This article presents an application of an active all-optical photoacoustic sensing system with four elements for steel rebar corrosion monitoring. The sensor utilized a photoacoustic mechanism of gold nanocomposites to generate 8 MHz broadband ultrasound pulses in 0.4 mm compact space. A nanosecond 532 nm pulsed laser and 400 μm multimode fiber were employed to incite an ultrasound reaction. The fiber Bragg gratings were used as distributed ultrasound detectors. Accelerated corrosion testing was applied to four sections of a single steel rebar with four different corrosion degrees. Our results demonstrated that the mass loss of steel rebar displayed an exponential growth with ultrasound frequency shifts. The sensitivity of the sensing system was such that 0.175 MHz central frequency reduction corresponded to 0.02 g mass loss of steel rebar corrosion. It was proved that the all-optical photoacoustic sensing system can actively evaluate the corrosion of steel rebar via ultrasound spectrum. This multipoint all-optical photoacoustic method is promising for embedment into a concrete structure for distributed corrosion monitoring.

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Surface and subsurface inspection of concrete structures provides useful information for the maintenance of these structures. Remote sensing techniques such as radar and microwave sensors enable engineers to assess structural condition with ease and efficiency. This paper reports the performance of a 10.5 GHz portable imaging radar system for the quantitative, surface, and subsurface sensing of concrete structures in field configuration. Ranging, size determination, crack imaging, and subsurface interface determination are conducted using stripmap synthetic aperture radar (SAR) images. Three concrete structures are selected for field measurements. Ranging accuracy, size determination procedure, and effect of background noise are studied. Issues with background subtraction are discussed. Image-based, quantitative condition assessment criteria for ranging and size determination are proposed.

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