Bei der Erhaltung von Stahlbetonbauwerken ist der Chloridgehalt im Beton ein wichtiger Parameter zur Festlegung geeigneter Instandsetzungsmaßnahmen. Die bisher etablierten Verfahren basieren auf der nasschemischen Analyse einer Probe, Bohrmehl oder aufgemahlener Segmente eines Bohrkerns. Das Verfahren liefert einen Chloridgehalt je Tiefensegment, was mit einer starken Homogenisierung der Probe einhergeht, wodurch detaillierte Informationen zu Chloridunterschieden im Millimeterbereich verloren gehen. Eine alternative Methode ist die laserinduzierte Plasmaspektroskopie (LIBS), die nicht nur die quantitative ortsaufgelöste Bestimmung des Chloridgehalts im Bezug zur Zementmasse ermöglicht, sondern auch simultan detaillierte Informationen über die Verteilung vieler anderer chemischer Elemente liefert. Die räumliche Auflösung liegt in der Regel im Bereich von einigen Millimetern, kann aber bei Bedarf auf 0,1 mm oder weniger erhöht werden. Das neue Merkblatt B14 „Quantifizierung von Chlorid in Beton mit der laserinduzierten Plasmaspektroskopie (LIBS)“ der Deutschen Gesellschaft für Zerstörungsfreie Prüfungen (DGZfP) regelt und vereinheitlicht erstmals die zuverlässige und reproduzierbare Ermittlung des Chloridgehalts mit LIBS. In diesem Beitrag wird das Merkblatt vorgestellt und anhand praktischer Anwendungsbeispiele das Potenzial von LIBS für Fragestellungen in der Betonerhaltung verdeutlicht.
The composition of concrete determines its resistance to various degradation mechanisms such as ingress of ions, carbonation or reinforcement corrosion. Knowledge of the composition of the hardened concrete is therefore helpful to assess the remaining service life of an existing structure or evaluate the damage observed during inspections. For example, for most existing concrete structures the type of cement originally used is not known and must therefore be determined afterwards. This paper presents a preliminary study on the application of laser-induced breakdown spectroscopy (LIBS) to identify the type of cement. For this purpose, ten different types of cement were investigated. For every type, three cement paste prisms were produced: (i) prisms dried, ground and pressed into tablets, (ii) prisms dried and (iii) prisms untreated. LIBS measurements were performed with a diode-pumped low energy laser (1064 nm, 3 mJ, 1.5 ns, 100 Hz) in combination with two compact spectrometers which cover the UV and NIR spectral range. A reduced subset of spectral features was used to build a classification model based on linear discriminant analysis. The results show that the classification of homogenized pressed cement powder samples provides a high accuracy, however, factors such as a different sample matrix and moisture content can affect the accuracy of the classification. The study demonstrates that LIBS is a promising tool to identify the type of cement.
Die chemische Analyse von Beton gehört bei der Ermittlung des Ist-Zustandes von Stahl- und Spannbetonbauwerken zu den grundlegenden Untersuchungen bei der Zustandserfassung. Der tiefenabhängige Chloridgehalt sowie der Karbonatisierungsfortschritt müssen bei der Bewertung des Ist-Zustandes, beim Instandsetzungsbedarf und ggf. bei der Planung eines erforderlichen Instandsetzungskonzeptes mit einbezogen werden. Seit Mitte der 1990er Jahre wird an der Bundesanstalt für Materialforschung und -prüfung die laserinduzierte Plasmaspektroskopie (engl. Laser-induced Breakdown Spectroscopy) – kurz LIBS – stetig für die chemische Analyse von Beton weiterentwickelt. In diesem Beitrag wird der aktuelle Stand der Forschung/ Technik und der Normungsarbeit präsentiert, sowie zukünftig mögliche Anwendungen vorgestellt. Mit den aktuellen LIBS-Systemen (mobiles LIBS-System sowie kommerziell erhältliche Laborsysteme) ist es möglich den Chloridgehalt ortsaufgelöst in wenigen Minuten zu quantifizieren, sogenannte Elementlandkarten zu erstellen und somit die Elementverteilung innerhalb des Betons zu visualisieren. Durch das Scannen eines Bohrkernquerschnittes und der simultanen Erfassung von mehreren Elementen mit einer Messung, ist es zudem möglich, die Gesteinskörnung in den Messdaten zu identifizieren und den Chloridgehalt auf die Zementsteinmatrix zu beziehen. Fehlerquellen aus der Umrechnung der betonbezogenen Chloridgehalte können dadurch minimiert werden. Neben den Grundlagen von LIBS werden vor allem aktuelle Praxisbeispiele vorgestellt, die die aktuellen Möglichkeiten des Verfahrens veranschaulichen.
This paper will address the relevance of chloride measurements in practice for durability condition assessment and a description of the working principle of LIBS will be given. Examples of chloride profiles obtained by the common wet chemical analysis will be shown and compared with results from LIBS measurements as to demonstrate the accuracy of the LIBS-technique. The latter will be supported by clear graphs. In addition examples on the extra information resulting from measurements performed with LIBS for improvement of condition assessment and prediction will be shown.
The presented work discusses the accuracy of Laser Induced Breakdown Spectroscopy (LIBS) in determining the total chloride content in cement pastes. LIBS as an emission spectroscopy method is used to detect simultaneously several elements present in cement-based materials. By scanning surfaces the variability in the spatial distribution of elements can be visualised. However, for a quantification of the results, studies are necessary to characterise possible influences due to the wide variation of the chemical compositions in which cement can occur. It is shown how the calibration can be done, how the calibration samples were produced, and which statistical parameters are necessary to describe the precision of the regression. The performance of LIBS is estimated by detecting chloride in validation samples. Therefore, 55 samples and 7 ets with changing mix ompositions were produced. The presented study deals with possible influences of different mix compositions, ncluding different cations of chloride, varying w/c-ratios and the artial replacement of Portland cement with last furnace slag (50% BFS) and limestone (30% LS). Comparing the LIBS results with otentiometric titration, n accuracy of±0.05 wt%/total has been determined.
A big part of the 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. In addition, the carbonation of the concrete may trigger the corrosion of the reinforcement. The ingress of alkalis from deicing 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 sulphates 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. The technique is established for automated laboratory use with high numbers of samples to investigate transport processes of harmful species (Cl-, CO2 , SO4 2-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 drill 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.
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.
The determination of chloride is still one of the main tasks for the evaluation of reinforced concrete structures.
The corrosion of the reinforcement induced by the penetrating chlorides is the dominant damage process affecting the lifetime of concrete structures. In the recent years different research groups demonstrated that LIBS can be a fast and reliable method to quantify chlorine in cement-bound materials. Because chlorine in concrete can only occur as solved ions in the pore solution or bound in salts or hydrated cement phases, the detected emission of chlorine can be correlated with the chloride concentration determined e.g. with potentiometric titration. This work inter alia describes the production of reference samples and possible side effects during the production process. Due to transport processes in the porous matrix of the cement a misinterpretation of the concentrations is possible. It is shown how to overcome these effects and higher precisions of the single measurements can be realised. Using the calibration method, blank sample method and noise method, three different ways of calculating the limit of detection (LOD) and limit of quantification (LOQ) are compared. Due to the preparation of the reference samples a precision of the whole calibration model of sx0 = 0.023 wt% is determined.
The validation of the model is based on different test sets, which are varying in their composition
(different Cl-salts, water-to-cement ratios and additives). The determined mean error of the validation is
0.595 ± 0.063 wt%, which is comparable to standardised methods like potentiometric titration, direct potentiometry
or photometry (0.40 ± 0.06 wt%) [1].
In den letzten Jahren sind im deutschen Bundesautobahnnetz vermehrt AKR-Schäden an Betonfahrbahndecken aufgetreten. Ursächlich hierfür ist die Verwendung alkaliempfindlicher Gesteinskörnung. Dies führt bei gleichzeitiger Anwesenheit von Wasser infolge der Exposition der Fahrbahndecke und des alkalischen Milieus durch den Einsatz alkalireicher Portlandzemente bei der Betonherstellung zu einer Alkali-Kieselsäure-Reaktion (AKR). Zusätzlich wird der AKR-Schädigungsprozess in Betonfahrbahndecken durch den externen Tausalzeintrag (primär NaCl) im Winter begünstigt. Vor diesem Hintergrund kommt der Ermittlung des Tausalzeintrags in den Fahrbahndeckenbeton eine große Bedeutung zu.
Die Analyse des Tausalzeintrags erfolgte bisher ausschließlich nasschemisch an gemahlenen Bohrkernsegmenten. Nachteilig sind hierbei der relativ hohe prüftechnische Aufwand und die eingeschränkte Ortsauflösung. Der alternative Einsatz von LIBS (Laser-induced breakdown spectroscopy) eröffnet in diesem Kontext neue Möglichkeiten. So wird in diesem Beitrag anhand von Bohrkernen aus einem repräsentativen AKR-geschädigten BAB-Abschnitt exemplarisch die Vorgehensweise bei der LIBS-Analyse zur Ermittlung der Na- und Cl-Verteilung an vertikalen Schnittflächen von Bohrkernen aufgezeigt. Da der Tausalzeintrag primär über den Zementstein erfolgt, wurde der verfälschende Na-Grundgehalt der Gesteinskörnung mittels eines Ausschlusskriteriums (Ca-Gehalt) eliminiert. Vergleichend durchgeführte Cl-Mappings mit Mikroröntgenfluoreszenzanalyse sowie nasschemische Analysen belegen die Güte der durchgeführten LIBS-Messungen. Allerdings bedarf es bei der quantitativen Ermittlung des Natriumgehaltes in der Betonfahrbahndecke noch weitergehender Untersuchungen.
The Laser-Induced Breakdown Spectroscopy (LIBS) is a laser spectroscopic method which allows a time efficient, minor-destructive, chemical analysis of materials. In principle all elements on the periodic table can be simultaneously analysed by using LIBS, regardless of the state of aggregation. LIBS offers numerous applications in the field of civil engineering; most importantly the analysis of building materials. This work will focus on the evaluation of concrete structures and harmful substances which can penetrate the concrete. A variety of information can be collected through LIBS. Determining the concentration of harmful substances like chloride, sodium or sulphur, the examination of the carbonation depth and the distinction between varying layers of materials (e.g. aggregates, cement paste, metals, etc.) are possible applications. All this information can be provided through one LIBS measurement in the form of a high resolution 2D element map, with resolutions up to 0.1 mm x 0.1 mm. To scan a concrete surface only an optical access is needed. To create a depth profile of an intruding substance the extraction of a drill core is necessary. Onsite measurements via LIBS can be conducted by using a mobile version of the LIBS system. Through using calibration curves LIBS allows not only the qualitative but also quantitative analysis of element concentrations. All those prospects make LIBS a trendsetting method to secure the integrity of infrastructures in a sustainable manner. A variety of examples which show the performance and results of LIBS analysis will be presented.
2D evaluation of spectral LIBS data derived from heterogeneous materials using cluster algorithm
(2017)
Laser-induced Breakdown Spectroscopy (LIBS) is capable of providing spatially resolved element maps in regard to the chemical composition of the sample. The evaluation of heterogeneous materials is often a challenging task, especially in the case of phase boundaries. In order to determine information about a certain phase of a material, the need for a method that offers an objective evaluation is necessary. This paper will introduce a cluster algorithm in the case of heterogeneous building materials (concrete) to separate the spectral information of non-relevant aggregates and cement matrix. In civil engineering, the information about the quantitative ingress of harmful species like Cl−, Na+ and SO2−4 is of great interest in the evaluation of the remaining lifetime of structures (Millar et al., 2015; Wilsch et al., 2005). These species trigger different damage processes such as the alkali-silica reaction (ASR) or the chloride-induced corrosion of the reinforcement. Therefore, a discrimination between the different phases, mainly cement matrix and aggregates, is highly important (Weritz et al., 2006). For the 2D evaluation, the expectation-maximizationalgorithm (EM algorithm; Ester and Sander, 2000) has been tested for the application presented in this work. The method has been introduced and different figures of merit have been presented according to recommendations given in Haddad et al. (2014). Advantages of this method will be highlighted. After phase separation, non-relevant information can be excluded and only the wanted phase displayed. Using a set of samples with known and unknown composition, the EM-clustering method has been validated regarding to Gustavo González and Ángeles Herrador (2007).