Conference Agenda

Creep of concrete refers to the progressive increase of deformation of a representative material volume subjected to a sustained stress. At low ratios between applied stress and material strength, an increase of the stress relates to an approximately linear increase of the deformation. Nonlinearities occur once the stress exceeds some 40% of the strength. In this work, the origins of such nonlinearities are explored, explicitly separating the contributions from viscoelastic processes and damage due to cracking. A multilevel uniaxial compressive creep test on a mature concrete [1] is re-analyzed. Additionally to documenting the prescribed loading and to measuring the strains, the acoustic emission technique was used to monitor the creation of microcracks. These data were used as input to formulate an analytical model as follows. The reported strength was related to the hydration degree of the material via a validated multiscale model. The obtained hydration degree, together with the mix design, served as input for quantifying linear, nonaging, basic creep properties of uncracked concrete by means of another well-validated multiscale model for creep of cementitious materials under saturated conditions. The obtained creep function was then extended to account for (i) different levels of time-invariant relative humidity (constant moisture during creep testing), by means of a creep reduction factor, (ii) nonlinear viscoelastic processes, by means of the affinity concept [2], and (iii) diffuse microcracking, by means of a micromechanics-motivated damage factor which relates the creation of microcracks to a proportional increase in the compliance of concrete. In [5], it was shown that cracks created during both quasi-static load application and sustained loading have an important influence on the deformation behavior of concrete. Herein, further experiments are analyzed. This confirms that nonlinear viscoelastic phenomena govern the creep behavior at medium stress levels, while cracking-induced damage dominates the behavior at high stress levels.

Currently, the development of innovative production methods for concrete structures is progressing rapidly. As an example, the state-of-the art on 3D printing of cementitious materials has progressed from the pioneering work of printing mortars to printing with conventionally proportioned concretes at structural scale. Recent advances have even enabled printing of concrete reinforced with steel fibers. With the rapid development, including the improvement of material toughness from the addition of fibers, the road is paved for advanced applications, including utilization of 3D printed concrete as the sole load bearing system of larger structures. With increasingly complex structures, an increased need for documentation of material properties at certain load conditions follows.
This paper presents an experimental investigation of the fatigue behavior of 3D printed fiber reinforced concrete in compression. The experimental program is to be considered as a preliminary test program, providing indications on the question if the fatigue life of the fiber reinforced 3D printed concrete material can be compared to the fatigue life of conventional concrete, e.g. as predicted by the Eurocode provisions. The results are compared with similar tests on 3D printed concrete without fibers to demonstrate that the inclusion of fibers does not impact the fatigue behavior negatively. From the experiments conducted, it is found that the behavior of the 3D printed materials exposed to fatigue loading, is comparable to the prediction models for fatigue life of conventional concrete.

The use of fiber-reinforced polymer (FRP) for strengthening concrete structures has gained significant interest. Given that the challenges and limitations of experimental investigations, numerical modeling is essential for studying FRP-reinforced concrete structures. However, simulating FRPconcrete debonding failure is challenging due to the complex nature of concrete damage, which often involves a thin layer of concrete substrate debonding with the FRP strip. This study investigates the bond behavior between steel-reinforced polymer (SRP) strip and concrete using a three-dimensional (3D) meso-scale lattice discrete particle model (LDPM). The model accounts for the inherent heterogeneity of concrete, including the distribution of coarse aggregates, and simulates the macroscopic debonding process as a propagating fracture inside the concrete substrate. In this research, the LDPM is calibrated and validated against experimental data and incorporates a meso-scale representation of concrete with SRP treated as a linear elastic material. This study investigates the bond behavior in single-lap shear tests, focusing on the load response and the initiation and propagation of cracks at the concrete substrate. A series of parametric studies were conducted to examine the width effect and the influence of the minimum modelled concrete coarse aggregate. The numerical simulations offer valuable insights into the mechanics of SRP-concrete interactions.

Accurate prediction of structural fatigue life under compression is crucial for infrastructure safety, yet the fatigue behavior of concrete remains insufficiently understood. This study proposes a dissipation hypothesis linking fatigue-induced degradation to cumulative inter-aggregate shear strain, leading to a pressure-sensitive interface model embedded in both discrete and microplane formulations. To validate this hypothesis, extensive experimental and numerical studies were conducted, including cylinder compression test of varying sizes, concrete mixes, and loading frequencies, alongside prestressed four-point bending tests representing structural compressive fatigue. Results indicate that direct transfer of fatigue data from cylinder tests to structural components is inadequate. Therefore, a detailed discrete mesoscale model of the prestressed four-point bending test was developed to further analyze and interpret structural fatigue damage. The mesoscale model, which uses a latticediscrete material idealization, is qualitatively compared with numerical studies performed using FE and the microplane model MS1. The studies include a comparison of the shape of hysteretic loops, stress redistribution along the cross-section, and the shape of energy dissipation profiles.

Concrete fatigue testing is costly and time-intensive, often requiring high loading frequencies to reduce durations, which impacts fatigue performance due to heat generation. This highlights the need for a robust model to capture the interaction between loading rate and temperature, aiding experimental interpretation and enabling faster testing—an area that is not yet addressed by current modeling approaches. This contribution aims to introduce a unified, thermodynamically-based constitutive model describing concrete behavior. The model integrates viscoplastic effects with cumulative sliding damage as mechanisms driving the material degradation. Additionally, the effect of temperature is integrated into the thermodynamic framework, coupled with viscoplastic strains, so that the intrinsic heat generation from internal friction is accounted for. Furthermore, Gibbs free energy-based formulation facilitates the stress-driven fatigue simulations at the single material point idealization. Elementary studies examining the behavior of concrete under uniaxial compression are used to analyze the effects of both loading-rate and thermal factors on both monotonic and fatigue behavior.