Microcrack Study of Cement-Based Materials by Means of Image Analysis
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YAHIA ALHASSANI, ALAIN BASCOUL, ERICK RINGOT L.M.D.C. INSA-UPS Genie Civil, complexe scientifique de rangueil, 31077 Toulouse CEDEX, France
ABSTRACT The relationship between microscopical damaging of concrete and its mechanical parameters such as Young's modulus and degree of reversibility is a basic issue. In the same time, transport properties and of course the durability of civil engineering and hydraulic structures are linked to the state of microcracking. The replica technique has been developed in our laboratory in order to study microcracks with a scanning electron microscope. This non-destructive method allows observation of the imprint of the concrete surface and to follow the evolution of the alterations with time. An objective quantitative analysis requires numerous replicas and many microscopic fields within each replica. After a brief state of the art of the automatic method of extraction of lines by image processing, we propose a specific algorithm of image analysis which allows the cleaning of the pictures and the extraction of the microcrack skeleton. Today, this procedure has been validated on cement paste samples. It has to be improved to be applied on concrete. The measured parameters are the microcrack specific lengths and their orientation. The objective values measured by means of this procedure compare favorably to results obtained by hand drawing. INTRODUCTION The aim of our work is to achieve an objective and systematic way to characterize microcracking and cracking in mortar and concrete. We have two goals. First, we try to link the microscopic damage of concrete to its mechanical parameters such as Young's modulus and the degree of reversibility [1]. Second, we wish to quantify the transport properties which are factors of major concern with regard to structural reliability. These properties are connected to the microcracking due to external as well as internal phenomenon. Another important issue is to improve our knowledge of the microcracking morphology in order to set up predictive models with a good degree of accuracy. We are working on these topics in our laboratory by means of the replica technique in conjunction with scanning electron microscopy (SEM) and image analysis. Until now, the replica technique was semi-automatic because hand drawing of the microcrack skeleton preceded computer analysis. In this paper we present a new stage of our investigation by replacing the manual operation with automatic extraction by image processing techniques.
REPLICA TECHNIQUE The replica technique gives the imprint of the polished surface of concrete[2, 3, 4, 5]. This imprint can be observed under SEM, because it is made of a material insensitive to the 43 Mat. Res. Soc. Symp. Proc. Vol. 370 01995 Materials Research Society
atmosphere of the SEM. Moreover, the replica method is not destructive and it allows to follow the evolution of the microstructure under loading by taking successive imprints of the surface. At usual magnification (xl00- x400), the minimum microcrack opening that is detect
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