A meso-scale analysis is performed to determine the fracture process zone of concrete subjected to uniaxial tension. The meso-structure of concrete is idealised as stiff aggregates embedded in a soft matrix and separated by weak interfaces. The mechanical response of the matrix, the inclusions and the interface between the matrix and the inclusions is modelled by a discrete lattice approach. The inelastic response of the lattice elements is described by a damage approach, which corresponds to a continuous reduction of the stiffness of the springs. The fracture process in uniaxial tension is approximated by an analysis of a two-dimensional cell with periodic boundary conditions. The spatial distribution of dissipated energy density at the meso-scale of concrete is determined. The size and shape of the deterministic FPZ is obtained as the average of random meso-scale analyses. Additionally, periodicity of the discretisation is prescribed to avoid influences of the boundaries of the periodic cell on fracture patterns. The results of these analyses are then used to calibrate an integral-type nonlocal model.

Meso-scale approach to modelling the fracture process zone of concrete subjected to uniaxial tension

Meso-scale modelling of the size effect on the fracture process zone of concrete

Reinforced concrete structures are frequently exposed to aggressive environmental conditions. Most notably, chloride ions from sea water or de-icing salts are potentially harmful since they promote corrosion of steel reinforcement. Concrete cover of sufficient quality and depth can ensure protection of the steel reinforcement. However, it is necessary to study the effects of material heterogeneity and cracking on chloride ingress in concrete. This is done herein by proposing a three-dimensional lattice model capable of simulating chloride transport in saturated sound and cracked concrete. Means of computationally determining transport properties of individual phases in heterogeneous concrete (aggregate, mortar, and interface), knowing the concrete composition and its averaged transport properties, are presented and discussed. Based on numerical experimentation and available literature, a relation between the effective diffusion coefficient of cracked lattice elements and the crack width was adopted. The proposed model is coupled with a lattice fracture model to enable simulation of chloride ingress in cracked concrete. The model was validated on data from the literature, showing good agreement with experimental results.

Lattice modeling of chloride diffusion in sound and cracked concrete

In this paper, a survey of the most relevant advances in natural neighbour Galerkin methods is presented. In these methods (also known as natural element methods, NEM), the Sibson and the Laplace (non-Sibsonian) interpolation schemes are used as trial and test functions in a Galerkin procedure. Natural neighbour-based methods have certain unique features among the wide family of so-called meshless methods: a well-defined and robust approximation with no user-defined parameters on non-uniform grids, and the ability to exactly impose essential (Dirichlet) boundary conditions are particularly noteworthy. A comprehensive review of the method is conducted, including a description of the Sibson and the Laplace interpolants in two- and three-dimensions. Application of the NEM to linear and non-linear problems in solid as well as fluid mechanics is studied. Other issues that are pertinent to the vast majority of meshless methods, such as numerical quadrature, imposing essential boundary conditions, and the handling of secondary variables are also addressed. The paper is concluded with some benchmark computations that demonstrate the accuracy and the key advantages of this numerical method.

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Overview and recent advances in natural neighbour galerkin methods

A random lattice is used to model moisture transport in cement-based composites. Model geometry, and the scaling of the elemental diffusivity terms, are based on a Voronoi discretization of the material domain. Steady-state and transient potential flow problems are simulated, and compared with theory, to demonstrate model accuracy and objectivity with respect to lattice random geometry. A novel routine is described for computing moisture flux values at the random lattice sites. Stress production, and potential shrinkage crack development, are driven by the associated drying processes. The random lattice modeling of moisture movement is coupled with rigid-body-spring networks (RBSN), which account for elasticity, creep, and fracture properties of the material. The RBSN is based on the same Voronoi discretization of the material as used to model moisture movement. Relative humidity contours, stress contours, and crack plots are produced for a cement composite overlay restrained by a mature concrete substrate. That example is based on a set of simulation results reported in the literature.

Simulation of shrinkage induced cracking in cement composite overlays

Crack band model of fracture in irregular lattices

Seismic protection systems include the use of special passive devices to introduce supplemental damping into the structure. In building applications, passive devices are normally used in brace configurations and activated by a component of the structural drift. However, in applications where small elastic interstory displacements are expected, the conventional usage of such devices may not be suitable. Lever mechanisms, which amplify the displacements and velocities affecting the damping device, can be employed to overcome this deficiency. This work describes a displacement amplification device (DAD) based on a gear-type mechanism and connected in series with a fluid viscous damper (FVD). Laboratory test results are given here for a small-scale version of the DAD-FVD system. Potential implementations of this amplification system in civil structures are briefly described.

Stefano Berton

Papers

Simulation of shrinkage induced cracking in cement composite overlays

Crack band model of fracture in irregular lattices

Amplification System for Supplemental Damping Devices in Seismic Applications