The number of steel structures and steel-concrete composite structures that are over 50 years old is increasing. Many of them have suffered deterioration damage due to corrosion. In addition, in consideration of the increase in vehicle weight, the B live load (TL-25) was specified as the design load in 1993. Furthermore, in response to the 1995 Southern Hyogo Prefecture Earthquake, seismic design standards were revised in 1996. This situation has increased the need for repair and reinforcement. Therefore, many studies have been conducted to apply carbon fiber reinforced polymers to steel components due to their light weight, high strength, high elasticity, and non-corrosive properties. This paper introduces the current state of the art and future prospects for the development of the applied technology.

 During fatigue damage to the steel deck, fatigue cracks propagate starting from the U-rib welds. As a countermeasure against such fatigue damage, SFRC pavement construction is a standard construction method.

 However, lane regulation is required and causes traffic congestion. Therefore, we are considering a reinforcement method using lightweight resin mortar filled in the U-ribs and CFRP molded material pasted between the U-ribs as a reinforcement method from the bottom of the steel deck slab. In this study, we performed a steel deck specimen with three U-ribs, filled the U-ribs with lightweight resin mortar, secondary injection of resin, and pasted CFRP molding material between the U-ribs. We confirmed the reinforcing effect. We also applied heat equivalent to that of goose asphalt pavement and confirmed changes in performance when affected by heat.

 GFRP (Glass Fibre Reinforced Polymer) is applied to civil engineering structures because of its excellent material properties such as light weight and high corrosion resistance. In this study, the effects of bolt arrangement in the slotted holes in the base and loading conditions on the stiffness of the joint and stress properties of the base were studied analytically for the base joint of a GFRP bridge barriers. As a result, when subjected to bending loads, higher stresses occur at the bolt positions close to the load, and yielding occurs at the notches. The initial stiffness was smaller for the negative loading than for the positive loading, and was smaller with the bolt arrangement on the outside than with the bolt arrangement on the inside. The bolt tension of tensile joints increase relatively evenly under positive loading, but become unequal under negative loading. This is because the asymmetry of the L-shaped members results in different neutral axis positions depending on the loading conditions, and therefore, two bolts instead of four are located within the tensile side in the negative loading case.

 The authors have proposed a concrete composite structure using I-beams with studs in the web as main steel members as an alternative to conventional RC structures. In this study, pullout tests were conducted to clarify the adhesion performance between the proposed structure and concrete and the effect of the Poisson effect. From the results of the pullout tests, it was confirmed that, in addition to the anti-shifting effect of the studs on the web, the Poisson effect causes a compressive force on the concrete between the flanges, which increases the frictional force between the inner surface of the flange and the concrete, thereby increasing the integrity of the proposed structure and the concrete. Furthermore, based on the adhesion characteristics of the proposed structure and concrete, an experiment was conducted to determine the anchorage length when the proposed structure is used as the main steel member. From the experimental results, it was confirmed that if the anchorage length is six times or more than the web height of the I-beam, the I-beam will not pull out until the steel yields, and the anchorage is secured.

 Aging of bridges and bridge attachments such as wall barriers has become a problem, and wall barriers made of GFRP, which are lightweight and highly corrosion resistant, are being used from the viewpoint of maintenance and workability. The structure of the GFRP bridge barrier base is composed of bent steel plates and high-strength bolts, and the surface is hot-dip galvanized. However, there is a concern that the bolt axial force may decrease due to creep deformation of the GFRP member. In this study, a loading experiment was conducted on a full-scale wall barrier in order to clarify the structural performance of the GFRP bridge barrier base under reduced bolt axial force. As a result, it was confirmed that the decrease in bolt axial force due to the presence or absence of phosphate treatment on the friction joint surface of the GFRP member had little effect on the base of the GFRP bridge barriers.

 Although it is desirable to examine the structural member level for fatigue verification of GFRP, it is often difficult to conduct member fatigue tests compared to material fatigue tests. Therefore, in this study, we proposed a method to numerically evaluate fatigue damage and member stiffness of GFRP members based on data obtained from material fatigue tests. In the proposed method, we focused on the material stiffness degradation of GFRP with fatigue damage. The stiffness degradation model was implemented in the shell element. The proposed method was examined through FEM analysis using single element model, showed good agreement with the experimental value. Furthermore, the bending fatigue test of a GFRP-concrete composite girder was simulated by the proposed method. The analysis results confirmed that fatigue damage did not occur even after two million loading cycles under the design load. In addition, analysis was performed in which the applied load and loading position were varied, and it was revealed that when fatigue damage occurs in a GFRP hollow member, tensile fatigue damage occurs first on the lower surface of the upper flange.

 Adding stiffening steel plates with high strength bolts is generally used to reinforce the load-bearing capacity of steel truss bridges. In contrast, this method has the following problems: the bolt holes cause cross-sectional defects in the base member, and the steel plates must be pre-fabricated and carried to narrow areas before being bolted. A reinforcement method using carbon fiber reinforced polymer (CFRP), which is stronger, more elastic, lighter, and less corrosive than steel, has been proposed as a potential solution to these problems. If a reinforcement method using CFRP could be applied, it would be a more rational method than reinforcement using steel plates.

 In this study, mainly for seismic reinforcement, CFRP sheets were attached to long column specimens that imitated the H-section diagonal members of steel truss bridges to examine the reinforcement effect on the coupled buckling of whole buckling and local buckling through compression loading tests.

 In this study, several stud arrangement patterns at the girder ends of the composite girder were assumed, and the shear force responses of the studs caused by the design action such as drying shrinkage and temperature change were obtained. The shear response of the studs was calculated using the rigid-body-spring model analysis.

 As a result, the shear forces of the studs in the composite girder model due to the effects of drying shrinkage and temperature change are very large at the girder ends, and superposed shear force does not satisfy the serviceability limit condition. However, when those actions are applied continuously for those stud arrangement patterns at the girder ends, the maximum shear force of the studs is considered to be acceptable for the serviceability limit condition.

 In the design of railway structures, the increment to the dynamic response due to train running is considered as the impact coefficient. In addition, railway bridges have nonstructural members such as wheel guard and bridge railing, which have a considerable impact on the rigidity of the main structure such as the main girder. If these are not taken into consideration, the impact coefficient will increase, the live load will increase, and as a result, the design may not be rational. In this study, FEM analysis considering nonstructural member was performed for the roadbed reinforced concrete integrated SRC girder, which was trial designed for the purpose of reducing the girder height. The impact factor was calculated by setting the Stiffness correction rate from the ratio of the actual stiffness and the stiffness of the main structural members only. As a result, it was confirmed that the impact factor, which is one of the design issues, could be kept below the upper limit of the design guideline value.

 In the early 2000s, a prefabricated composite girder (VFT-WIB technology) using T-beams, which was cut from an I-beam web in a corrugated shape, was developed in Germany. The first bridge to which this solution was applied was completed in 2004. Initially, this solution was applied with puzzle-shaped shear dowels. In recent years, the number of prefabricated hybrid bridges using clothoid-shape shear dowels with improved cut shape has been increasing in European countries, especially Poland.

 The design shear strength formula of the clothoid-shape shear dowels and its calculation example are shown in this paper. In addition, the recent hybrid bridges using clothoid-shape shear dowels constructed in Poland are reported.

 To solve the problem of increased weight in steel underdeck panel construction for RC slabs, we are developing a strengthening method using FRP girders, which are lightweight and easy to construct, are placed in a grid configuration from the underside of the RC slab. The FRP girders are planned to be applied to the I-shaped HFRP girders consisting of pultruded carbon fiber and glass fiber, and double web GFRP girders consisting of glass fibers. In this study, three- and four-point bending loading tests were conducted on full-scale FRP girders to experimentally investigate their bending properties such as bending strength, failure modes, and deflection, in preparation for the practical use of FRP girders for reinforcing RC slabs. The results showed that the FRP girders were safe enough for the design loads and could be applied to reinforcement of RC slabs. Also, the anisotropic material properties of the FRP girders were converted into isotropic ones, and vertical displacements were calculated by FEM analysis using shell elements based on isotropic materials and beam theory. The results showed that the experimental values could be evaluated with high accuracy.

 Shear joints of FRP members using bolts depend on the material strength of the FRP members, which is relatively small compared to steel. The authors proposed a high-strength bolted friction joint of GFRP plates reinforced by steel plate adhesion with steel plates as a method to improve the bearing capacity of the joint between GFRP plates and steel plates. In this study, specimens were fabricated by applying the proposed joint method to the joint between a hybrid FRP girder and a steel girder, and the load carrying capacity and fatigue durability of the girder joint were experimentally investigated. The static tests showed that the failure of girder joints was preceded by slip or delamination, and that the smaller the introduced rate of bolt axial force, the more the slip failure preceded the delamination. Fatigue tests showed that although initial delamination occurred at the adhesive joints in a small load range, the initial delamination did not affect the fatigue durability of the girder joints.

 In the repair method of bonding plates, data on delamination fatigue strength under cyclic loading are still limited, and fatigue design methods have not been established. In this study, specimens of steel plate or CFRP bonded to steel plate were fabricated using the Vacuum assisted Resin Transfer Molding (VaRTM). The specimens were subjected to cyclic loading, and delamination and its propagation were measured. As a result, S-N diagrams were generated by organizing the ratio of the principal stress range in the fatigue test to the principal stress in the debonding in the static test, and the bending fatigue properties were evaluated from these diagrams with high accuracy. Comparisons with previous experiments using flexural and tensile loading showed similar trends, suggesting the possibility of evaluation independent of adhesive, working force, and bonding method.

 In this study, we conducted a basic research aimed at designing an efficient friction bond between a GFRP plate and a steel plate. We have developed a screening method (slip tendency test) that allows easier experimental investigation of the effect of surface treatment on static friction force of joint surfaces and conducted friction welding joint tests on three test cases selected using the screening method. The highest slip coefficient was obtained in the case where the GFRP plate side and the steel plate side were coated with inorganic zinc rich paint.

 From the perspective of relaxation behavior, it is suggested that it may be efficient to input a larger bolt axial force during pre-tightening than the bolt axial force during main tightening.

 Permanent scaffolds must be safe not only from the load of inspectors, but also from the load of falling inspectors and tools during work. In addition, when scaffolds are installed on existing girders where damage to the slab is evident, they must be able to prevent damage to third parties below the roadway in the event of falling concrete fragments on the underside of the slab.

 In this study, the safety of GFRP permanent scaffolds against impact loads was verified by dropping an inspector, a rebar, and a piece of concrete that was assumed to fall, for each installation method. As a result, it was confirmed that GFRP permanent scaffolds can absorb impact loads to some extent due to the deformation of the panels, and that penetration does not occur, but concrete pieces falling on the panel joints may cause damage.

 In recent years, the aging of Japan’s social infrastructure has become an issue, and the replacement of steel and concrete structures constructed during Japan’s period of rapid economic growth has begun in earnest. In this context, the construction industry has been working to resolve issues with an eye to the future, such as the 17 goals of the SDGs, which aim to realize a sustainable society by 2030, and the proposal “Building a Nation for the 22nd Century” by the Japan Society of Civil Engineers (JSCE). This study is an extension of these efforts and is the first phase of a project on the super long sustainability of civil structures in response to future climate change and a declining population. Based on the results of research conducted in 2012 on test methods, etc., we have added studies from a structural perspective and have begun super long exposure tests for 300 years on construction materials and members. In the future, we hope that the results will be widely disseminated and developed into discussions and initiatives by the entire JSCE, including other research committees, toward the realization of structures with super long design service life.

 FRP is expected to be utilized for its outstanding performance such as light weight, high strength, and non-corrosion, and was first described in the Specification for Hybrid Structures in 2014 as a representative example of a material for constructing hybrid structures that are not limited to steel and concrete. However, several issues remained to be solved in the actual design, construction, and maintenance of FRP structures. Therefore the subcommittee on design and maintenance of FRP composite structures (H218) was established in the committee on hybrid structures in July 2017 to conduct research on some of these technical issues (i.e., (1) evaluation method of mechanical performance of members and verification of basic data, (2) investigation and research on design technology of FRP composite structures, (3) maintenance and management methods of FRP structures, etc.), and has conducted activities for more than five years. This report summarizes the main results of these activities.