Smart sensor tags for seepage sensing protected by 3D-printed case for embedding in concrete structures

Water seepage through cracked concrete structures often causes reinforcement corrosion as well as degradation of the strength of the primary material. Although there have been major tests of humidity sensors in concrete, based on radio-frequency identification, most such approaches rely on a vector network analyser to measure electromagnetic wave parameters as a proxy for the presence of water or moisture, and this is only suitable at laboratory scale. Therefore, the present researchers conducted an experimental study of sensor code-based smart sensor tags’ performance for long-term detection of seepage through cracks in concrete. Similar sensor code-based tags have been applied in many fields, but never previously for this purpose, i.e., as part of a concrete building's structural health monitoring (SHM) system. The researchers also propose 3D-printed cases made of polylactic acid to protect the sensor tags. Their experiments compared the performance of sensor tags encapsulated in two sizes of 3D-printed cases – one with an overall depth of 8 mm, and the other, of 3 mm – and embedded in concrete cylinders. This established that, while received signal strength and frequency both varied with soaking time, neither was satisfactory as a measurement of seepage. However, change in sensor code as soaking time increased following damage to the concrete specimens was effective as a seepage measurement. The sensor code-based damage index of those smart sensor tags protected by 3 mm-deep 3D-printed cases indicated better overall results than their 8 mm-deep counterparts, and the 3 mm-deep cases were also more cost-effective. In a subsequent experiment, the researchers embedded 24 of 3 mm-deep 3D-printed cases, each containing one smart sensor tag, in a reinforced-concrete beam. This established that only those smart sensor tags with one or more cracks touching them reached the damage-index threshold, confirming the viability of the sensor code-based damage index. A subsequent experiment's results also showed that, if one of the 24 smart sensor tags experienced an error, the remaining 23 were still able to verify seepage. In short, the proposed system using 3D-printed cases is expected to be a highly useful addition to SHM.