Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 62
62 CHAPTER 4 Conclusions, Recommendations, and Suggested Future Research 4.1 Conclusions details is determined using 4% of the prestressed force at transfer and 20 ksi allowable steel stress, which are the same Based on the work conducted in this project, the research criteria stated by the AASHTO LRFD specifications. How- team has developed a user's manual for acceptance criteria and ever, the proposed details require that at least 50% of the end repair materials and methods for prestressed concrete girders zone reinforcement be placed in the end h/8 of the member, experiencing end zone cracking due to transfer of the preten- where h is the member depth. The balance of the end zone re- sioning force. The manual consists of four criteria depending inforcement is recommended to be placed in the distance be- on the crack width. The first criterion is for crack widths less tween h/8 and h/2 from the member end. This distribution than 0.012 in., where no repair is recommended. The second concentrates the reinforcement where the highest bursting criterion is for crack widths from 0.012 in. to 0.025 in., where it stresses are expected to exist. The bursting reinforcement is recommended that the cracks be filled with a cementitious must be embedded into the top and bottom flanges such that packing material and then covered with a water-resistant it can develop at least 20 ksi at the junctions of the flanges surface sealant to keep water contaminated with corrosion- with the web. The anchorage is considerably less than that inducing chemicals from penetrating the concrete and reach- required to develop the full yield strength of the bars. ing the steel reinforcement. The third criterion is for crack An effective detail already in use by at least one producer widths from 0.025 in. to 0.050 in., where epoxy injection is rec- in the Northwest is to provide a single #8 bar in the center of ommended for cracks wider than 0.025 in. and cementitious the web at the end of the member. The bar is bent into a C in packing material for cracks narrower than 0.025 in. The man- the longitudinal direction of the girder to allow the bar ade- ual provides the provisions for successful epoxy injection. The quate anchorage. Another effective reinforcement at the end fourth criterion is for crack widths greater than 0.050 in., where h/8 of the member is a pair of 3/4-in.-diameter coil loop rods rejection of the girder is recommended, unless it can be shown with attached nuts to the top and bottom. Please note that the by detailed analysis that structural capacity and long-term dura- end h/8 is likely to be embedded in an end diaphragm and bility are not compromised. These criteria allow for acceptance should theoretically be exempt from minimum concrete side of girders with cracks wider than those implied for flexural cover between the web face and the bar. Also, regular #4 and members in the ACI-318 Building Code and the AASHTO #5 bars that are bent into the top and bottom flanges, and not LRFD Bridge Design Specifications. The nature and conse- necessarily projecting above the top flange, may be calculated quences of end zone cracking are quite different from those of to be adequately anchored. flexural cracking. Flexural cracking in beams has a different Based on the proposed end zone details, the research team impact on member behavior than does end zone cracking. For proposed changes to Article 188.8.131.52 of the AASHTO LRFD example, flexural cracks in beams tend to grow in width and specifications. depth with the application of superimposed loads. They may cause a negative impact on deflection, vibration, and fatigue 4.2 Implementation of Research behavior of the member. On the contrary, the width of end zone Findings in Highway cracks tends to decrease with the application of superimposed Communities loads and the development of time-dependent prestress losses. Based on the experience gained in this project, the research The research team will prepare at least one paper about the team was able to develop improved end zone details for use work conducted in this project and the recommendations in new girders. End zone reinforcement of the developed will be presented at the 2011 TRB Annual Meeting. Another