Fresh concrete used in 3D printing should ensure adequate yield stress, otherwise the printed concrete layer may suffer intolerable deformation or collapse during the printing process. In response to this issue, an analytical study was carried out to derive the initial yield stress and hardening coefficient of fresh concrete suitable for 3D printing. The maximum shear stress distribution of fresh concrete was calculated using a stress transformation equation derived from the equilibrium condition of forces. In addition, the elapsed time experienced by fresh concrete during the printing processes was estimated and was then substituted into the elapsed time-yield stress function to calculate the yield stress distribution. Based on these results, an algorithm capable of deriving both the initial yield stress and the hardening coefficient required for printing fresh concrete up to the target height was proposed and computational fluid dynamics (CFD) analyses were performed to verify the accuracy of the proposed model.

Corrosion of reinforcing bars deteriorates the bond between the rebar and concrete, consequently leading to the degradation of the strength and stiffness of reinforced concrete (RC). Because structural deterioration due to corrosion may cause failure and large-scale disaster, various methods of measuring the corrosion rate (electrochemical methods, electrical methods, fiber optic sensors, strain gauge, ultrasonic wave, and infrared thermography) have been proposed so far. The electrochemical methods are the most widely used for estimating corrosion rates but have the limitations that the measurement results are qualitative, and are affected by climatic parameters. To overcome these limitations, fiber Bragg grating (FBG) sensors to directly detect the expansion displacement of reinforcing bars due to corrosion have been developed. However, these sensors are susceptible to strain induced by heat and external loads, and they also present challenges in terms of miniaturization and installation. This study developed a new optical sensor that can overcome the limitations of existing sensors. The developed sensor detects the expansion displacement of the sacrificial steel plate due to corrosion and thus estimates the corrosion rate of rebar. A linear relationship between the displacement and the sensor response was observed, and the sensor was capable of measuring the displacement up to half the depth of the sensor. The sensor also showed a reversible response to repeated displacement and was stable even in alkaline and salty environments. Its practicality was also verified by accelerated corrosion tests.