Influence of fiber orientation on the microstructures of interfacial transition zones and pull-out behavior of steel fiber in cementitious composites

Abstract

The reinforcement of steel fibers on cementitious composites is mainly influenced by the bonding between the fiber and matrix. The steel fibers and matrix bonding depend on the interfacial transition zone (ITZ). A major cause of ITZ formation is microbleeding, i.e., the flow of free water around steel fibers at the early stage of bleeding may cause the redistribution of water and cement particles around the fibers, leading to the formation of ITZs. The fiber orientation in the matrix affects the flow of water and further influences the microstructure of the ITZ around the steel fiber and the bonding properties. This study proposes a theoretical model based on fluid mechanics to investigate the influence of fiber orientation on cement-water suspension migration around steel fibers. The microstructure of the ITZ around the steel fiber is then estimated based on the Powers model according to the water-cement ratio distribution. Finally, the interrelationships among the fiber orientation, ITZ porosity and bond strength are established by curve fitting. The results indicate that since free water with small particles flows past the steel fiber by flowing past circular (or elliptical) cylinders, a difference in the ITZ microstructures at the upper and lower surfaces of the steel fiber arises. As the inclined angle of the fiber increases, the upper surface porosity and the average porosity of the ITZ around the steel fiber decrease, leading to an increase in the fiber-matrix bond strength.