Durable Solutions

In today’s environment of enhanced dangers and devastating natural disasters, some facilities require added protective design to aid management of risk. Precast concrete creates an incredibly durable structure that aids in meeting a wide range of standards for resisting explosions, hurricanes and tornadoes, fires and seismic events. It also produces low maintenance needs throughout its service life.

Blast Resistance

As terrorism becomes more pervasive, building owners beyond those for government facilities and secure locations are looking to improve their blast resistance in particular.

Precast concrete provides a durable material that can withstand the explosive force of an external blast better than any other material. It also can be used to design aesthetically pleasing blockades that keep potential explosive devices at a distance, providing the best solution for meeting blast resistance.

Internal blast resistance requires more consideration. The objective of blast-resistant design is to provide an acceptable level of safety to building occupants in the event of an explosion. Considerable damage is usually acceptable as long as components remain attached to the building, and the building does not experience a progressive collapse.

Load-bearing precast panels must be designed to span failed areas through arching action, strengthened gravity connections, secondary support systems or other ways of providing an alternative load path.

Architectural precast concrete can be designed to mitigate the effects of an explosion and thereby satisfy requirements of the General Services Administration (GSA) and the Department of Defense (DOD). Protecting the entire façade, however, will impose a great cost regardless of the material used.

To provide the best protection for occupants, designers should plan for the building and its cladding to remain standing or attached long enough to protect occupants from injury or death resulting from flying debris and to evacuate everyone safely.

A minimum panel thickness of 5 inches, exclusive of reveals, should be designed. The panels also should include two-way, reinforcing bars spaced not greater than the panel’s thickness to increase ductility and reduce the chance of flying concrete fragments.

The thinnest panel thickness acceptable for conventional loads should be used. The objective is to reduce the loads transmitted into the connection, which must resist the panel’s ultimate flexural resistance.

All building components requiring blast resistance should meet the criteria required for GSA or DOD facilities. They should be designed using established methods and approaches for determining dynamic loads and dynamic structural response.

Seismic Resistance

Precast concrete panelized systems can meet any seismic zone’s requirements. And new connection techniques that help re-right buildings after a seismic event can ensure that buildings aren’t structurally damaged by an earthquake, allowing them to reopen quickly afterward to serve students and the community.

Recent tests have proven the validity of these approaches, and field work has supported those findings. Design guidelines have been developed from an 11-year, on-going study conducted by the Precast Seismic Structural Systems (PRESSS) research program under the auspices of the National Science Foundation, the Precast/Prestressed Concrete Institute and the Precast/Prestressed Concrete Manufacturers Association of California.

The study tested four frame systems and one shear wall system to determine their capability for withstanding seismic forces. All five systems were found to offer strong seismic performance that was as good as or better than that of cast-in-place systems.

Overall, studies have found, buildings constructed with precast concrete typically will sustain less seismic damage than conventional cast-in-place systems. They are suitable for low-, moderate- and high-seismic zones and use conventional hardware that is widely available. Because they are based on existing, known technologies, the systems offer simple designs that are easy to understand and implement.

Of most significance, three of the designs—the hybrid frame, pretensioned frame and shear wall—offer characteristics not available with any other type of seismic system currently on the market, regardless of material. These systems allow the building to flex with the earthquake force and then pull it back to its original position.

This self-righting characteristic, combined with the minimal damage these systems experience in general, allows immediate re-occupancy of the building after a major seismic event. That can create a compelling marketing advantage in high-seismic areas as well as major revenue generator for owners, who can be assured of few interruptions to the building’s occupancy.

Maintenance

Maintenance needs for precast concrete components are minimal, thanks to their durability. Panels require caulking only every 15 to 20 years to maintain their reliability. This makes precast far easier to maintain than brick mortar joints, plaster or exterior insulation finish systems (EIFS).

The panels’ fewer locations for moisture penetration prevent unsightly stains or damage to interiors while alleviating concerns about having to deal with mold.

Factory-produced and finished, precast panels are more weather- and impact-resistant than other materials. They require less on-going maintenance, such as repainting.

Insulated sandwich wall panels provide an interior wall that provides high durability, making it ideal for heavy-duty facilities such as manufacturing and warehouse facilities.

Panels can be washed with strong chemicals and cleaners without losing their color, and UV rays will not cause them to fade, as will happen with paint.

Parking structures, more than any other building type, require routine maintenance. The severity of the wear and exposure demands will depend on the structure’s location, environmental conditions and maintenance schedule.

Based on national surveys, precast concrete parking structures provide superior resistance to deterioration. Blakeslee Prestress, a PCI-certified plant produces components that use durable concrete mixes with specialty curing can be created that cannot be achieved in the field.

In general, there are three types of maintenance: housekeeping, for general upkeep; preventative maintenance, with periodic checkups and cleaning of components; and structural repairs, due to spalling, cracking or other wear issues. All of these are minimized with the use of precast concrete components.

Fire Resistance

Precast concrete is a noncombustible material that meets all fire-code provisions with no additional design or insulating material required. This resistance speeds construction, eliminates added trades from the site and provides an inherent level of protection that doesn’t need to be activated at the time of a fire.

Concrete doesn’t give off lethal smoke and maintains its structural integrity even when subjected to the most intense heat, unlike wood or steel structures. Designing with a total-precast system allows the durable structural framework and panels to work together to compartmentalize any fire, maximizing the time for detection, evacuation and suppression.

Noncombustible compartmentalization, combined with the inherently fire-resistant/tolerant structural frame that precast concrete provides, offers the best combination of economics and protection that owners and users seek. When this passive design combines with other safety measures, including sprinklers and early-warning detection systems, a balanced design approach is achieved.

A variety of precast concrete components can be used in creating a complete passive-design system for a building. Foremost among these are:

Hollowcore slabs, which serve as combined floor/ceiling systems and also can be used as wall panels in either vertical or horizontal configurations.
Wall panels, which offer high fire ratings and work with other components to create a noncombustible envelope. Insulated sandwich wall panels also can be used.
Double tees, which can be used similar to hollow-core planks for roofs, ceilings, floors or wall panels.
Columns and beams, which create a framework that will resist intense heat and will not add fuel to a fire.

A total-precast concrete system provides an effective design for minimizing fire damage and containing the effects within the smallest space possible for the longest time.

Precast concrete also eliminates the messy, time-consuming job of fire-proofing a building’s steel framing, as well as the need to patch and rework it after other trades finish at the project’s later stages. Precast members are naturally fire protected, because they will not burn.

The panels’ fewer locations for moisture penetration prevent unsightly stains or damage to interiors while alleviating concerns about having to deal with mold.

Factory-produced and finished, precast panels are more weather- and impact-resistant than other materials. They require less on-going maintenance, such as repainting.

Insulated sandwich wall panels provide an interior wall that provides high durability, making it ideal for heavy-duty facilities such as manufacturing and warehouse facilities.

Panels can be washed with strong chemicals and cleaners without losing their color, and UV rays will not cause them to fade, as will happen with paint.

Based on national surveys, precast concrete parking structures provide superior resistance to deterioration. Components produced by PCI-certified Blakeslee Prestress, Inc. in particular use durable concrete mixes with specialty curing can be created that cannot be achieved in the field.

Storm Resistance

Precast concrete’s durability allows it to stand up to the worst that nature can throw at it, including hurricanes and tornadoes. Many secure buildings are designed with precast concrete load-bearing panels because they withstand high winds better than other materials.

This is especially important for federal buildings as well as high-tech locations such as server and data centers that must be maintained and operational regardless of what weather hits. Schools also are turning to precast concrete more often to protect students and serve as a central base for after-storm coordination and a haven for those left homeless.

Designs can include thicker wall panels and heavy-duty connections, smaller areas of window glazing and spandrels fortified to hold them in place, and other features that ensure the building will remain standing.

Precast concrete structural systems can create designs specifically geared to withstanding hurricanes along the coast, which are especially devastating due to the water surge that often accompanies them. Columns combined with hollow-core planks and double-tee components can raise the ground level of the building to ensure it remains above water level on coastlines that are deemed susceptible to hurricane forces.

Even residential buildings can be designed using precast concrete to resist storms. In regions of the country where tornados can wreak havoc, precasters have worked with local communities and designers to create solutions. These include connections that securely tie the house together from roof to foundation, providing protection for winds up to 130 mph.