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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Steel Fiber Replacement of Mild Steel in Prestressed Concrete Beams
Author: Tadepalli, Padmanabha Rao University of Houston Hoffman, Norman University of Houston Hsu, Thomas T C University of Houston Mo, Y L University of Houston | Size: 8.96 MB | Format:PDF | Quality:Original preprint | Publisher: University of Houston | Year: 2011 | pages: 192
In traditional prestressed concrete beams, longitudinal prestressed tendons serve to resist bending moment and transverse mild steel bars (or stirrups) are used to carry shear forces. However, traditional prestressed concrete I-beams exhibit early-age cracking and brittle shear failure at the end zones despite the use of a high percentage of stirrups (4.2%). Moreover, producing and placing stirrups require costly labor and time. To overcome these difficulties, it is proposed to replace the stirrups in prestressed concrete beams with steel fibers. This replacement concept was shown to be feasible in a Texas Department of Transportation (TxDOT) project (TxDOT project 0-4819) recently completed at the University of Houston. The replacement of stirrups by steel fibers in highway beams requires a set of shear design provisions and guidelines for Prestressed Steel Fiber Concrete (PSFC) beams. The development of rational shear provisions with wide applications must be guided by a mechanics-based shear theory and must be validated by experimental tests on I- and box-beams. A rational shear theory, called the Softened Membrane Model (SMM), has been developed at the University of Houston for reinforced concrete beams. This theory satisfies Navier’s three principles of mechanics of materials, namely, stress equilibrium, strain compatibility and the constitutive relationship between stress and strain for the materials. The first phase of the research consisted of testing 10 full-size PSFC panels. This was done to establish the effect of fiber factor and the level of prestress on the constitutive models of steel fiber concrete and prestressing tendons. From the data a set of constitutive models was developed to predict the behavior of PSFC. Notable findings include the fact that increasing steel fiber content has a beneficial effect on the softening properties of PSFC. Additionally, the findings show that increasing steel fiber content increases tension stiffening in prestressed PSFC under tensile loading. The second phase of this research project generalizes the SMM shear theory for application to PSFC beams. This was achieved by feeding the new constitutive models of fiber concrete and prestressing tendons into a finite element program (OpenSees). The accuracy of the new shear theory was evaluated by testing full-size PSFC I- and box-beams that fail in shear modes. The developed finite element program was used to simulate the shear behavior of the beams with acceptable accuracy. Finally, a design equation and recommendations were provided for use when designing PSFC beams. Using the design equations, a series of four design examples was also provided.
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There are 16 small to medium simple span bridges in Larimer County that are currently load rated solely based on visual inspections. Most of these bridges are prestressed concrete bridges. The objective of this project is to load rate these bridges using structural analysis with very little to no information available related to their design. Larimer County provided everything available, which essentially was very limited plans and inspection reports for the bridges. The plans lacked details concerning prestress, cross-section dimensions, and material properties. The bridge (prestress concrete) manufacturer does not have records of the bridges built in the 1960’s or earlier. Due to these limitations, a basic structural analysis was performed using a program developed for the Colorado Department of Transportation (CDOT) in 2007 with rating-conservative assumptions in order to determine the capacities of the bridges. The influence of these assumptions on the conclusions is also discussed.
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Predicting Camber, Deflection, and Prestress Losses in Prestressed Concrete Members
Author: Rizkalla, Sami Zia, Paul Storm, Tyler | Size: 2.97 MB | Format:PDF | Quality:Original preprint | Publisher: North Carolina State University | Year: 2011 | pages: 174
Accurate predictions of camber and prestress losses for prestressed concrete bridge girders are essential to minimizing the frequency and cost of construction problems. The time-dependent nature of prestress losses, variable concrete properties, and problems related to production variables make it difficult to predict camber accurately. The recent problems experienced by NCDOT during construction are mainly related to inaccurate prediction of camber. In this report, several factors related to girder production are shown to have a significant impact on the prediction of camber. A detailed method and an approximate method for predicting camber that both utilize adjustments to account for the production factors are proposed. The detailed method uses time-dependent losses calculations and creep factors to predict camber, while the approximate method uses multipliers. The current NCDOT method and the proposed methods are analyzed and compared using an extensive database of field measurements. The proposed methods are shown to provide significant improvements to the camber predictions in comparison to the current NCDOT method. Recommendations for design and production practices are provided.
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