Unbonded Post TensioningEdit

Unbonded post tensioning is a practical method of strengthening concrete members by introducing prestress through tendons that are not bonded to the surrounding concrete along their length. After the concrete has gained sufficient strength, prestressing tendons are tensioned at the ends and anchored in place, while the tendons themselves remain protected from the concrete by coatings and barriers. This arrangement contrasts with bonded post tensioning, where the tendons are bonded to concrete along their length via grout or other joining media. In many applications, unbonded post tensioning enables longer spans, thinner sections, faster construction, and lower initial costs, but it also requires careful attention to durability, maintenance, and the long-term performance of protective coatings and end anchors. For a broader view of the technique, see post-tensioning and prestressing.

The core idea behind unbonded post tensioning is to provide a pre-compressed concrete member without relying on a continuous bond between steel and concrete along the tendon length. In unbonded systems, the prestressing steel—often high-strength strands or bars—is coated with protective grease and sometimes a polymer sheath to prevent bonding with the concrete. The tendon is placed in a duct or channel within the member, but the concrete does not grip the steel along its length. After the concrete reaches the required strength, hydraulic jacks tension the tendon and anchors hold the tendon in place at the ends. The force is then transferred to the concrete primarily at the anchor connections, and the tendon remains effectively unbonded for the life of the member. See tendons and anchors for related components, and duct (construction) for common duct configurations.

Principles and components

Tendons

Unbonded post tensioning uses prestressing steel consisting of one or more strands or bars gathered into a tendon. These tendons are designed for high tensile strength and durability under sustained loads. The tendons are chosen based on required prestress levels, target spans, and expected long-term losses. See prestressing steel and tendon for more details.

Coatings, sheathing, and protection

To maintain the unbonded condition, tendons are treated with protective grease and often polymer coatings, creating a barrier between steel and concrete. This barrier prevents the concrete from bonding to the steel along the length of the tendon. Some systems also use a protective outer sheath or cover to shield the tendon from moisture and chemicals. Durability depends on the integrity of these coatings over time. See grease (lubricant) and polymer coating for related topics.

Anchors and end details

The prestress is transferred to the concrete mainly through end anchors that grip the tendon ends once tensioned. End anchors and bearing areas must be designed to resist the force without slipping and to tolerate long-term loading and environmental exposure. See anchor and end anchor for related concepts.

Ducts, sleeves, and installation

In many unbonded installations, the tendon runs in a duct or sleeve that may or may not be filled during construction. The duct provides a path for placement and protects the tendon during assembly, while the coating on the tendon itself provides the unbonded interface with the concrete. See duct (construction).

Construction process

Placement: Tendons are laid out within the concrete formwork along the desired path, with ducts or sleeves as needed. The concrete is poured and cured around the tendons.
Tensioning: After the concrete has developed sufficient strength, technicians apply tension to the tendons at both ends using hydraulic jacks, then secure the tendons with end anchors.
Protection: The tendons remain coated and shielded to prevent bonding; enclosure and protective layers guard against moisture and chemical exposure.
Acceptance: Post-tensioning systems are tested to verify that the required prestress levels are achieved and that anchors and connections perform under anticipated loads. See load testing and structural testing for related topics.

Designers consider long-term prestress losses due to concrete creep, steel relaxation, shrinkage, friction, and anchor set. Because the tendon does not bond to the concrete along its length, losses must be anticipated and accounted for in the design. See prestress loss for a deeper discussion.

Advantages and limitations

Advantages:
- Greater span-to-depth ratios and thinner slabs in many cases.
- Potentially lower material and labor costs on a per-square-meter basis.
- Faster construction in some projects due to simplified erection sequences.
- Easier adjustment or replacement of specific tendons at end supports in some designs. See economic efficiency and construction management for related concepts.
Limitations:
- Durability depends on the integrity of coatings and end anchors; damage to protective systems can compromise performance.
- Long-term prestress losses still occur and must be accounted for in design.
- In aggressive or corrosive environments, unbonded systems may require more stringent coatings, covers, and maintenance regimes than bonded equivalents. See corrosion and durability (materials).

Durability, inspection, and maintenance

Durability focuses on protecting the steel tendons from moisture, chlorides, and other corrosive agents, as well as ensuring the integrity of the protective coatings and end anchors. Regular inspection of exposed anchors, corrosion protection, and the condition of coatings is essential. In some climates, protective measures and periodic non-destructive testing help confirm that prestress levels remain within design tolerances. See corrosion and non-destructive testing for related topics. The choice between unbonded and bonded systems often hinges on a balance between initial construction cost and long-term maintenance risk in a given environment.

Design and codes

Codes and standards governing unbonded post tensioning address material properties, installation tolerances, prestress levels, and durability requirements. In the United States, references include ACI 318 for concrete design, with specific provisions on post-tensioning, as well as regional standards such as AASHTO LRFD for bridges and similar structures. In Europe and other regions, codes like Eurocode 2 and associated post-tensioning guidelines provide parallel requirements. See code and standards for more context.

Controversies and debates

Durability vs. cost: Proponents of unbonded post tensioning emphasize lower initial costs, faster construction, and simpler installation, arguing that with proper materials and protection, long-term performance is reliable. Critics point to the reliance on protective grease and coatings, which can degrade over time, potentially increasing maintenance costs and raising questions about long-term durability in aggressive environments. See durability and maintenance.
Bonded vs unbonded in aggressive environments: In severely corrosive or highly chlorided environments, some practitioners favor bonded systems with grout that encapsulates the tendon, arguing it provides an extra layer of protection and monitoring. Supporters of unbonded counter that modern coatings and protective strategies can meet durability targets while keeping construction costs down. This debate often informs project-by-project decisions and code interpretations. See bonded post tensioning and grout.
Seismic performance and inspection: The behavior of unbonded systems under seismic loads and long-term monitoring requirements remains a topic of ongoing research. Advocates emphasize straightforward tendon replacement or adjustment opportunities in some designs, while others stress the need for rigorous inspection regimes and redundancy. See seismic design and structural health monitoring.
Regulatory and liability considerations: Because prestressed concrete elements carry significant structural responsibility, jurisdictions vary in how strictly they regulate unbonded systems, including requirements for coatings, corrosion protection, and maintenance documentation. Industry practitioners emphasize compliance with standards like ACI 318 and related guidance to manage risk and liability.