The connection between microscopic and macroscopic properties of ultra-high strength and ultra-high ductility cementitious composites (UHS-UHDCC)

There is an important connection between the microscopic and macroscopic properties of UHS-UHDCC with compressive strength (more than 160 MPa) and uniaxial tensile ductility (more than 6%). First, some microscopic properties were studied. For the fresh mortar matrix, the plastic viscosity within an optimal range is beneficial to achieve uniform fiber distribution. Compared with the fluorescence analysis, the backscattering analysis more truly displays the fiber distribution situation and cross section shapes of fibers. Based on the mesh size of three-dimensional grid formed by theoretically absolutely uniform distributed fibers, the river sand with an average particle size of 240 μm was selected as the aggregate. The addition of fibers slightly increases the porosity, the larger bubbles are not easy to be excreted due to the isolation of the fiber mesh. With the increasing of silica fume (SF) dosage and the decreasing of the water-binder ratio (w/b), the overall protruded length of fibers on fracture surface of tensile specimen reveals a decreasing trend, the hydration product attached to the fibers first increases and then decreases, and the decreasing accompanies a more severely scraped fiber surface, the fiber shedding changes from being no apparent to gradually increase. Increasing SF dosage or decreasing the w/b, the elastic modulus increases with different degrees, and the difference between the elastic modulus in interfacial transition zone (ITZ) of about 60 μm span and that of mortar matrix gets smaller and smaller. The elastic modulus in ITZ exhibits a gradient that first decreases and then increases. Further, the connection between microscopic and macroscopic (flexural、compressive、tensile) properties was analyzed. The highest fiber dispersion degree at mix proportion 0.4SF + W0.19 + PE and its suitable interfacial bonding strength together create the saturated multi-crack ductility of 6.53% and compressive strength of 163 MPa. When the strength growth benefitting from the increasing of interfacial bonding strength lowers than the strength loss due to the decreasing of the fiber dispersion degree, the ultimate tensile strength and the flexural strength of UHS-UHDCC decrease.