Natural Stone Tile Selection and Installation Guide
This standard addresses natural stone tiles that are adhered to a substrate. Natural stone tile, also called dimension stone, is stone that has been harvested from its in-situ position in the Earth and cut and machined into tiles without altering the natural fabric of the material. Dimension stone has a nominal thickness of less than 3/4 (19 mm) and does not have any facial dimension greater than 2 (610 mm). Agglomerates and other engineered products do not meet this definition and therefore are excluded. Natural stone tiles that meet the definition but not the dimensional requirements are also excluded.
Stone tiles are generally supplied in a gauged, or calibrated, thickness, which generally has a tolerance of 1/32 (0.8 mm) in thickness. Tolerances on face sizes will generally be 1/16 (1.5 mm), with some tiles available in tighter tolerances.
General
Stones, referred to as rocks by Earth scientists, are formed by three different methods. Sedimentary rocks are formed by clasts, or particles that settle and are then cemented together by another agent. Limestone, sandstone, travertine, and onyx are examples of sedimentary rock. Metamorphic stones are those that have undergone a change in composition resulting from intense heat and pressure, typically, a crystallization or recrystallization of the original rock fabric. Marble, slate, serpentine, quartzite, gneiss, and soapstone are examples of metamorphic rocks or stones. Igneous stones are formed by solidification, by cooling of molten or semimolten material. Igneous materials can be intrusive, meaning they cooled below the Earth's surface, or extrusive, meaning they cooled above ground. Granite and basalt are examples of igneous rocks or stones.
Of equal importance to the method of formation is the primary chemical makeup of the stone they are either calcareous or siliceous. Calcareous rocks, such as marble, travertine, and limestone, are made up primarily of calcium carbonate mineralogy. Because calcium carbonate, or the calcite crystal, can be attacked by relatively mild acids, such stones are predictably vulnerable in acidic environments.
Considerations When Selecting Natural Stone Tile
Water-Sensitivity
Many green-colored marbles and serpentines warp, or curl, when exposed to water or a water-based adhesive such as a cementitious mortar. To avoid having stones warp from the moisture in such setting materials, an epoxy adhesive bond coat, which does not contain water, may be required. Because some green marbles and serpentines do not warp, or only mildly exhibit this behavior, research is encouraged on the particular stone species selected. Some can be successfully installed with water-based portland cement mortars. For stones that are mildly water-sensitive, consider mixing the mortar with reduced water volumes as allowed by the mortar manufacturer or wetting the exposed surface of the tiles after installation to provide balanced saturation levels on both surfaces. There are a few stones other than green marbles or serpentines that will warp with unbalanced water exposures, so research is encouraged on any particular stone with which one has no direct experience.
Fiberglass Mesh Reinforced Stone
Many stone products have a mesh reinforcement adhered to the back surface of the tile. The adhesive used to attach the mesh is not standardized, but is commonly an epoxy or polyester-based adhesive that portland cement mortars will not reliably bond to. Such reinforced stone tiles require epoxy bonding mortar.
Filled Voids in Travertine
Travertine typically has voids, or holes, which are commonly filled at the fabrication site with cementitious or resinous filler. Travertine can be supplied in a vein cut, where the slab is sawn perpendicular to the bedding planes to expose the veined character, or in a fleuri cut, also called crosscut, where the slab is sawn parallel to the bedding planes to yield a random, flowery appearance. Particularly for fleuri cut tiles, many voids can exist just below the finished surface, concealed only by a slight shell of stone material. This concealment prevents detection and filling of the voids in the factory. Once in service, a concentrated load or point load will fracture the thin shell of stone, exposing the void below. High-heeled shoes and wheeled carts are examples of common causes of this occurrence, and maintenance to fill voids as they appear over time will be required when such stones are used.
Pinholes in Marble
On a much smaller scale than the voids found in travertine, many marbles have pinholes visible in the face of the material. Frequency can be as high as 100 or more per square foot. This is not considered a defect, but rather a natural characteristic of the material. Proper sampling is required to be aware of these occurrences prior to commencing with installation.
Viewing/Inspection Distance
As products of nature, dimension stones will have inherent characteristics that may be considered objectionable to some viewers. Inspection of the material should be done at reasonable distances, similar to how the material is viewed by building occupants. It is recommended that samples, range samples, mockups, and finished work be viewed for inspection at a distance of 6-1/2 from a position normal (perpendicular) to the stone face, and with natural lighting. Lighting of reduced angles of incidence, for instance down washed lights, can accentuate lippage and warpage that is within industry tolerances. Such lighting should be turned off for inspection purposes. See also Flatness and Lippage.
Variation of Abrasion Resistance in Patterned Floors
When two or more stone species are used together in a floor pattern, it is possible to have different rates of wear when the materials have different resistance to abrasion. If any of the materials have an abrasion resistance less than 20.0 when tested per ASTM C241 or C1353, the difference between the abrasion resistance of any two stones being used together should not differ in abrasion resistance by more than 5.0.
Acid Sensitivity
Stones resistant to acid are required where acid exposure is likely. Most notably, calcium carbonate and calcite-based stones will react to relatively mild acids like lemon juice, vinegar, and cut tomatoes. The result is not a stain, which could be removed, but an etching of the stone surface, which cannot be removed without refinishing the stone. Acid attack of polished stones will usually manifest itself as a dull spot within the surrounding glossy surface.
Anisotropy
An anisotropic material is one that has different physical or visual properties in different directions. Most stones, particularly sedimentary stones, have some degree of anisotropy. Therefore, it is common that they will have a strong axis and a weak axis. Attention must be paid to this feature, particularly when reviewing test results, to ensure that the direction tested is the same as the direction used.
Delamination
Riftis the direction along which a stone is most easily split, and many stone varieties have pronounced bedding planes, along which a predetermined rift exists. Bedding planes can be a region of weakness, and some stones, particularly sandstones and some slates, will spall or delaminate along bedding plane boundaries after installation. Delamination is when a portion of the stone separates from the rest of the stone typically the surface dislodges from the bottom, which remains bonded in place. Differential mineral expansion due to weathering and thermal cycles accelerate this occurrence, and freeze/thaw cycling is an additional factor. Additionally, floor traffic can cause spalling and delamination. Delamination is not related to any failure in adhesive; and research is required to verify the adequacy of stones with bedding planes for the intended application.
Shading and Variation
As products of nature, dimension stone will have inherent variation in color, shade, and character, including markings and veining, throughout any given lot of material. Variations from piece to piece are characteristic of the material and acceptable. Because of this variation, several pieces should be supplied as a sample, and because the material from a quarry can change over time, ensure samples are current before they are specified. Depending on the stone, it may be impossible for samples to represent all possible variations in color and character. Variations can be greater than or less than what is expressed by the sample pieces. The range of stones actually supplied should be reviewed and accepted prior to their installation. Dry laying pieces before installation helps ensure a pleasing blend of the tiles prior to permanently installing them, but is not always feasible, nor is doing so required unless specified in writing by the project superintendent or owner.
For viewing and installation, it is recommended that the material be blended from multiple packaging units to distribute the variation throughout the project and to avoid a blocking effect, where an area of one shade is distinctly bordered or surrounded by a different shade. For larger projects, it may not be feasible to view pieces from the full lot being supplied for the project. The project superintendant shall coordinate with the tile contractor to determine initial and subsequent viewing and acceptance procedures. As the owner's representative on the jobsite, the superintendant is responsible for checking the installation as it progresses and identifying shading issues immediately. If the superintendant does not initiate such procedures, the contractor is responsible for blending tiles, but not for future issues arising over shade differences or individual stone pieces supplied. Distributors and suppliers typically will not address issues of shade variation and acceptability after installation.
Ensure adequate quantities are ordered, including requirements for waste, breakage, and attic stock. While some stone deposits are uniform through hundreds of feet of depth times vast lateral distances, there are also quarries where the interblock, and even intra-block variability are so great that a given lot of material cannot be matched one month later.
Soundness of Marbles
Marbles are classified according to soundness based on the level of repair or reinforcement commonly required to facilitate their use. The soundness classifications A, B, C, and D are in descending order of soundness and ascending order of reinforcement and/or repair requirements. These classifications have no bearing on the commercial value of the stone. The lesser soundness classifications of C and D include many of the aesthetically prized, decorative marbles. However, these stones typically have too much repair or reinforcement to be used in wet or exterior areas. For more information about marble soundness, refer to the MIA's Dimension Stone Design Manual.
Efflorescence
Efflorescence results from migration of soluble salts originating from soils or masonry. These salts are carried to the surface where the water evaporates, leaving a white residue. Natural stones are not generally the source of efflorescence; however, they can sometimes provide efficient paths for the migration. In some high-porosity or open-veined stone varieties, efflorescence can travel through the stone tile in addition to traveling through the grout joint. Cleaning the salt deposit is generally easy and mechanical methods such as brushing are usually effective, but this only removes the symptom. The efflorescence will reoccur unless the source (water infiltration) can be eliminated.
Water Spots
Differing water chemistries may spot a given stone, which is generally most visible on dark, polished stones. This deposit will be on the surface of the stone, and can normally be removed mechanically by buffing with #000 steel wool, but in some cases requires buffing with a polishing powder.
Stone Finishes
A variety of standard stone finishes are available as well as some proprietary finishes. Only textured finishes are recommended for outdoor or wet area applications, for pedestrian safety. While the texture enhances the safety aspect of the walking surface, it may complicate the maintenance by requiring brushes for cleaning, as soft mops may snag by some of the sharper profile finishes. For more information on stone finishes, refer to ASTM C119 Standard Nomenclature for Dimension Stone and the MIA's Dimension Stone Design Manual.
Iron Staining
Some stones, slates in particular, contain soluble iron that can cause bleeding if installed in a wet area. The use of a sealer will not prevent this from occurring. Yellowing of light-colored marbles may also be due to iron oxide.
Mohs Scale of Relative Hardness
The Mohs scale of relative hardness is based on 10 minerals. Mohs scale is one of relative hardness, not absolute hardness, meaning it is not linear. By example, orthoclase is harder than apatite, which is harder than fluorite, which is harder than calcite. However, orthoclase is not twice as hard as calcite even though 3 doubled is 6. For perspective, a human fingernail would be about 2.5 on the Mohs scale, gold and silver would be 2.5 to 3, a copper penny would be about 3.5, an iron nail would be about 4, window glass would be about 5.5, and aluminum oxide abrasive would be about 8.
Mohs Scale of Relative Hardness
Hardness Mineral
1 Talc or Mica
2 Gypsum
3 Calcite
4 Fluorite
5 Apatite
6 Orthoclase
7 Quartz
8 Topaz
9 Corundum
10 Diamond
Considerations When Selecting Installation Materials Staining
Light-colored marbles and nearly all onyx and limestone must be installed with white mortars due to their susceptibility to staining from the grey portland cement components in grey bonding mortars.
Although silicone itself does not stain natural stone, silicone sealants can contain plasticizers that can wick into the stone and stain it. This is more problematic for higher-porosity stones, but can occur in dense stones as well. Some silicone manufacturers keep a database of projects where their sealants have been used with various species of stones so compatibility can be verified. In the absence of an exemplar project to view, compatibility can be verified via accelerated laboratory testing.
The use of dyes in grouts, epoxies, polyesters, and any other joint fillers should always be tested to verify that the dyes will not leach into the stone, causing a picture frame stain. This is most common when using materials of contrasting color, although dyes of matching color can also cause staining.
Sanded and Unsanded Grout
The sand in sanded grout is generally harder than any of the calcium-carbonate-based stones, such as limestone, travertine, marbles, and onyx. Masking the stone surface or the use of unsanded grout should be considered to avoid scratching the surface finish. This is particularly true of these stones when polished. If unsanded grout is required, the grout joint must be 1/8 or narrower.
Impregnators
It is a common, although not universal, practice to treat natural stone installations with impregnating repellants, often referred to as sealers, although this is a misnomer. The products typically used on natural stones are not truly sealers, in that they are formulated to allow the transmission of vapor, which maintains the breathability of the stone. The use of some impregnators with a particular stone species may create an undesirable color alteration, typically darkening. There area also products marketed as color enhancers, which are intentionally formulated to darken colors and accentuate natural veining and character of the stone.
All Information comes from Tile Council of North America Handbook
