LS-DYNA provides three methods for modeling reinforced concrete: integral model, composite model, and discrete model.
(1) Integral Model: Rebar and concrete are treated as a continuous homogeneous material
Introduction: Rebar is uniformly dispersed in concrete, approximately equivalent to a strength-enhanced concrete material.
Advantages and Disadvantages: This homogenized model is easy and fast to build, with high computational efficiency. However, it cannot characterize the interaction between rebar and concrete, nor can it reflect the influence of rebar arrangement, rebar stress and strain, and rebar on concrete cratering, spalling, and projectile attitude.
Application: Commonly used in large-scale reinforced concrete structure analysis, such as building demolition.
Keyword example: *MAT_PLASTIC_KINEMATIC
(2) Composite Model: The composite model is an improvement over the integral model
Introduction: This model can consider rebar and concrete separately within a single element. The two materials are assumed to have no-slip bond with consistent deformation, and the equivalent parameters of the reinforced concrete material model are calculated through volume weighting.
Advantages and Disadvantages: The composite model is an improvement over the integral model, but as an equivalent model, it has the same disadvantages as the integral model.
Application: Schwer believes that the composite model is only suitable for situations where the rebar is in elastic small deformation or minimal yielding, and does not recommend using this model in situations where rebar or concrete failure occurs.
Keywords: *MAT_PSEUDO_TENSOR, *MAT_CONCRETE_DAMAGE, *MAT_WINFRITH_CONCRETE used in combination with *MAT_WINFRITH_CONCRET_REINFORCEMEN, *MAT_BRITTLE_DAMAGE, *MAT_CONCRETE_EC2, *MAT_RC_BEAM, *MATRC_SHEAR_WALL and other material models all support the composite model.
(3) Discrete Model: Rebar uses LINK or BEAM elements, concrete uses SOLID elements
The discrete model uses shared nodes, coupling method, and Lagrangian contact method to define the interaction between rebar and concrete. Among them, the shared node method and coupling method assume complete bond between rebar and concrete; the CONTACT_1D method can consider the bond-slip of reinforced concrete by defining a slip-stress relationship curve.
3.1 Shared Node Discrete Model:
Introduction: Rebar and concrete are completely bonded, and both rebar and concrete must use Lagrangian elements or ALE elements simultaneously. Advantages and Disadvantages: The shared node discrete model using Lagrangian elements has relatively high computational efficiency, but the disadvantages are complex modeling, concrete mesh division is constrained by rebar arrangement, axial slip of rebar cannot be defined, and when rebar uses beam elements, the nodal degrees of freedom of the two conflict (rebar beam elements have six degrees of freedom, while concrete 3D elements have only three degrees of freedom). The shared node discrete model using ALE elements can define rebar through the *INITIAL_VOLUME_FRACTION keyword, simplifying the modeling process, but the disadvantages of ALE elements include unclear material interfaces, energy dissipation, and high computational cost, making them unsuitable for applications with large spatial and temporal spans.
3.2 Non-shared Node Discrete Model (Coupling Method)
Introduction: Rebar is constrained and coupled in concrete to simulate the interaction between rebar and concrete. In the constraint coupling relationship, rebar is the slave surface and concrete is the master surface.
Advantages and Disadvantages: (1) The modeling process of the discrete model is more flexible, with rebar and concrete meshed separately. Concrete mesh division is not limited by the geometric dimensions and positions of rebar, resulting in the minimum number of elements after meshing and the shortest solution time. Meanwhile, the coupling method allows mesh overlap, enabling its use in complex reinforced concrete models. The disadvantage is the inability to simulate the bond-slip phenomenon between rebar and concrete, causing energy loss while ensuring mass and momentum conservation. (2) In CLIS and ACNC keywords, rebar and concrete are completely bonded. Both Lagrangian constraint and fluid-structure coupling methods can be used. In the fluid-structure coupling method, concrete should use ALE elements, which also has the disadvantage of high computational cost. Furthermore, Schwer points out that when using these two types of keywords, rebar no longer carries axial load after concrete elements fail in tension, thereby underestimating the tensile strengthening effect of rebar on concrete. (3) The CBIS keyword is a Lagrangian constraint method, where in addition to normal bond between rebar and concrete, axial slip can also be customized, and it corrects some errors in the CLIS keyword with higher computational efficiency. (4) The DBSC keyword allows both Lagrangian constraint and penalty function methods.
Keywords: *CONSTRAINED_LAGRANGE_IN_SOLID, *ALE_COUPLING_NODAL_CONSTRAINT, *CONSTRAINED_BEAM_IN_SOLID, *DEFINE_BEAM_SOLID_COUPLING
3.3 Non-shared Node Discrete Model (Lagrangian Contact Method)
Introduction: *CONTACT_1D and *CONTACT_TIED are both Lagrangian contact methods. The CONTACT_1D modeling approach can be viewed as a combination of the shared node method and the coupling method, defining the bond-slip relationship between each rebar and adjacent concrete nodes through contact. The more rebar there is, the more cumbersome the contact definition process becomes.
Advantages and Disadvantages: *CONTACT_1D uses one-dimensional beam elements for rebar; *CONTACT_TIED can use one-dimensional beam elements or three-dimensional elements for rebar, but the disadvantages of using three-dimensional elements for rebar are obvious: the mesh size of rebar and adjacent concrete is very small, resulting in excessively small computational time steps and long computation time, and it is difficult to generate high-quality hexahedral meshes for concrete. The *CONTACT_1D method can simulate the bond-slip relationship between rebar and concrete, and the analysis results are closest to actual conditions. However, this method has the most complex modeling, the largest number of elements, and the most computationally time-consuming. Therefore, the Lagrangian contact method is used locally where slip is large, and the coupling method is used elsewhere.
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