Rebound and toughening mechanisms in steel fiber reinforced dry-mix shotcrete

Despite its worldwide application, dry-mix shotcrete is characterized by a 30 to 40% material loss due to rebound. For cases in which steel fibers are used, fiber rebound tends to be even greater at approximately 75%. This represents one of the main drawbacks to this technique and is one of the primary challenges facing the shotcrete industry today. Therefore, this work aimed at examining the fundamental mechanisms of aggregate and fiber rebound in dry-mix shotcrete and the parameters influencing them. In order to deal with aggregate rebound, a high speed camera was used to observe the shooting and rebound processes and an extensive shotcrete experimental program was carried out to investigate the various parameters of mix design and shooting technique that cause rebound. Additionally, using a theory of plasticity approach, a general model of aggregate rebound for shotcrete was developed and shown to be in good agreement with experiments. Shotcrete tests show that proper adjustment of the mix-design and shooting technique can lead to minimized aggregate rebound. The problem of fiber rebound was investigated using an experimental approach in which various mix designs and fiber geometries were produced and tested in actual dry-mix shotcrete conditions. It was found that steel fiber rebound is linearly related to a fiber aspect ratio given by the fiber length divided by the square root of its diameter. Special emphasis was given to the development of a steel fiber for dry-mix shotcrete with reduced rebound and optimized toughness performance. In order for this to be possible, a new concept in fiber anchorage was introduced and a computer model, capable of relating the pull-out of single fibers to the post-cracking flexural strength of shotcrete was developed and used to optimize this new fiber geometry for shotcrete conditions. Prototype tests using this new fiber in dry-mix shotcrete show significantly enhanced toughness performance when compared to the most efficient existing commercial fibers.Applied Science, Faculty ofCivil Engineering, Department ofGraduat