INVESTIGATION ON CRACK GROWTH IN CONCRETE BY MOMENT TENSOR ANALYSIS OF ACOUSTIC EMISSION

On meso-scale, concrete can be considered as a kind of non-homogeneous composite mainly composed of aggregates, cement paste and cracks. Under uniaxial quasi-static loading condition, quasi-brittle characteristics can be observed in the stress-strain curves. Failure process of concrete is essentially a process of nucleation, propagation and convergence of internal micro-cracks.Therefore, investigation on the crack growth process on meso-scale is beneficial to understand the deformation and failure process of concrete. Acoustic emission is a physical detection method which can be used to obtain the physical information of the mesoscopic damage evolution inside many kinds of materials. Acoustic emission technique, modified time of arrival approach and moment tensor theory were applied to analyze the AE sources obtained in the Brazilian test and the locations, types and orientations of cracks in the specimens were investigated. Relationship between the failure on macro-scale and the mechanisms of cracking on meso-scale of concrete was revealed. The results show that the micro-cracks generate near two contact surfaces between specimen and loading plates firstly. Then the nucleation of the micro-cracks occur at the local zone. Finally, cracks propagate from top and bottom to the center of specimen along the loading direction in the elevation view. In the side view, cracks propagated from the internal zone to the surface. Volume of micro-cracks could be regarded as a measure of elastic energy released from the nucleation of micro-cracks. The volume of micro-cracks is small at the early stage of nucleation. When the load reached its peak value, the maximum volume of the micro-crack was 5.93\times 10^ - 4 mm^3. Tensile cracks, shear cracks and mixed-mode cracks on meso-scale could be observed in the tensile failure process of concrete on macro-scale. The failure process of the concrete was dominated by tensile cracks (nearly 60%) and the shear cracks had the minimum proportion (nearly 10%). The motion of tensile cracks dominate the macro-scale failure of specimen.