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Bridges and other coastal structures in Georgia and throughout the Southeast are deteriorating prematurely due to corrosion. Numerous corrosion initiated failures have occurred in precast prestressed concrete (PSC) piles and reinforced concrete (RC) pile caps, leading to the costly repair and replacement of either the entire bridge or the affected members. With the Federal Highway Administration's goal of a 100-year bridge service life and recent legislative action such as the Bridge Life Extension Act, new emphasis has been placed on the development and implementation of new corrosion mitigation techniques. This thesis involves the mechanical testing, and proposed future test program of high-strength stainless steel (HSSS) prestressing strand to be used in prestressed marine bridge piles. The metallurgy for two types of HSSS strand was selected from a previous study of the corrosion resistance, mechanical properties, and feasibility of 6 candidate HSSS drawn wire samples. Duplex stainless steel (DSS) grades 2205 and 2304 were selected for production of 7-wire 1/2" diameter prestressing strand. DSS wire rod was drawn, stranded, and heat-treated using the same production methods and equipment as used for standard of practice, high carbon prestressing strand. The production process was documented to analyze the problems facing this production method and suggest improvement and optimization. After production, the strands were subjected to a series of mechanical tests. Tension testing was performed to provide a stress-strain curve for the strands and related mechanical properties. Wire samples were also taken at varying points in the drawing process to give more information about the work hardening of the stainless steels. Stress relaxation testing was performed on both strand and wire samples to assess the overall losses and to provide comparisons between strand and wire test results as well as drawn wires before and after heat-treatment. An experimental program for future study was designed to assess the HSSS prestressing strand behavior in precast piles. This testing involves assessment of pile driving performance, pile flexural and shear behavior, strand transfer and development length, long-term prestressing force losses, and material durability.
The use of stainless steel alloys in reinforced concrete structures has shown great success in mitigating corrosion in even the most severe of exposures. However, the use of high-strength stainless steels (HSSSs) for corrosion mitigation in prestressed concrete (PSC) structures has received limited attention. To address these deficiencies in knowledge, an experimental study was conducted to investigate the feasibility of using HSSSs for corrosion mitigation in PSC. The study examined mechanical behavior, corrosion resistance, and techniques for the production of HSSS prestressing strands. Stainless steel grades 304, 316, 2101, 2205, 2304, and 17-7 along with a 1080 prestressing steel control were included in the study. Tensile strengths of 1250 to 1550 MPa (181 to 225 ksi) were achieved in the cold-drawn HSSSs. 1000 hr stress relaxation of all candidate HSSSs was predicted to be between 6 and 8 % based on the results of 200 hr tests conducted at 70 % of the ultimate tensile strength. Residual stresses due to the cold drawing had a significant influence on stress vs. strain behavior and stress relaxation. Electrochemical corrosion testing found that in solutions simulating alkaline concrete, all HSSSs showed exceptional corrosion resistance at chloride (Cl- ) concentrations from zero to 0.25 M. However, when exposed to solutions simulating carbonated concrete, corrosion resistance was reduced and the only HSSSs with acceptable corrosion resistance were duplex grades 2205 and 2304, with 2205 resistant to corrosion initiation at Cl- concentrations up to 1.0 M (twice that in seawater). Based on these results, duplex grades 2205 and 2304 were identified as optimal HSSSs and were included in additional studies which found that: (1) 2304 is susceptible to corrosion when tested in a stranded geometry, (2) 2205 and 2304 are not susceptible to stress corrosion cracking, and (3) 2205 and 2304 are susceptible to hydrogen embrittlement. Efforts focused on the production of 2205 and 2304 prestressing strands showed that they could be produced as strands using existing ASTM A416 prestressing strand production facilities. Due to the ferromagnetic properties of 2205 and 2304, a low-relaxation heat treatment was found to be a viable option to reduce stress relaxation and improve mechanical properties. The overall conclusion of the study was that HSSSs, especially duplex grades 2205 and 2304, show excellent promise to mitigate corrosion if utilized as prestressing reinforcement in PSC structures exposed to severe marine environments.
This report provides practical tests to identify and measure residues (e.g., rust, lubricants used in manufacturing processes, or corrosion inhibitors) on the surface of steel prestressing strands and to establish thresholds for residue types found to affect the strength of the strand's bond to concrete. Key products presented here are four test methods suitable for use in a quality assurance program for the manufacture of steel prestressing strand.
Precast prestressed concrete members are widely used in civil engineering structures around the world with growing demand in the last few years due to the rapid urbanization and industrialization driving an increase in infrastructure projects. Precast pre-tensioned concrete members are especially beneficial for accelerated bridge construction (ABC) methods because of the reduction of cost, less traffic interference, and faster project development that are enabled by precasting the elements off-site. New technologies and emerging materials (e.g. high strength stainless steel strands) have recently been introduced to the construction industry as an alternative to corrosion resistance prestressing. However, the detensioning procedures during the load transfer in the precasting yard have not yet been studied for the implementation of such materials. In the casting beds, once the strands have been tensioned to the desired prestressing force and the concrete has been placed and achieved the necessary compressive strength, then the detensioning of strands takes place. This process transfers the prestressing force from the bulkhead of the precasting bed to the concrete member. This detensioning procedure can be accomplished by saw cutting the strands with a hydraulic pump, or by means of a flame torch. If the strands are abruptly cut using the saw cut method, the sudden release of pretensioning force may cause damage at the end of the member, slippage, and an increase in strand development length. Therefore, it is generally recommended to gradually detension the strands using a torch while following a symmetrical pattern to guarantee that the force is being transferred to the member with no damage. For efficiency reasons and to improve the economy of the process, multiple concrete members are usually cast in series using a long casting bed, in which all strands are placed along the length of the bed and tensioned. Once the concrete has reached the required strength for transfer (in some cases 24h after casting), the strands are cut in between and at the ends of the members. This transfers the load to each concrete member. For large members with multiple layers of prestressing strands, the strands sometimes break between the end of the concrete member and the bulkhead during the detensioning process. The strand rupture occurs due to the accumulation of elastic shortening of the concrete member as each strand transfers its force into the concrete. In consequence, the successively increased shortening of the concrete member amplifies the tensile strain in the remaining uncut strands. For concrete elements in harsh environments, high-strength stainless steel (HSSS) prestressing strands are being implemented with the goal of producing longer lasting structures. However, HSSS strands have some disadvantages compared to the generally used carbon steel strands. HSSS strands elongate more compared to regular carbon steel strands and have a much lower ultimate breaking strain. The risk of yielding leading to breakage during the detensioning process needs to be studied, as precasters are unfamiliar with this new material. The reduction in ultimate yielding strain significantly affects the pretensioning properties of the stainless steel strands, and it may increase the risk of rupture arising from the elastic shortening of the concrete member. This study aims to evaluate the yielding of the remaining uncut strands during the detensioning process based on the properties of both carbon steel strands and high strength stainless steel strands in large precast prestressed concrete members. Characteristics such as the number of strands, free length between the member and bulkhead, detensioning procedures, and the concrete member span length are considered the driving factors for the potential yielding and failure of strands during the load transfer process due to the additional tensile strain in the uncut strands caused by the shortening of the concrete member. A parametric study was created to extensively analyze the elastic shortening of the concrete using different beam sizes and number of strand. Finally, the strains in the uncut strands were compared to the yielding strain of HSSS strands to determine how many strands could be cut before the uncut strands yield. The yielding strain was chosen as the limiting strain, because thereafter strains are in the nonlinear range and cause permanent deformation, and because relatively little strain capacity remains after yielding until breakage occurs -- compared to carbon steel strands. Ultimately, this study provides conclusions and recommendations to improve the practices during the detensioning process to help ensure a smooth implementation of high-strength stainless steel strands in the precast prestressed concrete industry.
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.
This study demonstrated that Type 2205 stainless steel strand can be used as a replacement for conventional ASTM A416 steel strands where an increase in service life is required. The benefit and practicality of using Type 2205 stainless steel strand in prestressed piles were determined from two tasks performed in this study. First, a corrosion assessment was performed on stranded cold worked AISI 1080 carbon steel (hereinafter “SCW1080 steel”) (equivalent to conventional steel strands); cold worked austenitic stainless steel Type 201 modified (hereinafter “CW201 steel”); and stranded heavily cold worked Type 2205 duplex stainless steel (hereinafter “SCW2205 steel”), a currently available duplex stainless steel strand product. Second, the fabrication and placement of several prestressed piles reinforced with SCW2205 steel, which are now part of Virginia bridge structures, were observed. Laboratory corrosion testing under different exposure conditions was conducted on SCW1080 steel (as a baseline); CW201 steel (with some limited testing on non-cold worked Type 201 modified stainless steel to evaluate cold working effects); and SCW2205 steel. The laboratory studies were augmented with four-point bend and U-bend tests of specimens exposed to field conditions for 295 days. This study showed that in pore solution (strands embedded in quality concrete with no chloride), in concrete exposed to artificial seawater (strands embedded in concrete with the chloride concentration slowly increasing in pore solution), and in direct contact with artificial seawater (concrete damaged and strand exposed to artificial seawater), the SCW2205 steel outperformed the other steels tested. Based on the corrosion test results, it is expected that SCW2205 steel strand will provide a considerably more corrosion-resistant reinforcement option in prestressed concrete products as compared to conventional strand. Design, fabrication, and driving of concrete piles reinforced with SCW2205 steel strands and Type 304 stainless steel spirals were documented for three bridges; a fourth structure is currently under construction. Selected mechanical properties and estimated baseline costs were also determined for conventional ASTM A416 steel strands, SCW2205 steel strands, and carbon fiber reinforced polymer strands to facilitate lifecycle cost analysis by others. Based on this study, the Virginia Department of Transportation can implement the use of a corrosion-resistant strand in bridge elements in competition with carbon fiber reinforced polymer where corrosion is a concern, such as concrete elements exposed to brackish water, saltwater, or deicing salts.