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Simulations of concrete on the mesoscale require complex geometry models that resolve single aggregates. Since they fill up to 60-80% of the virtual specimen, efficient algorithms for their placement are necessary.
Event-driven molecular dynamics (EDMD) algorithms using growing spheres are proven to create dense polydisperse sphere packings. In this paper, a modified EDMD algorithm is presented and compared to the widely used random sequential addition algorithm. It reaches denser aggregate packings and saves computation time at high volume fractions.
When meshing the geometry, a minimal distance between the aggregates ensures undistorted elements. It is obtained by increasing the aggregate size during the placement process, which makes the packing problem more challenging. For a given aggregate volume fraction, the presented algorithm maximizes this distance and meshable mesoscale geometries for concrete specimens with more than 70% aggregate content are produced.
Carbon-based conductive coatings are complex composites, consisting of an organic or inorganic binder and conductive carbon components, for application as anodes in impressed current cathodic protection systems of reinforced concrete structures. The electrochemical properties of three coatings at different humidity and in saturated calcium hydroxide solution were studied by electrochemical methods, such as electrochemical impedance measurement, measuring of open circuit potential over time and galvanostatic polarization.
Thermo-mechanical and spalling behavior of normal weight and lightweight geopolymer concretes
(2016)
The mechanical and microstructural properties of two geopolymer concretes, produced with either quartz aggregate or expanded clay aggregate, were assessed before, during and after high-temperature exposure up to 750 °C in order to better understand the engineering properties of the material. SEM investigations were also undertaken to better understand the observed changes in the mechanical properties. It was found that dehydration of capillary water caused micro-cracking and strength losses at temperatures ≤ 300 °C. At higher temperatures (T ≥ 500 °C) sintering promoted strength increases, leading significant strength advantages over conventional concretes. Stress-mechanical strain curves, which are the basis of the fire design of concrete structures, were determined. In addition, the two geopolymer concretes where exposed to the ISO 834-1 standard fire curve in a small-scale spalling test set-up. Acoustic emission measurements during, and acoustic measurements and optical microscopy after heat exposure were employed to investigate crack formation during the tests. Both concretes did not spall, which is attributed to their comparatively high permeability and their low amount of chemically bound water. Significant crack formation was detected only around the temperature of the α–β quartz transition (573 °C) and on cooling. Because of aggregate deformations at the quartz transition temperature, deterioration after heating was more significant in the geopolymer concrete with quartz aggregates. Crack formation occurred also in the concrete with expanded clay aggregates, presumably caused by shrinkage of the geopolymer paste on cooling.
So far, only destructive measurement techniques are available for thickness determination of polymer based surface protection systems for concrete surfaces. Pulse thermography appears to be well suited for non-destructive thickness evaluation in these systems. Here, we present first results of the development of a respective measurement and analysis procedure. Since surface protection systems consist of a number of layers, a model for the calculation of the surface temperature of a multi-layer structure on an infinite (concrete) substrate in pulse thermography setup was developed. It considers semitransparency of the upmost layer and thermal losses at the surface. It also supports the use of an arbitrary temporal shape of the heating pulse to properly describe the measurement conditions for different heat sources. First experimental results regarding the verification of the model are presented.
Funding by the Federal Ministry for Economic Affairs and Energy is gratefully acknowledged.
One of the more difficult tasks for structural health monitoring is the continuous evaluation of the stability and load capacity of the building materials. This knowledge can be won, e.g., by taking material samples at the examining place with the drawback of partly destroying the structure. To avoid this, modern sensor and communication technologies offer promising methods for non-destructive testing. To address the tasks for monitoring of concrete structures, in the presented study, different sensors were combined with RFID transponders and embedded in concrete components.
Presentation on recent progress in ultrasinic testing and monitoring of concrete for massive structures. First, a new instrument (LAUS) for ultrasonic echo testing of thicknesses up to 5 m is shown. A new method to provide better images of the concrete interior, Reverse Mitem Migration (RTM)is presented. Second, the use of embedded ultrasonic transducers and data processing methods borrowed from seismology to detect subtle changes in concrete are documented.
Isotropic damage models are widely used for the finite element simulation of softening materials, e.g. in mesoscale simulations of concrete. Regularization techniques must be employed to obtain a physically meaningful fracture energy upon mesh refinement.
In regularized local damage models the strains localize in single elements allowing them to represent weak or strong discontinuities. In implicit integration schemes, these models can exhibit convergence Problems caused by an ill-conditioned tangent stiffness. This corresponds to the loss of ellipticity of the local rate equilibrium equations.
Oliver et al. developed the implicit/explicit (IMPL-EX) integration scheme which overcomes These problems in local damage models. The internal damage driving variable is extrapolated based on previous implicitly determined values. This provides two main benefits: First, it always results in a symmetric positive semi-definite algorithmic stiffness matrix which precludes ill-posedness. Second, the system becomes incrementally linear and converges in one Newton-Raphson iteration. Even though the IMPL-EX algorithm, like explicit algorithms in general, requires smaller time steps than implicit schemes to obtain the same accuracy, it leads to a computational speedup.
The gradient enhanced damage model by Peerlings is a nonlocal damage model that provides the regularization by limiting the curvature of the damage-driving strains. These models do not lose their ellipticity. However, structural instabilities often require tiny time steps and many iterations to obtain convergence. Here, the second aspect of the IMPL-EX scheme reduces the computational costs. This is shown in simulations of the complex geometry of concrete mesostructures, where only the gradient enhanced matrix material and linear elastic aggregates are considered.
With regard to future mesoscale simulations, the remaining component of the mesoscopic structure, the interfacial transition zone and its degradation, has to be included. This adds a local damage model to the nonlocal problem. Thus, an IMPL-EX implementation has to be provided for both models to benefit from the increase of robustness and performance.
Concrete is a complex material. Its properties evolve over time, especially at early age, and are dependent on environmental conditions, i.e. temperature and moisture conditions, as well as the composition of the material.
This leads to a variety of macroscopic phenomena such as hydration/solidification/hardening, creep and shrinkage, thermal strains, damage and inelastic deformations. Most of these phenomena are characterized by specific set of model assumptions and often an additive decomposition of strains into elastic, plastic, shrinkage and creep components is performed. Each of these phenomena are investigated separately and a number of respective independent models have been designed. The interactions are then accounted for by adding appropriate correction factors or additional models for the particular interaction. This paper discusses the importance of reconsider even in the experimental phase the model assumptions required to generalize the experimental data into models used in design codes. It is especially underlined that the complex macroscopic behaviour of concrete is strongly influenced by its multiscale and multiphyscis nature and two examples (shrinkage and fatigue) of interacting phenomena are discussed.
The current knowledge about fatigue behavior of concrete is still incomplete. This concerns especially the progress of fatigue which precedes the fatigue failure. Therefore, the process of fatigue itself under cyclic compressive loading was investigated in a systematic and comprehensive way. The aim of this investigation was to obtain a deeper insight and to provide a better understanding of the damage process occurring within the material during fatigue loading. Concrete cylinders were tested with a number of cycles to failure between 1E6 and 1E7. To investigate macroscopic and microscopic changes in the material, various methods of non-destructive and destructive testing were used. One main result was, in contrast to other authors, that the investigated changes in macroscopic material behavior could not be explained only by a development of micro-cracks. The results indicated rather, that the related changes in the fatigue behavior are mainly a result of viscous processes in the hardened cement paste, similar to the processes of creep. Based on the experimental results a description of the possible processes was derived which take place in the material structure during fatigue loading and also leads to the observed macroscopic changes in the material behavior. In this context, the results have shown that in case of isotropic material behavior a stiffness reduction related to a scalar value could not capture the damage effect on the stress-strain-relationship caused by fatigue.