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Rheology applied to Cement and Concrete Technology

Measure rheology of concrete in R&D and Quality Control without traditional inaccuracies created by segregation or improper mixing energy.
Overview 
 

Rheology measures the flow and deformation of matter. In the concrete industry, rheology is a crucial part of material science. The quest for concrete with a lesser water-to-cement ratio poses ever-increasing challenges to maintain adequate rheology during transportation, placement and finishing.  Whether it is in the development of new admixtures, the creation of industry norms and standards, the understanding of material interaction, or the complex mix designs and quality control required in technically challenging projects, it is important to measure rheology precisely and in conditions that mimic job site conditions in order to both optimize the financial return and the quality of concrete in built structures.

           

Traditional models to infer values of parameters such as viscosity, shear stress rely on specific impeller geometries and a laminar flow. But in inhomogeneous mixtures with a granular composition, like concrete, the flow of particles is not laminar. With common impeller geometries, particles in suspension will typically segregate, and at this point the instrument no longer measures the rheology of real concrete, but rather of one of the segregated phases. To ensure that granular matter remains in suspension, a more turbulent flow is needed, and then torque is measured as a proxy for shear stress, and rotation speed as a proxy for shear rate. Then, torque measurement accurately reflects the real rheological behavior of concrete (non-segregated). Therefore, in order to assess the rheology of concrete or other granular mixtures in suspension,  the rheometer used in such cases should be designed to impart a continuous mixing action while measuring torque. 

 

The Pheso rheometer is such a device designed for testing of inhomogeneous mixtures.

 

Typical uses of rheology measurements in concrete can be found in: 

 

  • Research and Development

    • Admixture formulation, such as slump-retaining admixtures

    • Characterization of Supplementary Cementitious Materials

    • Evaluation of new materials, or aggregates made of different materials, different shapes or different particle size distributions

    • Measure the Static and Dynamic Yield stress
       

  • Concrete Mix Design

    • Compare mixes using different sources of cement

    • Design mixtures for self-consolidating concrete, assess the thixotropy of a mix

    • Determine the best mixing profile or sequence for concrete with fibers

    • Compare different brands or types of admixtures

    • Compare mixes with various Fly Ash or Slag sources.  

    • Test the effect of different admixture addition times (upfront in mixing water or delayed), ...

 

  • Troubleshooting, Quality Control and Performance Limits

    • Determine the maximum dose of an admixture with a given cement before rheology gets affected. Especially recommended for high performance concrete.

    • Help determine the optimum dosage of admixtures or Supplementary Materials with different cements 

    • Understand the effect of a change in mixing energy on rheology 

    • Spot potential problems of adverse material interaction

    • Extended testing to evaluate the rheological profile during transportation of concrete in a mixer truck

    • Assess pumpability of concrete, e.g. for high rise construction or other mass concrete projects

 

 

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Example of test conducted in the Pheso rheometer

This example shows a study conducted by Roberto C. de Oliveira et al.* for mortars made with different aggregate sizes. The torque measurements show the sensitivity of rheology both to the particle size of aggregates and to dosage of air entraining agents. 

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The mortar with the lowest gap between the fine and coarse fractions of particles (C2 - red curve) has the highest torque and takes the longest to level out, indicating it is more difficult to mix and reach a stable rheological state. The addition of an air entraining agent has a large impact in all three mortars, generally improving the overall rheology and reducing the variations caused by the different aggregate sizes.

 

*Roberto C. de Olivera Romano, et al., "Impact of aggregate grading and air-entrainment on the properties of fresh and hardened mortars", Construction and Building Materials 82 (2015)

 

 

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