Fundamental experiments for monitoring water leakage of underground structures using plastic optical fibers

In coastal metropolises, strict regulations regarding the exploitation and utilization of groundwater have resulted in a continuous rise in groundwater levels over the past few decades. As a result, underground concrete structures are now at a higher risk of water leakage than ever before. To detect water leakage in underground structures as early as possible, this paper introduces a plastic optical fiber (POF) sensor. This sensor has a prism tip formed by two bonded parallel POFs, which enables it to detect water in real time. When the sensor comes into contact with water, the returning visible light changes due to the reflection and refraction at the prism interface. Fundamental experiments were conducted to verify the feasibility of the POF sensor. The results showed that the POF sensor is effective in detecting water in real time by measuring light intensity. Furthermore, the change of light intensity over time allowed for the observation of the water penetration process in hardened mortar specimens, enabling the estimation of the permeability of the mortar at a definite quantity. Finally, future work is discussed briefly.