دانلود رایگان مقاله پیش بینی رفتار خمشی عملکرد فوق العاده بالا بتن تقویت شده با الیاف

Abstract

To predict the flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) beams including straight steel fibers with various lengths, micromechanics-based sectional analysis was performed. A linear compressive modeling was adopted on the basis of experiments. The tensile behavior was modeled by considering both pre- and post-cracking tensile behaviors. Pre-cracking behavior was modeled by the rule of mixture. Post-cracking behavior was modeled by a bilinear matrix softening curve and fiber bridging curves, considering three different probability density functions (PDFs) for fiber orientation, i.e., the actual PDF from image analysis and PDFs assuming either random two-dimensional (2-D) or three-dimensional (3-D) fiber orientation. Analytical predictions using the fiber bridging curves with the actual PDF or the PDF assuming 2-D random fiber orientation showed fairly good agreement with the experimental results, whereas analysis using the PDF assuming 3-D random fiber orientation greatly underestimated the experimental results.

4. Conclusions

The flexural behaviors of UHPFRC beams including straight steel fibers with various lengths were numerically investigated. For the numerical analysis, sectional analyses incorporating linear compressive and tensile models before cracking and tensionsoftening curves obtained from micromechanical approach and previous models were adopted and verified through a comparison with the test data. From the above discussions, the following conclusions are made: 1) The use of longer steel fibers resulted in better flexural performance, but poorer fiber orientation, compared to that with shorter steel fibers. In contrast, the initial and descending slopes of the flexural load-CMOD curve, the fiber dispersion, and number of fibers were not significantly affected by the fiber length. 2) To obtain tension-softening curve, the fiber bridging curve was calculated based on the pullout model of straight steel fibers embedded in an ultra-high-strength cementitious matrix and a bilinear matrix softening curve was adopted. In particular, the fiber bridging strength was influenced by the fiber length and orientation. Higher fiber bridging strength was obtained with a higher fiber length. The order of fiber bridging strength was also found to be 2-D random fiber orientation > actual fiber orientation by image analysis > 3-D random fiber orientation. 3) The sectional analyses incorporating the fiber bridging curve based on the PDFs using fiber orientation from image analysis exhibited fairly good agreement with the test data. The use of PDFs assuming 2-D random fiber orientation showed slightly higher load carrying capacity, whereas the use of PDFs assuming 3-D random fiber orientation exhibited much lower load carrying capacity than the experimental results. From these results, it was concluded that the micromechanics-based sectional analysis using the PDFs obtained from image analysis is most suitable for predicting the flexural response of UHPFRC beams. However, if the actual PDF is not obtained from the image analysis, the assumption of 2-D random fiber orientation can be adopted as a substitute to predict the flexural behavior. This is more accurate than the assumption of 3-D random fiber orientation. 4) Most of the previously suggested tension-softening models were only appropriate to a specific type of UHPFRC, and they were not generally applied for UHPFRCs with various lengths, shapes, and volume contents of fibers. This limitation was able to be overcome by using the micromechanical approach although it required quite complicated equations for modeling fiber bridging curve.