STONE CURATORS | TECHNICAL BULLETIN #1

Limestone Classes: What They Mean and Where to Use Each

For landscape architects specifying limestone and contractors installing it 

Why this matters

Limestone is one of the most requested stones in architectural and landscape work for its warm color, fine texture, and timeless look. It is also one of the most variable. ASTM C568, Standard Specification for Limestone Dimension Stone, divides limestone into three classes by density (Table 1) [1]. Lower-density, more porous stone (Classes I and II) can perform well on walls and cladding, but is prone to problems in exterior horizontal hardscape, where it absorbs water, freezes, meets de-icing salts, and carries traffic (Table 2) [1] [4]. High-density Class III stone is generally the right choice for exterior paving, steps, and coping [1] [4]. We call this “hard limestone”. The working rule: choose the class for the exposure, then confirm the specific stone with current test data [3] [5].

Table 1. ASTM C568 physical requirements by class 

*Applies only to stone subject to foot traffic. C1353 is intended to replace C241; only one test is needed [1].

Values in Table 1 are from ASTM C568/C568M [1]. Unlike the granite (C615) and marble (C503) specifications, C568 sets no flexural strength (C880) minimum for limestone; modulus of rupture (C99), tested on a 2 1/4 in specimen, is its bending requirement [1] [2]. C880 testing at actual project thickness is still worth requiring for paving, treads, and coping [3]. The standard notes that finishes other than those in the test methods (such as thermal or bush-hammered) can change results [1].

Class does not guarantee durability. A stone can meet Class III density in Table 1 and still contain clay seams, stylolites, or weak bedding planes that fail under freeze-thaw or salt exposure [3] [7]. C568 separately requires stone to be “sound, durable, and free of spalls, cracks, open seams, pits, or other defects” [1]. Density and absorption are screening tools, not a performance guarantee [7].

Table 2. Where to use each class

*Avoid, or use only with project-specific testing and a proven track record in the same exposure and climate [4] [6].

Freeze-Thaw Conditions: Limestone with density and absorption values slightly below the Class II vs. Class III cutoff should not automatically be rejected for paving applications in freeze-thaw environments. In some cases, moisture mitigation (drainage and sealants) can make Class II suitable. Also, while the test numbers might indicate one thing, the actual performance history of a stone in freeze-thaw environments might tell a different story. This is where a trusted stone supplier can help you.

Do’s and don’ts

For specifiers:

• Do name the ASTM C568 class (Table 1). “Limestone” alone is not a specification [1].

• Do recommend Class III for exterior horizontal surfaces in freeze-thaw regions, pool environments, and salted areas (Table 2) [1] [4].

• Do request test reports for the specific quarry and bed, plus C880 flexural testing at project thickness and finish for paving, coping, and treads [3] [5].

• Don’t select paving or coping stone from a sample board or photo, or assume a stone that works as cladding will work as paving [4].

• Don’t treat density as proof of durability. For severe exposure, ask for freeze-thaw results (ASTM C666 or C1645) and petrography (ASTM C1721) [3] [7].

For installers:

• • Do check crate tags against the approved class, quarry, and finish before installing.

  • Do build in slope and drainage, and isolate stone from rising moisture (damp-proof unexposed faces or use pedestals per the design) [6].

  • Do use setting and jointing materials compatible with natural stone [4] [6].

  • Don’t substitute a softer look-alike stone without written approval and test data.

  • Don’t use chloride de-icing salts on limestone. Use sand or traction grit, and notify the owner in writing [6].

  • Don’t set exterior limestone saturated or in freezing conditions [6].

Sample specification language

(CSI Master Format Section 32 14 40)

2.1 LIMESTONE —
A. Limestone for exterior paving, steps, treads, pool coping, and wall caps should comply with ASTM C568/C568M, Classification III (High-Density), with the following Table 1 properties [1]:

1. Density: 160 lb/ft³ min.; Absorption: 3.0% max.; ASTM C97 [1].

2. Compressive strength: 8,000 psi min., ASTM C170 (wet and dry, parallel and perpendicular to bedding) [1] [3].

3. Modulus of rupture: 1,000 psi min., ASTM C99 [1].

4. Flexural strength: as required by the stone design for the specified thickness, ASTM C880, tested at project thickness and finish [3].

5. Abrasion resistance: Ha 10 min. (Ha 12 min. recommended for exterior paving and steps), ASTM C1353 (or C241) [1] [4].

6. Stone shall be sound, durable, and free of spalls, cracks, open seams, pits, and other defects likely to impair performance in its intended use [1].

1.4 SUBMITTALS —
A. Independent, accredited laboratory reports for each proposed quarry and bed, showing the Table 1 tests above, dated within three years of submittal [5]. B. For freeze-thaw or de-icing exposure: freeze-thaw results (ASTM C666 or C1645) and petrographic examination (ASTM C1721) [3] [7]. C. At least three references for the same stone in similar exposure and climate. D. Samples showing the full range of color and texture, to serve as the control standard.

How to verify what you are getting

• Ask for current, full lab reports, not a summary sheet. The Natural Stone Institute treats technical data more than three years old as outdated [5].

• Match the report to the product: same quarry, same bed or block selection, same finish. Honed-specimen results do not represent a thermal or bush-hammered surface [1] [3].

• Read the conditions. Look for wet and dry results, parallel and perpendicular to bedding. The lowest value governs [3].

• Check every Table 1 property, not just density. A stone that passes density but misses absorption or strength does not meet the class [1].

• Go beyond C568 for severe exposures: freeze-thaw and salt testing, petrography, and documented local track record are the best predictors of long-term performance [3] [7].

• Verify on delivery. Match crate tags and lot numbers to the approved submittal; pull samples for independent testing if in doubt.

Disclaimer

Stone Curators provides quarry-specific ASTM test reports, range samples, and fabrication guidance for every limestone and granite we supply. This bulletin is general technical guidance; it does not replace project-specific engineering, testing, or the current edition of the referenced standards.

Sources

[1] ASTM International, C568/C568M Standard Specification for Limestone Dimension Stone (2022). https://store.astm.org/c0568_c0568m-22.html

[2] Lawton Adams, Table of Dimension Stone Testing Values per ASTM Standard Specifications. https://www.lawtonadams.com/Customer-Content/www/CMS/files/StoneTestingValues-.pdf

[3] IIBEC, Dimension Stone Testing and Evaluation: The What, Why, When, and Where (2024). https://iibec.org/publication-post/dimension-stone/

[4] Natural Stone Institute, Limestone and Dolomitic Limestone Technical Guidance. https://www.naturalstoneinstitute.org/designprofessionals/varieties/limestone/

[5] Natural Stone Institute, Materials Test Laboratory: Natural Stone Testing Services. https://www.naturalstoneinstitute.org/resources/natural-stone-testing-services/

[6] Indiana Limestone Institute of America, Indiana Limestone Handbook (22nd ed.). https://www.indianalimestonecompany.com/Indiana_Limestone_Institute_of_America_ILIA_Handbook.pdf

[7] Wiss, Janney, Elstner Associates (WJE), Evaluating Stone Durability. https://www.wje.com/assets/pdfs/articles/WJE-PRIMER-StoneDurability.pdf