Study On Time-Dependent Seismic Performance Of Precast Prstressed Concrete Frames In Chloride Environment

Prefabricated buildings have been extensively investigated and applied for decades.By using durable and rapidly erectable prefabricated members,high-quality structures could be created at a relatively low cost.However,the seismic capacity of precast concrete structures has long been one major concern that prevents their wide application in seismic regions.Therefore,extensive investigations have been made to develop structures that have desirable seismic capacities especially at the joints of prefabricated components.Among these research efforts,the frame structure comprised of precast prestressed concrete components(SCOPE)has been put forward.For precast concrete structures,corrosion medium are easier to penetrate into structural joints where precast components are connected or post-casting is applied.The corrosion process and structural deterioration,however,highly depend on structural configuration and construction procedure.It is worth noting that the SCOPE system has been used in coastal region,where corrosion-induced deterioration may be a concern.However,so far no researches have been made with regard to the seismic performance of the corroded SCOPE system.This paper presents experimental study and numerical investigation on the seismic behaviors of corroded SCOPE system.In detail,this study includes contents as follows:(1)In order to provide basis for the use of SCOPE system in chloride area,an experimental study on the seismic behaviors of corroded beam-column joints of the SCOPE system was carried out.Five specimens with different levels of corrosion were tested under cyclic loading,and crack propagation,hysteretic loops,envelope curves,bearing capacity,ductility and energy dissipation were compared and analyzed.Based on the experimental results,the degradation characteristics and models of bearing capacity,ductility and energy dissipation were analyzed and developed.(2)According to previous research,the length of non-contact lapped splice between U-shape bars and the steel strands plays an important role in seismic performance of the beam column joint.However,the splice may deteriorate due to corrosion when the frame is used in coastal region.This paper presents an experimental investigation on seismic behaviors of the corroded beam-column joints of the SCOPE system.Ten specimens with different key slot lengths and different corrosion levels were tested under cyclic loading,and their hysteretic loops,envelop curves,bearing capacity,and energy dissipation were compared and analyzed.It is observed that the corrosion of steel bars would significantly affect seismic behaviors of the structure,and most specimens showed flexural failure,while for the seriously corroded specimens with long key slots,failure might occur at the interface between the precast beam and the key slot due to severe corrosion and bond failure of the lapped splice.In general,the energy dissipation of specimens with shorter slot length was greater than that of specimens with the longer key slot under the same corrosion level.However,as the corrosion level increases,the difference became smaller.Based on the experimental results,the design suggestion of the SCOPE system under corrosion environment is given.(3)This paper presents an experimental investigation on long-term behavior of PSC beams under the coupled effect of sustained load and corrosion.During the accelerated corrosion process,six PSC beams were subjected to different levels of sustained loads,and time-dependent prestress loss and concrete stains were recorded and analyzed.It is observed that the local corrosion(i.e.pitting corrosion)of the steel strands was significantly affected by the sustained load level,and the coupled effect led to more serious damage on the beams than individual sustained load or corrosion.The monitored data during the sustained load is very valuable for understanding and evaluating the long-term behavior of PSC beams.(4)To evaluate the time-dependent seismic performance of the corroded SCOPE system,a multi-scale finite element model is proposed to modeling of four-story moment frames with different levels.A fiber model is proposed to simulate the hysteretic behavior of corroded steel bars by using the User-defined material subroutine in the ABAQUS.The proposed multi-scale finite element model is validated through existing experiments,so that the balance between accuracy and computational costs is realized.Based on the proposed finite element model of frame,the influence of corrosion on seismic fragility of the frames is investigated through incremental dynamic analysis.Four limit states regarding the maximum interstory drift ratio are defined to evaluate the seismic fragility.The analysis results show that structural responses are highly random under different ground motions and corrosion levels.Meanwhile,the seismic fragility of the frame is significantly influenced by corrosion ratios,and the probabilities of exceedance show an exponential growth with the increase of corrosion level for all four limit states.(5)Based on the reliability theory and Fick's second law,the chloride diffusion law and the corrosion mechanism of steel bars are elaborated in detail.The probability distribution types and statistical characteristics of coastal environmental parameters and corrosion parameters are determined by combining duracrete model and previous test results.Based on the path probability model,the uncertainty analysis framework of time-dependent seismic performance of prefabricated concrete structure is established,which considers the multiple processes of ion transport,reinforcement corrosion and bearing capacity degradation.The time-dependent reliability analysis of the structure is carried out for each limit state.Finally,the whole life seismic performance evaluation model of prestressed concrete structure is established.