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Building Envelope Design Guide - Precast Concrete Wall Systems

by Paul E. Gaudette
Wiss, Janney, Elstner Associates, Inc.

Last updated: 06-01-2009

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:

  1. Cladding or curtain walls
  2. Load-bearing wall units
  3. Shear walls
  4. 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.

Fundamentals

Structural Aspects of Design

Precast concrete wall systems are most often constructed as a curtain wall or veneer, in which no building loads are transferred to the concrete panels. Most typically the precast concrete wall system must resist lateral loads directly imparted on it, such as from wind and earthquake; as well as vertical loads resulting from the self weight of the precast wall system. These loads must be transmitted through the wall system and secondary structural elements to the building's structure. Other loads such as erection, impact, construction related, and transportation must also be taken into account in the design. It is important to evaluate the design, detailing and erection of precast panels in order to avoid imposing unwanted loads onto the panels.

The concrete panels are designed in accordance with PCI Design Handbook-Precast and Prestressed Concrete (MNL 120), Design Responsibility for Architectural Precast Concrete Projects (ACI 533.1R-02), and ACI 318 Structural Concrete Building Code. Steel elements of a wall system are designed in accordance with AISC specifications for steel construction. Precast concrete elements are designed in accordance with ACI and PCI specifications.

Joints between panels must be wide enough to accommodate thermal expansion and differential movements between panels. Joints between panels are most commonly sealed with sealant to prevent water penetration in the wall cavity. The wall cavity space and back up wall which is usually covered with a water resistant membrane provide a secondary line of protection against water penetration into the building.

Performance Issues

Thermal Performance

Precast wall panels derive their thermal performance characteristics primarily from the amount of insulation placed in the cavity or within the backup wall, which is commonly a metal stud wall in commercial construction.

Moisture Protection

The most common moisture protection system used with precast concrete wall systems is a barrier system incorporating an adequate joint seal. In some cases where additional moisture protection is needed, the application of a sealer or a concrete coating is also used. Sealers can be either clear or pigmented if used as an enhancement of the precast appearance. Film-forming coatings usually offer a higher level of performance but will have a significant impact on the appearance of the precast concrete unit.

The precast concrete panel should also be designed to provide the appropriate level of durability for the planned exposure. Durability can be improved by specifying minimum compressive strengths, maximum water to cement ratios, and an appropriate range of entrained air.

Fire Safety

Precast concrete wall systems are not considered to provide any improvement in fire safety over cast-in-place concrete. In fact, for high-rise buildings precast concrete panels can pose a serious safety hazard when a fire occurs that damages the panel connections and causes a panel to then fall from the building. See Cast-In-Place Concrete Wall Systems for additional information, as well as the information included under Resources in this section.

Acoustics

A precast concrete wall system and cast-in-place facade will provide similar performance regarding sound transmission from the exterior to the interior of the building. However, distressed and open joints between panels can provide a condition in which sound transmission to the interior may be increased.

Material/Finish Durability

Precast concrete panels used in wall systems have many different finishes and shapes. Often the finish will include the abrasion or modifying of the surface by sandblasting, exposing aggregate, acid washing, bush-hammering, or other techniques. Each of these finishes presents a different challenge in producing a durable precast concrete panel. Sandblasting a concrete surface can produce a surface that is less resistant to moisture penetration. As a result, a surface treatment, such as a sealer, should be considered where this technique is used for finishing.

A precast panel with a highly architectural surface will present challenges in development of a concrete mix and placement of reinforcing steel. More complicated profiles in the surface of the panel usually require more workability in the concrete mix, better consolidation techniques, and often more post-production surface repairs. Precast panels with differing depths of surface profiling also require more care in maintaining sufficient concrete cover over the embedded reinforcing steel. In summary, the more complicated the appearance of a precast concrete panel, the more challenging and important the review and approval process and quality control program.

Most distress and deterioration encountered with precast concrete wall systems can be attributed to problems during erection, anchors used to attach panels to the structure, or corrosion of the embedded reinforcing steel. Panel cracking, displacements, or other distress conditions can occur at locations where anchors are inadequately or improperly connected. Poor construction is often the result of poor quality control and out of tolerance fabrication or erection of the panels. Also, damage from handling during construction can result in panel cracking, some of which may not become evident for several years.

Evaluation of future precast concrete durability is performed in several ways. Often requirements are specified (air entrainment, maximum absorption, minimum compressive strength, etc.) to enhance the durability of the concrete. History of the concrete mix and finish can also provide useful information. ACI 318 specifies various criteria for acceptance of a concrete mix. In addition, water-to-cement ratio, minimum compressive strength, air entrainment range and other criteria are also listed. If necessary, freeze-thaw testing can also be conducted in accordance with ASTM C666.

In addition to a precast concrete mix meeting the requirements and recommendations of ACI 318, evaluation and study of the historic performance of a particular concrete mix in a similar exterior environment can also be performed. Petrographic evaluation (ASTM C856) is also commonly used to evaluate aggregate in an effort to identify the mineral composition of the concrete and particularly the aggregate, and based on these observations and past knowledge of those characteristics, to predict future performance. Another method of evaluation is to expose samples of the concrete to an accelerated weathering procedure, and evaluate physical and mechanical properties for changes.

Maintainability

When properly constructed precast concrete panels systems require some maintenance. The most important maintenance item for precast panels is the sealant in joints and protection system, if used. If a sealer or concrete coating has been used for aesthetics or to minimize moisture penetration into the panel, the sealer or coating will require reapplication. The time frame for the sealant and surface protection systems varies widely but usually ranges from every 7 to 20 years.

Applications

Precast concrete wall systems allow a wide variety of colors, finishes and architectural shapes. Precast concrete can be used in environments that allow the use of conventional cast-in-place concrete. In addition, precast concrete may be made in a controlled environment and erected in an environment that would not allow site casting of concrete. The concrete used in precast panels should be designed to be durable in the environment in which it will be used.

See the tables for guidance by wall system type for climate-specific considerations that are imperative to the success of any enclosure design.

Details

DWG iconDWF iconPDF icon

The following details can be downloaded in DWG format or viewed online in DWF™ (Design Web Format™) or Adobe Acrobat PDF by clicking on the appropriate format to the right of the drawing title. Download Autodesk® DWF Viewer. Download Adobe Reader.

Architectural Precast Window Jamb and Sill  DWG | DWF | PDF

Architectural Precast Round Penetration  DWG | DWF | PDF

Architectural Precast Square Penetration  DWG | DWF | PDF

Architectural Precast Interface Between Vertical and Horizontal 2-Stage Joint  DWG | DWF | PDF

Architectural Precast Window Head and Jamb  DWG | DWF | PDF

Emerging Issues

The necessity to make building envelopes blast-resistant forces reconsideration of precast concrete joint and connection designs.

Relevant Codes and Standards

Additional Resources

WBDG

Design Objectives

Functional / Operational—Ensure Appropriate Product/Systems Integration

Products and Systems

Section 07 92 00: Joint Sealants, See appropriate sections under applicable guide specifications: Unified Facility Guide Specifications (UFGS), VA Guide Specifications (UFGS), DRAFT Federal Guide for Green Construction Specifications, MasterSpec®

American Concrete Institute (ACI)

ASTM International

Precast/Prestressed Concrete Institute (PCI)

Publications

  • CMHC Best Practice Guide—Architectural Precast Concrete Walls

Organizations