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.

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.

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.

Relatively low-cost and energy-efficient materials with desirable short-term mechanical properties can be constructed using cellulose fibers as cement reinforcement. There are, however, concerns regarding the long-term performance of cellulose fiber reinforced cement composites; some cellulose fibers tend to disintegrate in the alkaline environment of cement. The growth of cement hydration products within the hollow cellulose fibers may also lead to excessive fiber-to-matrix bonding and brittle failure after exposure to natural weathering. This paper presents the results of an experimental study concerned with the long-term performance of cellulose fiber reinforced cement composites. Cellulose fiber reinforced cement composites were investigated, using accelerated weathering conditions representing repeated wetting and drying of materials in outside exposure conditions. The cement composites considered in this investigation incorporated 2 percent mass fractions of kraft pulp. Comprehensive replicated flexural test data were generated for various test ages at different wetting-drying cycles and were analyzed statistically. The analysis of variance and multiple comparison techniques were employed to derive reliable conclusions regarding the effect of accelerated wetting-drying cycles on flexural strength and toughness characteristics of cellulose fiber reinforced cement composites. The results generated in this study showed, at 95 percent level of confidence, that accelerated aging under repeated wetting-drying cycles had negligible effects on flexural strength, but led to reduced toughness and embrittlement of cellulose fiber reinforced cement composites.