International Concrete Abstracts Portal

Concrete structures shrink when they are subjected to a drying environment. If this shrinkage is restrained, then tensile stresses develop and concrete may crack. One of the methods to reduce the adverse effects of shrinkage cracking is to reinforce concrete with short randomly distributed fibers. Another possible method is the use of wire mesh. The efficiency of fibers and wire mesh to arrest cracks in cementitious composites was studied. Different types of fibers (steel, polypropylene, and cellulose) with fiber content of 0.25 and 0.5 percent by volume of concrete were examined. Ring-type specimens were used for restrained shrinkage cracking tests. These fibers and wire mesh show significant reduction in crack width. Steel fiber reinforced concrete (0.5 percent addition) showed 80 percent reduction in maximum crack width and up to 90 percent reduction in average crack width. Concrete reinforced with 0.5 percent polypropylene or cellulose fibers was as effective as 0.25 percent steel fibers or wire mesh reinforced concrete (about 70 percent reduction in maximum and average crack width). Other properties, such as free (unrestrained) shrinkage and compressive strength were also investigated.

Discusses the results of a study of the effects of low addition rates of polypropylene fibers on plastic shrinkage cracking and mechanical properties of concrete. Addition rates of 0.75, 1.5, and 3.0 lb/yd 3 (0.05 to 0.2 volume percent) were used, with fiber lengths that varied between 0.5 and 2.0 in. Relatively low addition rates were shown to significantly reduce plastic shrinkage cracking. Freezing and thawing durability was not affected by the addition of fibers. Modulus of elasticity, flexural strength, and compressive strength were not changed by the addition rates of polypropylene fibers studied. At the addition rates of polypropylene fibers studied, ASTM Method C 1116 Level II I 5 toughness index values were satisfied. The drop weight hammer test, as described in ACI Committee 544, was utilized for determining the impact resistance of fiber reinforced concrete. Drop weight hammer impact results for fiber reinforced concrete at the fiber addition rate of 3.0 lb/yd 3 demonstrated a significant improvement.

Reports the results of an investigation on the strength and ductility of fiber reinforced high-strength concrete under direct shear. Both experimental and modeling studies were performed. In the experimental study, fiber reinforced high-strength concrete pushoff specimens were tested. Two fiber types, polypropylene and steel, were used with or without conventional stirrups. An existing model was further developed and used in the analytical prediction of the shear stress-strain relationships for these specimens. In general, fibers proved to be more effective in high-strength concrete than in normal strength concrete, increasing both ultimate load and overall ductility. This is attributed to the improved bond characteristics associated with the use of fibers in conjunction with high-strength concrete. For the specimens with steel fibers, significant increases in ultimate load and ductility were observed. With polypropylene fibers, a lower increase in ultimate load was obtained when compared to the increase due to steel fibers. Ductility of the polypropylene fiber reinforced specimens was greater than that of the steel fiber reinforced specimens. In the tests involving the combination of fibers and conventional stirrups, slight increases in ultimate load and major improvements in ductility were observed when compared to the values for plain concrete specimens with conventional stirrups. In general, good agreement between the model and the test results was found.

This research was specifically designed to test concrete in direct tension. Concrete prism specimens measured 4 x 4 x 16 in. in length. The specimens were first tested under monotonic loading conditions to determine ultimate stress-strain relationships. Samples were also tested under low-frequency high cyclic loading conditions to simulate concrete fatigue. Fibrous concrete containing steel, polypropylene, and composite fiber reinforcement made up the test groups. A closed-loop hydraulic test machine was used to develop a testing procedure to measure the monotonic and cyclic tension responses of fiber reinforced concrete. This procedure proved successful in determining the stress-strain relationship and cyclic behavior of the fiber reinforced concrete. The concrete evaluation included monitoring concrete in the plastic state. Concrete temperature, slump, air content, mix design, and mixing time were carefully controlled. The long-term concrete curing period lasted 150 days. The testing of cured samples included mechanical testing, statistical treatment evaluation, and scanning electron microscope analysis. The fiber reinforced concrete and composite fiber specimens provided substantial performance improvement when compared to the plain concrete specimens.

An innovative technique for reinforcing concrete to achieve extremely high flexural strengths has been developed. This technique utilizes a steel fiber mat instead of short, discrete steel fibers. The mat configuration is preplaced for infiltration with a concrete slurry to yield a composite with flexural strengths approaching ten times that of conventional concrete. Applications include high-performance bridge decks, earthquake-resistant structures, nuclear waste containment, military applications, and other innovative uses in which flexural strength is at a premium. Stainless steel mats or other advanced alloys can be provided where corrosion resistance or high temperature strength are required.