Experimental Study On Flexural Fatigue Behavior Of Steel-UHPC Composite Deck System With PBL Shear Connectors

Steel-UHPC composite bridge deck is a new bridge deck structure connected by shear connectors between ultra-high performance concrete（UHPC）and orthotropic steel deck.By this method,the technical problems such as fatigue damage of asphalt pavement,fatigue cracking of steel structure and excessive thickness of steel main girder can be solved effectively.The current steel-UHPC composite bridge deck usually uses studs as shear connectors,while PBL（Perfobond rib Connector）shear connectors are rarely used.However,PBL shear connectors have high bearing capacity,good ductility and excellent fatigue performance,so they have been widely used in steel-concrete composite structure.However,it is seldom used and studied in steel-UHPC composite bridge deck strucure.Therefore,this paper designs a new steel-UHPC composite bridge deck with PBL shear connectors based on Danjiangkou Reservoir Bridge.In this paper,experimental research and finite element model analysis were carried out on the fatigue performance of this kind of steel-UHPC composite bridge deck,in order to provide performance evaluation of this kind of structure.The chief contents and results are as follows:（1）The flexural fatigue tests of UHPC under different stress levels were carried out at-10℃,and the results were compared with those of UHPC at 20℃.The results show that,the degradation of UHPC flexural fatigue performance and the fatigue strength decrease under a cyclic load with the decrease of temperature due to the cold brittleness characteristics of steel fibers at the cracked section.Compared with the test results at 20℃,the fatigue strength of UHPC at–10℃is decreased by 15.6%.Although the fatigue strength of the specimens at low temperature is lower than that of the specimens at normal temperature,their fatigue deformation modulus ratios（En/E0）corresponding to their fatigue strengths are similar,which are both around 0.48.The residual strength of UHPC specimens undergoing 2 million cycles without flexural fatigue failure is close to the static strength under monotonic loading,whether at normal temperature or low temperature.Based on the results,a model of predicting the fatigue life of UHPC at 20℃and–10℃was proposed,and the predicted results could be a reference in the practical application.（2）The transverse bending fatigue tests of four steel-UHPC composite bridge decks were done with the main test parameters including shear connector form,the existance of penetrating rebar and the existance of joint.The ANSYS partial model was established,and the maximum tensile stress of UHPC layer under the most unfavorable condition was obtained by standard heavy truck loading.Considering the safety factor of 1.6,the maximum tensile stress of UHPC layer was used as the fatigue stress amplitude of steel-UHPC composite bridge deck.The results of specimens show that:under the predetermined stress amplitude,the stiffness of each specimen is not reduced,and the crack width of UHPC layer can meet the specification requirements.Under the stress amplitude corresponding to 0.05mm crack,all PBL specimens finally suffered fatigue failure of reinforcement fracture,and rebar strains was characterized by three stages under normalized coordinates.The first stage is the initial development stage（n₁/N=0～0.3）,the curve rises rapidly and then slows down gradually.The second stage:fatigue development stage（n₂/N=0.3～0.87）,the curve rises slowly and changes at a constant rate;The third stage:fatigue failure stage（n₃/N=0.87～1）,the change rate increases sharply,and diverges rapidly when reaching a certain limit value.Based on the test results,a method to evaluate the fatigue performance of reinforced steel-UHPC composite bridge deck was proposed,and a formula for calculating the reduction coefficient k of equivalent rectangular strain diagram of concrete in tension zone varied with the number of cycles N was given.（3）Based on the construction drawing design of engineering practise,the ANSYS finite element model of the standard beam segment was established to explore the hot spot stress amplitude response of six typical fatigue details of orthotropic steel bridge deck under the action of the standard heavy truck.At the same time,the variation characteristics of stress amplitude before and after laying 80mm UHPC layer are compared.The results show that the steel-UHPC composite bridge deck with80mm UHPC layer can effectively increase the local stiffness of bridge deck,and greatly reduce the hot spot stress level of typical fatigue detail position,which can be reduced by 41.8%-90.3%.According to the FAT90 standard of international Welding Institute IIW,it can be believed that this kind of structure can avoid the cracking problem of steel bridge deck.