Resistance to Tension in the Flexion

The evaluation of the resistance of rocky plates to the rupture by flexion, is more and more important because of the modern covering techniques in floors and façades. These techniques involve fixation of the coverings through metallic bulkheads, without direct contact of the plates with the floor base or the façades structure. In both situations it happens load effort perpendicularly to the biggest surface of the plate, caused by the pressure of the wind on the façades and the weight of objects set on the floors. Also for benches, the resistance to rupture by flexion is a very important factor on qualification of stones.

As much the fractures of stones in general, as the structures of the anisotropic stones - mainly the ones from gneissification and isorientation of the micas - consist of physical and mechanical weakness zones, and determine significant variations in the tests results. The plates with ruptures and/or perpendicular and transversal structures to its larger surface are less resistant, than the ones carrying parallel structural plans to these surfaces.

The stones with natural trend of plan-parallel displacement of the plates, even used for detachment in course of mining, such as the case of slates and some quartzites, are more flexible and resistant to rupture by traction, than the other lithological types.

The flexion essays may be performed by means of two procedures: the ASTM-C99 norm (ABNT-NBR 12763), brought about with three cutlasses and called "flexion traction"; and, the ASTM-C880 norm, performed with four cutlasses (two of action and two of reaction), called “flexion resistance”. The dimensions of the test bodies, used for each one of the procedures and the specifications of its results, are distinct.

There are specific software for dimensioning of plates on façades, providing parameters to the variables involved in the architectural project, and chiefly the flexion component of the selected stones. The enduring dimension of plates can then be compared, with the minimum demanded thickness for a reliable work.