International Concrete Abstracts Portal

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.

Results of an experimental investigation on the behavior of partially prestressed T-beams are presented. High-strength concrete with strengths higher than 8800 psi (60.6 MPa), mild steel with a yield strength of 60 ksi (413 MPa), 270 ksi (1,860 MPa) 7-wire strands, and 30-mm fibers with hooked ends were used for the entire investigation. Condensed silica fume and high-range water-reducing admixture were used to obtain the high-strength concrete. Six T-beams were tested using a simply supported span of 7 ft 6 in. (2286 mm) and two concentrated loads. The main variable was the fiber content that was varied from 0 to 250 lb/yd3 and (147.5 kg/m3). Only the minimum shear reinforcement (stirrups) was provided for all the beams. The flexural reinforcement was designed to create a shear failure to evaluate the fiber contribution to shear at low shear spans. The beams were instrumented to measure stresses in nonprestressed and prestressed reinforcement, curvature, crack spacing, crack width, and deflection. Companion cylinders were tested to obtain the compressive strength of concrete. Five out of six beams failed in shear mode. The fibers do contribute to the shear capacity. However, the contribution of fibers to shear is less for low shear spans, as compared to the contribution of fibers to shear capacity reported in the literature. The fiber reinforced concrete beams undergo more deformation before failure. The increases in fiber content result in consistent increase in flexural stiffness and cracking moment, decrease in crack spacing and maximum crack width, and reduction in reinforcement stresses and concrete strains.

Use of steel fiber reinforced concrete (SFRC) in mine roof support systems as a substitute of traditional wood construction has proven successful. Further improvements of this technology could be obtained using SFRC crib-blocks either consolidated by compaction or having a lightweight matrix. Test results indicate that both compacted and lightweight SFRC crib-blocks behave similarly to conventional SFRC construction, as reported in the literature. Half-scale crib models have a load-bearing strength of approximately 50 percent, the 28-day ultimate compressive strength of quality control specimens. The proposed compaction method has the advantages of decreasing portland cement content, relaxing aggregate gradation requirements, and easing fabrication. On the other hand, the use of lightweight aggregates in the conventional construction procedure facilitates product hauling and handling without impairing structural integrity.

To manufacture load-bearing structures of thin-walled concrete, a new production method called EKEBRO fiber shotcrete was developed. The essential part in the technology is a patented spray gun. Steel wire (0.5 mm), concrete, and compressed air are fed into the spray gun. A cutting wheel cuts the wire into fibers of the required length and shoots these in a stabilized direction on the form or surface together with the concrete. Through this method, a good distribution of steel fibers in the product is achieved, and long fibers can be utilized. Spraying with a variating steel fiber content is also possible. The method is used presently in Sweden for the production of balconies for apartment buildings and, further, for the production of various other products. The technique is also used in traditional shotcreting work, such as repair of deteriorated structures.