Tests were made on 92 knee-type steel fiber reinforced concrete (SFRC) beam-column connections to determine the effect of steel fibers on strength and behavior. Both beam and column had overall sections of 4 x 4 in. (101.6 x 101.6 mm) and length of 16 in. (406.4 mm) each. The column had main reinforcement comprised of two bars of «-in. (12.0 mm) diameter deformed steel bars having yield strength of 67.5 ksi (4745 kgf/cmý) near the outside face and two bars of ¬-in. (6.0-mm) diameter of deformed steel near the inside face of the column. The column had 3/16-in. (5.0-mm) diameter ties of plain mild steel at 3 in. (76.5 mm) center to center. The two bars of 1/2-in. (12.0-mm) diameter near the outside face of the column were continued into the top of the beam to provide main steel. The variables were M/P ratio (moment to axial load) type percentage and aspect ratio (length to diameter) of the fibers. Brass-coated high-strength steel plain fibers of size 1.0 x .01 in. (25.4 x 0.254 mm), « x 0.006 in. (12.7 x 0.152 mm), 1.0 x 0.016 in. (25.4 x 0.406 mm), and mild steel fibers of 0.011-in. (0.282-mm) diameter having aspect ratio of 10, 25, 50, 75, and 100 were used. The percentage of fibers (by volume of concrete) varied from 0.5 to 2.0. Connections having conventional reinforcement only were also tested. The test results indicated that steel fiber reinforced concrete is very effective in increasing ductility and crack resistance in the connection region. Ultimate rotation of SFRC connections was six to nine times that of conventional connections. There was an increase in moment capacity of 15 to 30 percent with increase in fibers from 0.5 to 2.0 percent by volume. Moment capacity increased by about 50 percent when the aspect ratio of the fibers was increased from 10 to 100.

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.

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.

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.

Article presents selected conclusions derived from a slurry-infiltrated fiber concrete (SIFCON) research program performed by a New Mexico research group under an Air Force contract. The selected conclusions discussed concern certain material properties of several SIFCON mixes tested under uniaxial, unconfined compression. The article concerns itself with the effect of varying the two parameters of water-cement and fly ash-cement ratios on the compressive strength.