Results of investigations carried out at the University of Innsbruck demonstrate that dust formation in the nozzle region is mainly influenced by the wetting level, resulting from the ratio of water content and solid matter particles. Due to the development of a new pre-wetting nozzle which injects compressed air and water into the locally narrowed material stream, dust formation can be lowered by over 60 % in the dry-mix shotcrete process. The amount of rebound in the dry-mix shotcrete process strongly correlates with the consistency of the sprayed concrete. In more than 120 laboratory tests the rheological properties of the mortars were correlated to the amount of rebound in the spraying process. Based upon these results a new powder admixture has been developed leading to a reduction of rebound of up to 45 % for the dry-mix shotcrete process combined with moist aggregates without altering water-cement ratio and without influencing strength development.
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