The rheology of fresh concrete and its effect on the shotcrete process

An experimental study has been carried out to investigate the effect of the rheological properties of fresh concrete on the shotcrete process. The experimental work was divided into two areas: investigations of the rheology of fresh cement paste and concrete; and studies of the bond strength between the concrete and the reinforcing steel for reinforced shotcrete. Finally, these two areas are integrated and technological applications are discussed. The rheology of fresh cement paste was researched to generate trends to guide in the selection of the mix proportions and admixtures for the study of the rheological properties of fresh concrete. The cement pastes were tested with an orifice rheometer. Four admixtures and their combinations were used to determine their applicability to the shotcrete process. The admixtures studied were high range water reducer (superplasticizer), fly ash, silica fume, and polysaccharide gum. The rheological properties of the mixes containing these additives were characterized, and some of the mix proportions were selected based on the rheological requirements of the shotcrete process. Further rheological measurements were taken employing a concentric cylinders viscometer for cement pastes containing polysaccharide gum. The inclusion of this additive would result in better mixes that follow closely the requirements imposed on the fresh concrete during the shotcrete operations.;Subsequently, a series of tests on fresh concrete using a concentric cylinders viscometer were performed. The rheology of fresh concrete showed similar trends to the test performed with cement pastes. It was found that the utilization of polysaccharide gums for shotcrete applications might prove to be beneficial for the rheology of fresh concrete. Therefore, an additional set of experiments was conducted with a concentric cylinders viscometer to further study the effect of polysaccharide gums addition on the flow properties of concrete. The concrete mixes containing this additive yielded concretes with a high flow resistance, and a relatively low consistency for higher shear rates.;The rheology of fresh concrete was also evaluated with conventional devices. These simple tests did not generate accurate rheological information, but they can be used as a control for the construction site operations. They can be the link between the laboratory experiments and the shotcrete operations. It was found that the quality of the shotcrete operations can be represented in terms of the quality of the concrete around the reinforcing steel bars. Thus, it was proposed to test the performance of the shotcrete process through the study of the bond strength between the shotcrete and the steel. Twelve beams were tested with mix proportions containing two different admixtures. The results showed that the performance of the shotcrete operations depends heavily on the rheology of the fresh concrete. Information on shotcrete repairs and the relative performance of shotcrete against the design theoretical calculations and the performance of regular cast in place concrete was also presented.