What is the difference between bonded and unbonded strands?

Understanding the Bonded and Unbonded Post-Tensioning Systems

When discussing post-tensioning systems, we often encounter two primary categories: bonded and unbonded tendons. Each serves distinct purposes and applications in construction.

Defining Unbonded Tendons

An unbonded tendon is designed not to form a connection with the surrounding concrete throughout its length. Typically composed of high-strength steel, these tendons are coated to resist corrosion and housed within a protective plastic sheathing. The tensile force exerted within these strands is transmitted to the concrete primarily through anchors at their ends. Most commonly, unbonded systems utilize mono-strands with dedicated anchors for each strand.

Characteristics of Bonded Tendons

In contrast, bonded tendons are intentionally designed to establish a continuous bond with the concrete along their entire length. This bond is facilitated by a cement-based material, known as grout, which fills the duct surrounding the strands. After the tendon is stressed, grout is injected into the duct, securing the pre-tensioning strands to the concrete. This method is frequently employed for beams and slabs where multi-strand configurations are common.

Advantages of Post-Tensioning Systems

Employing post-tensioning materials in construction designs offers significant cost benefits. Both bonded and unbonded systems contribute to quicker construction timelines and streamlined installation processes.

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Comparing Bonded and Unbonded Systems

Post-tensioning is an effective solution for various design and construction challenges. It involves the integration of concrete with pre-stressed cables which are tensioned and anchored within the structure. Once the concrete attains adequate strength, the tension is released, enhancing the structural integrity of the component.

Unbonded Post-Tensioning

This system generally consists of single strands or bars that do not bond with the concrete. The protective grease and outer plastic cover prevent corrosion, allowing unbonded strands to adjust their position relative to the structure. Ideal for new constructions, unbonded systems are lightweight and easily installed, providing a cost-effective solution for several applications.

Bonded Post-Tensioning

Bonded systems utilize tendons with one or more strands enclosed in metal or plastic ducts. After tensioning, this duct is filled with grout, ensuring a robust connection with the surrounding concrete. This method supplies ample corrosion protection for the prestressing steel while also enhancing the overall performance of the structure. Multi-strand systems are frequently applied to large elements like beams and are advantageous for their ability to span longer distances and support higher loads.

Key Differences

• Bonded systems generally require more reinforcement due to friction-related losses, meaning more tendons might be needed compared to unbonded systems.
• In corrosive settings, bonded systems are preferable due to their design advantages.
• Bonded tendons develop their force over shorter distances than unbonded ones.
• Flexibility for future alterations is often better with bonded systems compared to unbonded systems.
• Deflection, shear, top stress, bottom stress, and bending moments are typically lower in unbonded beams versus bonded beams.

Combining Both Systems

It's possible to integrate both bonded and unbonded post-tensioning systems within a single structure. For instance, unbonded systems might be employed for standard floors, while bonded systems could be strategically used for transfer girders to optimize structural performance.

Conclusion

It is projected that post-tensioning techniques, particularly in economical applications like hollow unit walls for gravity loads, will gain popularity. Although bonded systems often deliver enhanced strength and ductility, advancements in technology make hybrid models increasingly viable in modern construction.

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