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62 4.1 Conclusions Based on the work conducted in this project, the research team has developed a userâs 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 3â4-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.