Conference Agenda

Explicitly modeling embedded fibers in a concrete matrix subject to cracking, spalling, and localized crushing presents significant challenges. On top of that, a mesh conforming model of steel fibers embedded in a concrete matrix is computationally expensive. In this study, a hybrid contact model is used to generalize fiber-matrix coupling and remove the necessity of an excessively small mesh. The generalized model simulates the movement of embedded steel bars in a predefined tunnel within a discrete-mesoscopic matrix. There are limited reports on this slideline model, but a universal calibration, parameter fitting, and investigation of geometrical configurations are lacking. To address this and reveal the merits and limitations of the slidelineassisted coupled contact model, cases of steel fiber aligned pullouts are contrasted against the conventional mesh-conforming approach and experimental evidence. Future studies will extend these investigations to include inclined pullout cases, leading to a scalable fiber-lattice explicit coupling model to simulate SFRC composites.

In order to model concrete cracking, a phase-field model is retained which converges toward cohesive fracture when its regularisation length goes to zero. The damage threshold in stress-space is adapted from experimental failure surface. In addition, damage in compression is introduced to bound the stress re-sponse. The model is deliberately kept as simple as possible to preserve its robustness. Two test-cases are computed in order to assess its capabilities. The first one consists of a shear-mode pull-out test. It is focused on robustness in the presence of other sources of nonlinearity (finite strain plasticity, contact boundary condi-tions). The second one consists of a reinforced concrete beam submitted to four-point bending. It is character-ised by multi-cracking and enables a comparison with experimental results.

Concrete is one of the most commonly utilized building materials and has a significant environmental footprint due to its production process. Over the past decades, considerable progress has been made in developing high-performance and ultra-high-performance concretes (HPC/UHPC) and incorporating fiber reinforcement in structural concrete, resulting in improved strength and durability and opening the possibility of creating slenderer structures, leading to significant material savings. However, adequate models and design approaches must accompany these material improvements to fully realize the potential benefits. This work focuses on the material behavior aspect of high- and ultra-high-performance fiber-reinforced concrete (HPFRC/UHPFRC), investigating damage processes and mechanisms occurring at small scales, which are not readily observable during loading tests. To this end, it presents a framework based on image-derived mesoscale models generated from X-ray computed tomography (CT) scans of laboratory-scale specimens. These models incorporate realistic pore and steel fiber distributions, enabling detailed simulations of crack initiation and propagation processes that are not directly accessible in experiments. A finite element model utilizing zero-thickness cohesive interface elements is applied to simulate the cracking of fiber-reinforced concrete specimens. The zero-thickness interface elements are equipped with a cohesive-frictional traction-separation law. The steel fibers are considered explicitly and modeled as elastoplastic Timoshenko beam elements. The embedment of fibers into the cement matrix is facilitated via a penalty-based coupling algorithm that enables flexible placement of fibers without needing to conform with the background mesh. The bond between the cement matrix and fibers is modeled via an elastoplastic bond-slip law, whose parameters are calibrated based on single-fiber pullout experiments. The capabilities of the proposed framework are demonstrated by the reanalysis of an experimental scenario involving UHPFRC beam subjected to 3-point bending and comparison of the results with the available experimental data.

Elastoplastic constitutive models encompassing non-associated plastic flow are widely used for modeling the pressure-sensitive mechanical behavior of cohesive-frictional materials, such as concrete and rock. Although the potential instabilities of such models are well established in the literature, they remain a common—and often overlooked—source of numerical difficulties in simulations of cohesive-frictional materials. In the present contribution, we investigate the destabilizing effect of hyperbolic plastic potential functions, which were recently identified as a potential source of constitutive instability. To this end, we derive regions of critical constitutive states on the yield surface for non-associated Drucker-Prager plasticity with a hyperbolic plastic potential function. Moreover, a recently presented analytical solution for non-associated Drucker-Prager plasticity subjected to oedometric extension is extended to account for hyperbolic plastic potential functions. The results indicate a broadening of the region of critical constitutive states near the vertex of the yield surface, revealing a destabilizing effect of the hyperbolic plastic potential function. Moreover, softening in oedometric extension is observed where it would not occur with classical non-associated Drucker-Prager plasticity, further highlighting this destabilizing effect. These findings suggest that the near-vertex behavior of elastoplastic constitutive models plays a key role in the emergence of potentially unstable constitutive behavior linked to numerical difficulties in simulations of cohesive-frictional materials.

The paper contains a survey of several equivalent strain definitions and their use in the localizing gradient damage (LGD) model. When the conventional gradient damage (CGD) model with a constant internal length scale is applied, an artificially widened damage zone may appear. In the LGD model, the localization zone is controlled by a gradient activity function. As damage grows, the variable internal length scale decreases and the nonlocal interaction is reduced. Both versions of the model are implemented in the FEAP package, and the advantages of the LGD model are demonstrated. A range of equivalent strain measures is considered, and the resulting equivalent strain envelopes are illustrated with reference to the Kupfer test. The simulation of an eccentrically notched beam in three-point bending is also carried out. A mesh-sensitivity study is presented, indicating that the numerical response stabilizes and converges for sufficiently refined meshes. Generally, selected equivalent strain definitions are investigated in the numerical analysis, and they reproduce the experimental results reported by Garc´ıa-A´ lvarez et al.Moreover, the Häußler-Combe and Pröchtel definition provides the closest agreement and leads to representative crack patterns in concrete.