Effects of Increasing the Allowable Compressive Stress at Release on the Shear Strength of Prestressed Concrete Girders
Author: Heckmann, Christopher University of Texas, Austin Bayrak, Oguzhan University of Texas, Austin | Size: 9.46 MB | Format:PDF | Quality:Original preprint | Publisher: University of Texas, Austin | Year: 2008 | pages: 173
In recent years, several research projects have been conducted to study the feasibility of increasing the allowable compressive stress in concrete at prestress transfer, currently defined as 0.60f'ci in the AASHTO LRFD Bridge Design Specifications. Increasing the limit would result in many economical and design benefits for the precast concrete industry, such as increased span lengths and faster turnover of beams in stressing beds. This research study focuses on the effects of increasing the allowable compressive stress at release on the shear strength of prestressed concrete members, a topic which has not yet been explored by past research projects. The current experimental work is funded under Texas Department of Transportation (TxDOT) Project 5197, which initiated in 2004 at the University of Texas at Austin. In the shear performance evaluation, 18 shear tests were performed. In the shear tests, the beams were loaded to fail in web-shear, with a shear span to depth ratio of 2.22. The diagonal cracking shears and shear capacities were experimentally measured for all specimens tested. All test specimens were TxDOT Type-C highway bridge girders (40-inch deep pretensioned I-beams) and were fabricated by three different precast plants in Texas. The compressive stress at release for the test specimens ranged from 0.56f'ci to 0.76f'ci. The measured cracking shears and shear capacities were compared to the estimated cracking shears and shear capacities, as calculated using ACI 318-08 and AASHTO LRFD (2007), and the effects of higher release stresses on shear strength and serviceability were evaluated by examining the conservativeness and accuracy of the predictions. Based on the experimental results summarized in this report, an increase in the allowable maximum compressive stress in concrete in the end regions of prestressed concrete beams at prestress transfer to 0.65f'ci or 0.70f'ci can be justified.
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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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i need a paper which has examined behavior of sample 2d reinforced concrete moment frame under reverse load and .
i want to use this experimental model for modeling that in seismostruct software and check the experimental and analytical result of that under reverse load .
could you have this kind of paper?please if you have upload here ,
This report documents the findings of a Texas Department of Transportation sponsored research project to study self-consolidating concrete (SCC) for precast concrete structural applications. Self-consolidating concrete is a new, innovative construction material that can be placed into forms without the need for mechanical vibration. The mixture proportions are critical for producing quality SCC and require an optimized combination of coarse and fine aggregates, cement, water, and chemical and mineral admixtures. The required mixture constituents and proportions may affect the mechanical properties, bond characteristics, and long-term behavior, and SCC may not provide the same in-service performance as conventional concrete (CC). Different SCC mixture constituents and proportions were evaluated for mechanical properties, shear characteristics, bond characteristics, creep, and durability. Variables evaluated included mixture type (CC or SCC), coarse aggregate type (river gravel or limestone), and coarse aggregate volume. To correlate these results with full-scale samples and investigate structural behavior related to strand bond properties, four girder-deck systems, 40 ft (12 m) long, with CC and SCC pretensioned girders were fabricated and tested. Results from the research indicate that the American Association of State Highway Transportation Officials Load and Resistance Factor Design (AASHTO LRFD) Specifications can be used to estimate the mechanical properties of SCC for a concrete compressive strength range of 5 to 10 ksi (34 to 70 MPa). In addition, the research team developed prediction equations for concrete compressive strength ranges from 5 to 16 ksi (34 to 110 MPa). With respect to shear characteristics, a more appropriate expression is proposed to estimate the concrete shear strength for CC and SCC girders with a compressive strength greater than 10 ksi (70 MPa). The researchers found that girder-deck systems with Type A SCC girders exhibit similar flexural performance as deck systems with CC girders. The AASHTO LRFD (2006) equations for computing the cracking moment, nominal moment, transfer length, development length, and prestress losses may be used for SCC girder-deck systems similar to those tested in this study. For environments exhibiting freeze-thaw cycles, a minimum 16-hour release strength of 7 ksi (48 MPa) is recommended for SCC mixtures.
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Allowable Compressive Stress at Prestress Transfer
Author: Schnittker, Brian University of Texas, Austin Bayrak, Oguzhan University of Texas, Austin | Size: 7.94 MB | Format:PDF | Quality:Original preprint | Publisher: University of Texas, Austin | Year: 2008 | pages: 206
In 2004, the Texas Department of Transportation (TxDOT) initiated Project 5197 to investigate the feasibility of increasing the allowable compressive stress limit at prestress transfer. Initially, the live load performance of 36 specimens was evaluated by Birrcher and Bayrak (TxDOT Report 5197-1, 2007). Report 5197-4 presents the subsequent research conducted based on recommendations of Birrcher and Bayrak (2007). In this portion of TxDOT Project 5197, 45 Type-C beams and 10 4B28 box beams were tested to experimentally determine their cracking load. The Type-C beams were produced in four different fabrication plants using conventionally consolidated concrete. The 10 4B28 box beams were produced in two fabrication plants using concrete mixture designs of both self consolidating concrete as well as conventional concrete. For all specimens, measured cracking loads were compared to predicted cracking loads. The data from the 45 Type-C beams and 10 box beams were added to the 36 beams investigated by Birrcher and Bayrak (2007) to compile a comprehensive set of data from 91 specimens. An appropriate maximum compressive stress limit was determined from the ability to accurately predict the load at which cracking occurred. As the maximum compressive stress at prestress transfer was increased, a decline in cracking load prediction accuracy was observed. For the specimens subjected to high compressive stresses at release (greater than 0.65f’ci), the concrete in the pre-compressed tensile zone was subjected to the non-linear inelastic range causing microcracking to occur. This non-linear behavior (due to microcracking) was unaccounted for in prestress losses or standard design equations (P/A±Mc/I). Based on the analysis of the results, an increase of the allowable compressive stress limit at prestress transfer to 0.65f’ci is justified. Additionally, the use of self consolidating concrete with a maximum compressive stress of 0.65f’ci is not recommended.
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High-Strength Self-Consolidating Concrete Girders Subjected To Elevated Compressive Fiber Stresses
Author: Myers, John J | Size: 773 KB | Format:PDF | Quality:Original preprint | Publisher: Missouri University of Science and Technology, Rolla | Year: 2008 | pages: 70
There are limited measurements documented in the literature related to long-term prestress losses in self consolidated concrete (SCC) members. Recorded test data have shown variations in mechanical property behavior of SCC compared to conventional high strength concrete (HSC) mixtures in the 8-12 ksi range. Over the past year, precast manufacturers such as Coreslab Structures, Inc., in Marshall, MO have experienced inconsistencies in camber behavior with SCC which may be attributed to mechanical property variations, but variation in stress may also be a contributing factor. Additionally, increasing the allowable fiber stress limit is desired for full utilization of materials and members, as long as structural performance is maintained. Furthermore, accurate prediction of time-dependant prestress losses is essential for determination of the effective prestress force, which affects serviceability prediction and structural performance. Further investigation is required.
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Aggregate Distribution Investigation in Box Beams Fabricated with Self Consolidating Concrete
Author: Avendano, Alejandro University of Texas, Austin Bayrak, Oguzhan University of Texas, Austin | Size: 24.58 MB | Format:PDF | Quality:Original preprint | Publisher: University of Texas, Austin | Year: 2009 | pages: 67
In 2004, the Texas Department of Transportation (TxDOT) initiated Project 0-5197 to investigate the feasibility of increasing the allowable compressive stress limit at prestress transfer. Initially, the live load performance of 36 specimens was evaluated by Birrcher and Bayrak (TxDOT Report 5197-1, 2007). Report 5197-4 presents the subsequent research conducted based on recommendations of Birrcher and Bayrak (2007). In this portion of TxDOT Project 0-5197, 45 Type-C beams and 10 4B28 box beams were tested to experimentally determine their cracking load. The Type-C beams were produced in four different fabrication plants using conventionally consolidated concrete. The 10 4B28 box beams were produced in two fabrication plants using concrete mixture designs of both self consolidating concrete (SCC) as well as conventional concrete (Schnittker and Bayrak, CTR, 2008). After testing the 10 box beams procured in TxDOT Project 0-5197, Schnittker and Bayrak (2008) reported increased amounts of top flange cracking at release, substantially lower modulus of elasticity (along with increased deflections under live loading), slightly higher cambers near 28-days, and lower than expected flexural cracking loads under live loads. The present investigation is carried out in an effort to explain the poor performance of the beams fabricated with SCC as reported in research report 0-5197-4.
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Determination of Service Stresses in Pretensioned Beams
Author: Hamilton, H R O'Neill, Christina | Size: 7.73 MB | Format:PDF | Quality:Original preprint | Publisher: University of Florida, Gainesville | Year: 2009 | pages: 112
This report presents research on the evaluation of service flexural stresses and cracking moment in prestressed concrete members and on the minimum reinforcement requirements that are currently controlled by the flexural cracking moment. In prestressed concrete girders, the cracking moment changes when prestressing steel quantities are adjusted. If bonded prestressing steel is considered to contribute to the minimum reinforcement, then a single minimum reinforcement quantity is not possible. Furthermore, as bonded prestressing steel quantities are increased to satisfy the minimum reinforcement, the minimum steel requirement increases proportionately. A parametric study was conducted on hollow core, Florida bulb tee, and segmental box girders to evaluate the current minimum steel provisions. New minimum reinforcement provisions were then derived based on recommendations by Leonhardt 1964. These reinforcement provisions were then compared to the existing American Concrete Institute (ACI) 318 and American Association of State Highway and Transportation Officials (AASHTO) provisions using the sections from the parametric study. Ten precast, pretensioned pile cut offs from an FDOT construction project were salvaged and tested to determine cracking moment and to evaluate cracking behavior. Half of the piles were loaded monotonically to cracking and half were loaded cyclically to cracking. Cyclic loading was used in conjunction with AE monitoring and strain gage data to determine the initiation of microcracking. Structural cracking was determined using visual identification combined with interpolation from the load deflection plot. Six precast, pretensioned I-girders were also constructed and tested cyclically to determine cracking moment and evaluate cracking behavior. As both the piles and girders were loaded, microcracking and structural cracking were found to occur at lower stresses than would be calculated from the measured modulus of rupture and precompression. The stress range between the initiation of microcracking and the formation of a structural crack was found to increase with the prestress level. The current AASHTO provisions limiting tensile stress in harsh environments appear to be adequate in light of the girder test results.
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Performance of Self-Consolidating Concrete in Prestressed Girders
Author: Bhoem, Kurtis M Auburn University Barnes, Robert W Auburn University Schindler, Anton K Auburn University | Size: 4.04 MB | Format:PDF | Quality:Original preprint | Publisher: Auburn University | Year: 2010 | pages: 213
A structural investigation of self-consolidating concrete (SCC) in AASHTO Type I precast, prestressed girders was performed. Six test girders were subjected to transfer length and flexural testing. Three separate concrete mixtures, two girders per mixture, were used to construct these specimens. A moderate-strength, conventional-slump concrete mixture, similar to the concrete used in typical ALDOT girders was evaluated versus moderate-strength SCC and high-strength SCC. No significant difference in transfer bond behavior was found between the full-scale SCC girders and the conventional concrete girders. High-strength SCC girders had shorter transfer lengths than moderate-strength (SCC and conventional) girders. After normalization to account for the difference in prestress magnitude and concrete strength, there was no discernible difference in the magnitude of the transfer lengths between the concrete types. After a composite, cast-in-place concrete deck was added to each girder, flexural testing was performed near each girder end, resulting in two flexural tests per girder. Embedment lengths were varied for each test in order to bracket the AASHTO strand development length. Results indicated that the use of SCC had no adverse effects on the overall flexural performance, and the flexural bond lengths were conservatively predicted by the relevant ACI and AASHTO expressions. Similarly, the SCC girders exhibited comparable service-level performance to the conventional girders. Based on the work performed in this study SCC should perform well in prestressed concrete girder applications.
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Author: Iowa Department of Transportation | Size: 15.69 MB | Format:PDF | Quality:Original preprint | Publisher: Iowa Department of Transportation | Year: 2011 | pages: 470
This study manual is designed for self-study by individuals planning to seek certification as a Certified Prestress Quality Control Inspector under the guidelines of the Iowa Department of Transportation (Iowa DOT). Items that will be covered in this manual are intended to give the inspector some basic instruction and interpretation of the Iowa DOT methods and acceptable standard practices of prestress concrete unit fabrication. Successful completion of this 3 day instructional course, achieving a 80% or greater score on the final exam, and 40 hours of direct supervision by a Certified Inspector are the requirements needed to become certified. Recertification will be granted for a 5-year period.
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