International Concrete Abstracts Portal

The fracture process of high-strength concrete (HSC) submitted to uniaxial compression was analyzed by means of loading tests on cylinders under special closed loop control. Conclusions were drawn from axial and circumferential strain curves, cross sections of specimens impregnated with a fluorescent dye, and visual observations. The evolution of the internal crack extension was revealed and it was shown that under stable progressive fracture, predominantly aggregate bond failure and crack branching occur with the cracks passing around the coarse aggregates. the onset of damage is explained in terms of elementary force transfer concepts. The influence of maximum aggregate size and fiber addition are also discussed in this paper.

This study of the concrete/rock interface addresses primarily the interface of limestone and mortar (since no coarse aggregate was used in the mix) and, to a lesser extent, mortar and rock salt. Uniaxial tensile tests with closed-loop-control were used to determine the stress-crack opening displacement relationship in the softening regime. This relationship is proposed as the constitutive property in an interface cohesive zone model developed for interface fracture. The validity of such a model was investigated through testing and finite element analysis of compact tension specimens. A theoretical investigation of the effect of the complex singularity attributed to an interface crack was performed within the framework of the interface cohesive zone model. Although the theoretical analyses included only a semi-infinite geometry and was, therefore, limited in scope, it was found capable of addressing many of the characteristics of quasi-brittle fracture. Experimental tools used involved a scanning electron microscope to observe microscopic features of the interface that are responsible for strength and toughness. The electronic speckle pattern interferometry technique was used to evaluate pre-peak crack growth. Results indicate that the mechanisms responsible for strength and toughness at the interface are different and that the characteristics of the fracture at the interface is qualitatively similar to that of any other quasi-brittle material.

The global behavior of concrete is influenced by various scenarios of crack initiation and crack propagation. Recently, the study of the interface fracture and cracking in interfacial regions has emerged as an important research field, especially in the context of the development of high performance concrete composites. For a rigorous study, the use and further development of fracture mechanics based concepts are needed. The crack path criterion for elastically homogeneous materials is not valid when the crack advances at an interface because, in this case, the consideration of the relative magnitudes of the fracture toughnesses between the constituent materials and the interface is involved. In this paper, criteria based on energy release rate concepts are considered for the prediction of crack growth at the interfaces and an experimental/numerical study is presented on two-phase composite models of concrete to investigate the cracking scenarios in interfacial regions. From the testing and numerical analysis on physical models, the interface fracture and the crack propagation in concrete composites are studied, and the role of interface fracture toughness is discussed.

Crack propagation in composite materials is a very complex process. Of utmost importance seems the behavior of the interfaces between the constituting phases. In reinforced concrete, interfaces not only appear in the concrete itself, but also between the concrete and the steel reinforcement. Fracture of the steel-concrete interface can be seen as a combination of adhesion, mechanical interlock, and frictional stress transfer. In this paper, steel-concrete interface fracture is modelled at the meso level. At this level, a simple linear elastic fracture law seems to suffice to explain global fracture mechanisms of composite materials. Interfaces between aggregate and matrix and between matrix and reinforcing bars are simulated using a lattice model. In the model, the material is discretized as a lattice of brittle breaking beam elements. Disorder of the material is implemented by assigning different strength and stiffness properties to the beam elements. Cracking is simulated by removing in each load step the element with the highest stress over strength ratio. The model is applied to uniaxial tensile fracture of plain concrete specimens and to bond of steel to concrete. Comparison from the simulations presented in this paper with experimental data shows that crack mechanisms are simulated quite accurately. However, the bond-displacement behavior is still too brittle. This point can be improved when more detail is included in the material structure that is incorporated in the analysis. The macroscopic bond-slip behavior of a reinforcing bar depends strongly on the micro-cracking near the interfacial zone between concrete and reinforcing bar. The analyses clearly show the influence of adhesion between steel and concrete on the simulated crack patterns.

The bond between reinforcing bars and concrete under impact loading was studied both experimentally and by the finite element method. The experiments consisted of pullout tests and push-in tests, under three different types of loading: static, medium rate, and impact. Different concrete strengths (normal and high), types of fibers (polypropylene and steel), and fiber contents were considered. The study focused on the bond-slip relationships and the fracture energy in bond failure. The experimental results were compared with those obtained by the finite element method, in which a special "bond-link element" that was able to transmit both shear and normal forces was adopted to model the connection between the reinforcing bar and the concrete. It was found that higher loading rates, higher concrete compressive strengths, and the addition of steel fibers had significant effects on the bond resistance, the fracture energy, and the bond stress-slip relationship, especially for the push-in case. Reasonably good correspondence in the results between the two methods was also found, and a bond-stress-slip relationship under high rate loading could be established analytically.