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.

Concrete structures shrink when they are subjected to a drying environment. If this shrinkage is restrained, then tensile stresses develop and concrete may crack. One of the methods to reduce the adverse effects of shrinkage cracking is to reinforce concrete with short randomly distributed fibers. Another possible method is the use of wire mesh. The efficiency of fibers and wire mesh to arrest cracks in cementitious composites was studied. Different types of fibers (steel, polypropylene, and cellulose) with fiber content of 0.25 and 0.5 percent by volume of concrete were examined. Ring-type specimens were used for restrained shrinkage cracking tests. These fibers and wire mesh show significant reduction in crack width. Steel fiber reinforced concrete (0.5 percent addition) showed 80 percent reduction in maximum crack width and up to 90 percent reduction in average crack width. Concrete reinforced with 0.5 percent polypropylene or cellulose fibers was as effective as 0.25 percent steel fibers or wire mesh reinforced concrete (about 70 percent reduction in maximum and average crack width). Other properties, such as free (unrestrained) shrinkage and compressive strength were also investigated.

Discusses the results of a study of the effects of low addition rates of polypropylene fibers on plastic shrinkage cracking and mechanical properties of concrete. Addition rates of 0.75, 1.5, and 3.0 lb/yd 3 (0.05 to 0.2 volume percent) were used, with fiber lengths that varied between 0.5 and 2.0 in. Relatively low addition rates were shown to significantly reduce plastic shrinkage cracking. Freezing and thawing durability was not affected by the addition of fibers. Modulus of elasticity, flexural strength, and compressive strength were not changed by the addition rates of polypropylene fibers studied. At the addition rates of polypropylene fibers studied, ASTM Method C 1116 Level II I 5 toughness index values were satisfied. The drop weight hammer test, as described in ACI Committee 544, was utilized for determining the impact resistance of fiber reinforced concrete. Drop weight hammer impact results for fiber reinforced concrete at the fiber addition rate of 3.0 lb/yd 3 demonstrated a significant improvement.

It is well recognized that steel fiber reinforced concrete composites exhibit improved resistance to fracture and impact loads. Both fracture and impact resistance are primarily governed by the toughness characteristics of the material defined by its energy-absorption capacity. Toughness can be measured by carrying out tests involving direct tension, compression, or flexure. However, flexural tests are favored for measurement of toughness because of their simplicity and also their close representation of the stress conditions under field conditions. The test procedures for the measurement of toughness indexes given in codes of practice such as ASTM C 1018, JCI-SF4, JSCE-SF4, and ACI 544 help to obtain information on the qualitative performance of different materials and mix designs. Little information has been reported on the toughness characteristics of slurry-infiltrated fibrous concrete (SIFCON), which is basically a material formed by infiltrating a preplaced "fiber stack" with a cement slurry. This paper describes the details of toughness tests carried out on SIFCON at the Structural Engineering Research Centre, Madras, India, and summarizes the results of the investigations.