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1Precast/prestressed concrete bridge girders are widely used in the United States. Longi- tudinal web cracks, often called end zone cracks, at the ends of pretensioned concrete gird- ers are commonly observed at the time of strand detensioning, an event generally referred to as prestress transfer. During the last two decades, especially with the use of relatively high concrete strength, deep girders, and high levels of prestress, these cracks have become more prevalent. Longitudinal cracks will always develop in prestressed girders if the ver- tical bursting stresses generated by prestress transfer are greater than the tensile capacity of the concrete. Conventional reinforcement is generally placed to keep the cracks within acceptable width. In practice, there is no consistent understanding of the impact of end zone cracking on the strength and durability of the girders. Thus, the decisions made by bridge owners vary from doing nothing to total rejection of the girders. Other reactions include debonding of strands at the girder ends, limiting prestress levels, reducing allowable compression stress at the time of prestress transfer, injecting grout into the cracks, and coating the girdersâ ends with sealants. There is no consensus among owners on the level of tolerance to these longi- tudinal cracks. Concerns regarding end zone cracks are based on the possibility of having reduced struc- tural capacity and future durability issues from strand and bar corrosion. End zone cracks that run parallel with and intersect the prestressing strands, reflecting strand locations, could cause debonding. This would result in an increase in the transfer and development lengths, which may consequently reduce the shear and flexural capacity of the girder. Wide reflective cracks along the strands that are exposed to chloride solutions may cause strand deterioration. Therefore, a thorough understanding was needed to determine whether longitudinal web cracks are of structural significance. If these cracks are not structurally significant, an under- standing of whether they reduce durability was required. Published guidelines regarding acceptance and repair criteria of prestressed concrete gird- ers consider many types of cracking that may be reported but do not adequately address the uniqueness of end zone cracking. Also, most of these guidelines are greatly influenced by the criteria developed for flexural cracking in beams, which is fundamentally different in cause and effects from end zone cracking. For example, flexural cracks in beams tend to grow in width and depth with the application of superimposed loads. On the contrary, end zone cracks tend to become narrower with the application of superimposed loads and the development of long term prestress losses. The primary objective of the work conducted in this research project was to establish a userâs manual for the acceptance, repair, or rejection of precast/prestressed concrete girders S U M M A R Y Evaluation and Repair Procedures for Precast/Prestressed Concrete Girders with Longitudinal Cracking in the Web
with longitudinal web cracking. To achieve this objective, guidelines were required to be established for various cracking categories as follow: ⢠Cracks that are not required to be repaired, ⢠Cracks that are required to be repaired, including the methods and materials of repair, and ⢠Cracks that cause structural capacity to be compromised and thus cause the girders to be rejected. Additional objectives were to propose revisions to the AASHTO LRFD Bridge Design Specifications as warranted, and to develop improved crack control reinforcement details for use in new girders. The work conducted by the research team to achieve these objectives consisted of the following: 1. Structural investigation and full-scale girder testing to study the effect of end zone crack- ing on shear and flexural capacities, and to investigate the performance of different amounts and details of end zone reinforcement. 2. Epoxy injection testing to investigate the ability of epoxy injection to restore the tensile capacity of cracked concrete across the entire girder web width. 3. Durability testing to investigate what repair method and material should be used, if repair is required, and to investigate if the end zone surface should be sealed with a surface sealantâregardless of whether cracks are required to be filled with a patching material. 4. Field inspection of bridges to check if the in-service condition of end zone cracking changes with time. The field inspection also was used to investigate if unrepaired end zone cracking leads to corrosion of the reinforcement and/or delamination of the concrete. Based on the listed tasks and available previous work, the research resulted in establishing the following proposed cracking limits: ⢠Cracks narrower than 0.012 in. may be left unrepaired. ⢠Cracks ranging from 0.012 to 0.025 in. should be repaired by filling the cracks with approved specialty cementitious materials, and the end 4 ft of the girder side faces should be coated with an approved sealant. Recommendations are given about several products currently available for this repair and about repair procedure. ⢠Cracks ranging from 0.025 to 0.050 in. should be filled with either epoxy injection or cemen- titious patching material, depending on crack width, and then the surface should be coated with a sealant. ⢠For girders exhibiting cracks wider than 0.05 in., the research team recommends that the girder be rejected. For such girders, the research team believes that the cause of cracking may be beyond just the expected bursting force effects. If the owner wishes to reconsider these girders, it is recommended that a thorough structural analysis for the cause and effect of the cracking be conducted and appropriate measures taken. Based on full-scale experimental observations and previous research, it was found that end zone cracks can be effectively controlled by concentrating the reinforcement as near the girder ends as allowed by requirements for concrete cover to reinforcement and minimum reinforcement spacing. Further, reinforcement should be gradually reduced within a dis- tance approximately equal to half of the member depth in order to prevent cracks from reopening beyond the zone of concentrated reinforcement. It is thus recommended that the required reinforcement amount in the current AASHTO LRFD Bridge Design Specifications be retained, but the distribution of the reinforcement changed. The reinforcement is still 2
determined for a bursting force of 4% of the prestressing force and a stress limit of 20 ksi. But, at least 50% of that steel should be placed in the end h/8 of the member (where h is total girder depth). The full amount of bursting steel should be placed in the end h/2 of the mem- ber (not h/4) as currently specified. Anchorage of the steel into the top and bottom flanges is most critical for the bars in the h/8 zone as the steel stress rapidly diminishes beyond that zone. Even in the h/8 zone, it is not necessary to develop bars for yield strength as this rein- forcement is only for crack control and would experience its highest possible stress in the early stages of girder production and handling. End zone reinforcement should be anchored into the top and bottom flanges to develop at 30 ksi. Recommended reinforcement details are given in this report. 3
