Thin stone veneer

Thin stone veneer is a term used since the 1960s to describe stone that is less than 2 inches thick and is applied to the façade of a building in a non-load-bearing manner. A variety of stone, such as marble, granite, or travertine, are some of the most common types of materials used. Most thin stone veneer can be treated with a number of surface finishes and colors to achieve different architectural effects.

History
The first stone material used for construction was commonly thick slabs and blocks. A veneer was used to describe building stone and the term is traced to the 1890s. The first veneer stone was hand-cut and measures nearly four inches thick and could be used to construct tall buildings. Reliance Building in Chicago used stone veneer between two to four inches thick for the first two floors on the exterior portion. Thinner slabs were found in the early twentieth century for storefronts and building interiors. By the 1930s, stone veneer was being applied to large curtain walls and later became a popular technique for cladding entire building exteriors. The Rule-Page Building in Iowa and the Federal Reserve Bank in Michigan were some of the first buildings to use stone veneer to clad the entire building. Later, in the 1960s, the John F. Kennedy Center for Performing Arts in Washington, D.C., became one of the first buildings to receive thin stone veneer cladding. Marble veneer clads the entire building at roughly one inch of thickness. Because of advancements in knowledge about thin stone veneer, architectural preferences resulted in a dramatic increase in use.

Manufacturing Process
Before the 1900s, building stones were retrieved from quarries and were hand-finished into thick slabs or blocks. After the 20th century began, machines were introduced that could use multibladed frame saws that could simultaneously cut the blocks into smaller slabs. Water wheels were used to cool the blades and take away the resultant abrasive sludge. Eventually, water pumps and machines were refined to regulate the quantity of abrasive used. In 1932, the National Building Granite Quarries Association described the gang saw process as being almost entirely by machine. It used three to seven reciprocating steel blades running parallel to one another in the frame and could cut down through the stone to make uniform pieces. Several finishes were available at this time including: rock faced, sandblasted, peen hammered, rubbed, honed, and polished. The honed finished could be achieved using a fine grained grinding wheel while a polished finish required a heavy felt-coated wheel and jeweler’s rouge. Thin stone veneer was typically used for interiors, store fronts, and façades of buildings at the street level. Later, diamond-studded cables to slab blocks were introduced as well as a frame saw with metal abrasives, which could cut stone to 1/8 inch thick for composite panel use.

Uses and Installation
Veneer panels were usually laid up on mortar beds and the joints were finished in the same style of traditional masonry construction with mortar. Interior panels were attached to backup material of brick or block using wire anchors that were set in holes drilled into the panels and discrete mortar spots. Lateral anchors for exterior panels were made of galvanized and asphalt-coated steel rods that were bent and set into holes. The rods went into the stone roughly ¾ of an inch for stone that were two inches thick and 1 ¼ for panels four inches thick. Plaster of pairs could be used to quickly set spacers behind the panels. It was typical to use four anchors per panel, two to each side (either left to right or top to bottom). The Cold Spring and North Star Granite Company was one of the first companies to market the 1 inch thick granite veneer (1940), followed by the Aberene Stone Company who could manufacture granite veneers at 7/8 inch. A shelf angle was strongly recommended between each sheet to help carry the weight of the panels. Filling the joints with mortar was still the ideal way to finish the wall. Lateral anchors for thin stone were typically brass or steel rods that were set into holes filled with cement mortar. If the stone was more than 2 inches thick, it would traditionally be accompanied by steel straps. During the 1940s, the use of straps became the more popular form of lateral support for thin stone veneer. In recladding, the anchor was twisted 90 degrees and mortared or plastered into a slot in the backup masonry. New construction usually required the straps being mechanically fastened into the back material because they could be bent out of the way during the setting of the veneer and then bent into a slot for easy installation. In the 1950s, stone was usually produced in four different thickness; 7/8, 1 ¼, 1 ½, and 2 inches, though 1 ½ was suggested for veneer. 7/8 was thought to be more expensive to produce. Industry standards states tolerances being plus or minus ¼ inch and spacing between the panels at ¼ inch. Expansion joints were recommended to be placed every 30 feet vertically or every second floor horizontally. Creating a cavity (roughly 1 inch behind panels) filled with mortar and coating the back of the panel with damp proofing was also suggested. The popularity of the curtain wall and the improved manufacturing efficiencies promoted the use of thin stone veneer in the 1950s. Elastomeric sealants soon took the place of mortar in the horizontal joints. Aluminum, plastic or lead shims were placed in horizontal joints to transfer panel weights. Soon, manufacturers began suggesting that more lateral anchors be used. Different lateral anchors types emerged, including stainless steel straps, brass dowel and wires, and disks. The split-tail anchor gave anchors the ability to address gravity and lateral supports. In the later 1950s, stone was being used in composite building panels. Precast concrete panels faced with a thin stone veneer became the most popular. Hairpin wire anchors or dower anchors were preset into the back of the stone panel before casting. The thin stone veneer component was then formed by placing the concrete against the backside of the stone panel with the panel set face down in a form. A bond breaker sheet between the concrete and stone was not incorporated before casting the concrete. The technique of separating materials was more commonly use in the 1960s. Thin stone veneer became more standardized in the 1960s.

Conservation
Conservation of thin stone veneer if the cause of distress or deterioration is understood and the basic diagnostic and assessment techniques are well employed. Conservation techniques are wide ranging and thorough understanding of the techniques available will help in developing correct repair methods.

Deterioration
Connections and support are the most common causes of distress seen in stone cladding. The major types of distress observed with the typical anchoring systems include cracks around the anchors location, spalls at anchor locations, spalls at shelf angles supports, and cracks at panel corners. Rust staining is also common with these types of anchor systems. A variety of factors can cause these types of distress, including improperly designed anchors. Distress caused by improperly designed anchors can lead to cracking in adjacent stone of displacement of panels. Missing anchors can cause distress in the stone. Also, water penetration through the cladding can corrode the metal anchors and shims. Improper attachment of anchors, improper anchor positioning, and missing separators between concrete backup and panels are just a few other problems that can affect the structural integrity of the thin stone veneer structure. While most problems associated with stone veneer can be associated with improper installation, cyclical heating and cooling, water-penetration, freezing and thawing, and acid rain can also deteriorate building stone. Marble loses more strength than granite. The strength loss is related to the panel’s thickness, the thinnest panel is expected to be the most affected by strength loss. Stone curtain walls rely on the strength of the stone panel material; if it is not strong enough, it may present with cracking or bowing between supports. Marbles are not volume stable as the experience permanent volume growth as a result of heating and cooling. The marbles strength is decreased and can lead to warping or bowing as the opposite side of the panel experiences differential thermal expansion. Visible characteristics of stones have been linked to strength loss. Bowing, warping, sugaring of the surface, and exfoliation are just some surface appearances that imply strength loss. Composition of the stone is not limited to different varieties. Stones can experience different compositions in a single piece. All stones have a natural bedding orientation and their strength and durability can be affected. Distress in stone cladding components can also be related to buildings and wall movements or differential movement not accommodated by the cladding design. If insufficiently sized joints are used to accommodate building movement, panels can come in contact with another during thermal expansion which can result in cracking or spalling near the edges and corners. Staining is not usually a structural issue and can be related to preferential weathering of the stone’s mineral components, corrosion of ferrous components, or water runoff from surrounding minerals. Staining may also be caused by excessive water leakage through joints or cladding units and may indicate corrosion of embedded material. This can be a cause for concern because the structure may be suffering.

Conservation Techniques
Reviewing original construction documents and design calculations will assists in understanding the overall construction. Removing portions of the veneer for inspection or laboratory testing may be the only way to fully understand the construction of the veneer. Less intrusive ways should be attempted first because they might eliminate need for invasive procedures. Tapping or sounding with a hammer, metal detection tools, or field microscope are some of the less evasive ways to investigate the structural integrity of thin stone veneers. Conservation of the material usually involves the documentation the number of specific condition related to the veneer’s attachment to the building. It is important to determine if it is the anchors promoting distress, if there are enough anchors, if the anchors are properly sized and spaced, and if they are properly fastened to the structure. Corrosion of the materials should also be investigated as this can cause distress. Shim materials should be checked to ensure proper support. Shims should be properly placed in slots cut in the stone, between panels or panels and shelf angle supports. Panels should be checked for movement, cracking, or unusual surface conditions. Joints between veneer panels should be properly constructed to accommodate building movement and should be free from inappropriate materials. Sealant in the joints provides information relating to movement and distress. Repairs often involve replacement of connections or support components and may also involve selective or complete replacement of the cladding. Positive mechanical anchorage may be required; through-bolt anchors covered with sealants or mortar is commonly used if only a few components need repair. Expansion anchors can be attached to precast backup or dowels set in epoxy can be attached to the precast backup. Perimeter edge support systems attached to adjacent building construction can also be used. Cracked portions of panels can usually be reanchored. If the piece is severely distressed, the portion can be removed and a small panel piece (dutchman) can take its place. Dutchmen are pinned or fastened to the existing backup. If a dutchman is unavailable, mortar patches can be installed. Unaccomodated building and wall movements can be repaired by entail grinding the joints to make them wider, which allows for movement. If staining and discoloration are proof of distress and the cause determined, cleaning may be desired. Mild detergents and clean water washes or two-part chemical systems with an alkali prewash and an acidic afterwash can be used. Test pieces should be used to ensure no damage to the original structure.

Replacement
Removal and reinstallation should always be consider, but replacement of the stone cladding can also be considered. If the structure has undergone significant loss of material strength and additional anchors will not provide the appropriate support, the panels will most likely need to be replaced. The performance characteristics should be evaluated to ensure that it matches with the pre-existing piece. Testing of the material should be performed, including testing of anchorage components and whole assemblies. Tests, including those to determine the flexural strength and compressive strength should be performed in accordance with the standards established by the American Society for Testing and Materials. Also, testing the density, absorption, and aging characteristics is strongly suggested.