Measurement of Concrete Strength Using the Emission Intensity Ratio Between Ca(II) 396.8 nm and Ca(I) 422.6 nm in a Nd:YAG Laser-Induced Plasma KENICHIRO TSUYUKI, SATORU MIURA, NASRULLAH IDRIS, KOO HENDRIK KURNIAWAN, TJUNG JIE LIE, and KIICHIRO KAGAWA* Kajima Technical Research Institute, Kajima Corporation, Tokyo 182-0036, Japan (K.T., S.M.); Department of Physics, Faculty of Mathematics and Natural Sciences, Syiah Kuala University, Darussalam, Banda Aceh 23111, Indonesia (N.I.); Research Center of Maju Makmur Mandiri Foundation, 40 Srengseng Raya, Kembangan, Jakarta Barat 11630, Indonesia (K.H.K., T.J.L.); and Department of Physics, Faculty of Education and Regional Studies, University of Fukui, Fukui 910-8507, Japan (K.K.) An experiment to investigate the potential of a laser-induced plasma method for determining concrete compressive strength was conducted by focusing a Nd:YAG laser on concrete samples with different degrees of compressive strength. This technique was developed in light of the role of the shock wave in the generation of a laser-induced plasma. It was found that the speed of the shock front depends on the hardness of the sample. It was also found that a positive relationship exists between the speed of the shock front and the ionization rate of the ablated atoms. Hence, the ratio of the intensity between the Ca(II) 396.8 nm and Ca(I) 422.6 nm emission lines detected from the laser-induced plasma can be used to examine the hardness of the material. In fact, it was observed that the ratio changes with respect to the change in the concrete compressive strength. The findings also show that the ratio increases with time after the cement is mixed with water. Index Headings: Concrete compressive strength; Laser-induced plasma; LIP; Shock wave; Shock front speed; Ionization; Emission intensity ratio of Ca. INTRODUCTION Deterioration of concrete in aging structures is a significant problem in our present society. To elongate the service life of structures, appropriate and timely inspection of concrete integrity is essential. However, the current inspection method is usually time consuming and labor intensive. A rapid and remote concrete inspection method is therefore urgently required. From this point of view, we reported on a new technique for inspecting the carbonation of concrete, which induces its degradation, in terms of TEA CO 2 laser-induced plasma spectroscopy, based on detecting the emission line of C(I) 247.9 nm. 1 On the other hand, concrete hardness (compressive strength) inspection is absolutely important. It is assumed that a laser-induced plasma (LIP) technique could also be used to determine hardness because the plasma characteristics are inherently related to the hardness of the target. In this connection, it should be stressed that the mechanism by which a plasma is generated should be well understood. Based on recent experimental results, it has been concluded that the shock wave plays an important role in the production of a luminous plasma, even when the plasma is produced under atmospheric pressure. 2–4 As demonstrated in a previous study, 5 a luminous laser- induced plasma cannot be generated on soft samples using a TEA CO 2 laser. This was interpreted by assuming that no shock wave generation takes place due to the fact that the soft sample absorbs recoil energy of the ablated atoms, which lowers the ablating speed of the propelling atoms; on the other hand, with the help of a hard sub-target plasma generation easily takes place, even for a soft sample. In contrast to this, in the case where a Nd:YAG laser is used, a laser-induced plasma can be generated on any sample regardless of sample hardness because of its higher power density compared to a TEA CO 2 laser. However, it is believed that even in the case of a YAG laser there are some differences in the characteristics of the produced plasma, such as the plasma’s expansion speed, depending on the hardness of the sample. As discussed in a previous report, 6 when the laser power density is not too high, atoms come out from the target mostly in the form of neutral atoms, and ionization of the ablated atoms proceeds just behind the shock front during the plasma expansion process. When the shock expanding speed is very fast, the ionization of the ablated atoms is profound because of the high temperature generation just behind the shock wave, as expected from the Rankine–Hugoniot equation, 7 resulting in significant increase in ionic emission intensity in comparison with the intensity of neutral emission. This phenomenon is frequently observed for elements having a low ionization energy, such as Ca, which is a host element of cement. In fact, according to our preliminary experiments, the Ca ionic emission intensity from a hard stone sample is far stronger than the Ca neutral emission intensity, while for a soft sample containing Ca, ionization occurs weakly because the speed of the shock wave would be slowed down due to the lack of a repulsive force on the sample surface, resulting in a reduction in the ionic emission intensity. Therefore, it is expected that by measuring the ratio between the Ca emission intensities of the ionic line and the neutral line, it would be possible to estimate concrete compressive strength. In this paper, we demonstrate an innovative technique for measuring concrete compressive strength by detecting the Ca(II) 396.8 nm and Ca(I) 422.6 nm emission intensities in a YAG laser-induced plasma. This simple technique has great potential for use as a tool for making remote concrete compressive strength measurements. EXPERIMENTAL PROCEDURE The experimental set up used in this work is similar to that used in our previous work except for the laser source. 1 In this experiment, a Nd:YAG laser (Quanta Ray, LAB SERIES, 400 mJ, 5 ns) was operated in the Q-switched mode at a 10 Hz repetition rate with the laser output energy fixed at 50 mJ. The Received 31 August 2005; accepted 16 November 2005. * Author to whom correspondence should be sent. E-mail address: kagawa@edu00.f-edu.fukui-u.ac.jp. Volume 60, Number 1, 2006 APPLIED SPECTROSCOPY 61 0003-7028/06/6001-0061$2.00/0 Ó 2006 Society for Applied Spectroscopy