Clarification of Deformational Mechanism of Concrete by Laser Speckle Method

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In this research the laser speckle method, which is a non-contact type of measuring method for two dimensional displacements with accuracy of mum, was applied to observe non-linear deformational behavior of concrete as structural material and structural member.To investigate non-linear deformation of concrete as structural material, plate specimens of plain concrete with model coarse aggregate which was made of high strength mortar were tested with cyclic loadings. As a result of test series I in which 14 circular model aggregates were arranged in each specimen, it was observed that residual deformations in the direction vertical to the loading or to cracking were rather large but increased little with the loading cycles. Residual deformations in the direction of the loading, however, was found to be rather small but to increase steadily with loading cycles. Test series II,in which one square model aggregate was placed at the center of each specimen, was conducted to investigate effect … More of the interface between aggregate and morter on deformational characteristics of concrete. Separation at the interface which caused slip and opening displacements was first observed, then mortar crack was developed from the corners of the aggregate near the loading adges. Before the separation occurred, stiffness of the specimen was close to the stiffness calculated by a non-linear finite element method for the case where perfect bond was assumed at the interface. The specimen stiffness gradually decreased and approached to the calculated stiffness for the case where no bond was assumed at the interface. It was considered that the separation and the mortar cracking were developed not only with increase of load but also with increase of loading cycles.Beam specimens with shear reinforcement were tested under monotonic loading. Shear deformations of a shear span of the specimen were measured by using movement of points at the four corner of the shear span. Greater shear deformations were observed with a specimen with a smaller stiffness of longitudinal reinforcement which is a product of the reinforcement ratio and the Young's modulus. When flexural deformations of the shear span were defined as the total deformations minus the measured shear deformations, it could be said that the following model derived from truss analogy predict the deformations. The flexural deformations could be predicted by the conventional method, Branson's method, considering the moment shift. The moment shift could be calculated by using the measured direction of the principal compressive stress and the measured moment arm length which is the distance between longitudinal tensile and compressive forces. The shear deformations could be calculated as a part of deformations of the truss which is induced by the deformations of the diagonal tension and compression struts. Stiffness of the tension strut was assumed to be stiffness of the shear reinforcement plus tension stiffness of the surrounding concrete and to be reduced gradually to the stiffness of the shear reinforcement alone with increase of load. Less