Conclusion

This review has presented recent developments in the rheology of concrete, including:

• Tests that are commonly used
• Models and simulation of concrete flow
• Factors that affect the rheological properties of concrete
• Examination of two special applications: pumpable concrete and self-compacting concrete.

Concrete workability can be characterized in terms of the rheological parameters in either the Bingham or Herschel-Buckley equation. The theory exists but the measurements are not easy to obtain. The flow of a granular material like concrete needs to be defined by at least two parameters for instance yield stress and plastic viscosity as defined by the Bingham equation. While there are numerous tests to characterize the flow of concrete, few give results in fundamental units and therefore the measured rheological properties of concretes, using different tests, cannot be directly compared. Recently, new tests have attempted to characterize concrete using a more fundamental approach. While not all researchers agree on which test is the most suitable for the wide range of concretes in use today, all agree that tests that give results in fundamental units and that can be used on a construction site are needed.

Three different models allow the prediction of the flow of concrete from the mixture composition. The approaches of these models are based on:

• Packing density of all solids included in concrete, i.e., cement, sand and aggregates.
• Simulation of granular flow using Dissipative Particle Dynamic methods.
• Application of the Krieger-Dougherty equation.

Factors affecting the flow of concrete extend beyond mixture composition, with processing and the environmental factors also affecting rheological behavior.

The application of rheological principles to pumpable concrete and SCC seems to be a particularly promising field of investigation. The prediction of pressure/flow rate relationship in pumping networks is critical for today´s construction industry. Such a calculation appears possible, based upon scientific measurements of the concrete flow, but of interaction at the steel/concrete interface, which can be readily performed with a large-gap coaxial viscometer. In the case of SCC, the properties are heavily based on the viscosity of the cement paste and of the concrete. These properties need to be properly characterized to be able to design concrete mixtures with improved performance.

Progress in the areas discussed in this review will facilitate specification of concrete based on performance instead of prescriptive criteria. Research is being performed to validate the models and simulations that will provide new tools for use in optimization of day to day processing of concrete in ready-mixed concrete plants and other concreting operations.