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Showing 1-10 of 12 Abstracts search results

Document: 

SP156-11

Date: 

September 1, 1995

Author(s):

C. Perry and J. E. Gillott

Publication:

Special Publication

Volume:

156

Abstract:

Describes a small scale flexure test for the determination of cement- aggregate bond strength. Cylindrical test specimens were prepared by drilling cores in a perpendicular direction through slabs of rock against which mortar had been cast. A special casting procedure eliminated many sources of experimental variation and allowed the bond strengths of different mortars and rock types to be compared directly. Long term tests were conducted by coring the mortar/aggregate slabs at a number of curing times and coefficients of variation of 5 to 10 percent for bond and mortar strengths were obtained. The effect on cement-aggregate bond strength of partial cement replacement by silica fume was evaluated for a number of aggregate types. For siliceous aggregates (glass, obsidian, and quartzite), bond strength was increased significantly by the addition of silica fume; failure tended to occur away from the interface particularly in long term tests. For carbonate rocks (limestone and dolostone), similar bond strengths were obtained at seven days with and without the addition of silica fume. At later ages, silica fume interfered with strengthening of the cement-carbonate rock interface and lower bond strengths were obtained. For specimens not containing silica fume, bond strength increased more rapidly to glass and obsidian than to quartzite, which showed essentially "inert" behavior. This was tentatively attributed to strengthening of the transition zone by a pozzolanic mechanism involving reactive silica from the aggregate. A marked reduction in bond strength occurred with glass specimens containing boosted alkali content. This was attributed to alkali- silica reaction at the interface and was suppressed by the addition of 15 percent silica fume.

10.14359/946


Document: 

SP156-10

Date: 

September 1, 1995

Author(s):

M. Kawamura and S. Igarashi

Publication:

Special Publication

Volume:

156

Abstract:

Fracture of the interfacial zone between a fiber and the cementitious matrix plays a significant role in the mechanical behavior of fiber reinforced cementitious composites. For better understanding of debonding characteristics of a fiber in the composites, the behavior of the extension of cracks along the interface was examined under the fluorescence microscope. The correspondence between the features of fracture zones obtained by the microscopic study and the fracture toughness for the interfacial zone is discussed in this paper. Examinations under the microscope revealed that the debonding and the extension of interfacial cracks were not caused by a simple shear failure at the actual interface, but were accompanied by local failures over relatively wide regions surrounding the steel fibers. The incorporation of silica fume resulted in the reduction in areas containing local failures along the interface. Fewer local failures in the interfacial zone in the steel fiber-silica fume-bearing cement composite were reflected by the decrease in the fracture toughness for the interfacial zone.

10.14359/945


Document: 

SP156-09

Date: 

September 1, 1995

Author(s):

C. K. Y. Leung and Y. Geng

Publication:

Special Publication

Volume:

156

Abstract:

In many practical engineering applications of fiber reinforced concretes (FRC), fibers are subjected to significant lateral loading. The lateral stress may have a significant effect on fiber debonding and pullout, thus affecting the performance of FRC. In this investigation, a novel experimental set-up was developed to carry out fiber pullout tests under various levels of lateral compression. Interfacial properties for steel fiber in mortar were derived from the measured fiber load versus displacement curves, based on a unified fiber debonding theory. As expected, the interfacial friction at the onset of sliding increases with applied compressive stress. Surprisingly, the pre-sliding interfacial resistance (which can be either the interfacial strength or the interfacial fracture energy, depending on whether interfacial debonding is strength or fracture governed) is also found to increase with lateral compressive stress. Also, with higher compression, there is a more rapid decay of interfacial friction when the fiber is sliding out of its groove. As a result, while lateral compression can significantly increase the peak pullout load, the increase in total energy absorbed during the pullout process is much less drastic.

10.14359/944


Document: 

SP156-08

Date: 

September 1, 1995

Author(s):

S. H. Li, S. P. Shah, Z. Li, and T. Mura

Publication:

Special Publication

Volume:

156

Abstract:

A new method to predict the debonding behavior of fiber-matrix interface has been proposed by applying the principles of the micromechanics of inclusion and fracture mechanics. The validity of the mathematical model is further verified by uniaxial tension tests carried out on steel fiber reinforced cementitious composite specimens by employing a digitally controlled closed-loop MTS testing machine. It was demonstrated that the debonding occurs before the bend over point; the debonded lengths are largely influenced by the sequence of the occurrence of transverse matrix cracks and the loading stage. A stable growth of debonding was observed in the investigation. The measured debonded lengths were compared with the theoretical prediction of the proposed model. A reasonable agreement was observed.

10.14359/943


Document: 

SP156-07

Date: 

September 1, 1995

Author(s):

C. Yan and S. Mindess

Publication:

Special Publication

Volume:

156

Abstract:

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.

10.14359/942


Document: 

SP156-06

Date: 

September 1, 1995

Author(s):

A. Vervuurt and j. G. M. Van Mier

Publication:

Special Publication

Volume:

156

Abstract:

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.

10.14359/941


Document: 

SP156-05

Date: 

September 1, 1995

Author(s):

K. M. Lee, O. Buyukozturk, and Y. Kitsutaka

Publication:

Special Publication

Volume:

156

Abstract:

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.

10.14359/940


Document: 

SP156-04

Date: 

September 1, 1995

Author(s):

J. Wang and A. K. Maji

Publication:

Special Publication

Volume:

156

Abstract:

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.

10.14359/939


Document: 

SP156-03

Date: 

September 1, 1995

Author(s):

L. R. Taerwe

Publication:

Special Publication

Volume:

156

Abstract:

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.

10.14359/938


Document: 

SP156-02

Date: 

September 1, 1995

Author(s):

Z. P. Bazant and R. Desmorat

Publication:

Special Publication

Volume:

156

Abstract:

The size effect caused by post-peak softening in the relation of interface shear stress and slip displacement between a fiber or reinforcing bar and the surrounding matrix was analyzed. The problem was simplified as one- dimensional. It was shown that the post-peak softening leads to localization of slip. The larger the bar or fiber size, the stronger the localization. The size effect in geometrically similar pullout tests of different sizes was found to represent a transition from the case of no size effect for small sizes to the case of a size effect of the same type as in linear elastic fracture mechanics, in which the difference of the pullout stress in the fiber and the residual pullout stress corresponding to the residual interface shear stress is proportional to the inverse square root of bar or fiber size. An analytical expression for the transitional size effect was obtained and was found to approximately agree with the generalized form of the size effect law proposed earlier by Bazant. Measurements of the size effect can be used for identifying the interface properties.

10.14359/1019


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