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.

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.

Describes improvements in the performance characteristics of cements due to carbon fiber reinforcement. In particular, the structure, physical properties, mechanical behavior, and durability aspects of carbon-cement composites using pitch-based fibers are discussed. The various possible applications of these composites in structural and nonstructural applications are enumerated and future research needs to be identified.

Carbon fiber reinforced cement composite (CFRC) has outstanding advantages in its dynamic characteristics and durability. Among other characteristics, it has a flexural strength three to four times higher than that of ordinary concrete. Taking advantage of these characteristics of CFRC in designing curtain walls, the manufacturing of thin, lightweight curtain walls becomes possible. This paper describes experimental studies conducted using CFRC specimens to examine the effects of mixing and placing conditions upon the flexural strength of CFRC, scale effects, and the fatigue of CFRC subject to repetitive loads. Furthermore, based on the results of these experiments, allowable bending stress in designing curtain walls was determined and its authenticity verified by having a full-scale composite panel undergo a wind resistance test. Several examples of CFRC used as curtain walls are also introduced.