Principal Research Results

Measurement of Salt Concentration in Concrete by
Laser Induced Breakdown Spectroscopy− Sensitivity Evaluation of Chlorine Concentration Measurement−
Background
Since the durability of reinforced concrete structures is degraded due to corrosion of reinforcing bars by chloride ionspenetrated from outside, the quantitative measurement of the chlorine concentration in concrete is important in the evaluation of thedurability of concrete structures. So far, measurement of chlorine concentration in concrete has been carried out by a chemical analysisof samples picked out from the structures. However, since the chemical analysis is time-consuming, development of an on-sitemeasurement method for chlorine concentration is required. Laser-induced breakdown spectroscopy (LIBS) can identify and quantitatively measure elements contained in a sample by the analysis of the fluorescence from plasma produced by focusing the laser beam
on a measurement target, and can realize the remote and real time measurement on site.

Objectives
This study aims to evaluate the sensitivity of LIBS measurement of chlorine concentration in concrete using grinded andpressed samples of concrete containing salt;

Principal Results
We measured the chlorine fluorescence from the pressed sample by LIBS using single- or double-pulsed lasers ＊ 1 andobtained the following results (Fig. 1, 2).
1. Improvement of the sensitivity of chlorine concentration measurement by optimizing the timedelay between laser irradiation and fluorescence detection
White light noise occurred after laser irradiation was reduced by optimizing the time delay between laser irradiation andfluorescence detection, and the sensitivity of the measurement of chlorine fluorescence (wavelength: 837.59 nm) was improved. The
spectral intensity of the chlorine fluorescence line was successfully measured with signal-to-noise ratio of more than 2 even for thesample with a chlorine concentration of 0.18 kg/m3. These results show that the chlorine concentration of 0.6 kg/m3 ＊ 2, at which thecorrosion of reinforcing bars in concrete structures starts, can be detected by this method (Fig. 3).
2. Possibility of the quantitative measurement of chlorine concentration and enhancement of thefluorescence intensity by double-pulse LIBSLinearity between the spectral intensity of the chlorine fluorescence line and chlorine concentration was verified. Theseresults show the possibility of the quantitative measurement of the chlorine concentration in concrete. In addition, by optimizing thetime delay between the two laser pulses in double-pulse measurement, the spectral intensity of the chlorine fluorescence line wasenhanced by a factor of two by using double pulse instead of single pulse (Fig. 4).

Future Developments
The possibility of practical use of LIBS measurement of chlorine concentration in concrete will be verified by using aconcrete core sample.
Main Researcher: Takashi Fujii, Ph. D.,
Senior Research Scientist, Sector, Applied High Energy Physics, Electric Power Engineering Research Laboratory
ReferenceT. Fujii, et al., 2008, “Measurement of salt concentration in concrete by laser induced breakdown spectroscopy - Sensitivity evaluationof Cl concentration measurement using pressed sample -”, CRIEPI Report H07012 (in Japanese)＊1：Second-harmonic Nd: YAG lasers (wavelength: 532 nm) were used in this experiments.
＊2：It is defined by the Architectural Institute of Japan that the corrosion of reinforcing bars starts when the chloride ion concentration in concreteat the position of reinforcing bars is over 0.6 kg/m3.

126

9. Construction and Preservation of Electric Facilities

Bandpassfilter

Optical fiber

ICCD camera

Concretepressed sample

Fluorescence

Pressedsample

Spectrometer
Lens

PlasmaHe gasRotating
holderComputer

Laser beam

Timing controller
Lens
Movable stage

Laser 1

Laser 2

Fig.1 Experimental setup for LIBS measurement

Fig.2 Plasma generation on pressed sample

Chlorine fluorescence in plasma generated by irradiation of the laser beam on the sample is measured.
Laser 1 is used for the single-pulse measurement and laser 1 and 2 are used for the double-pulse
measurement. Plasma is generated using laser 1, and atoms and ions in plasma are re-excited or re-heatedby laser 2.

17000

Intensity (Arb. Units)

16000

5.40

Fluorescenceintensity

15000

14000

13000

12000
836

1.04
0.18
0.18

Signal intensity (Arb. Units)

5000

Cl: 837.59nm

4500●：Single pulse●：■：■：Double pulse

4000
3500
3000
2500
2000
1500
1000
500
0837

838

0839

123456

Cl concentration (kg/m3)

Wavelength (nm)Fig.3 Dependence of chlorine fluorescence
spectrum on chlorine concentration

Fig.4 Dependence of chlorine fluorescenceintensity on chlorine concentration

9Signals from 500 laser pulses are accumulated,and 5 successive data points are accumulated forwavelengths. The numbers in the figure showchlorine concentration (kg/m3).

Linearity between chlorine fluorescence intensity
and chlorine concentration is verified for singleand double-pulse measurements.

127