EN 12326 - The bendinging strength

With this test one can make statements how a slate resists against bending or flexural force. The bending strength of a slate mainly depends on the morphology of the fracture cleavage or mica layers, respectively. The smoother and denser the mica layers the higher the bending strength. On the other hand, there has to be sufficient hard and rigid minerals so that the slate resists against the load. A low spacing (=number of mica layers per mm) and the occurrence of carbonate can lower the bending strength and lead to a easier breakage of a slate.
According to the experiences, the average bending strength of a slate lies between 50-80 MPa. Compared to other rock types, roofing slates have the highest bending strength.

The determination of the bending strength is defined in the European Norm EN 12326-2 and is valid for all European countries. In this norm the preparation of the sample, measurements of the thickness, performing of the tests, the type of the results and the content of the test report is described.
For each sample the decrease of load and the bending strength has to be calculated. In addition, for each series of tests the average bending strength R has to be calculated from the single bending strengths. Afterwards, the characteristical bending strength R_c has to calculated. The average value and the standard deviation of the results has to compared by means of a two-tailed student's-t-test with a level of significance of 95%.