Research On Basic Performance Of Steel-thin UHPC Lightweight Composite Deck

A lightweight composite deck is a novel bridge deck system composted of an orthotropic steel deck(OSD)and a thin UHPC layer,in which UHPC denotes ultra-high performance concrete.The composite deck provides a new solution to alleviate the fatigue cracking issue of OSDs and the frequently reported damage issue of asphalt overlays.The UHPC layer is connected to the OSD through headed stud shear connectors.To allow for the thin thickness of the UHPC layer,the headed studs are small and short;meanwhile,to develop a robust composite action,a large number of headed studs are needed.As a consequence,two fatigue-prone details are generated by the massive welded studs:the headed stud itself and the deck plate with welded studs.So far very limited knowledge is available regarding the two details.To reveal the basic performance of the two details,this dissertation focuses on the following contents:(1)The static performance of short headed studs embedded in thin UHPC was revealed.Push-out tests were performed for short headed studs(d=13mm,h=35mm)embedded in 50-mm UHPC.The tests indicated that the studs fractured from the root,while the UHPC layer only developed minor deterioration around the root of the studs.Finite element models,developed based on the damage plasticitcity theory,were used to simulate the tests.The analysis results corresponded well with the test results.Parameter studies were carried out to explore the influence of the stud diameter,stud height,and compressive strength of concree on the shear behavior of headed studs.(2)The fatigue behavior of short headed studs embedded in thin UHPC was investigated.According to the fatigue tests on push-out specimens,the headed studs on one of the steel-UHPC interfaces fractured,while the UHPC layer did not develop appreciable fatigue damage.Based on linear fitting,a nominal stress S-N curve with 95%survival probability was built,which has a fatighe strength of 94.6 MPa at a fatigue life of 2 milion cycles,a value slighly higher than the strength of conventional headed studs embedded in normal concrete(90 MPa)as defined by Eurocode 4.(3)Hot-spot stress S-N curve was built for the detail of steel plate with welded studs.In a steel-thin UHPC lightweight composite deck,the behavior of the steel plate welded with studs is complex,making it difficult to determine an accurate nominal stress for the detail.Consequently,the hot spot stress method is more suitable when performing a fatigue analysis.To establish a hot spot stress S-N curve,fatigue test data based on the detail,reported in relevant literatures,were collected.Finite element models were built for each collected specimen and the hot spot stress concentration factor was calculated.Then the nominal stress range of each specimen was converted to the hot spot stress.Based on the converted data,three hot spot stress S-N curves were established by using three stress interpolation techniques.The S-N curves have comparable fatigue strength relative to the FAT 100 hot spot stress fatigue category defined by the international institute of welding.Thus,the S-N curve of FAT 100 can be used in the fatigue analysis.(4)Fatigue assessment was performed by using the multilevel finite element analysis method based on the Humen Bridge in Guangdong Province.The fatigue assessment for the headed studs was based on the nominal stress method,and the assessment for the deck plate with welded studs was based on the hot spot stress method.The analysis results indicated that for short headed studs,the most unfavorable position in terms of fatigue was predominately govered by the transversal position of the studs,and the longitudinal position had limited influences.While for the deck plate with welded studs,the fatigue behavior is influenced by its position.For the detail near the diaphragm,the longitudinal stress govers,while for the detail near the mid-span region,the longitudinal and transversal stresses are close to each other.The analysis also revealed that both details can meet the design requirements when the stud spacings were 155 mm×200mm(transversal×longitudinal).(5)Studies involved in the application of the new composite deck to the Mafang Bridge(the first pilot project)were presented,including the scheme design,finite element analysis,full-scale specimen tests,field construction and inspections.According to the full-scale specimen tests,the specimen failed because of local compressive yied of the lower flange of the cross beams,while the UHPC layer remained intact with no visible cracks developed even a maximum strain of 955 μεwas reached,indicating an impressive flexural behavior.Furter,field inspections performed by the third part showed that the new scheme can improve the stiffness of the bridge deck significantly,leading to an apparent stress reduction in the OSD.