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Sulfate attack constitutes a major risk of chemical aggression for concrete and other building materials. Mainly structures exposed to sewage waters and sulfate rich soils are affected. The measurement of sulfate ingress is important for damage assessment previous to repair works and for quality assurance of the effectiveness of protecting coatings or during concrete removal.
Laser induced breakdown spectroscopy is introduced for the quantitative measurement of sulfur contents in building materials, providing detailed depth profiles and imaging element distributions with 1 mm spatial resolution. Detailed statistical evaluation is possible due to the spatial resolution and the fast measurement of the method. Practical applications of the method for damage assessment are demonstrated for three different sewage plants.
Raman and laser-induced breakdown spectroscopy is integrated into a single system for molecular and elemental microanalyses. Both analyses are performed on the same ~ 0.002 mm² sample spot allowing the assessment of sample heterogeneity on a micrometric scale through mapping and scanning. The core of the spectrometer system is a novel high resolution dual arm Echelle spectrograph utilized for both techniques. In contrast to scanning Raman spectroscopy systems, the Echelle-Raman spectrograph provides a high resolution spectrum in a broad spectral range of 200-6000 cm- 1 without moving the dispersive element. The system displays comparable or better sensitivity and spectral resolution in comparison to a state-of-the-art scanning Raman microscope and allows short analysis times for both Raman and laser induced breakdown spectroscopy. The laser-induced breakdown spectroscopy performance of the system is characterized by ppm detection limits, high spectral resolving power (15,000), and broad spectral range (290-945 nm). The capability of the system is demonstrated with the mapping of heterogeneous mineral samples and layer by layer analysis of pigments revealing the advantages of combining the techniques in a single unified set-up.
The determination of water and salt distribution in brickwork and stonework is a frequent problem in cultural heritage protection. Conventional but reliable methods are usually based on the investigation of core samples and provide only punctual information. To avoid the destruction of valuable building fabric, a non-destructive approach using a combination of investigation methods is needed. A case study, demonstrating the possibilities of almost non-destructive testing methods and their combination is presented in this article.
Laser-induced breakdown spectroscopy has been employed for the investigation of the sulfur and chlorine content of building materials. Both, chloride and sulfate ions are major damaging species affecting the stability and lifetime of a structure. Chlorine and sulfur are mostly detected in the VUV and the NIR. In case of building materials the main elements like calcium or iron have many strong spectral lines over the whole spectral range, so that trace elements can only be detected in spectral windows unaffected from these lines. With regard to a preferably simply, robust against dust and vibrations and portable setup only the NIR spectral features are used for civil engineering applications.
Most detectors, mainly CCD cameras have rapidly decreasing quantum efficiency in the NIR. Also the quantum efficiency of the photocathode of CCD-Detectors with image intensifier is decreasing in the NIR. Different CCD-detectors were tested with respect to high quantum efficiency and high dynamic range, which is necessary for simultaneous detection of weak spectral lines from trace elements and intense spectral lines from main elements.
The measurements are made on reference samples consisting of cement, hydrated cement, cement mortar and concrete with well-defined amounts of the trace elements. Experimental conditions are chosen for an optimum intensity of the trace element spectral lines. The detector systems are compared by limit of detections and the signal to noise ratio.