Roman Lackner

Identification of four material phases in bitumen by atomic force microscopy

ABSTRACT The identification of material phases at the so called bitumen-scale in context of multiscale modeling of asphalt is presented. For this purpose, atomic force microscopy (AFM), providing insight into the surface topography and mechanical properties, is applied to different types of bitumen. Based on the obtained AFM results, four different material phases at the bitumen-scale are identified. These phases are related to the chemical composition of bitumen, characterized by a wide range of molecular mass. Within the anticipated multi-scale model, the properties of these phases serve as input for upscaling, providing material parameter of the bitumen phase at the next-higher scale, i.e., the mastic-scale.

Chemoplastic material model for the simulation of early-age cracking: From the constitutive law to numerical analyses of massive concrete structures

This paper deals with the development and application of a three-dimensional material model for the simulation of early-age cracking of concrete. The starting point is the determination of the intrinsic material function for the fracture energy of early-age concrete. For this purpose, results of beam bending tests reported in [Proceedings of the SEM/RILEM International Conference on Fracture of Concrete and Rock. Houston, Texas, USA: 1987. p. 409] are employed. The intrinsic material function serves as input for the calibration of the Rankine fracture criterion formulated in the framework of chemoplasticity. Finally, the developed 3D material model is employed for a chemomechanical analysis of a roller-compacted-concrete dam. The temperature fields and the field of the degree of hydration required for this analysis are obtained from a preceding thermochemical analysis of the dam.

Microstructure-based identification of bitumen performance

ABSTRACT Motivated by recent progress in both experimentation and the micromechanical description of the behavior of multi-composed materials, a multiscale model for asphalt is currently developed at the Christian Doppler laboratory for “Performance-Based Optimization of Flexible Pavements” at Vienna University of Technology. This model allows us to relate macroscopically observable asphalt properties to finer-scale information. Since the mechanical properties of asphalt and the influence of environmental conditions (temperature, aging) originate from the binder properties, a proper characterization of bitumen is essential for the predictive capability of the anticipated multiscale model. In this paper, the characterization of bitumen, comprising the bitumen chemistry, its microstructure, and its viscoelastic properties, is presented for two types of bitumen (standard and polymer-modified bitumen) considering the unaged and aged state. The obtained results provide new insight into the durability behavior of binders, explained by changes of the chemical composition, the microstructure, and the micromechanical properties of bitumen.

Microscale characterization of bitumen – back-analysis of viscoelastic properties by means of nanoindentation

Abstract In order to understand the complex thermo-rheological behavior of asphalt, stemming from the viscoelastic nature of bitumen, the nanoindentation (NI) technique is employed. Hereby, the load history applied onto the indenter tip is characterized by a loading, holding, and unloading phase. As regards the identification of viscoelastic properties, a recently published back-analysis scheme, employing the holding phase of the NI test, is extended towards fractional-creep models. In fact, this type of model is found to perfectly describe the viscoelastic behavior of bitumen. Based on the identified viscoelastic model parameters, the influence of loading rate, maximum load, and temperature on these parameters is investigated. Hereby, the temperature dependence of creep parameters follows an Arrhenius-type law. In addition to the model parameters, application of the so-called grid-indentation technique within NI testing provides insight into the bitumen microstructure and the mechanical behavior of the different bitumen phases. The results obtained indicate the existence of a string-like microstructure embedded into a less viscous matrix material.

Cracking in shotcrete tunnel shells

Abstract In this paper, a material model for the numerical simulation of cracking of shotcrete is presented. Cracking is described by means of a multi-surface chemoplasticity model formulated in the framework of thermodynamics of chemically reactive porous media. The material model is calibrated by means of the fracture energy concept. This concept is extended towards chemoplasticity accounting for early age cracking of shotcrete and towards consideration of the interaction between shotcrete and the reinforcement. The applicability of the material model is demonstrated by means of a numerical analysis of a shotcrete tunnel shell of the Sieberg tunnel, Lower Austria. For this purpose, a hybrid method proposed by [Rokahr RB, Zachow R. Ein neues Verfahren zur taglichen Kontrolle der Auslastung einer Spritzbetonschale. Felsbau 1997;15(6):430–4] is reformulated. The term “hybrid” refers to the combination of in situ displacement measurements and a material model for shotcrete. The amount of cracking in the shotcrete shell is investigated. From the obtained stress state, a “level of loading” is computed serving as safety measure of the shell.

Shapes of loading surfaces of concrete models and their influence on the peak load and failure mode in structural analyses

This paper focusses on the influence of the deviatoric shape of the employed loading surface on the predicted failure of concrete. For this purpose, the Extended Leon Model (ELM) [1] and a multi-surface model are considered. The multi-surface model consists of three Rankine surfaces and the Drucker–Prager surface. Whereas the Drucker–Prager surface is characterized by a circular shape in the deviatoric plane, the ELM accounts for the dependence of the strength of concrete on the Lode angle. It is characterized by an elliptic loading surface in the deviatoric plane. The eccentricity e defines the out-off-roundness. It allows a smooth transition from a circular (e=1) to an almost triangular shape (e≈0.5) of the loading surface in the deviatoric plane. The significance of the deviatoric shape of the loading surface on the predicted peak load and failure mode of concrete structures is investigated by means of two example problems: a plane-strain compression test characterized by compressive failure and an anchor-bolt test characterized by shear failure.

Creep in Shotcrete Tunnel Shells

Creep of shotcrete is modelled within the framework of thermodynamics of chemically reactive porous media. The process of creep is divided into a short-term and a long-term part. Short-term creep stems from stress-induced water movement within the capillary pores of shotcrete, located between the already formed hydrates which are the reaction products between cement and water. Thus, short-term creep is related to the accumulation of initially (micro)stress-free hydrates. Hence, it depends on increments of (macro)stress. Long-term creep results from dislocation-like processes within the (micro)stressed hydrates. Therefore, this process depends on the total (macro)stress. Microcracking of shotcrete is modelled by means of multisurface chemoplasticity. Moreover, the model accounts for chemical shrinkage and hydration heat. Finally, the significance of creep of shotcrete for real-life structures is shown by means of 3D hybrid analyses of a railway tunnel. The term ‘hybrid’ reflects the combination of advanced material modelling with in-situ displacement measurements in the framework of nonlinear Finite Element analyses.

Modeling of Early-Age Fracture of Shotcrete: Application to Tunneling

Shotcrete tunnel shells are mainly loaded by the inward moving soil masses and the temperature rise during the hydration process, both resulting in compressive loading of the shell. The decrease of temperature during the cooling process, however, leads to tensile loading which, favored by chemical shrinkage, may cause cracking of the early-age shotcrete. In this paper, the influence of temperature changes during the hydration process on cracking in shotcrete tunnel shells is investigated. The required temperature profiles through the tunnel shell are computed by means of a thermochemical analysis on the basis of an axisymmetric finite element (FE) model. The comparison of numerically-obtained temperature histories with respective in situ temperature measurements provides new insight into the composition of shotcrete after its application on the tunnel wall. For the first time, the loss of aggregates in consequence of rebound during shotcreting, causing an increase of the cement content, is quantified in the course of the presented analysis. In fact, the cement content strongly affects temperature changes during the hydration process. Hence, it has a strong influence on the loading of the shell. The obtained temperature profiles serve as input for the chemomechanical analysis. The chemomechanical analysis is based on a hybrid method recently developed at Vienna University of Technology. The term hybrid refers to the combination of in situ displacement measurements and a thermochemomechanical material model for shotcrete. The chemomechanical analysis provides the stress state in the tunnel shell, thus, giving insight into the load-carrying behavior of the shell.

Scaling relations for viscoelastic-cohesive conical indentation

Abstract Most geomaterials exhibit both viscoelastic and plastic material response when indented by a sharp conical tip. Lacking analytical solutions for viscoelastic – plastic material response, a numerical approach based on the finite element method is proposed for backcalculation of model parameters from indentation data. Departing from the analytical solutions recently obtained by Vandamme and Ulm, viscoelastic – cohesive conical indentation of a homogeneous material halfspace is dealt with in this paper. Results from finite element analysis, i. e., the load – penetration history for a rigid, conical indenter with linearly increasing load, are presented in dimensionless form. From the results, scaling relations are constructed for application to materials exhibiting viscoelastic – cohesive behavior.

Quantification of stress states in shotcrete shells

As reported in Chap. 3, monitoring of deformations which occur during the excavation of tunnels is essential for the success of the NATM. Nowadays it is performed at every NATM construction site. The combination of these monitored displacements with the material model for shotcrete outlined in Chap. 5 has led to the development of a hybrid method for the analysis of closed shotcrete tunnel shells by means of nonlinear Finite Element (FE) analyses. It allows determination of the stress state in the shotcrete shell. Knowing this stress state a level of loading can be computed, amounting to 0% for the unloaded shell and to 100% for shotcrete (locally) loaded up to its compressive strength. The analysis of segmented tunnel shells required further development of this hybrid method. The desire for real-time monitoring of the level of loading led to the development of a shell-theory-based hybrid method avoiding time-consuming FE analyses.