Determination of Water-Cement Ratio in Hardened Concrete by Optical Fluorescence Microscopy


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Title: Determination of Water-Cement Ratio in Hardened Concrete by Optical Fluorescence Microscopy

Author(s): U. H. Jakobsen, P. Laugesen, and N. Thaulow

Publication: Special Publication

Volume: 191


Appears on pages(s): 27-42

Keywords: field tests; fluorescence; microscopy; vacuum forming; water-cement ratio

Date: 12/1/1999

This paper describes a method for determining the water to cement ratio (w/c) of hardened concrete using optical fluorescence microscopy. The method is well established and has been used for many years. In Denmark the method is used for quality control of hardened concrete. The method is based on vacuum impregnation of concrete using a yellow fluorescent epoxy. During impregnation the capillary porosity, cracks, voids, and defects in the concrete are filled with epoxy. The amount of fluorescent dye entering the cement paste depends on the capillary porosity, which is determined by the w/c and the degree of hydration. After impregnation and hardening of the epoxy a thin section of concrete with a thickness of 0.020 mm (20 µm) is prepared. The thin section is analyzed under an optical microscope using a combination of a blue excitation filter and a yellow blocking filter. This is the fluorescent light mode in which epoxy filling air voids and cracks appears yellow, cement paste as shades of green, and aggregate black. The shade of green of the cement paste depends on the capillary porosity. A sample with low w/c appears dark green, i.e. has less fluorescence intensity due to a low amount of epoxy within the paste. A sample with high w/c appears light green, i.e. has high fluorescence intensity. These shades of green (fluorescence intensity) are used to determine the w/c by comparing the fluorescence intensity of the cement paste with the standards of known w/c. This paper describes the fluorescent impregnation technique, the thin section preparation, the visual determination of w/c and discusses the pitfalls in the w/c determination. Furthermore, the paper presents data from a quality assurance project and damage analysis and data of Round Robin Testing.