Architectural Precast Concrete

NOTE This standard governs precast concrete units whose exposed face is the architectural finish of the building: the delegated design of the unit and its connections, the face mix and its color, the finish and the sample and mockup it is accepted against, the reinforcement and hardware that must not stain or spall that face, the insulation of sandwich panels, the joints between units, the tolerances the finished facade is held to, the plant and erector that produce and set the units, and the repair and cleaning of the face after erection. (1.1)

NOTE Architectural precast differs from structural precast in what is being bought. A structural member is bought for its capacity, and its surface is whatever the form leaves; an architectural unit is bought for its surface, and its capacity is simply what it needs to survive stripping, shipping, erection, and the wind and seismic forces on a cladding element. That inversion is why the units are produced under PCI MNL-117 rather than MNL-116, why the accepted mockup rather than a strength test is the standard the work is judged against, and why color, texture, and dimensional precision are treated here as requirements with the same standing as strength. (1.2)

NOTE The units in scope are non-loadbearing cladding: they carry their own weight and the wind and seismic forces applied to them, and they transfer those forces to the building structure through connections designed for the purpose. The structure is designed by others to receive them. (1.3)

NOTE Where a unit both carries building load and presents an architectural face, as a loadbearing spandrel or a loadbearing insulated wall panel does, Precast ConcretePrecast ConcreteResolves to the current adopted revision.sync/precast-concrete governs its structural design, its production under PCI MNL-116, its release strength, and its erection as a structural member, and this standard governs its face mix, finish, color, sample and mockup acceptance, and the PCI MNL-117 tolerances of its architectural face. A plant producing such a unit holds certification for both. (1.4)

NOTE The following are coordinated with this standard but specified elsewhere. (1.5)

NOTE This standard states the project-wide policy for the units in the datasheet; the drawings carry the geometry of each unit, and the precast engineer's calculations carry the thickness, reinforcement, and connection of each unit. Where the drawings and the datasheet differ for a given unit, the drawings govern that unit. (1.8)

NOTE A dimensional conflict between precast and glazing found after casting is corrected in the precast only by recasting the unit, because the opening is formed by the unit itself. Resolving it on paper before either trade fabricates is the only inexpensive moment to do it. (3.1.4)

NOTE Review of the shop drawings confirms conformance with the design intent of the contract documents and does not relieve the Contractor of responsibility for dimensions, quantities, fit, and coordination with other trades. (3.1.5)

NOTE Category A1 certifies a plant against PCI MNL-117 in full, including the finish comparison, color control, and tolerance disciplines a non-loadbearing architectural unit is judged by, and it is the category the industry expects a cladding scope to be produced under. The A-suffixed commercial categories certify a structural plant that also produces finished faces, and they fit a project whose architectural units are loadbearing members that Precast ConcretePrecast ConcreteResolves to the current adopted revision.sync/precast-concrete governs structurally. CPCI certification is the Canadian equivalent, relevant where the nearest qualified plant is across the border. An audited program evaluated by the Engineer of Record is the arrangement where no certified plant is within economic haul distance, and it puts the burden of judging the plant's controls on the Engineer of Record rather than on a third-party audit. (4.1.2)

NOTE The Engineer of Record designs the structure to receive the panel loads shown on the structural drawings; the precast engineer designs the panel and every connection to carry those loads into that structure. The two designs meet at the connection load summary, which is why that summary is an action submittal rather than a calculation buried in the producer's package. (4.2.3)

NOTE PCI erector certification audits the erector's crews, rigging, and connection practice against a published program, and it is held by the erectors who do this work regularly; a project record puts the same questions to the Engineer of Record and fits a region where certified erectors are not available or where the producer erects its own product under its plant quality program. (4.3.2)

NOTE A sample unit cast at the plant shows the finish at full scale and under daylight before any production unit exists, and it can include a return, a reveal, and a joint so that the corners and edges the small sample cannot show are judged with the field. Holding the first production unit for acceptance costs no extra casting but puts the schedule at risk if it is rejected, because every later unit waits on the recast. A field mockup is the only one that shows the precast against the glazing, the flashing, and the sealant it will meet, and it is the one that catches an interface problem; it is also the most expensive, and it is the practice where the facade is a performance-tested assembly under Submittal And Quality ProceduresSubmittal and Quality ProceduresResolves to the current adopted revision.sync/submittal-and-quality-procedures. (4.5.4)

NOTE Twenty feet is the distance PCI MNL-117 uses to judge an architectural finish, and it is roughly the distance from which a pedestrian sees a facade. A shorter distance makes every bug hole and color mottle a defect that no one on the sidewalk will ever see, and a longer one lets a visibly uneven finish through on a low building. (4.5.8)

NOTE The classes follow from where the unit sits. A vertical cladding face in a freezing climate sheds water and is F1; a coping, sill, spandrel top, or soffit edge where water stands or drips is F2; a unit within splash reach of pavement treated with deicing salt is F3; an interior unit, or any unit in a climate that does not freeze, is F0. An exterior unit is C1 everywhere, and C2 on a coastal site exposed to salt spray or where deicing salt reaches it. A single project commonly carries two classes, one for the field of the wall and one for the base and the horizontal surfaces. (5.1.2)

NOTE A cladding panel attached rigidly at both top and bottom becomes a structural element the moment the frame drifts, and the forces that go into it were never in anyone's calculation. The convention is a bearing connection that carries gravity and an out-of-plane tieback that slides or flexes in plane, so that the frame moves and the panel rides along; which connection is fixed and which is released is the precast engineer's decision, but the principle that the panel is released is not. (5.2.5)

NOTE A separate face mix puts the white cement, selected aggregate, and pigment only where they are seen, and it is the way most architectural units are produced; a homogeneous mix is used where the unit is thin, where finished faces occur on several sides so that a face layer cannot be placed on all of them, or where the facing materials are inexpensive enough that the saving does not cover the second placement. (6.1.2)

NOTE A face layer that shrinks at a different rate from its backup bows the unit and can delaminate at the interface, and the defect appears months after erection. Compatibility is a matter of the two mixes sharing an aggregate source and a comparable paste content, which is why the producer is asked to show it rather than assert it. (6.1.6)

NOTE Non-conductive connectors carry no measurable heat through the insulation, so the panel's thermal resistance is close to that of the insulation alone, and they are the choice where the panel is the envelope's insulation layer; whether they make the wythes act compositely is a property of the specific connector and is settled by the precast engineer. Stainless pins conduct less than carbon steel and tolerate the moisture in the insulation plane, at a higher cost per connector. Steel trusses and solid concrete ribs give the stiffest composite panel and the cheapest connection, and they bridge the insulation so thoroughly that the panel's effective thermal resistance can fall well below the insulation's rated value. (6.2.3)

NOTE Extruded polystyrene holds its thermal resistance in the damp interior of a sandwich panel and resists the crushing of consolidation, and it is the most common choice. Expanded polystyrene is less costly per unit of thermal resistance and is available in greater thickness, at a lower resistance per inch and with more water absorption. Polyisocyanurate gives the highest resistance per inch, so it fits a panel whose total thickness is constrained, and its facers and its long-term thermal drift are the properties to check against the connector system and the energy calculation. (6.2.5)

NOTE Where the units are uninsulated, the wall insulation installed behind them is specified in Building Thermal InsulationBuilding Thermal InsulationResolves to the current adopted revision.sync/building-thermal-insulation, and the continuous insulation is coordinated with the connection hardware so that the connections do not require the insulation to be cut away. (6.2.9)

NOTE Color in an architectural face comes from the cement, the fine aggregate, the pigment, and the water-cementitious materials ratio in roughly that order of influence, and a change in any one of them between batches reads as a color change on the building. The single-source requirements hold the inputs constant for the length of the project; the mockup settles what those inputs look like. (7.1.7)

NOTE White cement is the base for light and pigmented colors and gives the most uniform color from batch to batch, at the highest material cost and with the least tolerance for a dirty aggregate. Gray cement varies in shade from mill to mill and from shipment to shipment, so it needs the single-source discipline most, and it is the base for natural gray and dark finishes at the lowest cost. A blend reaches the mid-tones that neither cement reaches alone, and it adds one more proportion the plant must hold constant. (7.2.2)

NOTE The color is recorded as the reference to the accepted sample, not as a description. A color named in words has no acceptance criterion; a color matched to an accepted sample at the specified viewing distance has one. (7.2.4)

NOTE A low water-cementitious materials ratio does more for an architectural face than durability: it reduces the paste that bleeds to the form face, which is the source of color mottling and of the surface voids every finish treatment is partly intended to hide. The ratio the exposure class requires is a ceiling, and a producer running a finish-grade face mix is usually well under it. (7.2.6)

NOTE Five thousand psi is the strength PCI MNL-117 sets for architectural precast, and it is reached without effort by a face mix proportioned for a low water-cementitious materials ratio. A higher strength is specified where the precast engineer needs it for a thin or long unit, not for durability, which the exposure class already covers. (7.3.3)

NOTE Uncoated carbon steel relies entirely on the cover and the density of the concrete, and in a C1 unit with the cover this standard requires that is the protection most cladding is built with. Epoxy coating adds a barrier that is only as good as its handling, because a nick at a tie or a bend is where the bar corrodes first, and the coating prevents welding. Galvanizing tolerates handling damage and can be welded with the right procedure, at a cost between epoxy and stainless, and it is the protection that fits a thin unit where the cover cannot be held. Stainless steel removes the corrosion question for the life of the building and fits a C2 unit or an exposed edge, at several times the bar cost. Fiber-reinforced polymer bars cannot corrode and weigh a quarter of steel, and they fit a thin unit where no steel cover can be achieved; they are not ductile, they cannot be bent in the field, and the precast engineer designs with them under a different code than ACI 318. (8.4)

NOTE The depth an exposed-aggregate or blasted finish removes is cover that no longer exists. A unit detailed with the code minimum cover to the form face and then blasted to a medium texture has less cover than the code requires, which is why the cover is measured to the finished surface and why the finish is settled before the reinforcement is detailed. (8.6)

NOTE Steel bar chairs and tie wire ends at a finished face telegraph through as rust spots within the first year, and no patch matches them. Plastic-tipped or stainless supports at finished faces, and tie wire turned away from them, are the production disciplines that prevent it. (8.9)

NOTE A primed embed is protected until the first field weld burns the primer off, and it is the choice where every connection is inside a conditioned, dry wall cavity and the primer is restored after welding. Galvanizing survives the cavity's dampness and most of the welding, with the welds repaired, and it is the protection most exterior cladding connections carry. Type 304 stainless removes the repair-after-welding step and resists the moisture in an exterior cavity indefinitely; Type 316 adds resistance to chloride, which is the difference on a coastal site or at a connection reached by deicing salt. Stainless is welded to a carbon steel frame through a transition that Welding RequirementsWelding RequirementsResolves to the current adopted revision.sync/welding-requirements governs, and the cost is several times that of galvanized. (9.4)

NOTE The producer furnishing the embeds is the common arrangement because the embed and the hardware it mates with are then one design, one material, and one coating; the trade placing the structure fabricating them fits a project where the structure is placed before the precast contract is let. Under either arrangement, the trade placing the structure sets them, under Cast In Place ConcreteCast-in-Place ConcreteResolves to the current adopted revision.sync/cast-in-place-concrete and Concrete AccessoriesConcrete Accessories and EmbedsResolves to the current adopted revision.sync/concrete-accessories for a concrete frame and Structural Steel FramingStructural Steel FramingResolves to the current adopted revision.sync/structural-steel-framing for a steel frame, and the precast producer surveys them before erection. (9.10)

NOTE An as-cast face shows every bug hole, form joint, and color mottle the form leaves, which makes it the least forgiving finish and the one that most needs the mockup to set a realistic standard; it costs nothing beyond the form. A form liner casts a texture or relief into the face and hides much of the mottling, at the cost of the liner and the draft it needs to strip, and the liner joints must be set out so that they fall at reveals. Sandblasting removes the surface paste by abrasion to a matte texture whose depth is set by the blast, and it is the most common treatment because it is uniform, repairable, and forgiving of minor form defects. Acid etching dissolves a thin skin of paste to expose the sand and leave a fine, even texture that does not change the color as much as blasting; it demands careful neutralization and it is not suited to calcareous aggregate, which the acid attacks. Retarder exposure washes unhydrated paste off the form face to reveal the coarse aggregate at a controllable depth, and it is the finish that shows the aggregate itself as the color. Bushhammering fractures the surface mechanically to a rough, deep texture, the most labor-intensive treatment and the only one that reads as rough stone. Honing grinds the face flat to a matte surface that shows the aggregate in cross section, and polishing continues the grinding to a reflective surface; both need a dense, void-free face mix and a hard aggregate, and both are the most costly treatments per square foot. (10.1.2)

NOTE A light exposure shows the color of the paste and sand and is the most uniform; a medium exposure shows the coarse aggregate as the dominant color, and it is where a selected facing aggregate earns its cost; a deep exposure shows the aggregate in relief and is the roughest and most costly, and it removes the most cover. (10.1.4)

NOTE Thin brick cast into the face gives a brick facade with the panel's erection speed and without the shelf angles, ties, and cavity of anchored masonry; the brick is set in a form liner that fixes the joint pattern, and the joints are the concrete itself, so the face reads as brick at the viewing distance and as a precast panel at the corner unless the return is also bricked. Stone veneer anchored to the unit gives a stone facade in large pieces set at the plant, with the stone isolated from the concrete by a bond breaker and held by stainless anchors so that the two can move independently; its weight and the anchor design belong to the precast engineer. Tile cast into the face behaves like thin brick with a thinner unit and a glazed surface, and it is the least common of the three. (10.2.2)

NOTE An unsealed face weathers as concrete does, darkening where water runs and lightening where it dries, which is the appearance many architectural precast facades are designed for. A penetrating silane or siloxane repellent leaves the surface appearance and vapor permeance unchanged while reducing water absorption and the dirt and efflorescence that come with it, and it is reapplied on an interval of several years. A film-forming acrylic changes the color and sheen of the face, which must be shown on the mockup, resists graffiti and staining better than a penetrant, and can trap moisture behind the film on a face that gets wet from behind. (10.3.2)

NOTE The joint has to absorb the product tolerance of both units, the erection tolerance, and the movement the sealant must take in service, and still leave a sealant bead within the width-to-depth ratio the sealant needs. Three quarters of an inch is the width that does this for most units up to about 30 ft, and it is what PCI MNL-122 points to as the practical minimum; a narrower joint is achievable on small trim units set to a tight tolerance, and a wider joint is needed on long units, on units with a large thermal movement, or where a lower-movement sealant is selected. The joint width is fixed when the units are cast and cannot be changed afterward. (11.2)

NOTE A single exterior seal is the convention, and it works where the seal is maintained; a two-stage joint puts the air seal at the inside face where it stays dry and lets the outer seal be a rain screen that drains, so that a failed outer seal does not become a leak, at the cost of a second bead of sealant that cannot be inspected after erection and of a drained cavity that must be flashed out at the base of each panel run. (11.5)

NOTE The sealant, its backing, and its installation are specified in Joint SealantsJoint SealantsResolves to the current adopted revision.sync/joint-sealants, and the joint faces of the units shall be finished to the condition that standard requires for sealant adhesion. (11.6)

NOTE The PCI tolerances are the ones a certified plant and a certified erector are audited against, and they are the basis every producer prices. A tighter tolerance is achievable on a specific unit with a specific form and a specific erection method, which is why it is confirmed by the producer before bid rather than imposed afterward; a tolerance tighter than the producer can hold becomes a schedule of rejected units. (12.2)

NOTE Every connection carries an adjustment range so that the erector can true the unit to the joint line despite the product and erection tolerances; a frame that is itself out of plumb consumes that range before the unit is ever set, and a connection forced to the end of its slot has no adjustment left and no clearance. The survey of the structure before erection is where this is found. (12.7)

NOTE A change in curing between units changes their color. Steam-cured and moist-cured units of the same mix differ visibly in shade, and so do units stripped at different ages, which is why curing is held constant across a finish rather than varied with the production schedule. (13.5)

NOTE The connection that is meant to release the panel from frame movement is the one most often locked solid in the field, by a weld across a slot or a bolt tightened on a connection designed to slide. The erection drawings state which connections are fixed and which are released, and the special inspector checks the released ones as closely as the fixed ones. (14.7)

NOTE A patch is judged against the same mockup and at the same viewing distance as the unit, and a patch that does not match is a defect in its own right. The constituents of the face mix are retained for this reason, and a patch that is mixed from a different cement or a different sand will never match however carefully it is proportioned. (15.3)

NOTE A finished face that is set down on gravel, leaned on a steel column, or stacked against another unit without a separator picks up a chip or a stain that is invisible until the unit is on the building and the sun is on it, and the repair then happens from a lift. Every handling rule in this section exists to keep the face off anything harder or dirtier than the separator. (16.5)

NOTE Weathering of the face within the accepted range, soiling, damage by other trades after acceptance, movement of the structure beyond the design basis, and the sealant in the joints, which is warranted under Joint SealantsJoint SealantsResolves to the current adopted revision.sync/joint-sealants, are outside this warranty. (17.5)

NOTE A repair made years after acceptance can match the face only if it is made from the same cement, sand, and pigment the face was cast from, and those materials are gone from the plant within a season. The patching stock, with the batch record from the closeout submittals, is what makes a future repair possible. (18.2)