The flexural behaviors and mechanism of wollastonite microfiber modified ultra-high performance concrete with steel fiber from micro to macro scale

Wollastonite microfiber (WF) is a naturally occurring calcium silicate (CaSiO3) produced in fibrous form and often used in ceramic industry as a cheap and valuable mineral. It is tried to be applied in ultra-high performance concrete (UHPC) in this study with expectation to improve the flexural performance and compensate for the deficiencies of steel fiber in enhancing UHPC from micro scale. The effects of WF on the flexural behaviors of UHPC with or without steel fibers were explored. The bonding behaviors of steel fiber in WF-modified ultra-high performance concrete (WFMUHPC) under different curing conditions were researched combined with single fiber pull-out tests. The results showed that WFs could significantly resist and delay the formation of microcracks in UHPC. When WFs were added to UHPC with steel fiber, the flexural properties of concrete were significantly improved from micro to macro scale. Accelerated curing contributed to the flexural strengths but deteriorate the toughness of WFMUHPC with steel fiber. The presented load-deflection curves proved that WFs had a significant improvement of first crack load and there were post-peak curve gaps because of the reinforcing effect of WFs on the frictional sliding behavior of steel fiber. The results of X-ray diffraction and scanning electron microscope showed that WFs had the bridging and filling effect and improved the interfacial transition zone between WFs and matrix. Meanwhile, the combined effect between WFs and high temperature that WFs provided sites for hydration products from cement particles and active minerals including silica fume and fly ash further improved the flexural behaviors. Moreover, a flexural strength model established could accurately describe the reinforcing effect of WFs on this particular UHPC and was expected to provide guidance for practical engineering applications.

Automated summary

The study found that adding WF could significantly resist and delay the formation of microcracks in UHPC, and when added to UHPC with steel fibers, it greatly improved the flexural properties of concrete from micro to macro scale. Accelerated curing improved flexural strengths but decreased the toughness of WFMUHPC with steel fiber. The load-deflection curves showed noticeable improvement in first crack load due to the reinforcement effect of WFs on steel fiber.