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Rheology of high performance shotcrete Beaupré, Denis


The main goal of this study was to develop high performance shotcrete and to provide a fundamental understanding of the shooting process. For this, a laboratory concrete pump, to pump and/or shoot a number of shotcrete mixes, and a rheometer, to measure the rheological properties on the fresh shotcrete, were designed and constructed. A model based on rheological behavior was finally developed to predict pumpability and shootability. The values of both flow resistance and torque viscosity were used to represent the rheological behavior of fresh shotcrete, which is similar to the Bingham model. Important fundamental relationships were obtained between rheological properties and pumping pressure, build-up thickness and compaction of shotcrete. With a new parameter, the fresh concrete aging rate, these relationships are used in a model which predicts pumpability and shootability. Production of high performance shotcrete can be carried out in two different ways: the “traditional method” consists of using superplasticizers while in the second method, the “concept of high initial air content” consists of using a very high air content to improve the workability. In both cases the requirements for good pumpability and shootability must be satisfied in order to be able to apply the shotcrete. These requirements are in conflict in terms of flow resistance: pumpability requires a low flow resistance while shootability requires a high flow resistance. The range of acceptable values for the flow resistance is reduced for mixes with high torque viscosity. The concept of a temporary high air content has some advantages over the traditional method: when compared to the use of superplasticizers only, the use of air, by reducing the torque viscosity, allows a wider range of acceptable values for the flow resistance to be maintained. Also, the compaction process allows a recovery of the flow resistance during shooting. Thus, this concept allows low water-cement ratio shotcrete having enhanced pumpability, shootability, strength, and durability to be produced. It could probably be an excellent way to avoid the use of accelerators which have adverse effects both on worker health and on concrete properties, especially the durability.

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