Prestress loss detection in concrete beams using fiber Bragg grating-differential settlement measurement technology

With the aim to avoid possible damages of bridges, it is necessary to ensure the safety conditions of the bridge beams during their full lifecycle. Prestressed concrete members are mainly applied in the highway, urban and railway bridges. During their design process, prestress forces are calculated based on existing standard and theoretical formulas, but during their serviceability life, many factors such as relaxation of tendons, shrinkage and creep of concrete, friction between tendons and tendon ducts, anchorage slip and ambient temperature can lead to the prestress losses. In order to effectively control the status and the bearing capacity of bridges, the prediction of the prestress losses is unavoidable. This work has mostly conducted the prestress load identifications by using fiber Bragg grating-differential settlement measurement (FBG-DSM) technology. In detail, the FBG-DSM monitoring system was used for the short-term deflection measurements along the axis of the post-tensioned beams under examination, after the application of an additional vertical load at their midspan. The utilized static method is in fact able to determine the prestress force and, consequently, the prestress loss making use of one or more vertical displacements. Laboratory tests on a post-tensioned concrete specimen and field tests on No. 24 bridge in Tainan (Taiwan) show the high accuracy of the deflections measured with FBG-DSM sensors, confirming additionally the potential of this new technology. With regards to the static method, the prestress load estimations require an accurate assumption of the beam end restraints and, moreover, an accurate identification of the beam flexural rigidity. In general, good prestress force detections are obtained when significant second-order effects are induced in the tests. The first laboratory results from the concrete specimen give reliable indications and, furthermore, allow the application progress of the non-destructive method on full-scale PCI beams in the laboratory.