How to Inspect a Post Tension Tendon
What is a Post-Tensioned 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 technique 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. Afterward, 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 susceptible 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. Infraspect has developed TendonScan®.
TendonScan® is a service that utilizes advanced magnetic flux leakage nondestructive testing technology 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 (Magnetic Flux Leakage) methods 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 pre-stressing 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. Infraspect’s TendonScan® was specifically calibrated with software written for the inspection of external post-tension tendons.
Perfectly tuned to external post-tension tendons, the software shows the location and percentage of loss of metallic area.
The unit has a speed indicator and high-intensity LED headlamps for dark box girders and an indicator when an issue is present. Wireless, Portable, and Accurate.
Potable, lightweight, wireless, and accurate. Infraspect’s TendonScan® will locate the loss of metallic area inside an external post-tension tendon.
According 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 the 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 known to work. Every structure being inspected is constructed differently. Even the grout being used today changes from batch to batch and manufacturer to manufacturer. Infraspect specially configured software can provide results long before a tendon needs to be repaired or replaced. Deterioration progression over time on additional inspections can help asset managers better budget for maintenance and repairs.
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, form a north pole and a south pole at each edge of the crack, as shown in the Figure below.
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 across 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.
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.
Infraspect is programmed through extensive R&D the software required to interpret the loss of metallic area in real-time while inspecting the tendon.
Advanced Robotic Bridge Inspections provide more quantitative data to better budget for maintenance and repairs.
Maintaining Our Infrastructure With TendonScan®
Post-Tension Tendon inspection service for bridge inspections helps keep our bridges and box girder roadways safe.
Large concrete slabs would crack and break under their own weight without being reinforced. Post Tensioning is a technique to reinforce concrete. Steel strands are placed in a duct, placed in the forms or outside the forms, and tensioned prior to loads being placed.
TendonScan® is an intelligent system to evaluate any damage done to the post-tension tendons in bridges and girder roadways. TendonScan® is a non-destructive post tendon inspection service that can be accomplished with relative ease.
Post Tensioned Tendon conduits are filled with grout to prevent air and water from corroding the steel cables. Tendons when they are placed are sometimes not grouted properly and sometimes due to age or environmental issues the grout may crack. In some cases, the grout does not cure properly and stays wet which can cause the beginnings of corrosion.
This corrosion can allow for air, water, and other environmental contaminants to enter the steel and weaken the overall structure. If the tendons are corrupted, then the concrete slabs overhead can collapse as well. Thus it is of utmost importance to regularly scan the post-tension tendons for faults. If there are known issues then the conventional method of visually inspecting tendons for faults or hammer sounding will not suffice.
TendonScan® peers through the post-tension tenon to look for loss of metallic area inside a post-tensioned duct. The portable unit utilizes magnetic flux leakage nondestructive evaluation to locate areas of concern inside a post-tensioned tendon.
The unit is a 16lb battery-powered robot, that crawls along the tendon. A report is generated with the collected data. This data highlights the faults and discontinuations along the tendon.
Based on the percentage and amount of loss, the asset owner can determine where further actions need to be taken.
The TendonScan® Robot
For post tension tendon inspection, the robot is attached to a particular tendon. The robot then runs along the tendon using its sensors to locate faults, openings, and corrosions. It also uses a system similar to magnetic resonance imaging that is used across hospitals. This MRI-like system scans the material inside the tendon itself.
The unit also transmits data wirelessly to the human inspector in a wheeled cart above. The human inspector receives and reviews the data transferred by the unit in real-time. The entire process is accomplished with non-destructive testing methods to ensure the best possible results. These methods the top of the line Electric Capacitance and Magnetic Flux. Both of these work in tandem to produce magnetic resonance imaging-like visuals.
The results speak for themselves as well. Internal structures and makeup of the tendons are revealed. This shows the human inspectors exactly where they need to repair or replace the tendons. It also identifies places where water or air might have collected in the post-tension tendons.
The entire inspection requires a minimum of 3 human personnel to operate and control the unit. The unit is automated and powered by a battery. As such it does not need constant supervision. The unit is attached to the post-tension tendon and the post-tension tendon inspection can begin.
A quantitative data report of the post-tension tendon inspection is generated and is provided to the relevant authorities. The data in the report correctly highlights exactly where any discontinuations are as well as other faults.
This allows any department to correctly allocate its resources towards the particular fault. They do not have to spend their money and resources trying to fix the entire structure but only the fault at hand.