Prestressed concrete is widely used in the construction of bridges, parking structures, and high-rise buildings due to its ability to withstand high loads and resist cracking. This is achieved by tensioning steel wires, or prestressing tendons, before or after concrete is poured, allowing the concrete to bear greater loads by balancing tensile and compressive forces. However, one of the key challenges in the durability of prestressed concrete structures is the corrosion of prestressing wire, which can significantly compromise the structural integrity of bridges and lead to catastrophic failures if not properly managed.

Corrosion in Prestressing Wire
Corrosion in prestressing wire is a common issue, particularly in structures exposed to harsh environmental conditions such as marine environments, de-icing salts, or areas with high humidity. Over time, the steel wires or tendons used to prestress the concrete can begin to corrode, weakening the wire and reducing the effectiveness of the prestressing load.

Consequences of Prestressing Wire Corrosion in Bridge Failures
Corrosion of prestressing wire can lead to significant structural issues and, in the worst cases, complete bridge failure. When the prestressing tendons lose their load bearing capacity due to corrosion, the bridge can experience a loss of stability and structural weakening. The main risks associated with corrosion in prestressing wire include:

• Reduced load-bearing capacity: As the prestressing wire corrodes, the bridge may no longer be able to support the loads it was originally designed for, leading to cracking, deflection, or failure.
• Cracking of concrete: The expansion of the corroding steel can induce tensile stresses in the surrounding concrete, resulting in cracks. Over time, these cracks can deepen and spread, further weakening the bridge.
• Loss of prestress force: The corrosion of the wire results in a reduction of the initial prestress force applied to the structure, compromising its ability to withstand applied loads and stresses.
• Spalling of concrete: As corrosion products build up around the wires, the concrete surrounding the tendons may begin to spall, creating gaps that expose the steel to further corrosion and leading to a loss of concrete coverage.

Case Studies of Bridge Failures

The Silver Bridge (1967): One of the most infamous examples of a bridge failure linked to corrosion of prestressing wire was the collapse of the Silver Bridge in the United States. Although the primary cause of the collapse was fatigue in a steel eye-bar, the bridge had a history of corroded prestressing tendons in the concrete supports. The corrosion-induced weakening of the bridge was a contributing factor in the eventual failure.

The Quebec Bridge (1907): Although not directly caused by corrosion of prestressing wires, a similar issue involving the weakening of steel due to corrosion and lack of proper inspection led to the collapse of the Quebec Bridge. This event highlighted the critical need for comprehensive maintenance and understanding of how environmental factors, such as water and salt, can affect bridge structures over time.

The Ponte Morandi Collapse (2018): In Italy, the collapse of the Morandi Bridge was attributed to a number of factors, including poor maintenance and the corrosion of prestressed cables. The bridge had shown signs of damage and deterioration due to the corrosion of both its steel reinforcement and prestressing tendons, with concrete cracking and steel degradation contributing to its failure.

Testing for Corrosion in Prestressed Concrete
In order to detect corrosion before it leads to failure, it is essential to regularly monitor the condition of prestressing wires in concrete structures. The process of inspecting and testing for corrosion in prestressed concrete bridges involves a combination of non-destructive testing (NDT) and destructive testing methods.
One of the key methods for detecting corrosion is removing samples of the prestressing wire for testing. This can be done by carefully extracting sections of the tendons or wires, which can then be subjected to laboratory analysis. Some common tests include:

• Visual inspection: Checking for surface cracks or visible signs of rusting on the surface of the prestressing wire.
• Tensile testing: Assessing the mechanical properties of the prestressing wire to determine the loss in breaking load caused by corrosion.
• Corrosion deposit analysis: Extracted samples are analysed using techniques like X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) to identify corrosion products and harmful contaminants.
• Metallographic examination: Samples are examined to evaluate the extent of corrosion damage and analyse the morphology, providing insight into the corrosion mechanism.

Conclusion
Corrosion of prestressing wires in bridges remains a critical concern for structural integrity, and failure to properly manage and monitor corrosion can lead to catastrophic consequences. Regular maintenance, early detection through sampling and testing, and understanding the environmental conditions that lead to corrosion are vital in ensuring the longevity and safety of prestressed concrete bridges. Advances in testing and monitoring techniques, along with proactive measures, can help mitigate the risks associated with corrosion and extend the service life of these important infrastructure assets.

For more information or expert advice on corrosion of prestressing wire, feel free to contact us at info@r-techmaterials.com. Our team of specialists can assist with material integrity challenges and provide support to ensure the safety and longevity of your engineering systems.