WHAT IS HIGH PERFORMANCE AND HIGH PERFORMANCE PRECAST?
What is a High Performance Structure?
The United States Federal Government defines a high performance structure as one that “…integrates and optimizes on a lifecycle basis all major high performance attributes including energy and water conservation, environment, safety, security, durability, accessibility, cost-benefit, productivity, sustainability, functionality, and operational considerations.” - Energy Independence and Security Act of 2007 - 401 PL 110-140
At the heart of this definition is a fundamental shift in perspective from sustainable design and construction to building sustainably and reducing building operating costs on a lifecycle basis. High performance structures use materials and systems that integrate and optimize high performance attributes and are built for long-term performance and durability. High performance also encompasses the concepts and practices of sustainability. However it goes beyond a “this” or “that” approach by requiring an optimization of all relevant attributes for a project. Hence, many attributes such as energy and water conservation, safety, security, durability, are no longer just options, but required to be integrated into a structure’s overall design, construction and performance.
Today, high performance structures are essential in order to meet a variety of demands, from economic to environmental. As a result, more codes, funding entities, and owners are requiring high performance structures. Examples include the new International Green Construction Code, the next version of LEED (v.4) and the mandate by the U.S. Federal Government Executive Order 13514 which requires government buildings to achieve net zero by 2030.
High performance structures require high performance materials and systems. These consist of integrated systems that allow for design versatility and are efficient, resilient and can be optimized to meet the multiple requirements and demands of high performance structures. Precast concrete is used more and more to help projects meet and exceed their high performance goals during design, construction and operation.
High Performance Precast Concrete
High performance precast concrete integrates easily with other systems and provides versatility, efficiency, and resiliency, while easily being optimized to meet the multiple requirements and demands of high performance structures. Many high performance attributes are inherent in precast concrete.
Precast is Versatile:
Precast concrete has excellent aesthetic versatility, providing an almost endless array of colors and textures and allowing for subtle variations through different techniques. These are achieved by using a variety of aggregates, finishing techniques, and sometimes colored pigments. Precast concrete can also be embedded or veneered with many natural materials such as clay thin-brick and natural stone. This provides all of the natural beauty of these materials, while adding all of the high performance benefits of precast concrete.
Precast concrete allows the designer to create a variety of shapes, patterns, and details, which are difficult - if not impossible - to achieve with other materials.
Multiple finishes can be combined in one panel, which reduces detailing, joints, flashing and maintenance costs, as well as the associated liability risks that are a frequent by-product of facades created from a variety of different products.
Precast concrete is also a structural material, which means it can be used as the structural and cladding/envelope system simultaneously, minimizing material usage and accelerating construction. In today’s financial environment, reduction in the overall construction schedule results in quicker return on investment – making precast the optimal solution.
Since precast concrete is essentially custom designed and manufactured for projects, it allows for design flexibility needed to overcome unique situations, ultimately allowing for easier project optimization. Precast/prestressed concrete also provides for longer, open-spans with less columns and obstructions, as well as smaller sections through the use of high-strength concrete and prestressing. This allows floor plans to be more easily adapted as a building’s functional use changes throughout its service life.
Its versatility also allows for scalable performance. New energy codes and ASHRAE requirements demand increasing thicknesses of continuous insulation. Precast insulation is easily scalable to meet these requirements, without many of the detailing difficulties present in cavity wall construction.
Precast is Efficient:
High performance materials and systems must be efficient; both in their use, and in helping to reduce a structure’s overall life-cycle costs. Precast concrete is manufactured offsite in a controlled environment. This helps minimize negative impact to a construction site, while maximizing quality. Precast concrete does not require onsite storage or staging, and is delivered to the project site as it is needed. It is typically erected with a crane and a relatively small crew, which allows for construction within a tight perimeter of the project. This can greatly decrease the footprint requirements of a project site and help minimize disruption to the surrounding areas.
Precast concrete arrives to a project ready for installation and does not require protection from rain, sun, snow, wind, and extreme temperatures. This saves time and money, and reduces the potential for winter conditions-related change orders. Furthermore, precast concrete is one of the fastest building systems available and helps reduce the construction schedule by as much as 40% over steel construction.
Precast concrete helps reduce a project’s overall life-cycle costs, particularly with regard to energy usage. Because it is made of concrete, precast has thermal mass, which can be used to reduce the total energy needed to heat and cool a building. Concrete has a high heat capacity, which means it slowly absorbs and stores heat energy, and likewise, slowly releases it when the temperature differential shifts. This helps reduce the magnitude of the heating and cooling peaks shifting peak loads to times when utility rates are less expensive. Thermal mass provides for a greater effective R-value which allows for the downsizing of the HVAC systems. This all translates to less energy needed to maintain a comfortable and more uniform indoor temperature.
Precast concrete sandwich wall panels combine thermal mass with a layer of continuous, edge-to-edge insulation meeting the new ci requirements. Composite connectors between the wythes eliminate thermal bridges or cold spots. This has been verified using thermal imaging.
Since energy efficiency is one the greatest factors contributing to total life cycle costs and environmental impact, this is a vital area to optimize. Precast concrete sandwich wall panels can greatly contribute to the optimization of a building’s envelope and often help reduce total energy consumption of a building by as much as 30%.
Precast concrete can have over a 100 year service life. It does not rot, rust, or degrade in the presence of moisture or when exposed to sunlight. Precast structures typically have fewer joints and do not require painting. All of these attributes contribute to reducing life cycle maintenance costs.
Precast is Resilient:
High performance systems must also be durable and able to resist natural and manmade forces. Resiliency expands on durability and sustainability to include a structure’s ability to be restored to its full function with minimal effort and resources following a major disaster such as an earthquake or hurricane. This also helps minimize negative effects to the environment after such an event, since resilient structures do not need to be completely rebuilt.
Precast concrete provides resilience against earthquakes. PCI has conducted extensive research on seismic design and the performance of precast concrete structures in earthquakes. This research has resulted in innovative design methodologies including the Precast Seismic Structural Systems (PRESSS). Precast systems are designed to withstand high seismic forces and allow a building to “self-right” after a seismic event.
Several of these concepts have been adopted into building codes such as ACI-318 which is referenced by the International Building Code. These systems now are used by developers across the country, including in high-seismic zones such as California.
Precast concrete also offers protection from high winds and flying debris. Impact studies highlight the protection that precast concrete construction provides. Three different wall systems were tested by launching lengths of 2 x 4 projectiles at 100 mph at each test panel. Projectiles easily penetrated through both stud and cavity wall system test panels. Each test panel had ¾ in. OSB sheeting over a 2 x 4 stud frame. One was then finished with vinyl siding, and the other with a 4 in. brick veneer. However, the projectile did not penetrate or crack the precast concrete sandwich wall panel system. In fact, after multiple tests, the precast panel suffered only minor blemishes.
Precast concrete also provides resiliency against blasts. Recent tests conducted by the Air Force Research Laboratory demonstrate the capabilities of precast concrete to withstand blast forces. This research examined the blast resistance of insulated precast concrete sandwich wall-panel construction under full-scale blast conditions in a multi-story structure.
These results help designers utilize precast concrete to protect against threats often required in the design of structures. Results also show that due to the inherent strength and resiliency of precast, the required stand-off distance can be reduced when using precast concrete compared to traditional design requirements.
Precast Also Provides a Safe Environment:
By designing and building with high performance precast concrete, we help ensure that occupants inside remain safe. Precast concrete not only provides the resilience needed to protect against disasters such as earthquakes and hurricanes, but it also provides great protection against fire. Concrete does not combust, and it provides passive fire protection. This protection does not rely on other systems such as mechanical or water pipes that sometimes can fail to operate. Precast concrete can also easily achieve building code required fire ratings.
Precast concrete is an inert material and does not contribute to poor indoor air quality through VOC offgassing or mold growth. Furthermore, since precast concrete can be erected so quickly, buildings are enclosed quickly, providing a barrier between outside contaminates and moisture, while shaving months off of a project’s schedule. Precast concrete’s high STC ratings provide effective acoustic separation of occupiable spaces from exterior and interior noises. In many cases, precast concrete can be left exposed on the interior of a building, eliminating the need for drywall, reducing waste, and increasing the overall durability of the interior finish. Overall, precast concrete provides a safe and quality indoor environment.
Precast concrete is a versatile, efficient, and resilient high performance system that allows designers and owners to integrate and optimize important attributes for their projects.
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