For the safety of a prestressed structure, prestress loss is a critical issue. Unfortunately, up to date, with consideration of the harsh environment and the durability, no qualified technique is available. This paper proposes a novel technique to monitor prestress loss in RC beams by using combined Brillouin and fiber Bragg grating sensors on a single optical fiber. A smart steel strand based on the newly developed sensor was introduced. Laboratory tests of a series of prestressed RC beams were used to verify the concept of monitoring of prestress loss by using the smart steel strands. The prestress loss obtained from the Brillouin and the fiber Bragg grating sensors are compared with those from the load cell and the structural analysis, which validated the feasibility of the proposed sensing system. The monitoring results from the proposed optical sensors can reveal both the spatial distribution and the time history of prestress loss during the construction and the in-service phase.

Polarization is an important property of electromagnetic (EM) wave and different polarization manipulations are required for varied optical applications. Here we report a reconfigurable metasurface which achieves both the polarization conversion and the polarization rotation in THz regime. The metasurface is reconfigured through the micro-electro-mechanical-systems (MEMS) actuation. The cross polarization transmittance from a linear polarized incidence is experimentally tuned from 0 to 28% at 2.66 THz. In addition, the polarization rotation angle is effectively changed from −12.8° to 13.1° at 1.78 THz. The tunable bi-functional metasurface for polarization conversion and the polarization rotation can be flexibly applied in various applications such as imaging, polarization microscopy and material analysis, etc.

For the safety of prestressed structures, prestress loss is a critical issue that will increase with structural damage, so it is necessary to investigate prestress loss of prestressed structures under different damage scenarios. Unfortunately, to date, no qualified techniques are available due to difficulty for sensors to survive in harsh construction environments of long service life and large span. In this paper, a novel smart steel strand based on the Brillouin optical time domain analysis (BOTDA) sensing technique was designed and manufactured, and then series of tests were used to characterize properties of the smart steel strands. Based on prestress loss principle analysis of damaged structures, laboratory tests of two similar beams with different damages were used to verify the concept of full-scale prestress loss monitoring of damaged reinforced concrete (RC) beams by using the smart steel strands. The prestress losses obtained from the Brillouin sensors are compared with that from conventional sensors, which provided the evolution law of prestress losses of damaged RC beams. The monitoring results from the proposed smart strand can reveal both spatial distribution and time history of prestress losses of damaged RC beams.

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