| In recent years,with the rapid economic development of China,the requirements for the span and the construction speed of bridges have become higher and higher.Therefore,some world-class long-span continuous rigid frame arch bridges have been built in China continuously.As important parts of the continuous rigid frame arch bridge,the pre-stressed concrete structure of the main beam and the concrete-filled steel tube(CFST)structure of the arch rib play a key role in the construction speed and the excellent performance of the bridge.Both the pre-stressed concrete structure and the CFST structure are internal grouting structures.The prerequisite for the excellent performance to be achieved is that the internal grouting of the steel tube and pre-stressed tendon duct must be dense.If there are grouting defects inside the tendon ducts in the pre-stressed concrete structure,steel strands will be exposed to the air.With the infiltration of external chloride ions and moistures,steel strands at the defect position are prone to corrosion,which will sharply reduce the mechanical properties of steel strands,greatly weaken the preloading effect on the concrete,and reduce the stiffness and durability of the structure.If subsurface cavities appear in the CFST structure,it will reduce the restraining capacity of the steel tube to the concrete and the adhesion between them,destroy the integrity of the composite structure of the CFST,and then reduce the bearing capacity of the bridge.Therefore,it is necessary to detect the possible internal grouting defects in the continuous rigid frame arch bridge as soon as possible,and carry out remedial measures such as secondary grouting to prevent serious accidents during the long-term service of the bridge.However,the current detection methods for grouting defects in pre-stressed tendon ducts and subsurface cavities in CFST structures are mostly point measurement methods.The signal wave is excited on the surface of the structure to enter the internal,and whether there are grouting defects in the structure is judged by receiving the signal of transmitted wave or reflected wave.For the main beam and the arch rib of the long-span continuous rigid frame arch bridge,the existing detection methods need to measure the whole structure point by point because the possible location of defects cannot be known in advance,resulting in low detection efficiency.Therefore,a simultaneous quantitative detection method for grouting defects in pre-stressed tendon ducts and subsurface cavities in CFST structures at multiple positions based on thermally driven distributed fiber optic sensors(DFOSs)was proposed in this study,and the evaluation method for performance degradation of the prestressed concrete beam with grouting defects was established.The main research contents and conclusions of the full text are summarized as follows.(1)Aiming at the problem of grouting defects in the main beam of the long-span continuous rigid frame arch bridge,a quantitative detection method for grouting defects in pre-stressed tendon ducts based on thermally driven Brillouin optical fiber sensing technology was proposed.According to the need of defect detection,a thermally driven Brillouin fiber optic sensor with both active heating and distributed temperature measurement functions was designed,and the sensor layout method matching with the construction process of pre-stressed concrete structure was proposed.Through the heat transfer theoretical analysis of the grouting defect in the tendon duct,the mapping relationship between the defect size and the equivalent thermal conductivity was established,and the solution method of the equivalent thermal conductivity was explored.The results of the physical test and the numerical simulation show that multiple grouting defects can be detected and accurately located at the same time by identifying temperature abnormal areas on the temperature distribution cloud map of the DFOS.Based on the established theoretical model and DFOS temperature rise data,the equivalent thermal conductivity at any longitudinal position of the tendon duct can be determined.Then,according to the mapping relationship between the defect size and the equivalent thermal conductivity in theoretical analysis,the quantitative identification of the grouting defect size of the tendon duct is realized.The minimum defect height and length that can be identified by the proposed detection method are 13.5 mm and 5 cm respectively,and the maximum defect location deviation is only 3.8 cm.(2)Aiming at the problem of subsurface cavities at the top of the arch rib in the long-span continuous rigid frame arch bridge,a detection method for subsurface cavities in the CFST structure based on line distributed optical fiber sensing technology was proposed.A thermally-driven distributed optical fiber sensing scheme was designed,which can easily measure the linear distribution temperature data on the top of the CFST structure in a large range.Based on the analysis of the heat transfer characteristics of the CFST structure,a method for the location and length identification of the subsurface cavity was proposed.On the basis of considering the air heat convection in the cavity,the mapping relationship between the cavity height and the equivalent heat absorption coefficient was established,and the solution method of the equivalent heat absorption coefficient was proposed.The results of the physical test and the numerical simulation show that multiple cavities can be quickly detected at the same time by identifying abnormal temperature areas on the temperature distribution curve of the top of the CFST structure.The local temperature maximum point and the maximum temperature change rate point in the abnormal temperature area are extracted as the location features of the center and the edge of the cavity,so as to realize the accurate location and length identification of the cavity.There is an obvious linear relationship between the temperature rise at the top of the CFST structure and the square root of the heating time.According to the temperature rise characteristics of the distributed measurement data,the equivalent heat absorption coefficient of each position can be solved to realize the quantitative detection of the cavity height.The minimum cavity height that can be quantitatively identified by the proposed detection method is 5 mm.(3)In order that the optical fiber sensing technology is suitable for the detection of subsurface cavities at any position of the CFST,a contour identification method for the subsurface cavity in the CFST based on the regional sensing and deep learning was proposed.The change of the subsurface cavity height in the CFST will not only cause different temperature values on the top of the structure,but also change the abnormal temperature area on the surface of the CFST.By obtaining the temperature distribution data of the surface of the CFST under thermal drive and directly identifying the contour of the abnormal temperature area on the surface of the steel tube formed by the cavity,the size of the cavity can be determined more accurately.However,due to the complicated fluid-solid coupling heat transfer at the cavity position,the temperature abnormal area on the outer surface of the steel tube is not completely consistent with the internal cavity area,and it is quite difficult to establish the relationship between them through the three-dimensional heat transfer theory.Therefore,by drawing the collected temperature data of the upper surface of the CFST into a distributed thermal image,the cavity size recognition problem was transformed into an image object detection problem.Further,the deep learning method was used to automatically extract the temperature data characteristic of each cavity position,and the mapping relationship between the DFOS measured temperature data and the cavity size was established.The test results show that the improved IVGG-16 network structure is more suitable for the Faster R-CNN algorithm to identify the size of the subsurface cavity in the CFST.The use of the deep learning algorithm for distributed thermal image detection can detect all cavity regions,realizing accurate and quantitative identification of cavity length and height.(4)Aiming at the problem that the tendon duct grouting defect in the main beam of the long-span continuous rigid frame arch bridge is difficult to be repaired by the secondary grouting method,based on the detection of grouting defects and the time-varying model of the steel strand corrosion,an evaluation methods for the performance degradation of the prestressed concrete beam with grouting defects was established,the influence law of the steel strand corrosion on the long-term performance of the main beam in the continuous rigid frame arch bridge was explored,and the specific preventive reinforcement schemes was proposed.By analyzing the corrosion mechanism of the steel strand in the tendon duct grouting defect,a time-varying model of the cross-section loss and fracture of the steel strand was established and applied to the numerical simulation model.The numerical simulation results show that the two indexes of vertical displacement and axial stress are suitable for evaluating the performance degradation of the prestressed concrete beam with grouting defects.The corrosion of steel strands of different positions,lengths and numbers has different effects on the distribution of the vertical displacement and axial stress of the main beam.Under the sole effect of the corrosion of the 16 steel strands in the bottom plate of the middle span,the vertical displacement change value at the middle position of the main beam within 30 years of completion of the bridge reaches 17.55 mm.Under the combined action of concrete shrinkage and creep and the corrosion of 16 steel strands in the middle span bottom plate,the axial compressive stress at the middle of the main beam is reduced from 6.30 MPa to 0.79 MPa within 30 years of completion of the bridge.Finally,according to the detection results of grouting defects and the influence law of steel strand corrosion on the long-term performance degradation of the prestressed concrete beam,a specific scheme for preventive reinforcement of the bridge by external prestressing method was proposed.The numerical simulation results show that the reinforcement method has a good repair effect on the performance degradation of the main girder caused by the corrosion of steel strands. |
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