: ALTERABILITY
The
objective of these tests is to enter the structure
of the stone and try to describe it, to be able
to do this description there exist a series of
more or less standard tests among them to be found
are:
The
petrographic tests describe the mineral composition
of the stone, both its basic components as well
as its crystallisation and structure .
The
analysis using X -rays that complement the possibility
of determining the mineral structure of natural
stone .
The
chemical analysis that permits knowing the basic
elements which compose the structure of stone.
The
use of these tests today is very low because they
require a good knowledge of geology, therefore
the use is restricted to those who study stone.
If in principal the practical use of these tests
seems limited, it is after the tests that the
concept of alteration of stone is determined,
that is to say, the capacity of the stone to resist
the physical medium in which it is found without
deterioration. The measurement of alteration is
a parameter that is important even though little
used in the world of stone.
An
example of not considering these parameters we
have in the appearance of problems that appear
in the walls of buildings because of using a high
level of alterability
