Numerical and experimental investigations for the modeling of the time-dependent deformation characteristics of concrete on the mesoscale with coupled models for mechanical and hygric effects

• Applicant: Dr.-Ing. Jörg F. Unger
• Subject Area: Construction Material Sciences, Chemistry, Building Physics ; Applied Mechanics, Statics and Dynamics ; Mechanics
• Term: from 2014 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 252766671

The modeling of time-dependent deformations of concrete is commonly performed with pure phenomenological approaches, where usually additional deformations due to creep and shrinkage are introduced. However there is experimental evidence that these effects are coupled in a nonlinear way (Picket effect) and are strongly related to mechanically induced damage and the moisture distribution within the specimen.The purpose of the project is the development and implementation of a numerical model for concrete that is able to simulate the coupled effects of mechanical loading and time-dependent local moisture distribution. The simulation will be performed with an explicit representation of the heterogeneous mesoscale structure based on a geometry model for concrete developed by the author. Concrete is modeled as a three phase composite with aggregates, cement paste and the interfacial transition zone (ITZ) as an additional weak link. The nonlinear behavior of the cement paste including the softening is modeled with a combined damage-plasticity model, which is regularized with a gradient damage formulation. This reduces the numerical effort compared to the nonlocal approach used in previous versions for large nonlocal radii. The viscous character of concrete is modeled with an extension of the plasticity model using a Perzyna-type creep model with hardening. This allows for a direct coupling between the mechanically induced damage and viscous creep deformations. The development of the time-dependent macroscopic properties (strength, stiffness) as a function of the moisture content during hardening of the cement paste is modeled with a modified solidification theory while ensuring to satisfy thermodynamic principles. The interaction between the mechanical model and the moisture content is realized by modeling concrete as a porous medium with a decomposition of the macroscopic stress components related to the skeleton and the capillary pressure. In addition, the moisture content influences the solidification rate. The ITZ is modeled using a cohesive interface approach that is extended to accurately describe the moisture transport as a function of the crack opening.The objective of the project is the development of a physics-based numerical model for creep and shrinkage that is able to accurately capture the complex macroscopic effects. It should be investigated if the modeling of coupled physical effects (viscous cement matrix, moisture transport, cement hydration) and the direct representation of the mesostructure are able to explain the complex phenomenological properties and interactions. As a result, the interpretation of experimental results with a realistic understanding of the physical processes on the mesoscale is facilitated. Additionally, the calibration process of the model is simplified due to clear physical meaning of the constitutive parameters.

• DFG Programme: Research Grants