International Concrete Abstracts Portal

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.

Partially prestressed beams contain both prestressed and non-prestressed reinforcement. Addition of steel fibers results in an increase in first crack moment and flexural strength and a decrease in deflection and reinforcement stresses. This paper presents an analytical method to compute the deformation of partially prestressed beams made with fiber reinforced concrete. A computer program was developed to evaluate the theoretical moment-curvature and moment-deflection relationships. It uses the linear and nonlinear stress-strain relationships of the composite materials. Strain compatibility concept is incorporated to obtain the stresses in concrete, prestressed steel, and non-prestressed steel. The cracking moment and the nominal flexural strength are also computed. The method can analyze prestressed sections of rectangular, T, I, and box shapes. The analytical predictions of the proposed method agree well with experimental results.

Compares load-deflection responses obtained using deflection control and crack-mouth opening displacement (CMOD) control. CMOD control provides a more stable response in the immediate post-peak regime of the load-deflection response than deflection control. The differences in the responses recorded using these two types of test control are more pronounced for the more brittle mixes. Results reported and discussed in this paper were obtained using third-point loading in flexure. Deflection controlled tests were performed using manual control on a stiff million-lb-capacity machine. This is similar to the manner in which most commercial laboratories perform deflection controlled tests on concrete specimens. CMOD controlled tests were conducted using a servo-controlled machine. Normal and lightweight aggregate concrete mixes were evaluated with polymeric fiber loadings of 1, 2, 3, and 4 lb/yd 3 (0.6, 1.2, 1.8, 2.4 kg/m 3). Overall load-deflection response and material toughness values are compared and discussed. Beams reinforced with low volume contents of polymeric fibers typically exhibit a sharp drop in load carrying capacity after first crack. The shape of the load-deflection response in the initial portion of the softening regime is important for toughness computations, particularly for the smaller ASTM indices, such as I 5 and I 10. Since the type of test control and the level of post-peak stability provided by the test set-up influence the shape of the load-deflection response in this regime of interest, there are questions regarding the objectivity of toughness indexes computed at small limiting deflections.

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.