International Concrete Abstracts Portal

Compares load-deflection responses obtained using deflection control and crack-mouth opening displacement (CMOD) control. CMOD control provides a more stable response in the immediate post-peak regime of the load-deflection response than deflection control. The differences in the responses recorded using these two types of test control are more pronounced for the more brittle mixes. Results reported and discussed in this paper were obtained using third-point loading in flexure. Deflection controlled tests were performed using manual control on a stiff million-lb-capacity machine. This is similar to the manner in which most commercial laboratories perform deflection controlled tests on concrete specimens. CMOD controlled tests were conducted using a servo-controlled machine. Normal and lightweight aggregate concrete mixes were evaluated with polymeric fiber loadings of 1, 2, 3, and 4 lb/yd 3 (0.6, 1.2, 1.8, 2.4 kg/m 3). Overall load-deflection response and material toughness values are compared and discussed. Beams reinforced with low volume contents of polymeric fibers typically exhibit a sharp drop in load carrying capacity after first crack. The shape of the load-deflection response in the initial portion of the softening regime is important for toughness computations, particularly for the smaller ASTM indices, such as I 5 and I 10. Since the type of test control and the level of post-peak stability provided by the test set-up influence the shape of the load-deflection response in this regime of interest, there are questions regarding the objectivity of toughness indexes computed at small limiting deflections.

Experiments were performed on concrete specimens reinforced randomly with polypropylene fibers. To obtain the true properties of the fibers, their tensile stress-strain diagram was obtained through tests. The fibers used had a tensile strength of 2800 kgf/cm 2 (40,000 psi), a failure strain of about 11 percent, and a modulus of elasticity of 2.55 X 10 5 kgf/cm 2 (3,642,857 psi). Then, the 7- and 28-day polypropylene fiber reinforced concrete (PFRC) samples with 0, 0.5, 1.0, 1.5, 2.0, and 2.5 percent by volume of fibers were tested in splitting tensile and compressive strength tests, and the tensile strength, maximum tensile strain, and compressive strength versus percentage by volume of fibers diagrams were plotted. The results show that compressive strength did not change significantly, but tensile strength had an increase of about 80 percent. Significant improvement in ductility was also achieved. The tests also showed that the best improvement was obtained at an optimum percentage by volume of fibers of about 1.5 percent.

It is well recognized that steel fiber reinforced concrete composites exhibit improved resistance to fracture and impact loads. Both fracture and impact resistance are primarily governed by the toughness characteristics of the material defined by its energy-absorption capacity. Toughness can be measured by carrying out tests involving direct tension, compression, or flexure. However, flexural tests are favored for measurement of toughness because of their simplicity and also their close representation of the stress conditions under field conditions. The test procedures for the measurement of toughness indexes given in codes of practice such as ASTM C 1018, JCI-SF4, JSCE-SF4, and ACI 544 help to obtain information on the qualitative performance of different materials and mix designs. Little information has been reported on the toughness characteristics of slurry-infiltrated fibrous concrete (SIFCON), which is basically a material formed by infiltrating a preplaced "fiber stack" with a cement slurry. This paper describes the details of toughness tests carried out on SIFCON at the Structural Engineering Research Centre, Madras, India, and summarizes the results of the investigations.

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.

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.