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New possibilities for concrete analysis 4.0 with the Laser-Induced Breakdown Spectroscopy (LIBS)
(2020)
In civil engineering the damage assessment of concrete infrastructures is an important task to monitor and ensure the estimated life-time. The aging of concrete is caused by different damage processes like the chloride induced pitting corrosion of the reinforcement. The penetration depth and the concentration of harmful species are crucial factors in the damage assessment. As a highly cost and time-consuming standard procedure, the analysis of concrete drill cores or drilling by wet-chemistry is widely used. This method provides element concentration to the total mass as aggregates and binder are homogenized. In order to provide a method that is capable to detect the element concentration regarding the cement content only, the laser-induced breakdown spectroscopy (LIBS) will be presented. The LIBS method uses a focused pulsed laser on the sample surface to ablate material. The high-power density and the laser-material interaction causes a laser-induced plasma that emits elemental and molecular line emission due to energy transition of the excited species in the plasma during the cooling phase.
As each element provides element-specific line emission, it is in principle possible to detect any element on the periodic table (spectroscopic fingerprint) with one laser shot. In combination with a translation stage the sample under investigation can be spatially resolved using a scan raster with a resolution up to 100 µm (element mapping). Due to the high spatial resolution, the element distribution and the heterogeneity of the concrete can be evaluated. By using chemometrics the non-relevant aggregates can be excluded from the data set and the element concentration can be quantified and referred to a specific solid phase like the binding matrix (cement) only. In order to analyze transport processes like diffusion and migration the twodimensional element distributions can provide deep insight into the transport through the pore space and local enrichments of elements. As LIBS is a multi-elemental method it is also possible to compare the ingress and transport process of different elements like Cl, Na, K, S, C, and Li simultaneously and evaluate cross-correlations between the different ions. Furthermore, the element mapping allows to visualize the transport along cracks. This work will show the state of the art in terms of hardware and software for an automated LIBS system as well as different application for a concrete analysis 4.0. Focus will be the application of LIBS for a fast concrete analysis.
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.
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.
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.
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.