Conference Agenda

Concrete’s complex heterogeneous internal structure leads to an involved quasi-brittle response where a progressive loss of material integrity is observed. Additionally, in real-life applications, concrete structures are under loading conditions resulting in complex mixed-mode fracture patterns. Hence, prediction of crack behavior in concrete structures is a challenging task. Owing to the high costs of experimental testing, computational modeling has emerged as a viable alternative for studying concrete fracture. The phase field approach has proven to be a well-established formulation for simulating different fracture phenomena, where crack propagation is tracked implicitly using an additional independent field that diffuses the damage. Previously we introduced a phase field model investigating various crack driving forces considering only the elastic response of concrete. Following a thermodynamically consistent approach, we extend that model to an elastoplastic formulation which can accurately capture the quasi-brittle response of concrete, including the pressure dependency of strength. We formulate the equations within a generalized continuum framework, which accounts for the microstructure of the solid, naturally captures the size effect of materials, and addresses stability issues arising from complex plastic formulations. We demonstrate that employing this framework captures the internal microstructure of concrete by incorporating an internal length scale, which characterizes the microstructural fracture response and represents the finite size of the fracture process zone ahead of the crack tip. A comparison with experimental results confirms the good performance of the model in capturing mixed-mode I-II or I-III failures of concrete.

The modeling and simulation of concrete structures at high-loading rates is an important topic in computational mechanics, as it can be relevant to improving the safety and durability of structures. High-loading rates on concrete structures may occur during explosions, or impacts. Meshbased methods often encounter difficulties in these scenarios due to potentially high mesh distortion in these regions. However, the Material Point Method (MPM) is well-suited for modeling situations involving large deformations, as it uses a continuously reset computational mesh. Additionally, modeling concrete that exhibits strain softening behavior requires regularization methods to solve strain localization and mesh dependency issues. One of the leading methods is implicit gradient enhancement, which is based on a nonlocal formulation, where an additional degree of freedom is introduced to be solved in the linearized system of equations. In this work, the MPM is used with a regularized microplane damage material model at finite
deformation to describe concrete behavior at high loading rates.

The slab-column connection is a critical point in the design of a structure, mainly buildings, since a high concentration of shear stresses can lead to punching, which is a localised failure mode that can take place in a brittle manner and with no previous warning. Currently, structural standards provide recommendations and general expressions to help design these structural connections, where the Critical Shear Crack Theory (CSCT), proposed by Muttoni, stands out. This approach considers the shear strength as dependent on the crack width developing in the shear-critical region and uses a control perimeter (b0) that delimits the cracking region. Many efforts have been devoted to understanding the failure mechanisms involved in punching and to propose tools, such as CSCT, for a safe and efficient design; nevertheless, there is still no consensus on the mechanics governing this phenomenon. The present contribution uses the finite element method and takes advantage of material models based on fracture mechanics to reproduce punching failure in reinforced concrete slabs. This approach is not new and has been employed in the past, but with limitations and some issues still not completely solved. The aim of this work is to analyse different possible modelling techniques in order to obtain a numerical model that reproduces this phenomenon with accuracy. A major advantage of using a finite element model in this case is that the main fracture mechanisms involved in the failure process, which are varied and complex, can be identified. Bidimensional and tridimensional models are discussed, and the possibility of taking into account the slip between concrete and the reinforcement bars, which turns out to be a key mechanism in the evolution of punching failure, is analysed.