This document discusses post-tension tendons and methods for inspecting them. Post-tension tendons are steel strands encased in concrete that are tensioned to reinforce large concrete structures. They are susceptible to corrosion over time. Infrastructure Preservation Corporation developed TendonScan, which uses magnetic flux leakage testing technology combined with robotics and interpretation software to inspect external post-tension tendons and provide detailed condition assessments to help with maintenance planning. Magnetic flux leakage testing can detect corrosion and breaks in prestressing strands by measuring changes in the magnetic field caused by defects, but specialized systems calibrated for specific structures are needed to properly interpret the results.
1. Post Tension Tendon -How to inspect a post tension
tendon
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 infrastructure 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 post tension tendon 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.
2. 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.
3. 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 bridge inspections tool.
4. 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.
