Some Considerations on Yield Stress: A Modeler's Perspective

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Some Considerations on Yield StressA Modeler's
Perspective

• Nicos Martys
• NIST

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Why study the rheology of concrete?

• Concrete is the most widely used building
  material.
• Ease of flow and placement can significantly
  reduce costs.
• Architectural benefits Improving workability and
  mechanical properties allows for more
  flexibility in design.

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Viscosity
F/A
V/L
F/A
F applied force A area V velocity L gap
width
V
L
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Yield Stress
F/A
V0
L
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Interaction potential
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ModelMeasurement

• Bingham model
• Hershel Buckley
• Modified Casson Equation

.
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Stress Growth
v
100 10 1 .1 .01 .005
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Bingham Model
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Low Shear Rate Limit
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Comparison of models
Casson qp2
Brinkman
Hershel Buckley
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Role of force vs. potential
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Stress Growth-Raw Data
V/KT
40 40 10
5
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Scaled Data (I)
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Scale Data (II)
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Radial distribution functionV(r)
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Radial distribution function for scaled
potential V(3r)
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Observations on Yield Stress

• 1)Over small strains (ltlt1) stress growth
  simulation data can be collapsed using maximum
  force and interaction potential width as scaling
  parameters (at fixed concentration).
• 2)Over larger strains (0.1) entropic barriers
  begin to complicate analysis.

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Mean Squared Displacements Volume
fraction0.52
Conrad and Weitz
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Yield Stress?
V1 V.01
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Some possible questions

• 1)Further validation steps?
• 2)What are the most reliable measurements to
  compare to?
• 3)Other improvements?
• 4)Potential use of AFM or other methods to
  determine inter-particle forces.

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Realistic interaction forcesNonat et al.
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Cement Particles
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Cement Particles
N N
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Yield Stress vs. Volume Fraction
Flatt and Bowen
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Acknowledgements

• Ray Mountain Numerical algorithms
• Julien Lancien, Jim Sims and Bill George
  parallel code development
• Steve Satterfield, John Hagedorn and Tere
  Griffin visualization
• Ed Garboczi realistic aggregate shapes
• Clarissa Ferraris, Robert Flatt (Sika),Vijay
  Gupta (W.R. Grace), David Fowler (ICAR), John
  Winpigler, Daniel Constantiner (Degussa)
• VCCTL, ACBM
• NASA

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Sheared Cubes
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Spheres and Cubes
Open symbolssimulation Filled symbolsExperiment
Koehler and Erdogan
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Revisit Bingham Model
Y8 Y3 Y1.6