International Concrete Abstracts Portal

Carbon fiber reinforced cement composite (CFRC) has outstanding advantages in its dynamic characteristics and durability. Among other characteristics, it has a flexural strength three to four times higher than that of ordinary concrete. Taking advantage of these characteristics of CFRC in designing curtain walls, the manufacturing of thin, lightweight curtain walls becomes possible. This paper describes experimental studies conducted using CFRC specimens to examine the effects of mixing and placing conditions upon the flexural strength of CFRC, scale effects, and the fatigue of CFRC subject to repetitive loads. Furthermore, based on the results of these experiments, allowable bending stress in designing curtain walls was determined and its authenticity verified by having a full-scale composite panel undergo a wind resistance test. Several examples of CFRC used as curtain walls are also introduced.

The effectiveness of steel fibers as shear reinforcement to replace and/or augment conventional stirrups in concrete beams with flexural reinforcement has been demonstrated by Batson, et al. (1972), J. Craig (1984), and other researchers. The current thinking within ACI Committee 544 is to adjust the limiting values of the empirical equations for shear design in ACI 318. However, a rational basis for the design or analysis of steel fibers as shear reinforcement has not been developed. Possible approaches can be based on the plasticity of concrete, Chen (1978) and Nielsen (1984); and limit state analysis and the modified compression field theory, Marti (1986) and Collins (1984). Test data for flexural reinforced concrete beams using steel fibers as the shear reinforcement match the lower bound solution for the shear strength as a function of the shear span-depth ratio, based on limit states analysis of concrete by Nielsen and Braestrup (1978) and Kemp and Al-Safi (1981). The test data agree well with the theoretical predicted strength, assuming the steel fiber concrete is rigid-plastic with a modified Coulomb failure criterion for the yield condition, no tensile strength, and the compressive strength is the effective compressive strength. The plasticity assumption for steel fiber reinforced concrete is supported by research reported on its torsional strength by Narayanan et al. (1979 and 1983), in which the torsional strength was best predicted by the Nadai's "sand heap" plastic model for a variety of steel fiber volumes in the concrete. The random distribution of the steel fibers at relatively close spacing provides a very ductile mode of failure that is in good agreement with strength theories based on plasticity theory and limit states. The initial test results suggest that a rational design procedure for the shear strength of steel fiber concrete can be based on a modified compression field theory that will be accepted by design engineers. This paper briefly reviews the current thinking on shear design of beams using steel fibers as the shear reinforcement, plastic material response of steel fiber concrete, and test data that agrees with the plasticity properties and limit theorems proposed by Nielsen and Braestrup and by Kemp and Al-Safi for reinforced and prestressed concrete beams without shear reinforcement.

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.