End-region Behavior and Shear Strength of Pretensioned Concrete Girders Employing 0.7-in. Diameter Strands PDF Download

Author: Hossein Yousefpour
Publisher:
ISBN:
Category : Concrete bridges
Languages : en
Pages : 177

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Author: Jessica Lauren Salazar
Publisher:
ISBN:
Category :
Languages : en
Pages : 352

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Pretensioned concrete girders are currently fabricated using 0.5- or 0.6-in. diameter prestressing strands. In recent years, however, it has become of interest to employ larger-diameter 0.7-in. diameter strands to reduce the number of strands and improve the efficiency of pretensioned concrete members. Such a transition requires a considerable initial investment that needs to be justified based on the benefits obtained. Furthermore, the use of 0.7-in. strands would increase the stresses within the end-region of pretensioned elements, which could lead to undesirable cracking and impact the serviceability of the girders. The work presented in this thesis consists of 1) a comprehensive parametric investigation to evaluate the benefits and limitations of using 0.7-in. strands in pretensioned bridge girders, and 2) a full-scale experimental study to investigate the behavior of pretensioned concrete girders with 0.7-in. strands at the time of prestress transfer. The parametric investigation was accomplished by designing thousands of bridge girders with different span lengths, concrete release strengths, and transverse spacings. The results showed that the most noticeable benefit of 0.7-in. strands over 0.6-in. strands was a reduction of up to 35 percent in the number of strands. However, the difference in the total weight of prestressing steel was insignificant. Increasing the release strength of concrete, at least to 7.5 ksi, was found essential to observe benefits in design aspects other than the number of strands. The experimental investigation involved the fabrication of two Tx46 and two Tx70 specimens at the Ferguson Structural Engineering Laboratory. All specimens employed 0.7-in. strands on a 2- by 2-in. grid and the standard detailing currently used for girders with smaller-diameter strands. The observed crack widths in the specimens upon prestress transfer did not exceed those typically observed in Tx-girders with smaller-diameter strands. Therefore, the use of 0.7-in. strands does not seem to trigger a need to modify the end-region detailing in Tx-girders. However, noticeably greater bursting and spalling forces were observed in the end regions of the specimens compared to the demands predicted by AASHTO LRFD provisions. The measured 24-hour transfer length from the specimens also exceeded estimates by AASHTO LRFD and ACI 318-14 provisions.

Author: Roya Alirezaei Abyaneh
Publisher:
ISBN:
Category :
Languages : en
Pages : 190

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Prestressed concrete girders are commonly fabricated with 0.5-in. (12.7-mm) or 0.6-in. (15.2-mm) diameter prestressing strands. Recent interest in the use of larger (0.7-in. (17.8-mm) diameter) strands has been driven by potential benefits associated with reduction of the required number of strands and fabrication time, or potential increases in the workable range of prestressed concrete girders (i.e., greater capacities and span capabilities). A limited number of experiments on full-scale specimens with 0.7-in. (17.8-mm) diameter strands have shown that the load-carrying capacity and strand transfer length of specimens with 0.7-in. (17.8-mm) diameter strands can be conservatively estimated using existing AASHTO LRFD provisions. However, performance at prestress transfer requires further investigation to ensure that application of the strands with standard 2-in. (50-mm) spacing and conventional concrete release strength does not increase the end-region cracking that is characteristic of prestressed girders. It must be verified that the development of such cracks does not stimulate anchorage-driven or premature shear failures prior to yielding of the shear reinforcement. Previous research lacks in monitoring of reinforcement stresses and evaluation of end-region cracking which has long been a durability concern. A reliable finite element model that captures the behavior of the specimen at prestress transfer with consideration of performance from construction stages, over the course of the service life, and up to the ultimate limit state can provide key insight into the suitability of using of 0.7-in. (17.8-mm) diameter strands. Further, it could serve as an economical tool for the investigation and proposal of efficient end-region reinforcing details to reduce concrete cracking and enhance durability. Finite element analyses of prestressed I-girder end-regions encompassing cracking and long-term creep- and shrinkage-induced damage, especially in girders fabricated with large diameter strands, have been limited. This research program assessed the limitations of 0.7-in. (17.8-mm) diameter strands at prestress transfer up to limit state response and investigated measures for enhancing the serviceability of the girders through finite element analyses using the commercial software, ATENA 3D. The finite element study was complemented with a full-scale experimental program which was used to validate the numerical results. This paper lays out a validated procedure for modeling the construction stages of prestressed girders and load testing. The model was then used as a tool for investigating alternative end-region reinforcement details for improved end-region serviceability. The most promising options are presented for consideration in further experimental studies and future implementation

Author: Alex Tyler Katz
Publisher:
ISBN:
Category :
Languages : en
Pages : 500

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The majority of precast, pretensioned concrete elements are currently fabricated using 0.5- or 0.6-in. diameter prestressing strands. However, in recent years, potential benefits such as reduced fabrication costs and extended span capabilities have led to an interest in using larger-diameter 0.7-in. strands in the pretensioning industry. Such an increase in the diameter of strands might impact the shear strength of pretensioned girders due to the possibility of atypical failure modes that are not considered in current design provisions. An experimental program was conducted to study the effects of using 0.7-in. prestressing strands on the performance of precast, prestressed concrete I-girders under shear-critical loading conditions. Four full-scale pretensioned Texas bulb-tee girders (Tx-girders) employing 0.7-in. strands were fabricated and tested at Ferguson Structural Engineering Laboratory at the University of Texas at Austin. The mild steel reinforcement in the specimens was detailed according to standard drawings by the Texas Department of Transportation for girders employing 0.6-in. strands. The test program investigated the shear failure in girders with different concrete release strengths, overall member depths, shear span-to-depth ratios, and strand patterns. Analysis of the results revealed clear signs of atypical shear failure mechanisms in all specimens. Considerable strand slip was recorded at both ends of the specimens prior to peak load. In three of the specimens, the shear failure resulted in prominent horizontal cracks at the interface between the web and the bottom flange. However, all specimens demonstrated significant diagonal cracking prior to failure. Yielding of the stirrups was also confirmed in all specimens, indicating a shear-tension failure. The capacities of all specimens were conservatively estimated using the general procedure in AASHTO LRFD Bridge Design Specifications and the detailed method in ACI 318-14. The findings of this study reveal no concerns regarding the performance of existing design provisions in predicting the shear strength of Tx-girders that employ 0.7-in. diameter prestressing strands.

Author:
Publisher:
ISBN:
Category : Girders
Languages : en
Pages : 83

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End region detailing has a significant effect on the serviceability, behavior, and capacity of pretensioned concrete girders. In this project, experimental and analytical research programs were conducted to evaluate and quantify the effects of different end region detailing schemes. Using results from these programs, two end region design models were developed. The first model can be used to design confinement reinforcement to prevent lateral-splitting failure at ultimate strength. The second model focuses on serviceability criteria and can be used to calculate bottom flange stresses due to prestressing and thereby assess the likelihood of bottom flange cracking in the end region. The experimental program was conducted using fourteen Florida I-Beam (FIB) specimens. Cracking and strain data were collected during prestress transfer and during the months following transfer. These data were used to evaluate serviceability criteria. Following serviceability evaluations, specimens were load-tested to determine capacity and behavior due to applied loads. Specimens were loaded in three-point bending at a shear-span-to-depth (a/d) ratio of approximately 2.0. Variables considered in the experimental work included confinement reinforcement, steel bearing plates, horizontal reinforcement, vertical reinforcement, strand quantity, strand shielding, and strand layout. The analytical program was conducted using finite element analysis (FEA). FEA models were validated using data from the experimental program. Variables considered in the analytical program included bearing pad geometry, bearing pad stiffness, steel bearing plates, transfer length, and prestress release sequence. A test program was also conducted to evaluate the shear strength of 1950s era pretension girders used in the Florida highway system. These girders are of interest because they have thin 4-inch webs and very little specified shear reinforcement. Six test girders were removed from an existing bridge and were tested to failure in the laboratory. Results from the testing will be useful in determining the shear strength of similar pretensioned girders. Recommendations are provided with regard to detailing of confinement reinforcement, embedded bearing plates, strand shielding, and crack control. Recommendations are also given regarding evaluation of early pretensioned girders.

Author: Alistair Thornton Longshaw
Publisher:
ISBN:
Category :
Languages : en
Pages : 270

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Although 0.5- and 0.6-in. diameter strands are commonly used in the prestressing industry, there is a growing interest in the implementation of 0.7-in. diameter strands. However, the greater prestressing force induced poses several potential implications, particularly when the strands are placed on a 2- by 2-in. grid. One such issue is end-region cracking, an occurrence that is common in pretensioned girders, regardless of strand size. These cracks tend to grow in width, length, and number over time due to time-dependent effects such as shrinkage or creep. Additionally, the cracks tend to close under an applied load when placed in a service-state condition. End-region crack widths are often used to evaluate the condition of pretensioned girders, so a thorough understanding of the development of these cracks is essential to applying crack width criteria appropriately. A multifaceted experimental program was conducted at the Ferguson Structural Engineering Laboratory at the University of Texas at Austin. A series of seven Texas bulb-tee girders employing 0.7-in. diameter strands was fabricated, monitored, and load tested under shear-critical conditions. The end-region cracks of three specimens were measured immediately after prestress transfer and monitored for at least 28 days, showing that the crack widths grew significantly over time. This growth corresponded closely with the shrinkage strain measured at midspan of each girder, indicating that shrinkage is the primary cause of end-region crack growth. A significant amount of transverse reinforcement is placed in end-regions to restrict cracks immediately after prestress transfer, but this same reinforcement also provides a large amount of restraint against concrete shrinkage, exacerbating crack growth. End-region cracks were also measured during the shear-critical load test for two specimens. Although they closed in a linear manner, they were not completely closed at an expected service load. At ultimate load, the cracks never closed entirely, as the imperfect concrete surfaces bore against each other shortly after initial diagonal shear cracking. Based on both of these findings, future end-region crack widths can be more accurately predicted from any point in the lifespan of a pretensioned girder, allowing for more appropriate applications of permissible crack width limits.

Author: Rafik Y. Itani
Publisher:
ISBN:
Category : Concrete beams
Languages : en
Pages : 140

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Author: Julio A. Ramirez
Publisher: Transportation Research Board
ISBN: 030911747X
Category : Concrete
Languages : en
Pages : 131

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"This report documents research performed to develop recommended revisions to the AASHTO LRFD Bridge Design Specifications to extend the applicability of the transfer, development, and splice length provisions for prestressed and non-prestressed concrete members to concrete strengths greater than 10 ksi. The report details the research performed and includes recommended revisions to the AASHTO LRFD Bridge Design Specifications. The material in this report will be of immediate interest to bridge designers."--Foreword.

Author: Anwer Al-Kaimakchi
Publisher:
ISBN:
Category : Civil engineering
Languages : en
Pages : 0

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Prestressed concrete is used in structures because of its versatility, adaptability, and durability. Durability of prestressed concrete bridges in extremely aggressive environments is of increasing concern because of corrosion of the carbon steel strands that are typically used for prestressing. Concrete is a permeable material where chloride ions can penetrate through and reach the internal reinforcement and carbon steel strands are highly susceptible to corrosion. Thus, prestressed concrete bridges located in areas with high exposure to environmental factors (e.g., marine environments) deteriorate due to corrosion of carbon steel strands. For example, Florida has a long coastline, with many concrete bridges over coastal water. Among the 12,518 bridges in Florida, 6,303 are prestressed concrete, and almost half of them are older than 40 years. One solution to overcome the early deterioration of coastal bridges is to use corrosion-resistant strands, such as Duplex High-Strength Stainless Steel (HSSS) strands.HSSS strands have high corrosion resistance and are an alternative to carbon steel strands in concrete bridges in extremely aggressive environments. The growing interest in using stainless steel strands has led to the development of the ASTM A1114. In 2020, ASTM A1114 was released as a standard specification for low-relaxation, seven-wire, Grade 240, stainless steel strands for prestressed concrete. Stainless steel is made from different alloys compared to carbon steel, and thus the mechanical properties of stainless steel strands are fundamentally different than those of carbon steel strands. The most significant difference is in the guaranteed ultimate strain: the value for stainless steel strands is only 1.4%. Several departments of transportation (DOTs) have already used or allowed the use of HSSS strands in prestressed piles. As of 2020, a total of 17 projects have used stainless steel strands, a majority of them in piles. Those projects are in areas with high exposure to environmental factors. The use of HSSS strands in flexural members has been hindered by the lack of full-scale test results, structural design approaches, and/or design guidelines. The main concern in using HSSS strands in flexural members is their low ductility. Concrete members prestressed with HSSS strands, if not properly designed, might fail suddenly without adequate warning. There have been no attempts to address this problem in full-scale research studies. The goals of this research project were to investigate the use of HSSS strands in flexural members and to develop design guidelines that could be used by bridge engineers. A total of thirteen (13) 42-ft-long AASHTO Type II girders were designed, fabricated, and tested in flexure or shear. Ten (10) girders were prestressed with HSSS strands, while the other three (3) were prestressed with carbon steel strands and served as control girders. This research program included experimental activities to determine the mechanical and bond strength characteristics, prestress losses, and transfer length of 0.6-in-diameter HSSS strands. Twenty HSSS strands from two spools were tested in direct tension. A stress-strain equation is proposed for the 0.6-in.-diameter HSSS strands, which satisfied all ASTM A1114 requirements. The minimum and average bond strengths, following ASTM A1081, of six 0.6-in.-diameter HSSS strands were 15.8 kips and 17.9 kips, respectively. The minimum and average experimental ASTM A1081 bond strengths were 23.4% and 19.8% greater than the recommended values by PCI Strand Bond Task Group. The maximum measured transfer length of 0.6-in.-diameter HSSS strands was 21.5 inches, which was less than the value predicted by AASHTO LRFD Bridge Design Specifications' equation for carbon steel strands. Experimental flexural and shear results showed that the post-cracking behavior of girders prestressed with HSSS strands continued to increase up to failure with no discernible plateau. The behavior is attributed to the stress-strain behavior of the HSSS strands. Also, flexural results revealed that, although HSSS strands have low ductility and all composite girders failed due to rupture of strands, the girders exhibited large reserve deflection and strength beyond the cracking load and provided significant and substantial warning through large deflection, as well as well-distributed and extensive flexural cracking, before failure. A non-linear analytical model and an iterative numerical model were developed to predict the flexural behavior of concrete members prestressed with HSSS strands. Although the analytical model gave better predictions, the iterative numerical approach is slightly conservative and is easier to use for design - designers prefer to use an equation type of approach to perform preliminary designs. Numerical equations were developed to calculate the nominal flexural resistance for flexural members prestressed with HSSS strands. The proposed equations are only valid for rectangular sections. In the case of flanged sections, iterative numerical approaches were also introduced. Because HSSS strand is a brittle material, the design must consider the strain capacity of the strand and must be balanced between flexural strength and ductility. Based on the flexural design philosophy for using carbon steel strands in prestressed concrete girders, along with experimentally-observed behaviors and analytical results for concrete members prestressed with HSSS strands, flexural design guidelines were developed for the use of HSSS strands in flexural members. For I-girders, rupture of strands failure mode is recommended by assuring that concrete in the extreme compression fiber reaches considerable inelastic stresses, at least 0.7f_c^'. For slab beams (e.g. Florida Slab Beam), crushing of concrete failure mode is recommended by assuring that the net tensile strain in the HSSS strand is greater than 0.005. The recommended maximum allowable jacking stress and stress immediately prior to transfer are 75% and 70%, respectively. A resistance factor of 0.75 is recommended for both rupture of strand and crushing of concrete failure modes. AASHTO equations conservatively estimated the measured transfer length and prestress losses of 0.6-inches-diameter HSSS strands. The ACI 318-19 and AASHTO LRFD conservatively predicted the shear capacity of concrete girders prestressed with HSSS strands.

Author: Emre Kizilarslan
Publisher:
ISBN:
Category :
Languages : en
Pages : 202

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Wisconsin bulb tee pretensioned concrete girders have being used for bridges. Their effective spans to depth ratios and higher durability have made prestressed concrete girders desirable. However, cracks were observed at the anchorage zones of these girders because of the demand. To satisfy demand, these girders are heavily prestressed. Cracks initiate during detensioning of pretensioned strands and grow more while transporting them to the resting beds. These cracks create durability concerns as cracks lead aggressive salty water to the steel strands, endangering structures' stability. Especially, cracks in the bottom flange closer to the strands are main concerns in this research. This research primarily focused on the analyses of prestressed girder ends with modelling with nonlinear material properties to understand and recommend control methods for girder end cracking. The end zone behavior of the pretensioned girder was modelled using nonlinear material properties. The concrete nonlinearity, strain softening and stress redistribution upon cracking were also included in the behavior and the verification of tests were done by real tests on these girders. Finally, the reasons for cracks were explained by examining the principal tensile strain directions. The results of previous study showed that debonding strands can effectively control cracking. In this thesis, only debonding for cracking control method, therefore, was tested on 72W with 48 strands and 54W with 42 strands WI girders to see the real effect of debonding on anchorage zone cracks. After getting good results from tests and verifying them with Finite Element Analysis models, exact debonding percentages for other girders to eliminate cracks were presented by giving results of FEA models built for each of them.