Influence of Key Parameters on Quality of Dry-Mix Shotcrete

Two main shotcreting processes exist. In the dry-mix process, all the components are mixed dry, and then a jet of water is added to the mixture as it leaves the shooting nozzle. In the wet-mix process, components are mixed wet and then shot through the nozzle. However, relatively few data are pertinent to factors that have a vital effect on the shotcrete quality when the shotcrete is applied using prepackaged (preprepared) materials or materials prepared on site. The effects of several parameters on the quality of dry-mix shotcreting were investigated: shooting distance, water-to-cementitious materials ratio, and mix proportion. Panels were shotcreted using conventional on-site mixtures and prepackaged mixtures. Cores were extracted from the surface, and beams were sawed from the panels for testing. Concrete absorption, concrete unit weight, rebound losses, compressive strength, flexural strength, and Schmidt (rebound) hammer tests were performed on both mixtures. Results indicate that the overall quality of the prepackaged material is superior to that of the on-site mix. They also suggest that a shooting distance exists at which mechanical properties as well as losses may be at an optimum. Findings strongly indicate that the traditional nondestructive tests (e.g., Schmidt hammer and ultrasonic pulse velocity) should be applied to shotcrete with some precautions because of changes in the nature of the concrete surface. Additional work should be carried out to elaborate on the shotcrete parameters and investigate their effects when other parameters and techniques are implemented.