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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Author: Zuk, W Sinha, R | Size: 1.56 MB | Format:PDF | Quality:Original preprint | Publisher: Virginia Highway Research Council | Year: 1971 | pages: 46
PRESENTED IS AN ENTIRELY NEW TYPE OF BRIDGE DECK, CONSISTING OF AN UNREINFORCED LIGHTWEIGHT CONCRETE SLAB MADE OF EXPANDING CEMENT SANDWICHED BETWEEN TWO THIN PLATES OF STEEL. THE EXPANDING CORE SERVES TO PRESTRESS THE PANEL. LABORATORY TESTS WERE CONDUCTED ON THE CONCRETE TO DETERMINE PREDICTABLE RELATIONSHIPS FOR EXPANSION. LABORATORY TESTS WERE ALSO CONDUCTED ON TEN SMALL SCALE PANELS, SOME LOADED WITH A CONCENTRATED LOAD AND OTHERS WITH A UNIFORMLY DISTRIBUTED LOAD. INSTRUMENTATION WAS INSTALLED TO MEASURE STRAINS AND DEFLECTIONS. MATHEMATICAL THEORIES WERE ALSO DEVELOPED TO PREDICT BOTH THE PRESTRESSING AND EXTERNAL LOAD BEHAVIOR OF THE PANELS. A SATISFACTORY CORRELATION WAS FOUND BETWEEN THE TEST RESULTS AND THE THEORY. A COMPARATIVE STUDY OF THE PROPOSED SANDWICH PANELS AND STANDARD REINFORCED CONCRETE SLABS INDICATED THAT THE PROPOSED SANDWICH PANELS ARE SUBSTANTIALLY STRONGER AND STIFFER THAN CONCRETE SLABS USING THE SAME QUANTITY OF CONCRETE AND STEEL. /AUTHOR/
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Posted by: mahyarov - 11-10-2012, 11:54 AM - Forum: Steel
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Effects of Bending and Heat on the Ductility and Fracture Toughness of Flange Plate
Author: Keating, Peter B Christian, Lee C | Size: 1.83 MB | Format:PDF | Quality:Original preprint | Publisher: Texas Transportation Institute | Year: 2012 | pages: 94
Bridge fabricators for the Texas Department of Transportation (TxDOT) have occasionally experienced the formation of cracks in flange plate during bending operations, particularly when heat is applied. Bending the flange plate is necessary for certain details used in the fabrication of steel highway girders such as dapped end details. Heat is sometimes used to assist in the bending operation, particularly to help reduce the forces required to bend the plate. This report documents the findings of a TxDOT-sponsored research project that investigated the possible causes of the cracking and developed recommendations to prevent the occurrence of such cracking. The research project investigated the cracking problem using both experimental and analytical studies. The experimental study involved the use of small tensile specimens loaded to different strain levels under varying temperature conditions. Strain levels up to 15 percent were investigated. Temperature conditions included testing at room temperature, 450ºF, and 1150ºF. The results showed that strain levels above 10 percent generally reduced the ductility and fracture toughness of the plate. Additionally, it was found that the application of heat during the bending process significantly reduced ductility and was the major contributor to the formation of cracks. A finite element study of the heating process was used to extend the results for the experimental study. As a secondary study, the fatigue behavior of non-loaded bolted connection details was investigated along with the influence of plate thickness. These details occur when gusset plates are bolted to flanges of girders. Unlike flange or web splices where the load must transfer out of the main member, the load in these details passes through the main plate, resulting in higher stresses around the bolt holes. Pre-tensioning the bolts normally shields the bolt holes from fatigue damage due to the resulting compressive stress field. This study found that for plate thicknesses greater than 1.0 inch, a reduction in fatigue strength was warranted.
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Porous Asphalt Pavement Performance in Cold Regions
Author: Lebens, Matthew A Troyer, Brett | Size: 16.98 MB | Format:PDF | Quality:Original preprint | Publisher: Minnesota Department of Transportation | Year: 2012 | pages: 204
Local agencies are interested in porous asphalt pavement’s potential for improved water quality and quantity control by allowing direct infiltration through the pavement structure. However, prior research in the seasonally diverse Minnesota climate is lacking. The purpose of this research is to study the durability, maintenance requirements, hydrologic benefits, and environmental considerations of a full-depth porous asphalt pavement, installed on a low-volume roadway in a cold climate. This report includes the design, construction, and performance of two porous asphalt test cells and one dense graded asphalt control cell constructed at MnROAD in 2008. These cells were constructed to evaluate the performance of porous asphalt pavements on a low-volume road in a cold-weather climate. The cells discussed in this report are as follows: full-depth porous asphalt over granular subgrade, full-depth porous asphalt over cohesive subgrade, dense graded hot mix asphalt (HMA) on mixed materials subgrade.
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Author: Cenek, P D Opus International Consultants Sutherland, A J Opus International Consultants McIver, I R GREENBeing Consulting Engineers | Size: 3.15 MB | Format:PDF | Quality:Original preprint | Publisher: New Zealand Transport Agency | Year: 2012 | pages: 79 | ISBN: 978-0-478-39444-3
There is an increasing requirement to control and manage ground vibrations generated by road construction and maintenance activities through project specific construction management plans. The objective is to minimise any potential adverse effects. The ability to reliably estimate vibration levels of specific construction activities at the project planning stage and to assess their likely effect on structures and their occupants is therefore required. Typical vibration characteristics for various activities, including site preparation, dynamic compaction and piling were measured for representative equipment and soil types to obtain baseline values for use in preconstruction assessments and to enable validation of available prediction methods. A review of international standards was also undertaken leading to two proposed criteria against which predicted vibrations can be assessed for damage and human perception. The possible application of data acquired from commonly used geotechnical methods, notably scala penetrometer for estimating soil attenuation and falling weight deflectometer to generate site-specific predictor curves for impact-related construction activity, was additionally investigated. This led to the recommendation of three methods, which make use of readily available data, for estimating vibration levels from construction activity at any specified distance from the vibration source.
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Developing a Mixture Design Specification for Flexible Base Construction
Author: Epps, Jon Sebesta, Stephen Sahin, Hakan Button, Joe Luo, Rong Lytton, Robert | Size: 2.18 MB | Format:PDF | Quality:Original preprint | Publisher: Texas Transportation Institute | Year: 2012 | pages: 172
In the Texas Department of Transportation (TxDOT), flexible base producers typically generate large stockpiles of material exclusively for TxDOT projects. This large “state-only” inventory often maintained by producers, along with time requirements for testing and reduced manpower within TxDOT offices, resulted in this project’s investigation into a mixture-design specification for flexible base. The goals of this project are to develop a flexible base specification that reduces TxDOT’s burden for sampling and testing, reduces time and space burdens on producers for maintaining TxDOT-only stockpiles, and maintains or improves the quality of the base. To facilitate accomplishing these goals, this project developed a draft quality control/quality assurance (QC/QA) specification for flexible base. Based on a thorough review of background information, sampling and testing placeholders for placement and production aggregate and mixture tests were selected and incorporated into a classical QC/QA format draft specification. While the current draft specification largely uses existing TxDOT test procedures, improvements may be realized by considering the Grace methylene blue method and the use of index tests as surrogates for modulus and permanent deformation properties. The methylene blue method may be able to better control the quantity and amount of fines, while linking index tests to mechanistic properties could enable acceptance testing better linked to design assumptions. The second year of this project will sample and test flexible base quarries around the state to determine material variability that is attainable in the real world without compromising the design strength.
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Posted by: mahyarov - 11-10-2012, 08:07 AM - Forum: Concrete
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Performance of Lap Splices in Large-Scale Column Specimens Affected by ASR and/or DEF
Author: Bracci, Joseph M Gardoni, Paolo Eck, M Kathleen Trejo, David | Size: 26.59 MB | Format:PDF | Quality:Original preprint | Publisher: Texas Transportation Institute | Year: 2012 | pages: 380
This research program conducted a large experimental program, which consisted of the design, construction, curing, deterioration, and structural load testing of 16 large-scale column specimens with a critical lap splice region, and then compared and calibrated models developed in the analytical program with the experimental behavior. Specimens were carefully instrumented both internally and externally to monitor the strain behavior of the concrete and reinforcing steel from specimen construction, curing, deterioration, and final structural load testing.
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Posted by: mahyarov - 11-10-2012, 08:05 AM - Forum: Concrete
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Preparation and Testing of Drilled Shafts with Self-Consolidating Concrete
Author: Ozyildirim, H Celik Sharp, Stephen R | Size: 1.20 MB | Format:PDF | Quality:Original preprint | Publisher: Virginia Center for Transportation Innovation and Research | Year: 2012 | pages: 36
In this study, self-consolidating concrete (SCC) was evaluated in drilled shafts and the integrity of drilled shafts was determined using cross-hole sonic logging (CSL), a low-strain nondestructive integrity testing technique. SCC has very high flowability. It was placed in the drilled shafts of the bridge on Route 28 over Broad Run in Bristow in Prince William County, Virginia. There were two bridges at the site; the one carrying the northbound traffic had drilled shafts using conventional concrete with high consistency (i.e., flowability). Half of the shafts of the bridge carrying the southbound traffic were cast with SCC. During placement, properties of the fresh concrete were tested and specimens were prepared to determine the hardened properties. The integrity of the shafts within the reinforcing cage was determined using CSL, with sonic echo/impulse response also used to evaluate several test shafts. The use of acousto-ultrasonic (AU) measurements to determine the cover depth outside the reinforcing cage was also evaluated during laboratory testing. In addition to the Route 28 shafts, three test shafts with conventional and SCC concretes were cast in an area headquarters. These shafts had intentional voids created through the use of sand bags and Styrofoam to investigate further the ability of the nondestructive test equipment. The results indicated that SCC is highly desirable for drilled shafts; it flows easily, filling the hole, and the removal of the temporary casing is facilitated by this highly workable material. CSL is a satisfactory nondestructive method to determine the integrity of shafts. Sonic echo/impulse response also showed promise as a method that complements CSL for determining the integrity of a shaft.
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