Compact fiber optic system for in-situ concrete condition assessment

Concrete structures require precise temperature and humidity monitoring during curing to ensure optimal strength and to prevent defects such as cracking. A compact optical sensing system was developed using a single fiber that can be embedded directly within the concrete. When paired with a spectral interrogation unit operating in the low-loss single-mode communications band of 1500–1600 nm, the system functions as both a temperature and relative humidity (RH) sensor. Temperature monitoring is achieved using a Fiber Bragg Grating, while humidity sensing is provided by a Fabry-Perot interferometer (FPI) at the fiber tip. The interferometer cavity is formed with a layer of polyvinylpyrrolidone (PVP), an RH-sensitive polymer. The system exhibited a response time of approximately 3 h and operated over a relative humidity range of 50–95% RH and a temperature range of 30–60 ∘ C, with normalized free spectral range sensitivities of the FPI up to 35 m − 1 /%RH at high humidity levels. The system was validated using a small-scale 50-day in-concrete test.