This study presents a new way to improve masonry wall behavior. Masonry structures comprise a significant part of the world’s structures. These structures are very vulnerable to earthquakes, and ...their performances need to be improved. One way to enhance the performances of such types of structures is the use of post-tensioning reinforcements. In the current study, the effects of shape memory alloy as post-tensioning reinforcements on originally unreinforced masonry walls were investigated using finite element simulations in Abaqus. The developed models were validated based on experimental results in the literature. Iron-based shape memory alloy strips were installed on masonry walls by three different configurations, namely in cross or vertical forms. Seven macroscopic masonry walls were modeled in Abaqus software and were subjected to cyclic loading protocol. Parameters such as stiffness, strength, durability, and energy dissipation of these models were then compared. According to the results, the Fe-based strips increased the strength, stiffness, and energy dissipation capacity. So that in the vertical-strip walls, the stiffness increases by 98.1%, and in the cross-strip model’s position, the stiffness increases by 127.9%. In the vertical-strip model, the maximum resistance is equal to 108 kN, while in the end cycle, this number is reduced by almost half and reaches 40 kN, in the cross-strip model, the maximum resistance is equal to 104 kN, and in the final cycle, this number decreases by only 13.5% and reaches 90 kN. The scattering of Fe-based strips plays an important role in energy dissipation. Based on the observed behaviors, the greater the scattering, the higher the energy dissipation. The increase was more visible in the walls with the configuration of the crossed Fe-based strips.
Shape memory alloys (SMAs) have been widely used in civil engineering applications including active and passive control of structures, sensors and actuators and strengthening of reinforced concrete ...(RC) structures owing to unique features such as the shape memory effect and pseudo-elasticity. Iron-based shape memory alloys (Fe-SMAs) have become popular in recent years. Use of iron-based SMAs for strengthening RC structures has received attention in the recent decade due to the advantages it presents, that is, no ducts or anchor heads are required, friction losses do not occur and no space is needed for a hydraulic device to exert force. Accordingly, Fe-SMAs embedded in a shotcrete layer have been used for pre-stressing RC beams at Empa. The aim of this study is to present an approach to model and analyze the behavior of RC members strengthened and pre-stressed with Fe-SMA rebars embedded in a shotcrete layer. The lack of research on developing finite element models for studying the behavior of concrete structures strengthened by iron-based shape memory alloys is addressed. Three-dimensional finite element models were developed in the commercial finite element code ABAQUS, using the concrete damaged plasticity model to predict the studied beams’ load–displacement response. The results of the finite element analyses show a considerably good agreement with the experimental data in terms of the beams’ cracking load and ultimate load capacity. The effects of different strengthening parameters, including SMA rebar diameter, steel rebar diameter and pre-stressing force level on the beam behavior, were investigated based on the verified finite element models. The results were compared. The load-displacement response of an 18-m concrete girder strengthened and pre-stressed with iron-based SMA bars was examined by the developed finite element model as a case study.
Civil structures such as bridges and buildings can be strengthened with prestressed fibre reinforced polymer (FRP) strips to enhance both their stiffness and load-bearing capacity. End anchorage is a ...crucial issue for prestressed FRP strips. An innovative anchorage procedure, called the “gradient anchorage method” and based on the possible accelerated curing of the epoxy-resin in the end region of the FRP strip, has recently been conceived with the aim of avoiding more invasive mechanical fastening systems. An in-depth knowledge of the actual development of the key mechanical properties of resins under different curing conditions (i.e., in terms of curing temperature) is of paramount importance for employing the above mentioned gradient method in practical applications. This paper presents experimental results and analytical investigations aimed at developing a better understanding of the strength development of a commercial adhesive under different curing times and temperatures. Firstly, direct tensile tests on epoxy specimens were performed at different curing temperatures. It was shown that the necessary curing time to reach the maximum tensile strength can be significantly reduced from several hours at room temperature to approximately 30min at 90°C. Furthermore, higher curing temperatures reduced the activation time after which strength starts to increase. The experimental observations are shown graphically with both the activation time and reaction duration at different curing temperatures. Secondly, pull-off bond tests were conducted on 100mm wide and 1.2mm thick FRP strips bonded to concrete using epoxy adhesives cured either at 90°C for different durations or at room temperature. An optical image correlation system (ICS) allowed the load transfer behaviour of the inhomogeneous cured adhesive between the FRP strip(s) and concrete to be studied. Finally, using the experimental measurements, the bond shear stress–slip interface relationships for the different test specimens were identified in order to present the effect of elevated curing temperatures and curing durations.
In civil engineering today, only 20 to 30% of the strength of carbon-fiber-reinforced polymer (CFRP) strips is used when they are applied as externally bonded strips for flexural and shear ...strengthening or in confinement of reinforced concrete (RC) structural elements. The strips are better used when the CFRP material is prestressed. This offers several advantages, including reduced crack widths, reduced deflections, reduced stress in the internal steel, and possibly increased fatigue resistance. In this paper, recent developments in the field of RC strengthening using prestressed CFRP are presented. The paper focuses on developments in flexural and shear strengthening and column confinement made at the Swiss Federal Laboratory for Materials Testing and Research (Empa). Several innovative ideas have been successfully realized in the laboratory. For example, a gradient prestressing technique without end anchorage plates was developed and successfully applied to a 17 m RC bridge girder. A confinement technique using nonlaminated thermoplastic CFRP straps was also investigated and applied to 2 m high RC columns. These results are encouraging, although practical and theoretical problems remain to be solved before these techniques can be fully applied.
Most research on fatigue strengthening of steel has focused on carbon fiber-reinforced polymer (CFRP) strengthening of steel members with existing cracks. However, in many practical cases, aging ...steel members do not yet have existing cracks but rather are nearing the end of their designed fatigue life. Therefore, there is a need to develop a "proactive" retrofit solution that can prevent fatigue crack initiation in aging bridge members. Such a proactive retrofit approach can be applied to bridge members that have been identified to be deficient, based on structural standards, to enhance their safety margins by extending the design service life. This paper explains a proactive retrofit design approach based on constant life diagram (CLD) methodology. The CLD approach is a method that can take into account the combined effect of alternating and mean stress magnitudes to predict the high-cycle fatigue life of a material. To validate the retrofit model, a series of new fatigue tests on steel I-beams retrofitted by the non-prestressed un-bonded CFRP plates have been conducted. Furthermore, this paper attempts to provide a better understanding of the behavior of un-bonded retrofit (UR) and bonded retrofit (BR) systems. Retrofitting the steel beams using the UR system took less than half of the time that was needed for strengthening with the BR system. The results show that the non-prestressed un-bonded ultra-high modulus (UHM) CFRP plates can be effective in preventing fatigue crack initiation in steel members.
This paper presents a study on the load carrying capacity of reinforced concrete (RC) beams strengthened with externally bonded (EB) carbon fiber reinforced polymer (CFRP) strips prestressed up to ...0.6% in strain. At the strip ends, the innovative gradient anchorage is used instead of conventional mechanical fasteners. This method, based on the epoxy resin's ability to rapidly cure under high temperatures, foresees a sector-wise heating followed by a gradual decrease of the initial prestress force towards the strip ends. The experimental investigation shows a promising structural behavior, resulting in high strip tensile strains, eventually almost reaching tensile failure of the composite strip. Additionally, ductility when considering deflection at steel yielding and at ultimate load is satisfying, too. From a practical point of view, it is demonstrated that premature strip grinding in the anchorage zone is not beneficial. In addition, a non-commercial 1D finite element code has been enlarged to an EB reinforcement with prestressed composite strips. A bilinear bond stress-slip relation obtained in earlier investigations is introduced as an additional failure criterion to the code. The numerical code is able to almost perfectly predict the overall structural behavior. Furthermore, the calculations are used for comparison purposes between an initially unstressed and a prestressed externally bonded composite reinforcement. The increase in cracking and yielding load, as well as differences in structural stiffness are apparent.
Carbon fiber reinforced polymer (CFRP) composites have exhibited a great potential for strengthening of steel structures. In the current study, an innovative prestressed unbonded reinforcement (PUR) ...system is introduced for fatigue strengthening of existing steel members. The system relies on a pair of mechanical clamps; each holds multiple CFRP plates and anchors their prestressing forces to the steel substrate via friction. A finite element model was established to optimize the design of the required mechanical components of the system. A set of static and fatigue tests was conducted on the developed mechanical clamps as the key elements of the proposed PUR system. The performance of the PUR system was then evaluated using a set of fatigue tests on two precracked steel plate specimens, one without any strengthening system and the other one strengthened with the proposed PUR system. In the latter specimen, the CFRP plates were prestressed up to about 800 MPa (approximately 30% of the CFRP tensile strength), which resulted in a complete fatigue crack arrest in the precracked steel plate. Furthermore, neither slippage of the mechanical clamps nor any prestress loss in the CFRP plates was observed after 7.5 million fatigue cycles. Based on the promising experimental results, obtained from the sets of fatigue tests performed in the current study, it can be concluded that the proposed PUR system can be considered as an efficient alternative to the conventional bonded reinforcement solutions for fatigue strengthening of damaged steel members.
•Normal, high and ultra-high modulus CFRP laminates for strengthening of steel beams.•Bonded and un-bonded reinforcement systems for steel beams.•Comparisons between the static behavior of the bonded ...and un-bonded systems.
This paper studies the elastic behavior of steel beams strengthened with normal, high and ultra-high modulus CFRP laminates using bonded and un-bonded systems. The elastic behavior of retrofitted beams provides useful information for design of fatigue strengthening systems. A total of seven steel beams including one control unstrengthened beam and six strengthened beams were tested statically until failure in a simply supported four-point bending set-up. The steel beams were retrofitted by normal modulus (NM), high modulus (HM) and ultra-high modulus (UHM) carbon fiber-reinforced polymer (CFRP) laminates with nominal Young’s moduli, ranging from 165 to 440GPa. Each type of laminate was attached to the steel beams using bonded reinforcement (BR) and un-bonded reinforcement (UR) systems. There is no direct comparison between the BR and the UR systems in the literature. The main goal of the paper is to provide a better understating about the stress distribution along the beam bottom flange when the BR and the UR systems are used for strengthening. All specimens failed due to lateral-torsional buckling (LTB). The effect of different strengthening methods on buckling capacity of the retrofitted specimens was also studied. Experimental results have shown that strengthening using bonded UHM laminates could increase the stiffness of the composite section so that the steel profile has yielded prior to buckling and a larger reinforcement efficacy was then achieved.
•Pre-stressed CFRP laminates for strengthening of steel beams.•Bonded and un-bonded retrofit systems.•Comparisons between the static behaviour of the bonded and un-bonded systems.•Effect of CFRP ...strengthening on lateral-torsional buckling capacity of steel beams.
Lateral-torsional buckling (LTB) of steel beams that are strengthened by normal modulus (NM) carbon fibre-reinforced polymer (CFRP) laminates was studied experimentally and numerically. A total of seven steel beams, which included a reference unstrengthened beam, were tested statically until failure. The steel beams were strengthened by bonded and un-bonded CFRP laminates. For each type, three steel beams that were strengthened by CFRP laminates with 0%, 20% and 40% pre-stress levels were prepared and tested. The effects of various strengthening schemes on the linear elastic behaviour, as well as the buckling strength of the retrofitted beams, were investigated. The test results showed that retrofitting the steel beams using CFRP laminates increases the elastic stiffness of the specimens almost the same for both bonded and un-bonded systems compared to the reference beam. The pre-stressing of CFRP laminate has no effect on the elastic stiffness of the retrofitted beams but substantially influences the buckling strength. Higher pre-stress levels do not necessarily lead to an increased buckling strength in the retrofitted beams. In most of the tests, the specimens that were strengthened with the un-bonded CFRP laminate showed slightly higher strength than the specimens that were strengthened by bonded CFRP laminate. Preparation of the bonded specimens took approximately twice as long as that of the un-bonded specimens. Finite element (FE) models of the retrofitted beams with pre-stressed bonded and un-bonded laminates were created. The results from the FE analysis were compared with the results obtained from the experiments.
PDF
