The primary rationale for pre-stressing steel is to enhance both the quality and the resistance to tension and compression properties of the material.

While pre-stressed steel may seem like a recent development, its roots extend far into the past. The credit for adopting pre-stressing techniques is often given to Paxton, who employed this method in 1851 to construct the Crystal Palace, unaware of the groundbreaking discovery he had made.

In 1907, Koenen became the first to suggest pre-stressing steel bars before applying concrete to prevent crack formation and unwittingly stumbled upon the innovation of reinforced concrete. Unfortunately, his attempts were thwarted as the phenomena of fluage and shrinkage were unknown at that time.

The true pioneer of pre-stressing, Eugène Freyssinet, emerged in 1928, defining it as a technique involving subjecting a material—reinforced concrete in his case—to loads that generate stresses opposing those in operation, using cables laid in the stressed mass.

The motivation behind pre-stressing lies in the mechanical characteristics of concrete, which exhibits excellent compression force absorption but low tension resistance, delegated to metallic reinforcement. The latter, however, lengthens under tension and, due to bonding, pulls the concrete with it.

In contrast to reinforced concrete, pre-stressed steel boasts high resistance to both tension and compression. Consequently, a comparison between pre-stressed steel and reinforced concrete reveals that this technique not only enhances the quality and resistance of steel but also significantly improves the tension resistance of reinforced concrete, which is otherwise negligible.

Types of Pre-stressing Steel

The steel undergoes various treatment processes to achieve the desired properties. The following procedures are employed:

Cold Working (Cold Drawing):
Cold working involves rolling the bars through a series of dies. This process realigns the crystals within the steel and enhances its strength.

Stress Relieving:
Stress relieving is carried out by heating the strand to approximately 350°C and then allowing it to cool slowly. This reduces the plastic deformation of the steel after the onset of yielding.

Strain Tempering for Low Relaxation:
Strain tempering for low relaxation is accomplished by heating the strand to around 350°C while it is under tension. This process not only improves the stress-strain behavior of the steel by minimizing plastic deformation after the onset of yielding but also reduces relaxation.

Properties of Pre-stressing Steel

High-quality steel is essential for pre-stressed applications, necessitating specific attributes:

1. High Strength: The steel must exhibit high strength to withstand the forces involved in pre-stressed structures.

2. Adequate Ductility: Ductility is crucial to ensure that the steel can deform without failure under the applied stresses.

3. Bendability: Bendability is required, especially at harping points and near the anchorage, to facilitate the shaping of the steel as needed.

4. High Bond: Pre-tensioned members rely on high bond strength to ensure effective transfer of stresses between the steel and surrounding materials.

5. Low Relaxation: Minimizing relaxation is essential to reduce losses over time, ensuring the long-term effectiveness of the pre-stressed elements.

6. Minimum Corrosion: The steel should have resistance to corrosion, safeguarding its structural integrity and longevity in diverse environmental conditions.

The main applications for prestressing steel include:

1. Bridge Building: Prestressing steel finds extensive use in the construction of bridges, where its high strength and ability to withstand tension contribute to the structural integrity and load-bearing capacity of the bridge components.

2. Industrial and Residential Construction: In both industrial and residential construction, prestressing steel is employed to enhance the strength and durability of structural elements, ensuring they can withstand varying loads and conditions.

3. Railway Construction: Prestressing steel is utilized in the construction of railway structures, such as viaducts and overpasses, to reinforce and support the infrastructure, accommodating the dynamic loads associated with railway operations.

4. Wind Power Stations: The construction of foundations and support structures for wind power stations benefits from prestressing steel, providing the necessary strength and stability to withstand the mechanical forces and vibrations associated with wind turbine operations.