Browsing by Subject "Prestress losses"

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    Effect of Temperature on Prestressed Concrete Bridge Girder Strand Stress During Fabrication
    (Center for Transportation Studies, University of Minnesota, 2015-12) Swenson, Tanner W.; French, Catherine E. W.
    The Minnesota Department of Transportation has reported erection cambers of many prestressed concrete bridge girders that were much lower than anticipated. A previous University of Minnesota study (O’Neill and French, MnDOT 2012-16) attributed the discrepancies to inaccurate estimates of the concrete strength and stiffness at release and strand force loss due to temperature during fabrication. The objective of this study was to further investigate the effects of temperature on strand force and camber during precast, prestressed girder fabrication and to make recommendations for the design and fabrication processes to reduce the potential loss of prestress due to temperature effects during fabrication and to improve the release camber estimation. A thermal effects analysis was developed based on four key steps in the girder fabrication process: tensioning, concrete-steel bond, release, and normalization. The study included fabricating six short prestressed concrete segments released at early ages to determine the time/temperature associated with bonding the prestressing strand to the concrete. To investigate the non-recoverable prestress losses during girder fabrication, four sets of girders (MN54 and 82MW) were instrumented with thermocouples, strain gages, and in some cases load cells, that were monitored during the fabrication process to separate the thermal and mechanical strain components. Effects investigated included casting during a cold season, casting during a warm season, casting with the free length of strand covered, and casting with different bed occupancy during any season. A recommended procedure for adjusting strand force during tensioning was proposed to account for non-recoverable strand force changes due to temperature changes between tensioning and bond.
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    Self-Compacting Concrete (SCC) for Prestressed Bridge Girders
    (Minnesota Department of Transportation, 2008-10) Erkmen, Bulent; Shield, Carol K.; French, Catherine E.
    Researchers conducted an experimental program to investigate the viability of producing self-consolidating concrete (SCC) using locally available aggregate, and the viability of its use in the production of precast prestressed concrete bridge girders for the State of Minnesota. Six precast prestressed bridge girders were cast using four SCC and two conventional concrete mixes. Variations in the mixes included cementitious materials (ASTM Type I and III cement and Class C fly ash), natural gravel and crushed stone as coarse aggregate, and several admixtures. The girders were instrumented to monitor transfer length, camber, and prestress losses. In addition, companion cylinders were cast to measure the compressive strength and modulus of elasticity, and to monitor the creep and shrinkage over time. The viability of using several test methods to evaluate SCC fresh properties was also investigated. The test results indicated that the overall performance of the SCC girders was comparable to that of the conventional concrete girders. The measured, predicted, and calculated prestress losses were generally in good agreement. The study indicated that creep and shrinkage material models developed based on companion cylinder creep and shrinkage data can be used to reasonably predict measured prestress losses of both conventional and SCC prestressed bridge girders.
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    Self-Compacting Concrete for Prestressed Bridge Girders
    (2008-10) Erkmen, Bulent
    Self-consolidating concrete (SCC), which is different from conventional concrete especially in its fresh state, is a highly workable concrete that flows through congested reinforcement under its own weight alone, filling the formwork without segregation of its constituent materials with a void-free structure, and can be placed without any vibration. Self-consolidating concrete was first developed in Japan in the early 1980s, and the main issues that promoted the development of SCC were the shortage of skilled labor and the emergence of heavily reinforced structures that made it difficult to sufficiently consolidate the concrete which is crucial for its durability. Although some raw materials and chemical admixtures may increase the initial cost, its use is on the rise worldwide for precast concrete construction mainly due to its ease of placement over conventional concrete. Some benefits of using SCC for precast concrete applications are easily quantified such as faster construction, reduced noise level, and improved surface finish which eliminates the need for patching. Other less tangible benefits include worker safety improvements and extended life of the precasting forms. Although SCC has been developed and successfully used for numerous precast and cast-in-place applications worldwide, and both fresh and hardened properties of SCC have been investigated, concerns have remained regarding mix proportioning, acceptance criteria of SCC in its plastic state, and long term behavior (e.g., creep and shrinkage) of SCC precast/pretensioned elements in service. Limited literature is available to evaluate the hardened and long-term behavior of SCC members, particularly creep, shrinkage, and elastic modulus. Furthermore, there is a wide variation in the findings regarding the long-term behavior of SCC. Due to these reasons, many state departments of transportation, including the Minnesota Department of Transportation (Mn/DOT), have been hesitant to allow SCC for precast bridge girder applications. This study was initiated with the intent to investigate the viability of using SCC developed at local precast plants with locally available materials for the construction of precast prestressed SCC girders in the State of Minnesota. The primary objective of the research was to determine both short-term and long-term properties of SCC bridge girders, evaluate the applicability and accuracy of available test procedures, design equations, and material models for SCC bridge girders. The research was divided into several phases. In the first phase, SCC trial mixes were developed using locally available materials from two local precast concrete plants (Plant-A and Plan-B). The developed trial SCC mixes were studied to identify the main parameters that affect the performance of SCC in its fresh state (e.g., flowability and segregation resistance) such as cement, high-range water reducing admixture dosage, and fresh concrete temperature. It was found that variations in cement from the same supplier with no difference in the cement mill report can significantly affect the flowability of SCC, and recommendations were included for the effect of concrete temperature and admixture dosage on fresh concrete properties. In addition, a testing program was undertaken to evaluate the static and dynamic one-dimensional free flow and flow through reinforcing obstacle segregation resistances of SCC and passing ability of coarse aggregate through reinforcing obstacles. Correlations between different test results were investigated to minimize the required number of test methods to adequately evaluate SCC mixtures. The next phase involved casting four SCC and two conventional concrete precast prestressed bridge girders using locally available materials from Plant-A and Plant-B (three girders per plant). The girders were Mn/DOT 36M I-girders with a span length of 38 ft, and design concrete compressive strengths of 7.5 ksi at release and 9.0 ksi at 28 days. The girders were designed incorporating 36 straight strands in the bottom flange, and four strands in the top flange to avoid the need to drape strands (total of 40 strands). This large amount of prestressed strand was used to create a situation with congested reinforcement to challenge the SCC flow. In addition, the large amount of prestress maximized the allowable compressive stresses at release in the bottom concrete fiber to maximize the concrete creep. The section represented one of the most severe cases for the application of SCC. In addition to the girders, companion cylinders were cast to monitor compressive strength, modulus of elasticity, creep, and shrinkage over time. The girders were instrumented and stored in an outdoor storage site for a period of approximately 2 years to monitor both short-term and long-term performance, which included transfer length, camber, and prestress losses. Both short-term (e.g., elastic shortening) and long-term performance of the girders (e.g., prestress losses) were measured and compared to AASHTO (2004 and 2007), PCI Design Handbook 6th Edition (2004), and PCI General Method (PCI, 1975) predictions. The results indicated that the predicted total long-term prestress losses calculated with AASHTO 2004, PCI Design Handbook 6th Edition (2004), and PCI General Method (PCI, 1975) using measured material properties obtained from conventional cylinders were conservative for both SCC and conventional concrete girders (Note that the SCC conventional cylinders were fabricated with a slightly modified process; rather than rodding the cylinders after each lift, the sides of the mold were tapped with a rubber mallet). The predicted long-term losses at the end of the monitoring periods (i.e., approximately 600 days and 450 days for Plant-A and Plant-B, respectively) were larger than measured losses by 2 to 5% for AASHTO 2004 Lump Sum Method, 12 to 15% for AASHTO 2004 Refined Method, 4 to 7% for PCI General Method, and 8 to 11% for PCI Design Handbook Method for all girders. However, the long-term prestress losses computed with AASHTO-2007 (Approximate Estimate of Time-Dependent Losses) were either not conservative or very close to the measured losses for both the SCC and conventional concrete girders at the end of the monitoring periods. The magnitude of the difference between the measured and predicted losses was comparable for both the conventional and SCC girders. Finally the girders were tested in three-point bending to determine the cracking and crack re-opening loads at the University of Minnesota Structures Laboratory. The experimentally measured crack re-opening loads were used to indirectly calculate the remaining effective prestressing forces and total prestress losses. Also, a semi-destructive test method was used to experimentally measure the remaining tendon forces to verify the field measured losses. The measured girder prestress losses were compared to those determined from a fiber-based finite element analysis incorporating time-dependent creep and shrinkage models based on companion cylinder data. The measured, predicted, and calculated prestress losses were generally in good agreement. The study indicated that creep and shrinkage material models developed based on measured companion cylinder creep and shrinkage data can be used to reasonably predict measured field prestress losses of both conventional and SCC prestressed bridge girders.