Experimental study of shear strength and failure mechanisms in RC beams scaled along height or length

摘要

The paper presents results of laboratory experiments carried out on longitudinally reinforced concrete beams subjected to four-point bending. Beams of separately varying height and length were analyzed to investigate the size effect on nominal strength and post-critical brittleness. Beams were scaled in the height direction in the first test series and in the length direction in the second series. Due to lack of geometrical similarity, different failure mechanisms were exhibited. Load-deflection diagrams and crack paths were registered during experiments. The digital image correlation technique was applied to visualize strain localization on the concrete surface. The crack opening and crack slip displacements were also measured. The beam response was characterized by two non dimensional parameters eta(a) = a/D and eta(b) = b/D defined as the ratios of shear and bending spans to the beam depth D assumed as the size parameter and the reinforcement position parameter eta(c) = c'/D. Two major failure mechanisms were observed: flexural failure in the central beam zone combined with plastic yielding of the reinforcement and the diagonal shear crack failure in external shear zones. Two distinct modes of shear failure can be specified depending on the dominance of crack opening or crack closure contact zones. Two different effective stresses associated with failure mechanisms were defined to specify the beam strength's dependence on eta(a), eta(b), eta(c) and D. Some analytical formulae specifying the critical shear stress dependence on eta(a), eta(c) and strengths ratio of reinforcement and concrete were presented at the end of paper and compared with experimental data.