International Concrete Abstracts Portal

Infrastructural application of steel fiber reinforced concrete to access hole covers is explored. It has been shown that bar reinforced concrete covers further reinforced with fibers are suitable candidates for access hole covers in medium- and heavy-duty applications. The covers possess increased cracking strength due to the crack-arresting mechanism of the fibers. A greater ultimate load also results, thanks to the increase in shear strength afforded by the fibers. Further, these covers also possess good energy absorption capacity and local anti-splitting characteristics due to the presence of fibers everywhere in the mass.

Theoretical analysis is used to predict bending properties of strain-softening materials from known stress-strain relationships in uniaxial tension and compression. Conversely, given the bending load-displacement relation, it is possible to predict the entire tensile strain-softening response. Bending properties of a polymer concrete have been obtained using the proposed theory and given stress-strain relationships. It is shown that the bending strength is higher than the tensile strength due to the strain-softening effect.

Tests have shown that steel fibers increase the tensile strength of concrete and reduce the sudden failure in tension when bonding is adequate; impact resistance is also greatly increased. On a lesser scale, polypropylene fibers also increase the impact resistance of concrete. Compressive strengths of concrete containing either type of fiber are not increased. Since creep is a fundamental property of concrete, a test program was initiated to measure the effect of both steel and polypropylene fibers on plain concretes and on concretes containing silica fume. The addition of fibers, polypropylene or steel, increased substantially (20 to 40 percent) the creep of plain concrete and, to a lesser extent, the creep of concretes containing 5 to 10 percent silica fume. It was found that creep of concrete with or without fibers was decreased by at least 20 percent when 5 to 10 percent of cement was replaced by silica fume.

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.