The hybrid structure combining steel and concrete has been adopted on expressway bridges as a excellent in economy, seismic design and maintenance. These hybrid structures can be classified into composite structures such as corrugated steel web bridges and composite truss bridges, and mixed structures in which steel girder and RC pier or steel girder and concrete girder. This paper describes each case, development history and characteristic, review and perspectives. In particular, regarding the corrugated steel web structure, its buckling, jointing method and dynamics were described. For the composite truss structure, the characteristic of the truss joint and the review in construction are described. For the mixed structure, the jointing method of the composite rigid frame structure are described. For the mixed structure where the steel girder and the concrete girder are joined, the review and the characteristics of the jointing method are described. Finally, perspective for composite structures are described.

 In replacing the slab of the girder bridge, a joint structure with the combination of the perfobond strip and the steel fiber reinforced mortar, have been proposed to connect precast slabs on the site. In previous study, the bending behaviour of this joint structure was confirmed experimentally. In this paper, the bending tests of the joint structure are carried out with two parameters, first is the presence or absence of a chin instead of formwork at the undersurface of the joint structure when placing mortar, and second is the mixture rate of steel fiber in the mortar. The influence of these factors on the bending behaviour of joint structure are considered. In addition, the pull-out tests of the perfobond strip with the steel fiber reinforced mortar are also carried out. The shear resistance of the perfobond strip from the pull-out tests are compared with the estimated value from the bending tests to confirm the corresponding of both tests.

 The perfobond strip is intended to integrate composite structural members by filling the hole put in the rib steel plate with concrete. In this study, an experimental study was conducted to understand the shear resistance mechanism of the perfobond strip without the penetrating rebar. In particular, the influence of the confining force acting the hole part on the shear strength was focused.

 In the experiment, the element test specimen that can subject the confining force only to the hole and eliminate the friction force between the steel plate and concrete around was used. From the experimental results, it was clarified that shear strength increases as the confining force increases but has a certain upper limit. Based on this result, the shear strength formula of perfobond strip without the penetrating rebar, which eliminates the effects of friction, was derived. In addition, regarding the slip displacement in the shear direction, the influence of the confining force was incorporated into the coefficient of the model formula in the Standard Specification for Hybrid Structures.

 In Japan, the repair and strengthening method by bonding carbon fiber reinforced plastic (CFRP) of lightweight and high strength to corrosion damaged steel member has begun to be applied. In 2018, JSCE issued “Guidelines for Repair and Strengthening of Structures using Externally Bonded FRP”, and the energy release rate was employed to the checking of debonding of CFRP. On the other hand, the multi-step CFRP bonding repair was proposed for the reduction of stress concentration in the adhesive at the end of CFRP. However, the calculation procedure of the energy release rate of the multi-step CFRP and the shorter step length of CFRP has not been proposed. In this paper, it was clarified that the energy release rate of any step condition of CFRP bonded steel plate under uni-axial loading can be calculated by applying the eigenvalue analysis and using the simple equation of shear stress in the adhesive.

 In this study, the fatigue strength of welded joints in steel-concrete composite slabs was investigated by fatigue testing. In addition, the local stress around the welded joint was evaluated through finite element analysis. In the test, fatigue cracks occurred at the toes of boxing welds of L-shaped shear connectors as a result of stress concentration due to the local out-of-plane bending deformation of the steel plates around the boxing weld. This local deformation can be produced by the steel and concrete behaving independent of one another locally near welds when the bond at their interface is lost. The test results demonstrated that the fatigue assessment of welded joints in a composite slab can be performed using the hot-spot stress approach. Then, actual composite slabs with different structural characteristics were analyzed to investigate the local stress behavior around welded joints, and their fatigue performance was assessed based on the hot-spot stress approach.

 The purpose of this study is to examine the effect of the studs’ specifications on the shear capacity of the composite members of steel plate and concrete, which adopt headed studs as the shear connectors. A quantitative evaluation of shear capacity was conducted. In current design formula, a reduction factor is simply multiplied to the strength of reinforced concrete structures to consider the shear capacity where the composite members are treated as one-way plate, owing to that the mechanism of the shear capacity has not been fully understood. However, in this design formula, the shear reinforcement effect due to shear connectors is not taken into account, and the design capacity is often underestimated on the safe side. Therefore, in this study, the loading test was conducted with different parameters such as the arrangement spacing, height and shaft diameter of the headed studs. Based on the test results, a new formula for calculated the shear capacity was proposed which could consider the shear reinforcement effect from shear connectors.

 As an effective measure to prevent the fatigue cracks, steel-fiber reinforced concrete (SFRC) overlays have been used for existing damaged OSDs. It has been confirmed that they can drastically decrease the out-of-plane deformation of the deck plate and greatly reduce the local stress of the weld root portion where the cracks initiate. However, there is no sufficient data to confirm its durability. It is important to investigate the durability performance under the effect of wheel loading and environmental action. For the purpose of evaluating the retrofit performance under severe service condition, shear fatigue tests of the adhesively bonded joints at deck-to-overlay connection were carried out using small size specimens with two different adhesives. This paper reports the trends of S-N curves and their fatigue behavior.

 An inspection passage made of glass fiber reinforced plastic (GFRP) has been applied to road bridge. GFRP is lightweight and has excellent corrosion resistance. The inspection passage utilizing the characteristics of GFRP has been developed, and ensured serviceability and safety. In this study, a sandwich panel slab which is lighter and more economical than the conventional assembled sandwich panel slab, has been investigated. The slab is made by integral molding of core material, skin plate and flange. The core material is made of urethane and the skin plate and flange is made of GFRP. There are two types of inspection passage: truss type and girder type, and in the case of truss type, the orientation of the slab was studied as a parameter. Based on the NEXCO (Nippon Expressway Company) test method, a series of tests under design and impact loads were conducted on GFRP test channels with integrally molded sandwich panel slab, and serviceability and safety were verified. As a result, it is confirmed that the GFRP inspection passage with integrated molding sandwich panel slab is applicable because test results show sufficient safety and serviceability.

 In this study, strength of bearing-type bolted joints between pultruded hybrid FRP and steel was evaluated by applying tensile load to bolted specimens. The number of bolt rows, the type of bolt, the number of shearing surfaces, the presence or absence of bolt axial force, and the type of washer were parameters of specimens. Effects of these parameters on joint strength were investigated experimentally. As a result of the experiment, it was confirmed that by introducing the bolt axial force, the out-of-plane deformation near the bolt hole can be suppressed, and the joint strength can be about twice as large as the case without axial force. In addition, it was confirmed that the joint strength of single-lap shear was smaller by 17% at most than that of double-lap shear. Finally, a clear difference in joint strength was not confirmed in the comparison between knurling bolts for bearing-type connections and high strength bolts for friction-type connections.

 In the early stage of the introduction of the concept of green infrastructures to Japan, concrete or steel structures were named gray infrastructures, and considered as the exact opposite concept against green infrastructures. However, appropriate combination based on the consideration of the each character of green and gray infrastructures is recently expected to create new values that will lead safe and sustainable land.

 Based on the background above, the task committee for the hybrid of green and gray infrastructures carried out the extensive study on the hybrid (combination and fusion) of green and gray infrastructures from June 2018 to May 2019. This report shows the summary of the activities of the task committee.

 This report summarizes the technologies for waterproofing and drainage of composite structures combining steel and concrete. It can be said that water intrusion is an important issue in the maintenance of composite structures. Here, we focused on bridges among composite structures, and investigated the current status of technology related to waterproofing and drainage, and examined the mechanism elucidation through tests. In the current state of technology survey, we mainly conducted a survey based on literature, summarized the drainage technology used for floor slabs, and introduced a measurement method using electromagnetic wave radar, which has been attracting attention in recent years. In addition, several tests have been conducted on the corrosion of the triple contact point, which is the interface between steel and concrete, in order to elucidate the mechanism, and the results of these tests have been reported.