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The laser-induced breakdown spectroscopy (LIBS) is a fast method to provide multi-elemental analysis of any sample. At the Federal Institute for Materials Research and Testing (BAM) the LIBS technique is applied on building materials to measure ingress profiles of harmful species like chloride and alkalis. The ingress depth and the quantitative amount is important for the evaluation of the potential for damage processes like the alkali-silica reaction or chloride-induced corrosion. Concrete as an example is a highly heterogeneous material with 1/7 cement (major component CaO) and 6/7 aggregates (SiO2) with different grain sizes. Due to a scanning procedure a two dimensional element distribution of a concrete surface can be measured. In order to have an automated Separation method to evaluate heterogeneous materials, different cluster algorithm have been tested. Best results have been achieved with the Expectation-Maximization-Algorithm (EM-Algorithm).
The information about the chemical composition of coal is of great interest during mining, sorting and processing. Due to great variations of the soil layers the quality of coal suffers great fluctuations during mining because of adjoining rocks [1-6]. To provide on-site analysis of coal the application of LIBS as a multi-element method has been tested. Therefore different qualities of coal have been chemically analyzed to get reference values for the water content as well as for the major (carbon) and minor components (ash = Ca, Mg, S, Si). For the LIBS analysis a system using a low energy laser with a wavelength of 1064 nm, pulse energy of 3 mJ, pulse length of 1.5 ns and a repetition rate of 100 Hz have been used. A compact czerny turner spectrometer operating in the UV range (Δλ = 170 nm – 350 nm) have been applied. To provide quantitative concentrations different calibration curves have been evaluated using chemometrics. The system has been validated using internal cross-validation as well as an external set of samples. For the evaluation of the method different figures of merit will be presented. After applying LIBS for coal analysis in the laboratory a LIBS system for the on-site analysis has been tested during the mining process.
In civil engineering the information about the quantitative ingress of harmful species like Cl⁻, Na⁺ and SO²⁻₄ is of great interest to evaluate the remaining life time of structures. These species are triggering different damage processes like the alkali-silica reaction (ASR) or the chloride-induced corrosion of the reinforcement. For the evaluation of the heterogeneous concrete it is necessary to discriminate between the different phases mainly cement matrix and aggregates. The transport processes are only proceeding in the cement matrix therefore the measured concentrations should be regarded to the cement content. For the 2D evaluation of element distributions different multivariate cluster-algorithms like k-means and Expectation-Maximization-algorithm (EM-algorithm) have been tested. The methods are compared and different figures of merit will be presented. After phase separation non-relevant information of the aggregates can be excluded. The ingress of harmful species is then quantified using chemometrics. Due to concrete cores from a parking deck the methods have been validated and verified with standard methods of wet-chemistry.
Reinforced concrete structures like bridges and parking decks are subject of corrosion of the reinforcement if chlorides from thawing salts or sea water penetrating into the concrete. For the determination of the expected life time normally samples are taken by borehole cuts or drilling of cores. These samples are taken to the lab and standard chemical methods are used for evaluation. As an alternative a fast and reliable onsite technique which gives results just in time is needed. In cooperation with industrial partners and BAM a mobile LIBS system for the on-site application of LIBS in civil engineering has been developed. The mobile system uses a low energy laser with a pulse energy of < 3 mJ, a wavelength of 1064 nm, a pulse width of 1.5 ns, a repetition rate of 100 Hz and a NIR-spectrometer in combination with a scanner to map areas of up 170 mm x 140 mm. In this work the correlation between exposures of the concrete surface due to chlorides and their ingress profiles will be shown. Quantitative ingress profiles are evaluated and compared with standard analysis.
In civil engineering, the laser-induced breakdown spectroscopy has been applied as a fast and reliable method for a quantitative evaluation of concrete cores. Due to a two-dimensional scanning, the heterogeneity of concrete can be evaluated and elements like Cl, Na, and S are related to the cement matrix only. This study deals with the temporal evaluation and imaging of laser-induced plasmas on cement-based materials, in order to investigate the impact of aggregates with diffrent grain size on the spectral response in LIBS.
For concrete production aggregates, cement and water are mixed together and after 28 days of hydration a porous and multiphase material is formed. To consider the heterogeneity of concrete a 2D scanning system is used. The coarse aggregates (limit > 2 X d ) can be excluded. laser spot The remaining content is a mixture of flour grains and cement particles (micro heterogeneity). Harmful species like chlorides may penetrate together with water through the capillary pore space. A quantification of Cl regarding to the cement content only (European standard EN 206) is necessary for the damage assessment. A LIBS system operating with a NdCr:YAG laser (pulse energy of 3 mJ, a wavelength of 1064 nm, a pulse width of 1.5 ns, a repetition rate of 100 Hz) and two Czerny-Turner spectrometer (UV and NIR range) have been used.
Concrete is a multiphase material made of cement, aggregates and water. The heterogeneity of concrete is a result of mixing aggregates (grain size between 0.125 mm to 32 mm) and cement (grain size < 0.04 mm) together. There are always aggregates with a grain size below 0.125 mm (flour grains) therefore the cement matrix is always a mixture of these small particles. Different grain size distributions by making concrete are important to ensure the needed density and com-pressive strength. In order to estimate the remaining lifetime of concrete structures the quantification of element concentra-tions of alkali and chlorides regarding to the cement matrix only (1/3 of the total mass), is a major concern in civil engineer-ing. Due to two-dimensional scanning with LIBS the coarse aggregates can be evaluated and excluded from the analysis. In the case of particles (flour grains and cement) smaller than the laser spot size, the microscopic hetero-geneity influences the laser-plasma interaction and has therefore an impact on the results. In this study the influence of the micro-heterogeneity and the impact on the laser-material interaction is examined. The effect of changing ratios between cement particles and aggregate particles in the laser-induced plasma has been analyzed (see figure 1, right). Therefore, different samples with defined grain sizes are prepared and the obtained distributions were analyzed with x-ray diffraction (XRD). For the LIBS measurements, an automated system that operates with a low energy NdCr:YAG laser (pulse energy of 3 mJ, a wavelength of 1064 nm, a pulse width of 1.5 ns, a repetition rate of 100 Hz) and a NIR Czerny-Turner spectrometer has been used.