International Concrete Abstracts Portal

The recent introduction of epoxy coating to reinforcing steel has made the study of bond, and the effect of this coating, all the more important. A recent large scale study of bond performance of epoxy-coated and uncoated reinforcement conducted at the University of Kansas using modified cantilever beam-end specimens has shown the effects of various parameters on bond. These specimens placed the bar and surrounding concrete in tension, simulating the situation in actual members. A prescribed bond test region, the bonded length, was placed at a discrete distance, the lead length, from the front of the specimen to prevent surface effects from interfering with the test region. The experimental work has provided ample evidence of the fundamental fracture mechanics aspects of bond failure and the subsequent specimen failure. Splitting failure of the beam-end specimens was observed consistently in all tests where a fracture plane formed above the bond test region and propagated quickly through the tension region of the specimen. Tests indicated that the bonded length of the bar, the value of the lead length, and the amount of cover were all important parameters. The paper presents the results of an analytical evaluation of the bond process and specimen fracture that was observed in the laboratory, using nonlinear finite element analysis to study the effects of interface properties on the fracture behavior and failure load. The majority of the beam-end specimen was modeled using linear elastic elements representing one-half of the symmetric experimental specimen. The actual bar-concrete interface was modeled using link elements and a Mohr-Coulomb failure model. Rod elements joined the specimen to the specified crack plane located at the center line of the specimen. The fracture process was modeled using Hillerborg's fictitious crack model. The behavior observed in the laboratory for coated and uncoated bars has been accurately predicted using this procedure. The fracture process and resulting overall bond performance has been studied analytically to assess the effects of interface properties on the fracture behavior. The analytical studies confirmed that coating reduced the relative bond strength with respect to that of an uncoated bar, while the absolute bond strength was found to increase with additional cover and lead length.

Reviews recent theoretical and experimental results on the size effect in brittle failures of reinforced concrete structures caused by the release of stored energy After summarizing the size effect law and explaining the novel concept of a brittleness number, the results of recent tests of diagonal shear failure, punching shear failure, torsional failure, and pullout failure are discussed. These results, which were obtained on geometrically similar specimens with a broad range of sizes, are found to be in excellent agreement with the theoretical size effect law. The experimental evidence is much stronger than that which was previously obtained by analyzing a large amount of test results from the literature, which were not obtained on geometrically similar specimens and were limited to a narrow size range. It is also pointed out that the test data on diagonal shear disagree with the classical Weibull-type theory of size effect, thus strengthening the theoretical argument against using this theory for the size effect in concrete structures whose maximum load is much larger than the cracking initiation load. The test results indicate that the presently considered fracture mechanics size effect ought to be incorporated into the formulas for the contribution of concrete to the ultimate load capacity in brittle failures of concrete structures. It is shown that such formulas can be based on the brittleness number. For any given structure shape, this number can be determined from size effect tests. However, prediction of this number without such test data will require some further research.

The fracture mechanics size effect in unreinforced concrete beams has been clearly demonstrated by Bazant. The effect of reinforcement on the fracture mechanics size effect has not been demonstrated quite as clearly. The bending failure of a singly reinforced concrete beam serves to illustrate the effect of reinforcement in the fracture mechanics size effect. The effect of prenotched and unprenotched beams is also considered. A simple analytical model has been developed for the behavior (up to peak load and beyond) of a singly reinforced concrete beam. This model takes into account the existence of an initial traction-free crack and assumes linear elastic behavior of concrete, elastic-plastic response of the steel, crushing of concrete, and simplified bond-slip between the steel and concrete. The model employs the fictitious crack model to determine the crack growth in small beams and linear elastic fracture mechanics to determine crack growth in large beams. The model demonstrates a size effect which starts with a high nominal strength for low values of á (small beams) and a low nominal strength for high values of á (large beams). Between these shelves, in the neighborhood of log(á) = 0, there is an S-shaped transition region, but not well-approximated by a line with a slope of negative one-half, as for unreinforced, prenotched concrete beams. Example problems show the importance of the size effect in design.

Discusses the shear design problem in concrete in the context of mixed mode crack propagation in concrete structures. Shear behavior and fracture of precast concrete segmental bridges are presented as a design case study. Joints between the precast segments of these bridges are critical locations through which large shear stresses, combined with normal stresses, must be transmitted. Crack initiation and propagation at these locations represent a mixed mode concrete fracture problem. General concepts for the representation of mixed mode fracture in concrete are briefly discussed, and a combined analytical and experimental methodology is presented for predicting this cracking behavior. Finally, using the developed fracture mechanics approach, a preliminary design concept is proposed for the shear design of prestressed concrete elements.

Progressive microcracking and crack closure effects are the most important phenomena which need to be described in finite element calculations of reinforced concrete structures subjected to cyclic or seismic loads. Microcracking produces a loss of stiffness which is usually modeled with continuous damage mechanics. Crack closure effects such as inelastic deformations and stiffness recovery remain features that must be incorporated in the constitutive relations describing the response of concrete under cyclic loadings. These effects are introduced into a novel damage model in a rigorous, consistent fashion. An attempt to derive the constitutive relations for fiber reinforced concrete using this model is also described. The implementation of these constitutive relations into a layered beam finite element code is discussed, and computations on medium-size bending beams and a beam-column joint subjected to cyclic loading are compared with experiments. Although the computational method remains simple and sufficiently fast for engineering applications, the good agreement obtained with test data shows that the constitutive relations capture very well the main characteristics of the behavior of concrete.