Compressive Strength of Dimension Stone: ASTM C170 Explained

Almost every stone data sheet that crosses a fabricator’s desk carries a line reading “Compressive strength, ASTM C170” followed by a large number in psi or MPa. It is usually the biggest figure on the page, and it is the one customers, designers and sometimes sales people reach for when they want to argue that one stone is “stronger” than another. The number is real and the test behind it is carefully defined, but it answers a narrower question than most people assume. ASTM C170 tells you how much crushing load a small, carefully prepared block of stone carried before it failed in a laboratory press. It does not tell you how a cantilevered overhang, a narrow sink rail or a long vanity top will behave.

This article walks through what the C170 test method actually covers, how the specimens are prepared and conditioned, why results are reported wet and dry and both parallel and perpendicular to the rift, and which minimum values the ASTM material specifications set for granite, marble, limestone, quartz-based stone and travertine. It then looks at the figure from the shop floor: what compressive strength can and cannot tell a fabricator, why flexural strength is usually the more relevant property for countertops and thin panels, and how to read a supplier’s data sheet without being misled by a single headline number.

What ASTM C170 Measures and Where It Sits Among the Stone Standards

ASTM C170 is a test method, not a pass-or-fail specification. Its scope covers the sampling, the preparation of specimens and the determination of the compressive strength of dimension stone. Compressive strength here means the maximum load a specimen sustains in a compression machine divided by the area of the face that carried the load, which is why the result is expressed as a stress in pounds per square inch or megapascals. The standard’s own statement of significance is modest: the method is useful in indicating the differences in compressive strength between the various dimension stones, and in comparing stones of the same type with one another.

The pass-or-fail numbers live in a separate family of documents, the ASTM material specifications. Each covers one commercial stone group and lists the minimum or maximum physical properties a stone must show to be sold under that classification: ASTM C615 for granite, C503 for marble, C568 for limestone, C616 for quartz-based stone such as sandstone and quartzite, C1526 for serpentine and C1527 for travertine. Each specification points back to test methods for its values. Absorption and density are measured by ASTM C97, compressive strength by C170, modulus of rupture by C99 and flexural strength by C880.

How the Test Is Run: Specimens, Conditioning and Loading

Specimen shape and preparation

C170 specimens are cubes or cylinders, sawn or core-drilled from a sample chosen to represent the true average of the stone type or grade being evaluated. The lateral dimension of a cube, or the diameter of a cylinder, is not less than 2 inches, so in practice most laboratories work with blocks of roughly two to three inches, about 50 mm, on a side. The small size is deliberate. It lets a modest sample yield a full set of specimens and keeps the failure loads within the range of a normal laboratory compression machine even for very strong granites and quartzites.

Surface preparation of those blocks is where a careless laboratory, or a poorly cut sample from a shop, can spoil a result. The two faces that will bear against the machine platens are ground flat and parallel, and the specimen needs to be free of nicks, chipped corners and saw damage. Any high spot concentrates load into a small area and starts a crack early, so the block fails at a lower figure than the stone deserves. The dimensions of each specimen are measured and recorded so the load-bearing area can be calculated, and both the bearing faces and the direction of the rift are marked on every piece.

Dry and wet conditioning

Stone is tested in two moisture states because water changes how many stones behave. For the dry condition, specimens are oven-dried for 48 hours at about 140 °F (60 °C). They are weighed at the 46th, 47th and 48th hours, and if the weight is still falling the drying continues until three successive hourly readings agree. The blocks are then cooled to room temperature in a desiccator so they do not pick up moisture from the air before they reach the press. The intent is a specimen with a known, stable, essentially moisture-free condition.

For the wet condition, specimens are immersed in water for 48 hours at roughly room temperature, about 72 °F (22 °C), and tested promptly after they come out of the bath. A dense, low-absorption granite may show little difference between its wet and dry figures. A porous limestone or sandstone can lose a noticeable share of its dry strength when saturated, which is exactly the information a designer needs for paving, copings, pool surrounds and any exterior stone that will spend part of its life soaked. A data sheet that reports only a dry value has left out the less flattering half of the picture.

Parallel and perpendicular to the rift

Stone is rarely the same in every direction. Sedimentary stones have bedding planes, metamorphic stones can have foliation, and even granites have a rift, the direction along which the stone splits most easily. C170 therefore calls for specimens to be loaded both parallel and perpendicular to the rift. Loading across the layers generally gives the higher figure; loading along them can let the block split apart at a lower load. The direction of load relative to bedding, where bedding is present, has a strong influence on the result.

Put the two moisture states and the two load directions together and there are four test conditions: dry parallel, dry perpendicular, wet parallel and wet perpendicular. The method calls for at least five specimens in each condition, so a full C170 programme typically consumes a minimum of twenty specimens from the sample. A report built on fewer pieces, or on a single orientation, is still information, but it is not the complete test, and it should not be compared line for line with a full report on a competing stone.

Loading, calculation and reporting

Each specimen is centred in the compression machine and loaded at a uniform rate not exceeding 100 psi per second until it fails. The maximum load is recorded and divided by the calculated bearing area to give the compressive strength of that specimen. A thorough report identifies the stone, lists specimen dimensions and orientation, states the conditioning used, and gives the individual results along with the average for each condition. The scatter between individual specimens deserves a look: a stone whose five results sit close together is more predictable than one with the same average and a wide spread.

The table lists only the classifications whose minimums could be cross-checked for this article. Several specifications contain further classes. ASTM C568 sorts limestone into low-density, medium-density and high-density classifications, with the required compressive strength stepping up as density rises, and C616 also includes a sandstone class below quartzitic sandstone with a lower requirement. ASTM C1526 sets its own minimum for serpentine. Specifications are revised periodically, so for contract work always confirm the figures against the current edition of the relevant standard instead of a chart copied from an old submittal.

Reading the Minimum Values in the Material Specifications

The spread of the minimums says a lot about how differently the stone groups are built. Granite, under C615, must reach 19,000 psi, and quartzite under C616 must reach 20,000 psi, while marble under C503 qualifies at 7,500 psi and medium-density limestone under C568 at 4,000 psi. None of the lower figures means the stone is unfit for building. They reflect what sound, commercially proven material of each type actually delivers, and each specification pairs its strength requirement with limits on absorption and density that describe the same stone from another angle.

ASTM C616 is a good illustration of how classification and strength travel together. Quartz-based stone is sorted by its free silica content: a stone needs at least 90 percent free silica to be classed as quartzitic sandstone, and the quartzite class sits above that. The required compressive strength doubles from 10,000 psi for quartzitic sandstone to 20,000 psi for quartzite. A slab sold under the trade name “quartzite” that is really a softer sandstone or a dolomitic marble would not come close to the quartzite line, which is one reason a current test report is more trustworthy than a name on a bundle tag.

What Compressive Strength Tells a Fabricator, and What It Does Not

Stone in service almost never fails by being crushed. Even the minimum for medium-density limestone, 4,000 psi, is a stress far beyond anything a countertop, a hearth or a floor tile experiences from the weight of appliances, people or stored goods. In structural stonework designers do not even use the test value directly; they divide it by an established safety factor for the stone type to arrive at an allowable working stress. For a fabricator, the practical reading is simple: if a stone meets the compressive requirement for its class, crushing is not the failure to worry about.

The failures a shop actually sees are bending failures. A slab cracks across a sink cutout when it is carried flat, a narrow rail in front of a cooktop snaps during installation, an unsupported overhang breaks when someone sits on it. In every one of those cases the stone failed in tension on the stretched side of a bend, and stone is far weaker in tension and bending than in compression. That is why the material specifications also carry requirements tested by two bending methods, and why those rows of the data sheet deserve more of a fabricator’s attention than the compressive line.

ASTM C99, modulus of rupture, breaks a short, thick specimen of 4 by 8 by 2¼ inches under a single load applied at midspan. ASTM C880, flexural strength, uses a longer, thinner specimen under quarter-point loading, in which the load is split between two points so that the middle portion of the span sees the same maximum bending moment. Because a larger length of stone is put under peak stress, C880 is better at finding local weaknesses such as veins and fissures, and it can be run on specimens of the actual job thickness and finish. For thin stone, C880 is the more informative of the two.

Compressive strength also says nothing about several properties fabricators are asked about every day. It is not hardness, so it does not predict scratch resistance or how fast a blade or polishing pad will wear. It is not abrasion resistance, which the specifications address separately for stone subject to foot traffic. It is not absorption, so it does not predict staining. And it is not resistance to acids, which depends on mineralogy: a marble and a granite with respectable strength figures behave completely differently when lemon juice sits on them.

Trade Tips for Reading a Stone Data Sheet

Start by checking that every value names its test method and its units. A sheet that lists “compressive strength” with no reference to C170, or to a European or other national method, cannot be compared with one that does, because specimen size, shape and conditioning all influence the number. Different test methods are not interchangeable, and a figure obtained under one cannot be judged against a minimum written around another. If the method is missing, ask the supplier for the underlying laboratory report.

Then go straight to the bending rows. If the sheet gives only modulus of rupture by C99, treat it as an optimistic indication for slab work, since the short, thick specimen does not behave like a thin countertop. If it gives flexural strength by C880, note whether wet and dry conditions and both orientations are reported, and whether the test thickness is stated. For large-format panels, mitred waterfall ends and long unsupported spans, the wet flexural value in the weaker direction is the conservative figure to carry into the conversation with the designer.

Because bending, not crushing, is the working risk, the practical response in the shop is about support and reinforcement. Slabs and cut pieces travel on edge, not flat, so their own weight does not bend them across a cutout. Narrow rails at sinks and cooktops are the classic place for rodding: a slot is cut in the underside with a rodding blade and a rod is bedded in adhesive so that it carries tension the stone cannot. Stones with low flexural values, heavy veining or visible fissures are the ones where that step should not be skipped.

Keeping Test Records Useful Over the Long Term

A shop that handles commercial or specified work should keep data sheets and laboratory reports with the job file, tied to the supplier, the stone’s commercial name and, where available, the block or bundle numbers. If a question comes up years later about a cracked panel or a rejected submittal, the ability to show which report accompanied which material is worth far more than a general statement that the stone “meets ASTM.” Keeping the edition year of the specification on file matters too, because the standards are revised and requirements can change between editions.

For the reinforcement side of that work, Dynamic Stone Tools stocks rodding blades for cutting reinforcement slots, along with rod stock such as fiberglass rodding for stone slab reinforcement. Bedding and lamination materials are grouped under epoxy adhesives, cantilevers and breakfast bars can be backed up with overhang supports, and on-edge transport is covered in the material handling range.