Interfacial bond characteristics of steel fibers embedded in cementitious composites at high rates

In this study, the source of the rate-sensitive steel fiber pullout response in cement-based matrices was investigated at pullout rates of 0.0167–500 mm/s. A new pullout impact machine utilizing elastic strain energy to generate high-rate pullout loads was used in high-rate fiber pullout tests. In ultra-high-performance concrete (UHPC), smooth (S) steel fibers generated a higher peak bond strength dynamic increase factor (DIF, 1.9) than hooked steel fibers (DIF, 1.25), whereas S fibers in high-strength mortar (96 MPa) did not exhibit noticeable rate sensitivity. Furthermore, the addition of a shrinkage-reducing agent to UHPC clearly decreased the rate sensitivity of S fibers owing to the reduced matrix shrinkage: the peak bond strength DIF decreased from 1.9 to 1.77. The theoretical analysis revealed that the fiber pullout rate sensitivity was correlated with the interfacial crack path in a manner that depended on the fiber geometry as well as the matrix composition.