Journal of architectural technology published by Hoffmann Architects, specialists in exterior rehabilitation Third Issue 1997 Volume 15 Number 3

Thin-Stone Veneers Offer Unique Design and Performance Demands

relatively new product known as rein- Theodore F. Babbitt, AIA forced stone veneer is also available. Made of very thin stone veneers N ew stone-cutting technologies laminated with epoxy to a reinforcing have allowed for ever-thinner stone expanded-steel mesh, these reinforced veneers to be used as a curtain-wall stone panels come in 5/16" and 3/16" component on high-rise and other thicknesses and offer the strength of buildings, a radical departure from the stone veneers twice their thickness. traditional use of dimension stone as This new technology was developed in a load-bearing element or a heavy- Turin, Italy and to date has been used in weight, non-load bearing facade. But Europe and City. while these veneers offer an excellent design opportunity at a cost lower An Intrinsic Danger than dimension stone, relatively little While the engineering principles and data is available on their long-term physical properties of dimension stone performance. And problems are are well understood and tested by starting to crop up after just two time, thin-stone veneers offer a new decades of use. Owners, architects, realm of design, engineering, fabrica- and builders should be forewarned tion, transport, and installation when choosing thin-stone veneers: demands. Testing prior to installation precise detailing is essential to may show no problems, but under prevent displacement, cracking, real-world conditions, veneers may spalling, water intrusion, and, ulti- begin to display unexpected and mately, failure. serious potential for failure. Actual failure has occurred in many cases. Thin-stone veneers are commonly defined as dimension stone cut to The very thinness which makes thicknesses of 2" or less (dimension veneers so economically attractive is stone is typically more than 2" thick). also the instrument of their downfall. As with dimension stone, veneers are That is because stone’s relative available in granite, limestone, marble, thickness has a direct effect on its sandstone, slate, and travertine. A performance. Thin veneers suffer more than dimension stone — and Thin-stone veneer being prepared for face- pinning. perform differently — under the Mr. Babbitt is an expert with more than onslaught of thermal movement, 45 years of experience in design and gravity, wind, seismic, and handling technical issues for exterior rehabilitation. loads, and with the settlement of As Director of Architectural Services, he surrounding building materials. oversees stone, curtain wall, and Veneers are also more subject to wear rehabilitation services for Hoffmann from environmental conditions. Even Architects. the building interior becomes more

1 Copyright Hoffmann Architects 1997 Hoffmann Architects / Journal

Common Problems creating weak points. The most common source of prob- lems in thin-stone cladding comes Stone texture can also play a role in from: performance. A thermal finish on • Anchorage failure due to placement granite veneer, for example, reduces of the kerf, slot, or hole too close to its thickness by up to 1/8th of an inch. the edge of stone. This will cause the In turn, that loss can reduce bending stone to break and lead to loss of strength by up to 30%, increasing the securement to the back-up structure. risk of cracking under seismic or wind • Anchorage failure due to corrosion, loads. Thermal finishes also increase material failure, incorrect installation, the risk of water infiltration, as the or unanticipated shear force (e.g. dramatic temperature differential An anchor set in a slot in the top of a deflection). created in applying the finish can cause thin-stone panel. • Anchorage failure due to warping or micro-cracking within the crystalline bending of the stone. structure of the granite. Those micro- prone to moisture damage when a • Cracking and spalling due to cracks can allow water to penetrate up thin-stone cladding is used. There is insufficient joint width or the use of to 1/4" deep into the stone, leading to simply that much less between the rigid grout or epoxy where a flexible cracking and spalling through the stress building interior and the elements. sealant should have been used. Both of freeze-thaw cycles. A similar Dimension stone is also far better able situations fail to accommodate thermal weakening effect occurs when brown- to handle the wide variances in or dynamic movement. stone is traumatized by bush-hammer- tolerances of companion building • Cracking and spalling due to water ing, a type of decorative finish. materials. The connections between infiltration behind the stone, which will panels require new and complex also lead to damage to the back-up Handling and fabrication concerns techniques. Maintenance demands are structure through subsequent freeze- Thin-stone veneers are particularly more critical. All of which requires thaw cycles. subject to handling damage, which can expert knowledge on the part of the • Cracking and spalling due to occur at the quarry, at the panelization design and construction team. improper installation, including over- drilling, or micro-cracking caused by Image and Endurance dry-sawing and dry-drilling. So why use thin-stone veneers at all? Simply put — aesthetics, performance, These common problem areas, and and economics. Stone, whether thin others, are looked at in more detail Kerf or thick, is the hallmark of classical below. design — timeless, monumental, and enduring. As a design medium, it Fabrication issues offers the architect and owner a stellar Stone is essentially a brittle material array of color, texture, and finish that will crack, spall, or shatter under choices, and can be used for exterior stress. How well stone performs over cladding, interior flooring and walls, time is as much a question of its Slot and ornamentation. Yet it’s not all interior composition as it is of its beauty. Stone provides a stable intended use. Strength, density, and thermal mass which is energy-efficient, composition can vary widely, even relatively resistant to environmental with the same type of stone. Natural damage, and easy to maintain. With weak points within the stone must be the advent of thin-stone veneers, all analyzed and addressed in both this becomes much more affordable to fabrication and design to reduce the Hole the building owner, as the lighter risk of cracking along rift planes. The weight of the stone will mean savings stone must be cut at the proper angle on structural costs. and to the correct dimension to avoid Illustration A: Anchorage Types

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or cutting plant, during transport to on high-rise buildings, as the panels Anchorage the job site, or during installation. The can be installed from the interior. Anchors connect the thin stone to the risk of cracking increases when the These panels frequently serve as structure of the building — the steel stone is transported flat or when lifted spandrels or in-fill panels on curtain or concrete spandrels and columns from a horizontal position. Even walls. The stone is attached to the which are the ultimate support of any vehicular vibration can cause damage. back-up (the structure of the panel) facade. Choosing the right anchorage Proper packing materials should be using stainless steel pins set in epoxy. is vital to the performance of thin- used to cushion the stone during stone veneers. Adhesives alone are transport. When a pre-cast concrete back-up is not enough to secure the stone to the used, the pins can be set in the stone, building. Even in the case of panelized Hysteresis with the concrete then cast against stone, the pins create a positive Most thin-veneer stones remain the stone to create a monolithic mechanical anchorage to the back-up dimensionally stable, even with thermal panel. For aesthetic reasons, “false” panel. In general, anchors should be expansion and contraction. Some joints are often created between made of stainless steel to avoid relatively pure, fine-grained marble stones that are attached to the same corrosive interaction with any metallic veneers with uniform texture, how- panel. In most situations, the panels elements that may be present in the ever, may gradually expand with each span one structural bay, and are stone. thermal change, a condition know as secured at the column lines to allow hysteresis. This “growth” occurs in the columns to directly carry the There are two types of anchors used minuscule increments, which nonethe- gravity load. with thin-stone veneers: anchors which less add up over time, causing the are inserted into kerfs and anchors veneer to cup or warp. That in turn Less commonly used are stone panels which are inserted into holes drilled increases the stress on both the “glazed” directly into mullions (such into the sides or rear of the stone anchorage system which holds the as the technology from Turin, Italy panel. (A kerf is a narrow slot cut veneer in place and on the supporting described earlier) or exterior stone longitudinally into the edge of the structure. If analysis of the selected tile set with tile techniques. The stone panel, and is used to hold an stone panel shows a propensity to latter is not recommended due to its anchor which secures the stone slab to hysteresis, specifying a thicker panel lack of durability. its support backing. Illustration A on can help mitigate its effects.

Design and Installation Issues There are three basic installation systems used in thin-stone cladding:

1. Hand-set stone with mortar joints and anchors that connect to the structure. This system is more typical of thicker dimension stone systems, and more likely to be found with a concrete or masonry back-up structure.

2. Hand-set stone with sealant joints and anchors that connect to the structure. This system is an adaptation of the above system and is commonly found on steel-framed buildings.

3. Prefabricated panels with steel frame, steel truss, or precast concrete back-up. This method is often used A pull-test in the back-up wall is used to evaluate anchorage performance.

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the stone during fabrication. Pre- will inevitably cause some water drilled holes should have a diameter of penetration into the veneer. Flashings not more than one-fourth of the and weepholes should be specifically stone’s thickness, and the anchor detailed to give this moisture a direct should be placed in the center-third of route to the outside. the stone’s thickness. A common failure problem is breakage at the Kerfs must also be carefully designed inboard side of the kerf, as shown in and fabricated to avoid pooling of B.1. Illustration B.2. on this page. water. This is a particularly common Anchor slot has filled with water. problem area, where a poorly cut kerf During fabrication, kerfs are often dry- will increase the chance for water sawn or dry-drilled. The thermal absorption into the stone. (Please see differences between the drilled area Illustration B.1. on this page.) Elasto- and surrounding stone during this meric sealants or caulks compatible work can cause minute fractures in the with stone should be used to keep kerf. If cleavage planes are present, moisture out of the kerf. Mortars and this cracking can travel far beyond the expanding mortars should not be used, drilled area. The result is a significant as they will crack and fail under B.2. loss in the veneer’s bending strength, thermal stress. Anchor is binding on edge of slot. increasing its vulnerability to dynamic loads. Accommodating movement Thermal movement can have a When using hand-set thin stone, dramatic and negative effect on thin- anchors are set into holes, slots, or stone veneers, particularly on the kerfs. Relieving angles can be incorpo- anchorage system. Because the rated into the overall anchoring stone, anchorage system, and support scheme, as they provide positive structure each experience different gravity-load support at each floor level. degrees of movement during normal B.3. Incorrect slot placement leads to breakage. In some situations, a small block of thermal change, or from seismic or stone is pinned and epoxied to the wind loading, enormous stress is back of a thin-stone slab and allowed placed on the anchorage system. The to rest on the relieving angle to anchorage system design must support the stone. anticipate and accommodate these diverse rates of change. Environmental risks and water control Surface damage to the stone isn’t the Horizontal expansion joints should be only result of freeze-thaw cycles and placed along the horizontal gap B.4. Mortar in slot prevents water intrusion and acid rains. These and other environ- between the stone panels and beneath binding. Sealant is used in some designs. mental factors also pose a serious risk any supporting anchors or relieving to the stone’s flexural strength due to angles. This will help minimize the Illustration B: Anchorage Design the weakened bonds between mineral stress of vertical movement between particles in the stone. Hoffmann the veneer and the support structure, Architects recommends that a flexural especially the shrinkage movement page 2 shows an example of a kerf.) strength test be conducted twice on common to concrete structures. the selected stone veneer: once after Both kerfs and holes should be pre- fabrication and once after the veneer is In turn, vertical expansion joints will sawn and pre-drilled as part of shop subjected to the “accelerated aging” accommodate the effects of any fabrication. The kerf’s depth is vital — test recommended by ASTM. horizontal movement of the stone deep enough to hold the anchor but caused by thermal change or by shallow enough to avoid weakening Wind-driven rain and condensation deflection of the building frame.

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As a general precaution, anchorage should not be over-restrictive. If Face-pinning Stone Veneers anchors are mortared, grouted, or epoxied, then movement must be addressed and allowed for elsewhere F ace-pinning of thin-stone veneers is in the curtain wall design. Conversely, a remedial strategy most often under- Face-pinning anchors that are loose or set in sealant taken as a safety measure to prevent must not be too permissive of random loose, displaced, or cracked stone from movements, such as those caused by falling. The following steps are recom- wind gusts hitting the stone. This mended for face-pinning: dichotomy is a critical design issue, and a careful balance between the two Identifying the need must be achieved. Anchors should be installed wherever stones are cracked or dislodged. The Remediation Strategies architectural elevations should desig- One primary maintenance rule in thin- nate which stones are to be pinned. stone veneers is the full replacement of all joint sealant every 15 to 20 years. Preparing the stone Step 1: Drilling the stone and back-up wall Mortar joints, if present, should be re- A counterbore hole approximately 1" in preparation for the anchor. pointed as needed, usually every 20 to in diameter by 3/4" deep should be 25 years. drilled in the stone to accommodate the nut and washers. A 3/8"-diameter Patching may be required for a specific corresponding hole should be drilled in crack, break, or spall in the stone. the concrete or masonry back-up to (Please see the related article, ”Face- hold the anchor rod. A depth gauge pinning Stone Veneers,” on page this used during drilling will ensure the page.) One repair technique, known proper depth of both openings. If the as a “dutchman repair,” calls for sawing stone is less than 2" thick, however, out the damaged area of the stone, counter-boring may not be possible, inserting a new, custom-fit slab into the and the nut will remain exposed, a opening, and securing it with epoxy and definite aesthetic disadvantage. Step 2: Injecting the epoxy adhesive into steel pins. It is a more expensive the anchorage hole. solution than patching, but may be Preparing for installation necessary for large repair areas. When using an adhesive anchoring Matching the appearance of new and system, the holes in the back-up existing stone is difficult, however. structure are cleaned first of dust and stone fragments (using a brush and Face-pinning can be used to re-secure compressed air) before the adhesive a cracked piece of stone which is in is injected or inserted. This will help danger of breaking loose or an entire achieve a solid bond between the piece of stone which has suffered adhesive and the anchor rod. The anchorage failure. While this repair anchor rod should be left undisturbed technique may mar the appearance of during the setting time specified by the the stone, it is often undertaken as an adhesive manufacturer. Once the Step 3: Inserting the anchor rod with nut. essential safety measure. More exten- adhesive has set, a stainless steel sive repair projects may involve washer and nut should be installed drilling dust from the hole. The removing and re-setting the stone or over a neoprene pad. anchor should be pre-assembled, using installing a replacement stone. Re- two neoprene pads and a stainless setting stone is often impractical, When using an expansion anchoring steel washer, with the nut engaging (continued on page 8) system, a syringe is used to remove two or three threads of the anchor

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rod. The anchor should be set to its The Facility Manager’s Bookshelf: Thin-Stone Facades full depth with light hammer blows. The anchor is then tightened to the A. Periodicals specified torque, using a calibrated torque wrench, causing the expansion 1. Dimenionsal Stone (monthly). Subscriptions: $50/year. Dimensional Stone shield to grip. Magazine, 6300 Variel Avenue, Suite I, Woodland Hills, CA 91367. 818- 704-5555. Once the anchorage is in place, a patching material is applied to the 2. Building Stone Magazine (quarterly). Subscriptions: $65/year. Building Stone counterbore hole over the nut and Institute, P.O. Box 507, Purdys, New York 10578. 914-232-5725. rod. For limestone and marble, Hoffmann Architects recommends the 3. Stone World (monthly). Subscriptions: $65/year. Stone World, 299 Market European-made JAHN materials. Street, Third Floor, Saddle Brook, New Jersey 07663. 847-291-5224, fax 847-291-4816. These come in a variety of colors which can be custom-blended for an 4. Journal of Performance of Constructed Facilities (quarterly). Subscriptions: exact match. $76/year for non-members. American Society of Civil Engineers, Technical Council on Forensic Engineering, 345 East 47th Street, New York, NY Patching granite requires a slightly 10017-2398. more complex approach, primarily because of the difficulties in matching 5. ENR: Engineering News-Record (weekly). Subscriptions: $74/year. The the patch material to the original McGraw-Hill Companies, Fulfilment Manager, ENR, P.O. Box 518, stone. Typically, the architect deter- Hightstown, New Jersey 08520. 800-525-5003. mines the number of samples needed to conduct an accurate color review, B. Technical Articles from The Construction Specifier which is done prior to the actual patching. The color testing is done To order reprints, contact the Construction Specifications Institute at (800) 689-2900. Reprints are $4.00 each, $10.00 minimum order. using an Akemi epoxy (with a coloring paste kit), iron oxide pigments, and 1. Algeo, Harold. “What the Specification Writer Needs from the Stone granite chips that are similar in Industry.” The Construction Specifier, April 1961, p. 41. appearance to the original granite. The samples of epoxy patching 2. Hoigard, K. R. and Raths, C. H. “Specifying Stone Testing.” The Construction Specifier, December 1991, p. 108. material are prepared for the owner’s review, using the coloring paste in 3. Hurd, Robert. “Commercial Uses for Marble.” The Construction Specifier, some samples and the iron oxide August 1986, p. 76. pigments in others, and adding in granite chips as needed to improve the 4. Pritsker, Ellen. “Coloring the Skyline with Granite.” The Construction color match. Specifier, February 1985, p. 40.

The patching surfaces must be dry and C. Technical Articles - Miscellaneous free of grease and dust, with smooth surfaces roughened first to improve 1. Dorris, Virginia Kent. “Anchoring Thin-Stone Veneers.” Architecture, bonding. Once the patch has been December 1993, p. 105-107. applied and hardened, it can be polished to match the surrounding 2. Grassi, Ennio. “Stone Materials in Curtain Walls: Design Criteria, Technical Performance, Use of Stone in Pre-fabricated Systems.” Booklet published by surfaces. Internazionale Marmi E Macchine Carrara S.P.A., Ente Minerario Sardo for the Italian Marble Council, December 1992. Distributed by Italian Marble This process is also used in other Centre, London. granite patching situations that are unrelated to face pinning, such as the 3. “Shop Drawing Process of Stone Veneered Cladding Systems.” Journal of repair of cracks and spalls. ■ Architectural Engineering, June 1997.

Compiled by Alan P. Eddy, Technical Information Specialist ■

6 Volume 15 Number 3

R E P R E S E N T A T I V E P R O J E C T S

Rehabilitation of Thin-Stone Veneers

Resolving deterioration and failure Lincoln Center for the problems in thin-stone veneer cladding Performing Arts, Inc. requires extensive investigative work Avery Fisher Hall into the underlying causes of the New York, New York problems. Hoffmann Architects begins (Lincoln Center for the each rehabilitation project with an Performing Arts, Inc.) exhaustive study of existing conditions, original contract documents, and other One Magnificent Mile historical data on the building. From Chicago, Illinois there, the firm’s team of architects and (Teachers Insurance and Annuity engineers begin to formulate repair Association) strategies that address those root The Executive Building causes. The final rehabilitation recom- Washington, District of Columbia mendations reflect both the technical (Prudential Insurance Company) and aesthetic requirements of the project, budgetary and facility life-cycle 28 State Street considerations, and occupancy needs New York, New York during construction. The Philip Morris Building, 120 Park (Prudential Insurance Company) Avenue, New York, New York. Once the repair method has been 335 Madison Avenue selected, the firm prepares comprehen- Building # 76 New York, New York sive plans and specifications for Hoffmann-La Roche (Cushman & Wakefield, Inc.) competitive bidding. On-site project Nutley, New Jersey staff and contract administrators track (Hoffmann-La Roche) 1251 Avenue of the Americas the progress and quality of the work New York, New York throughout the project. The Chase Manhattan Bank, N.A. ( Management New Rochelle, New York Corporation) Hoffmann Architects has been called in (The Chase Manhattan Bank, N.A.) by major corporations, private institu- American Express Company tions, and real estate owners to provide 110 South Bedford Road 200 Vesey Street specialized consulting on the rehabilita- White Plains, New York New York, New York tion of thin-stone facades. The (Edward S. Gordon Company, Inc.) (American Express Company) ■ following is a sampling of the firm’s work in this field: Pfizer, Inc. World Headquarters New York, New York (Pfizer, Inc.)

Rockefeller Center Lower Plaza and Skating Rink Area New York, New York (Rockefeller Center Management Corporation) Bristol-Myers Squibb Company, Wallingford, .

7 Hoffmann Architects / Journal FORWARDING AND ADDRESS CORRECTION REQUESTED however, as breakage frequently Any water trapped within the wall is (203) 239-6660 North Haven, Connecticut 06473 432 Washington Avenue occurs during removal. guaranteed to cause trouble — including freeze-thaw damage, Another key maintenance and repair corrosion, efflorescence, and many consideration is at flashings and weeps. other problems. Prevention is the best solution. Check for clogged weep holes during routine inspections and be sure to include flashing repairs in any remedial work.

Conclusion All told, thin-stone veneers can offer enormous design and image opportu- nities to owners at a more affordable cost than traditional dimension stone. Avoiding the pitfalls of this relatively new technology is the challenge. The owner’s best bet is to work with A worker removes failed patching material qualified architects and engineers who from a crack in the limestone facade. have an in-depth understanding of the quirks and demands of thin-stone design. ■ ......

JOURNAL is a publication of Hoffmann Architects, specialists in investigative and rehabilitative architecture/ engineering services, including the analysis and resolution of problems within roofs, exterior walls, glazing, and structural systems of existing buildings, plazas, and parking garages. Damaged stone has been removed, exposing the concrete back-up wall. Please send news, technical informa- tion, address changes, or requests for free subscriptions to Emily D. Dowden, Editor, Hoffmann Architects, 432 Washington Avenue, North Haven, Connecticut 06473.

For more information on the technical issues discussed in this issue or on the specific services we offer, please call Brian Schafer at (203) 239-6660...... A crack in the granite facade.

Editor: Emily D. Dowden Graphic Services: Bligh Graphics Editorial Services: Jonelle Lawhorn Hobbs Communications 8