Detection of chlorine with concentration of 0.18 kg/m3 in concrete by laser-induced breakdown spectroscopy

The chlorine concentration in concrete samples was measured by laser-induced breakdown spectroscopy (LIBS). One or two pulsed second harmonic Nd:YAG lasers (λ=532 nm) were used for the generation of laser-induced breakdown, and an intensified CCD camera, spectrometer, and optical bundle fiber were used for spectral measurement. To maximize the spectral intensity of the chlorine fluorescence line at a wavelength of 837.59 nm, the time delay between laser irradiation and spectral measurement, the time delay between the two laser pulses in double-pulse measurement, and the gate width of the spectral measurement were optimized. The linear relationship between the spectral intensity of the chlorine fluorescence line and the chlorine concentration was verified for pressed samples with chlorine concentrations from 0.18 to 5.4 kg/m3. The signal-to-noise ratio was higher than 2 for the sample with a chlorine concentration of 0.18 kg/m3 (0.008 wt. %). Thus, a chlorine concentration of 0.6 kg/m3, at which the reinforcing bars in concrete structures start to corrode, can be detected. These results show that LIBS is effective for the quantitative measurement of chlorine concentration in concrete with high sensitivity.