Rheological analysis of wet-mix shotcrete with various viscosity-modifiying admixtures

Shotcrete must be suitable for the process of transport (pumping) and application (spraying) on site. Therefore, it is necessary to obtain a suitable consistency and rheology for its placement. This paper evaluates the influence of various viscosity modifying admixtures (VMAs) on the rheology and thixotropy for application in wet-mix shotcrete. Six VMAs were used and classified into three groups, according to their composition: additives based on silica, phyllosilicates, and polymeric additives. These rheology modifiers were studied in depth in mortars, and the values of the yield stress (tau(o)) and plastic viscosity (mu) of the material were obtained, as well as the thixotropy (hysteresis areas), which represents the energy required for the structural recovery of the fluid. To obtain these parameters, the fluid was tested under dynamic state with a laboratory rheometer, applying a ramp of shear rates. Additionally, the consistency in mortars was determined by obtaining the flow table diameter on the flow table test. This evaluation was carried out in mortars with different contents of superplasticizer (SP). All this information made it possible to evaluate the influence of SP in combination with each of the VMAs, obtaining a workability box, determining the hysteresis areas and verifying which combinations obtained a rheological behaviour superior to the control mix (without VMAs). The results described were correlated with the rebound index obtained in the shotcrete mixes performed on-site. The thixotropy in mortars and the values of the rebound rate on site led to the most accurate correlation that allowed the selection of the most effective VMAs for use in shotcrete. Lastly, both combined results (laboratory and on-site) permitted a process that facilitated analysis of the wet-mix shotcrete to be designed and optimised.