Stone Veneer-Faced Precast Concrete Panels

Stone veneer-faced precast concrete panels have been used successfully to clad buildings in North America for the past 40 years. Their popularity is due to the aesthetics, strength, durability, substantial benefits and low maintenance cost of such panels. However, their successful implementation requires careful planning, proper stone selection, and skillful workmanship in producing the panels. This article provides information on stone properties, design considerations, anchorage of stone facing, panel watertightness, Sidney Freedman veneer jointing, handling, storage and shipping of panels and repair Director Architectural Precast of panels, if needed. A wide variety of building applications of stone Concrete Services veneer-faced precast panels are given. Throughout the article, it is Precast/Prestressed Concrete Institute Chicago, Illinois emphasized that for optimum results, close coordination is needed between the architect, precaster and stone supplier.

atural stone has been used 1. Veneer stock can be used in thin widely in building construction ner sections because anchoring points Nfor centuries due to its strength, may be placed closer together. durability, aesthetic effect, general 2. Multiplane units such as column availability, and inherent low mainte covers, spandrels with integral soffit nance costs. In the 1960s, the practice and sill sections, deep reveal window of facing skeleton-frame structures frames, inside and outside corners, with large prefabricated concrete com projections and setbacks, and parapet ponents to decrease construction time sections are more economically as and reduce costs resulted in a combi sembled as veneer units on precast nation of the rich beauty of natural concrete panels (see Fig. 2). Often, it stone veneer and the strength, versatil is desirable to use one of the veneer ity, and economy of precast concrete materials in a traditional manner (see Fig. 1). around the lower portion of a building Stone veneer-faced precast con and extend a similar finish with ye crete panels offer many benefits. neered precast concrete panels up the These include: exterior walls.

72 PCIJOURNAL 3. A precast concrete backup system permits faster enclosure, allowing ear lier work by other trades and subse quent earlier occupancy, because each of the larger panels incorporates a number of veneer pieces. The overall size and weight of the panels are gen erally limited to what can be conve niently and economically handled by available transportation and erection equipment. In general, panels span be tween columns, usually spaced 20 to 30 ft (6 to 9 m) on centers, although spandrel pieces have been made as largeas6x43ftand8x35ft(l.8x 13.1 m and 2.4 x 10.7 m). Typically, a single-story panel has been 13 to 30 ft (4.0 x 9.1 m) by story height. 4. Veneered precast concrete panels can be used to span column-to-col umn, thereby reducing floor-edge loading and eliminating elaborate tem porary scaffolding.

GENERAL CONSIDERATIONS The purchaser of the stone should appoint a qualified individual to be re sponsible for coordination. This per son should oversee delivery and scheduling responsibility and should ensure acceptable color uniformity. Color control or blending of the stone veneer should take place at the stone fabricator’s plant, where ranges of color and shade, finishes, and mark ings such as veining, seams and intru sions are viewed most easily. The amount of color control and blending to which the stone lends itself varies depending upon the type of stone used on the project. Acceptable stone color should be judged for an entire building elevation rather than as individual panels. The responsibility for coordination should be written into the specifica tions so its cost can be bid. The owner, architect, and precaster should visit Fig. 1. The Georgia Center, Atlanta, Georgia, a 29-story building, was completed in the stone fabricator’s plant to view the 1964 and has 27 in. (690 mm) deep single window box units, each with 16 pieces of 1 stone veneer and establish criteria and in. (25 mm) marble veneer. Architects: Bodin and Lamberson; and Eggersand Higgins. methods for color range blending on the project. With proper coordination and advance planning, fabrication and All testing to determine the physical will reduce the need for potentially shipments of the stone veneer to the properties of the stone veneer with the costly repairs or replacement should precaster will proceed smoothly. If same thickness and finish as will be deficiencies in the stone veneer be communication is lacking, major prob used on the structure should be con found after the start of fabrication. lems in scheduling and delivery may ducted by the owner prior to the award Because of the need for close coor occur. of the precast concrete contract. This dination between the precast manufac

July-August2000 73 used to select colors of stone, win dows, and caulk as well as judging overall building appearance. Mock ups should be built to test wall, win dow and joint performance under the most severe wind and rain conditions. Acceptance criteria for the stone as Stone well as the anchorage should be estab Veneer lished in the project specifications. Pond breaker STONE PROPERTIES Stone is a product of geologic evo lution and, therefore, does not demon Fig.2. strate the consistent behavior that may Typical spandrel and column cover apply to manufactured building mate panels. rials, such as concrete. The strength of natural stone depends on several fac tors: the size, rift and cleavage of crys turer and stone veneer supplier, shop The precaster and stone fabricator tals, the degree of cohesion, the inter drawing preparation and submissions should coordinate packaging require locking geometry of crystals, the may vary from procedures established ments to minimize handling and nature of natural cementing materials for non-veneered precast panels. breakage. Extra stone (approximately present and the type of crystal. The Checking and approval of these details 2 to 5 percent) should be supplied to stone’s properties will vary with the and shop drawings will be simplified the precaster to allow immediate re locality from which it is quarried. and expedited if they can be combined placement of damaged stone pieces, Therefore, it is important that current and/or submitted simultaneously. Sep particularly if the stone is not supplied testing is performed on stone quarried arate subcontracts and advance awards from a domestic source. The extra for a specific project. often occur in projects with stone-ye stone should be the largest sized Sedimentary and metamorphic neered panels. While these procedures pieces to be used on the project. De rocks, such as limestone and marble, may affect normal submission rou liveries should be scheduled to corre will exhibit different strengths when tines, it is not intended that responsi spond as closely as possible to actual measured parallel and perpendicular to bilities for accuracy be transferred, or fabrication schedules. their original bedding planes reassigned. The precaster is responsi Samples and mock-up units are par (anisotropic). Igneous rocks, such as ble for precast concrete details and di ticularly important for evaluating granite, may or may not exhibit rela mensions, while the stone-veneer fab stone finishes and acceptable color tively uniform strength characteristics ricator is responsible for stone details, variations. Fig. 3 shows a mock-up on the various planes (isotropic). In ad- dimensions, and drilling of anchor holes. The production of stone veneer pan Fig. 3. Mock-up of els requires adequate lead-time in Cityfront Center — order to avoid construction delays. NBC Building, Therefore, it is important that ap Chicago, used to provals for shop drawings be obtained select colors of expeditiously. Furthermore, it is rec stone, windows ommended that the designer allow the and caulk as submission of shop drawings in prede well as to judge termined stages so production can overall building begin as soon as possible and ensure appearance. there is a steady and timely flow of Architect: approved information to allow unin Skidmore, Owings terrupted fabrication. & Merrill, Chicago, Illinois. The precast concrete producer pro vides the stone quantity and sequence requirements to meet the erection se quences, which are determined by mutual agreement. For reasons of pro duction efficiency, some concrete panels may be produced out of se quence relative to erection sequence.

74 PCIJOURNAL dition, the surface finish, freezing and Table 1. Permeability of commercial building stones, Cu in./sq ft/hr thawing, and large temperature fluctu for 1/2 in. thickness. 7 ations may affect the strength and in Water psi turn influence the anchorage system. pressure, To the degree possible, information Stone type 1.2 50 100 on the durability of the specified stone should be obtained through current Granite ‘‘ 0.06-0.08 0.11 0.28 testing in conjunction with observa Limestone 0.36-2.24 4.2-44.8 0,9-109 tions of existing installations of that particular stone. This information Marble 0.06-0.35 1.3-16.8 0.9-28.0 should include such factors as ten dency to warp, reaction to weathering Sandstone 4.2-174.0 51.2 221 forces, resistance to chemical pollu Slate 0.006-0.008 0.08-0.11 0.11 tants, resistance to chemical reaction from adjacent materials, and reduction Note:I cuinisq ft/hrP/2 in. = 16.393m/hr/13 mm; 1 psi = 0.006895 MPa: I in:= 25.4 mm. in strength from the effects of weath ering or wetting and drying. Prior to awarding the precast con fractures in the stone. Absorption testing (ASTM C97), as crete contract, tests should be per Thermal or flame finishing of gran mentioned, helps evaluate freeze- formed to determine the physical ite surfaces causes microfracturing, thaw durability of the stone. properties of the stone being consid particularly of quartz and feldspars. Stones which have a satisfactory ered. The testing should be done on These microcracks permit absorption perfomance record in thicknesses, stone with the same finish and thick of water to a depth of about 1/4 in. (6 sizes and climates similar to those en ness to be used on the structure. Flex mm) in the distressed surface region visioned for a project may at the op ural tests (ASTM 1C880) should be of the stone which can result in degra tion of the designer be exempted from used to evaluate the physical proper dation by cyclic freezing and a further the above testing requirements. ties and obtain the required design val reduction in bending strength. For most types of stone, tempera ues. Absorption testing (ASTM 2C97) Weathering affects different stones ture induced movements are theoreti may help evaluate freeze-thaw dura in different ways. It can cause both a cally reversible. However, certain bility. These properties, along with the chemical decomposition and physical stones, particularly marble, when properties of the anchor system, disintegration in some stones. The subjected to a large number of ther should be used to ensure adequate thinner the stone is sliced, the more mal cycles, develop an irreversible strength of the panel to resist loads susceptible it may be to weathering. expansion in the material amounting during handling, transportation, erec Most natural stones lose strength as a to as much as 20 percent of the total tion and in-service conditions. result of aging [thermal cycling, e.g., original thermal expansion. This The process used to obtain a thermal heating to 170°F (77°C) and cooling to residual growth is caused by break or flame finish on granite veneers re -10°F (-23°C), and wet/dry cycling]. ing of crystal bonds. Such growth, duces the effective thickness by about The modulus of rupture of5building if not considered789 in the stone size, 1/ in. (3 mm) ’ and the physical strength stone can also be affected by freezing may result in curling or bowing of to a measurable degree. Bushham and thawing of the stone. thin marble. For relatively thick mar mered and other 3similar surface the expansion effects are fin Flexural tests (ASTM C880) ble veneers, ishes also reduce the effective thick should be conducted on the selected restrained or accommodated by the ness. For 11/4 in. (3 cm) thick veneers, stone, at the thickness and surface unaffected portion of the veneer. a reduction in thickness of ‘/8 in. (3 finish to be used, in both the new Tests should be performed to estab mm) reduces the theoretical bending condition and the condition after 100 lish the minimum thickness required strength by about 20 percent and in cycles of laboratory accelerated aging to obtain satisfactory serviceability. creases the elastic deflection under (weathering) tests to determine the Stone can be exposed to differential wind loads by about 37 percent. reduction in strength, if any. Sug accelerated heating and cooling cy Laboratory tests on 1V4in. (3 cm) gested weathering test procedures in cles and measured for deformation thick specimens of unaged thermally clude (1) cycling between 170°F and (bowing/hysteresis). finished granite revealed that the ef -10°F (77°C and -23°C), while the face Volume changes due to moisture fects of the thermal finish reduced the of the stone is submerged in a 4 pH changes should be considered in de bending strength of the specimens by sulfurous acid solution that simulates sign, especially for joint size. Moisture as much as 25 to 30 percent. The loss chemical weathering. For warm cli permeability of stone veneers is gener of strength depends4 mainly on the mates, the6test procedure can be mod ally not a problem (see Table 1). physical properties of the stone form ified to cycle between 41°F and However, as stone veneers become ing minerals, on the coherence of the 170°F (5°C and 77°C). Also, in areas thinner, water may penetrate in greater crystalline structure of the stone, and where the pH of rainfall is above 6, amounts and at faster rates than nor on the presence of micro and macro the acid solution can be eliminated. mally expected, and damp appearing

July-August 2000 75 areas of moisture on the exterior sur Thickness variations are less impor casters have developed design and face of thin stone veneers will fre tant, since concrete will provide a uni production procedures to minimize quently occur. These damp areas re form back face except at corner butt bowing. The following paragraphs ex suit when the rate of evaporation of joints. In such cases, the finished plain how this may be accomplished. water from the stone surface is slower edges should be within ±V16 in. (±1.6 The precaster and designer should than the rate at which the water moves mm) of the specified thickness. How consider the following factors in de to the surface. ever, large thickness variations may sign and production in order to mini lead to the stone being encased with mize or eliminate panel bowing. concrete and thus restrict the relative 1. Temperature differential (exterior STONE SIZES movement of the materials. The aes to interior) Stone veneers used for precast fac thetic problems that occur with toler 2. Coefficients of thermal expansion ing are usually thinner than those used ances concern the variation from a flat of materials for conventionally set stone with the surface on an exposed face and stone 3. Ratio of cross-sectional areas of maximum size generally determined pieces being out-of-square. the materials and their moduli of elas by the stone strength. Table 2 summa ticity (axial tension, flexure, and axial rizes typical dimensions. Veneers thin compression) ner than those listed can result in an DESIGN CONSIDERATIONS 4. Amount, location and type of re chors being reflected on the exposed Structural design, fabrication, han inforcement in concrete panel surface, excessive breakage or perme dling and erection considerations for 5. Use of prestressing ability problems. veneered precast concrete units are 6. Type and location of connections The length and width of veneer ma similar to those for other precast con to structure terials should be sized to a tolerance of crete wall panels, except that special 7. Rigidity of connection between +0, -‘Is in. (+0, -3 mm) since a plus consideration must be given to the ve stone veneer and concrete backup (too tolerance can present problems on pre neer material and its attachment to the rigid may cause problems) cast concrete panels. This tolerance concrete (see Appendix for typical 8. Shrinkage of the concrete becomes important when trying to line production practices). The physical Panel design must also take into up the false joints on one panel with properties of the stone facing material consideration the conditions that pan those on the panel above or below, must be compared with the properties els will encounter when in their final particularly when there are a large of the concrete backup. location in the structure and subjected number of pieces of stone on each These properties include: to the wide range of seasonal and daily panel. Tolerance allowance for out-of- 1. Tensile (axial and flexural), com temperatures. In general, interior sur square is ±Vl6 in. (±1.6 mm) differ pressive and shear strength faces of panels are subjected to a very ence in length of the two diagonal 2. Modulus of elasticity (axial ten small temperature range while the ex measurements. sion, flexure, and axial compression) terior surfaces may be exposed to a Flatness tolerances for finished sur 3. Coefficient of thermal expansion large daily or seasonal range. The tem faces depend on the type of stone and 4. Volume change perature differential is tempered by finish. For example, the granite in Because of the differences in mate “thermal lag” due to the mass of the dustry’s flatness tolerances vary from rial properties between natural stone veneer and the concrete. ‘/16 in. (1.6 mm) for a polished sur and concrete, veneered panels are The likelihood that a panel will bow face to 3/, in. (4.8 mm) for flame more susceptible to bowing than ho depends on the design of the panel and (thermal) finish when measured with mogeneous concrete units; also, the its relative stiffness or ability to resist a 4 ft (1.2 m) straightedge.’° Toler flat surfaces of cut stone reveal bow deflection as a plate member. Critical ances should be clearly specified in ing more prominently than homoge panel lengths for bowing depend on the contract documents. neous concrete panels. However, pre temperature and moisture gradients, panel thickness and concrete’s modulus of elasticity. Panels that are relatively Table 2. Dimensional parameters of various stone materials. thin in cross section as compared to their Minimum overall plan dimensions are more likely recommended Length Width Maximum to bow as a result of design, manufactur thickness range range area ing and environmental conditions. Stone type in. (cm) ft (m) ft (m) sq ft )2(m Minimum thickness of backup con Marble 1.25 (3) 3-5 (0.9-1.5) 2-5 (0.6-1.5) 20 (1.9) crete of flat panels to control bowing is usually 5 to 6 in. (125 to 150 mm), Travertine* 1.25 (3) 2-5 (0.6-1.5) 1-4 (0.3-1.2) 16 (1.5) but a 4 in. (100 mm) thickness can be used where the panel is small or where Granite 1.25 (3) 3-7 (0.9-2.1) 1-5 (0.3-1.5) 30 (2.8) it has adequate rigidity obtained Limestone 1.75 (4.5) 4-5 (1.2-1.5) 2-4 (0.6-1.2) 15 (1.4) through panel shape or thickness of

*Sufface voids filled front and back. natural stone. If the panel’s thickness tlndiana Limestone Institute recommends 2 in. (5 cm). is sufficient, usually 6 in. (150 mm) or

76 PCI JOURNAL more, two layers of reinforcement should be used, as this helps to reduce Typical Flat Mold bowing caused by differential shrink age or temperature changes. Volume changes due to moisture changes in most stones are relatively small and are usually not a critical item in de sign, except that bowing of the stone Flat Mold For Natural Stone can occur. Little or no addition cost Reinforcement of the precast con crete backup should follow recom mendations for precast concrete wall panels relative to design, cover and placement. Cover depth of uncoated Clearancesbuiltinform face for natural stone reinforcement must be a minimum of 1 handlingdevice ’1‘/2 in. (28 mm) to the back of the ye Return Mold For Natural Stone neered surface. or epoxy- Galvanized Stone must be heldinplace Additionalmold coated reinforcement is recommended withclampsor disposable cost for back- 3/4 (galvfSt. StI.)threaded at cover depths of in. (19 mm). formingand fasteners joints) are Prestressing of panels has been ef dailylaborto (at that fective in controlling bowing of long, attach stone removedafter each pour, leasinga threaded wing flat, relatively thin panels.’ Such pan to return wall assemblyembeddedinthe els are generally more2 susceptible to concrete panel bowing. As with any multi-layer panel, trial runs may be necessary to verify analysis as to the best prestress ing strand location in order to avoid an Fig. 4. Mold types showing degree of complexity. increase in bowing. Unrestrained bowing of a panel in duces no stresses. If the bowing is re cient of expansion of 2.8 x lightweight concrete are frequently strained by end connections that resist in./in./F (5.0 x 10.6 mm/mm/°C), used. rotation, significant stresses may de while granite has 4.5 x 10.6 (8.1 x 106) As individual stone pieces become velop over time. If excessive bowing and marble 7.3 x 10-6(13.1 x 10j. larger or thinner, the coefficient of ex is taken out after the panel has been Coefficients of 6 x 10-6in./in./°F (10.8 pansion differentials become more im erected, then cracking of the panel x 106 mmlmm!°C) for normal weight portant because the stone has less may occur. The force necessary to and 5 x 10.6 (9.0 x 10-6) for sand- rigidity to resist bowing. It is desir straighten a bowed panel, and the re sulting stresses, can be determined Table 3. Coefficients of linear thermal expansion of aggregate easily. Midpoint tie back connections and concrete.’ can help minimize convex bowing. 1 Average coefficient After initial set, concrete begins to of thermal expansion shrink as it loses excess water to the x 10.6 injin.I’F surrounding environment. The stone veneer, especially with an imperme Type of rock * able bondbreaker, limits drying on the (aggregate) Aggregate Concrete veneered side of the backup concrete. Quartzite, cherts 6.1-7.0 6.6-7.1 The resulting differential shrinkage of Sandstones 5.6-6.7 5.6-6.5 the concrete and stone veneer can Quartz sands and gravels 5.5-7.1 6.0-8.7 cause outward bowing in a simple Granites and gneisses 3.2-5.3 3.8-5.3 span panel. While homogeneous con Syenites, crete panels usually bow in response Diorites, andesite 3.0-4.5 4.4-5.3 Gabbros, diabase, basalt to thermal gradients through the panel Limestones 2.0-3.6 3.4-5.1 thickness, stone veneered concrete Marbles 2.2-3.9 2.3 may also bow when the temperature is Dolomites 3.9-5.5 uniform through the panel thickness. Expanded shale, This bowing is caused by differences Clay and slate 3.6-4.3 Expanded slag 3.9-6.2 in the coefficients of expansion of the Blast-furnace slag 5.1-5.9 stone and concrete. *coefflcientsfor concretesmade with aggregatesfrom differentsourcesvary from thesevalues,especiallythose for Limestone has an average coeffi gravels,granites,and limestones.Fine aggregatesgenerallyare the samematerialas coarse aggregates.

July-August 2000 77 able, therefore, to have a backup con humidity and temperature conditions). ANCHORAGE OF crete with low shrinkage, and a ther The application of a curing compound STONE FACING mal expansion coefficient that closely on all exposed concrete surfaces, e.g., approximates that of the stone veneer. back surface and panel edges will The responsibility for determining The coefficient of thermal expansion minimize shrinkage. the type of anchorage between the of concrete can be varied by changing Precasters may compensate for stone and concrete backup varies on the aggregate type (see Table 3). bowing by using cambered forms, different projects. The stone fabricator or precaster appear to have the domi The most important single factor af e.g., 1 in. (25 mm) for 40 ft (12 m), to fecting shrinkage is the amount of produce panels initially bowed in nant responsibility for conducting the water placed in the mix per unit vol ward. Also, in some cases, reinforc anchor tests. The architect or engineer ume of concrete. This is because ing trusses may be used to add stiff of record will occasionally determine shrinkage of concrete is due mainly to ness. the type of anchorage. However, it is the evaporation of the mixing water. In others, vertical and/or horizontal preferable for the architect to deter As a result, the humidity of the sur concrete ribs that run continuously mine anchor spacing so that consistent rounding air for a given concrete mix from one end of the panel to the other information can be supplied to all bid ders (refer to ASTM C1242). affects, to a large extent, the magni may be formed on the back of the 13 tude of the resulting shrinkage. panel to increase stiffness. This will Contract documents should define Control of concrete shrinkage ne require backforming, however, which clearly who drills the anchor holes in cessitates close attention to concrete is more costly. See Fig. 4 for mold the stone; type, number and location mix design, and curing regime (proper considerations. of anchors; and who supplies the an chors. In most cases, the stone fabrica tor drills the anchor holes in the stone according to architectural specifica 4N tions and drawings using a diamond- core bit with a non-percussive tool. It is recommended that the precast manufacturer detail all precast units to the point where the fabricator of the veneer is able to incorporate details, sizes and anchor holes for the individ ual stone pieces. It is also recommended that there be no bonding between the stone veneer 11/4 mm.Preferred and concrete backup in order to mini mize bowing, cracking, or staining of the veneer. Even with concrete shrink age kept to the lowest possible level, there may still be some interaction with the facing material either through Fig. 5. Typical anchor for marble veneer. bond or mechanical anchors of the stone veneer. This interaction is mini mized by the use of a bondbreaker be tween the facing material and the con crete. Connections of natural stone to the concrete should be made with flex ible mechanical anchors which can ac commodate some relative in-plane movement. Two methods may be used to pre vent bond between the veneer and concrete to allow for independent movement: Prferre 1. A 6 to 10 mil. polyethylene sheet. 2. A closed cell /8 to 1/4 in. (3 to 6 mm) polyethylene foam pad. Using a compressible foam pad bondbreaker is 1o Edge of Stone preferred because it allows for move ment of stones with uneven surfaces, either on individual pieces or between Fig. 6. Typical anchor for granite veneer.

78 PCI JOURNAL stone pieces on a panel. Preformed anchors, usually 1/8 to /8 I in. (3 to 16 mm) in diameter, fabri ‘A

S,S,Anchor . cated from Type 304 stainless steel, . .. ‘ . . . .. are supplied by the stone fabricator or, =3/lGtoS/& , ‘ . . /_..., .

21/2”Min. . in some cases, by the precaster de Depth of concrete . ‘ pending on the contract document re varies accor1ing quirements. The number and location to tlesign of anchors should be determined by a 35 minimum of five shear and tension tests conducted on a single anchor em 0 bedded in a stone/precast concrete test t (Variee) Sreaker with tape sample using 0 ASTM 14E488 or ASTM •, 0 0 —r C1354’ and the anticipated applied Hole DowelsMay J Face of 5loads, both normal and transverse to 1/16”> ct be Epoxies Stone Veneer the panel. Care should be taken in grasping the anchor to assure direct Fig. 7. Typical cross anchor dowels for stone veneer. tension.’ Anchor size and spacing in ”67veneers of questionable strengths or with natural planes of weakness may Sotid breaker require special analysis. Deformed Varies Four anchors usually are used per 55. Anchortl= 3/16 to 5/8° /q stone piece with a minimum of two 0 Concrete ••.o °I recommended. The number of anchors 07 t2 Dowels may has varied from 1 per l’/2 sq ft (1 per wanes . 0.1 )2m of stone to 1 per 6 sq ft (1 per hole>4 :1:em: 0.6 2m with 1 per 2 to 3 sq ft (1 per ) Stone 0.2 to 0.3 2m being the most — in 5” stone ) I common. Anchors should be 6 to 9 Deformed 12 ond breaker 5.5. Anchor = 3/16 to 5/8° in. (152 to 229 mm) from an edge with 15° not more than 24 to 30 in. (610 to 760 Concrete :: mm) between anchors depending on Varies the local building code. The shear ca pacity of the spring slip (hairpin) an chors perpendicular to the anchor legs is greater than when they are parallel (see Table 4) and capacity depends on 1”to2” the strength of the stone. 30°to4S° A typical marble veneer anchor de Useanchorsat opposingangles tail with a toe-in spring clip (hairpin) anchor is shown in Fig. 5, while a typ Fig. 8. Typical anchors for limestone veneer. ical granite veneer anchor detail is shown in Fig. 6. The toe-out anchor in the plane of the stone. Holes, approxi granite may have as much as 50 per mately 50 percent oversize, have been cent more tensile capacity than a toe- used to allow for differential move in anchor depending on the stone ment between the stone and the con strength. The stone anchorages on crete. However, holes ‘/16 in. (2 mm) most precast panels are conservatively larger than the anchor are common, as designed with significant redundancy excessive looseness reduces holding and excess capacity. power. Anchor holes should be within The depth of anchor holes should be ±/,6 in. (5 mm) of the specified hole approximately one-half the thickness spacing, particularly for the spring clip of the veneer 3/4 [minimum depth of anchors. in. (19 mm)]. Minimum concrete Stainless steel dowels, smooth or cover over the drilled hole should be threaded, may be installed to a depth 3/ in. (9.5 mm) to avoid spalling dur of two thirds of the stone thickness, ing drilling and spotting from ab Fig. 9. Example of a compressible with a maximum depth of 2 in. (50 sleeve used to reduce stone anchor sorbed moisture. The holes should be mm) at 45 to 60-deg. angles to the rigidity when the anchors are epoxied drilled at an angle of 30 to 45 deg. to plane of the stone. The minimum em- in the stone.

July-August2000 79 _

Table 4. Ultimate shear capacity of spring clip (hairpin) anchors in granite veneer from the alkali salts in the from various sources.* concrete. Shear parallel to anchor, lb (kg) Shear perpendicular to anchor, lb (kg) Moisture barrier/bonding agent ma terials include portland cement con taining less than 0.03 percent water Stone soluble alkalies; waterproof cementi tious stone backing; non-staining as

1 2400 to 2650 (1090 to 1200) 3200 to 3500 (1450 1o1590) phaltic or bituminous dampproofing; or an epoxy bonding agent that cures 2 1800 (815) 2500 (1135) in the presence of moisture. Dowels and spring clip anchors can be used 3 1500 (680) 1500 (680) to anchor limestone. Typical dowel 4 2500(1135) 3400(1540) details for limestone veneers are shown in Figs. 7 and 8. The dowels in 5 2800 (1270) 4000 (1815) Fig. 8 should be inserted at opposing angles to secure stone facing to 3400(1540) 4200(1905) 6 backup concrete.

7 1000 (455) (660(725) Some flexibility should be intro duced with all anchors by minimizing *Need to apply safety factor. the anchor’s diameter to allow for the inevitable relative movements that bedment in the concrete backup to de of 3 to 5 in. (75 to 125 mm), but it is occur with temperature variations and velop the required bond length is now being used as thin as 1/4 in. (44 concrete shrinkage. Unaccommodated shown in Fig. 7. Dowel size varies mm), although one limestone group relative movements can result in ex from /16 to 5/ in. (5 to 16 mm) for recommends a minimum of 2 in. (50 cessive stresses and eventual failure at most stones, except that it varies from mm). 18 an anchor location. Depending on the ‘/4 to /8 in. (6 to 16 mm) for soft lime When limestone is 2 in. (50 mm) size of the project, consideration may stone and sandstone and depends on or thinner, it is prudent to use a be given to accelerated cyclic temper the thickness and strength of the stone. bondbreaker, along with mechanical ature tests on the stone-concrete as Limestone traditionally has been anchors. If limestone is to be bonded, sembly to determine the effect of bonded and anchored to the concrete it is desirable to use a moisture bar strength loss on the shear and tensile because it has the lowest coefficient rier/bonding agent on the back side strengths of the anchors. of expansion. Limestone also has of the stone that has been proven to Some designers use two-part been used traditionally in thicknesses eliminate the staining of the stone polyester or epoxy to fill the anchor

U.. 42 -

q) 36 -

30 - - 0 x 24 - Sand-flHepofiIIedepo

12 - — Concrete

• . . • • —. • •• • —. • .. — . .. • —. . • •. • . • • • • • • • • • I • •—• • •• • • • •. • • .—• •• • •• •• .—• •• •. . I I 0 I I I I I I I I I I I I I I

1 2 3 4 5 6 7 & 9 10

Aggregate-bilicler ratio

Fig. 10. Effectof changes in the sand aggregate binder ratio on the thermal coefficientof an epoxy.

80 PCI JOURNAL holes in order to eliminate intrusion of overcome by using 1/2 in. (13 mm) cracking of the stone veneer. Epoxies water into the holes and to prevent the long compressible (60 durometer) rub yield under stress, and, if properly for possible dark, damp appearance of ber or elastomeric grommets or mulated, they will accommodate rela moisture on the exposed stone surface. sleeves on the anchor at the back sur tively large dimensional changes re The polyester or epoxy increases the face of the stone, as shown in Fig. 9. sulting from thermal effects. It is shear capacity and rigidity of the an Differential thermal expansion of the necessary to closely match the coeffi chors. This rigidity may be partially stone and unfilled epoxy may cause cients of expansion of the stone and

Exterior Interior Continuou WindowUnit Self-Leveli Continuous Airtight Seal Urethane Vented Air Sloping Gutter to Seal Vertical Joint (optional) Air Chamber (optional) Stone Veneer

Concrete ‘ond Sreaker

Airtight Seal at Horizontal Joint Sealant Urethane w/ackii Horizontal Joint Sealant w/acking Air Chamber Stone (optional) Air Chamber (optional) Veneer t-Va ries

1/&”to 1/4 Section at Veriical Joint Plan Fig. 11. Stone veneer precast concrete panel with modifiedjoint.

Exterior Interior

Window Unit

Continuous Airtight Seal

Sloping Gutter to Vertical Joint (optional)

Air Chamber (optional) Concrete Horizontal Joint Airtight Seal at Horizontal Joint Sealant w/acking Air Chamber (optional)

Section at Vertical Joint Plan

Fig. 12. Insulatedsandwich-veneer precast concrete panel with modified joint.

July-August2000 81 Fig. 13(a) and (b). Insulated sandwich- veneer precast concrete panel with two stage joint.

Polyethylene ope epoxy. However, this may be over come by keeping the oversizing of the hole to a minimum, thereby reducing epoxy volume and using stone flour or fines or fine sand as a filler for the epoxy to reduce the coefficient of ther Neoprene Pd t mal expansion of the epoxy and the Joint Intersection shrinkage (see Fig. lO).19 It may be more desirable to fill the anchor hole with a low modulus polyurethane sealant. The overall ef fect of either polyester, epoxy or sealant materials on the behavior of the entire veneer should be evaluated prior to their use. At best, the long- term service life of adhesive-embed ded anchors is questionable; therefore, any increase in pull-out strength of the anchors should not be used in cal culating long-term anchor capacity. When using polyester or epoxy in an chor holes, the precaster needs to fol low the manufacturer’s recommenda tions as to mixing and curing temperature limitations. The design of anchorage and size of the stone should always be based on specific test values for the actual stone to be installed. Test samples for an chor tests should be a typical panel section of about 1 sq ft (0.09 )2m and () approximate as closely as possible ac tual panel anchoring conditions. A

82 PCIJOURNAL ______

1. From 0-4 (a) Epoxy edge of granite to existing granite only

Granite Existing 2. From 4-12” (b) Epoxy edge of granite and provide one 1/4 or 3/8” 0 Stainless Steel Anchor as shown

Epoxy crack and finish to match acceptal2ly 1/4” 0 Vent. Hole — 3. Greater than 12” (c) Epoxy edge of granite and use A.. detail shown on (Lv)or (c) Cracked 4 Granite — — 3/4” 0 holefield drilled to PIece * S le filled with non-shrink grout , prior to placement of granite

- - .“, , I I

• I II flate - Washer 3” x 3” x 3/8” complete with 7/16” 0 hole at Grommet center line 1/4” 0 Stainless Steel (a) Epoxy anchor into granite prior Anchor complete with to replacement of granite threaded end and 1/4” 0 nut

Non-Shrink Grout ondbreaker

Plate-2” x 3/8” x 10” 1/4”0 vent 7/16”0 holeat center

3/8” 0 ii’E Galvanized • :‘. -Form olt with : _-—----—----m up back Flat Washer EP0xY<ç /‘ of cored and Nut

Granite - —- •• V V Shrink Stainless : Grout Steel Anchor Grommet

R.oughen cored hole Surface 02)

(c)

Fig. 14(a),(b)and (C). Granite slab repairs where access to back of precast concrete is possible.

July-August2000 83 ondbreaker glued to stone ——1,-----—- 1. From 0-4’ I. (a) . : 20 core drilled Epoxyedgeofgraniteto existing granite only T” hole - 3” deep 2. From 4-12” . : : (17) Epoxyedgeofgraniteand provide one 3/16” 0 Stainless • : • . . • Grommet Steel Anchor as shown

i• •• : A • : . Note: Epoxyedge 3. Greater than 12” :. • ••

• • of stone and (c) Epoxyedgeofgraniteand use • • finish to match detail shown on (17)or (c) A •

(a) broken Granite Slab Repairs

,_ Roughen surFace 0 .... bond breaker / with a chisel glued stone Stone to 4” drilled .. 0 core 3 1/2 deep /_hi5 (.eyed _\r/ Anchors

7 See 0••• Anchor — Non-shrink Petail grout Section A ñns

3/1611 to 5/S’ Stainless (17) •:.:...... , Steel Pin3-1/4” long

Fig. 15(a),(b)and (C). Granite slab repairs where access to back of precast concrete is not possible. bondbreaker should be placed between stone and concrete during sample manufacture to eliminate any bond be tween veneer and concrete surface. Each test sample should contain one anchor connecting stone to concrete backup and a minimum of five tests are needed to determine tensile (pull out) and shear strength of each type of (c) anchor. Depending on the size of the project, it may be desirable to perform shear and tensile tests of the anchors at intervals during the fabrication period.

84 PCI JOURNAL The stone trade associations and the veneer and the concrete backup at the Fig. 12 shows an insulated sand suppliers of different kinds of building bondbreaker. These gaps could allow wich-veneer precast panel constructed stones recommend safety factors. Be moisture penetration due to capillary using a logical extension of the modi cause of the expected variation in the action and gravity, particularly where fied rain-screen joint. The free move physical properties of natural stone the window or roof design allows ment of the stone veneer is provided and to account for the risks of brittle water to puddle on top of the panel. by the insulation itself with anchorage failure and for possible weathering ef One method that has been used to of the concrete to the stone similar to fects, recommended safety factors are solve this problem is a modified rain- Figs. 5 to 8. An air space is not pro greater than those used for manufac screen joint (two-stage joint) as vided and the bottom part of the panel tured building materials, such as steel shown in Fig. 11. This approach pro is open at the insulation to drain any and concrete. The minimum recom vides an air-tight 1 in. (25 mm) wide possible moisture. mended°2 safety factor, based on the av urethane seal, bonded to the stone ve The construction of an insulated erage of the test results, is 4 for an neer and concrete backup, and contin sandwich-veneer precast panel with a chorage components. If the range of uous along both sides and top of the 1/2 to 3/4 in. (13 to 19 mm) air space is test values exceeds the average by panel. Other designers have used a shown in Figs. 13(a) and (b). In order more than ±20 percent, then the safety sealant applied to the top and side to minimize bending of the stone wire factor should be applied to the lower edges of the stone/concrete interface anchors, the anchors are embedded in bound value (see the Appendix to after the panels are cast. Care must be 4 in. (100 mm) diameter concrete ASTM C 1242 for a discussion on taken to ensure that the sealant used is plugs, which penetrate the insulation. safety 13factors). compatible with the sealant to be ap The plug is separated from the back Finite element analysis may be a plied to panel joints after erection of side of the stone by a small section of a useful technique for evaluating stress the panels. corrugated plastic formliner or voided in a veneer panel 17system. This ne The bondbreaker should not be plastic eggcrate to allow air circula cessitates testing 2 to determine the sealed at the bottom of the panel. This tion, or by a polyethylene foam pad. In spring constant values’ for the panel’s ensures any moisture that somehow most cases, it has been found that since material components to model the as penetrates behind the stone veneer, the concrete plug is separated from the sembly. Stone veneer should be tested can drain freely. In the case of long stone, it does not represent a serious in flexure (ASTM C 1352)22 and the panels, a sloping gutter is sometimes thermal bridge and to date, major con section properties and modulus of used not only under the window but densation or discoloration of the exte elasticity should be determined. For also at every horizontal joint. rior wall has not been reported. some stones, the modulus of elasticity varies with stress levels. Granite rift (bedding planes), direction and grain size influence modulus of elasticity. Shear and tensile tests are required for the anchors. The spring constant of a compress ible bondbreaker should be deter mined. For insulation, compressive spring and shear-spring constants should be determined if no bondbreaker is used. The 4 in. (100 mm) diameter concrete plugs encasing the N anchors (see Fig. 9) when an air space is used, should be treated as a short circular beam. The circular beam and concrete backup can have their proper ties determined by calculation for use in modeling. N

PANELWATERTIGHTNESS The bondbreaker between the stone veneer and concrete backup.may func tion as a vapor barrier on the con crete’s exterior face, keeping mois ture in the veneer or at the interface unless drainage provisions are pro vided. After some period of time, gaps Fig. 16. Granite anchor detail for post applied granite slab (stone piece not available also may develop between the stone to be cast in precast concrete panel).

July-August2000 85 -_

The air space, which is vented flashing details, or by using VENEERJOINTING through the jointing to the outside en shiplapped horizontal panel joints vironment, forms a pressure equalizer. which are also left open. With pres In the form, the stone veneer pieces Pressure equalization is achieved by sure equalization, water should not are temporarily spaced with a non- leaving an open horizontal joint at the penetrate the wall system far enough staining, compressible spacing mate windows, which necessitates proper to cause any problems. rial, such as rubber, neoprene, or soft

Fig. 17(a). Portland Oregon Temple for the Church of Jesus Christ of Latter Day Saints, Lake Oswego, Oregon. Architects: Lee, Ruff, Stark Architects, LakeOswego, Oregon and Leland Gray Architects, Salt LakeCity, Utah.

:—.1, ,Th

1/2 Fig. 17(b). Marble faced precast concrete fins are 3 x 6 ft Fig. 17(c). Intermediate panels are 5 ft 10 in. x 1 ft 4 in. (0.9 x 1.8 m) x variable length. (1.8 x 0.4 m) x variable length.

86 PCI JOURNAL plastic wedges, or a chemically neu cess to receive the sealant and also after the panel has been removed from tral, resilient, non-removable gasket, prevent any of the concrete backup the form unless it is a resilient sealant such as sealant backer rod, which will from entering the joints between the backup. not stain the veneer or adversely affect veneer units. Non-acidic based mask Joints between veneer pieces on a the sealant to be applied later. Shore A ing or duct tape (other types will stain precast element are typically a mini hardness of the gasket should be less stone) may also be used to keep con mum of 1/4 in. (6 mm) with /8 in. (9.5 than 20. crete out of the stone joints so as to mm) preferred although they have The gaskets should be of a size and avoid limiting stone movement. been specified equal to the joint width configuration that will provide a re Spacer material should be removed between precast elements, usually ‘/2,

Fig. 18. Hospital Corporation of America Data Center, Nashville, Tennessee. Architect: Gresham, Smith and Partners, Nashville, Tennessee.

1/4 3/4 Fig. 19(a). The new wing for the Joslyn Art Museum, Omaha, Nebraska has 1 in. (3 cm) marble with 6 in. (170 mm) concrete backup. Lead Designer: Sir Norman Foster and Partners, London, United Kingdom; Architect of Record: Henningson, Durham & Richardson, Inc., Omaha, Nebraska.

Fig.19(b). Panels ranged from 20 to 218 sq ft (1.8 to 20.3 2).m

July-August2000 87 joint pattern with a complementary joint size. When stone veneer is used as an ac cent or feature strip on precast con crete panels, a 1/2 in. (13 mm) space is left between the edge of the stone and the precast concrete to allow for dif ferential movements of the materials. This space is then caulked as if it were a conventional joint. The sealant between stones or pan els should be an elastomeric, usually urethane, polysulfide, or silicone, that will not stain the stone-veneer mate rial. Some grades of silicone sealants are not recommended by their manu facturers for applications on stone, as they may stain light colored stones or may cause a change in surface mois ture absorption characteristics that can be seen whenever the stone is wet. Fig. 20. In some projects, caulking between Southwestern Bell stone pieces on a panel may be in Texas stalled more economically and satis Headquarters, factorily at the same time as the caulk Dallas, Texas. ing between precast elements. On Architect: JPL other projects, consideration may be Architects. given to caulking the veneer material at the plant. Plant caulking of stone-to- 3/4 stone joints is recommended in areas or 1 in. (13, 19 or 25 mm), depend is simply not possible because the subject to freezing and thawing, if ing on the panel size. Because the ac joint must have the width necessary panels are to be left in prolonged tual joint width between precast pan to allow for movements, tolerances, stor age during the winter. els, as erected, depends largely on the and other dimensional or volumetric accuracy of the main supporting struc changes. Also, due to tolerances and ture, it is not realistic to require natural warping, adjacent panels HANDUNG, STORAGE matching joint widths between stone may not be completely flush at the pieces and between panels. joint, and shadow lines will appear. AND SHIPPING Often, an invisible joint is speci Rather than attempting to hide the In all operations after removal from fied, e.g., less than /16 in. (5 mm), joint, the joint should be emphasized forms, the veneer-faced precast panels especially on polished veneer. This by finding an aesthetically pleasing should be handled, stored and shipped

1/4 Fig. 21(a). Airline Pilots Association Headquarters, Washington, Fig. 21(b). Closeup of 1 in. (3 cm) travertine clad mullions D.C. Architect: Vlastimil Koubek, Washington, D.C. meeting spandrel at third floor.

88 PCIJOURNAL Fig. 22. Roseville Telephone Company, Roseville, California. Architect: Williams + Paddon Architects & Planners/Inc., Roseville, California.

Fig. 23(a). Collier Center, Phoenix, Arizona. Architect: Opus Architects & Engineers, Phoenix, Arizona.

July-August2000 89 Fig. 24. Sacramento Municipal Utility District Customer Service Center, Sacramento, California. Architect: Williams + Paddori Architects & Planners/Inc., Roseville, California.

Fig. 25. 388 Market Street Building, San Francisco, California. Architect: Skidmore Owings & Merrill, San Francisco, California.

90 PCIJOURNAL on the concrete edge of the panel or on ping of edges and damage to returns. APPLICATION S their backs with the stone facing up. Long returns at sills and soffits gen Over the last 40 years many struc The panels should not at any time rest erally create handling problems, un tures have been constructed with on the veneer face or on any of the ve less proper procedures are worked stone-veneer-faced precast concrete neer edges or corners. To minimize Out ahead of time. panels. Several examples are shown to the effects of the sun on bowing, pan illustrate the use of the various types els are sometimes stored on edge with of stone. the length oriented to north and south. REPAIR In order to prevent staining, wood In the event minor damage occurs blocking should be covered with a to the veneer stone during shipping, Marble plastic film or some other non-staining handling or erection, field remedial The base structure of the Portland material to prevent contact with the work can be performed successfully. Oregon Temple for the Church of stone veneer. Also, contact between The precaster normally does such re Jesus Christ of Latter Day Saints, the stone and oil and asphalt-based pairs, with repair procedures devel Lake Oswego, Oregon consists of 11/4 compounds should be avoided. oped in consultation with the stone in. (3 cm) Vermont marble facing Once the panels are ready for load fabricator. backed with 4 in. (100 mm) of precast ing, they may be cleaned (if part of Epoxy, stone dust, and a coloring concrete [see Figs. 17(a), (b) and (c)]. the contractual obligations) with stiff agent, if necessary, are used to re The roof and base of the entry over fiber, or stainless steel or bronze wire pair small chips or spalls. These hang has l’/8 to 1V2 in. (29 to 38 mm) brushes, a mild soap powder or deter patches can be finished to the same green slate with a 4 in. (100 mm) con gent and clean water using high pres surface texture as the stone facing. crete backup. sure, if 23necessary. No acid or other If it is necessary to replace a stone The 10 x 15 ft (3 x 4.5 m) panels on strong chemicals that might damage piece, satisfactory techniques have the Hospital Corporation of America’s or stain the veneer should be used. In been developed for when the back Data Center, Nashville, Tennessee formation from stone suppliers or of the panel is accessible or after the have 1 in. (25 mm) thick marble ve trade associations on methods of panel has been erected and the back neer on a 5 in. (125 mm) precast con cleaning oil, rust and dirt stains on of the panel is inaccessible (see crete backup (see Fig. 18). The con stones should be made available to the Figs. 14, 15 and 16). Note that each crete has two layers of reinforcement. precaster. anchor should be oriented so that The project was completed on a 14- During shipping, the panels may be when the panel is erected on the month fast track schedule. placed on special rubber padded building, the two anchor prongs will Architectural precast panels for the racks and care taken to prevent chip- be horizontal. new wing of the Joslyn Art Museum,

July-August 2000 91 Omaha, Nebraska are clad in pink 11/4 Sandstone is clad with 1915 panels that are faced with 1/4 in. (3 cm) red granite in. (3 cm) Etowah Fleuri Georgia mar Precast concrete panels are inte cast in concrete [see Figs. ble to match the original stone build grally cast with 11/4 to 1/4 in. (3 cm to precast 25(a) and (b)]. ing constructed in 1931 [see Figs. 44 mm) Arizona Red Sandstone on 4 The first three the 19(a) and (b)]. Labor and material in. (100 mm) thick concrete backup floors of Prome costs were reduced using this system for the Roseville Telephone Company nence in Buckhead office building in Georgia is clad with 1’/4in. compared to traditional stone cladding in Roseville, California (see Fig. 22). Atlanta, (3 cm) flame finished granite on 43/4 systems, There were 199 panels with Two in. (50 mm ) thick red sand in. (120 mm) [see the heaviest piece weighing 22,100 lbs stone was anchored to 4 in. (100 mm) precast concrete (10023 kg). thick precast concrete on the Collier Fig. 26(a)1. Above the third floor, the is Center, Phoenix, Arizona (see Fig. 23). building clad with precast panels a sandblast finish to match Two panel types were specified for with heavy Travertine the granite [see Fig. 26(b)]. the Sacramento Municipal Utility Dis Over 73,000 pieces of 11/4 in. (3 cm) trict’s Customer Service Center, The 30-story State Office Tower II 11/4 travertine were anchored to 7055 pre Sacramento, California (see Fig. 24). in Columbus, Ohio has in. (3 cm) thick granite on 5 and 7 in. (125 and cast units to produce 600,000 sq ft One panel was integrally cast with 11/4 175 mm) thick precast concrete (55800 )2m of cladding for the South to l/4 in. (3 cm to 44 mm) thick Ari backup [see Figs. 27(a) and (b)j. western Bell Texas Headquarters, Dal zona Red sandstone facing on 4 in. las, Texas (see Fig. 20). (100 mm) thick concrete while the Limestone The Airline Pilots Association other panel was cast with a light sand Headquarters, Washington, D.C., has blast finish. The darker sandstone pan The two 18-story towers for the 330 precast concrete units with 11/4 in. els are featured on the lower levels of GSA Federal Building in Oakland, (3 cm) thick travertine inset on 43/4 in. the seven building complex. California are clad with l/4 in. (44 (121 mm) thick concrete [see Figs. mm) beige and white-hued limestone 2 1(a) and (b)1. The precast concrete supported on 2208 precast concrete units with 4400 travertine pieces clad Granite panels [see Fig. 28(a)]. Precast con over 30,000 sq ft (2787 )2m and were The 26-story 388 Market Street crete was selected as the backing be erected in less than 6 weeks. building in San Francisco, California cause of the plastic shaping possibili

Fig, 27. State Office Tower II,Columbus, Ohio. Architect: Bohm-NBBJ, Columbus, Ohio.

92 PC!JOURNAL ties that allowed substantial in-and out relief in the exterior plane [see Fig. 28(b)]. The 38-storyNBC Tower at Cityfront Center, Chicago, Illinois is clad with some 2500 precast concrete panels, each of which has a 2 in. (50 mm) or 2I4 (70 mm) thick limestone veneer cast integrally with 5 in. (125 mm) thick concrete backing (see Fig. 29). The Terry Sanford Institute of Public Policy, Duke University, Durham, North Carolina had approximately 4500 pieces of custom-fabricated gray German limestone inset into a system of precast concrete panels during fabri cation (see Fig. 30). Rather than sim ply butt against each other, the lime stone inset corner pieces were cast in a dovetail pattern replicating stone ma sonry construction. This distinctive look was achieved by casting one leg of the corner joint and then rotating the cured piece and casting the second leg with a cold joint (sequential casting).

Accents or Feature Strips There are a variety of ways that stone veneer can be used as an accent

July-August 2000 93 Fig. 29. NBC Tower at Cityfront Center, Chicago, Illinois. Architect: Skidmore, Owings, & Merrill, Chicago, Illinois.

Fig. 30. The Terry Sanford Institute of Public Policy, Duke University, Durham, North Carolina. Architect: Architectural Resources Cambridge Inc., Cambridge, Massachusetts.

94 PCIJOURNAL Fig. 31(b). Close-up of black granite accents.

Fig. 32. Mountain Fuel North Service Center, Salt Lake City, Utah. Architect: Richardson Companies, Salt Lake City, Utah.

July-August2000 95

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1986,

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4. 11.

3. 10. 2. APPENDIX TYPICALPRODUCTION PRACTICESFOR STONE VENEER-FACEDPRECASTCONCRETEPANELS

(Sequences shown are from several projects and are used to illustratespecific points.)

1. Stone is carefully placed in the form either manually or with a vacuum lifter. Since cut stones can be stained by oil and rust, the formsfor the precast concrete should be lined with polyethylene sheets or other non-staining materials. 2. Compressible spacer material is placed between the stone slabs in the form. 3. Bondbreaker is placed over the back of the stone; spring clip anchors can be seen penetrating through, together with the connection hardware in place, and the prestressingstrand already stressed.

4. Foran insulated sandwich panel with an airspace, rubber stripsare placed to create the airspace (to be removed when panel is stripped), anchors are inserted and a polyethylene foam pad bondbreaker is placed and taped. 5. Two layers of insulation are placed, and insulation joints are taped and caulking is used between insulation layers. 6. All reinforcement, prestressingstrand, connection and liftinginserts,and additional attachments such as window washer insertsor tracks are assembled in the form prior to placing the concrete.

7. Backforms,if necessary, are fixed in the form and concrete is placed and vibrated. 8. Finishedpanel being lifted.

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