No abstract

Ultra-high performance concrete (UHPC) is promising in construction of concrete structures that suffer impact and explosive loads. In order to make UHPC structures more ductile and cost-effective, hybrid fiber reinforcements are often incorporated. In this study, a reference UHPC mixture with no fiber reinforcement and five mixtures with a single type of fiber reinforcement or hybrid steel fiber reinforcements of 6 and 13 mm in length at a total dosage of 2%, by the volume of concrete, were prepared. Quasi-static compressive and flexural properties of those mixtures were investigated. Split Hopkinson press bar (SHPB) testing was adopted to evaluate their dynamic compressive properties under three impact velocities. Test results indicated that UHPC with 1.5% long fiber reinforcements and 0.5% short fiber reinforcements demonstrated the best static and dynamic mechanical properties. The static compressive and flexural strengths of UHPC with 2% long fiber reinforcements were greater than those with 2% short fiber reinforcements, whereas comparable dynamic compressive properties were observed. Strain rate effect was observed for the dynamic compressive properties, including peak stress, dynamic increase factor, and absorbed energy. The reinforcing mechanisms of hybrid fiber reinforcements in UHPC were eventually discussed.

Novel hierarchical cobalt phosphate (α- and β-Co2P2O7) and cobalt hydrogen phosphate hydroxide [Co11(HPO3)8(OH)6] three-dimensional (3D) architectures with different morphologies were synthesized under a solvothemal condition. The form and shape of cobalt phosphate can be readily tuned by adjusting experimental parameters of the reaction system. The novel hierarchical α-Co2P2O7 microcrystals with basketry-like microstructures were obtained with a molar ratio of Co2+/H3PO4/CH3NH2/HO(CH2)2OH of 1:1.2:1:108, the reaction temperature of 180 °C, and the reaction time of 5 days; dumbbell-like β-Co2P2O7 microspheres were formed with the same reaction temperature and time but the molar ratio of 1:1.2:1:54, and Co11(HPO3)8(OH)6 with peony-like nanostructures was prepared with the same molar ratio and reaction time as β-Co2P2O7 but the reaction temperature of 120 °C. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectra (FT-IR) technologies, a Quantum Design superconducting quantum interference device (SQUID) magnetometer, and a network analyzer. The formation mechanism of cobalt phosphate microstructures was investigated in detail.