Testing

For more details about the various tests see the TI 228 of the BBRI (www.bbri.be). A few tests mentioned in the fact sheets are described here.

Bulk density (NBN EN 1936)

The apparent bulk density reflects the degree of compactness of a stone. The principle of this method consists in calculating the volume of the void fraction and the apparent volume of the sample to be analysed. The method is applied to at least 6 cubes with an edge length of 50 mm. After desiccation until constant mass is reached, the test tubes are placed in a sealed container from which the air is evacuated.

A vacuum is maintained for 24 hours so as to eliminate the air contained in the test tube pores. After 24 hours, you slowly pour demineralised water into the container. Vacuum and water are kept in the container for 24 hours. After these 24 hours, the container is brought back to atmospheric pressure and the test tubes remain immersed for another 24 hours. Based on the measure of the bulk density after desiccation (at the beginning of the experiment), that of the bulk density in water and that of the bulk density of the test tube after being removed from water, you can calculate the bulk specific density and the true specific density. This test should be mainly considered as an identification test.

If the value of a sample received differs widely from the values given in the reference sample (for example the value given in a fact sheet), it is not the expected natural stone which was delivered to you.

Open porosity (NBN EN 1936)

Open porosity can easily be inferred by determining the calculation of bulk density and is expressed in percentage of the volume (calculated on the basis of the bulk densities measured in the previous test). This figure indicates the quantity (the volume) of open void in natural stone.

This quantity is in direct relation to the resistance and the susceptibility to absorption of liquids (for instance staining) of natural stone. The higher the porosity, the more easily the stone absorbs liquids (and pollution) and the more it is susceptible to staining.

This experiment should also be mainly considered as an identification test. If the value of the result you get differs widely from the values given in the reference fact sheet, it is not the expected natural stone which was delivered to you.

Resistance to compression (NBN EN 1926)

The test is done on at least 6 cubes with an edge length of 50 mm. The test is normally done perpendicularly to the quarry face or to anisotropy, identified by at least 2 parallel lines on the test tubes. The sides subjected to compression are rectified before so as to make them plane and parallel. After desiccation until constant mass is reached, you place the test tube between the two platens of a hydraulic press fitted with a ball joint.

The test tube is subjected to an increasing compression without any shock, with constant strain speed, until it breaks. From the maximum compression (that at which the test tube gives way, the breaking load) and the test tube surface, you can calculate resistance to compression (expressed in N/mm²).

Resistance to compression indicates to what extent natural stone can sustain compression strain (overload). With most natural stones, resistance to compression is high enough to be able to sustain most strains. The higher the resistance to compression is, the better natural stone resists overload.

Resistance to bending under centric load (NBN EN 12372)

The test is done on a minimum of 10 test tubes whose dimensions are 300x50x50 mm. The test tubes are first dried until constant mass is reached. The test tube is placed on two rollers distant one from the other by at least 5 times h and at most (length -2cm).

The load is applied via a central roller placed halfway between the two rollers. The load is increased consistently until the test tube breaks. Resistance to bending, expressed in N/mm², is calculated by the breaking load, the distance between the support rollers, the width and the thickness of the test tube.

Resistance to bending indicates to what extent natural stone can sustain a pressure (overload). The higher the resistance to compression is, the better natural stone resists overload. Resistance to bending mainly shows the resistance to distortion under load of stone matter.

Resistance to wear (NBN EN 14157)

European standard NBN EN 14157 regarding natural stone tiling ascertains Capon test as the European testing for resistance to wear. At the moment this testing method only exists as a pre-norm (prEN 14057).

In this test, a 70mm-thick disk turns in very specific conditions (with fixed rotation speed). The natural stone subjected to the test is drawn towards the disk’s edge thanks to a preset counterweight. After 75 revolutions, you measure the height of the disk imprint on the test tube (expressed in mm). The Belgian test method, which has been widely used, is Amsler test (NBN B15-223). This test consists in measuring the wear of a test tube subjected to the friction of a special cast iron table following a pre-set journey (generally 1000 m). Results are expressed in mm of loss of thickness for each journey done (mm/1000m).

On the occasion of revision of TI 205 of BBRI, a relation is being established between the two tests.
Resistance to erosion indicates the susceptibility of stone to erosion due to friction because of repeated crossing (for instance traffic). The higher the figures obtained for these two tests, the quicker the stone will become eroded. On the basis of the values obtained during wear tests, you can also classify stones according to their use on some premises, and for a determined use of the premises (intense collective use or individual lodgings).

This classification of natural stone for floor covering is indicated in TI 213 of BBRI, based on experience over several years and in situ observation.

Frost resistance (NBN EN 12371)

The Belgian norm NBN B 27-009 determines frost resistance for natural stones thanks to frost-defrost cycles. After desiccation until constant mass is reached, 5 test tubes of 200 mm x 200 mm x thickness of use are impregnated with demineralised water under a specific vacuum in relation to the intended use of the stone. If the stone is destined for an outside terrace for instance, it cannot show any visible damage after 25 cycles and impregnation under the highest depression (650 mm Hg), and the loss of the dynamic elasticity modulus cannot be higher than 20%.

The criteria in relation with the location of natural stone in the structure are indicated in the NIT 205 of BBRI. Then, the test tubes are placed horizontally in a tray of siliceous aggregates, the tray having thermal insulation and being drained, and you can see the upper part of the test tubes showing on the surface of the aggregates. The test tubes are then subjected to 25 specific frost-defrost cycles. At the end of these 25 cycles, the test tubes are visually inspected and potential damage is described (number and length of cracks, splinters...).

After being dried prior to impregnation with water, before the frost-defrost cycles, and after these cycles as well as after another drying, the test tubes are tested for their dynamic elasticity modulus. The difference in the dynamic elasticity modulus also serves as a gauge for the assessment of frost resistance of the material.

This frost-defrost test allows to control if a specific stone is appropriate for a given outside use (for example on a terrace, or cladding...).
A recently approved frost-defrost test (NBN EN 12371) looks very much like the French test. This test is also based on cycles of frost-defrost, but what is different in the Belgian test is that frost is applied on all sides (test tubes are not placed on siliceous aggregates) and that many more cycles are scheduled. Test tubes are prisms and not tiles. This norm does not yet include criteria in relation to outside use of natural stone (outside terrace, cladding...). Tests are being done in the BBRI to develop this kind of criteria.