BACKGROUND: Grouted tendons (bonded and unbonded) are predominantly used for post-tensioned concrete bridge elements in the United States. However, because of potential durability issues with grouted tendons, some highway agencies recently opted to use ungrouted tendons for post-tensioning to facilitate replacement. Combinations of bonded and unbonded tendons as well as internal and external tendons or combinations thereof are used for such applications.
The AASHTO LRFD Bridge Design Specifications address the design of post-tensioned concrete bridge elements. However, some design aspects pertaining to this variety of tendon applications are not adequately addressed or evaluated (e.g., shear and torsion for elements post-tensioned with unbonded internal tendons, combined unbonded internal and external tendons, or combined bonded and unbonded tendons; and shear and flexure for elements post-tensioned with combined bonded and unbonded tendons). Research is needed to review available information, perform necessary analytical and experimental evaluations, and propose revisions to the AASHTO LRFD Bridge Design Specifications and the AASHTO LRFD Bridge Construction Specifications, if necessary. These revisions will ensure appropriate consideration of the effects of tendon configuration on the design of post-tensioned concrete bridge elements.
OBJECTIVE: The objective of this research is to propose revisions to (1) the AASHTO LRFD Bridge Design Specifications design procedures for post-tensioned concrete bridge elements with unbonded tendons or a combination of bonded (pretensioned or post-tensioned) and unbonded tendons, and (2) the AASHTO LRFD Bridge Construction Specifications as needed. The research shall address both internal and external tendons.
Accomplishment of the project objective will require at least the following tasks.
Phase I: (1) Collect and review information relevant to the design and construction of post-tensioned concrete bridge elements with unbonded tendons or a combination of bonded (pretensioned or post-tensioned) and unbonded tendons (internal and external). The review shall consider shear, torsion, flexure, construction components and materials, and other relevant items. This information may be assembled from published and unpublished reports, contacts with transportation agencies and industry organizations, and other domestic and foreign sources. (2) Based on the information gathered in Task 1, identify and categorize the items necessary to address the gaps in the AASHTO LRFD Bridge Design Specifications for the design of post-tensioned concrete bridge elements with internal/external and bonded/unbonded tendon configurations. Consideration should be given to post-tensioning system details that will facilitate replaceability, such as anchorages, deviator, and duct material. (3) Based on the information obtained in Tasks 1 and 2, update the research plan that will be executed in Phase II for (a) developing and validating revisions to the AASHTO LRFD Bridge Design Specifications design procedures for post-tensioned concrete bridge elements to address the identified gaps; (b) proposing revisions to the AASHTO LRFD Bridge Construction Specifications as needed; and (c) preparing design examples to illustrate applicability of the proposed revisions. It is anticipated that the research plan will include experimental and analytical investigations. (4) Prepare an interim report that documents the research performed in Phase I and includes the updated work plan for Phase II. Following review of the interim report by the panel, the research team will meet with the project panel. Work on Phase II of the project will not begin until the interim report is approved and the Phase II work plan is authorized by the NCHRP.
Note: The research plan prepared in Task 3 must provide details on the work anticipated in Phase II. The work proposed for Task 5 must be divided into subtasks, and the work proposed in each subtask must be described in detail (e.g., details of the proposed experimental and analytical investigations).
Phase II: (5) Execute the work approved in Task 4. Based on the results of this work, propose changes to the AASHTO LRFD Bridge Design Specifications and the AASHTO LRFD Bridge Construction Specifications and submit for NCHRP review a technical memorandum summarizing the recommended changes. Work on Task 6 will proceed upon completion of NCHRP review of the memorandum. (6) Review and consider the review comments, and revise the proposed changes. The proposed changes shall be presented in AASHTO LRFD format and include commentary. (7) Prepare design examples to illustrate use of the proposed changes. (8) Submit a final deliverable that documents the entire research effort. The deliverable shall include (a) a research report documenting the work performed leading to the proposed changes to the current specifications; (b) the proposed changes to the AASHTO LRFD Bridge Design Specifications and the AASHTO LRFD Bridge Construction Specifications; and (c) the design examples. The proposed changes to the specifications shall be presented in AASHTO LRFD format and include a commentary and be prepared as a stand-alone deliverable appended to the report.
Note: A review meeting that includes a presentation by the research team to the project panel should be scheduled as part of Task 5, about 1 month after submittal of the technical memorandum deliverables.
STATUS: Research in progress.