Location of Prestressing Steel Fractures in Concrete

Zenglongyu Li, Sommersemester 2020/2021

Comparing with steel structures and normal concrete structures, pre-stressed concrete structures are one of the most common forms of buildings in modern age. Pre-stressed concrete is massively used in high stress-demanding structures such as skyscrapers and bridges. The fracture on pre-stressed tendons may lead to disastrous failure to the whole pre-stressed concrete structure. To locate these fractures four testing methods, The Remanent -Magnetism method (RM method) alongside with other three methods, will be introduced in this article.

Existing Location Method

There are four main methods which have been applied in tendon fractures detection. They are:

1. Visual inspection after opening the concrete
2. X-Ray method
3. Acoustic Monitoring System
4. Remanent -Magnetism method

The first visual inspection is a destructive method as it requires an opening to the tendon, while other three methods are non-destructive.

Brief Introduction to all four Methods

Visual inspection needs an opening in the concrete surface. The main disadvantage of visual inspection is its local limitation. Limited by its opening, only fractures of unbonded tendons around the opening location can be detected. For damage status of the whole concrete body and fractures on inner wires of a bonded tendon, this method is not so effective. Additionally, the testing opening must be sealed carefully, as this defect can be the hot spot of harmful ion intrusion and stress concentration. ^[1]

The X-ray method applies X-rays to scan the whole structure. Because of the difference between materials the film will be exposed also differently. To apply X-ray method the wires of a pre-stressed tendon should not be bounded. If the fractured wire is covered with other intact wires, its fracture will be difficult to locate. Additionally, as the concrete is actually a shielding material against X-ray, in real application people must use high energy X-ray and expose the concrete for a long time, which is relative time and energy costy.

Acoustic Monitoring catches the characteristic sound emission of wire fracture, and locates the fracture by different travel times of sound wave to sensors. Acoustic Monitoring requires two prerequisites: a series of sensors which must be installed before the concrete is putting into use and a long-term continuous monitoring. If the fracture happened ahead of installation or intervals of data collection period, it will not be recorded. ^[2]

The Remanent -Magnetism method detects disturbance of magnetic field. The magnet field of originally intact wire will change rapidly comparing to fractured wire. This disturbance is permanent so don't need long-time monitoring. This characteristic change of magnet flux can be easily identified in Remanent -Magnetism method, even the fractured wire is coupled with other complete wires and sealed in sheath, shown as in Figure 1.

Points to note about the RM method

Material Magnetization

The first step of RM method is to magnetize the testing body. Magnetization of steel bars is usually conducted using electromagnet on the testing surface. Magnetizing the tendons before casting is not advisable, because its magnetisation will be affected easily by mechanical impacts in installing and storage. ^[4]

This step is aiming to overwrite the existing impact induced magnetization state of testing body and provide a magnet field to manifest the defect.

Eliminate the interference of general reinforcement

Comparing to reinforcement bars, pre-stressed tendons are magnetically soft material due to their alloy components. To distinguish the signal caused by reinforcements and one by tendons, a multi-stage magnetization can be applied, as soft magnetic materials are suspectable both to be magnetized and demagnetized. After a series of procedures, the remanent magnetism can be maximized in tendons and minimized in reinforcements. ^[5]

After these selective procedures the signals from two materials are divided, but further numerical process is needed to eventually eliminate the influences from ordinary steel.

Factors affecting the RM method

Comparing with X-Ray method, RM method is relatively unsusceptible to application conditions. The result of locating will be affected by following factors:

• Thickness of concrete cover
• Number of wires in tendon
• Number of fractured wires in the same location
• Magnetic material properties of tendon
• Status of normal reinforcement

Overall

The advantages of RM method compared with other non-destructive testing method are various. The application of RM method is fast and easy as well as low investment demand. What's more, the biggest advantage is its sensibility of detection. As the change in magnet flux can be induced by only slight deformation of wires, fractures can be detected even it’s is not optical visible (for human eye or normal magnifier) due to its very limited width. Limitation of RM method is mostly from the interference by normal reinforcement, but this can be overcome with proper data processing method.

Literature

1. Bligh, R. P., Nakirekanti, S., Bray, D. E., and James, R.W. Evaluationof NDE techniques for detecting grout defects in cable stays, Materials Evaluation (1994), 508–514.
2. Halsall, A. P.; Welch, W. E.; Trépanier, S. M., Acoustic Monitoring Technology for Post-tensioned Structures, Symposium Papers of the FIP Symposium 1996 on Post-Tensioned Concrete Structures (1996) 521-527
3. Scheel, B. Hillemeier, Fast Location of Prestressing Steel Fractures in Bridge Decks and Parking Lots. https://www.ndt.net/article/ndtce03/papers/v103/v103.htm
4. Scheel, B. Hillemeier: Capacity of the remanent magnetism method to detect fractures of steel in tendons embedded in prestressed concrete, NDT & E International, Volume 30, Issue 4 (1997), p. 211-216.
5. Scheel, B. Hillemeier: Location of prestressing steel fractures in concrete, Journal of Materials in Civil Engineering (2003, 15 (3), p. 228-234: