Section 5: Types of Tiles for Mass Transit Applications
5.1 Why Use Ceramic Tile in Mass Transit Applications
Mass transit floor applications can be exposed to some of the harshest and most extreme conditions of any system in a building structure. In general, many types of ceramic tile, porcelain tile, quarry tile, pavers, klinker tile, brick, and granite stone pavers are suitable for these applications. However, there is no standard formula or recommendation for the selection of these finishes types. Selection must be made by an assessment of the individual finish material’s functional and aesthetic characteristics in relation to the performance requirements. A discussion of the aesthetic merits of different finish materials is highly subjective and beyond the technical focus of this manual.
Why select ceramic tile as the finish material for these applications?
Ceramic tile adds value to any structure, requires very little maintenance and costs less per square foot (m2) than other permanent and long use flooring and wall options.
The following are some beneficial features of ceramic tile cited by the Tile Council of North America:
Low Life-Cycle Costs – ceramic tile costs less per year than all other flooring finishes over the life of a building. In fact, ceramic tile can cost up to 5 times less than other finish types over the life of a building.
Durability – maintains its original qualities and lasts longer than other cladding options
Water Resistant – ceramic tile installations can be waterproof and are suitable for use in areas exposed to wet or damp conditions.
Clean and Healthy – easy to clean and resistant to dirt, grime, mold and bacteria.
Green Product – ceramic tile is inherently green and an important part of sustainable construction. The use of ceramic tile can contribute towards Leadership in Energy and Environmental Design (LEED) credits and other green building certification programs.
Good Indoor Air Quality – ceramic tile is inherently good for the indoor environment. It has negligible volatile organic compound (VOC) emissions. Ceramic tile is virtually inert, with no off-gassing.
Low Maintenance – ceramic tile is virtually maintenance free, as it resists dirt, grime and stains.
Fade Resistant – properly rated ceramic tile will not fade in sunlight like the dyes used in other floor and wall cladding finishes.
Fire Resistant – non-combustible and will not give off toxic fumes when exposed to fire.
Exterior Use – fade-resistant, frost-resistant, durable, enhanced traction and low maintenance are all characteristic of ceramic tile finishes that makes it the preferred choice of specifiers and designers.
In fact, an addendum to the 2009 LEED Reference Guide for Green Building Design and Construction states that mineral-based finish flooring such as tile, masonry terrazzo, and cut-stone without integral organic-based coatings and sealants…qualify for credit without any IAQ testing requirements. However, associated site-applied adhesives, grouts, finishes, and sealers must be compliant for a mineral based…flooring system to qualify for credit.
This section will focus primarily on the functional criteria necessary to determine whether a finish material’s physical characteristics satisfy the performance requirements of a mass transit application’s design and location. While every application can be unique, the following are criteria that can be used to determine general functional suitability of the finish materials:
5.2 Selection Criteria for Finished Materials
Low Water Absorption Rate
Thermal Movement Compatibility With Adhesive and Substrate
High Breaking Strength
Chemical Resistance
Thermal Movement and Shock Resistance
Adhesive Compatibility
Dimensional Stability (Heat and Moisture Insensitivity, Moisture Expansion)
Frost Resistance (Where Required)
Dimension and Surface Quality/Tolerance
Low Water Absorption Rate
The rate of water absorption is one of the most significant physical characteristics. This characteristic provides an indication of material structure and overall performance, and has significant influence on many other physical characteristics that are desirable for mass transit applications.
Water absorption, also known as porosity, is defined as a measure of the amount of water that can be absorbed through pores of a material. The absorption rate is measured as a percentage difference between tested dry and wet (saturated) weight of the material. Generally, the lower the water absorption rate the greater the frost, stain, chemical, abrasion and breaking strength resistance; all desirable qualities for a material exposed to heavy traffic and loads. Tile and stone absorption rates less than 3% would be considered suitable for mass transit floor and base applications. Quarry tile, brick pavers and stone should have an absorption rate of less than 3%. Porcelain tiles and pavers generally have an absorption rate of less than 0.5%.
Thermal Movement Compatibility
The tile or stone’s rate of expansion and contraction due to temperature changes must be relatively compatible with the tile/stone adhesive mortar. Significant differences could cause excessive stress in the adhesive interface and lead to delamination or bond failure (see Section 7). Minor differences in thermal compatibility are acceptable and the selection of a suitable flexible adhesive (see Section 7) plays a critical role in distributing minor differential movement. Accurate prediction of thermal behavior is extremely complex, considering the rate and fluctuation of temperatures, thermal gradients and lag that exists, plus other potential factors. Figure 5.1 shows typical rates of thermal movement of materials commonly used.
High Breaking Strength (Modulus of Rupture)
The breaking strength resistance of a finish material is important primarily due to the type of handling that is necessary for installation within or on a structure. Once adhered in place to a suitable, rigid substrate, a properly installed tile or stone has up to ten times the breaking strength resistance compared to the unbonded tile or stone alone. The natural fragility and cleavage of many type of stone makes them particularly susceptible to breakage. Because the direct adhered method of installation allow relatively thin tile or stone to be used, a careful assessment of breaking strength relative to the tile or stone’s thickness and dimension (facial area) will eliminate unforeseen call backs, high waste factors and increased costs.
Chemical Resistance
The finish materials must have good chemical resistance to prevent deterioration from environmental pollutants, spills, and chemicals that may be used in cleaning and maintenance not only of the tile or stone, but also other components of the buildings structure. Keep in mind the application, using a polished marble in an airport concourse would not be a great idea because inevitably someone will spill a cola based drink that will quickly etch the surface of the marble. Porcelain tile is a much better choice for these conditions.
Thermal Shock Resistance
Any tile or stone installation which may be exposed to a wide range of temperatures, and a rapid rate of temperature change, requires that the entire installation be able to accommodate the movement caused by thermal shock. Thermal shock refers to the rate and range of temperature fluctuation within short periods of time. For example, an airport granite facade in Texas, with a southern or western solar orientation, exposed to a sudden cool rainstorm can send the temperature of a cladding material plunging within a matter of minutes. The stresses caused by this sudden and rapid thermal movement may have a detrimental effect on the stone façade if proper knowledge of the thermal movement capabilities of the stone are not known and proper allowance for this movement are not taken into consideration.
Compatibility with Adhesive
The suitability of adhesives for the proposed application must be evaluated taking into consideration the criteria listed in Section 7 – Types of Mortars / Adhesives / Grouts. Part of that process is evaluating an adhesive’s compatibility with the tile or stone composition, surface texture, and other physical characteristics, such as translucency. For example, lighter colored marble stones are translucent, and the transmission of color from the underlying adhesive can have significant aesthetic consequences. Similarly, adhesives should not stain the cladding material, or contribute indirectly to staining by solubility or reaction of chemicals with water. For example, certain silicone or urethane adhesives may be absorbed by stone causing permanent discoloration. Polymers of some latex additives which are not intended for exterior applications may be soluble in water and could cause staining problems. Another concern is a calcium chloride accelerant that may be used in some latex cement adhesive mortars. This additive could contribute soluble salts to the system and result in efflorescence after repeated water infiltration to the adhesive layer. Depending on the texture and porosity of the tile or stone’s bonding surface, certain adhesives may require more or less dwell time in order to allow absorption of adhesive, a process known as “wetting out” a surface.
Dimensional Stability (Moisture and Heat Sensitivity)
Generally, the dense and compact nature of a low absorption material will impart good dimensional stability to a material, thereby making the finish material suitable for a mass transit application. However, there are certain exceptions where low absorption is not necessarily an indicator of dimensional stability. Certain types of marble and agglomerates, while water absorption rate may be favorable, exhibit internal crystal growth when exposed to moisture and can warp, spall or deteriorate rapidly when exposed to the weather in exterior applications. The plastic resins used in many agglomerates have a significantly higher rate of thermal expansion when exposed to the sun or other heat source. Similarly, clay brick can undergo permanent volume expansion after prolonged exposure to moisture.
Frost Resistance
Generally, frost resistance is a function of the water absorption characteristics of a tile or stone. Any cladding material with water absorption under 3% is typically considered frost (freeze) resistant. However, the pore structure of brick and certain stone may allow water absorption greater than 3% and still be considered frost resistant. Nonetheless, high water absorption will still reduce durability and resistance to weathering in general. Polishing of a stone surface can reduce surface porosity and increase resistance to weathering.
Dimension and Surface Quality
Ceramic tile and thin brick masonry are manufactured materials, and therefore dimensional and surface tolerances required for direct adhesion can be assured by selecting materials in compliance with established standards. For ceramic tile the applicable standards would be ISO 10545-2 "Standard for Dimension and Surface Quality" and ANSI A137.1, which incorporates ASTM C499 “Standard Test Method for Facial Dimensions and Thickness of Flat, Rectangular Ceramic Wall and Floor Tile”. For thin brick, ASTM Standard C1088 Type TBX “Standard Specification for Thin Veneer Brick Units Made From Clay or Shale” governs dimension and surface quality. Stone is generally fabricated to specification for a variety of methods of installation. There are uniform standards for dimension and surface quality of stone tiles or slabs listed for individual varieties of stone in Section 5.10. It is recommended that the back side of an exterior cladding material have a key back or dovetail configuration in order to develop a mechanical lock with the bonding adhesive (or concrete in the case of negative cast pre-cast concrete panels). Grooved or rib-back tile or stone will also improve the factor of safety in the event of adhesive bond failure. Ceramic tile manufacturers currently offer this technology, primarily with ceramic tile manufactured by the extruded method. Porcelain tile manufacturers are expanding this concept to thinner (as thin as 1/8" [3 mm]) and larger module tiles (as large as 4′ x 8′ [1.2 x 2.4 m]) manufactured with the dust pressed method.
5.3 Ceramic Tile
The beauty, durability, and functional qualities of ceramic tile make it one of the most suitable finishes for mass transit applications. While some other finish materials (e.g. terrazzo, stained concrete, etc…) may possess some of these qualities, none are as versatile and affordable as ceramic tile. As you might expect, there are an extraordinary number of different types and sizes of ceramic tile. Ceramic tile cladding can range in size from 1" x 1" (25 x 25 mm) nominal mosaics up to 48" x 48" (1200 x 1200 mm) nominal porcelain stoneware tile. The raw materials for ceramic tile are a mixture of clay (to give plasticity), quartz sand (to give structural strength and act as an economical filler), and carbonates or feldspars (to provide fluxing/fusing action). Glazes are formed from sand, kaolinitic clay, prepared glasses (frit), and oxide based pigments to provide color. The raw materials are ground together with water added. The raw material for the majority of ceramic tiles used for mass transit applications are typically dried to a moisture content of 4–7% and shaped by the dust pressed method at pressures of 300 kg/cm2 or higher. Some tiles used for mass transit facilities may be formed by the extrusion method, where clay with a moisture content of 15–20% is extruded through a die of desired shape. After forming, the raw tile or “bisque” is dried to remove excess water and fired in kilns operating at temperatures of 1,750 – 2,200°F (954 – 1200°C). This results in the vitrification or fusing of the clay and fillers, producing a tile product that is dense and non-porous. As mentioned previously, low water absorption is a key physical characteristic of external cladding materials, and has significant influence on the other physical characteristics.
Characteristics of Ceramic Tile
In order to select the most suitable type of ceramic tile or pavers for a mass transit application, and to understand the technical considerations for adhesive compatibility and installation, the specifier must have a general understanding of the classifications and physical properties of ceramic tile.
Water Absorption (Body of Tile)
The definition of water absorption is the measure of the amount of water that can be absorbed through pores of the ceramic tile. This characteristic is an indication of ceramic tiles’ structure and overall performance. Water absorption is measured by ASTM C373 “Standard Test Method for Water Absorption, Bulk Density, Apparent Porosity, and Apparent Specific Gravity of Fired Whiteware Products” and ISO 10545–3 “Determination of Water Absorption, Apparent Porosity, Apparent Relative Density, and Bulk Density” as a percentage difference between dry and wet weight of tile. The water absorption characteristics of ceramic tile have significant influence on many other physical characteristics that are important to proper performance in industrial applications. Water absorption of ceramic tile for mass transit (exterior and interior) applications should be 3% or less. However, precision manufacturing processes and advancements in technology now allow for production of porcelain tiles with under 0.05% (negligible) water absorption rates. While this creates an extremely durable product, it makes adhesion with traditional portland cement adhesives extremely difficult, because these types of adhesives rely on absorption of cement paste to provide mechanical locking of crystals within the pore structure of the tile body. Porcelain tiles require the additional adhesive power of latex thin-set mortars or epoxy adhesives in order to develop the high bond strength and flexibility required for mass transit applications. Currently, porcelain is the most popular ceramic tile choice for mass transit, commercial, industrial and other high traffic installations.
Thermal Shock
The definition of thermal shock is the resistance to internal stress when a tile undergoes rapid changes in temperature. The significance of this characteristic is that it provides an indication of good performance in demanding mass transit applications where there are constant cycles of thermal shock. Thermal shock is measured by ASTM C484 “Standard Test Method for Thermal Shock Resistance of Glazed Ceramic Tile” and ISO 10545-9 “Determination of Resistance to Thermal Shock” where there are no defects after 10 cycles of sudden temperature changes to and from 60 to 220°F (15 to 105°C). Many mass transit applications can experience sudden temperature changes on a repeated basis. Hot or cold liquid spills can subject the tile to thermal shock. Therefore, this consideration of thermal shock is critical in determining the suitability of a tile for the intended purposes.
Thermal Expansion/Contraction
The definition of thermal movement is the amount of expansion or contraction a tile undergoes from temperature changes. The significance of this characteristic is that tiles expand with temperature increases, and contract with temperature decreases. The measurement of a tile or stone’s thermal coefficient of expansion provides the designer with the information necessary to determine compatibility of the tile with the substrate and adhesive materials, to calculate movement differentials, and for the design of movement (expansion) joints. Thermal expansion is measured by ASTM C372 “Standard Test Method for Linear Thermal Expansion of Porcelain Enamel and Glaze Frit and Fired Ceramic Whiteware Products by the Dilatometer Method”, and ISO 10545–8 “Determination of Linear Thermal Expansion” and expressed as the linear coefficient of thermal expansion in units of in/in/°F (mm/m/°C).
Frost Resistance
Frost resistance measures the ability of the ceramic tile to resist the expansive action of freezing water. This characteristic is dependent on the tile absorption rate and the shape and size of pores. Frost resistance is measured by ASTM C1026 “Standard Test Method for Measuring the Resistance of Ceramic Tile to Freeze-Thaw Cycling” and ISO 10545-12 “Determination of Frost Resistance”.
Breaking Strength (Modulus of Rupture)
Breaking strength primarily determines resistance to the handling and installation process. This characteristic is a measure of the tile material and not the tile itself. For example, if you compared two tiles of the same material with one being twice as thick, both would have the same unit breaking strength, but the thinner tile would require 75% less load or force to break. Impact resistance in service (fully adhered) is approximately 10 times greater than the minimum standard. Breaking strength is measured by ASTM C648 “Standard Test Method for Breaking Strength of Ceramic Tile” and ISO 10545-4 “Determination of Modulus of Rupture and Breaking Strength” which requires a minimum strength for all floor tile of 250 psi (1.75 MPa). Minimum breaking strength requirements for tile can be found in ANSI A137.1 “American National Standard Specifications for Ceramic Tile”.
Moisture Expansion
Moisture expansion is the dimensional change of building materials as a result of changes in moisture. This is a significant characteristic for ceramic tile used in mass transit applications, because moisture expansion of clay is irreversible. It is measured by ASTM C370 “Standard Test Method for Moisture Expansion of Fired Whiteware Products” and ISO 10545–10 “Determination of Moisture Expansion”. Moisture expansion is directly proportional to absorption; the lower the absorption, the greater resistance to moisture expansion and vice versa. In order to accommodate moisture expansion, there must be properly placed expansion joints within the installation itself to prevent heaving or failure due to the moisture expansion of the finish material.
Chemical and Stain Resistance
The definition of chemical resistance is the behavior of tile when it comes into contact with aggressive chemicals. Chemical resistance actually measures deterioration caused by two mechanisms; 1) chemical reaction resulting in alteration of tile, and, 2) penetration of a chemical or stain below the tile surface along with the difficulty of removal resulting in long term deterioration, or, effect on materials in contact with the surface. Chemical and stain resistance is measured by ISO 10545-13 “Determination of Chemical Resistance” by determining visual deterioration after exposure to standard chemical solutions (cleaning detergents, bleach, lactic and sulfuric acid, potassium hydroxide/alkali). The importance of this characteristic for mass transit applications is the resistance to deterioration and staining caused by exposure to various cleaning chemicals necessary for normal maintenance, or to fluids used for operation and maintenance of vehicles.
5.4 Use of Ceramic Tile in Roadway and Metro Tunnels
Ceramic tile and stone have traditionally been used to line roadway tunnels, bridges and metro stations. In addition, to the aforementioned reasons, ceramic tile is also used for several of the following reasons in these specific applications:
Ease of Maintenance – ceramic tile is very easy to clean and maintain when compared to other finish types in these applications. In fact, ceramic tile can be used in virtually every climate type; including areas exposed to severe freeze/thaw conditions and areas of continued or intermittent submersion. Ceramic tile with an appropriate finish (generally glazed) will not allow small particles of dirt, debris, exhaust fumes and residue, roadway salts and other contaminants / pollutants to accumulate on its surface. Ceramic tile is generally very easy to clean (typically with high-pressure water wash, scrub brushes and neutral pH detergent) and will maintain its durability even under the harshest conditions.
Conforms to Contour of Tunnel Substrates – ceramic tile is a great choice to clad tunnel walls due to the fact that this finish can conform to cover almost any desired substrate shape. Tiles are typically used to clad tunnel walls and ceilings. The tiles can be installed to follow the contour of curves, multiple curves, cylindrical and other geometric shapes.
Easy to Repair – ceramic tiles can also be repaired in spot areas or over large areas as required. If localized damage occurs or repairs are required for any reason, then individual ceramic tiles can be removed and easily replaced when compared to other finish cladding types (e.g. metal panels).
Provides Safe Driving Conditions – light colored ceramic tile provides a very good reflective surface. In roadway tunnels, this makes for safer driving conditions using less artificial lighting. Municipal lighting within the tunnels, as well as the cars' headlights, reflect light off the ceramic tile clad walls, illuminating all areas of the tunnel.
5.5 Quarry Tile/Klinker Tile
The durability and functional qualities of quarry tile make it one of the most suitable finishes for mass transit applications. As you might expect, there are an extraordinary number of different types and sizes of tile, only some types of tile have the physical characteristics required to be used in mass transit applications. Generally, quarry/klinker tile ranges in size from 4" x 8" (100 mm x 200 mm), to 6" x 6" (150 mm x 150 mm),to 8" x 8" (200 mm x 200 mm).
The raw materials for quarry tile are a mixture of clay (to give plasticity), quartz sand (to give structural strength and an economical filler), and carbonates or feldspars (to provide fluxing/fusing action). Glazes are formed from sand, kaolinitic clay, prepared glasses (frit), and oxide based pigments to provide color. The raw materials are ground together with water. The raw material for ceramic tiles are typically dried to a moisture content between 4–7% and shaped by the dust pressed method at pressures of 4,270 psi (29.5 MPa) or higher.
Some tiles used may be formed by the extrusion method, where clay with a moisture content of 15–20% is extruded through a die of desired shape. After forming, the raw tile or “bisque” is dried to remove excess water and fired in kilns operating at temperatures of 1,750–2,200°F (950–1,200°C). This results in vitrification or fusing of the clay and fillers, producing a tile product that is dense and non-porous. As mentioned previously, low water absorption is a key physical characteristic of tile for use in mass transit applications, and has significant influence on the other physical characteristics.
Klinker tiles are defined as red body tiles formed by either the extrusion process or dust pressing. Klinker tiles can also be referred to as red stoneware. This tile type can be glazed or unglazed and generally has a water absorption rate of less than 0.7%.
5.6 Porcelain Tile and Pavers
Porcelain pavers generally have a very low absorption rate just like porcelain tile (less than 0.5%).
Porcelain pavers are generally considered full body porcelain products which means the middle of the porcelain paver looks just like the outside of the porcelain paver. This works well in virtually all mass transit applications due to the fact that if a tile is chipped or broken, it is more difficult to notice the defect. Porcelain tiles also come in a variety of finish types ranging from polished (suitable for vertical applications) to matte and textured finishes (ideal for heavy traffic floor applications). Due to its dense nature, porcelain tiles are able to resist abrasive environments which can be found in demanding mass transit applications.
5.7 Abrasive Tile and Treatments
Abrasive tiles are used in areas to prevent slipping and where greater traction is required. Abrasive tiles are manufactured to have a high coefficient of friction or slip resistance built into the product. These types of tiles are frequently installed in mass transit, commercial and industrial applications, because of the potential amount of water, dirt, fluids and other debris, to help prevent pedestrians and facility personnel from being injured by slipping on a slick surface. In addition, abrasive tile is used on ramp areas to increase traction and reduce the potential for slip / fall accidents. Generally, corundum or metallic shavings are mixed in with the clay prior to the firing or extrusion process. These shavings become an integral part of the tile body. Abrasive tile can be manufactured in the form of quarry tile, klinker tile, dairy brick and porcelain pavers. In extreme cases where extra slip resistance is required, double abrasive tile can be used.
There are also treatments that will aid in slip resistance for tiles that do not have a high coefficient of friction. The treatments are typically spray applied to the tile. The treatment eventually becomes part of the tile itself. These treatments can last for years and can help to reduce the possibility of slip/fall accidents. Abrasive treatments usually require regular cleaning with non-abrasive cleaners. Static coefficient of friction tests are performed according to ASTM C 1028-89 “Standard Test Method for Determining the Static Coefficient of Friction of Ceramic Tile and Other Like Surfaces by the Horizontal Dynamometer Pull-Meter Method”. Currently, there is no ANSI Standard for slip resistance. OSHA recommends a static C.O.F. of .50 minimum for dry surfaces. ADA recommends, on dry surfaces, a 0.60 for accessible routes and 0.80 for ramp surfaces. See Section 9 Protection and Sealing – Water Repellant Sealers and Coatings for more information on applying surface sealers to tile surfaces.
5.8 Detectable Warning Tiles
To comply with Americans with Disabilities Act (ADA) requirements (Accessibility Guidelines for Buildings and Facilities Federal Register Volume 56 No. 173, Section 4.29 dated September 1994 [ADDAG]), detectable warning tiles are mandated at railway tracks, ramps, landings and other critical areas to warn pedestrians that they are approaching an area that requires caution. These tiles are designed to trigger three senses; sight, sound and touch.
There Are 3 types of Warning Tiles:
Wet Set Concrete Type – embedded into wet concrete and fastened in place. Many of these tile types can be removed and replaced if damaged.
Surface Mount Type – this tile type is generally bedded in a latex fortified portland cement based mortar and is then fastened in place with masonry anchors.
Cast in Place Type – this tile type is similar to the wet set tile type in that it is embedded into wet concrete and fastened in place. However, this tile type is not removable.
The tile material composition should be UV resistant, color fast, scratch resistant and have excellent slip resistant characteristics. These dense co-polymer compound tiles are generally bright yellow in color and are designed to have a truncated dome pattern to alert pedestrians. The detectable warning tiles are generally installed within 6" to 8" (150–200 mm), or as specified, of curb lines, railway tracks and other critical areas.
5.9 Natural Stone and Agglomerates
There is a wide variety of stone and stone agglomerate tile used in building construction which are suitable for mass transit floor and wall installations. However, determining suitability of stone for use in heavy traffic mass transit projects requires more careful analysis than manufactured materials like ceramic tile because it is a heterogeneous, natural material. In fact, different pieces of the same type of stone will exhibit varying properties. Aside from aesthetic characteristics of color and texture, which again are not the focus of this manual, the porosity of stone is one of the key physical characteristics which determines the durability and suitability of the stone and agglomerates as a finish which can be used in mass transit projects. The effects of moisture on stone or stone agglomerates are varied. Moisture absorbed in a stone may be heated by solar radiation or frozen by cold temperatures and exert pressure in excess of the tensile strength of the stone (water increases 9% in volume when frozen!). Moisture will act also as a vehicle for transport of salts and contamination from other surfaces, from pollutants, or from weathering of the stone. Rupture or breaking strength is also an important characteristic of stone and agglomerates used in exterior applications. Good breaking strength is required to resist reflection of thermal or moisture (shrinkage) movement in the underlying substrates, and to resist potential breakage of stone during handling and installation. In order to select the most suitable type of stone for an application, and understand the technical requirements for adhesive installation of a particular stone, the specifier must have a general understanding of the classifications and physical properties of stone and stone agglomerates.
Types of Natural Stone – Geological Classification
Natural stone is classified geologically in three categories, also known as the “Three Great Classes” of natural stone:
Igneous – solidified rock from molten state
Types – Granite, Basalt
Sedimentary – cementing, consolidation and crystallization of chemical solutions and biological deposits
Types – Limestone, Sandstone
Metamorphic – change or alteration of solidified rock by heat, pressure, or intrusion of other rock
Types – Marble, Slate, Quartzite, Serpentinite
Granite
Geologic and Commercial Classification
Granite is classified as an igneous stone, and has a primary mineral composition of feldspar and quartz. Black granite, also known as trap rock, has a completely different mineral composition than granite, but is commercially classified as granite. Black granite actually has a completely different mineral composition of hornblende and biotite and is not necessarily black in color. Some varieties of granite contain trace minerals which can cause discoloration or exfoliation after prolonged exposure to the weather.
Granite – Characteristics
Granite has a distinct crystalline appearance and is hard, dense, and resistant to scratches and acids. Of all the stone types, granite is the most suitable stone for direct adhered exterior/interior walls and floors in mass transit applications. This is because the density and hardness of granite impart stability and high breaking strength resistance (minimum requirement 1500 psi [10 MPa]) when fabricated in thin slabs or tiles that are necessary for cost effective installations. Laboratory research has also demonstrated that most granite fabricated in sections as thin as 7/16" (10 mm) have low moisture sensitivity and undergo minimum distortion or hysteresis growth when adhered with latex cement adhesive mortars. Granites used in building construction, especially exterior walls and floors, should have a maximum absorption rate of 0.40% by weight according to ASTM standards. The low absorption rate of most building granites require that cement adhesive mortars, which rely on absorption of cement paste and subsequent locking effect of crystal growth into the stone pores, utilize a latex additive or epoxy adhesives to insure proper adhesion. A latex additive will retard the evaporation of moisture needed to allow maximum absorption of cement paste and allow cement crystals to grow and produce a locking effect, and also impart pure adhesive bond. Because of the translucency of the minerals in some varieties, together with the thin widths typically used with the direct adhered method, some granite can darken temporarily from exposure to moisture (including the moisture in adhesive mortars). Granite may also darken permanently from reflection of dark or inconsistent coverage of underlying adhesives, or even darken or stain permanently from absorption of chemicals, such as the plasticizers, found in some (silicone) sealants. In selecting a thin granite for direct adhesion, it is recommended to avoid large grained granites relative to thickness; grain size should be less than 1/10 the stone thickness to maintain structural integrity of the vitrification between grain boundaries. While finishes of stone are primarily an aesthetic consideration, a textured (or thermal) finish is well suited for mass transit floor use and the preferred finish type. The thermal finish provides better slip resistance than other finish types. However, it should be noted that thermal finishes may require more maintenance as dirt and debris may be trapped within the stone finish. In addition, a thermal finish on granite induces thermal shock damage to the first 1/8" (3 mm) depth of the stone face, and should be taken into account by deducting this layer when calculating thickness specifications. Other common granite finishes are polished, honed, sandblasted and bush hammered. Commercially, granite is available in hundreds of varieties, differentiated primarily by color (a function of the mineral composition) and geographic origin.
Limestone
Geologic and Commercial Classification
Limestone is classified as a sedimentary stone with a primary mineral composition of calcite and dolomite. Limestone is geologically categorized as either oolitic (granular composition) or dolomitic (non-granular composition), and are commercially categorized as a building stone according to American Society for Testing and Materials standard ASTM C 568 (Standard Specification for Limestone Dimension Stone) by density properties:
Low Density – Category I
Medium Density – Category II
High Density – Category III
High density – Category III has an absorption rate of <3% and a minimum Modulus of Rupture of 1,000 psi (7 MPa), and is considered the best choice for interior floors and exterior walls, especially in colder climates (see characteristics below). Similar to other natural building stones, limestone is further differentiated by color (white, cream, buff or rose) and geographic origin. Special varieties of limestone include travertine, a limestone which is formed by the precipitation of minerals in hot springs. Travertine, while geologically classified as a limestone, is commercially classified as a marble (see marble) because it can be polished. Onyx is a type of translucent limestone which is formed by precipitation of calcite in cold water found in limestone caves.
Limestone – Characteristics
Limestone is characterized by the relatively loose cementing or consolidation of the minerals calcite and dolomite originating from biological deposits such as shells and sediments. As a general rule, lower density limestone (as classified above) has less desirable physical characteristics for exterior applications (especially in cold climates) such as a higher water absorption rate (7–12% by weight). Conversely, lower density limestone may possess better adhesive characteristics, especially with lower cost cement based adhesives. High density limestone has low absorption rates (<3% by weight) which impart good freeze thaw resistance and moisture stability.
Sandstone
Geologic and Commercial Classification
Sandstone is geologically classified as a sedimentary stone with a primary mineral composition of quartz. Sandstone is commercially categorized by mineral content (the percentage of quartz) according to the following three categories;
Sandstone (60% Quartz)
Quartzitic Sandstone (90% Quartz)
Quartzite (95% Quartz)
Sandstone is further classified by varieties according to their color and geographic origin. For example, bluestone is a dense, fine grained quartzite, and brownstone is loose, rough textured sandstone. Therefore, of all the sandstone types, dense species of bluestone and quartzite would be considered the only suitable sandstone types for demanding horizontal mass transit applications.
Sandstone – Characteristics
Sandstones are typically characterized by a loose or rough texture. Standard sandstone may have water absorption rates as high as 20% by weight, while quartzite, a more homogeneous composition of mainly quartz cemented with silica, has absorption <1% by weight. Sandstones (<60% quartz), are sensitive to weathering and cut relative to bedding planes.
Marble
Geologic and Commercial Classification
Marble is geologically classified as a metamorphic stone with a primary mineral composition of calcite and dolomite. Geologically, marble is actually a limestone that has been re-crystallized by heat, pressure, and intrusion of other minerals (thus the term “metamorphic”). The term “marble” is a commercial category of natural stone. Geologically marble is a metamorphic limestone of sufficient hardness which is capable of taking a polish. Commercially, there are over 8,000 varieties of marble, based on mineral content, color and geographic origin. According to American Society for Testing and Materials standard ASTM C 503 “Standard Specification for Marble Dimension Stone”, there are four classifications of marble building stone for interior and exterior vertical cladding and heavy traffic floor installations:
Marble Classification
Class I – Calcite
Class II – Dolomite
Class III – Serpentine
Class IV – Travertine
The percentage of magnesium carbonate in marble generally determines its strength, color, texture and variety. Calcite marbles have <5% of magnesium carbonate, and dolomite marbles have >40% magnesium carbonate. Travertine is geologically a limestone, and serpentine is geologically an igneous stone, both capable of taking a polish, and therefore commercially classified as a marble. Stone industry organizations such as the Marble Institute of America further classify marble according to fabrication, handling and working qualities according to the following categories:
Marble – Fabrication and Working Quality Classification
Group A – Sound Stone With Uniform Characteristics and Favorable Working Qualities
Group B – Stone Similar to Group A; May Have Some Natural Faults
Group C – Stone With Variations in Working Qualities, Containing Geological Flaws, Voids and Veins
Group D – Contains Many of the Most Highly Colored, Veined, and Decorative Marbles With Substantial Natural Cleavage Faults
While fabrication classifications are not necessarily an indication of the physical properties or durability of stone, it is generally recommended that only Group A and Group B marble are suitable for use on a high traffic mass transit floor or for interior or exterior vertical cladding. This is especially true with the thinner stone modules typical with the direct adhered methods of installation. However, one of the advantages of the direct adhesion of stone is that the entire surface of the stone is adhered, which allows stone that may normally be too fragile for mechanical anchorage to be considered for direct adhesion.
Marble – Characteristics
Marble is a relatively soft stone (approximately 3 on the Mohs hardness scale), and the surface is easily scratched by abrasives or etched by acidic materials. Marble is not particularly durable as an exterior vertical / horizontal cladding in harsh climates or for floor applications in mass transit applications.
Slate
Geologic and Commercial Classification
Slate is geologically classified as a metamorphic stone with a primary mineral composition of quartz and mica. According to ASTM C629 “Standard Specification for Slate Dimension Stone”, slate is commercially classified as either:
Type I – Interior
Type II – Exterior
Slate is available in a variety of colors and from numerous geographic regions of the world.
Slate – Characteristics
Slate is characterized by a sheet-like structure with cleavage parallel to the grain. Slate is normally fabricated with a natural cleft surface, although some slates can be sanded smooth. There are a wide variety of slates, and even some type II slates do not have suitable characteristics for use in exterior or vertical applications. Relatively “young” slates typically have a high percentage of mica and, as a result, lower density. This characteristic results in easy parallel cleavage and susceptibility to cohesive shear failure when exposed to shear forces common in mass transit floor applications. The high percentage of mica can also result in a friable, dust-like surface which prevents good adhesion to the body of the slate, even after proper washing and preparation. Conversely, “old” slates have a dense, compact structure, and are better suited for direct adhesion. Only laboratory or field shear bond and tensile strength testing can ascertain suitability of slates for direct adhesion in all application types.
Agglomerate Stones
Classification
Agglomerate is a term used to describe a man-made stone slab or tile product that typically consists of natural stone pieces and/or aggregates held together in a synthetic binder (e.g. polyester resin or epoxy resin). While there is no geologic classification for agglomerates, many of these products have physical characteristics similar to the type of stone pieces used in the matrix, and are often commercially classified as granites or marbles. In some agglomerate products, the characteristics of the binder may have a dominant effect on the behavior and performance of the product. Polyester resins have a high thermal coefficient of expansion and can present problems of significant differential movement when installed on exterior facade.
Agglomerates – Characteristics
There are hundreds of proprietary agglomerate products on the market. Each of these roducts has its own physical characteristics dependent on the type of stone fragments or aggregates, type of binder, and percentage of each material. The most popular agglomerate tiles typically consist of natural stone pieces in a 4–8% polyester resin binder. It is important to verify the suitability of agglomerates for mass transit applications based on agglomerate stone hardness.
5.10 Color, Temperature and Moisture Sensitivity
Moisture Sensitivity of Stones
Modern stone fabrication technology now allows production of stone tiles as thin as 1/4–1/2" (6–13 mm). While this technology has made direct adhered stone technically feasible and affordable, it presents problems of moisture permeability and sensitivity that previously, were of little concern with traditional thick (2–4"/50–100 mm) stable slabs of natural stone. Known by the term “hysteresis,” some thin stone (primarily marble) can bow or warp from crystal growth as a result of differential temperature or moisture change through its thickness. Some stone, especially dark, highly colored marbles and certain slates, contain minerals such as serpentine which are reactive with water. This simply means that crystal growth occurs on the side exposed to water, and the volume of the stone literally expands. This results in two problems that may occur if thin, moisture sensitive stone is installed using the direct adhered method:
Progress of installation – if water based cement or latex cement adhesive mortars are used, the side in contact with the adhesive may expand, and the outer surface will remain dry, resulting in differential movement with enough pressure to cause a thin stone to warp or distort from a flat plane. A thick section of stone would not be affected because the high ratio of unaffected or dry cross section to wet setting surface would not generate enough expansive force to overcome the resistance of the mass of stone.
The solution to this problem has been to either use accelerated latex cement adhesive mortars (e.g. LATICRETE® 254R Platinum Rapid) which mechanically lock the surface of the stone before distortion by expansive forces begins, or in highly sensitive stones where reaction to moisture is rapid, to use a 100% solids, chemically reactive epoxy adhesive (e.g. LATAPOXY® 300 Adhesive) which contains no water. However, these types of adhesives, and the labor techniques required, are typically more costly.
Post installation – Even if moisture sensitive stone is successfully installed in exterior mass transit applications, the stone may still be subject to cracking, spalling or adhesive bond failure from excessive volume expansion caused by exposure to constant high humidity or repeated cycles of rain.
Cladding Temperature and Color
A dark stone, such as black granite, or dark ceramic tile can become extremely hot from absorption of solar radiation. Color selection of a cladding material requires special consideration for expansion and contraction, as well as differential movement between the cooler underlying substrate. Dark colored tiles, stone or thin brick can easily reach a surface temperature of 170–190°F (80–90°C) with 3–4 hours of exposure to the sun in a hot, arid desert climate. When the sun sets, the ambient air temperature can drop to 60–70°F (15–20°C) in 1–2 hours, resulting in a temperature drop of 90°F (50°C) or more in the cladding material. This means that a dark stone, with an average coefficient of linear expansion of 7.3 x 10-6"/°F could expand and contract up to 7/8" (20 mm) over a distance of 100′ (30 m) in as little as 2 hours!! This is not only a graphic example of the importance of movement joints, but also the importance of using a flexible, low modulus, high shear bond strength adhesive that can absorb the differential movement between the cladding material and the underlying substrate.
5.11 Expansion and Contraction of Tile Finishes
Certain tile finishes can permanently increase in volume as a result of absorption of atmospheric moisture after removal from the kiln after firing. The total recommended design coefficient for moisture expansion as recommended by the Brick Institute of America is 3 – 4 x 10–4" per inch of length. Factors affecting moisture expansion are:
Time of Exposure – 40% of the total expansion will occur within three months of firing and 50% will occur within one year of firing.
Time of Installation – moisture expansion will depend on the age of the tile and the remaining potential for expansion.
Temperature – the rate of expansion increases as temperatures rise and when moisture is present.
Humidity – the rate of expansion increases with relative humidity. Tile exposed to relative humidity of 70% can have moisture expansion rates two to four times as great.
In addition to permanent moisture expansion, brick will undergo reversible expansion and contraction due to changes in ambient air and surface temperatures. It is not uncommon for dark tile or stone surface temperatures to reach 170°F (75°C) on hot summer days and below freezing (32°F [0°C] or colder) on winter nights.
5.12 Panelized Vertical Tile and Stone Systems
For mass transit installations of ceramic tile, stone, and thin brick, clad pre-cast concrete panels combine durability and tremendous design flexibility with the strength and economy of pre-cast concrete. The primary advantage of this type of backup wall construction is the economy of pre-fabricated, panelized construction. Pre-fabrication permits construction of panels well in advance of the normal sequencing of on-site construction of a building’s exterior wall. Once the proper stage in the sequence of construction is reached, panels can be erected quickly, without weather or scaffolding erection delays. Pre-cast concrete also allows more stringent quality control afforded by plant production of both the batching and casting of the concrete, as well as the installation of the cladding material. The considerations for clad pre-cast concrete panels are generally the same as those for unfinished panels, with two exceptions; the method of installation for the cladding material, and investigation of differential thermal and moisture movement between the pre-cast concrete and the cladding material.
Pre-Cast Concrete Panels – Negative and Positive Cast Methods
There are two methods for installation of tile on pre-cast concrete panels; the negative cast, and the positive cast methods.
Negative cast panels involve the casting of the concrete and bonding of the cladding in one step. The cladding material is placed face down over the face of the panel mold; joint width and configuration are typically controlled by a grid to insure proper location, uniform jointing and secure fit during the casting operation. Joints are typically cast recessed, and pointed or grouted after the panel is cured and removed from the mold. This method requires the use of a cladding with a dovetail or key back configuration on the back of the tile (as shown in Figure 5.13) in order to provide a mechanical locking action between the cladding and the concrete. The mechanical bond strength afforded by the integral locking of the concrete to the back is often augmented by the use of latex portland cement slurry bond coats or polymeric bonding agents just prior to casting of the panel.
Positive cast panels are prefabricated in two separate processes. The pre-cast panel is cast, cured, and removed from the mold, and the cladding material is then installed using an adhesive in the production plant in similar fashion to in-situ construction. Installation of the cladding after erection and attachment to the structure on-site is viable, but this sequencing defeats the goal of economy and quality control provided by prefabrication.
Pre-Cast Concrete Panels – Differential Movement (Internal to Panel)
Differences in the physical characteristics of the pre-cast concrete and the cladding material make this type of back-up construction more susceptible to problems of panel bowing or excessive shear stress at the adhesive interface. Bowing of panels can occur from several mechanisms. In negative cast panels, the concrete shrinks as it hydrates and excess water evaporates. The cladding, being dimensionally stable, can restrain the shrinkage of the concrete. The result is compressive stress in the cladding, and tensile stress at the adhesive interface, with the potential for outward bowing of the cladding surface. The best techniques in preventing panel bowing is to control the concrete shrinkage and to provide the proper ratio of cross sectional area to stiffness (modulus of elasticity) of the panel. Avoid flat panels less than 5–6" (125–150 mm) thick; concrete as thin as 4" (100 mm) can be used in panels with small areas, or in panels where stiffness is increased by configuration or composite action with a thick cladding material. Concrete mix design and curing conditions can be adjusted to minimize shrinkage. Several other techniques, such as the amount, location, and type of (pre-stressing) reinforcement, or introduction of camber to the panel, have been developed to compensate for possible bowing caused by shrinkage. Differential movement caused by different coefficients of thermal expansion between the cladding material and the concrete can also result in panel bowing. The optimum condition is for the concrete to have a rate of thermal expansion that closely matches that of the cladding material. The thermal coefficient of expansion of concrete can be modified slightly by adjustment of aggregate type, size and proportion to provide compatibility with the cladding, and, minimize differential movement under temperature changes.
Pre-cast Glass Fiber Reinforced Concrete Wall Panels (GFRC)
Pre-cast glass fiber reinforced concrete (GFRC) is the term applied to a material which is fabricated from a cement aggregate slurry and reinforced with alkali resistant glass fibers. Mix composition and types of applications vary, but for installation of direct adhered cladding, GFRC panel consist of mix which contains 5% by weight of glass fibers combined with a portland cement-sand slurry which is spray applied onto a form. The form may contain a cladding material (negative cast method) to which a bond coat of latex portland cement is applied just prior to application of the GFRC material, or the panel is cast, cured and removed from the form for subsequent application of a cladding material in a separate process (positive cast method). A single skin GFRC panel is the most common type of panel construction. This type of panel has a thickness of approximately 1/2" (13 mm); however, it is recommended to increase the thickness of the GFRC panel, to approximately 1" (25 mm) to reduce and better resist differential movement stress. GFRC panels rely on a structural backing or stiffener of a steel stud framework. The steel frame is commonly separated from the GFRC by an air space and attached to the GFRC by means of 1/4" (6 mm) diameter rods called flex anchors, which are imbedded into the GFRC and welded to the framework. These anchors, while rigidly attached, have flexibility inherent by diameter and orientation of the rods, which allow some panel movement to accommodate thermal and moisture movement. Heavier panels, or those requiring seismic bracing, also require additional anchors known as gravity or seismic anchors, and are differentiated from flex anchors by their size, configuration, and connection orientation to the GFRC. It is very important to consider the additional weight of the cladding material during the design and engineering of a GFRC panel; you cannot install direct adhered cladding using the positive method unless the panel was engineered specifically for that purpose. Properly engineered and constructed GFRC panels have extremely high strength and good physical characteristics. However, due to the thin sections employed in GFRC panels, differential thermal and moisture movement can cause panel bowing, resulting in cracking. Because GFRC expands and contracts from wet-dry cycling, the adhesion of a cladding can result in a different rate of moisture gain or loss between the front and back of the panel, which, in turn, can induce bowing stress. Therefore, careful attention to detailing to prevent rain infiltration and condensation within the wall are important. Similarly, cladding materials with incompatible coefficients of thermal movement can induce stress. So thermal and moisture movement compatibility with cladding is important, as are low modulus adhesives and movement joints.
Pre-fabricated steel frame and cement backer unit (CBU) wall panels
Pre-fabricated steel frame and CBU wall panels rely on the same construction principles as in-situ light gauge steel stud construct. Another advantage of this type of wall construction is the economy of the pre-fabricated, panelized construction. In similar fashion to pre-cast concrete wall panels, pre-fabrication permits construction of panels well in advance of the normal sequencing of on-site construction of a building’s exterior wall. Once the proper stage in the sequence of construction is reached, panels can be erected quickly, without weather or scaffolding erection delays. Pre-fabricated steel frame and CBU wall panels also allows more stringent quality control afforded by plant production in the sequencing of building elements (including fastening and taping of the CBU, air and vapor barrier placement, etc…) as well as the subsequent installation of the cladding material. Positive side waterproofing membrane is required for exterior panels. Thin, load bearing waterproofing membranes (e.g. LATICRETE® 9235 Waterproofing Membrane or LATICRETE Hydro Ban™) are generally specified to be installed directly under the cladding finish. A compatible latex-fortified portland cement thin bed adhesive mortar is used to adhere the cladding material to the waterproofed panel assembly. Critical detailing and sequencing is required when erecting the panels to:
1. Ensure that waterproofing integrity is maintained between adjacent panels
2. Proper expansion allowance is maintained with the appropriate flexible sealant while still maintaining waterproofing integrity (see Section 10 expansion joint with waterproofing looped into movement joint EJ-10 and EJ-11)
3. Waterproofing membrane “touch-ups” may be required after panels are erected into place
† United States Patent No.: 6881768 (and other Patents).