دانلود رایگان مقاله خواص مکانیکی فوق العاده بالا عملکرد بتن تقویت شده با الیاف

Abstract

A comprehensive investigation into the mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC), considering various influential factors, is imperative in order to obtain fundamental information for its practical utilization. Therefore, this paper reviewed the early-age strength (or setting) development and mechanical properties of hardened UHPFRC. In connection with the latter, the effects of the curing conditions, coarse aggregate, mineral admixtures, fiber properties, specimen size, and strain-rate on the mechanical performance of UHPFRC were specifically investigated. It was obvious that (1) heat treatment accelerates the hydration process, leading to higher strength; (2) a portion of the silica fume can be replaced by fly ash, slag, and rice husk ash in mechanical perspective; (3) the use of deformed (hooked and twisted) or long straight steel fibers improves the mechanical properties at a static rate; and (4) high rate loading provides a noticeable increase in the mechanical properties. Alternatively, there are some disagreements between the results from various ‘size effect’ tests and the effectiveness of using twisted steel fibers at static and high rate loadings. Further research to reduce the production cost of UHPFRC is also addressed in an attempt to make its widespread use more practical.

5. Conclusion

This paper reviewed the state-of-the-art on the mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC). Based on the literature review and discussions, the following conclusions can be drawn: 1) Heat treatment on UHPFRC resulted in acceleration of the hydration process and an increased density, which led to the ultrahigh strength. A lower curing temperature generally required a longer curing period to achieve strengths similar to those of heat-treated UHPFRC, i.e., wet curing at 20 C required 91 days to provide a compressive strength of approximately 200 MPa. In addition, the replacement of fine sand with coarse aggregate with a maximum size of 8 mm has no significant effect on the compressive strength, whereas the flexural strength was decreased. 2) Using silica fume (SF) accelerated the hydration process of UHPFRC, whereas the use of fly ash (FA) and slag delayed the hydration process. Increasing the SF content up to 30% led to an increase in the bond strength and pullout energy. Replacing the SF with FA or slag up to 40% has no significant effect on the compressive strength, whereas using the FA and slag positively affected the flexural strength and toughness. In addition, due to a synergic effect, the combination of 10% rice husk ash (RHA) and 10% SF improved the compressive strength, compared to using only SF. 3) The use of deformed (hooked-end and twisted) steel fibers improved the mechanical properties compared to the performance of straight steel fibers, i.e., the use of twisted steel fibers increased the tensile strength, strain capacity, and flexural strength by about 32%, 205%, and 167%, respectively, compared to short straight steel fibers. By increasing the length of the straight steel fibers at an identical diameter, higher flexural strength, deflection capacity, toughness, and fracture energy were obtained; this is caused by the better fiber pullout performance without any significant change of the number of fibers at the crack surfaces. 4) The flexural strength of UHPFRC was clearly influenced by the casting method, whereas the fracture energy was not. By increasing the casting speed of the layer-casting method for uniaxial beams, a preferred fiber alignment was observed. This led to the improved flexural performance. The biaxial panels cast in the center had more fibers aligned perpendicular to the crack surfaces; thus, a higher flexural strength was obtained compared to counterpart panels made with other placement methods. Even though the actual probability density function (PDF) for fiber orientation distribution was different to that of 2- D and 3-D random fiber orientations, the assumption of 2-D random fiber orientation was reasonable for simulating the flexural behavior of uniaxial UHPFRC beams without reinforcing bars.