International Concrete Abstracts Portal

The composite material concept is commonly used to predict the tensile strength of fiber reinforced concrete. Application of this concept implies that the fiber pullout resistance is mobilized to a large extent when the peak tensile strength is reached. The validity of this assumption is checked in this paper using the available results of pullout tests on steel fibers and direct tension tests on steel fiber reinforced concrete. The measured values of fiber pullout stiffness and fiber reinforced concrete strain at peak stress are used to derive conclusions regarding the contribution of the pullout mechanism of fibers to the tensile strength of fiber reinforced concrete.

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.

Behavior of soil-cement that has been modified by the addition of fiber reinforcing is investigated. Two different soil mixtures were used--one containing a sand aggregate and the other containing a clayey sand aggregate. Four different types of fibers were examined--straight steel, hooked steel, polypropylene, and fiberglass. All fiber mixtures were evaluated based on a comparison with a control mixture containing no fiber reinforcing. The material properties examined were: 1) compressive strength; 2) splitting tensile strength; 3) shear strength; 4) compressive stress-strain behavior; 5) wet-dry durability; and 6) freeze-thaw durability. Overall, fiberglass reinforcing was found to be most effective in improving the strength properties of the soil-cement. An increase in tensile splitting strength of up to 140 percent was observed. Ductility was greatly enhanced for all the fiber mixtures, as indicated by higher post-peak strengths. Also because of the presence of fibers, the confinement of specimens was improved. For some fiber/soil combinations, increases in compressive strength and shear strength were also observed. The wet-dry and freeze-thaw tests showed that all the fiber types except fiberglass improve the durability of soil-cement. Fiberglass fibers, however, were generally found to be detrimental to durability. In view of substantial increases observed in tensile splitting strength, ductility, durability, and confinement, it is suggested that fiber reinforced soil-cement might be applied in the construction of soil-cement liners for reservoirs and landfills.

Paper presents the results of an experimental investigation to determine the flexural fatigue strength of concrete reinforced with collated fibrillated polypropylene fibers. The performance of fresh concrete and the elastic and mechanical properties of hardened concrete are compared for concretes with and without fibers. The test program included 1) flexural fatigue and endurance limit; 2) static flexural strength including load-deflection curve, determination of first-crack load, and toughness index; 3) compressive strength; 4) static modulus; 5) pulse velocity; 6) unit weight, workability, and finishability of fresh concrete. The complete series of tests was run for three concentrations of fibers. Special care was taken to insure consistency with cement, aggregates, admixtures, procedure, and mix temperatures. There was no "balling" or tangling of the fibers during mixing and placing. Fiber reinforced concretes had better finishability and were easy to work with even at higher fiber concentrations. Due to the addition of fibers, the ductility and the post-crack energy absorption capacity was increased. There was a slight increase in the static flexural strength and a moderate increase in the flexural fatigue strength. When compared to plain concrete, there was a positive improvement in the endurance limit (for 2 million cycles).