Post Tension Tendon -How to inspect a post tension tendon

November 18th, 2018 Author: Doug Thaler

A post tensioned tendon is a bundle of steel strands encased in grouted concrete and wrapped in a protective sheathing and then tensioned. Post tensioning is a tehnique for reinforcing concrete. According to Concretenetwork “Post tensioning is a technique for reinforcing concrete. Post-tensioning tendons, which are prestressing steel cables inside plastic ducts or sleeves, are positioned in the forms before the concrete is placed. Afterwards, once the concrete has gained strength but before the service loads are applied, the cables are pulled tight, or tensioned, and anchored against the outer edges of the concrete.” Without post tension tendons large slabs of concrete would crack and collapse under load. Post tensioned tendons are used in bridge construction, building construction, box girder roadways, tennis courts, dams and other large concrete structures.

Once in place grouted post tension tendons are succeptible to corrosion and deterioration. Many of these external post tension tendons that are in our bridges are exposed to chloride, air, salt water and other elements that can aid in their corrosion. Regular inspections are imperative to keeping the structure and the traveling public safe.

Up till now almost all post tension tendon inspections continue to be manual and subjective in nature with very little quantitative data. Infrastructure Preservation Corporation’s has developed TendonScan®. TendonScan® is a service that is composed of two services. One that utilizes advanced magnetic flux leakage nondestructive testing technology and one that utilizes magnetic flux leakage combined with robotics and interpretation software to peer through an external post tension tendon and provide the asset manager with an advanced condition assessment report to help budget for maintenance.

According to FHWA The MFL method can be used to detect the location and extent of corrosion in post-tensioned and precast prestressed strands in concrete girders, and breakage of wires and strands in post-tensioning tendons and prestressing strands. MFL is also commonly used to test the cables of suspension or cable stayed bridges. MFL units can be clamped onto a cable as part of a climbing module or rolled across a surface.

Where most mfl units cannot provide proper data for post tension tendons. IPC’s TendonScan® was specifically callibrated with software written for the inspection of external post tension tendons.

Perfectly tuned to external post tension tendons, the software shows location and percentage of loss of metallic area.

The unit has a speed indicator, headlamps for dark box girders and an indicator when an issue is present. Wireless, Portable and Accurate.

Potable, light weight, wireless and accurate. IPC’s TendonScan® will locate loss of metallic area inside an external post tension tendon.

According to The Subcommittee Magnetic Processes for Pre-stressing Steel Breakage Detection of the Technical Committee for NDT in Construction of the German Society for Non-Destructive Testing deals with the further development and dissemination of the procedure.

“Statements about the condition of prestressed steel reinforcements are not only important for ensuring the stability and usability of structures under known load levels but also for the conversion and renovation of structures. With the prestressing steel breakage detection based on MFL technique it is possible to make nondestructive test statements. It is essential to coordinate with the client regarding the possibilities and limits of the respective examination task”

Without designing a system and conducting the testing for a specific component the nondestructive testing technology by itself will not give you the results required to properly assess the structural item in question. Although the technology by itself is know to work. Every structure being inspected is constructed differently. Even the grout being used today changes from batch to batch and manufacturer to manufacturer. Without conducting the R&D and properly configuring software to be specific to that component mfl would just not be a useful inspection tool.

A research article written by Seunghee Park and Changgill Lee offers the Principals of Magnetic Flux Leakage to help explain.

Principle of Magnetic Flux Leakage

A steel portion that is magnetized contains a magnetic field both in and around itself, and additionally any place where a magnetic line – of force- exits or enters the specimen or what is called a pole. If you take a magnet that is cracked, but not broken completely in two, forms a north pole and a south pole at each edge of the crack, as shown in Figure 2. The field or magnetic field exits in the north pole and reenters in the south pole. The magnetic field will spread out when it comes accross the small air gap that is created by the crack, because the air which cannot support as much magnetic field per the unit volume as the magnet can. When the field spreads out across the air, it begins to leak out and creates what is called a flux leakage field.

Figure 2: Principle of magnetic flux leakage.

2.2. Magnetic Flux Leakage-Based Local Fault Damage Detection Technique

If you take a strong permanent magnet or an electromagnet to establish a magnetic flux in the material to be inspected. If there is no defect, the flux in the output from the metal remains uniform. In contrast, when there is damage, due to broken wire or wear. The flux leaks out of the metal near the defect.

The sensors that can detect this flux leakage are placed between the two poles of the magnet and it is there that they generate a signal that is proportional to the magnetic flux leakage.

Infrastructure Preservation Corporation programmed through extensive R&D the software required to interpret the loss of metallic area in real time while inspecting the tendon.