New Energy Dissipation Mechanisms for Steel Fiber Reinforcement in Ultra High-Performance Concrete

This research focused on improving ultra high-performance concrete (UHPC) toughness through the addition of annealed plain carbon steel fibers and stainless steel fibers that both exhibit increased ductility and strain hardening compared with conventional steel fibers used for concrete reinforcement. Implementing optimized heat treatments and selecting proper alternative alloys can improve the postyield carrying capacity of UHPCs through plastic deformations, phase transformations, and fiber pullout. This research focused on the flexural response and dynamic penetration resistance of a UHPC known as Cor-Tuf with three different steel fiber types: plain carbon steel, stainless steel that can exhibit phase transformation-induced plasticity, and annealed carbon steel with reduced tensile strength but increased ductility and strain hardening. Annealed carbon steel fibers were able to reduce mass loss by 0.8 % for 5-cm-thick dynamic impact panels. By using a phase-transformable stainless steel, the ultimate flexural strength increased from 32.0 to 42.5 MPa (33% increase) and the postimpact velocity decreased an average of 31.5 m/s for 2.5 and 5-cm-thick dynamic impact panels. Phase transformations (austenitic to martensitic) were quantified in the stainless steel fibers of postyielded UHPC specimens using a vibrating sample magnetometer. Stainless steel fibers sampled from the postyielded tensile face of a flexural beam increased from 75 to 107 emu/g. The results of the study evidence improvements in tensile properties and toughness that can be accomplished by modifying the stress versus strain response of steel fiber reinforcement and including new energy dissipation mechanisms such as phase transformation.