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Selection of Stone Type and Finish


1 Quarry assessment. Prior to selection of the stone, it is advisable to obtain reliable information to determine if the quantity and the largest stone sizes required are readily available from the quarry source. Rates of block production on seasonal basis shall be evaluated. The uniformity of the color, texture, and physical properties of the rock must be checked as well. It is also advisable to establish a mutually acceptable range of color and texture to prevent possible later dissatisfaction or dispute.

2 Manufacturing plant assessment. Past and current performance on similar projects shall be evaluated. Fabrication capacity within the required time frame shall be examined. Crating and transportation facilities should also be examined. Quality control during fabrication must be insured.

3 In addition to aesthetic considerations, the selected stone has to resist possible present and future environmental attacks during the planned life span of the building. Exposure to weather may cause changes in shade or coloration. Polluted air, acidic and sulfuric rainwater may cause changes in appearance. Serious and repeated environmental attacks, combined with freeze-thaw action may cause spalling and slow deterioration if the improper type of stone and/or finish is selected.

3.1 Failure investigations and research work do not justify the use of thin marble veneer for exterior cladding high-rise buildings in an environment where air pollutants, carbon dioxide (C02) or sulfur dioxide (S02) are present. Atmospheric water dissolves these gasses creating "acid rain" which, in turn, can cause degradation of the marble veneer.

3.2 Freeze and thaw cycles can also change the original characteristics of the thin marble veneer. Due to thermal expansion or contraction, and moisture, some of the thin (1" to 1-1/2", or 2.54 to 3.81cm) crystalline marble tile will release their stress of geological origin and when cooling off or warming up, will not fully return to their original position (hysteresis).
The volumetric content of natural cementation in the marble and the size of the crystals may play important roles in the moisture activated bowing of the thin marble veneer.

3.3 Warping is also caused by unequal moisture absorption which happens when one side of the slab stays dry while the other side becomes wet. This type of warping can be prevented by providing adequate ventilation, using weep slots (weep holes) to keep the cavity, behind the marble veneer, dry.

4 In selecting panel sizes, consideration must be given to the capability of the quarry and the fabricating plant to economically produce the selected sizes and thicknesses.

The selected thickness shall be proportionally adequate for the panel size, anchoring system and the finish, without losing sight of economic considerations. Where wind loading criteria is established by the specification, the selected thickness as well as selected anchoring system shall be substantiated with calculations by a licensed Professional Engineer. Tests shall be conducted by an independent testing laboratory.

4.1 Thermal (flamed) finish will effect the strength and durability of thin granite veneer. Flame treatment of granite will produce a type of finish which is desirable to many architects and building owners. However, it will reduce the effective thickness of the thin granite tile, together with its bending strength. This may become critical for the long term durability of the thin granite veneer.

Freeze and thaw cycles of flame-treated thin granite may alter the always present micro-cracks to macro-cracks*, making it more vulnerable to further deterioration, permitting absorption of water to a depth of about 1/4" which may freeze within the flamed thin granite tile.

*MICRO FRACTURES were formed when the molten rock (magma) came to the surface of the earth and it cooled off. Micro fractures can not be seen by the naked eye.

*MACRO FRACTURES have very limited depth and width, however they are visible with the naked eye. Macro fractures do not impair the structural integrity of the intended use of the rock.

4.2 Functionally, a polished finish is the most desirable finish of thin granite veneer used for exterior cladding. Polishing procedures close the pores of the thin stone slab, protecting its surface from deterioration caused by hostile environmental weathering conditions.

4.3 Polished granite and marble are recommended for interior vertical walls.

For interior flooring, polished granite is preferred, but good quality polished marble is often used. The use of flamed, honed, sawn or sandblasted oolitic or dolomitic limestone for wet room flooring such as baths or showers is NOT recommended.

It is NOT desirable to combine granite and marble for a floor pattern due to the difference in abrasion rate. If repolishing is needed, it will be difficult to handle with mixed materials.

In the selection of marble flooring material for high traffic areas, the quality of the marble pavers and their resistance to heavy foot traffic should be carefully considered. Stratified marble or conglomerates are often contaminated with clay, sand, and other such minerals, which after being subject to foot traffic and maintenance procedures, may become fissures on the finished surface.

*FISSURES are narrow openings in the rock, having occasionally more depth than width. Fissures are very common in travertine marble and are caused by entrapped gases.





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