PSI - Issue 22

184 J. Gonçalves et al. / Procedia Structural Integrity 22 (2019) 181–188 J. Gonçalves, C. Oliveira, C. Reis, A. Soares, P.Silva / First Internati nal Symposi m on Risk and Safety of Complex Structures and Components (2019), Porto, Portugal 2.2. Mechanical Surface Finishes Granite sawing: The sawing is performed on a gangsawn. In this device the blocks are placed and cut through a constant speed back and forth horizontal movement and a downward movement of the blades, in which a variable speed determined by the user is defined. In this process a water dispersion system is indispensable, which is used for cooling the blades, spreading and to remove debris from the cutting zone (Frazão, 2005). This is the most economical finish, and the resulting surface is neither smooth nor completely slip resistant, making it suitable for exterior coatings. After sawing the granite another surface finish can be applied. Following is the characterization of some surface finishes used in the industry. Polishing : is a surface treatment widely used in granite because it highlights the colours and petrographic textures. The polish process is performed with abrasives of different roughness (Transgranites uses abrasive sequences that vary, in decreasing grain size, - 120, 220, 320, 400, 1200 mesh and gloss). During this process, pores in the rock usually close because the existing voids are filled with the crushed material. This process makes the material more resistant to external aggression as it reduces the absorption capacity of water. The result of this treatment is a smooth and shiny surface with a noticeably darker shade than that the one obtained with other treatments (Rojo et al., 2003 - Pietro, B. et al. 2011). This type of finish allows displaying the original coloration of the material thus transmitting the nobility and refinement, revealing all the beauty and aesthetics of the natural stone. Honing : The process of honing an ornamental rock is very similar to polishing and the main difference is that the surface does not acquire gloss (this surface finish is also performed on polishers). This finish uses the same abrasives as polishing, except that the polishing abrasive (Pietro, B. et al. 2011). This finish gives a smooth and flat surface that shows the true coloration of the materials without the shine and the detail of the polished finish. Flaming : flaming is performed heating the sawn surface with a blowtorch, using a mixture of propane gas and oxygen. Thermal shock causes some mineral grains to be eliminated from the surface. The result is a rough surface with a glassy appearance. This finish has some limitations as its uses in brownish or yellowish granites (the granites look like this due to the presence of ferrous minerals) causes them to change the surface appearance heterogeneously as a result of chemical reactions. (Pietro, B. et al. 2011). This finish is unique in its effects because it makes surfaces non-slip while attenuates the original colour of the material, giving it several variations. Shot blasting : Steel blasting is used to perform the blasting projecting onto the stone surface. The degree of blasting varies with several factors: - Diameter of steel balls; -Speed attack -Speed of the steel Balls; -Advance of the plate in the shot blast. This finish allows obtaining rough surfaces and the resulting material is being used mainly in exterior coatings. (Sousa, L. 2012) 2.3. Colour Measurements The colour of the rocks depends on their petrographic characteristics: mineral composition, texture and structure. The minerals present are of prime importance, especially the colouring substances they may contain. Iron oxides, which give a reddish colour, are the most common pigments. Grain size influences colour to a lesser extent; however, finer grain rocks have more homogeneous colours. Hue (colour), lightness (light / dark) and saturation (purity) are variables that allow us to classify colours (Konica Minolta, 1998).