The Importance of Testing When Specifying Natural Stone  

Natural stone refers to stone that is quarried from the earth. Though it may be cut into shapes, including slabs and tiles, and a resin or sealer may be applied to its face, the internal fabric of the stone is unchanged.

It is important to understand what natural stone is not. Engineered products that are made from aggregates bound together with cementitious or resin binders do not meet the definition of natural stone.

Prized for its beauty, strength, and longevity, natural stone is one of the oldest building materials, and its applications are limited only by the imagination. Inside, stone can be installed as flooring and countertops, as well as in vertical applications such as wall cladding. Other options include mantels, fireplace surrounds, bath surrounds, and furniture.

All images courtesy of Natural Stone Institute

When a stone is tested for compressive strength, a ram is used to crush the stone until it fails.

Stone has just as many exterior applications. It is used extensively in landscaping, serving as pavers, walkways, walls, and other structures. Historically, stone served as the structure for many buildings; today, it is more typically used as non-structural cladding.

Dimension stone is stone that has been cut or sawn into specific shapes. How dimension stone is finished—whether honed, polished, or flamed—affects the stone’s appearance. Many types of natural stone, including granite, marble, and limestone, can be fabricated into thin panels and used on the exterior of buildings.

How Natural Stone Is Classified

When specifying natural stone, it is important to understand the material’s inherent characteristics so that you make an appropriate choice for the application. Knowing about the stone’s chemistry as well as how it was formed provides some basic information about how the stone will perform.

Natural stone can be divided into two broad categories based on its chemistry, or mineral composition. Calcareous stones are made of calcium carbonate and tend to be vulnerable to acids, even mild ones like lemon juice. Common types of calcareous stones include marble, limestone, travertine, onyx, and occasionally types of sandstone.

Siliceous stones are made of silica or silicates. These minerals make up close to 95 percent of the earth’s crust and include such common minerals as quartz, feldspar, and mica. Siliceous stones tend to be strong and resistant to acids; examples of siliceous stones include slate, quartzite, granite, and most types of sandstone.

Natural stone also can be categorized by how it is formed. Sedimentary stones are formed from preexisting rocks or the bodies of once-living marine organisms. Many sedimentary stones start out as sediments carried by rivers or glaciers that are deposited in lakes or oceans. The particles are “stacked” layer by layer; when trapped and buried, the particles become compacted and cement together. Common examples of sedimentary stones include sandstone and limestone.

Metamorphic stones are created through a combination of heat, pressure, and time. Metamorphic stones start out as sedimentary, igneous, or earlier metamorphic stones, but over a long period of time and under intense heat and pressure the crystals rearrange themselves or react with fluids that enter the rocks. Metamorphic stones, which include such common varieties as schist, gneiss, quartzite, and marble are usually denser and more compact than their earlier forms.

Natural stone comes in a variety of colors and patterns that tell us the story of the stone’s origins.

Igneous stones have their start as molten material that originates from deep within the earth near active plate boundaries. The lava is exposed to great heat and pressure, then cools as it rises to the surface. Lava that cools very slowly forms large grains, while igneous stones that cool quickly have a fine-grained or glassy texture. Igneous stones include gabbro and basalt; many of these are included in the granite group because they perform similarly to granites.

This brings up an important point. The stone industry uses commercial definitions for dimension stone types. Although these certainly overlap with scientific and geologic definitions, there are some differences.

The scientific descriptions focus primarily on a stone’s geographic locality and mineralogical composition, while commercial descriptions focus on workability and general performance of the stone. Hence, the commercial industry groups stones according to those which behave and perform similarly. While the stone industry recognizes all rock types, they consolidate them into just 10 groupings of stones: granite, limestone, marble, onyx, quartzite, sandstone, serpentine, slate, soapstone, and travertine.

We can use the two categories of chemical composition (calcareous and siliceous) and the three categories of formation (sedimentary, metamorphic, and igneous) to create a matrix to help organize the various stone types. Stones within each category have properties that are generally predictable. Later in this course, we will see how the design professional can use this matrix to create a plan to guide future specifications.

All stones are unique; consequently, expect variation in the properties and behavior between stone products, even if they are of the same type. While this variation makes stone an appealing and stimulating material, it also adds challenge. Research is critical, as the specifier must understand a stone’s strengths, vulnerabilities, and limitations.

There are several factors to consider when specifying natural stone. The first and most obvious is aesthetics. Will the stone help achieve the desired look? A stone’s aesthetic is determined by its inherent properties—color, grain size, and veining, for example—but also by how it is cut and finished. For example, a highly honed or polished finish will impart a more refined look.

Performance is just as important as aesthetics. Will the stone perform adequately for the desired application? For example, if the stone is to be used as a countertop, will it resist staining and hold up to abuse from steel knives? If installed on a building exterior, will it hold up to the elements?

It is also important to choose stone that is available in the dimensions and quantities desired, and within the allotted timeframe. Since by its nature stone is a heavy material, it is preferable to source it locally or regionally, although this is not always possible. Some highly desirable stone types—certain marbles, for instance—are only found in a few places in the world. In another example, most travertine used in the United States used to be imported almost exclusively from Italy and a few other countries.

Finally, cost is a factor. Even if the stone is the perfect material from an aesthetic and performance standpoint, it may not fit within the project’s budget.

Though all of these considerations are important, this course focuses on performance, and the design professional’s role in obtaining and interpreting information about how a given material will perform.

ASTM Standards

Did you know that ASTM C1799 states that test results for stone should not be more than three years old? One of the most important resources for the design professional is the set of standards and testing procedures developed by ASTM International and the American National Standards Institute (ANSI). These standards guide the natural stone industry and help protect end users; in fact, most architectural specifications require that stone meet certain specified ASTM or other testing standards before it will be accepted for use.

ASTM publishes about 25 documents related to natural stone. These are organized into four categories: Standard Guides, Standard Materials Specification, and Standard Test Methods. Standard Guides documents provide basic information and definitions of terms used in the stone industry. The Standard Materials Specifications are specific to each of the commercially defined groups of dimension stone—marble or travertine, for example. (Note: There is no Materials Specification for onyx or soapstone.) These documents define the acceptable range of values for the physical and mechanical properties for these products and explain which test methods should be used to evaluate these properties.

Standard Test Methods describe the test protocols that are used to evaluate certain properties of a stone material—compressive strength and abrasive hardness, for example. We will be looking at these tests in greater detail soon, but for now, let’s look at a simple example that shows the relationship between the Materials Specifications and Standard Test Methods.

If we look at the Standard Materials Specification for limestone, we see that limestone is classified into three types based on density. The standard sets minimum or maximum values for several ASTM tests for each type of limestone. For example, a type II or medium-density limestone should have an abrasion resistance of at least 10 and compressive strength of at least 4,000 psi. Later we will look at how these test results help specifers.

A number of ASTM test standards pertain to natural stone; above all, these test standards help ensure an acceptable level of safety and quality for a given application. However, if you are new to this area, it can be intimidating if not overwhelming to determine which tests are applicable to your project.

Fortunately, design professionals have many helpful resources at their disposal, starting with the accredited quarry, material supplier, or fabricator. Not only will the supplier have test data, they will have anecdotal and historical information about the performance of a given material. They often can point to specific projects that have used the same stone in a similar application.

The Dimension Stone Design Manual is the “bible” of the natural stone industry. This invaluable resource is a one-stop reference manual that includes a comprehensive Illustrated Glossary of Stone Industry Terms, descriptions and technical data for individual stone varieties, and installation guidelines.

The Natural Stone Resource Library is a resource created for architects, designers, and contractors. This free online database includes documents on all varieties of natural stone, including suggested uses and applications for both residential and commercial settings. Users can refine their searches by using filters such as the type of stone, application, and document type.

The Role of the Design Professional

It is important to understand the roles and responsibilities of each of the key players in the process of specifying, installing, and maintaining a natural stone product. These include the design professional, general contractor, materials supplier, installing contractor, and end user.

As the specifier, the design professional is responsible for material selection based on aesthetics and performance criteria. He or she must obtain visual and physical samples and also acquire the test results that back up claims about the material’s performance for that particular application. It is the design professional’s responsibility to choose which tests are included in the specification, and it should include everything necessary to obtain an informed bid.

The material supplier furnishes the material to the installing contractor. Most suppliers hold test data from a standard series of tests for their main products. They must be able to obtain material that meets or exceeds the performance parameters outlined in the specification. They should also provide specimens and perform testing to document that the material meets those parameters. Realistically, the supplier will not always have the full spectrum of test results required on hand.

The general contractor oversees construction and coordinates with the installing contractor. The installing contractor must be able to install the material as prescribed. Often it is up to the installer to obtain testing if it is required, though this task may also fall to the specifier or general contractor.

The end user also has a role to play. The person must understand the material and his/her role in its maintenance; ideally, this will be established early in the design process so the client is not caught off guard.

There are several situations where you might have to seek testing. Perhaps you are introducing a new stone for which testing has not been conducted. You might have outdated technical data—test results that are more than three years old. Or you might have a special project that requires specific testing not included in the usual scope of tests.

You have several options for seeking the required tests. You may request the quarry or representative company to conduct them, but be prepared to encounter some resistance, depending on the quarry or provider, as they can sell directly to homeowners and residential projects that do not require performance testing. Independent labs also can perform the tests. If you anticipate that the stone product you are specifying may require additional testing, be sure to account for this in the specification, as these tests involve a cost, both in terms of time and money, and must be considered during the bid.

ASTM Test Standards Relevant to Natural Stone

Now let’s take a look at the commonly used ASTM Test Standards relevant to natural stone. The tests can be divided into two broad categories: those that pertain to the stone’s physical performance, and those that tell us something about how the stone will perform under certain circumstances.

ASTM C97: Absorption and Bulk Specific Gravity of Dimension Stone: Absorption is a measure of a stone’s porosity; this characteristic is an indicator of both its stain resistance and possibly its performance in cold, wet environments where it will be subjected to freeze-thaw cycles.

ASTM C170: Standard Test Method for Compressive Strength of Dimension Stone: This test measures a stone’s resistance to crushing pressure.

ASTM C99: Standard Test Method for Modulus of Rupture: This test measures the bending or flexural strength of a stone material under a single-point load.

ASTM C880: Standard Test Method for Flexural Strength of Dimension Stone: This test evaluates bending strength using two points of loading and a sample that is the actual thickness of the stone to be installed.

ASTM C1353: Standard Test Method for Abrasion Resistance of Dimension Stone Subjected to Foot Traffic Using a Rotary Platform, Double-Head Abraser: This test determines the degree to which a stone material can withstand scratching or abrasion.

ANSI DCOF (dynamic coefficient of friction): This test measures slip resistance of an object in motion.

ASTM C1354: Strength of Individual Stone Anchorages in Dimension Stone: This test evaluates the performance of individual anchors with a specific stone product.

Which Tests for Which Application?

Exterior and interior applications each have their own specific considerations. In general, if the stone is going to be installed outside, it must be resistant to weathering and decay, as it could be exposed to temperature extremes, freeze-thaw cycles, pollutants, and chemicals such as deicers. Because stones in interior applications are sheltered from the elements, the range of choices is much wider; however, there are special considerations for interior stones that are to be used in flooring applications or installed in locations where they could be exposed to chemicals, grease, or prolonged moisture.

Once you have determined the application and whether the stone is to be used indoors or outside, you can begin to formulate a plan to help guide future specifications. The plan basically outlines the process that you will follow for each general application. It will help ensure that your specification is thorough and that you have considered all of the pertinent tests for that particular application. While we will look at this in more detail soon, following are some general considerations.

If the stone is being used in a structural capacity, you should seek test data for compressive strength and flexural strength. If the stone is to be exposed to the elements, study the stone’s absorption, porosity, and permeability. In addition, you may want to seek test data for how it reacts to freeze-thaw cycles. If the stone is to be installed in a vertical application, study the anchorage system and specify ASTM C1354. If the stone is to be used in a floor areas which receive foot traffic, it should be tested for abrasive hardness and slip resistance. Absorption and density are also important considerations.

Natural Stone Testing: Testing for Building Stone Applications

Exterior stones must be able to withstand the full range of stresses to which they will be exposed. Depending on the application and location of the project, these may include various loads and environmental stresses.

Gravity exerts a downward load on a stone or assembly, and in vertical applications must be borne by the anchorage system. If used structurally, stones on the bottom of a wall must be strong enough to withstand the weight of the material bearing on it. Stones used in vertical applications experience wind loads; these exert both positive and negative pressures and tend to be stronger at building corners or near discontinuities. Wind loads are perpendicular to the stone face or wall assembly. Seismic loads result from accelerations of the earth during an earthquake or aftershock. These forces may be perpendicular or parallel to the stone face. Design of assemblies is usually governed by the windload, as it is typically greater than the seismic load, although exceptions do occur.

Exterior stones may be exposed to freeze-thaw cycles and to thermal expansion and contraction. They may be exposed to rain, snow, or hail, as well as pollutants and chemicals such as carbon monoxide and sulfates. Exterior stones used as pavers may experience heavy compressive loads from vehicle traffic; they also must be able to resist abrasion and provide slip resistance.

Compressive and Bending Strength

Because strength is such an important consideration for outdoor applications, let’s take a closer look at ASTM tests that evaluate compressive and flexural strength.

ASTM C170: Standard Test Method for Compressive Strength of Dimension Stone

This test is used to measure a stone’s resistance to crushing. It requires cubic or cylindrical specimens that measure between 2 and 3 inches in all dimensions. A ram is used to apply pressure to the stone until it fails or “smashes.” The compressive strength of the sample is reported as the failure stress in either pounds per square inch (lbs/in²) or MPa.

At least five specimens for each of four conditions should be tested, and they should be as flat as possible and free of nicks or other flaws, as these can negatively impact strength. In some instances, the testing laboratory may have to refinish the specimens to produce acceptable surfaces.

A higher compressive strength indicates that the stone can withstand a higher crushing load. The Materials Specifications put forth minimum values for compressive strength for each stone type; for example, at least 1,800 psi (12.45 MPa) for marble and 19,000 psi (131 MPa) for granite. Compressive strength—or the load at which the stone fails—is usually much higher than the actual load the stone must resist. The industry has established safety factors for various stone types, which help determine the allowable load for that stone. For example, if the safety factor for a given stone is 4, you would divide the compressive strength for that stone by four to obtain the maximum allowable load to which that stone should be subjected.

ASTM C170 measures a stone’s resistance to crushing pressure, or compressive strength.

Bending Strength: ASTM C99 and C880

Two ASTM tests evaluate the bending or flexural strength of a stone material. The first is ASTM C99: Standard Test Method for Modulus of Rupture. For this test, stone specimens are supported near the ends, creating a kind of “bridge,” and a downward load is applied to the top until the stone fails. This test was thought to have been influenced by a preexisting test that was developed to test bricks, and it uses specimens that measure 8 by 4 by 2¼ inches—the dimensions of a standard brick.

The test has some shortcomings. The short, thick test specimen behaves like a “thick beam,” with a high shear-to-moment ratio, and so does not experience the flexural stresses that a thinner specimen would. For this reason, the test results tend to be high. Second, because the load is applied at a single point mid-span, the bending stresses are concentrated in the middle of the specimen. However, stones are heterogeneous materials with varying grain sizes. A specimen may contain weaknesses near the ends that will not be exposed during ASTM C99 testing because of the mid-span loading.

During the ASTM C99 test, a downward load is applied to the middle of a short, thick stone specimen.

ASTM C880: Standard Test Method for Flexural Strength of Dimension Stone

The second difference that distinguishes ASTM C880 from the modulus of rupture tests is that the flexural strength test is conducted with quarter-point loading. Instead of applying the full load at single location at the midpoint, the total test load is split: half is applied one-quarter of the distance from one support; the other half one-quarter of the distance from the other support. This way, the entire center half of the span is subjected to the same maximum bending moment, or stresses. This method is more likely to expose any local weakness in the stone. For these reasons, C880 is superior to C99.

As with C99, the load in C880 is increased until the specimen breaks. Flexural strength is then calculated using a formula based on the geometry of the test conditions.

Flexural strength is expressed as lbs/in² or MPa. A higher flexural strength or modulus of rupture indicates a higher bending strength. The required minimum values, as defined in the Materials Specifications, range from 400 psi (2.8 MPa) for low-density limestone to 1500 psi (10.3 MPa) for granite.

As with compressive strength, a safety factor is used to ensure the actual loads never come close to the flexural strength, or the load at which the stone will fail.

ASTM C880 measures flexural strength by applying a split load at two points until the stone fails.

Absorption and Density

The absorption or porosity of a stone will affect its durability and long-term appearance. If a stone readily absorbs water, it may be more susceptible to damage during freezing weather. Similarly, a highly absorbent stone may be more prone to staining. In general, a low absorption value is desirable. It is also important to know how dense or “heavy” a stone material is, especially when designing structural walls or assemblies that use stone cladding. This is why ASTM C97: Absorption and Bulk Specific Gravity Testing of Dimension Stone is one of the most important and universally administered of the ASTM tests.

For this test, at least five test specimens are required, each of which should be a cube or cylinder that measures at least 2 inches but not more than 3 inches. The stones are dried in a ventilated oven for 48 hours and weighed, then submerged in water for 48 hours and weighed again. The difference between the dry weight and wet saturated weight is the absorption value and is expressed as a percentage.

The maximum allowable water absorption is prescribed in the Materials Specification for that stone. These values range from 0.20 percent for marble to 12 percent for low-density limestone.

It is important to note that absorption value is based on the stone’s weight, not its volume. To understand this, consider two stones of the same volume but different weights—in other words, one is denser than the other. Let’s assume that both stones absorb the same amount of water. The denser stone will have a lower absorption value than the lighter stone, since the volume of water absorbed will make up a smaller percentage of the total saturated weight.

For the final part of the test, the stone is suspended by fine wire and weighed. By comparing the stone’s weight to the unit weight of water, we can obtain its specific gravity. This in turn can be used to calculate the stone’s density. This is important information that can help when designing support systems, for example.

The specific gravity of stones ranges from 2 to 3; a stone with a specific gravity of 2.6, for example, is 2.6 times as heavy as water. Specific gravity is a unitless ratio, while density is expressed in either pounds per cubic foot (lbs/ft³) or kilograms per cubic meter (kg/m³). Density can be calculated by multiplying a stone’s specific gravity by 62.4 (for lbs/ft³) or 1000 (for kg/m³).

The Materials Specifications also prescribe minimum densities. Generally, a higher-density stone is probably harder, less porous, and stronger, but this is not always the case.

Specimens are dried in an oven for 48 hours before obtaining their dry weight.

Specimens are soaked in a tub for 48 hours and weighed again; the difference between their dry and saturated weight represents the absorption value.

Anchorage Systems for Stone Cladding

Stone that is used as cladding is usually part of a curtain wall assembly that does not bear the load of the building; instead, it is attached to the structure in one of two ways. In the “hand set” method, each individual stone is individually placed and attached to either the primary building structure or a secondary wall framing system. In a panelized installation, stone panels are preinstalled into a frame or attached to a precast concrete panel. The panels are then attached to the building’s primary structure or to a secondary framing system.

In either case, anchors are used to attach the stones or panels. Anchorage systems must withstand the many forces that impact natural stone cladding. There are a number of anchorage system types, but in general, the best anchors are the simplest and designed with the fewest components. Anchors also should be carefully designed to prevent galvanic corrosion, which can cause the anchors to fail years after they have been installed.

Because anchorage systems are so critical, special ASTM tests have been developed to evaluate them.

ASTM C1354: Strength of Individual Stone Anchorages in Dimension Stone

This method is used to test a particular anchorage in a prepared stone specimen. This small-scale test is used to establish the “ultimate capacity” of a given anchor in a given stone—the point at which the anchor fails under load. It is recommended that any filler that is to be used must be omitted for the test, or prevented from bonding to either the stone or the anchor during the test. This ensures that the anchor is being tested on its own merits and is not being “helped” by the filler.

There is no “pass/fail” criteria for this test. ASTM C1242: Standard Guide for Selection, Design, and Installation of Dimension Stone Attachment Systems can be used to select the safety factor and calculate the allowable capacity of the anchor, which should be significantly lower than the ultimate capacity, as determined by then anchorage test.

A kerf anchor being tested according to ASTM 1354.

There is a test that can be used to evaluate entire assemblies instead of individual anchorages. ASTM C1201: Standard Test Method for Structural Performance of Exterior Dimension Stone Cladding Systems by Uniform Static Air Pressure requires a full-scale mockup of a stone panel. The actual stone to be installed, cut to the specified dimensions and with the correct finish, is used for the mockup. The anchorages are the actual types recommended in the design. The anchorage hardware is attached to a rigid frame which simulates the building structure. The mockup is enclosed in an air chamber and subjected to a simulated wind load.

This test provides a more accurate picture of how the assembly will behave in the field. However, because this test is quite expensive, it is not used extensively.

Other Considerations for Exterior Stone

One important consideration for exterior stone applications is how the stone will hold up to repeated cycles of freezing and thawing. In general, less dense stones with greater absorption rates may be more vulnerable to degradation. Although there is no specific ASTM test to evaluate a stone subjected to freeze-thaw cycles, the specifier does have options. Samples can be sent to an independent lab for an Accelerated Weathering test. There stones are placed in an environmental simulation chamber and subjected to any number of conditions, including temperature and humidity cycling. The stones are tested for flexural strength before and after their time in the chamber to evaluate the rate of degradation.

Pavers (both interior and exterior) should also be tested for Abrasive Hardness. This test is covered in the following section.

Developing a Plan

Now let’s consider how you can begin to create a plan for each general exterior application. The plan is the process you will follow to ensure the stone is tested and placed into the specification correctly. Consequently, you will need to decide which test data you will need for each exterior application. For most, you will need data on absorption and density, compressive strength, and flexural strength; however, if the application is for stone pavers you may also seek test data for Abrasive Hardness, or if a cladding application, consider specifying the anchorage test. If the project is located in an extreme climate, you might want to specify an accelerated weathering test. When developing your plans, factor in cost and time for additional testing and be sure to understand your options for testing.

TESTING EQUIPMENT

Highly specialized equipment is used for testing stone. The use of these machines ensures that test conditions are uniform and results are accurate.

ASTM C1353: The Taber Abraser is a specialized machine used to test stone (and many other materials) for abrasion resistance. During the test, the stone subjected to the wear action of two abrasive wheels that rotate in opposite directions.

DCOF: A portable machine called the BOT3000 is used to test walking surfaces in the field. A lubricant is used to simulate a floor that is wetted with a residual detergent level.

ASTM C1354: A specialized machine tests a kerf anchor to failure during this test.

Testing for Interior and Walking Surface Applications

Because stone installed inside of buildings is protected from many of the forces and elements to which exterior stone is exposed, design professionals have much greater freedom when specifying stone for interior applications. In general terms, the biggest consideration is whether the stone will be used in a vertical or horizontal application. Horizontal applications include flooring, countertops, or lavatory tops. Vertical applications include walls, backsplashes, mantels, and tub surrounds.

If the stone is going to be used as a decorative element, the choices are virtually unlimited. At the very least, the specifier still should obtain basic test data, including absorption and density, for these stones. Stone used as flooring or countertops have special considerations, as do stones that are frequently exposed to water or other liquids.

Test Methods for (Horizontal) Flooring Applications

There are two major considerations specific to stone used for flooring: its wear or abrasion resistance and its slip resistance. These are measured using two separate and distinct tests.

Long prized for its beauty and ease of maintenance, marble can help create graceful and timeless interiors and is often used in horizontal (flooring) applications.

Abrasion Resistance

Abrasion or wear resistance is crucial for stone products that are to be used in flooring applications. There are two tests used to test the abrasion resistance of stone that is to be used as a walking surface. ASTM C241: Standard Test Method for Abrasion Resistance of Stone Subjected to Foot Traffic was developed in the 1950s; however, this test had several limitations, one being that only one commercial testing laboratory owned the special machine required for the test. ASTM C241 is not widely used today. Instead, it has been largely replaced by ASTM C1353: Standard Test Method for Abrasion Resistance of Dimension Stone Subjected to Foot Traffic Using a Rotary Platform, Double-Head Abraser. Both of these test are used to evaluate a stone product’s abrasion resistance, as measured by an indexed value called Abrasive Hardness, or Ha. The higher the number, the more resistant the stone.

During the ASTM C1353 test, specimens are abraded using a Taber Abraser, a specialized machine with a rotating turntable and two rotating heads. Three specimens are tested, each measuring 4 inches square and less than ½ inch thick. One face should be flatsawn or ground and smoothed with 120 mesh grit. Rough surfaces, such as thermal, bush-hammered, gang-sawn, and cleft finishes, are not suitable. The stones are dried for 48 hours and weighed, abraded in the machine for 1,000 revolutions, dusted off, and weighed again. The function of the actual test is to measure how much mass was abraded from the stone. The resultant index value is proportional to this mass. It is important to note that ASTM C241 should be used for very hard stone because the testing methods for ASTM C1353 is not capable of abrading them.

The ASTM Materials Specifications include a minimum Ha value for each type of stone. You can refer to the DSDM for recommendations of minimum Ha by application.

In addition, the stone industry has established general thresholds for abrasive hardness for light, medium, and heavy traffic: •

• Light traffic: residential applications; minimum Ha of 6
• Moderate traffic: residential entries and small commercial installations; minimum Ha of 7
• Heavy traffic: commercial installations with 50-plus people per minute; minimum Ha of 10
• High-volume areas: lobbies, thresholds, and stairwells; minimum Ha of 12
• Exterior pavers: minimum Ha of 12

Slip Resistance

When specifying natural stone as flooring, it is important that it provides adequate traction, even when it is wet or greasy. Be aware that a floor’s finish can impact both its slip resistance and its tendency to attract dirt or stains. A more heavily textured floor may provide more “grip,” but it also has more topography that can catch particles.

In the past, the industry used a value called the static coefficient of friction (SCOF) to evaluate a material surface’s frictional resistance, or “slipperiness.” A test called ASTM C1028 was used to determine SCOF, which is defined as the amount of force required to displace a weighted plate that is set on a wet or dry surface. The higher the number, the more friction (i.e., less slippery)the surface. However, this test had some shortcomings. Because the application of the load was not automated, the test operator had considerable influence over the load application rate, directional bias, and uniformity. Also, the wet condition test sometimes created a suction that prevented it from providing reliable data on smooth and polished surfaces in wet conditions.

To address the shortcomings of the C1028 test, an ANSI accredited standards committee developed a new standard for measuring the slip resistance of floor tile. ANSI 326.3: Standard Test Method for Measuring Dynamic Coefficient of Friction of Hard Surface Flooring Materials describes a method for measuring the dynamic coefficient of friction, or DCOF. This is defined as the amount of force required to keep an object in motion as it slides over a tile. A 0.05 percent sodium lauryl sulfate solution is used to create a thin film on the stone surface, simulating a wet floor with a very low level of residual detergent.

The standard states that hard flooring suitable for level interior spaces expected to be walked on when wet from water shall have a “wet DCOF” of 0.42 or greater when tested.

It is important to understand that the 0.42 value represents a minimum level of slip resistance for level interior floors that are wet because of water. When this same flooring is contaminated with another “slipperier” substance—oil or grease, for instance—this level of friction may not be adequate.

It is also important to understand that 0.42 is not a magic number. Flooring with values below 0.42 may be safe for certain situations, and flooring with a DCOF greater than 0.42 may be unsafe in others. When evaluating safety, you will need to consider other factors, such as use and traffic, likelihood of spills, nature of substances that are likely to come into contact with the flooring, surrounding environment, and the slope of the floor.

Countertops

Though it is by no means the only choice, granite is an extremely popular countertop material, in part because it is hard, stable, and with a low porosity, as well as beautiful.

The design professional must weigh several factors when specifying stone for countertop applications. Will it hold up to abuse from stainless steel knives? Will it stain? Does it react to acids? Will the material harbor bacteria?

First, let’s consider hardness. While you can seek data for the Abrasive Hardness test, you can quickly evaluate a material using the Mohs Hardness Scale. This scale rates the relative hardness of a material between one and ten, with one being the softest and ten being the hardest.

In most cases, a less porous material will be more resistant to staining and bacterial growth, so test data for absorption and specific gravity is key when selecting countertop material. However, when it comes to chemical or acid resistance, this is closely related to the stone’s mineral makeup, so be sure to research the material for chemical reactivity.

Wet Areas

Showers, spas, and areas around urinals deserve special consideration. Not only are these areas subjected to continual streams of water, but some also must be able to withstand steam. Wet areas can be a breeding ground for mold and mildew. The Dimension Stone Design Manual provides guidance for best installation practices that dissuade mold and mildew growth, proper backerboard, ventilation fans, and the use of hard grout that is the full depth of the stone.

Some specifiers look to the absorption test, C97, to evaluate how a material will perform with regard to mold and mildew. However, installation materials and methods used to play a much larger role in potential damage. That does not mean you should not refer to C97 when specifying stone for wet areas. You should make sure that the material falls below the maximum value for absorption, as determined by testing and outlined in the Materials Specifications. You also can use C97 test data to compare one stone to another.

The soundness of the stone (the absence of flaws) and the construction of the stone (the elements that constitute the stone) are important factors in how the stone will perform in wet areas. Elements of iron and other minerals in stone can be unstable when exposed to moisture, leading to surface staining and stone degradation. The action of water on polished marble or limestone can cause surface dulling, spalling, warpage, or deterioration over time.

Here are the stones commonly used in showers and the allowable absorption values by weight:

• Marble: 0.20 percent maximum
• Limestone (Group 3 only): 3 percent maximum
• Granite: 0.40 percent maximum
• Quartz-based stone (quartzite): 1 percent maximum
• Slate 2: 0.25 percent maximum

Developing a Plan

You are beginning to see how you can start to create a plan for each general interior application that will ensure you use a thoroughly vetted material. For example, understand that the specification for a flooring application will require different and more extensive test data than for a decorative interior wall. In either case, you will want basic data such as absorption, density, and hardness for all of the stones you are considering. Still, your plan for flooring applications should include using data from the abrasive hardness requirements to winnow down your choices and DCOF test results to compare those that remain. Finally, do not forget to consult with the quarry or fabricator, as they have deep knowledge and valuable anecdotal information that can help inform your choices.

Conclusion

Natural stone is a durable, beautiful, and timeless material with a stunning variety of choices and applications. The design professional has a key role to play in making sure that the right stone is used in the right place. ASTM test data can guide the specifier’s choices, but it is critically important that he/she understand which tests are relevant to which application, and how to interpret the test data and use it in tandem with the Materials Specifications and knowledgeable advice from the supplier. By developing plans for each general type of application, the design professional can ensure that he/she is creating a thorough specification that will ultimately result in a successful installation—and a happy client.