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.

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.

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.

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.