Currently, the bridge type that is widely used in the construction of small and medium span bridges uses prestressed concrete (PSC). This technology was first introduced in Korea in 1962, and PSC is widely applied to crucial infrastructures such as bridges, nuclear power plants, and buildings. In particular, PSC bridges are excellent in terms of safety, economy, maintenance, and aesthetics; therefore, their proportion in bridges is steadily increasing.
PSC bridge is a bridge type that improves structural efficiency by introducing prestress to concrete. However, prestress losses occur throughout the life cycle, from the construction stage to the maintenance stage. PSC structures must be properly maintained throughout their design life to ensure public safety and provide reliable service.
However, the loss in PSC bridges continuously occurs throughout the life cycle from the public construction stage to the maintenance stage; however, measuring the loss is difficult. Therefore, the amount of change in prestress must be measured to ensure the continuous safety of the PSC structure. Accordingly, the interest in stress management in the construction stage and effective prestress measurement in the maintenance stage is increasing.
The objective of this study is to develop an embedded sensor module to evaluate the prestress inside the PSC structure using a hetero-core optical fiber. A concrete-embedded sensor module based on strain measurement was constructed by utilizing the linear relationship between the bending curvature of the hetero-core optical fiber and the light loss. In addition, to examine the performance of the developed sensor module, sensor module exposure and watertightness tests were conducted. To confirm the possibility of actual application, an embedding test was conducted using a concrete specimen.
In the exposure state performance test, the power consumption was measured by transforming it into a displacement control method using a universal testing machine (MTS). As a result of the performance test, the coefficient of reliability determination (R2) for each module showed high reliability with an average of 0.98–0.99. As a result of the watertightness test using the indicator material, the necessary watertightness was observed to be secured when the module was reinforced with a silicon pad.
The embedding test of the sensor module was conducted by embedding the sensor module in the specimen mold, pouring concrete, and using the universal testing machine as in the exposure test. As a result of the embedding test, the bending and unfolding of the optical fiber attached to the inside of the module could be effectively measured according to the displacement, and R2 showed reliability of 0.78–0.93. In addition, the correlation analysis between power and displacement for the test results of the concrete embedding state and verification through regression analysis were performed. Consequently, R2 was observed to be 0.92, and the coefficient of determination of the measured value and the calculated value using the regression equation was adequate.
Two methods exist for evaluating prestress: an indirect method of calculating the cross-sectional analysis after measuring the tensile force of the tension member and a direct method of evaluating the prestress by measuring the compressive strain of the PSC structure. This study confirmed the applicability of the sensor module that can directly measure the prestress inside the PSC as an embedded sensor module that can measure the deformation inside the concrete specimen through experimental analysis. Moreover, the following conclusions were obtained regarding the performance test of the developed embedded sensor module.
In the exposure state performance test, the linearity was satisfactory in the coefficient of determination and repeated measurements, and watertightness was also secured, indicating that it could be used as an embedded sensor module. In the embedding test, as a result of the embedded state test in the specimen, the prestress could be directly evaluated by measuring the amount of internal strain of the PSC structure. To verify the embedding test, a regression equation between displacement and power was derived. As a result of pairwise comparison between the displacement obtained by calculating the regression equation and the measured displacement, R2 was 0.90, proving that the regression equation had sufficient field applicability.
With the recent development in IT technology, research on the convergence of structure health monitoring (SHM) technology and IT technology is being actively conducted. In the case of the embedded sensor module using a hetero-core optical fiber developed in this study, the stress of the concrete could be determined by measuring the amount of deformation inside the PSC structure. In conclusion, the sensor module developed in this study can be used as a useful data for the development of a sensor module for internal embedding in the PSC partial girder model experiment.