What is Post-Tensioning?

Post-tensioning is a method used to strengthen concrete. Tendons made of prestressed steel cables enclosed in plastic ducts or sleeves are positioned in the formwork prior to pouring concrete. After the concrete has hardened but before loads are applied, the tendons are tightened and anchored at the edges of the concrete.

Post-tensioning is a type of prestressing where steel is tightened before the concrete bears service loads. Unlike pre-tensioning where steel is tightened before the concrete is poured, in post-tensioning, concrete is poured first and then the steel is tightened, making it still prestressed.

Advantages & Applications Of Post-Tensioning

Post-tensioning is a type of prestressing that offers several benefits compared to traditional reinforcement with rebars:

• Minimizes or eliminates shrinkage cracking, resulting in fewer or no joints required.
• Tightly holds cracks that do form, reducing potential damage.
• Enables the use of thinner slabs and other structural elements.
• Allows for construction on expansive or soft soil.
• Supports longer spans in elevated members such as beams and floors.

Common Applications for PT

Post-tensioning (PT) has gained widespread popularity over the past few decades due to advancements in technology. In the past, there were issues with cable corrosion, particularly in parking structures exposed to deicing salt, but these problems have largely been resolved through the use of better materials like Anchor Cone, and LRPC Strands to improved construction techniques, and effective training and certification programs.

Some of the more common applications are:

• Ground Slabs: PT is widely utilized for slabs on the ground in areas where soils are prone to movement (such as expansive soils), particularly in the American Southwest. As per Jim Rogers, editor and publisher of Post Tension Magazine, roughly half of all post-tensioning work was for residential ground slabs before the housing construction slowdown occurred last year. The September 2006 issue of Residential Concrete magazine offers a comprehensive review of this application.
• Bridge Design with PT: Both cast-in-place concrete and precast segmental construction techniques have utilized PT in bridge design. This technique enables the creation of longer spans while maintaining tight crack control.
• Masonry walls can be reinforced through post-tensioning by using a solid steel bar anchored to the foundation and tightened with a nut at the top of the wall.

Post-Tensioning Construction Basics

Building post-tensioned slabs on the ground is similar to using rebars, with the exception of the tensioning stage. The cables are placed as per the engineer’s specifications and positioned to run through the center of the slab. For residential construction, tendons spaced 48 inches apart are common, while commercial foundations require more steel. The tendons can be easily navigated around obstacles.

Residential post-tensioned concrete slabs are typically 8 inches thick and constructed with 3000 psi concrete. The tendons are tightened once the concrete reaches a strength of 2000 psi, typically within the recommended 3-10 day period by the Post-Tensioning Institute (PTI).

Tendons, which are made of seven high-strength steel wires wound together and encased in a plastic duct, are anchored at each end in pockets within the slab edge. When the strands are tightened, the wires stretch (about 4 inches for a 50-foot strand) to apply 33,000 pounds of load. Only qualified workers should perform the tensioning. After stressing, the tendon is cut and the anchor pocket is filled with grout to prevent corrosion.

Large concrete structures, such as bridges and parking structure floors and beams, can also utilize post-tensioning techniques. The process is similar to that of post-tensioning slabs, but on a larger scale. An interesting aspect is that tendons are often “draped” in a way that they are low in the center of a beam and high at the supports, as this allows the steel to reinforce the concrete where tension is greatest.

In structural members, the ducts are often filled with grout after stressing to secure the tendon to the concrete along its entire length, which results in a “bonded tendon.” On the other hand, “unbonded tendons” used in residential slabs are left to move within the duct and protected from corrosion through the use of grease.

Decorative Post-Tensioned Concrete

Post-tensioning is used mainly for reinforcement and does not have specific decorative applications. Its benefits include minimal cracking and the ability to span greater distances. PT slabs can be finished and treated like regular concrete slabs, with the exception of not cutting or drilling into the slab. Some slabs may have a warning stamp to indicate that it is post-tensioned.

Pre-Tensioning in Prestressed Concrete

In pre-tensioning, wires or strands called tendons are tensioned to a specified amount before the concrete is placed. The process involves stretching the tendons between anchor points and securing them in place. The concrete is then poured and the tendons become bonded to it. After the concrete hardens, the tendons are cut at the anchor points and regain their original length, compressing the concrete in the process.

Hydraulic jacks are typically used to stress the tendons. The tendons remain under tension during concrete placement and curing, thanks to the anchor points located as far apart as 200m. The anchor points and other support structures used in this process are known as prestressing beds or benches.

Post-Tensioning in Prestressed Concrete

The alternative to pre-tensioning is post-tensioning. In a post-tensioned beam, the tendons are stressed after the concrete has been cast and has attained sufficient strength to safely withstand the prestressing force. Another method used in preventing the tendons from bonding to the concrete during placing and curing of concrete is to encase the tendon in a flexible metal hose before placing it in the forms. The metal hose is referred to as sheath or duct and remains in the structure.

Post-tensioning can be done at the site. It may become necessary or desirable in certain cases to use prestressing for heavy loads and large spans in buildings or bridges. For pre-tensioning in precast as well as cast-in-place construction, some types of devices are needed to attach or anchor the ends of the tendons to the concrete sections.

Author: Vikas