Steven G. Naggatz, AIA, NCARB
Time is money. Developments in technology and production continue to push the envelope and advance the speed at which buildings are constructed. When properly designed, specified, and installed, thoughtful details and materials can assist the construction team by significantly reducing the installation time. Even when well-intentioned, the use of incompatible materials to increase productivity when installing cladding can result in disastrous situations.
Dimension stone and limestone cladding
Stone has been used as a building material for thousands of years and continues to be utilized for decorative and functional purposes. Its esthetics and sense of permanence have continued to make it a popular material choice among architects and owners. Buildings of great significance, such as churches, civic centres, and government facilities, were constructed of stone to reflect the structure’s importance within its urban context and culture.
The term “dimension stone” refers to natural stone fabricated to specific sizes or shapes with or without a dressed or finished surface. Dimension stone cladding relies on inexpensive backup materials in combination with more expensive facing. Early 20th century stone structures relied on cladding panels that were at least 100 mm (4 in.) thick. These stone-clad buildings were commonly multi-wythe loadbearing assemblies combining high quality stone finished to very strict tolerances with a looser and less expensive rubble or brick backup assembly. Often, cladding of varying thicknesses was keyed into a masonry backup assembly. Iron, galvanized steel, copper alloys, and aluminum have also been used as dimension stone cladding attachments. Advances in fabrication technology and the introduction of new systems have changed how stone is anchored to substrates. Consequently, stone panels have decreased in thickness over the past 50 years. Modern dimension stone cladding panels are commonly between 30 and 50 mm (1 ¼ and 2 in.) thick and are anchored with stainless steel—typically rods, straps, dowels, or a combination thereof. Recently, composite assemblies have been introduced in flat panel applications. These panels combine stiff backer materials, such as aluminum honeycomb and adhesives, and have reduced stone thickness to as little as 5 mm (¼ in.). The use of ornamental or decorative stone still requires the use of elements that are more than 100 mm thick.
Limestone is a sedimentary rock consisting of calcium carbonate (calcite). Dolostone is a similar sedimentary rock consisting of mainly dolomite, essentially limestone with some amount of magnesium. They are suitable for many architectural and structural applications. For the purposes of this article limestone will refer to either sedimentary rock. Considering their sedimentary formation, limestone and dolomite have a natural lamination or bed (stratum), and in many cases, the bedding is sufficiently uniform, so it can be machined or cut with little risk of splitting. Limestone tends to be fairly uniform in strength (isotropic) when loaded parallel or perpendicular to its natural bedding plane compared to other types of dimension stone, such as marble, granite, or slate. For durability, it is preferential to install limestone cladding panels with its natural bed, horizontally, in a manner consistent with its original formation. ‘Face-bedded’ panels (i.e. bedding planes are vertical) are more vulnerable to damage (delamination) through various mechanisms, such as crystallization of salts, dissolution of clay layers, and freeze-thaw cycles. These processes can result in separation at the bedding layers within the panel.
Mechanical properties of limestone vary between quarries, and to some extent, within the quarry itself. The range and variability of mechanical properties of the stone proposed for use should, therefore, be determined by testing.
Dimension stone attachments
The test standards and specifications most commonly used to evaluate dimension stone in the United States and Canada are published by ASTM International as well as CSA S304, Design of Masonry Structures. Cladding elements should be evaluated further with statistical methods and attachments and designed using sound engineering principles, and building code requirements. Metal anchors in direct contact with limestone should consist of American Iron and Steel Institute (AISI) Type 304 or 316 stainless steel. Other noncorrosive anchors, such as copper or bronze, are also acceptable. Due to the potential for chemical attack and degradation of the material itself, anchors in direct contact with limestone should not be fabricated with aluminum unless the latter is protected with a corrosion-inhibiting coating. Mill finish or anodized aluminum can be used with granite. The anchor type varies, depending on the application, but generally consists of wires, dowels, or straps set into holes, kerfs, or other sinkages. The hole, kerf, or sinkage will routinely be oversized to accommodate installation and construction tolerances, and accommodate in-service behaviour and/or incidental cladding movement. The ancillary space between the anchor itself and the hole or slot in which it is embedded should be filled with compatible material to prevent moisture accumulation within the void space.
ASTM C1242, Standard Guide for Selection, Design, and Installation of Dimension Stone Attachment Systems, recognizes the use of epoxy-filled holes for liner blocks, or in the case of precast concrete backup, as an adhesive bond for primary anchorage between the stone and stainless steel dowel. However, some epoxy adhesives may be unsuitable for limestone attachments or specific applications (Consult the 22nd edition of the Indiana Limestone Handbook by the Limestone Institute of America, Inc.).
