Smart structures of the future will require a cost-effective, easily deployable solution for structural health monitoring. High loads on structures cause stresses that may lead to expansion of gaps, which are of utmost importance when it comes to overall structural health, as they absorb excess stress. Existing methods for direct displacement measurement of expansion joints are not ideal, as they operate under line-of-sight assumptions, are sensitive to moisture, or employ moving parts. In addition, the majority of existing sensors for structural health monitoring are uniaxial, and hence are fundamentally unable to measure 3-D displacement. Importantly, none of the existing wireless sensors for structural health monitoring can be embedded in concrete. We propose a system that uses low-frequency magnetic fields to conduct 3-D displacement measurement directly from within concrete, with a median displacement error of 0.5 mm in all directions, with a maximum separation distance of 50 mm between the transmitter and the receiver. The sensors can be attached to the concrete surface after the building is erected, or can be included in the concrete mix at manufacture, to monitor displacement between gaps in expansion joints, perform crack detection in concrete ties for railroads and in pavements, as well as aid position measurement for the assembly of premanufactured concrete blocks. Embedment in concrete allows operation throughout the lifetime of a structure, providing early warning of impending disaster and helping to inform repair operations.
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