PAINTSQUARE BLOG

Stone, in general, is considered a durable material; however, not all stones are as durable as one might anticipate.

Stone durability is dependent on its composition, which can affect strength and appearance, and environmental exposure factors. Even though a particular stone may be thought of as uniform, it is still a natural material with varying composition and physical properties. These properties can vary significantly, even for stone removed from slightly different areas of a single quarry or formation.

Small variations in some stone types can result in significant differences in durability performance in service.

This blog post explores the stone selection process and provides a systematic approach for evaluating a stone type for durability.

Use and Exposure

Stone durability is greatly influenced by its use and exposure. A stone that performs well in one application or geographic location will not necessarily perform well in all applications and locations. Factors that contribute to accelerated deterioration of stone include exposure to moisture, freeze/thaw cycles, temperature extremes, rain acidity, and crystallization of salts. While all of these factors are influenced by project location and local climate, several of these factors are further influenced by application or location on the building.

For instance, horizontal applications such as paving or coping units that are exposed to standing water or snow/ice are typically considered more severe exposures than vertical applications. Severe exposures that are influenced by location on the building include panels near grade that are exposed to deicing salts; locations with greater frequency of freezing and thawing such as southern exposures where surface temperatures are significantly greater during the day than at night; and locations that are not moderated by the interior building temperature such as parapet walls.

Color and Thickness

The color and thickness of the stone may, in certain situations, affect its durability, as a dark colored stone will experience greater temperature extremes than light colored stone, and a thin stone will not moderate temperature extremes through its thickness as much as a thicker stone.

Photos courtesy of Wiss, Janney, Elstner Associates Inc.

Examples of deterioration that are influenced by location on a building. Marble deterioration at the base of a building due to deicing salts (left). Typical freeze/thaw damage to a travertine coping stone (right).

Given sufficient time and certain exposure conditions, any stone is vulnerable to some degree of deterioration when exposed to environmental weathering. Test methods and evaluations that provide a better understanding of the potential for poor in-service durability are a critical part of the stone selection process.

The following is a systematic approach for evaluating a stone type for durability. Each task provides a progressively rigorous review. However; if one of the first tasks indicates a marginal or poor quality stone, it is likely time/cost effective to consider an alternative stone.

Task 1 – Project Review

Where a stone has an established history of use (20 or more years) in an environment and application similar to that being considered, the long-term durability of the stone is most accurately assessed by previous performance of the stone in similar applications. The applicability of this evaluation (and testing summarized below) depends on whether the material is representative of the range of material to be provided.

Task 2 – Petrographic Evaluation

Petrographic evaluation is a qualitative review of stone features or constituents that may affect stone strength and durability. Petrographic evaluation of representative specimens provides detailed information on the stone mineralogy and structure of the stone for identification of minerals that may cause spalling, staining, or other serviceability issues (e.g. iron inclusions or poor weathering minerals such as clay).

Petrographic evaluation can also identify mineral sizes, grain structure, susceptibility to cleaving, orientation of bedding/foliation/rift, or other variations in the stone fabric that may contribute to strength variations, bowing, or low durability. It can also be used to help determine the cause of distress in an existing application. Petrographic evaluation is typically performed in accordance with the pertinent sections of ASTM C1721, Standard Guide for Petrographic Examination of Dimension Stone.

Thin-section view of stone with adjacent porous (upper left; void space filled with blue epoxy) and non-porous (lower right) layers. Variable absorption presents a potential concern under cyclic freezing and thawing.

Task 3 – Quarry Evaluation

Petrographic evaluation of a quarry is performed to provide a broad overview of the uniformity of the exposed stone or blocks within the quarry and presence of visible features or textures that may cause serviceability issues for use in establishing quality control measures or determination of where material is to be removed. Quarry evaluation can also help to ensure that test specimens are representative of project material.

Effusive igneous rock formation showing several layers from which material is being quarried.

Task 4 – Physical/Mechanical Testing

Physical/mechanical properties of dimension stone are typically assessed through testing in accordance with the following ASTM procedures: absorption and specific gravity (C97), modulus of rupture (C99), compressive strength (C170), and flexural strength (C880) testing. Testing may also include abrasion resistance (C1353) testing if the stone is to be used in a flooring or paving application.

ASTM publishes material specifications for granite, limestone, marble, sandstone, serpentine, slate, and travertine. Minimum values listed in these specifications are based on stone that has traditionally performed well in service for general building and structural purposes.

Stone with absorption and density values that meet ASTM requirements are typically considered to have properties that are consistent with and compare favorably to other stones of that type that have been shown to perform satisfactorily in weathering environments. However, this testing is not comprehensive, as the composition and void structure of the stone are not assessed through this testing; factors that can significantly affect durability. Because of this, simulated weathering testing or other durability testing is more predictive of weathering durability

Task 5 – Durability Testing

Durability testing is particularly useful for a stone with unknown or limited performance history, or for stone that does not meet the minimum specified physical properties for that stone type, to reduce the risk of using a poor durability stone on a project.

These tests are greater cost and longer duration than the standard physical/mechanical tests, but provide the most accurate durability assessment of a stone with limited historical precedent in severe weathering applications. While durability tests may not be of great value on a new project when using a stone that has an established history of use in the environment similar to that being considered, they may be appropriate for large or high profile projects where testing represents a small percentage of the total project cost regardless of the stone type.

Simulated weathering testing that is most often performed is used to evaluate the effect of temperature extremes and freeze/thaw cycles in the presence of moisture on stone durability. For some stone types, durability can be assessed to a certain degree using procedures for evaluating concrete durability outlined in ASTM C666, Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing. However, the temperature range in this test (0 to 40 degrees Fahrenheit) does not represent service temperature ranges known to affect some stone types (particularly marble) or the potential effects of acid rain (particularly for marble and limestone).

While there is no ASTM test method for simulated weathering of dimension stone, an “accelerated weathering” test method has been used to evaluate durability of stone proposed for use in severe weathering environments for more than 30 years and employs procedures similar to those outlined in ASTM C666. This testing involves cyclical exposure of stone specimens to extreme temperatures (typically -10 to 170 degrees Fahrenheit) while they are partially immersed in water or a 4pH acid solution to simulate acid rain exposure. The flexural strength of specimens tested after exposure to 100 (or in some cases 300) temperature cycles is compared to the strength of specimens not exposed. Dynamic modulus of elasticity testing is also often performed to non-destructively evaluate rate of deterioration at periodic intervals in accordance with ASTM C215, Test Method for Fundamental Transverse, Longitudinal, and Torsional Frequencies of Concrete Specimens.

Other durability tests include salt crystallization to evaluate resistance of a stone to crystallization of salts within its pore structure (EN 12370 - Determination of resistance to salt crystallization), and bow testing to evaluate the potential for marble bowing (EN 16306 - Determination of resistance of marble to thermal and moisture cycles).

Varying degrees of deterioration exhibited by specimens after accelerated weathering exposure. (Clockwise from top left) Heavily deteriorated and cracked marble specimen; surface spalling of sandstone specimen; more advanced surface spalling of limestone specimen; stain spot in granite specimen.

Summary

A poor durability stone can result in diminished building appearance, may require supplemental repairs to address deterioration, or in extreme cases may require replacement of panels or entire facades.

While durability assessment of stone may seem like an unnecessary expense during the stone selection process, identification of poor performing stone or features within a stone early in a project can result in significant savings down the road.

About the Author

Since joining WJE in 2003, Daren Kneezel has been involved in projects ranging from investigation of high-rise building facades and masonry deterioration to laboratory testing of stone and other construction materials and insitu testing of scaffolding davits and retrofit anchorage systems. He has also performed structural and finite element analyses and developed repairs for wood trusses and thin stone cladding and its anchorage systems. You can reach Daren at dkneezel@wje.com

ABOUT THE THE BLOGGER Wiss, Janney, Elstner Associates Inc. “Solving for Why” is written by professionals at Wiss, Janney, Elstner Associates Inc. Since 1956, WJE’s primary goal has been to provide the best solutions for its clients’ new and existing construction-related problems. The firm’s highly qualified engineers, architects, and materials scientists possess a collective knowledge gained from solving, as well as helping clients avoid, thousands of problems. Author information is available at the bottom of each blog entry. SEE ALL CONTENT FROM THIS CONTRIBUTOR