Aging tests in stone

Accelerated stone aging, artificial aging or durability tests are a set of tests based on an accelerated process to expose a material to environmental changes; these are made to predict the behaviour of the different natural stones against different environmental conditions, such as changes in temperature and humidity, exposure to soluble salts, etc.

Rocks, like all other materials, undergo changes when exposed to environmental conditions, more or less important depending on the type of stone and the aggressiveness from the climatic conditions to which it is exposed.

The durability is the measure of the stone’s ability to maintain its essential characteristics of appearance, strength and resistance to decomposition (ASTM, 2005), i.e. the time a stone can maintain its intrinsic characteristics during its use and it depends on:

The relationship of these factors should be considered; for example, an exterior pavement intended for pedestrian traffic that is in an area of ​​frequent frosts, can be treated with salt to avoid slipping, so it should be, in addition to being resistant to freezing, resistant to the crystallization of salts.

To predict how a stone will behave in a work and in a place, the European Committee for Standardization CEN / TC 246 Natural stone has developed several durability testing standards:

Considerations for the design of natural stone exterior flooring:

• Resistance to the freezing, determining the variation of the resistance to flexion after 56 ice-thaw cycles; for specific cases, the number of cycles is calculated based on ice index in the location and the number of useful life years defined by the designer.
• Resistance to thermal shock, determining at least the variation of flexural strength and apparent volume after 20 cycles of testing.
• Resistance to the crystallization of salts. It is determined in stone varieties with an open porosity of more than 5%, and whose floor is directly supported on soil containing soluble salts or with atmospheres rich in particles with this type of salts. Mass variations up to 1% are allowed after 15 test cycles.

Considerations for the design of a natural stone facade:

In addition to the considerations taken into account for the design of the exterior pavement (14 freeze/thaw cycles in this case), it must also be studied:

• Sensitivity to the changes in the appearance produced by thermal cycles, to evaluate the possible occurrence of oxidations, changes of tone, etc.
• Resistance of the marble to the cycles of heat and humidity, if any, since some varieties could exhibit situations of granular decohesion and give rise to warpings.

The granites in general, are characterized by their high durability presenting very good behaviour in the tests of aging; this is due, among others, to its low water absorption and open porosity.

 The excellent durability in addition to its aesthetic characteristics makes granite without hesitation the best choice for outdoor use.

Nuria Sánchez Delgado
Doctor in Geology
GRANITE TECHNOLOGICAL CENTER OF GALICIA