Micro-crack detection in high-performance cementitious materials

Detection and quantification of microcracks due to autogenous shrinkage in high-performance concrete represents a problematic issue. Techniques based on crack impregnation typically require drying of the samples, which may introduce further cracks. Other non-destructive techniques, such as x-ray tomography, do not allow sufficient resolution of microcracks. A new technique presented in this paper allows detection of microcracks in cement paste while avoiding artefacts induced by unwanted restraint, drying or temperature variations. The technique consists in casting small circular cylindrical samples of high-performance cement pastes in silicone moulds that exert minimal external restraint. Cast-in steel rods with varying diameter internally restrain the autogenous shrinkage and lead to crack formation. Dimensions of the steel rods are chosen so that the size of this restraining inclusion resembles aggregate size. Gallium intrusion of the cracks and subsequent examination by electron probe micro analysis, EPMA, are used to identify the cracks. The gallium intrusion technique allows controllable impregnation of cracks in the cement paste. A distinct contrast between gallium and the surrounding material and a µm-resolution are ensured by the EPMA technique. Results of crack detection on cement pastes with different autogenous shrinkage behaviour are presented to show applicability and potentialities of the technique.