A meso-mechanical model for concrete under dynamic tensile and compressive loading

We present a computational model, which combines interface debonding and frictional contact, in order to investigate the response of concrete specimens subjected to dynamic tensile and compressive loading. Concrete is modeled using a meso-mechanical approach in which aggregates and mortar are represented explicitly, thus allowing all material parameters to be physically identified. The material phases are considered to behave elastically, while initiation, coalescence and propagation of cracks are modeled by dynamically inserted cohesive elements. The impenetrability condition is enforced by a contact algorithm that resorts to the classical law of Coulomb friction. We show that the proposed model is able to capture the general increase in strength with increasing rate of loading and the tension/compression asymmetry. Moreover, we simulate compression with lateral confinement showing that the model reproduces the increase in peak strength with increasing confinement level. We also quantify the increase in the ratio between dissipated frictional energy and dissipated fracture energy as the confining pressure is augmented. Our results demonstrate the fundamental importance of capturing frictional mechanisms, which appear to dissipate a similar amount of energy when compared to cracking under compressive loading.

Micro-mechanical finite element modeling for in-plane behavior of historical masonry

Shenghan Zhang

Seismic assessment of unreinforced masonry structures remains a challenge. This dissertation investigates the in-plane behavior of historical masonry elements and proposes an advanced simulation method based on the cohesive zone model (CZM) in a finite element framework. Cohesive elements are often used to simulate dynamic fracture. To consider contact and friction, a new node-to-node algorithm is implemented in the open-source code Akantu, which offers a parallel computing model. Based on the proposed framework, two types of tests are investigated: the shear test, which is a basic material test to determine parameters such as cohesion and friction coefficient, and the diagonal compression test, which is a standard masonry material test to determine the nominal diagonal tensile strength and shear modulus of masonry. The simulation results correspond well with experimental results. One potential advantage of the detailed micro-modeling method is the ability to represent and study the influence of the micro-structure of the stone masonry on the force-displacement behavior. However, previous studies focused only on one specific block layout. This constraint was caused by the absence of a stone masonry typology generator. Based on related research in computer vision and geology, a versatile typology generator is developed. To quantify the micro-structure of a stone masonry typology, the line of minimum trace (LMT) is often used. Determining the LMT manually is time consuming and subjective. To facilitate the calculation process, an algorithm to calculate LMT automatically from pictures of masonry patterns is developed. Based on the proposed typology generator, the influence of the masonry typology on the masonry shear strength under shear-compression boundary condition is then systematically investigated. Starting with regular brick masonry patterns, the roundness and the arrangement of the units are altered gradually. The commonly used shear failure criterion by Mann-Müller is investigated in detail and it is shown that the assumed stress distributions are good approximations of the stress state in the linear elastic range but deviate significantly from the stress distribution at peak shear strength. The numerical results also highlight that the effect of the interlocking between stones on the shear strength is overestimated by the Mann-Müller model. For irregular stone masonry, the correlation between LMT and shear resistance is studied. While CZM has been widely used to simulate tensile damage dominated fracture and inter-facial failure, continuum modeling method, i.e., concrete damage plasticity (CDP), is more commonly used for simulating the compressive behavior of quasi-brittle materials, e.g., mortar in masonry and concrete. While existing studies already established the equivalence between the continuum modeling method and the CZM under tension dominated fracture process, the behavior of CZM under compression has not been investigated. To consider a wider range of loading conditions, a new traction-separation law based on the plasticity theory was proposed and therefore expanded the usage of CZM.

EPFL2018

A meso-scale computational approach to dynamic failure in concrete

Leonardo Snozzi

The aim of the present thesis is to provide a numerical tool for the mechanical modeling of dynamic failure in concrete. This widely used construction material is characterized by a non-linear failure behavior, which is consequently difficult to describe with macroscopic quantities. In order to grasp the mechanical damage response of this heterogeneous brittle material, our approach is based on a meso-mechanical point of view. The whole research work is, therefore, mainly focused on the understanding of the mechanisms that are linked with the material’s heterogeneous composition. The advantage of this level of observation is that it allows to represent the most important concrete constituents (e.g. aggregates and cement paste), thus facilitating the physical identification of the material parameters of the model and of the mechanisms (interaction between matrix and inclusions) that characterize its constitutive behavior. For this purpose we exploit the capabilities of a two-dimensional finite-element model. The onset of fracture is explicitly modeled using the well-known cohesive approach. We first investigate the dynamic tensile response of concrete specimens. Different simulations are carried out to assess the influence of aggregates properties on peak strength and dissipated fracture energy at different strain rates. The model aims to explain the strain-rate strengthening through structural effects. However, to capture the full extent observed experimentally, our results suggest that is not possible to discard the combination of inertial with material rate hardening mechanisms. Next, in order to account for crack-interactions as well as path dependent behavior, the model is enriched with the introduction of an explicit contact algorithm and a mode-dependent fracture energy. To demonstrate the capability of the proposed approach to provide accurate results, the model is first applied to two benchmark tests in masonry engineering. Afterwards, the developed framework is applied to reproduce dynamic compressive failure of meso-scale concrete samples. Simulations involving different strain rates and levels of lateral confinement are conducted. An energetic analysis shows that dissipation of energy through fracture and friction is an increasing function of the applied confinement and strain rate. Our results underline thus the importance of capturing frictional mechanisms, which appear to dissipate a raising amount of frictional energy with increasing strain and applied pressure. Finally, a multi-scale computational framework is developed to up scale the obtained fine scale response to the coarse scale. The selected approach considers concrete macroscopically as being homogeneous and to behave linear elastically aside from the propagating cracks. The cohesive macroscopic tractions are however extracted from the response of meso-scale representative volume elements, which are activated on the fly when a macroscopic discontinuity propagates. Parallel simulation shows the capability of the model to predict the structural response of a tested unit including the physical mechanisms occurring at the fine scale.

EPFL2013

Structural Behavior of R-UHPFRC - RC Composite Slabs

Maléna Bastien Masse

The application on existing Reinforced Concrete (RC) slabs of cast-on-site Ultra-High Performance Fiber Reinforced cement-based Composite (UHPFRC) layers is an efficient reinforcement technique, currently spreading. The thin layer of UHPFRC, with or without steel rebars, serves as a tensile reinforcement for the RC slab, creating a composite element. This thesis combines material and structural engineering to study the behavior and resistance of two-way spanning composite slabs, with a certain focus on punching shear resistance. When analysing the behavior of composite elements, the effect of fiber orientation on the in-plane tensile response of the UHPFRC layer needs to be accounted for. Theoretical tools are derived herein to analyze the complementarity of fiber orientation in perpendicular directions and determine the average effect of fiber orientation on fiber efficiency at pull-out. A comprehensive material testing campaign on a strain-hardening UHPFRC is carried out on specimens with various thicknesses and casting processes. The most likely fiber orientation in a layer of UHPFRC for the casting method considered herein is finally estimated with the results of the theoretical and experimental work and a representative tensile response is scaled accordingly. An experimental campaign is carried out on six composite slabs without transverse reinforcement. The parameters of the tests include the thickness of the UHPFRC layer and the amount of reinforcement in it. The punching shear resistance of all composite slabs is higher than the resistance of the reference RC slab. The layer of UHPFRC increases the rigidity of the slab and provides added shear resistance to the cracked RC section by out-of-plane bending accompanied by limited or inexistent Near Interface Cracking (NIC) in the concrete section prior to failure. By doing so, it allows more deformation to take place in the RC section before punching shear failure. This results in rotations at maximum resistance close to what is observed for the reference RC slab. An analytical model is then developed to predict the global bending behavior of the composite slab and the punching shear resistance. A multilinear moment-curvature relation for composite sections is used to calculate the force-rotation curve of a slab. The intersection between this curve and a deformation based composite failure criterion predicts the punching shear resistance. This criterion combines the concrete and the UHPFRC layer contributions. The latter resists to punching shear by out-of-plane bending over a limited length. This mechanism induces tensile stresses perpendicularly to the interface with the concrete. The contribution of the UHPFRC layer to the punching shear resistance thus depends on the tensile strength of concrete. In the final section of the work, a description of the parameters influencing the shear resistance of composite elements is done. With the tools developed in this work, the effects of fiber orientation in a layer can be mastered and the analytical models allow to simply verify the resistance of a composite section.

EPFL2015