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In construction and demolition waste (CDW) recycling, the preference to date has been to apply simple but proven techniques to sort and process large quantities of construction rubble in a short time. This contrasts with the increasingly complex composite materials and structures in the mineral building materials industry. An automated, sensor-based sorting of these building materials could complement or replace the practice of manual sorting to improve processing speed, recycling rates, sorting quality, and prevailing health conditions for the executing staff.
A joint project of partners from industry and research institutions approaches this task by investigating and testing the combination of laser-induced breakdown spectroscopy (LIBS) with near-infrared (NIR) spectroscopy and visual imaging. Joint processing of information (data fusion) is expected to significantly improve the sorting quality of CDW, and may enable the detection and separation of impurities such as SO3-cotaining building materials (gypsum, aerated concrete, etc.)
We present current advances and results about the methodological development combining LIBS with NIR spectroscopy and visual imaging. Here, applying data fusion proves itself beneficial to improve recognition rates. In the future, a laboratory prototype will serve as a fully automated measurement setup to allow real-time classification of CDW on a conveyor belt.
Geschlossene Materialkreisläufe und sortenreine Materialfraktionen sind erforderlich, um hohe Verwertungs und Recyclingquoten in der Bauindustrie zu erreichen Beim Recycling von Bau und Abbruchabfällen wurden bisher bevorzugt einfache, aber bewährte Techniken eingesetzt, um große Mengen Bauschutt in kurzer Zeit zu verarbeiten Dies steht im Gegensatz zu den immer komplexer werdenden Verbundwerkstoffen in der Mineralbaustoffindustrie Die aktuell oft praktizierte händische Klaubung bürgt viele Risiken und Gefahren für das ausführende Personal und basiert lediglich auf offensichtlichen, visuell erkennbaren Unterschieden zur Trennung Eine automatisierte, sensorgestützte Sortierung dieser Baustoffe könnte diese Praxis ergänzen oder ersetzen, um die Verarbeitungsgeschwindigkeit, die Recyclingraten, die Sortierqualität und die vorherrschenden Gesundheitsbedingungen zu verbessern.
Responsible treatment of the environment and resources is a key element of sustainability. The building and construction industry is one of the largest consumers of natural resources. Consequently, there is a particular need for regulations and technologies that help to create closed material cycles. From the technological point of view, such efforts are complicated by the growing material diversity and the amount of composites contained in present and future construction and demolition waste (CDW). Nowadays, simple but proven techniques like manual sorting are mainly used. However, this practice not only poses health risks and dangers to the staff performing the work, but also relies on merely obvious, visually striking differences. Automated, sensor-based sorting of these building materials could complement or replace this practice to improve processing speed, recycling rates, sorting quality, and prevailing health conditions. The preliminary results for the identification of a wide variety of building materials with LIBS are presented.
To conduct a reliable, repeatable and accurate LIBS analysis, the optimization of the experimental setup is an important task. Hardware parameters of lasers and spectrometers used in the setup as well as additionally required components such as optics or process gas pipes must be carefully aligned and adjusted because factors like (i) focal conditions of the optics, (ii) alignment of the sample, (iii) process purge gas (types, flow rate) or (iv) measurement settings (integration time, accumulation of pulses) have a big impact on the signal quality. Therefore, in most cases the effect of different factors is evaluated empirically due to changing one factor at a time while keeping the overall configuration the same. In the end, the optimal configuration is selected based on the best parameters for each influencing factor. During optimization, a configuration of the experimental setup is aimed at, which allows e.g. the highest signal intensity or the lowest variation. In most cases, cross-correlation, interference and interaction among the various factors are not considered. For this reason, the possibilities of using Design of Experiment (DoE) to optimize a LIBS experiment will be shown and advantages of (i) reduction of testing plans using a feature space, (ii) identifying and considering cross-correlations and interactions, (iii) evaluating individual impacts on the measurement (e.g. contour and surface plots) as well as (iv) multivariate models for prediction of impacts will be presented.
In civil engineering, clear regulations and standards, such as the European standard DIN EN 206 apply to ensure that the existing infrastructure is sufficiently resistant to a wide range of exposure conditions. Despite these regulations, in practice concrete structures often show severe damage during their service life. One of these damages is pitting corrosion of the reinforcement, which can be caused by chloride ingress into the structure. Therefore, determining the distribution and depth of chloride ingress is important in predicting the service life. LIBS provides an alternative method to conventional wet chemistry in civil engineering. Despite many advantages, the use of LIBS has been severely limited due to a lack of regulations. Together with project partners from research and industry, we are currently working on a leaflet that will regulate chloride analysis in civil engineering using LIBS. This year, an international round robin test was organized to evaluate the performance of LIBS for chloride analysis in cement pastes. No specifications were given for the experimental LIBS setup and data evaluation. The preliminary results are presented.
Measurement of hydrogen concentration in steels by laser-induced breakdown spectroscopy (LIBS)
(2021)
Current efforts to achieve lightweight construction and the required reduction in CO2 emissions and an increase in energy and resource efficiency call for the increasing use of high-strength fine-grain structural steels. However, as the strength of higher-strength fine-grain structural steels increases, so do the associated joining challenges. Particular attention must be paid to hydrogen-assisted cold cracking. The influence of hydrogen reveals itself less in the strength but has a significant effect on the deformability [1, 2]. The degradation of the material properties can lead to zero ductility, where the values of the yield strengths coincide with the tensile strengths.
Laser-induced breakdown spectroscopy (LIBS) is a spectroscopic technique that can be used to determine elemental compositions without pre-treatment of the samples. Short, high-energy laser pulses ablate a small volume (< 0.1 mm3) of the examined material and ionize it to form a plasma. The decaying plasma emits element-specific light. This light is spectroscopically analysed and allows to detect qualitatively the present elements and to quantify them with help of a standardization routine. The simple experimental set-up and the fast, nearly non-destructive analysis procedure characterize the LIBS analysis. The LIBS method allows a time and spatially resolved in situ measurement of steel components in use.
Even low hydrogen concentrations (~ 1 wt.-ppm) in steel can be measured with the LIBS method and can be quantified with help of certified reference materials. The results are compared with results gained with the well-established carrier gas hot extraction method.
The spatial heterodyne spectrometer (SHS) concept, which is based on an interferometricoptical setup, boasts both the Fellgett and Jacquinot advantages. Theoretically it can provideboosted sensitivity and spectral resolution with respect to dispersion spectrometers in acompact, reasonably cheap arrangement without any moving components – this set ofcharacteristics can be attractive to a number of industrial, space and other field applications. The potential of SHS has already been demonstrated in IR and Raman spectroscopies(e.g.), and more recently also in LIBS. The scientific goal of our present project is toapply the SHS concept to the development of an optimized, but practical dual-grating,tunable SH-LIBS setup, which would possess appealing spectroscopic characteristics.During this development, we extensively rely on the computer-based simulation of theoptical setup, which is an efficient approach that we found to have been missing from earlierSHS efforts published. It can provide application-specific optimization of the SHS systemand predict the performance of the final system. In particular, we use optical simulation tostudy the effect of various important parameters on the relevant spectroscopic figures ofmerits of the system. We used the non-sequential ray-tracing mode of the Zemax/OpticStudio software for optical modelling of the SH-LIBS setup (Optical distortions were studiedin sequential mode). Characteristics of the setup and interferograms were calculated with atleast one million rays. All calculations were carried out for the visible spectral range(400-700 nm), using stepwise extension of monochromatic simulations with 5 nm steps.Wherever applicable, characteristic discrete visible wavelengths from the emissionspectrum of Hg discharge lamps (404.7 nm, 435.8 nm, 546.1 nm, 579.0 nm) were used for thecalculation of spectroscopy figures of merit and for the validation of simulation.
Asymmetrical laser-induced plasmas were investigated by a tomography approach based onthe inverse Radon transform. Two distinct sources of asymmetricity were investigated:double-pulsed laser-induced plasmas in the orthogonal configuration and single-pulsedlaser-induced plasmas under an inclined incidence angle. Both cases were observed atvarious delay times. The optical thinness of the laser-induced plasmas was achieved byappropriately adjusting the pulse energies. High temporal resolution was achieved by agated intensified charge-coupled-device camera. The asymmetrical laser-induced plasmaswere investigated in terms of their total emissivity, spectrally resolved emissivity, andtemperature. The latter was obtained by the Saha–Boltzmann plot method. The imagesrequired for the inverse Radon transform technique were obtained with a high angularaccuracy and reproducibility provided by mounting the spectrometer on a high-precisionnano-positioning rotary stage. The plasmas were induced in the center of rotation of thestage. This arrangement enabled the reconstruction of emissivity which was integrated overthe full spectral range (200–800 nm) or over a desired spectral range selected by a bandpassfilter (~10 nm). It also allowed for the reconstruction of spectrally-resolved emissivity ineach cross-sectional plasma slice by scanning the plasma across a spectrometer slit. The 3Dmaps of the temperature and electron density were thus obtained for different types ofasymmetric plasmas. The work will provide a more detailed description of the twoasymmetrical laser-induced plasmas. This might help with the development of LIBSinstrumentation using the orthogonal double-pulse geometry, or remote LIBS applicationswhich inherently rely on inclined-angle ablation.
The elemental analysis of seawater is often critical to the understanding of marinechemistry, marine geochemistry, and the deep-sea ecosystems. Laser-induced breakdownspectroscopy (LIBS) with the advantage of rapid multi-elements detection, has a greatpotential for in-situ elemental analysis of seawater. In practice, it is crucial to create acompact, low cost and power saving instrument for the long-term deep-sea observation. Arecently appeared diode-pumped solid-state (DPSS) laser seems to be a promising candidateas it is both compact and robust. Additionally, its high repetition rate up to hundreds of kHzcan provide a considerable throughput for LIBS analysis. However, the DPSS lasers operateat moderate pulse energies, usually less than one mJ, which cannot sustain stablebreakdowns in bulk water. To ensure stable laser-induced plasmas underwater with such aμJ-DPSS laser, we introduced an ultrasound source to assist the breakdown process. Thephase interface and mass flow generated by the near-field ultrasound can greatly reduce thebreakdown threshold and enhance element-specific emissions. Meanwhile, the highrepetition-rate pulses can also improve the breakdown probability and generate uniqueemission lines originated from the water molecule. We further demonstrate that the highrepetition-rate DPSS laser combined with the Echelle spectrometer can provide effectivequantitative analysis for metal elements in bulk water.
In civil engineering, the investigation of existing infrastructure is of major importance for maintaining and ensuring stability of the structures. To ensure durability, uniform regulations and standards apply e.g. the European standard EN 206-1. In some countries, the EN-standard is supplemented by additional standards, as in Germany with DIN 1045-2. Here, specific application rules are described, e.g. for the cement type, to ensure the resistant to different exposures. Therefore, the knowledge of the materials originally used is important in assessing the condition of existing concrete structure. Unfortunately, these are often unknown and must be determined retrospectively. Therefore, we present the application of the laser-induced breakdown spectroscopy to distinguish between different types of cement. Spectral information’s are used to build a classification model. First, the accuracy of the classification is analyzed on ten pure laboratory cement samples. To investigate possible sources of error, the model was then applied to cement samples with different moisture content. The study shows that LIBS is a promising tool for distinguishing between cement types. For further industrial application, however, factors influencing the LIBS signal must be included to ensure a robust model.
19th and 20th centuries glass paint layers consist of a colour body and a colourless lead silicate flux, in which borax or boric acid was added as further component to improve the paint ability and to reduce the firing temperature for multiple layers of paint. Model glasses were used in laboratory tests to investigate the stability of glass paints with additions of boron oxide. To determine boron in paint layers, a LIBS-system with pulsed NdYAG-laser was used.
The spatial heterodyne detection principle has a great potential in spectroscopy. It has an optical setup similar to that of a Michelson interferometer, with the mirrors replaced by diffraction gratings positioned at fixed, equal distances from the beamsplitter and are slightly tilted. The resulting interference pattern is recorded by a digital camera and the spectrum is recovered by using Fourier transformation. Although SHS was initially developed for astronomical and satellite-based atmospheric measurements, but in recent years it has been started to be applied in other branches of spectroscopy too. Recently the area of laser-induced breakdown spectroscopy (LIBS) has also discovered the potential of SHS. The main appeal of SHS detection in LIBS includes the compactness and robustness of the setup (in view of field applications) and the flexibility to optimize the setup for either high sensitivity or for high resolution, which can be benficially exploited in applications like stand-off measurements, quantitative analysis with isotope resolution, etc.
In the present work, we have improved and further optimized our initial LIBS-SHS setup described in a previous conference. By using optical simulations, we have modelled the light transmission efficiency, instrumental function and imaging properties of the system. We significantly improved and automated the spectral and image data processing sequence. The optimizations carried out resulted in an improved spectral resolution and repeatability, a lower spectral background and the elimination of the central line artifact originating from the Fourier transformation procedure. A detailed characterization of the LIBS spectroscopy performance (e.g. resolution, spectral coverage, tuning range, linearity, etc.), including a comparison with that of a LIBS setup based on a conventional dispersion CCD spectrometer was also performed.
Over the last few years, there has been a growing interest to apply spectroscopic methods to the agricultural field for better understanding of soil properties and for efficient, sustainable management of arable land. Within the project I4S (intelligence for soil), funded by the BMBF, an integrated system for site-specific soil fertility management is developed, consisting of different sensors like X-Ray fluorescence analysis (XRF), near-infrared spectroscopy (NIR) and laser-induced breakdown spectroscopy (LIBS). LIBS provides a fast and simultaneous multi-element analysis with little to no sample preparation, which makes it a suitable method for real-time analysis on the field.
The quantification of macro and micro nutrients in soils with LIBS is challenging due to matrix effects, different levels of moisture content and varying grain sizes. First studies revealed that the problems with matrix effects can be overcome by using well characterised soils as reference materials and chemometric tools like Partial Least Squares Regression (PLSR) for calibration.
The next step was to investigate the influence of moisture and grain sizes on the LIBS signal, which is a big issue when measuring directly on the field. The results showed that the LIBS signal decreases exponentially with increasing moisture content, as most of the laser energy is used for vaporising the water. With moisture contents of 30 % or higher almost no signal can be detected. This decrease is more severe for sandy soils than for clay soils. First tests of different grain size distributions indicate that the variation of the LIBS signal increases with growing amounts of larger grains. This results in a higher standard deviation, because of a poorer reproducibility of the plasma formation and plasma characteristic. With the help of chemometric tools the influence of moisture and grain sizes should be implemented in the calibration model for accurate analysis of nutrient composition in agricultural soils.
Over the last few years, there has been a growing interest to apply spectroscopic methods to the agricultural field for better understanding of soil properties and for efficient, sustainable management of arable land. Within the project I4S (intelligence for soil), funded by the BMBF, an integrated system for site-specific soil fertility management is developed, consisting of different sensors like X-Ray fluorescence analysis (XRF), near-infrared spectroscopy (NIR) and laser-induced breakdown spectroscopy (LIBS). LIBS provides a fast and simultaneous multi-element analysis with little to no sample preparation, which makes it a suitable method for real-time analysis on the field.
The quantification of macro and micro nutrients in soils with LIBS is challenging due to matrix effects, different levels of moisture content and varying grain sizes. First studies revealed that the problems with matrix effects can be overcome by using well characterised soils as reference materials and chemometric tools like Partial Least Squares Regression (PLSR) for calibration.
The next step was to investigate the influence of moisture and grain sizes on the LIBS signal, which is a big issue when measuring directly on the field. The results showed that the LIBS signal decreases exponentially with increasing moisture content, as most of the laser energy is used for vaporising the water. With moisture contents of 30 % or higher almost no signal can be detected. This decrease is more severe for sandy soils than for clay soils. First tests of different grain size distributions indicate that the variation of the LIBS signal increases with growing amounts of larger grains. This results in a higher standard deviation, because of a poorer reproducibility of the plasma formation and plasma characteristic. With the help of chemometric tools the influence of moisture and grain sizes should be implemented in the calibration model for accurate analysis of nutrient composition in agricultural soils.
Over the last few years, there has been a growing interest to apply spectroscopic methods to the agricultural field for better understanding of soil properties and for efficient, sustainable management of arable land. Within the project I4S (intelligence for soil), funded by the BMBF, an integrated system for site-specific soil fertility management is developed, consisting of different sensors like X-Ray fluorescence analysis (XRF), near-infrared spectroscopy (NIR) and laser-induced breakdown spectroscopy (LIBS). LIBS provides a fast and simultaneous multi-element analysis with little to no sample preparation, which makes it a suitable method for real-time analysis on the field.
The quantification of macro and micro nutrients in soils with LIBS is challenging due to matrix effects, different levels of moisture content and varying grain sizes. First studies revealed that the problems with matrix effects can be overcome by using well characterised soils as reference materials and chemometric tools like Partial Least Squares Regression (PLSR) for calibration.
The next step was to investigate the influence of moisture and grain sizes on the LIBS signal, which is a big issue when measuring directly on the field. The results showed that the LIBS signal decreases exponentially with increasing moisture content, as most of the laser energy is used for vaporising the water. With moisture contents of 30 % or higher almost no signal can be detected. This decrease is more severe for sandy soils than for clay soils. First tests of different grain size distributions indicate that the variation of the LIBS signal increases with growing amounts of larger grains. This results in a higher standard deviation, because of a poorer reproducibility of the plasma formation and plasma characteristic. With the help of chemometric tools the influence of moisture and grain sizes should be implemented in the calibration model for accurate analysis of nutrient composition in agricultural soils.
The relevance of chloride measurements in practice for durability condition assessment and a description of the working principle of LIBS will be given. A comparison of chloride profiles obtained by the common wet chemical analysis and results from LIBS measurements are shown to demonstrate the accuracy of the LIBS-technique. In addition, an example on the extra information resulting from measurements performed with LIBS for improvement of condition assessment and prediction will be shown.
Influence of moisture and grain sizes on the analysis of nutrients in agricultural soils using LIBS
(2018)
Over the last few years, there has been a growing interest to apply spectroscopic methods to the agricultural field for better understanding of soil properties and for efficient, sustainable management of arable land. Within the project I4S (intelligence for soil), funded by the BMBF, an integrated system for site-specific soil fertility management is developed, consisting of different sensors like X-Ray fluorescence analysis (XRF), near-infrared spectroscopy (NIR) and laser-induced breakdown spectroscopy (LIBS). LIBS provides a fast and simultaneous multi-element analysis with little to no sample preparation, which makes it a suitable method for real-time analysis on the field.
The quantification of macro and micro nutrients in soils with LIBS is challenging due to matrix effects, different levels of moisture content and varying grain sizes. First studies revealed that the problems with matrix effects can be overcome by using well characterised soils as reference materials and chemometric tools like Partial Least Squares Regression(PLSR) for calibration.
The next step was to investigate the influence of moisture and grain sizes on the LIBS signal, which is a big issue when measuring directly on the field. The results showed that the LIBS signal decreases exponentially with increasing moisture content, as most of the laser energy is used for vaporising the water. With moisture contents of 30 % or higher almost no signal can be detected. This decrease is more severe for sandy soils than for clay soils. First tests of different grain size distributions indicate that the variation of the LIBS signal increases with growing amounts of larger grains. This results in a higher standard deviation, because of a poorer reproducibility of the plasma formation and plasma characteristic. With the help of chemometric tools the influence of moisture and grain sizes should be implemented in the calibration model for accurate analysis of nutrient composition in agricultural soils.
Two calibration-free (CF) LIBS approaches are used for the quantitative analysis of cement samples: the CF-LIBS based on the Boltzmann plot method and the Monte Carlo (MC) LIBS based on the iterative spectrum fitting. In CF-LIBS, the inverse problem is solved, i.e. the elemental concentrations are determined by the reconstruction of plasma parameters from spectra. The MC-LIBS technique solves the direct problem by finding the highest correlation between the model-generated and experimental spectrum. The accuracy of both calibration-free LIBS methods suffers from factors such as inaccurately determined instrumental function, the deviation of experimental plasma from the mathematical model used, not taking into account the collection geometry, and from the uncertainty of spectroscopic data. The both calibration-free LIBS approaches are first applied to synthetic spectra which perfectly suit the mathematical model of the method, i.e. the model of the uniform, isothermal, and stationary plasma. This test yields the accuracy of both the approaches for the ideal case. In addition, the accuracy of both the methods is investigated for non-uniform and non-isothermal plasma, because real laser-induced plasma often has high gradients in temperature and particle number densities. Finally, both calibration-free LIBS approaches are applied to experimental spectra obtained from cement samples. The figures of merits of two approaches are compared when working with both synthetic and experimental spectra.
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 the determination of the remaining life-time and the degree of damage of reinforced concrete structures such as marine construction, bridges or parking decks, a highly precise measurement of harmful species is also required for trace elements. One of the most interesting elements is chlorine, because above a certain threshold corrosion is triggered. To increase the intensity of the chlorine line, helium is usually used, which is costly. To overcome this problem, low electrical discharge reheating is used which operates in air atmosphere. A comparison between results obtained by measuring with helium and reheating by electrical discharge is presented. The performance is compared by the resulting calibration curves and the calculated limit of detection obtained by 15 reference samples based on cement with NaCl. Concentrations of reference samples range from 0.05 to 2.5 wt% chlorine.
The durability and the lifetime of reinforced concrete structures can be drastically reduced by the influence of damage processes. One of the most common causes is chloride-induced corrosion, which is triggered by increased chloride content near the reinforcement. In a study, the chloride content should be determined directly at corrosion areas. The LIBS system used consists of a micro-chip laser (3 mJ, 100 Hz, 1.5ins) and two compact spectrometers covering the wavelength range of 177-355 nm (UV) and 750-940 nm (NIR). The analysis of the chloride content was carried out via the atomic chlorine spectral line 837.59 nm, using helium for signal amplification. Calibrations were carried out for quantitative chlorine measurements with 15 reference samples in the working range of 0.05 to 6.00 wt% chlorine. The calibration of the LIBS system was done according to DIN 32 645 and was tested for linearity. The determination of the quantitative chlorine contents was carried out on samples which were broken and thus have a high surface roughness. This requires real time correction of the focus point to compensate for the roughness of the samples. The poster shows spatially resolved element distributions and determined quantitative chloride concentrations near the corrosion Areas.
Quantitative on-site analysis of harmful elements in building structures with a mobile LIBS-System
(2018)
Environmental influences and damage processes drastically reduce the durability of concrete and reinforced concrete structures. Much of this damage can be traced back to the penetration of harmful elements into the concrete. These elements cause damage processes such as the chlorine-induced corrosion, the alkali-silica reaction or the carbonization. For the maintenance the knowledge of the element concentration and the penetration depth of these harmful elements is essential. Based on this data, a maintenance concept can be developed and an estimation of the remaining service life-time can be carried out.
Typically, chemical analyzes are used to determine harmful elements in concrete, which are costly and time consuming. As an alternative method LIBS is used. On-site measurements were carried out with a mobile LIBS-System to determine harmful elements in a parking garage. A scanner is used to obtain a two-dimensional element mapping. For the quantitative analysis, a calibration of the system is carried out with 15 reference samples based on cement with NaCl with a concentration range of chlorine of 0.05 to 2.5 wt.%. To determine the penetration profile of the harmful element chlorine, concrete cores were drilled, split and analyzed directly on-site.
Detection of ion ingress by LIBS for Evaluation of the remaining lifetime of a concrete structure
(2018)
The majority of the built infrastructure is made of concrete, which is a multiphase system made of cement, aggregates, water and pores. Concrete is often used in combination with steel as reinforced concrete. Environmental influences, especially the ingress of harmful ions in combination with the ingress of water, trigger different damage processes which reduce the designed lifetime of a structure. The ingress of chlorides from de-icing salt or sea water leads to corrosion of the reinforcement. Also the carbonation of the concrete may trigger the corrosion of the reinforcement. The ingress of alkalis from de-icing salts may cause the expansion of the amorphous silica aggregates (alkali-silica reaction) through formation of a swelling gel of calcium silicate hydrate if water is present. The ingress of sulfates may cause spalling of the concrete surface due to ettringite formation.
For the standard procedure in civil engineering cores are taken, cut in slices, grinded and the obtained homogenized powder is solved in acid and investigated by standard procedures.
BAM has developed the LIBS technique for the 2D evaluation of the chemical composition of concrete [1-11]. The technique is established for automated laboratory use with high numbers of samples to investigate transport processes of harmful species (Cl-, CO2, SO42- and alkalis) in concrete. Information about ingress depth and the quantitative values are important to estimate the remaining lifetime of the infrastructure. LIBS is a surface technique. To get information about the ingress depth, a core has to be taken and cut in the middle. The measurements are carried out at the cross section. The main advantages of LIBS are the direct measurement on the surface of the concrete, fast analysis (sample rate 100 Hz) with a spatial resolution of up to 100 µm, the consideration of the heterogeneity of the concrete and the possibility of automated measurements which save a lot of manpower and time. As an example the investigation of ingress profiles for standard diffusion and migration tests in civil engineering takes hours in comparison to just a few minutes using LIBS. At the same time a 2D-evaluation provides information about hot spots of elemental concentration which may not be found by standard methods.
Ingress of chlorides due to a crack in a repair mortar. Left: Photo of the cross section of a concrete core and the surface investigated by LIBS (area 70 mm x 70 mm). Right: Color coded chlorine intensity on the cross section of a concrete core, dark red represents high chlorine content.
The state of the art of LIBS technique for applications in civil engineering will be presented, including typical results of 2D investigation of concrete in laboratory. The performance is also demonstrated by examples for onsite applications using a mobile LIBS system. The road map to standardization is presented as well.
Die Laserinduzierte Plasmaspektroskopie (engl.: Laser-induced Breakdown Spectroscopy, kurz: LIBS) ist eine Kombination aus Laserablation mittels eines energiereichen Laser-Pulses, der Erzeugung eines Plasmas auf der zu untersuchenden Oberfläche und dem quantitativen Nachweis der Elementzusammensetzung durch spektroskopische Untersuchung der vom Plasma emittierten Strahlung. Ein LIBS-Gerät liefert dem sachkundigen Planer Daten für die Bewertung des Ist-Zustandes von Bauwerken unserer Infrastruktur durch die zweidimensionale Erfassung von Elementverteilungen. Typische Anwendungen sind die Erfassung von Chlor, Schwefel, Kohlenstoff, Natrium, Kalium und Lithium in Beton oder als – Qualitätssicherung – der Nachweis der mit einem Marker versehenen Tiefenhydrophobierung.
In Zusammenarbeit zwischen Industriepartnern und der BAM wurden ein Labor und ein mobiles LIBS-System für die Vor-Ort-Messungen entwickelt. Die Analyse erfolgt vollautomatisch. Die Ortsauflösung beträgt bis zu 0,1 mm x 0,1 mm. Es können Messflächen von 140 mm x 170 mm gescannt werden. Die Erstellung von 2D-Elementverteilungen im Beispiel unten benötigt weniger als 10 Minuten.
Two calibration-free LIBS techniques are used for the quantitative analysis of synthetic cement samples: the CF-LIBS based on the Boltzmann plot method and the Monte Carlo (MC) LIBS based on the iterative spectrum fitting. In CF-LIBS, the inverse problem is solved, i.e. the elemental concentrations are determined by the reconstruction of plasma parameters from spectra. The MC-LIBS technique solves the direct problem by finding the highest correlation between the model-generated and experimental spectrum. The accuracy of both calibration-free LIBS methods suffers from factors such as inaccurately determined instrumental function, the deviation of experimental plasma from the mathematical model used, not taking into account the collection geometry and from the uncertainty of spectroscopic data. Therefore, the both calibration-free LIBS approaches are applied to synthetic spectra which perfectly suit the mathematical model of the method. This test yields the accuracy of both the approaches for the ideal case. In addition, the accuracy of both methods is investigated for non-isothermal plasma, because real laser-induced plasma often has high gradients in temperature. Both methods assume an isothermal plasma.
In respect of an efficient cultivation of agricultural cropland, a site-specific fertility management is necessary. Therefore, affordable and extensive mapping methods are needed. For this purpose, the research project I4S (intelligence for soil) has the goal to develop an integrated system. This system includes a sensor platform, which contains different sensors, like XRF, VIS-NIR, Gamma and LIBS.
LIBS (laser-induced breakdown spectroscopy) is known as a fast and simultaneous multi-element analysis with little or no sample preparation. The main task of LIBS measurements in this project is the real time determination of the elemental contents of nutrients in soils, like calcium, magnesium, potassium. For this purpose, a special setup has been designed. The sample uptake operates with the help of a rotatable sample plate which circulates with different velocities to simulate the application on the field. To provide a higher intensity and a better reproducibility of the obtained signal, a double-pulse Nd:YAG laser (1064 nm) was used. In order to minimize dust formation from the soil during the operation of the laser, a dust removal by suction has been integrated.[1] When using relative methods such as LIBS, a suitable calibration curve is needed for absolute quantification. The complex matrix of soils, as well as the influence of moisture and grain size in soils makes the absolute quantification by LIBS challenging. To overcome these influences, chemometric methods were used. With the principal component analysis (PCA) a classification of soils into different soil types was performed and a calibration curve based on partial least squares regression (PLSR) was generated. With this calibration model’s elemental distribution maps for different German agricultural fields were created.
In respect of an efficient cultivation of agricultural cropland, a site-specific fertility management is necessary. Therefore, affordable and extensive mapping methods are needed. The research projects I4S (intelligence for soil) has the goal to develop a system for this purpose. I4S is one of ten interdisciplinary research project associations of the innovation programme called BonaRes, which is funded by the German Federal Ministry of Education and Research (BMBF).
The system includes a sensor platform, which contains different sensors, like XRF, VIS-NIR, Gamma and LIBS. The main task of LIBS measurements in this project is the real-time determination of the elemental contents of major and minor nutrients in soils, like calcium, magnesium, potassium. LIBS (laser-induced breakdown spectroscopy) is known as a fast and simultaneous multi-element analysis with little or no sample preparation. The main task of LIBS measurements in this project is the real-time determination of the elemental contents of nutrients in soils, like calcium, magnesium, potassium. For this purpose, a special setup has been designed. The sample uptake operates with the help of a rotatable sample plate which circulates with different velocities to simulate the application on the field. To provide a higher intensity and a better reproducibility of the obtained signal, a double-pulse Nd:YAG laser (1064 nm)was used. In order to minimize dust formation from the soil during the operation of the laser, a dust removal by suction has been integrated. When using relative methods such as LIBS, a suitable calibration curve is needed for absolute quantification. The complex matrix of soils, as well as the influence of moisture and grain size in soils makes the absolute quantification by LIBS challenging. To overcome these influences, chemometric methods were used. With the principal component analysis (PCA) a classification of soils into different soil types was performed and a calibration curve based on partial least squares regression (PLSR) was generated. With this calibration model’s elemental distribution maps for different German agricultural fields were created.
Die Probenpräparation ist ein Teil im Prozess der Baustoffuntersuchung und muss dementsprechend sorgfältig geplant werden. Die Probenpräparation ist letztendlich der Grundstein für alle folgenden Untersuchungen, somit wirken sich alle hier getroffenen Entscheidungen direkt auf die Qualität der Ergebnisse aus.
Introduction: Laser Induced Breakdown Spectroscopy (LIBS) is an atomic emission analytical technique, wide spreading in laboratories and industries. One way to dramatically increase its analytical results is to deposit metal NPs on the sample surface, resulting in an better version called Nanoparticle Enhanced LIBS (NELIBS). In order to better know and use this technique, the evolution of the plasma has been studied with Tomography.
In order to determine the remaining life-time of reinforced concrete infrastructure like marine constructions, bridges or parking decks a fast and reliable detection of harmful species is necessary. If, as an example chlorine exceeds a specific concentration threshold, the result can be pitting corrosion of the reinforcement, which affects the stability and lifetime of building structure. In cooperation with industrial partners and BAM a mobile LIBS system for the on-site measurements has been developed. The mobile system uses a low energy laser with a pulse energy of 3 mJ and a NIR-spectrometer in combination with a scanner for two-dimensional mapping (140 x 170mm).
This work presents the mobile LIBS system, the calibration and an on-site applications in a parking garage. A correlation between surface measurements on a parking deck and ingress profiles obtained due to concrete cores are presented. LIBS results using the mobile LIBS system are able to provide in-time information about the Cl concentration and can therefore be used for quality assurance during reinstatement work.
Die Gehalte von für das Pflanzenwachstum relevanten Nährstoffen divergieren innerhalb einer landwirtschaftlichen Nutzfläche sehr stark. Für eine ertragssteigernde Bewirtschaftung ist daher eine gezielte Düngung unabdingbar. Die Nachfrage nach einer kostengünstigen, flächendeckenden Kartierung von Ackerflächen in Bezug auf die im Boden enthaltenen Nährstoffe steigt demnach. Hierzu werden schnelle, mobil einsetzbare und verlässliche Messmethoden benötigt.
Eine geeignete Methode stellt die laserinduzierte Plasmaspektroskopie (kurz: LIBS aus dem Englischen für laser-induced breakdown spectroscopy) dar. Die LIBS ermöglicht eine schnelle und simultane Multielementanalyse und ist dabei nahezu zerstörungsfrei. Des Weiteren bedarf es für die Analyse kaum bis keine Probenvorbereitung was den Einsatz auf dem Feld begünstigt. Die Methode beruht auf der Ionisierung des Probenmaterials (Plasmabildung) durch den Beschuss dieser mit kurzen Laserpulsen. Während der Expansion des Plasmas wird Strahlung emittiert, welche charakteristisch für die in der Probe enthaltenen Elemente ist.
Das hier verwendete LIBS-System verfügt über einen Doppelpuls-Laser, bei dem zwei Laserpulse im kurzen Abstand hintereinander ausgesendet werden. Der erste Laserpuls ist für die “Vorbehandlung“ der Probe und der zweite Laserpuls für die Erwärmung des Plasmas. Dadurch können die Nachweisgrenzen im Vergleich zu einem einfach gepulsten Laser deutlich verbessert werden.
Die Probenzufuhr findet durch einen drehbaren Probenteller statt auf dem der lose Boden in Form einer Spur aufgetragen wird. Der Probenteller kann in verschiedenen Geschwindigkeiten betrieben werden, um die Anwendung auf dem Feld zu simulieren.
Da es sich bei der LIBS um eine Relativmethode handelt, ist eine Kalibrierung mit exakten Referenzwerten von verschiedenen Bodenproben notwendig. Böden weisen eine sehr komplexe Matrix auf, was die verlässliche Analyse mittels LIBS erschwert. Daher ist zunächst das Ziel die verschiedenen Messparameter des LIBS-Systems für unterschiedliche Böden zu optimieren und mögliche Störeffekte, wie Korngrößen und Feuchtigkeitsgrad, durch eine geeignete Probenkonditionierung zu eliminieren.
LIBS (laser-induced breakdown spectroscopy) is known as a fast and simultaneous multi-element analysis with little or no sample preparation. In the last few years there has been a growing interest in applications of LIBS in the field of agriculture. As part of the National Research Strategy BioEconomy 2030 the German Federal Ministry of Education and Research (BMBF) started an innovation programme called BonaRes. BonaRes consists of ten interdisciplinary research project associations which are dealing with soil as a sustainable resource for the bio-economy. One of these research projects is I4S (intelligence for soil) which has the goal to develop an integrated system for site-specific soil fertility management. This system includes a sensor platform, which contains different sensors, like XRF, VIS-NIR, Gamma and LIBS. The main task of LIBS measurements in this project is the real time determination of the elemental contents of nutrients in soils, like calcium, magnesium, potassium. For this purpose, a special setup has been designed. The sample uptake operates with the help of a rotatable sample plate which circulates with different velocities to simulate the application on the field. To provide a higher intensity and a better reproducibility of the obtained signal, a double-pulse Nd:YAG laser was used. In order to minimize dust formation from the soil during the operation of the laser, a dust removal by suction has been integrated. When using relative methods such as LIBS, a suitable calibration curve is needed for absolute quantification. With the help of 16 certified reference soils, calibration curves for different elements were initially calculated and used for the quantification of seven soil samples from different testing grounds in Germany. The complex matrix of soils, as well as the influence of moisture and grain size in soils makes the absolute quantification by LIBS challenging. To overcome these influences, a calibration curve based on multivariate analysis (partial least square regression) was generated.
LIBS (laserinduzierte Plasmaspektroskopie) ist bekannt für eine schnelle, simultane Multielementanalyse, welche kaum bis keine Probenvorbereitung benötigt. Im Hinblick dessen ist das Interesse an LIBS als Online-Analysentechnik für den Einsatz auf Agrarflächen in den letzten Jahren stark gestiegen.
Im Rahmen des Projekts I4S (engl. für: intelligence for soil) soll mithilfe von LIBS der Elementgehalt von Makro- und Mikronährstoffen in Böden in Echtzeit bestimmt werden. I4S gehört zu den zehn interdisziplinären Forschungsprojekten des Innovationsprogramms BonaRes und arbeitet an der Entwicklung einer Sensorplattform für ein ortsspezifisches Management der Bodenfruchtbarkeit. BonaRes ist vom Bundesministerium für Bildung und Forschung (BMBF) gefördert und beschäftigt sich mit Boden als nachhaltige Ressource für die Bioökonomie.
Für die Anwendung von LIBS an Böden wurde ein spezieller Aufbau konstruiert. Die Probenzufuhr erfolgt über einen drehbaren Probenteller, auf den der lose Boden in Form einer Spur aufgetragen oder als gepresste Tablette platziert werden kann. Der Probenteller kann in unterschiedlichen Geschwindigkeitsstufen betrieben werden, um die Anwendung auf dem Feld zu simulieren. Für eine höhere Signalintensität und eine bessere Reproduzierbarkeit wird eine Doppel-Puls (DP) Nd:YAG Lasereinheit verwendet. Weiterhin besitzt der LIBS-Messkopf eine integrierte Absaugvorrichtung, um bei dem Ablationsprozess entstehende Stäube zu minimieren und eine störungsfreie Detektion des Signals zu gewährleisten.
Da es sich bei LIBS um eine Relativmethode handelt, muss das System für eine absolute Quantifizierung hinsichtlich der Zielanalyten kalibriert werden. Hierfür werden realitätsnahe Proben benötigt. Es wurden 16 freiverkäufliche, zertifizierte Referenzmaterialien gemessen und für die Kalibrierung verwendet. Aufgrund der komplexen Bodenmatrix und Einflüsse, wie Korngrößeneffekte und Feuchtigkeit, stellt die absolute Quantifizierung mittels LIBS eine Herausforderung dar. Für die Kalibrierung wurde sowohl ein univariater als auch ein multivariater Analyseansatz (Partial least square regression) verwendet. Mittels der multivariaten Auswertung konnte ein robusteres Kalibriermodell aufgrund einer besseren Korrelation zwischen Signalintensität und Elementkonzentration erstellt werden.
Innerhalb des I4S-Verbundes gibt es eine Vielzahl von Bodenproben von unterschiedlichen Testackerflächen mit bereits bekannten Informationen über die chemische Zusammensetzung, Textur und Korngrößenverteilung. Diese Bodenproben sollen im nächsten Schritt für weitere Berechnungen und zu Validierungszwecken verwendet werden.
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.
One important aim of precision agriculture (PA) is the optimization of crop growth by means of site-specific measures, e.g. fertilizer application. Thus, PA should contribute to a resource efficient and ecofriendly soil management. Due to the expenses associated with traditional methods of soil analysis, requiring sample collection and laboratory analysis, PA technologies are still not in widespread use. Therefore, the aim of the project “I4S – Integrated System for Site-Specific Soil Fertility Management” is the development of a field-deployed, sensor-based analysis system offering rapid, cost-effective and spatially-resolved fertilizer recommendations.
In this system, laser-induced breakdown spectroscopy (LIBS) could be ideally suited to assessing elemental nutrient contents of soils. In addition to low cost, durability and small size, a reliable quantification procedure is a crucial requirement for such a system. However, the texture as well as the composition of the soil can affect the spectra. This matrix dependence is the key challenge to be addressed in the application of LIBS for soil evaluation. The focus of this work was the establishment of a LIBS method for soil analysis in a laboratory environment for future field application.
Natural soil samples of various origins, textures and compositions were used to characterize the matrix dependence of the LIBS spectra. Reference samples were prepared by adding defined amounts of the target elements to the soils (standard addition). Signals not superimposed by peaks of other elements were identified for each element. The reference samples also provided calibration curves for the respective soil type when the initial concentrations in the soils were taken into account. Additionally, the common laboratory method ICP-OES following aqua regia extraction was used to obtain reference values. Various approaches of calibration free evaluation of the data were also evaluated. In addition to traditional single-pulse experiments, the advantages of dual-pulse LIBS in relation to signal intensity, reproducibility as well as overcoming the matrix dependence of soil spectra were investigated. These methods were subsequently applied to validation samples collected on a dense grid within a field.
LIBS (laser-induced breakdown spectroscopy) is known as a fast and simultaneous multi-element analysis with little or no sample preparation. Because of that, over the last few years there has been a growing interest in applications of LIBS in the field of agriculture.
As part of the National Research Strategy BioEconomy 2030 the German Federal Ministry of Education and Research (BMBF) started an innovation programme called BonaRes. BonaRes consists of ten interdisciplinary research project as-sociations which are dealing with soil as a sustainable resource for the bio-economy. One of these research projects is I4S (intelligence for soil) which has the goal to develop an integrated system for site-specific soil fertility management. This system includes a sensor platform, which contains different sensors, like XRF, VIS-NIR, Gamma and LIBS. The main task of LIBS measurements in this project is the real time determination of the elemental contents of major and minor nutrients in soils, like calcium, magnesium, potassium. For this purpose, a special setup has been designed. The sample uptake operates with the help of a rotatable sample plate, on which the loose soil sample can be placed in form of a track. The sample plate circulates with different velocities to simulate the application on the field. To provide a higher intensity and a better re-producibility of the obtained signal, a double-pulse Nd:YAG laser (1064 nm) was used. In order to minimize dust formation from the soil during the operation of the laser, a dust removal by suction has been integrated When using relative methods such as LIBS, a suitable calibration curve is needed for absolute quantification. Within the I4S project a large number of soil samples from different testing grounds with known data of chemical composition, texture etc. is available. For the first calibration curves seven soil samples from different grounds in Germany were prepared as reference materials and four certified reference materials from China and Canada were purchased. With the help of these reference materials, calibration curves for different elements were initially calculated based on internal standard addi-tion. Copper was used as internal standard because of its low concentration in soils. The complex matrix of soils, as well as the influence of moisture and grain size in soils makes the absolute quantification by LIBS challenging. To overcome these influences, a calibration curve based on multivariate analysis was generated. Therefore, baseline correction on the second derivative with a Savitzky-Golay filter was followed by a partial least squares regression (PLSR). Multivariate analysis leads to noise reduction and neglection of interfering signals. Therefore, a better correlation between signal intensity and nutrient concentration is observed and a robust calibration curve is obtained.
Time- and space-resolved in situ LIBS measurements of chemical compositions during TIG-welding
(2017)
An in situ monitoring of chemical compositions in the weld pool and the heat affected zone (HAZ) can enable the control of the welding process through the regulation of the welding parameters, and thus can prevent possible weld defects.
The most critical parameter for hot cracking -from a metallurgical point of view- is the chemical composition of the weld pool.
Chemical composition can be measured and quantified during the welding process with the LIBS technique having the appropriate calibration measurements.
The main task of BAM in the I4S project is the development of online analytics for reliable monitoring of major and minor nutrients like K, P, Ca in soil using mobile-XRF (X-ray fluorescence spectroscopy) and mobile-LIBS (laser-induced breakdown spectroscopy). Each method provides fast and simultaneous multi-element analyses which are neccessary for mapping agricultural cropland. The first objective was to design a combined measurement setup for XRF and LIBS. The centerpiece is the rotatable sample plate which enables the measurement of moving samples. Current steps are the optimisation of the parameters for the analysis of soils.
Production of soil reference materials (for project partners) is integrated into the project „X-ray fluorescence, laser-induced breakdown spectroscopy (and Raman spectroscopy) as tools for a site-specific management of soil fertility“. BAM provides good analytics which is necessary for measuring the right values. Why is it important to have reference values? They enable the connection of own measuring results to the SI (international system of units) and the comparison with results from other laboratories. The soil reference materials are also needed for the calibration of the matrix dependent methods laser-induced breakdown spectroscopy (LIBS) and X-ray fluorescence analysis (XRF). For the production of soil reference materials different steps are required, for example drying, sieving and homogenisation.
The main task of BAM in the I4S project is the development of online analytics for reliable monitoring of major and minor nutrients like K, P, Ca in soil using mobile-XRF (X-ray fluorescence spectroscopy) and mobile-LIBS (laser-induced breakdown spectroscopy). Each method provides fast and simultaneous multi-element analyses which are neccessary for mapping agricultural cropland. The first objective was to design a combined measurement setup for XRF and LIBS. The centerpiece is the rotatable sample plate which enables the measurement of moving samples. Current steps are the optimisation of the parameters for the analysis of soils.
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.
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.
A laser induced breakdown spectroscopy (LIBS) system was combined with the Gas Tungsten Arc (GTA) Welding process for the in situ measurement of chemical compositions in austenitic stainless steels during welding. The purpose of this project is to develop a prototype of an online chemical composition control system for welding applications. One of the use cases of this new LIBS-based measurement system can be the controlling of potentially dangerous emissions during the welding process in order to improve the working safety conditions. Further, the control of the weld pool composition allows governing the weld pool solidification behavior and thus allows reducing the resultant susceptibility for solidification cracking. The composition of the weld seam is from the metallurgical point of view the most critical parameter for the occurrence of weld defects1. Conventional inspection methods for weld seams as e.g. ultrasonic inspection are ex situ and cannot prevent the defects. For that reason automated in situ analysis systems for weld defect detection are much more efficient2. Beyond that, the proposed LIBS-based analysis system allows to study the correlation between the occurrence of weld defects and the chemical composition of the weld pool or the region where solid and liquid phase coexist.
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).