Application research on internal prestress monitoring of box girder based on FBG self-sensing steel strand

Internal prestressing is an effective prestressing approach in the concrete box girder that ultimately plays a vital role in their construction. In practical engineering, accurate measurement and long-term monitoring are still challenging. FBG-based self-sensing steel strand (FSS) possesses the advantages of quasi-distributed monitoring of internal cable prestress and long-term stable monitoring. To examine its application in monitoring the internal prestressing of the actual box girder project, as well as to reveal the law of prestressing distribution and prestressing loss in the box girder, the self-sensing steel strand is appropriately experimented by using the cyclic tensile test. Precast and cast-in-place beams are then selected to examine the prestressing process and long-term prestressing monitoring of box girders with single-ended and double-ended tensioning processes. The results reveal that the linearity of FSS is 0.84%, the hysteresis is 0.75%, and the repeatability is 1.87%, which exhibits a good prestress monitoring performance. In the comparative application with the Elasto-Magnetic sensor and the reverse tension method, the instantaneous prestress monitoring results are basically consistent with the Elasto-Magnetic sensor. Due to the influence of the friction resistance of the anchorage, it is slightly lower than the measured value of the reverse tension method. Therefore, the FSS exhibits the accuracy of prestress monitoring and consistency of the actual box girder engineering application. In the monitoring of instantaneous prestress loss, such a loss at the tension end of the box girder mostly caused by the internal shrinkage of prestressed tendon, accounting for 81.3 % on average. The mid-span and the fixed end of the box girder, which experience tension under the single-ended tension conditions, are affected by friction and concrete compression loss. Therefore, reducing the amount of shrinkage of steel strands and adopting the double-ended tension method is still the key to reducing the instantaneous prestress loss. In the long-term prestress loss, the measured value exhibits a certain discontinuity. Compared to the theoretical value, the amount of prestress loss of the box girder tensioned at both ends is linearly positively correlated with the magnitude of applied effective prestress, and the amount of loss is negatively correlated with channel curvature.