Introduction
Architectural precast concrete has been used since the early twentieth century and came into wide use in the 1960s. The exterior surface of precast concrete can vary from an exposed aggregate finish that is highly ornamental to a form face finish that is similar to cast-in-place. Some precast panels act as column covers while others extend over several floors in height and incorporate window openings.
In most cases, the architect selects the cladding material for appearance, provides details for weatherproofing, and specifies performance criteria. The structural engineer designs the structure to hold the cladding, designates connection points, and evaluates the effects of structural movement on the cladding. The precast concrete manufacturer designs the cladding for the specified loads, erection loads, connection details, and provides for the weatherproofing, performance and durability of the cladding itself.
Precast concrete wall systems offer a wide variety of shapes, colors, textures, and finishes to the designer. As a result, the assessment of samples is a key component in the use of precast concrete. The majority of the review and approval process is conducted at the precast plant prior to precast panel production. This assessment is in addition to the quality control and field testing that takes place during the production phase.
Typically, each precast panel is independently supported to the building structure using an assemblage of metal components and anchors. Joints around each of the precast panels are usually filled with sealant.
Description
Precast Panel Types for Building Envelopes
There are generally four types of precast panels used as part of building envelopes:
Cladding or curtain walls
Load-bearing wall units
Shear walls
Formwork for cast-in-place concrete
Precast cladding or curtain walls are the most common use of precast concrete for building envelopes. These types of precast concrete panels do not transfer vertical loads but simply enclose the space. They are only designed to resist wind, seismic forces generated by their own weight, and forces required to transfer the weight of the panel to the support. Common cladding units include wall panels, window wall units, spandrels, mullions, and column covers. These units can usually be removed individually if necessary.
Load-bearing wall units resist and transfer loads from other elements and cannot be removed without affecting the strength or stability of the building. Typical load-bearing wall units include solid wall panels, and window wall and spandrel panels.
Precast concrete shear wall panels are used to provide lateral load resisting system when combined with diaphragm action of the floor construction. The effectiveness of precast shear walls is largely dependent upon the panel-to-panel connections.
In some cases, precast panels are used as formwork for cast-in-place concrete. The precast panels act as a form, providing the visible aesthetics of the system, while the cast-in-place portion provides the structural component of the system.
Support and Anchorage Systems
The connections for precast concrete panels are an important component of the envelope system. Precast manufacturers utilize numerous different types of anchors but they are often characterized as gravity and lateral types of connections.
The primary purposes of the connection are to transfer load to the supporting structure and provide stability. The criteria used to design precast connections including but not limited to:
Strength
Ductility
Volume change accommodations
Durability
Fire resistance
Constructability
Joints and Joint Treatments
The numerous joints in a precast concrete envelope are an important aspect of the facade design. The joints between precast units or between precast and other building components must be maintained to prevent leakage through the precast wall system.
Joint design should consider the structural, thermal, and all other factors that affect the performance and movement of a joint. The joint seal should of course be adequately designed to withstand the movement of the joint.
Common Backup Wall Elements
In commercial construction, the most common back-up wall element for architectural precast concrete wall systems is an insulated, metal stud back-up wall assembly.
Source : http://www.wbdg.org/design/env_wall_precast_concrete.php
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