Magnetic Flux Methods: Detecting Corrosion in Post-Tensioned Bridges
FHWA developed and evaluated a proof-of-concept prototype for the magnetic-based, nondestructive evaluation technique called the return flux method. Laboratory test results prove encouraging in identifying corrosion damage in post-tensioned bridges.
Post-tensioning in bridge structures offers many benefits. It provides better performance during seismic activity; it reduces or eliminates shrinkage cracking, therefore requiring fewer or no joints; it holds cracks tightly together; and it enables slabs and other structural members to be thinner. Post-tensioning is a constructive technology in modern bridge structures including segmental box girder bridges and cable-stayed bridges. However, the potential for corrosion of the steel strands that provide post-tensioning in prestressed concrete bridges continues to be a concern.
|FHWA is exploring a nondestructive evaluation method to identify corrosion in post-tensioned bridges, which are common in the United States.
Shown here is the Ringling Causeway Bridge in Sarasota, FL, which needed to have many external post-tensioned tendons replaced.
Among nondestructive evaluation (NDE) technologies, magnetic-based methods have evolved to become promising techniques to identify corrosion of metallic members embedded in concrete structures. An 18-month laboratory study conducted at the Federal Highway Administration’s NDE Laboratory developed and evaluated a proof-of-concept prototype based on the return flux method.
Progressive Corrosion of Post-Tensioned Tendons
Post-tensioned strands are protected from corrosion by a passive film formed in cementitious grout, which also serves as a physical barrier to water, oxygen, and carbon dioxide. However, post-tensioned tendons have been discovered frequently to contain grout deficiencies such as segregated grout and grout voids that can indicate high-risk areas of corrosion. In other words, corrosion susceptibility of the highly stressed post-tensioned strands in the grouted tendons increases as grout quality surrounding the strands decreases.
Post-tensioned bridges in the United States have experienced tendon failures or serious corrosion problems since 1999. On November 13, 2009, the Indiana Department of Transportation (INDOT) closed the Cline Avenue (SR—912) bridge over the Indiana Harbor Ship Canal after a routine inspection revealed significant corrosion of the steel tensioning cables and rebar within the box girders because of water seeping through cracks in the bridge deck. After determining that the level of corrosion had compromised the bridge’s structural integrity beyond repair, INDOT decided to permanently close and eventually demolish the entire bridge to build a new one.
More recently, in June 2020, detailed inspections of the temporarily closed Roosevelt Bridge in Stuart, FL, revealed severe corrosion and ruptured steel strands in the southernmost portion of the 23-year-old bridge’s southbound span.
|In June 2020, an inspection of the temporarily closed Roosevelt Bridge in Florida revealed corrosion and ruptured steel strands. Shown here is the exterior view of the damaged location.|
Developing an NDE Technique for Internal Post-Tensioned Tendons
While tendon corrosion can occur in both external and internal post-tensioned tendons, even careful monitoring of internal tendons embedded in the concrete may not reveal corrosion problems until it is too late.
“As in-service post-tensioned bridges containing internal tendons get older, the need for reliable ways to assess for this type of structure grows. Effective NDE methods can help,” says Joseph Hartmann, the director of FHWA’s Office of Bridges and Structures. “In addition, repairing or replacing corroded internal post-tensioned tendons is cumbersome or, in many cases, nearly impossible compared to similar work for external tendons.”
To overcome these problems and difficulties in the field, many agencies have employed NDE technologies such as ground penetrating radar, impact-echo, ultrasonic surface waves, and ultrasonic tomography to inspect internal post-tensioned tendons. However, current techniques may be able to detect some types of grout deficiencies but not corrosion of the metal tendon components themselves, and therefore detecting metal corrosions is difficult and inconclusive.
|The return flux magnetizer in FHWA’s NDE Laboratory.|
Return flux technology can detect corrosion of steel strands in the internal post-tensioned tendons. FHWA’s NDE Laboratory, in collaboration with a contracted manufacturing company, developed the concept of the return flux system based on the fundamental principle of magnetic main flux: when a ferromagnetic material, such as steel strands, is magnetized close to a saturation level, the magnitude of the magnetic flux going into the material is proportional to its cross-sectional area. If corrosion damage reduces the cross-sectional area, the magnetic flux decreases accordingly. With the technology, the internal tendons embedded in concrete are magnetized using a specially designed yoke-type magnetizer and the system measures return flux via multiple Hall-effect sensors and search coils.
Magnetic main flux is measured after wrapping a magnetizer around an external post-tensioned tendon. In contrast, the return flux method uses a two-yoke-type magnetizer that is placed on the concrete surface directly over an internal post-tensioned tendon. After magnetizing the buried tendon from one yoke to the opposite yoke, researchers can measure the magnetic flux on the return yoke.
Because concrete is essentially a nonmagnetic material with a relative magnetic permeability of unity, it exerts a negligible influence on the magnetic measurements through the gap that can be clear concrete cover in actual post-tensioned structures plus an air gap between the yoke bottom and concrete surface.
The research team conducted an extensive numerical simulation to maximize the effectiveness of the system in terms of the strength of magnetic fields. The final prototype consists of a pair of solenoid coils in series layout between two yokes. The yoke-type solenoid magnetizer was able to exert a strong magnetic field through air gaps and different concrete cover depths.
There is an inherent gap between the pair of yokes and the concrete over the internal tendons, which affects the measurement accuracy by leaking flux through the air. To address this, the researchers elongated the length of the coils to minimize the magnetic flux leaking in the air by increasing resistance in the air between the yokes. The team found that the optimal diameter and length of the solenoid coils are 4.7 inches (11.9 centimeters) and 20.6 inches (52.3 centimeters), respectively.
“Condition assessment of embedded pretensioned strands and post-tensioning tendons in prestressed concrete bridges is one of the high priority bridge performance issues identified by FHWA’s Long-Term Bridge Performance Program and its stakeholders—mainly the State DOTs,” says Dr. Jean Nehme, the team leader for FHWA’s Long-Term Infrastructure Performance Team. “Performance of embedded prestressing strands and post-tensioned tendons will be assessed in detail as part of the program. Therefore, this technology will be helpful in acquiring the data necessary to assess these components.”
|The lab’s bridge mockup is composed of post-tensioned tendons and an anchorage zone.|
The design of the mockup became crucial to the system development. The research team considered key features of a typical web of segmental box girders containing internal tendons. They included sufficient concrete cover between a mockup internal tendon and the magnetizer, two types of duct material (metal and plastic), and horizontal and vertical reinforcing bars. The team fabricated two mockup tendons. Each could accommodate up to 19 7-wire strands with different simulated cross-sectional loss and a real anchorage zone composed of wedge plate, bearing plate, transition tube, and spiral confinement reinforcement in a realistic configuration.
The team evaluated various test parameters, such as return flux and leaked magnetic flux, using strategically placed search coils and axial and radial Hall-effect sensors. Test results showed that the proof-of-concept prototype successfully detected 15.3 percent or larger section loss introduced in the mockup internal tendons surrounded by vertical rebars at 6-inch (15-centimeter) or wider spacing and clear concrete cover less than 7.4 inches (18.8 centimeters) for metal ducts, and 6.4 inches (16.3 centimeters) for plastic ducts.
“These initial results are promising,” says Cheryl Richter, the director of FHWA’s Office of Infrastructure Research and Development, “and suggest that the return flux method is viable as the basis for field-deployable NDE systems to detect section loss in post-tensioning strands.”
Hoda Azari is the manager of the NDE Research Program and FHWA’s NDE Laboratory at the Turner-Fairbank Highway Research Center. She holds a Ph.D. in civil engineering from the University of Texas at El Paso.
Seung-Kyoung (SK) Lee is the founder and president of a private consulting firm. Dr. Lee has been working on corrosion of different types of reinforcing steel and prestressed strands, protective steel coatings, cathodic protection, corrosion monitoring sensors, and non-destructive evaluation of post-tensioned tendons. He is a former chair of the Transportation Research Board’s Corrosion Committee and Steel Bridge Coating Subcommittee. He holds a Ph.D. in ocean engineering from Florida Atlantic University.
For more information, contact Hoda Azari at 202–493–3064 or Hoda.Azari@dot.gov.