Do you know the cause of prestressed steel strand breakage?

Prestressed steel strand plays an irreplaceable role in modern engineering through its high strength, high toughness and flexible construction adaptability, and has significant advantages. 

However, in actual engineering, it may still break or slip due to the following reasons, which will bring serious consequences once it breaks. 

The reasons for its fracture are complex, mainly analyzed from three major directions: the material itself, construction operation and external environment:

- Material quality problems:

- Steel wire quality defects: Steel strands are composed of steel wires. If there are defects such as internal cracks and inclusions in the steel wires during production, stress concentration points will be formed. When bearing loads, these weak points are prone to crack first and gradually expand, eventually leading to the fracture of the steel strands. 

For example, incomplete deoxidation during steelmaking, the oxides produced are mixed in the steel wires, reducing the strength and toughness of the steel wires. Low-quality steel wires may suddenly fail due to insufficient strength during tensioning.

- Unstandardized twisting of steel strands: Improper twisting process, such as uneven twisting distance, will cause uneven force on each steel wire. The stressed steel wire reaches the ultimate strength too early and breaks, which then causes the entire steel strand to break. 

For example, the twist pitch fluctuates, causing some steel wires to be overstressed.

- ‌Uncompacted grouting‌: In anchor cable projects, if there are cavities in the grouting body or it is not firmly bonded to the steel strand, external corrosive media can easily invade and accelerate the corrosion of the steel wire‌.

- Improper construction:

- Tensioning operation errors: Over-tensioning during tensioning, the applied stress far exceeds the design value, the yield strength of the steel strand decreases or even breaks directly. In addition, if the tensioning speed is too fast, the strain of the steel strand cannot be evenly distributed in time, and local stress concentration can also cause fracture. 

Over-tensioning or under-tensioning may cause the actual force of the steel strand to deviate from the design requirements, and long-term non-uniform stress state is prone to fatigue fracture‌. 

For example, the operator arbitrarily increases the tension force based on experience.

- Anchoring defects: Poor handling of the anchor end, such as deviation in anchor installation or wear of the clip, can cause the steel strand to slip, weaken the prestressing effect and increase the risk of fracture. In some projects, the friction between the clip and the steel strand is insufficient because the anchor is not cleaned before installation.

- Installation damage: During construction, the steel strand rubs and collides with other objects, resulting in scratches and pits on the surface. 

These damaged areas will become stress concentration sources and cause fractures when subjected to subsequent forces. 

For example, during the bundle threading process, the steel strand rubs and scratches against the inner wall of the pipe.

- External environmental impact:

- Corrosion: When the steel strand is exposed to moisture, salt spray or chemical corrosion, if the surface protective layer is damaged, the steel wire will gradually lose strength due to electrochemical corrosion. The volume of rust expands, generating internal stress, weakening the effective cross-sectional area of the steel strand, reducing the bearing capacity, and eventually leading to fracture. For example, the steel strand of the cross-sea bridge has been corroded by seawater for a long time.

- Stress concentration and local overload：At the structural nodes or bending parts, the steel strand is prone to stress concentration due to the sudden change of geometric shape. If the stress is not dispersed through reasonable design, local overload will directly lead to steel wire fracture.

- Fatigue load effect: in an environment of frequent vibration, repeated loading and unloading, such as bridges subject to frequent vehicle traffic, steel strands subject to alternating stress, and long-term exposure to alternating loads (such as wind vibration, vehicle loads) may lead to fatigue accumulation of steel wires and eventually brittle fracture. ‌‌

- ‌External impact and accidental damage‌：During the construction process, mechanical collisions, welding sparks or improper maintenance in the later stage (such as drilling to accidentally damage steel strands) may directly damage the surface of the steel wires, forming a risk of fracture. ‌

Due to its unique advantages, prestressed steel strands are widely used in modern engineering. 

In order to ensure the safety of the project, it is necessary to strengthen management and control from aspects such as raw material testing, refined construction, and regular maintenance to achieve the optimization of steel strand performance and structural life.