Hydration-Drucker-Prager Model

The mechanical aspects of hydration in FLAC3D are handled by a modified Drucker-Prager constitutive model where elastic and strength properties depend on the hydration grade, α (Hinze 1987). [*]

Due to a dormant phase, the evolution of strength and stiffness starts with some delay. This is taken into account by the minimum degree of hydration, α0. This value marks the transition between the suspension and solid-state behavior. Beyond α0, strength and stiffness do not always depend linearly on hydration grade. Thus, a relationship is introduced, based on the idea of a multiplicative split of the final values of material properties and the degree of hydration, including the minimum degree of hydration according to the power law in Equation (1):

(1)f(α)=max(1×104,(αα01α0)a)

During evolving, (αα0) may be less than zero when α<α0. This is avoided by enforcing (αα0)(αα0)min, where (αα0)min is an input with a default value 1e-6.

With this formulation, the actual (and initial) Young’s modulus, E, during the hydration process is

(2)E(α)=f(α)Ecte

where Ecte is the Young’s modulus (stress unit) after complete hydration, and a is the power exponent (no unit).

The actual uniaxial compressive strength σc and the uniaxial strength σt also depend on the function in Equation (1).

(3)σc(α)=0.85fctec(αα01α0)3/2
(4)σt(α)=fcte(αα01α0)

where fcte is the uniaxial strength (stress unit) after total completion of the hydration process, and c is a material parameter (no unit). In the above two equations, (αα0)(αα0)min is enforced as well.

The yield criterion in the Drucker-Prager model is

(5)0=τ+qσk

where q and k are material parameters, and τ and σ are stress invariants. q and k can be derived from the actual uniaxial compressive and tensile strengths, σc and σt.

(6)q=3(σcσt)σc+σt
(7)k=2σcσt3(σc+σt)

In this model, the compression strength is assumed no less than one third of k/q (σck/q) and greater than the extension strength (σc1.001×σt is used).


[*]During the hydration process, the values of elastic material parameters can vary over several orders of magnitude. Accordingly, the gridpoint masses have to be adjusted for numerical stability in both small-strain mode and large-strain mode. The frequency of the update can be set by the user with the zone geometry-update command.

Reference

Hinze, D. “Zur Beurteilung des phsikalischen nicht-linearen Betonverhaltens bei mehrachsigem Spannungszustand mit Hilfe differenzeiller Stoffgesetze unter Anwendung der FEM,” Thesis, Hochschule für Architektur und Bauwesen, Weimar (1987).


Hydration-Drucker-Prager model properties

Use the following keywords with the zone property command to set these properties of the Hydration-Drucker-Prager model.

hydration-drucker-prager
bulk f

bulk modulus, K

bulk-reference f

reference bulk modulus for α = 1, Kcte

cohesion-drucker f

Drucker-Prager material parameter, kφ

compression f

compressive strength limit, σc

constant-a f

material parameter, a

constant-c f

material parameter, c

dilation-drucker f

Drucker-Prager material parameter, qψ

friction-drucker f

Drucker-Prager material parameter, qφ

hydration-minimum f

minimum hydration grade, α0

hydration-difference-minimum f

minimum difference of (αα0)min

poisson f

Poisson’s ratio, ν

shear f

shear modulus, G

shear-reference f

reference shear modulus for α = 1, Gcte

tension f

tension cut-off, σt

tension-reference f

reference tensile strength for α = 1, fcte

young f

Young’s modulus, E

young-reference f

reference Young’s modulus for α = 1, Ecte

Key

(r) Read-only property.
This property cannot be set by the user. Instead, it can be listed, plotted, or accessed through FISH.

Notes

  • Only one of the two options is required to define the elasticity: reference bulk modulus Kcte and reference shear modulus Gcte, or reference Young’s modulus Ecte and Poisson’s ratio v. When choosing the latter, reference Young’s modulus E must be assigned in advance of Poisson’s ratio v.
  • The tension cut-off is σt = min (σt(α)/3.0, kϕ(α)/qϕ(α)).