Description:
Approximately 66% of existing concrete bridges consist of prestressed concrete components (calculated by deck area). Prestressed concrete is constructed using either pre tensioned or post tensioned steel tendons as tensile reinforcement. Similar to traditional reinforcement (rebar), these tendons experience degradation due to corrosion and carbonation. However, unlike typical structural concrete, prestressed concrete is more difficult to inspect using nondestructive evaluation techniques. This difficulty arises from the fact that tendons cannot be easily distinguished from other reinforcement, are inaccessible, and are often encased in ductwork. There is a need for new and improved methods, techniques, and technologies to efficiently and effectively inspect these components.
There are multiple existing methods to inspect prestressed concrete components. These methods include, but are not limited to, the nondestructive evaluation (NDE) techniques of magnetic methods (magnetic flux leakage (MFL) and the main magnetic flux method (MMFM)), acoustic methods (impact echo, impulse response, etc.), and nuclear methods (gamma ray and x-ray). Although these methods have proven some successes, there reliability and reproducibility is limited.
Additionally, there are a variety of structures and structural elements that are comprised of prestressed concrete. This population includes a variety of configurations. These configurations range from pretensioned concrete girders and slabs to post tensioned concrete girders and column caps (this list is not all inclusive). The pretensioned concrete is comprised of steel tendons that are incased in concrete and are typically surrounded by a dense mesh of traditional reinforcement. Post tensioned configurations typically contain tendons incased in long ducts. These ducts are either incased in the concrete structure or run from adjacent piers on the internal sections of hollow shaped girders (box girders, pie girders, etc.). Thus, it is easier to inspect ducts that are not incased in material. There are currently very few procedures to inspect any of these configurations, especially post tensioned steel tendons incased in concrete.
The Federal Highway Administration’s (FHWA) NDE and Long Term Bridge Performance (LTBP) programs have identified, through coordination with key stakeholders, that the improved investigation of prestressed concrete is of great importance to the infrastructure of the United States.
Expected Phase I Outcomes:
The objective of this phase is to identify new and improved methods to inspect prestressed concrete nondestructively. The outcome expected from Phase I is a detailed concept that demonstrates the viability of creating a prototype that satisfies the issues identified above. The four areas of concentration should be:
- Inspection of tendons incased in concrete (typical of pretensioned concrete configurations),
- Inspection of grouted tendons in ducts incased in concrete (typical of post tensioned concrete I beam girder and pier cap configurations),
- Inspection of grouted tendons in ducts not incased in concrete (typical of post tensioned concrete hollow girder configurations), and
- A risk based approach to inspection of prestressed concrete that will determine element level inspection criteria and assign ratings to each element with regard to high probability of failure and subsequent high consequence of failure. This approach would result in a rating system to be used by bridge inspectors to determine the frequency of required inspection of prestressed concrete elements.
The inspection procedures should focus on identifying cross section loss of individual tendons as well as variation in grout density, if possible. Phase I deliverables should include a demonstration proving the method is field deployable with a high probability of detection. This demonstration should include a statistically significant number of trials showing a high percentage of true positives and true negatives with a low percentage of associated false positives and false negatives proving the probability of detection using this method. Only methods with a high probability of detection will be granted Phase II awards.
Expected Phase II Outcomes:
The Phase 2 outcomes build upon the lessons learned in Phase 1 and will result in a full optimization development of the NDE methods identified in Phase 1. The final product would be a technology and associated deployable equipment that could be used for inspection. This technology and equipment would include all appropriate analysis software and decision making framework that could be used by state bridge inspectors to determine the level of section loss of a prestressed tendon.
