ANISOTROPY OF THE STRZEGOM GRANITE

The ancient stonemasons of Strzegom knew well from which side and which angle to strike the stone so that it would break evenly. It was also no coincidence that the finished products were usually assembled in a certain way starting from the base. The stonemason knew in which direction the granite was stronger and from which angle it was easier to work.  

These observations are related to the anisotropy of granite, i.e. the different properties of granite depending on the method of examination. They are closely linked to the direction of deposition, i.e. the location in relation to where the stone was located before it was quarried. We can speak of different planes of anisotropy, which will serve as reference points in describing the various ways of examining the Strzegom granite.

For the purpose of this study, I have selected four most often tested functional properties of the stone. The result of these tests depends on the direction of measurement:

– flexural strength according to EN 12372

– compressive strength according to EN 1926

– capillary water absorption coefficient according to EN 1925

– tearing load in the pin hole according to EN 13364

Taking into account the fact that the individual tests have already been discussed in the past on the pages of Kurier Kamieniarski, we will not discuss their methodology here.

Flexural strength

This test is typically used for flat and oblong products such as kerbs, paving stones, floor tiles or veneer. Three possibilities are considered in the study: fracture load is directed perpendicular to the anisotropic planes (Figure 1), fracture load is directed parallel to the anisotropic planes (Figure 2), fracture load is directed perpendicular to the edges of the anisotropic planes (Figure 3).

Figure 1 Determining the bending strength under concentric loading (according to EN 12372)

Figure 2 Determining the bending strength under concentric loading (according to EN 12372)

Figure 3 Determining the bending strength under concentric loading (according to EN 12372)

Fig. 4 Determining the simple compressive strength according to EN 1926

Fig. 5 Determining the simple compressive strength according to EN 1926

Figure 6 Determining the tearing load in the pin hole according to EN 13364

Figure 7 Determining the tearing load in the pin hole according to EN 13364

Figure 8 Determining the tearing load in the pin hole according to EN 13364

  Table 1 shows that the drop in strength is significant.

Table 1. Results of the flexural strength test in different directions  

Compressive strength

Compressive strength is a characteristic property of cubic-shaped products, such as setts. In this test, we consider two possibilities of applying destructive force: perpendicular (Figure 4) and parallel (Figure 5) to the planes of anisotropy. The test results in Table 2 again indicate a significant reduction in strength.

Table 2. Results of the compressive strength test in different directions  

Tearing load in the pin hole

The purpose of this test is to check that a facade panel suspended from anchors will not be damaged at the place of anchor and that its use will not pose a hazard to the user. The test will be threefold: loads perpendicular to the anisotropy planes (Figure 6), loads parallel to the anisotropy planes (Figure 7) and loads parallel to the edges of the anisotropy planes (Figure 8). As can be seen from the data in Table 3, the force that must be applied to the mounting pin to cause it to break away depends, among other things, on the direction of the test.

Table 3. Results of the tearing load limit in different directions  

Capillary absorption

It is clear that capillary absorption differs for different planes of anisotropy. We consider two cases of capillary arrangement. Vertical to the water surface (Fig. 5) and horizontal to the water surface (Fig. 4). The photograph on the next page shows two samples placed in a cuvette of water for 60 minutes – depending on the arrangement of the capillaries, the rate of water absorption varied.

Table 4. Capillary absorption  

Modern technology and efficient machines allow for a wide variety of stone processing methods. The work that these machines do is no longer just the result of human hands. Perhaps that is why ancient stone techniques are falling into oblivion. The ancient stonemasons of Strzegom knew that we should care for more than just economy and yield, since the direction of the cut in relation to the rock layout has a significant impact on the workability and durability of the product and its subsequent appearance. Unfortunately, in current stone practice, anisotropy is often neglected and the yield of blocks and slabs seems to be the only parameter taken into account in stone processing. 

Source: Kurier kamieniarski

Author: Michał Firlej | Published: 07.05.2018