International Concrete Abstracts Portal

SIFCON (slurry-infiltrated fiber concrete) is a type of fiber reinforced concrete construction in which formwork molds are filled to capacity with fibers and the resulting network is infiltrated by a cement-based slurry under vibration. The main objective of the paper is to investigate the compressive properties of SIFCON and to develop constitutive models to describe them. Primary focus is on the results of the experimental phase of the program. The experimental program consists of compression tests on 3 x 6 in. cylindrical specimens. Parameters under investigation include the following variables: four compressive strengths for the slurry (with and without fly ash and microsilica), three types of fibers (deformed, hooked, and crimped), orientation of fiber axis to direction of loading (normal and parallel), and the effect of mold walls (cored versus molded specimens). The instrumentation used comprises a servohydraulic testing machine with closed-loop control capability, a data acquisition system, data transfer devices, and data plotting. Results include the stress-strain response as well as strength, stiffness, and ductility properties. To model the descending branch of the stress-strain curve that exhibits a plateau value up to very high strains (10 percent), an analytical relationship with asymptotic behavior is proposed.

Several types of failure mechanisms and fracture of fiber reinforced concrete (FRC) composites are discussed. These include: multiple fracture of the matrix prior to composite fracture; catastrophic failure of the composite immediately following matrix cracking due to inadequate reinforcing; fiber pullout following matrix cracking providing significant energy absorption with or without substantial strengthening of the matrix; and fracture of short fibers bridging the matrix crack without multiple fracture of the matrix. Aspects relating to the modeling of the two major causes for nonlinearities associated with fiber concrete composites, namely interfacial bond-slip and matrix softening, are also discussed. Analytical models available for predicting the tensile response of such composites are examined in light of the above mechanisms of failure.

In analytical modeling of crack growth resistance (KR) curves for fiber cements, it is important to determine the size of the matrix fracture process zone (FPZ), in addition to the characteristics of the fiber-bridging zone. New experimental techniques are given for identifying and measuring crack growth and FPZ in a low-modulus wood-fiber cement. A computerized data acquisition system has been developed to investigate the nature of crack growth with a grid of closely spaced conductive bars screen-printed onto the specimen surface using colloidal graphite. As the crack path progresses through the grid, the position of the crack tip is automatically recorded and the discrete cracking behavior of crack growth is shown. Crack lengths measured in this way are in good agreement with results obtained using optics. The extent of the FPZ can be determined by cutting thin strips of the specimen normal to the crack path in the vicinity of the crack tip and measuring the bending stiffness of each strip as a function of distance away from the tip. The presence of microcracking is easily detected by this technique and the size of the FPZ can be determined. Experimental results show that the process zone is approximately 30 to 40 mm in a compact tension geometry.

Use of reinforced fiber concrete in buildings, for construction of an adequate section to resist a flexural failure, has been under investigation by engineers in the past decade. Design and analysis methodologies are discussed in the paper so this type of construction can be developed successfully by engineers. In the first part of the paper, results from 13 beams that were tested at New Jersey Institute of Technology are presented to show the nature of the flexural behavior. These beams are for: 1) normal concrete, 2) high-strength concrete, and 3) lightweight concrete with and without fibers. Most of the results from these tests have not been reported previously. A computer program will also be shown that accurately predicts the flexural behavior of these beams and other reinforced fiber concrete members. Using these test results and the computer program, inelastic and elastic behavior in flexure are discussed. The majority of the paper deals with analysis and design methods. All past methods of analysis are discussed briefly. A method that has been developed by NJIT is explained for analyzing regular singly reinforced, doubly reinforced, and T-beams. The {rho}b criteria is explained for each case, and analysis equations and design methodology are shown for each type of beam. Hence, the paper shows the state-of-the-art in analysis with the examples, and presents a rational design scheme for use by the design engineer that will help in the adoption of such a construction material.

Paper reviews seven test specimen geometries that have been used to determine the shear performance of fiber reinforced concrete (FRC) materials. All the geometries are modified standard quality control test specimens--modified cubes, beams, or cylinders. The performance of FRC materials can be characterized by two fracture parameters--fracture toughness, which gives the resistance to cracking, and toughness index, which quantifies the post-first-crack toughness. The shear strength results are similar for the various test geometries used in the study. The shear strength of steel FRC mixes is shown to be independent of fiber content, whereas the shear strength of polypropylene FRC decreases with increasing fiber content and the shear strength of glass FRC increases with increasing fiber content. The post-cracking toughness increases uniformly with increasing fiber content over the range of fiber contents studied. This increase in toughness is observed for all three fibers--steel, polypropylene, and glass fiber.