13th International Conference on Fracture June16–21, 2013, Beijing, China -3concurrent inelastic flexural loading and 15 percent sodium chloride (NaCl) solution application. The results provided insight into the sustained robustness of UHPC structural members loaded beyond their tensile cracking strength [10]. Test results from Lee et al. [11] revealed that UHPC specimen (5 cm × 10 cm cylinder) was still in good condition after 600 cycles of freezing and thawing in accordance with ASTM C 666-97. After freezing and thawing test, the non-shrinkage high strength mortar showed a reduction in compressive strength, slant shear strength, steel pull out strength, and dynamic modulus by 17, 21, 24, and 25 %, compared with the corresponding values of 6, 7, 5, and 10 %, respectively, for UHPC. Specimens of normal strength concrete were used as reference specimens. For the normal strength concrete, the average value of relative dynamic modulus of elasticity based on resonant frequencies after 600 freeze-thaw cycles was 55%, compared with the corresponding value of 92% for UHPC. The goal of this paper is to describe the research done toward the realization of UHPC as an option for precast concrete products. Special concern will be given to the steel fibres by volume were chosen and used in UHPC precast products in severe freeze-thaw conditions. In addition to the above, this experimental study also focuses on evaluating the the optimum steel fibre of UHPC precast products. 2. Experiments 2.1. Materials and mix design The cement used in this study was type II Portland cement conforming to ASTM C 150 which was manufactured by the Taiwan cement company. Table 1 displays its chemical composition and physical properties. The silica fume used was supplied by Sun-Li Trade Company in Taiwan. The specific gravity of the silica fume is 2.25. The physical and chemical properties of silica fume are displayed in Table 2. The UHPC to be used as a prospective material of precast product, contains type II Portland cement, silica fume, silica sand, quartz powder, steel fiber and a superplasticizer. The UHPC to be used as a precast product material contains a large amount of cement (type II Portland cement), silica fume, silica sand, quartz powder, steel fiber, superplasticizer, and a very small amount of water. The UHPC mixes with eight different volumes of steel fibres are shown in Table 3, were designed, tested and evaluated to find the optimum quality of precast production. Table 1. Chemical composition and physical properties of Portland cement II . Chemical composition (%) Physical properties C3S C2S C3A C4AF CSH2 1-day hydration heat Blaine 1-day strength 50 24 4.9 12.2 5 250 J/G 250 cm2/g 60 kgf/cm2 CSH2: gypsum
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