Diagnosis of Concrete Infrastructure Deterioration Using Near-Infrared Spectroscopy - Effects of Cement and Mortar Types on the Determination of the Chloride Ion Concentration-

Concrete is widely used in the construction of infrastructure, which ultimately deteriorates as a result of chemical damage. To prevent serious accidents caused by concrete deterioration, developing rapid, inexpensive analytical tools is necessary. Some nondestructive methods that detect the physical deterioration of concrete infrastructure, such as cavities and cracks, are already in practical use, while few reports have described the nondestructive analysis of chemical deterioration, such as chlorination and carbonization.
We have been investigating the application of near-infrared spectroscopy (NIRS) to estimate the chloride ion concentration in cement because the deterioration caused by chloride ions takes place in cement only. We used cements made by the same manufacturer as samples, although many types of cement exist and they are usually used as mortar and contain sand and crushed stone as aggregate. Therefore, we investigated the effects of these factors on the estimation of the chloride ion concentration using NIRS.
The spectrometers used in the study were MicroNIR 1700/2200 (JDSU, Milpitas, CA, USA), which use a linear variable filter (LVF) as the dispersing element and a 128-pixel uncooled InGaAs linear array as the detector. The light source was a pair of integrated vacuum tungsten lamps. The wavelength range was 908–1676 nm at 6.2 nm intervals for the MicroNIR 1700 and 1150–2150 nm at 8.1-nm intervals for the MicroNIR 2200.
The chloride ion concentration in the samples was measured using thiocyanic acid mercury (II) absorptiometry. Partial least-squares regression (PLSR) with a full cross-validation method was used to develop and validate the calibration models using The Unscrambler software v9.8 (CAMO Software, Oslo, Norway). The wavelength ranges used for calibration were 1100–1676 nm for the MicroNIR 1700 and 1353–2150 nm for the MicroNIR 2200, since the spectra were noisy at the edges of the wavelength range of both spectrometers.
The results were R2 = 0.64 and SECV = 0.40% for the MicroNIR 1700 and R2 = 0.91 and SECV = 0.20% for the MicroNIR 2200, which were inferior to the results for cement-only samples but still sufficient for the qualitative analysis.