Ultra-high performance fibre-reinforced concrete (UHPFRC) is an emergent material with outstanding mechanical properties which still needs research regarding the characterisation and modelling of its fracture behaviour. The present work focusses on the determination of the tensile strength, conceived as the stress corresponding to crack initiation. The Brazilian test is a convenient experimental option in the case of plain concrete which provides a sufficient accurate value of the tensile strength obtained from the maximum load of the test, provided that it is performed with the adequate conditions of geometry and load rate, because the peak load occurs very soon after crack initiation. However, in the case of fibre-reinforced materials, the peak load can occur much after crack initiation and so the apparent splitting strength be much higher than the true tensile strength. Nevertheless, in previous experimental and numerical works it has been assessed that a local maximum occurs in many cases at a splitting stress close to the tensile strength of the material. It means that the stress corresponding to initiation of cracking can be determined in Brazilian tests provided that the test is properly designed and instrumented taking into account the size-effect to ensure detection of the local maximum. As shown in a previous numerical study, numerical simulations are an essential tool for this purpose; however, a systematic parametric study is still necessary. The current work analyses the main alternatives for dimensionless simulations within the finite element framework COFE (Continuum Oriented Finite Element). In this study, the results of simulations of the Brazilian tests by considering a domain in which all the elements can crack are compared with those obtained by using joint elements with a cohesive behaviour along of the foreseeable cracking plane, and the main aspects of the dimensionless simulations are analysed in detail.

A tensorial continuum damage model for concrete which can correctly predict the failure stress states and failure modes in general multiaxial stress states is presented. The model is thermodynamically consistent and is based on proper expressions for the specific Gibbs free energy and the complementary form of the dissipation potential. Damaging of the material is described by a symmetric positive definite second order damage tensor. Invariant theory is used in construction of the potential functions which guarantees that the proper symmetry behaviour is satisfied and no artificial symmetrization operations need not to be done. The failure surface is formulated in a way that it mimics the behaviour of the well-known Ottosen’s four parameter failure surface. The predictions of the proposed model are compared to the Concrete Damaged Plasticity (CDP) model available in the commercial finite element software Abaqus in uniaxial and equibiaxial cases. The CDP model is calibrated against the uniaxial test results. However, for the CDP model the strain in the loading direction in the biaxial case starts to deviate from the experimental results already before the peak stress, while the present model yields accurate prediction. The constitutive model is implemented as an UMAT subroutine to be used in Abaqus.

The significant influence of boundary and loading conditions on the shear response of slender reinforced concrete beams without shear reinforcement has been substantiated by recent research. It would be of interest to examine whether this influence is also present in the case of beams with low shear reinforcement less than the minimum shear reinforcement ratio. This study presents a numerical investigation of the shear behavior of slender reinforced concrete beams with low shear reinforcement under various boundary and loading conditions. An experimental study from the literature is considered, comprising three beam series types: four simply supported beams subjected to a point load, three simply supported beams subjected to a distributed load, and four cantilevers subjected to a distributed load. The GID and ATENA software are utilized for 3D modeling and nonlinear finite element analysis. A novel approach to modelling distributed loads is proposed. By evaluating load-displacement curves, nominal shear stresses and shear transfer actions along the shear cracks, the influence of loading and support conditions on the shear behavior of slender reinforced concrete beams with low amounts of shear reinforcement is investigated. Due to boundary and loading conditions, normalized nominal shear stress increases up to 124%. The benefit of numerical modeling is leveraged to perform a sensitivity analysis of varying shear reinforcement locations on the shear resistance of specimens.

Explosive effects are commonly encountered on structures. The 2020 Beirut explosion, 9/11 attack in USA, or current Russia-Ukraine war cause significant structural destruction. Reinforced concrete (RC) members in compression are most vulnerable to blast loads. This paper deals with a numerical investigation on the propagation of damage and its effects due to distribution of explosives at different positions around a compression member for close range explosion. In this analysis, explosive weight and scaled distance are maintained constant throughout. The Arbitrary Lagrange Euler (ALE) algorithm along with Fluid-Structure Interaction (FSI) and Erosion algorithms, to accurately capture the blast effects have been employed. Models have been validated by assessing the damage profile and blast over-pressure. The concrete and reinforcement have been modeled using the Lagrange approach, while Eulerian method is adopted to model air and explosives. Coupling between the Lagrange and Eulerian part have been considered along with the contact between concrete and reinforcement. The strain contour reveals that damage propagation in a compression member varies with its cross-sectional size and the placement of explosives. Compression members with larger cross-sections exhibit a pronounced effect of explosive distribution, with damage penetrating to the core from the explosion side. In contrast, compression members with small cross-sections show reduced effectiveness of explosive distribution due to the small cross section size. In C900, plastic strain extends toward the core from the side of the explosion, whereas in C450, damage reaches the core from the opposite side of the explosion, leading to complete failure of compression member.