Highly-sensitive near-infrared spectroscopy system for remote monitoring of concrete structures

Salt- or neutralization-induced damage of reinforced concrete structures significantly affects their durability. Because most large-scale infrastructures, such as bridges, urban tunnels, and seawalls, are made of reinforced concrete, developing efficient and accurate methods or devices for early detection of concrete structure damage is important for disaster prevention. Moreover, the cost of a life cycle of a concrete structure can be much lower if surface degradation can be detected and preventive maintenance performed. In this paper, we report the development status of a remote monitoring system that implements remote imaging and diagnosis of the concrete surface degradation. The system consists of a high-sensitivity spectrometer and signal analysis software. The first prototype of the spectrometer was developed using a large-aperture and lossless optical system and a near-infrared camera. Near infrared spectra of samples that simulated salt damage were captured and diagnosed, to analyze the system’s performance. Then, specimens of pure materials constituting concrete were measured, and their absorption spectra were analyzed. Based on the absorption spectra it was concluded that the absorption characteristics of these substances had no influence on the quantification of Friedel’s salt. The prototype machine for field testing has been developed. Using this machine, a series of remote measurement of surface water with up to 10m distance was conducted. And also, measurement of surface degradation on actual concrete structures has been started, to validate the approach. The developed system is expected to rationalize inspection of concrete structures by enabling effective imaging and objective diagnosis of degradation, which will help to clarify which parts or sections of structures need intensive inspection for preventive maintenance.