Post-Tensioned Concrete: Durability, Design, and Structural Performance

Introduction

The process of designing a structural building system involves careful selection and design by both engineers and architects, coupled with a proper understanding of the materials used. The appropriate utilization of structures such as post-tensioned concrete, along with consideration of their effects on durability and stability, is one of the most important concepts for designers. A proper analysis and assembly of concrete structures, combined with high-quality materials that provide unique and robust properties, results in superior combinations of materials for better durability, sound control, and standard fire safety—all highly required in the contemporary building market.

Current construction market conditions and global economics highlight the need to pay great attention to factors such as construction cost, material supply processes, and reduced floor-to-floor heights. These factors, combined with limited developer financing mechanisms, lead contractors to select concrete as the more cost-effective material in construction over steel (Olson and Smith, 1997).

How Post-Tensioned Concrete Works

Technological advancement is one of the main drivers for the continuing increase in the use of post-tensioned concrete systems (Crigler, 2007). Technology has continually expanded the limits of durability of structures designed and developed using post-tensioned concrete, making it an increasingly attractive option for modern construction.

It is paramount to understand what post-tensioned concrete is composed of and how it achieves its working condition. Concrete is very strong under compression but weak in tensile properties; steel, conversely, is very strong in tension while relatively weak in compression. However, framing members in all construction structures must resist various loads through combined action of both compressive and tensile forces. Various forms of concrete framing are reinforced with steel to improve their load-bearing capabilities.

The development of post-tensioned concrete was designed to compensate for concrete's structural weakness under tension through the imposition of a permanently compressive load on its main structural members. This type of concrete arrangement is made up of extremely durable steel tendons with increased strength. This arrangement, in conjunction with several reinforcing steel bars, is embedded and securely anchored in the solid concrete.

Benefits of Post-Tensioned Concrete

After the concrete has attained adequate strength—a process that typically occurs within 3 or 4 days from placement—the steel tendons are appropriately tensioned through the imposition of a compressive force on the concrete structure. Thereafter, the tendons remain stressed throughout the life of the structure, producing a counterbalancing tension for future loads. The tendons, together with the usual rebar reinforcing, allow the engineer to design shallower structural members that possess increased load capacities with characteristically small deflection under loading.

Post-tensioned structures provide several significant advantages. Among these are reduced dead load coupled with member depth due to the decreased amount of concrete needed. Another key advantage is better control of deflection and far greater crack control. The better crack control leads to improved durability, an added merit. Beyond these benefits, post-tensioning allows for increased span-to-depth ratios, which effectively lowers building heights and reduces both heating and cooling volume. The overall achievement is a decrease in the façade area of the whole structure.

For developers and owners of commercial buildings, the advantages of post-tensioning can make it a preferred reinforcing system. As noted by Scott Greenhaus, president of VSL: "Since the slab thickness is reduced, a developer building a high-rise structure can easily add more floors without increasing the overall building height. Over the course of the building's life, this can represent significantly increased leasing revenue for the owner."

A traditional reinforced concrete building with two-way slabs requires more concrete and thus more weight. As a lighter alternative, post-tensioned slabs require less concrete to achieve the same performance, thereby creating a structure with fewer shear walls, smaller columns, and lower foundation loads. This results in more durable, efficient structures with longer clear spans and greater usable space within the building envelope.

In corrosive environments, encapsulated bonded systems offer significant design advantages that lead to life-cycle savings. Because the amount of mild steel is reduced—particularly in the top zone of slabs—there is less steel to corrode should the concrete crack or spall. This is particularly important in parking garages where significant maintenance costs result from repairs associated with spalled concrete from corroded rebar.

Another advantage of bonded post-tensioning is the inherent capacity to provide resistance to progressive collapse, especially important in the event of localized blast loading. Like mild steel reinforcement, a bonded post-tensioning tendon can develop its force in a relatively short distance along its length. In the event that an anchorage fails or a tendon is severed, the loss of tendon force would be localized. The remainder of the tendon would retain its force at the development length away from the failure point and remain functional. This functionality can be considered in the design of a structure.