Browsing by Author "Dr. Sami Rizkalla, Committee Chair"

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Behavior of FRP Repair/Strengthening Systems for Prestressed Concrete
(2006-08-23) Rosenboom, Owen Arthur; Dr. Mervyn Kowalsky, Committee Member; Dr. Kara Peters, Committee Member; Dr. Paul Zia, Committee Member; Dr. Sami Rizkalla, Committee Chair
This research study examines the behavior of prestressed concrete beams retrofitted with Fiber Reinforced Polymer (FRP) materials. Due to deficiencies in the built environment, engineers may be asked to retrofit or upgrade the capacity of an existing concrete structural member. This could be a result of new demands on the structure, or a repair of damage from an unforeseen event. Retrofits are possible using the traditional building materials of concrete and steel. The cross-section of the structural element can be increased, or steel plates can be bolted or adhesively affixed to the structure to increase capacity. Many of these techniques are costly, and some perform poorly under service conditions. The main benefit for using FRP materials for the strengthening of existing structures is the lightweight nature of the composite material, which makes the use of extensive scaffolding (required in the installation of steel plates) obsolete. The objectives of this research are twofold. First, the overall structural behavior of an FRP strengthened or repaired concrete beam is studied. Two different loading conditions are examined: extreme loading simulated by a monotonic load to failure, and fatigue loading designed to simulate service loads. The structural behavior of the system is evaluated under these different conditions, and an analytical model is presented which predicts the flexural behavior of the system assuming certain failure modes. The second objective of this research is to evaluate the bond behavior of an FRP strengthened reinforced or prestressed concrete flexural member. A database of experimental failures was constructed, and an analytical model is proposed which predicts the bond failure of the FRP strengthening system.
Behavior of GFRP Bridge Decks for Highway Bridges
(2005-12-06) Nelson, James Lee; Dr. Eric Klang, Committee Member; Dr. Emmett Sumner, Committee Member; Dr. Sami Rizkalla, Committee Chair
This research presents the results of an experimental program undertaken at the North Carolina State University (NCSU) Constructed Facilities Laboratory (CFL) to evaluate the performance of a new innovative glass fiber reinforced polymer (GFRP) bridge deck. This bridge deck is produced commercially by Martin Marietta Composites of Raleigh, NC under the trade name of DuraSpan. The experimental program involved examining the behavior of 5.00 inch deep and 7.66 inch deep DuraSpan bridge deck profiles. The program included numerous quasi-static flexural tests, testing of connection details to facilitate the development of a railing system, evaluation of the performance of the bond lines in negative moment regions, and an evaluation of the coefficient of thermal expansion. A finite element model was developed to predict the stiffness of the bridge deck at service load levels. Finite element optimization techniques were used in conjunction with coupon test data and the large scale flexural test data to calibrate the model.
Behavior of Infill Masonry Walls Strengthened with FRP Materials
(2009-05-05) Lunn, Dillon Stewart; Dr. Sami Rizkalla, Committee Chair; Dr. Abhinav Gupta, Committee Member; Dr. Rudolf Seracino, Committee Member
Collapse of unreinforced masonry (URM) structures, including infill walls, is a leading cause of property damage and loss of life during extreme loading events. Many existing structures are in need of retrofit to bring them in compliance with modern design code provisions. Conventional strengthening techniques are often time-consuming, costly, and add significant weight to the structure. These limitations have driven the development of alternatives such as externally bonded (EB) glass fiber reinforced polymer (GFRP) strengthening systems, which are not only lightweight, but can be rapidly applied and do not require prolonged evacuation of the structure. The objective of this research program was to evaluate the effectiveness of strengthening infill masonry walls with externally bonded GFRP sheets to increase their out-of-plane resistance to loading. The experimental program comprises fourteen full-scale specimens, including four un-strengthened (control) specimens and ten strengthened specimens. All specimens consisted of a reinforced concrete (RC) frame (which simulates the supporting RC elements of a building superstructure) that was in-filled with solid concrete brick masonry. The specimens were loaded by out-of-plane uniformly distributed pressure in cycles up to failure. Parameters investigated include the aspect ratio, the strengthening ratio, the number of wythes, and the type of FRP anchorage used. The type of FRP anchorage was found to greatly influence the failure mode. Un-strengthened specimens failed in flexure. However, strengthened specimens without overlap of the FRP onto the RC frame failed due to sliding shear along the bed joints which allowed the walls to push out from the RC frames in a rigid body fashion. In the case where GFRP sheets were overlapped onto the RC frames, the aforementioned sliding shear caused delamination of the GFRP sheets from the RC frames. Use of steel angles anchored along the perimeter of the walls as shear restraints allowed these walls to achieve three times the service load without any visible signs of distress. GFRP strengthening of infill masonry walls was found to be effective, provided that proper anchorage of the FRP laminate was assured.
Behavior of Precast L-shaped Spandrel Beams
(2009-08-11) Hariharan, Vivek; Dr. Sami Rizkalla, Committee Chair; Dr. Emmett Sumner, Committee Member; Dr. Rudolf Seracino, Committee Member
Bond Behavior of High Performance Reinforcing Bars for Concrete Structures
(2007-05-08) Hosny, Amr; Dr. Sami Rizkalla, Committee Chair; Dr. Emmett Sumner, Committee Member; Dr. Vernon Matzen, Committee Member
Bond between the concrete and the reinforcing steel is a major factor affecting the performance of reinforced concrete structures. Advances in material science led to the production of High Performance Steel that has enhanced corrosion resistance and higher strength compared to conventional Grade 60 steel. Such material can lead to more economical design reducing the material requirements for a particular project and expanding its life span. The objective of this research is to study the bond behavior of High Performance reinforcing bars for concrete structures and to evaluate the effect of different parameters believed to affect the bond characteristics. Twenty-two large scale reinforced concrete splice beams were constructed using No.8 and No.11 reinforcing bars, having different cross-sections with varying concrete compressive strengths and development lengths. The beams were tested using four point bending setup to provide a constant moment region over the splice zone. Test results indicate that stresses up 90 ksi can be achieved in the No.8 bars and up to 70 ksi in the No.11 bars without confinement; however, it is recommended to use transverse reinforcement to confine the High Performance bars in order to ensure ductility. These stresses can be evaluated at failure using a simple proposed equation. Test results were used to extend the current ACI Committee 408 equations to better predict the stresses in the High Performance Steel.
Bond Characteristics and Environmental Durability of CFRP Materials for Strengthening Steel Bridges and Structures
(2008-08-19) Dawood, Mina Magdy Riad; Dr. Sami Rizkalla, Committee Chair; Dr. Emmett Sumner, Committee Member; Dr. Murthy Gudatti, Committee Member; Dr. Kara Peters, Committee Member
This dissertation presents the findings of a research program that was conducted in two parts to investigate the bond behavior and environmental durability of carbon fiber reinforced polymer (CFRP) materials for strengthening steel bridges and structures. The first part of the research consisted of an experimental and analytical research program to investigate the bond characteristics of CFRP lap-splice joints. The experimental program included a total of eight double-lap shear tests and ten steel beam tests. The main parameters considered include the geometric configuration of the plate ends, the length of the splice plates and the use of mechanical anchorage near the plate ends. A finite element analysis was conducted to determine the distribution of the stresses within the adhesive layer for different splice configurations. The findings indicate that the presence of the reverse tapered plate end reduced the magnitude of the peak stresses in the adhesive layer thereby increasing the tension strength of the splice joints. Increasing the splice length and installing additional mechanical anchorage did not enhance the strength of the joints. Based on the findings, a method is proposed to design lap-splice joints for implementation of the proposed CFRP system on longer-span flexural members. The second part of the research consisted of a total of 44 steel-CFRP double-lap shear tests to study the environmental durability of the proposed CFRP strengthening system. The specimens were exposed to accelerated corrosion conditions and subsequently loaded monotonically to failure. The additional use of a silane adhesion promoter or a glass fiber insulating layer, to enhance the bond durability, was also studied. The findings indicate that the presence of the glass fiber layer enhanced the initial bond strength of the system, while the use of a silane adhesion promoter was essential to ensure the durability of the system. The findings of this research program demonstrate that, with proper detailing, the proposed CFRP system can be effectively used for strengthening of steel bridges and structures.
Bond Characteristics and Qualifications of Adhesives for Marine Applications and Steel Pipe Repair
(2005-07-19) Smith, Glen Eugene; Dr. Sami Rizkalla, Committee Chair; Emmett A. Sumner, Committee Member; James M. Nau, Committee Member
Performance of adhesives significantly affects the overall behavior of structural elements, especially in the process of developing innovative designs using new materials. Adhesives that bond metals, plastics, FRP and other materials have been used in transportation, industrial and marine applications. Fundamental understanding of the bond behavior and load transfer mechanisms of different adhesives is essential prior to their use in civil infrastructure applications. This thesis presents the results of an extensive research program conducted to determine the engineering properties of different structural adhesives under normal and severe environmental conditions. The bond characteristics between composite-to-composite and composite-to-steel materials are investigated. The performances of two different structural adhesives are evaluated by testing 105 single-lap shear specimens. The research includes the effect of severe environmental conditions using the classical direct lap-shear tests. The factor considered is time-to-failure in which the specimens are submerged in de-ionized water with different pH values and subjected to different temperatures at different stresses. The program includes also examining the cleavage peel and short term creep properties. Adhesive behaviors, as well as the failure modes for each adhesive, are presented. The influence of preparation procedures of the substrate on the shear strength of the adhesives is also studied. Test results show a significant impact of the pH level and temperature on the shear strength and bond characteristics of the two structural adhesives considered in this investigation. Test results were used to determine the most appropriate adhesive for marine application and repair of steel pipeline.
Bond Characteristics of Micro-Composite Multi-Structural Formable Steel Used in Reinforced Concrete Structures
(2003-07-28) Elagroudy, Hossam Aly; Dr. Sami Rizkalla, Committee Chair
The bond performance of a unique type of reinforcing steel rebars, claimed to have high corrosion resistance as well as high tensile strength, with concrete was studied. The objective was to investigate the bond behavior of straight rebars made out of this steel, named MMFX, embedded in concrete flexural members and to examine the applicability of the current expressions for bond force to predict the bond capacity of the MMFX bars embedded in concrete. Two phases of experimental investigation was conducted. In the first phase, four beam end specimens were tested and in the second phase eight splice beams were studied. The bond behavior of the MMFX steel bars was found to be similar to that of carbon steel. The bond strength of the MMFX is significantly reduced as the tensile stresses developed in the bar went beyond the proportional limit. Both the ACI code 318-02 equation for bond force and the current equation proposed by the ACI committee 408 for bond force gave conservative prediction for bond force for low stress levels. However, at high stress levels, the prediction of the two equations went to the unconservative side. The non linear behavior of the MMFX stress-strain curve was the reason behind the unconservative prediction. The above two equations were modified to ensure conservative prediction at high stress levels. A second degree best fitting curve was found to be the best to describe the relationship between the splice length and the bond force capacity for both # 6 and # 8 MMFX bars.
Evaluation of MMFX Steel for Concrete Bridge Decks
(2006-05-19) Lucier, Gregory William; Dr. Paul Zia, Committee Member; Dr. Sami Rizkalla, Committee Chair; Dr. Emmett Sumner, Committee Member
Commercially available Micro-Composite, Multi-Structural, Formable (MMFX) steel reinforcing bars are a proprietary product claiming to provide both high tensile strength and excellent corrosion-resistance when compared to conventional Grade 60 steel. Use of MMFX steel reinforcement could potentially lead to significant cost savings due to these unique characteristics. Recently, many state transportation departments, including the North Carolina Department of Transportation (NCDOT), have begun to use MMFX steel as a direct replacement for conventional steel in reinforced concrete bridge decks. However, the behavior of bridge decks reinforced with MMFX steel bars was previously undocumented. In light of this fact, a research program was commissioned by the NCDOT in which three concrete bridge decks, each having a span-to-depth ratio of 12.5, were constructed and tested to failure under simulated truck wheel loads. Two of the decks were constructed with identical reinforcement ratios, one utilizing conventional Grade 60 steel and the other MMFX steel. The third bridge deck was also reinforced with MMFX steel, but was reinforced with 33 percent less steel in an attempt to utilize the higher tensile strength offered by MMFX. The results of the experimental program demonstrate that taking advantage of lowered reinforcement ratios by utilizing the higher strength of MMFX steel is a definite possibility in reinforced concrete bridge decks. The bridge deck reinforced with 33 percent less MMFX steel developed the same load-carrying capacity as the deck reinforced with conventional Grade 60 steel. In addition, the same deck showed the same deflections at service load as the deck reinforced with Grade 60 steel.
Fabrication and Behavior of 3D Orthogonal Woven FRP/Concrete Bridge Deck
(2007-04-24) Johnson, Charles Miller; Dr. Sami Rizkalla, Committee Chair; Dr. Emmett Sumner, Committee Member; Dr. James Nau, Committee Member
In recent years, glass fiber reinforced polymer bridge decks have been considered as an alternative to conventional steel and concrete bridge decking. The research presented in this thesis investigated the behavior of an innovative 3-dimensionally woven bridge deck. The proposed bridge decks were woven at a local textile company, using the 3-D capabilities of the company. Typical decks consisted of two skins of E-glass fabric, and each skin consisted of two fabric layers. The fabric was woven with warp, weft, and vertical z-yarns. Additional z-yarns were used to form a longitudinal joint between the skins. Balsa wood cores were inserted between the skins and the entire deck was vacuum infused with an epoxy resin system. The research included fabrication of two deck panels with appropriate shear connectors to provide composite action with the top concrete slab. The study included a special study to investigate the behavior of three types of shear connector configurations. Modeling of the behavior is based on the test results of the measured material properties. These tests included tensile coupon testing, fiber volume fraction by burn-off method, tensile tests of the steel reinforcement, and compressive strength of the concrete to define the complete stress-strain relationship of the concrete using concrete cylinders. Predictions of the behavior were based on simple flexural member behavior and finite element analysis using ANSYS computer program. A U.S. patent is currently filed for the proposed innovative bridge deck panel.
Fabrication and Behavior of Three-Dimensionally Woven Glass Fiber Reinforced Polymeric Bridge Deck.
(2007-03-27) Mohamed, Tarek Said; Dr. Murthy Guddati, Committee Member; Dr. Emmett Sumner, Committee Member; Dr. Sami Rizkalla, Committee Chair
Deterioration of many bridge decks due to corrosion of steel triggered civil engineering researchers to consider other alternatives to the current conventional reinforced concrete and steel bridge decks. During the past two decades, researchers have intensively investigated the use of fiber reinforced polymeric (FRP) bridge decks as an alternative to the current conventional concrete and steel bridge decks. This research explored the feasibility of three-dimensional woven glass fiber reinforced bridge decks (3-D GFRP), fabricated using textile machine and resin infusion process. The research investigated the mechanical properties of 3-D GFRP including: Tensile properties, compression properties, and flexural properties. Three bridge decks were fabricated and tested up to failure to access the applicability of this new concept for bridge deck. The use of epoxy resin versus vinyl ester resin in the fabrication process was examined. For design purposes, the overall elastic modulus of the 3-D GFRP has been investigated using various methods. Test results confirm the effectiveness of the proposed concept in producing bridge decks for highway bridges. The use of 3-D weaving technique eliminates the typical delamination observed for the current pultruded GFRP bridge decks. The behavior of the 3-D GFRP bridge decks indicated promising potential, and lead to filing a US patent for this innovative concept in coordination with the local textile company collaborated in providing the bridge deck.
Flexural Behavior of Prestressed Girder with High Strength Concrete
(2006-12-08) Choi, Wonchang; Dr.Paul Zia, Committee Member; Dr. Amir Mirmiran, Committee Member; Dr. Sami Rizkalla, Committee Chair; Dr.Kara Peters, Committee Member
The advantages of using high strength concrete (HSC) have led to an increase in the typical span and a reduction of the weight of prestressed girders used for bridges. However, growing demands to utilize HSC require a reassessment of current provisions of the design codes. The objective of one of the research projects, recently initiated and sponsored by the National Cooperative Highway Research Program (NCHRP), NCHRP Project 12-64, conducted at North Carolina State University is to extend the use of the current AASHTO LRFD design specifications to include compressive strength up to 18,000 psi (124 MPa) for reinforced and prestressed concrete members in flexure and compression. This thesis deals with one part of this project. Nine full-size AASHTO girders are examined to investigate the behavior of using different concrete compressive strength and subjected to the flexural loadings. The experimental program includes three different configurations of prestressed girders with and without a deck slab to investigate the behavior for the following cases: 1) the compression zone consists of normal strength concrete (NSC) only; 2) the compression zone consists of HSC only; and 3) the compression zone consists of a combination of two different strengths of concrete. An analytical model is developed to determine the ultimate flexural resistance for prestressed girders with and without normal compressive strength concrete. The research also includes investigation of the transfer length and the prestress losses of HSC prestressed girders. Based on materials testing and extensive data collected from the literature, a new equation is proposed to calculate the elastic modulus for normal and high strength concrete.
Fundamental Behavior of Steel-Concrete Composite Beams Strengthened with High Modulus Carbon Fiber Reinforced Polymer (CFRP) Materials
(2005-06-30) Dawood, Mina Magdy Riad; Dr. Emmett Sumner, Committee Member; Dr. Vernon Matzen, Committee Member; Dr. Sami Rizkalla, Committee Chair
There is a growing need for a cost-effective, durable repair system that can be used for the repair and strengthening of steel bridges. Recently, high modulus carbon fiber reinforced polymers (CFRP) have been developed with a modulus of elasticity approximately two times greater than that of steel. Externally bonded high modulus CFRP materials have successfully been used to increase the elastic stiffness and ultimate capacity of steel-concrete composite beams However, since the technology is relatively new, the detailed behavior of steel bridge members strengthened with high modulus CFRP is not yet well understood. The current research investigates three aspects of the behavior of steel-concrete composite beams in detail. An experimental program was conducted to investigate the behavior of steel-concrete composite beams strengthened with high modulus CFRP materials. In the first phase of the study the behavior under overloading conditions was investigated. In the second phase of the research, the fatigue durability of the system was examined. In the third phase, the possible presence of shear-lag between the steel beam and the CFRP materials was investigated in detail. An analytical model was developed which can be used to determine the ultimate capacity and elastic stiffness increase for steel beams strengthened with high modulus CFRP materials. Additionally, a set of criteria are proposed which can be used to determine the allowable increase in the live load level for steel beams strengthened with high modulus CFRP materials.
Fundamental Characteristics of 3-D GFRP Pultruded Sandwich Panels
(2008-05-10) Patrick, Jason Fredrick; Dr. Paul Zia, Committee Member; Dr. Sami Rizkalla, Committee Chair; Dr. Vernon Matzen, Committee Member
This research presents the behavior of proposed 3-D glass fiber reinforced polymer (GFRP) pultruded sandwich panels designed to enhance the structural efficiency and to overcome delamination problems typically exhibited by traditional FRP panels. The sandwich panels consist of GFRP laminate plates at the top and bottom, separated by a polyurethane foam core, and connected by through-thickness fibers to achieve composite action. The use of the through-thickness fibers prevents delamination-type failures, increases the out-of-plane properties of the panels, allows low cost manufacturing, and ensures full utilization of the individual material strengths. The fundamental material characteristics of the sandwich panels are evaluated in three phases. The first phase evaluates the in-plane tensile properties of the GFRP laminate face sheets to determine the effects, if any, of the through-thickness fiber insertion pattern and test direction. The second phase investigates the shear behavior of the tested panels in order to evaluate the effects of various parameters, including through-thickness fiber insertion pattern, corresponding fiber insertions per square inch (fipsi), and testing direction (parallel and perpendicular to pultrusion direction). The third phase focuses on the flexural behavior of the sandwich panels in order to evaluate the effects of the same parameters in addition to the effects of varying panel widths and span lengths. The analytical phase of the research investigation incorporates the measured tensile and shear material characteristics in conjunction with Elementary and Advanced Sandwich Theories to predict the flexural behavior of the 3-D GFRP panels. Based upon these research findings, recommendations are proposed to the manufacturer and design engineers planning to use these sandwich panels in structural applications. The current research has shown these panels are a viable, cost-effective alternative which can be customized for various applications, such as pedestrian bridge decks, construction and industrial mats, truck trailer or rail car components, and marine environment applications.
Fundamental Properties and Bond Characteristics of Chlorinated Polyvinyl Chloride (CPVC) and SS340 Adhesive for Evaluation of Steel Tank Linings
(2006-12-28) Vickery, John D; Dr. Emmett Sumner, Committee Member; Dr. James Nau, Committee Member; Dr. Sami Rizkalla, Committee Chair
Tank linings are used to extend the service life of tanks and to avoid replacement of damaged tanks. Adhesive materials play a major role in the effectiveness of the liner. The main objective of this research is to understand the behavior of the different materials proposed for lining typical bleach tanks used in the pulp and paper industry. This research will focus on the behavior of the selected Structural Adhesive Series 340 (SS340) for a proposed lining system and its interaction with Chlorinated Polyvinyl Chloride material (CPVC) as well as the steel material at various temperatures and sustained stress levels. The experimental program was designed to subject the specimens to a combination of corrosive environments and sustained stresses in order to study their behavior and to determine the effect of those conditions on the behavior and service life. Research findings provide better understanding of the adhesive when subjected to severe environmental conditions and sustained loads. The overall composite behavior of the proposed lining for steel tanks based on testing of small-scale models subjected to high temperature is investigated. The experimental program focuses on the fundamental material properties and bond characteristics of SS340 when subjected to severe conditions. Test variables include temperature and applied sustained loads, as well as time and soaking solution. Sixty-six SS340 specimens and sixty-six CPVC specimens were examined under severe environmental exposure to determine the effects of these conditions on the tensile strength. The bond characteristics were investigated using fifty double lap shear specimens to examine the bond behavior of CPVC-to-steel surfaces. The investigation focused on the influence of the environmental exposure on the shear strength of the adhesive bond. Results obtained from tension and lap shear tests were used to identify the performance associated with each variable. Such influences are critical to the future field applications of these materials in extreme environments. Conclusions are focused on ultimate tensile strength and shear strength under various environmental conditions. Test results also provided limitations of the temperature and sustained load level, which can be used safely under service loading conditions. The final phase of the experimental program consists of small-scale specimens to simulate the lining proposed for a typical steel bleach tank. The specimens were subjected to extreme temperature changes to examine the thermal gradient distribution and the composite interaction of the CPVC liner to the steel wall under the effect of temperature. Strains and temperatures were recorded at each interface. Finite element analysis was conducted to validate test results. The analytical model was used to study the effect of key parameters believed to affect the behavior including boundary conditions and specimen dimensions.
Repair of Impact-Damaged Prestressed Concrete Bridge Girders Using Carbon Fiber Reinforced Polymer (CFRP) Materials
(2006-12-13) Miller, Anthony David; Dr. Emmett Sumner, Committee Member; Dr. Sami Rizkalla, Committee Chair; Dr. James Nau, Committee Member
Over-height vehicles impacting prestressed concrete (PS) and reinforced concrete (RC) bridge girders is a frequent problem experienced by the majority of transportation departments all over the world. The most common practice used to restore a damaged bridge is to cut out the damaged girder and replace it with a new one. More recently, alternative methods have been examined to help decrease the costs of replacing damaged girders and minimizing closure time. The research reported in this thesis considered three scenarios to examine the effectiveness of using Carbon Fiber Reinforced Polymers (CFRP) to restore impact-damaged PS girders to their original capacity. The first scenario investigated the effectiveness of CFRP sheets to repair a 54 ft (16.4 m) long girder with one ruptured prestressing strand caused by an over-height vehicle impact. The second scenario investigated the effectiveness of CFRP sheets to repair two 54 ft (16.4 m) long girders with various numbers of prestressing strands ruptured artificially at midspan. The final scenario examined the effectiveness of CFRP sheets to repair a shear-critical specimen with four prestressing strands artificially ruptured near the support. The design of all CFRP repair systems was conducted using a cracked section analysis and/or guidelines for shear capacity of prestressed members. The predictions according to the two approaches compared well with the measure values. The designs were compared to current codes and a recently developed debonding model. All of the repaired girders were able to reach and surpass their respective undamaged capacities. All of the flexural tests failed due to crushing of concrete and exhibited ductility even higher than the predicted value for the undamaged specimens.
Shear Behavior of Concrete Beams Reinforced with High Performance Steel Shear Reinforcement
(2008-08-12) Munikrishna, Aruna; Dr. Sami Rizkalla, Committee Chair; Dr. Emmett Sumner, Committee Member; Dr. Rudi Seracino, Committee Member
The current shear design provisions of the ACI 318 specifications limit the yield strength in transverse reinforcement to 60 ksi. Advancement in technology has led to the fabrication of High Performance steel. Use of HP steel in reinforced concrete could lead to cost savings by reducing the amount of steel required due to the inherited high strength and increase of the service life of structural members due to its enhanced corrosion resistance. This research is undertaken to examine the use of high performance steel as a feasible reinforcement material for reinforced concrete structures. Commercially available steel, Micro-Composite Multi-Structural Formable (MMFX), conforming to ASTM A 1035, was selected for this study. MMFX steel has minimum yield strength of 100 ksi. This experimental program comprised eighteen tests using nine large-scale reinforced concrete beams subjected to static loading up to failure. The key parameters considered in experimental program were the steel type and the amount of shear reinforcement. This research investigated crack width, modes of failure, deflection, stirrup strain, ultimate load carrying capacity and the behavior of the MMFX steel as transverse reinforcement for concrete beams. Results from the experimental program show that by utilizing the higher yield strength and consequently reducing the reinforcement ratio of MMFX steel, the beams can achieve almost the same load-carrying capacity as the beams reinforced with conventional Grade 60 steel. Also, beams reinforced with MMFX showed similar deflections at service load as the beams reinforced with Grade 60 steel. Therefore, reduction in the reinforcement ratio of MMFX steel, did not affect the serviceability of these beams. Analysis shows that the ACI 318, CSA, and AASHTO LRFD design codes can closely predict the ultimate shear strength for beams reinforced with high performance steel having yield strength up to 100 ksi. The beams were also analyzed using a well-established Modified Compression Field Theory (MCFT) to predict the shear strength of beam reinforced with high-performance steel. MCFT can be used to accurately predict the shear behavior of the beams. Based on the results and findings of the experimental and analytical research, design guidelines are proposed for the use MMFX steel as shear reinforcement in concrete beams.
Strengthening of Steel Structures with High Modulus Carbon Fiber Reinforced Polymer (CFRP) Materials
(2005-06-15) Schnerch, David; Dr. Sami Rizkalla, Committee Chair; Dr. Emmett Sumner, Committee Member; Dr. Mervyn Kowalsky, Committee Member; Dr. Kara Peters, Committee Member
Transportation departments and the telecommunications industry are currently demanding cost-effective rehabilitation and/or strengthening techniques for steel structures, including bridges and monopole towers. Rehabilitation is often required due to cross-section losses resulting from corrosion damage and strengthening may be required due to changes in the use of a structure. Current strengthening techniques, have several disadvantages including their cost, need to match the surface configuration of the existing structure, poor fatigue performance and the need for ongoing maintenance due to continued corrosion attack. The current research program makes use of new high modulus types of carbon fiber for strengthening steel structures. The experimental program was developed in four phases. These phases included the selection of suitable resins and adhesives for bonding the CFRP sheets and strips to the steel, characterization the bond to the steel through testing of the development length, performing large-scale tests on strengthened steel monopole towers and also determining the behavior of strengthened steel-concrete composite beams that are typical of bridge structures. The result of the experimental program was the demonstration of sizeable strength and stiffness increases for the steel structures, strengthened with the developed system. Analytical work has also been completed to predict these strength and stiffness increases as well as to determine the bond stresses to ensure the avoidance of a debonding failure, which is detrimental to the effective use of the high modulus CFRP material.
Strengthening of Steel Structures with High Modulus Carbon Fiber Reinforced Polymers (CRRP) Materials: Bond and Development Length Study
(2009-04-28) Stanford, Kirk Alan; Dr. Sami Rizkalla, Committee Chair; Dr. James Nau, Committee Member; Dr. Emmett Sumner, Committee Member
Cost-effective solutions for the rehabilitation and strengthening of steel structures, such as steel bridges and steel monopole towers used for cellular phone antennas, are greatly needed by government transportation departments and industry. Rehabilitation is often required due to loss of cross-section from corrosion and/or changes of the demand or use of a structure. Current techniques for strengthening steel structures have several drawbacks including requiring heavy equipment for installation, their fatigue performance, in addition to the need for ongoing maintenance due to continued corrosion attack. The current research program proposed the use of a new high modulus carbon fiber reinforced polymer (CFRP) for strengthening of steel structures. This program includes extensive research to select the resin for wet lay-up of carbon fiber sheets and the adhesives for bonding of pre-cured laminate strips. The bond behavior of FRP materials to steel structures is quite different from that of concrete structures. Preliminary test results showed the occurrence of very high bond stresses for most strengthening applications due to the amount of strengthening required for developing the material for steel structures and bridges.
Study in the Improvement in Strength and Stiffness Capacity of Steel Multi-sided Monopole Towers Utilizing Carbon Fiber Reinforced Polymers as a Retrofitting Mechanism
(2005-01-07) Lanier, Bryan Keith; Dr. William Rasdorf, Committee Member; Dr. James Nau, Committee Member; Dr. Sami Rizkalla, Committee Chair
Wireless service is a fast developing market which places inherent demands on providers to maintain constant, reliable networks through which the service is offered. In order to facilitate this growing need, wireless providers must install equipment which creates and strengthens these networks. Telecommunication towers are popular solutions for placing antennas at elevations which develop the line of sight trajectory and signal coverage the networks demand. However, as telecommunication towers have a finite limit to the amount of equipment installation, they must be strengthened to support additional equipment expansion. Research completed at North Carolina State University proposes a strengthening solution utilizing high-modulus carbon fiber polymers as a retrofitting mechanism for monopole telecommunication towers. The experimental program, along with development of an analytical model, investigates the behavior and validates the effectiveness of carbon fiber in increasing the flexural capacity of existing monopole tower structures. The experimental program consists of testing three large scale monopole towers using high-modulus sheets, high-modulus strips and intermediate-modulus strips to determine their respective effectiveness in increasing the flexural strength enhancement. The three tests are designed using approximately the same reinforcement ratios, as well as identically sized monopole towers, to compare the effectiveness of the three strengthening systems regarding the increase in strength and stiffness. Design nominal strength and stiffness increases were in the range of 20 to 50% which was found in the measured values. The three tests were subjected to the same load setup and tested until failure to capture the elastic and inelastic behavior and the strength increases, as well as the failure mode of the strengthened tower. The analytical models were designed to simulate the monopole?s behavior before and after strengthening using conventional methods of analysis typically applied to tower design. The analytical model is based on moment-area and transformed section theories to predict the strain and deflection behavior in the elastic range of the steel and carbon fiber. Parametric studies are conducted to study the effect of the numerous variables with respect to strengthening these types of towers.