TESTS IN NATURAL STONE

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 :

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.

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Last Updated: July 2008

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