National Academies Press: OpenBook

Evaluation and Repair Procedures for Precast/Prestressed Concrete Girders with Longitudinal Cracking in the Web (2010)

Chapter: Chapter 4 - Conclusions, Recommendations, and Suggested Future Research

« Previous: Chapter 3 - Research Findings
Page 62
Suggested Citation:"Chapter 4 - Conclusions, Recommendations, and Suggested Future Research." National Academies of Sciences, Engineering, and Medicine. 2010. Evaluation and Repair Procedures for Precast/Prestressed Concrete Girders with Longitudinal Cracking in the Web. Washington, DC: The National Academies Press. doi: 10.17226/14380.
×
Page 62
Page 63
Suggested Citation:"Chapter 4 - Conclusions, Recommendations, and Suggested Future Research." National Academies of Sciences, Engineering, and Medicine. 2010. Evaluation and Repair Procedures for Precast/Prestressed Concrete Girders with Longitudinal Cracking in the Web. Washington, DC: The National Academies Press. doi: 10.17226/14380.
×
Page 63

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

62 4.1 Conclusions Based on the work conducted in this project, the research team has developed a user’s manual for acceptance criteria and repair materials and methods for prestressed concrete girders experiencing end zone cracking due to transfer of the preten- sioning force. The manual consists of four criteria depending on the crack width. The first criterion is for crack widths less than 0.012 in., where no repair is recommended. The second criterion is for crack widths from 0.012 in. to 0.025 in., where it is recommended that the cracks be filled with a cementitious packing material and then covered with a water-resistant surface sealant to keep water contaminated with corrosion- inducing chemicals from penetrating the concrete and reach- ing the steel reinforcement. The third criterion is for crack widths from 0.025 in. to 0.050 in., where epoxy injection is rec- ommended for cracks wider than 0.025 in. and cementitious packing material for cracks narrower than 0.025 in. The man- ual provides the provisions for successful epoxy injection. The fourth criterion is for crack widths greater than 0.050 in., where rejection of the girder is recommended, unless it can be shown by detailed analysis that structural capacity and long-term dura- bility are not compromised. These criteria allow for acceptance of girders with cracks wider than those implied for flexural members in the ACI-318 Building Code and the AASHTO LRFD Bridge Design Specifications. The nature and conse- quences of end zone cracking are quite different from those of flexural cracking. Flexural cracking in beams has a different impact on member behavior than does end zone cracking. For example, flexural cracks in beams tend to grow in width and depth with the application of superimposed loads. They may cause a negative impact on deflection, vibration, and fatigue behavior of the member. On the contrary, the width of end zone cracks tends to decrease with the application of superimposed loads and the development of time-dependent prestress losses. Based on the experience gained in this project, the research team was able to develop improved end zone details for use in new girders. End zone reinforcement of the developed details is determined using 4% of the prestressed force at transfer and 20 ksi allowable steel stress, which are the same criteria stated by the AASHTO LRFD specifications. How- ever, the proposed details require that at least 50% of the end zone reinforcement be placed in the end h/8 of the member, where h is the member depth. The balance of the end zone re- inforcement is recommended to be placed in the distance be- tween h/8 and h/2 from the member end. This distribution concentrates the reinforcement where the highest bursting stresses are expected to exist. The bursting reinforcement must be embedded into the top and bottom flanges such that it can develop at least 20 ksi at the junctions of the flanges with the web. The anchorage is considerably less than that required to develop the full yield strength of the bars. An effective detail already in use by at least one producer in the Northwest is to provide a single #8 bar in the center of the web at the end of the member. The bar is bent into a C in the longitudinal direction of the girder to allow the bar ade- quate anchorage. Another effective reinforcement at the end h/8 of the member is a pair of 3⁄4-in.-diameter coil loop rods with attached nuts to the top and bottom. Please note that the end h/8 is likely to be embedded in an end diaphragm and should theoretically be exempt from minimum concrete side cover between the web face and the bar. Also, regular #4 and #5 bars that are bent into the top and bottom flanges, and not necessarily projecting above the top flange, may be calculated to be adequately anchored. Based on the proposed end zone details, the research team proposed changes to Article 5.10.10.1 of the AASHTO LRFD specifications. 4.2 Implementation of Research Findings in Highway Communities The research team will prepare at least one paper about the work conducted in this project and the recommendations will be presented at the 2011 TRB Annual Meeting. Another C H A P T E R 4 Conclusions, Recommendations, and Suggested Future Research

63 paper will be prepared for publication in the PCI Journal or the ACI Structural Journal. Also, the research team will approach NDOR and VDOT to present the findings and recommendations of this project. This may be done through direct meetings with the bridge division engineers or through arranged seminars. The research team will submit the proposed change to Article 5.10.10.1 of the AASHTO LRFD specifications to the T-10 AASHTO Committee for possible adoption. 4.3 Suggestions for Future Research No further research into the acceptance, repair, and rejec- tion of web end cracking due to prestress transfer is sug- gested. Further research to develop finite element modeling of the end zone of pretensioned members should be of value in optimizing the bursting reinforcement, especially as larger than 0.6-in.-diameter strands and higher than 15 ksi concrete become more common in practice.

Next: References »
Evaluation and Repair Procedures for Precast/Prestressed Concrete Girders with Longitudinal Cracking in the Web Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB’s National Cooperative Highway Research Program (NCHRP) Report 654: Evaluation and Repair Procedures for Precast/Prestressed Concrete Girders with Longitudinal Cracking in the Web explores the acceptance, repair, or rejection of precast/prestressed concrete girders with longitudinal web cracking. The report also examines suggested revisions to the American Association of State Highway and Transportation Officials’ Load Resistance Factor Design Bridge Design Specifications and measures to develop improved crack control reinforcement details for use in new girders.

Appendices A through G for NCHRP Report 654 are available online.

Appendix A—Literature Review

Appendix B—National Survey

Appendix C—Structural Investigation and Full-Scale Girder Testing

Appendix D—Sealant Specifications

Appendix E—ASTM Specifications

Appendix F—Field Inspection of Bridges

Appendix G—Design Examples of End Zone Reinforcement

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!