Prestressed concrete structures with post-tensioned (PT) tendons have been extensively used for bridges because they facilitate the creation of long-span superstructures. Post-tensioned tendons provide a specific load carrying capacity to the bridge structure. However, inspecting and monitoring the post-tensioning tendons, ducts (plastic or metal) and grouting materials to encapsulate the steel strands of the tendons remained a concern, and no nondestructive testing (NDT) or inspection techniques were available to determine if the ducts were intact and if any of the steel strands had begun to corrode.
To address this concern, a new methodology of electrically isolating the tendon system was developed. This new technique completely isolates the post-tensioned metallic tendons from the surrounding reinforced concrete and regular reinforcement steel (rebar). Thanks to this electrical isolation, the post-tensioned steel is unable to form a corrosion cell with the regular reinforcement steel and the concrete because there is no flow path for electrons and because no oxygen is present in the duct (moisture and air cannot percolate into the duct). The electrically-isolated tendon also facilitates monitoring the effectiveness of the electrical isolation.
What is an Electrically Isolated Tendon (EIT) System?
Prestressed cement concrete is often preferred over traditional reinforced concrete construction for flyover, road bridge and rail bridge applications. It is a type of concrete that is compressed during installation in order to provide additional strength against the tensile load that it will be subjected to during its service life.
Compression is achieved by tensioning the tendons, which may be pre-tension tendons or post-tension tendons.
For pre-tensioned prestressed concrete, the tendons must be tensioned before casting the concrete, whereas for post-tensioned prestressed concrete, the tendons are tensioned only after the concrete around it has already been cast. In this case, the tendons can’t be placed into contact with the concrete, so instead they are placed in protective polymer or galvanized steel ducts, in encapsulated condition. The tendon is firmly connected to the surrounding concrete at both ends through anchorages.
An electrically isolated tendon (EIT) system facilitates corrosion detection of load-bearing members (post-tension tendons) in prestressed concrete structures by using electrical isolation. In other words, the prestressed metal connected to anchorages remains electrically isolated (fully disconnected) from its environment throughout the length of the tendon, which is encapsulated in polymer ducts (or galvanized steel ducts in some cases). In addition, the tendon also remains electrically isolated from the ground (earthing). This new system is used to monitor the integrity of corrosion protection in post-tensioned prestressed cement concrete structures.
Principles of Electrically Isolated Tendon (EIT) Monitoring
Post-tensioned tendons system normally use polymer duct couplers, half shells and ducts for the length of the tendon. The polymer half shells are inserted between polymer ducts and the normal reinforcement steel to protect the polymer duct during tensioning and fabrication. A fiber-reinforced plastic plate placed below the anchorage acts as an isolating plate.
The effectiveness of the electrically isolated tendon's corrosion protection can be verified and monitored by measuring the electrical impedance between strands of post-tensioned tendons and the normal reinforcement steel structure.
The conditions of the tendons and the ducts are assessed by measuring the impedance of the tendons with respect to the normal reinforcement steel structure. The impedance value signifies the opposition to current flow when a voltage is applied across the circuit. The anchorage's electrical terminal is removed to facilitate impedance measurement of tendons without interfering with the prestressed concrete structure. The circuit's impedance value includes the high resistance value of the grout and the reinforced prestressed concrete in series, as well as the capacitance of the polymeric duct. However, defects in the duct and vents in the grout create a low resistance parallel path (parallel to capacitance) for current flow, thus reducing the effective impedance by short-circuiting the capacitance. A significant reduction in the impedance of the tendons indicates that the corrosion protection system is at risk because moisture and chlorides can reach the strands of the tendons via the defects.
Impedance measurements are taken at various stages of construction, such as:
- Installation of the tendon
- Installation of the grout
- During tensioning
Measurements are also taken throughout the structure's service life. To determine the condition of the corrosion protection system, the impedance values are compared with the acceptance criteria (revised 2007 Swiss guidelines issued by jointly Swiss Federal Road Administration (ASTRA) and Swiss Federal Railways (SBB)).
History of EIT Development
The EIT system was initially developed in Switzerland by the efforts of several organizations. The Swiss Federal Road Administration (ASTRA), Swiss Federal Railways (SBB), IfB @ ETH, Zurich (a research organization) and industry (VSL) collaborated on developing the design, components of the system, condition monitoring and inspection procedures. The Swiss guideline, “Measures to ensure durability of post tensioning tendons in structures,” which included acceptance criteria, was issued in 2001 and subsequently revised in 2007.
Initially the new technique was tested on two flyover construction projects. On the basis of practical knowledge and insights gained in these pilot projects, the initial guidelines were published in 2001. The same concept is used in the fib bulletin no. 33 “Durability of post-tensioning tendons” 2005. (fib stands for International Federation for Structural Concrete). The fib bulletin 33 gives details of PL 3, the highest corrosion protection for PT tendons, which can be attained through adoption of EIT monitoring systems.
Case Study of a Concrete Bridge
Carnegie Mellon University's Mobility 21 program describes a replacement project for a 1930s era bridge between Lehigh and Northampton Counties in Pennsylvania that will be the very first to use electrically insulated tendon system (EIT) technology for corrosion detection on the post-tension tendons.
The US Federal Highway Administration, in consultation with the Pennsylvania Department of Transportation, selected this bridge replacement project to demonstrate the use of EIT technology to monitor the integrity of corrosion protection of post-tensioned tendons.
The new technology will be used to detect and monitor the breaches occurring in the corrosion prevention system during the structure's lifetime, as well as to monitor the quality of corrosion prevention efforts during the construction phase. (For another concrete case study, see Case Study: Remediation of 1960s-Era Concrete Silos.)
Advantages of an EIT System
The system of post-tensioned electrically isolated tendons has the following advantages:
- Facilitates assessing and monitoring the integrity of post-tensioned tendon encapsulation
- Improves fretting fatigue resistance and related corrosion resistance
- Enables improved maintainability, reliability, durability and higher level of corrosion protection for the post-tension prestressed concrete bridges and other structures
- Insulates and effectively minimizes the risk of stray current corrosion. (The risk of stray current corrosion is high for rail bridges where electric trams and locomotives operate. To learn more, read Electric Transportation Systems and Stray Current Corrosion.)
- Enables designers to specify thinner slabs and larger distances between beam supports for bridges and flyovers
Limitations of EIT
Researchers have noted certain limitations on the performance of EIT post-tension tendon systems.
They found that the use of high performance grout is critical for the effective anti-corrosion performance of EIT systems. Standard grout was found to be less effective.
Specifying polymer ducts is important for applications involving severe exposure to corrosive environments. The use of galvanized steel ducts is restricted to mildly corrosive environments for post-tensioned tendons. When used in corrosive environment, galvanized steel ducts rapidly deteriorate due to corrosion.
Research is needed to develop better splicing techniques for polymer ducts to minimize the risk of chlorides and moisture ingress.
Epoxy coated and galvanized post-tensioning steel bars showed minimal corrosion risk as post-tensioned tendons, except in weak spots such as cement concrete cracks and duct cracks, in which case localized corrosion was seen in every case. In general, the performance of an EIT system depends upon the use of the proper materials and perfect setup. (Learn more about epoxy coatings for concrete structures in Review of Solvent-Free Epoxy Protection for Secondary Containment.)
The temperature limitations for the polymer being used to make the ducts must also be considered, because some polymers may not bend at low temperatures and may melt at high temperatures.
Is EIT the Future for Bridge Design?
According to one study, EIT systems have been adopted in post-tension tendon monitoring of pre-stressed concrete bridges and flyovers in Europe for over 20 years. EIT has the capability and potential to facilitate condition monitoring of load-bearing post-tension tendons over the entire lifetime of the structures. There are now several suppliers producing the component parts needed for the EIT systems.
Based upon the field experience gained after applying the acceptance criteria according to the 2001 guidelines, the guidelines were revised in 2007 because the acceptance criteria was difficult to fulfill in the case of short tendons. According to the revised guideline, the specific resistance (i.e., the resistance per unit length of tendon) and the specific capacitance (i.e., the capacitance per unit length of tendon) are to be computed.
Thus, in spite of some initial limitations, more and more countries have begun adopting EIT corrosion detection systems, and EIT looks poised to be the technology of the future for post-tensioned concrete structures.