International Concrete Abstracts Portal

Reports the results of an investigation on the strength and ductility of fiber reinforced high-strength concrete under direct shear. Both experimental and modeling studies were performed. In the experimental study, fiber reinforced high-strength concrete pushoff specimens were tested. Two fiber types, polypropylene and steel, were used with or without conventional stirrups. An existing model was further developed and used in the analytical prediction of the shear stress-strain relationships for these specimens. In general, fibers proved to be more effective in high-strength concrete than in normal strength concrete, increasing both ultimate load and overall ductility. This is attributed to the improved bond characteristics associated with the use of fibers in conjunction with high-strength concrete. For the specimens with steel fibers, significant increases in ultimate load and ductility were observed. With polypropylene fibers, a lower increase in ultimate load was obtained when compared to the increase due to steel fibers. Ductility of the polypropylene fiber reinforced specimens was greater than that of the steel fiber reinforced specimens. In the tests involving the combination of fibers and conventional stirrups, slight increases in ultimate load and major improvements in ductility were observed when compared to the values for plain concrete specimens with conventional stirrups. In general, good agreement between the model and the test results was found.

An innovative technique for reinforcing concrete to achieve extremely high flexural strengths has been developed. This technique utilizes a steel fiber mat instead of short, discrete steel fibers. The mat configuration is preplaced for infiltration with a concrete slurry to yield a composite with flexural strengths approaching ten times that of conventional concrete. Applications include high-performance bridge decks, earthquake-resistant structures, nuclear waste containment, military applications, and other innovative uses in which flexural strength is at a premium. Stainless steel mats or other advanced alloys can be provided where corrosion resistance or high temperature strength are required.

This research was specifically designed to test concrete in direct tension. Concrete prism specimens measured 4 x 4 x 16 in. in length. The specimens were first tested under monotonic loading conditions to determine ultimate stress-strain relationships. Samples were also tested under low-frequency high cyclic loading conditions to simulate concrete fatigue. Fibrous concrete containing steel, polypropylene, and composite fiber reinforcement made up the test groups. A closed-loop hydraulic test machine was used to develop a testing procedure to measure the monotonic and cyclic tension responses of fiber reinforced concrete. This procedure proved successful in determining the stress-strain relationship and cyclic behavior of the fiber reinforced concrete. The concrete evaluation included monitoring concrete in the plastic state. Concrete temperature, slump, air content, mix design, and mixing time were carefully controlled. The long-term concrete curing period lasted 150 days. The testing of cured samples included mechanical testing, statistical treatment evaluation, and scanning electron microscope analysis. The fiber reinforced concrete and composite fiber specimens provided substantial performance improvement when compared to the plain concrete specimens.

Reports on an investigation on the behavior of high-strength concrete beams (with concrete compression strength equal to 11,600 psi or 80 MPa), with and without steel fiber reinforcement, to determine their diagonal cracking strength as well as nominal shear strength. Experimental data on the shear strength of steel fiber reinforced high-strength concrete beams are currently scarce to nonexistent. Twenty-two beam specimens were tested under monotonically increasing loads applied at midspan. The major test parameters included the volumetric ratio of steel fibers, the shear span-to-depth ratio, the amount of longitudinal reinforcement, and the amount of shear reinforcement. It was found that steel fiber reinforced high-strength concrete beams effectively resist abrupt shear failure. Such beams exhibit higher cracking loads and energy-absorption capabilities than comparable high-strength concrete beams without fibers. Empirical prediction equations are suggested for evaluating the diagonal cracking strength as well as nominal shear strength of steel fiber reinforced high-strength concrete beams.

It is well recognized that fiber reinforced concrete (FRC) exhibits a number of superior properties relative to plain concrete, such as improved strength, ductility, impact resistance, and failure toughness. These advantageous features of FRC can lead to novel structural applications, for which standard design and analysis procedures must be supplemented by numerical modeling (for example, the finite element method). This, in turn, makes necessary the development of satisfactory constitutive models that can predict the behavior of FRC under different load conditions, both monotonic and cyclic. In this paper, a constitutive model for FRC is developed loosely within the theory of mixtures. For plain concrete, an anisotropic, strain-based, continuum damage/plasticity model with kinematic and isotropic damage surfaces is developed. To represent the effect of the fibers, a simplified model that accounts for the tensile resistance of the fibers and the enhanced tensile resistance of the plain concrete is proposed. The predictions of the FRC constitutive model are compared to data from laboratory tests of steel fiber reinforced concrete (SFRC) specimens under uniaxial and biaxial loadings.