This study proposes a novel discrete hygro-thermo-mechanical model for concrete under freeze-thaw cycles (FTCs) within the framework of the Multi-physics Lattice Discrete Particle Model. The evolution of ice content and its hysteretic behavior are simulated using a modified liquid-solid interfacial energy function. The non-uniform local eigenstrains induced by temperature variation and ice formation govern the FTC-induced cracking behavior; accordingly, these eigenstrains are incorporated into the mechanical constitutive model via a one-way coupling scheme. Notably, the model successfully captures cracking patterns, which initiate at the surface and propagate inwards over hundreds of FTCs. It also accurately predicts the associated degradation in both tensile and compressive strength. Therefore, this study finds that an external compressive load exerts a complex influence on FTC-induced degradation. Specifically, a moderate compressive load equal to 50% of the material’s compressive strength increases the residual strength by approximately 1.5 MPa after 50 FTCs. This beneficial effect arises because the applied external load restrains frost-heaving deformation and thereby suppresses the initiation of FTC-induced micro-cracks. In contrast, when the external load exceeds 70% of the compressive strength, degradation is significantly accelerated.

This study investigates various types of numerical integration techniques in mechanical lattice-type discrete models for concrete. Using the Lattice Discrete Particle Model (LDPM) as a~reference, the geometry is reformulated into a~set lattice elements connecting two particles with an~associated area that represents mutual particle contact. The original LDPM constitutive functions are preserved, including the effect of volumetric strain, while different integration schemes are introduced to control rotational stiffness, ranging from single- to multi-point formulations. The approach is assessed through simulations of a~dogbone tension test, three-point bending, unconfined compression, and hardening response. Results show that tensile, bending, and hardening behavior are largely insensitive to the integration choice, in contrast to unconfined compression, which is strongly affected: single-point integration underestimates stiffness due to the absence of rotational resistance, whereas multi-point integration yields consistent predictions. Simulations with arbitrarily constrained rotations further confirm the decisive role of rotational stiffness. The findings underline that an~adequate representation of rotations and aggregate interactions is essential for reliable modeling of compression-dominated behavior in lattice-type frameworks.

Although CO2 curing of cement-based materials offers a promising avenue for mitigating carbon emissions in construction, its optimization is limited by the lack of mechanistic understanding of the interaction between hydration and carbonation reactions during curing. This study establishes a unified physicochemical reaction kinetics framework that explicitly captures the coupled interaction between hydration, carbonation, water content evolution, and reaction heat within a thermodynamically consistent formulation. Based on a representative volume element (RVE) at the cement paste level, microdiffusion-reaction equations are developed to describe the coupled reaction kinetics. The interaction is characterized by three dominant mechanisms, including competitive reactant consumption, heat evolution, and water content evolution, each providing physicochemical feedback to the reaction kinetics. The proposed model quantitatively captures the influence of pre-hydration on carbonation kinetics and the evolving influence of carbonation on subsequent hydration. Furthermore, the results reveal that although both hydration and carbonation initially consume water, the carbonation of hydration products subsequently releases chemically bound water, while carbonation ultimately reduces the equilibrium moisture content through pore refinement and microstructural densification. The proposed framework provides a physics-based foundation for understanding and optimizing early-age CO2 curing conditions.

Microfibers (less than 100 lm in diameter) are commonly employed in structural applications to minimize early shrinkage cracking and lower pore pressure during fires. For any application, micro fiber-reinforced concrete (FRC) structural behavior and durability must be estimated using the mechanical constitutive law. Formulating a mechanical constitutive law for FRC presents several difficulties in terms of comprehending the physical principles and employing suitable numerical techniques. A novel model called "lattice discrete particle model for micro-FRC (LDPM-MicroF)" is presented to simulate the fracture behavior of micro-FRC. An equivalent fiber diameter coefficient has been defined to balance modeling accuracy and computational cost so that the LDPM-MicroF model can simulate the mechanical responses of engineered cementitious composites. The unimodal variation in tensile strength caused by the increase in microfiber dose is assessed and quantitatively reproduced by LDPM-MicroF predictions. This phenomenon is explained by a combination of mesoscopic mechanisms and the "near-field effect" of the fibers. A small number of microfibers can improve the strength of the matrix and thus slightly the tensile strength. However, when the dosage of microfibers exceeds a certain amount, the tensile strength decreases as the contribution of the fiber bridging force to the strength becomes lower than that of the replaced matrix. This research has provided new insights into the physical comprehension of the mechanical properties of micro-FRC, which has significant implications for the field of study. (c) 2024 THE AUTHORS. Published by Elsevier LTD on behalf of Chinese Academy of Engineering and Higher Education Press Limited Company. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

This article answers the question of whether homogenization of discrete fine-scale mechanical models, such as particle or lattice models, gives rise to an equivalent continuum that is of Cauchy-type or Cosserat-type. The study employs the machinery of asymptotic expansion homogenization to analyze discrete mechanical models with rotational degrees of freedom commonly used to simulate the mechanical behavior of heterogeneous solids. The proposed derivation has general validity in both stationary (steady-state) and transient conditions (assuming wavelength much larger that particle size) and for arbitrary nonlinear, inelastic fine-scale constitutive equations. The results show that the unit cell problem is always stationary, and the only inertia term appears in the linear momentum balance equation at the coarse scale. Depending on the magnitude of the local bending stiffness, mathematical homogenization rigorously identifies two limiting conditions that correspond to the Cauchy continuum and the Cosserat continuum. A heuristic combination of these two limiting conditions provides very accurate results also in the transition from one limiting case to the other. Finally, the study demonstrates that cases for which the Cosserat character of the homogenized response is significant are associated with non-physically high fine-scale bending stiffness and, as such, are of no interest in practice.

This article presents a comparison of various implementations of the Lattice Discrete Particle Model (LDPM) for the numerical simulation of concrete and other heterogeneous quasibrittle materials. The comparison involves the use of transient implicit and explicit solvers and steady-state (static) solvers as well as implementations for central processing unit (CPU) and graphics processing unit (GPU). The various implementations are compared on the basis of a set of benchmarks tests describing behaviors of increasing computational complexity. They include elastic vibrations, confined strain-hardening compressive response, tensile fracture, and unconfined strain-softening compressive response. Metrics of interest extracted from the simulations include macroscopic stress versus strain responses, computational times, number of iterations, and energy balance error. Pairwise comparison of final crack patterns is provided through the correlation coefficient and normalized root mean square error of the crack opening vectors. Moreover, for the most numerically challenging case of unconfined compression with sliding boundary conditions, the stability of the strain-softening response is tested by perturbing the solutions as well as changing the convergence criteria and time step size. Attached to this paper is the complete input data of the benchmark tests; this will allow researchers to run the examples and compare them with their own implementations. In addition, most of the reported implementations are publicly available in open source packages.

Lattice modeling of quasi-brittle materials, such as concrete, is a discrete mesoscale description of a material, where constitutive relations are defined at a lower scale compared to the continuum-based approaches. Over the years, these lattice discrete models have become increasingly efficient, and they are expected to be useful for generating high-fidelity databases of complex material responses. Such databases can be exploited in two ways: either to inform data-driven approaches or to calibrate macroscale models. In this paper, we focus on the latter. Macroscopic stress and strain responses are obtained by coarse-graining lattice discrete particle model (LDPM) responses. Stresses and strains are coarse-grained independently from computations on bending beams. Local and nonlocal scalar damage models are used to fit these data. The evolution of damage is constructed from these stress-strain responses by computing the pairs composed of damage and the history variable that govern its growth. Model parameters in the nonlocal model, including the internal length, are then obtained by fitting the macroscale constitutive model to the coarse-grained results. The global response of the bending beam (load vs. displacement) and the energy dissipation profiles provided by the calibrated nonlocal damage model are found to be consistent with LDPM results.

Lightweight aggregate concrete (LWAC) offers clear advantages for sustainable construction, including reduced density and improved thermal insulation. However, its mechanical and fracture behavior is difficult to characterize due to the heterogeneity and brittle crushing of porous lightweight aggregates. This study examines the mechanical response and fracture behavior of ultra-high-performance concrete with foam glass aggregates (UHPC-FGAs) as a representative LWAC system by combining targeted experiments with mesostructure-resolved numerical simulations. Experimental investigations included single-particle crushing tests on FGAs, uniaxial compression tests on the UHPC matrix, and three-point bending (TPB) tests on the UHPC matrix. These data informed parameter identification for the Polymaterial Lattice Discrete Particle Model. Aggregate-related parameters were calibrated under joint constraints to reproduce both FGA crushing behavior and the compressive response of UHPC-FGA composites. Realistic mesostructures were generated from voxel-based microstructures produced by the Virtual Cement and Concrete Testing Laboratory and mapped into the numerical model. TPB simulations of UHPC-FGA composites were then performed to quantify fracture energy. Results show that cracking initiates within porous FGAs and propagates transgranularly into the UHPC matrix, rather than along interfaces as in normal-weight concrete. The fracture energy of UHPC-FGAs is approximately 50% lower than that of plain UHPC, reflecting limited crack deflection and bridging. Parametric analyses indicate that aggregate stiffness and tensile strength primarily govern fracture energy and post-peak ductility, while the shear-to-tensile strength ratio controls compressive strength and peak strain. The proposed experimental-numerical framework offers practical guidance for optimizing lightweight concrete systems by balancing strength and ductility.

Alkali-silica reaction (ASR) in concrete produces mechanical responses that are highly sensitive to environmental conditions. Although multiscale modeling has shown promise in linking laboratory-scale ASR degradation to field-scale structural performance, a comprehensive theoretical framework for this connection remains underdeveloped. This study introduces a multiscale homogenization approach to upscale coupled mechanical and mass transport processes in ASR-affected concrete within a dual-lattice framework. At the aggregate level, ASR-induced expansion is represented by eigenstrain imposition within a micromechanics model, while at the macroscale, a representative volume element (RVE) formulation employs asymptotic expansion homogenization and coarse-graining method to upscale governing equations for momentum equilibrium and mass balance. The model is validated against the long-term performance of Fontana Dam, demonstrating its robustness in predicting damage evolution during hydration and ASR development. Simulation results highlight that the spatiotemporal progression of ASR-induced cracking is strongly governed by the anisotropic distributions of multiphysics fields under coupled mechanical and environmental conditions.

The Lattice Discrete Particle Model (LDPM) provides a robust computational framework for modeling the behavior of quasi-brittle cementitious composites, excelling at simulating fracture processes, crack initiation and propagation, and material failure mechanisms at the mesoscopic scale of concrete, which schematizes the material at the level of coarse aggregate and mortar paste. However, LDPM remains computationally expensive, particularly when modeling large-scale structural elements under complex dynamic conditions. This study utilizes Proper Orthogonal Decomposition (POD) to develop a reduced-order model (ROM) for the LDPM integration solver employing the central difference scheme. A novel two-stage projection strategy is introduced, enabling direct and consistent enforcement of boundary conditions in the reduced subspace, while maintaining compatibility with the original solver. The objective is to balance accuracy and computational efficiency. In constructing the ROM, both offline and online modes are presented and discussed in detail, including the demonstration of offline ROM for mesoscale parameter calibration to enhance predictive capabilities. The proposed methodology is validated through various independent tests involving highly nonlinear behavior. The results demonstrate significant computational savings without compromising the accuracy of the numerical predictions, highlighting the potential to apply ROM techniques to the LDPM framework.

This study shows that nonlocal models for fracture can be calibrated from field data, e.g., strain fields, stress and other internal variable fields, including their internal length. There is no need to consider specimens of various geometries or size effect data. However, such a calibration should be performed with caution as the fracture process zone ought to be well described. This work starts from a discrete model (LDPM) viewed as a high-fidelity model, and uses it to generate a synthetic data set for the calibration of a macroscale damage model. Compared to the methodology where the internal length and material parameters are obtained from fitting field data altogether, the present study shows that constraining the internal length in the nonlocal model to fit the width of the FPZ prior to calibration yields more accurate results. Consistent structural responses and correct size effects predictions are obtained.

Many studies have demonstrated the Lattice Discrete Particle Model (LDPM) to be an accurate methodology for simulating the fracture behavior of quasi-brittle materials at the spatial scale of major heterogeneities, often referred to as mesoscale. Despite the availability of various solution schemes for LDPM, dedicated static solvers that allow for displacement controlled loading conditions are not available in the literature due to several challenges associated with enforcing convergence for highly complex 3-dimensional stress states at the mesoscopic level. To address this knowledge gap and enhance solution stability while mitigating time step sensitivity, this study presents a newly developed arc length-based static solver for LDPM capable of direct enforcement of displacement constraints. Both residual- and increment-based convergence criteria are integrated to facilitate numerical convergence, while adaptive strategies are introduced to ensure compatibility of the arc length equation with the LDPM framework. Two solution schemes are proposed, namely the consistent and non-consistent schemes. To achieve satisfactory convergence rates, modifications to the standard LDPM constitutive laws are proposed to ensure continuity in the transition between tensile and compressive stress states. Validation against three tests with distinct failure mechanisms demonstrates that the developed static solver achieves excellent accuracy and high stability, and robust convergence behavior with respect to different solver settings. A parametric investigation is also presented to quantify the effects of different parameter choices in the proposed arc length equation.

Understanding mesoscopic component migration in fresh fiber-reinforced concrete (FRC) helps to control concrete construction quality. Studying of components' movement in non-transparent flow requires a high-fidelity fluid-solid interaction method. This study employs a two-way coupled approach based on the SPH (Smoothed Particle Hydrodynamics) and DEM (Discrete Element Method) to simulate the rheological behavior of fresh FRC at mesoscale. The Herschel-Bulkley model is implemented to represent the shear thinning of mortar and aggregates settlement during vibrating compaction. With this high-fidelity framework, both the macroscale rheological behavior of fresh FRC as free surface flow and the mesoscale movements of coarse aggregate and fibers as suspended components can be well captured. After performing the high-fidelity preparations of fresh FRC, good agreements between experimental and numerical L-box tests and smart aggregate vibrating tests are reached. The proposed approach, in accordance with experimental evidence, shows that the fibers' orientation strongly depends on the flow speed. Moreover, the fibers' orientation in high-speed flows tends to follow the same direction of flow, while fibers in low-speed flows tends to follow a random orientation pattern and block the flow.

The response of fiber-reinforced concrete (FRC) members to cyclic or seismic loading has been extensively studied in recent years. However, these experiments cannot fully elucidate the impact of cyclic fiber-bridging criteria on the macroscopic mechanical properties of FRC, particularly for hybrid fiber FRC after cracking. To explicit the cycling fiber-bridging behavior of steel fibers in the ultra-high-performance concrete (UHPC) and polyethylene (PE) fibers in the engineered cementitious composite (ECC), a series of single fiber pull-out test programs considering fiber types, embedding length, and loading scheme were carried out. The test results indicate that the relatively high flexural stiffness of steel fiber results in the bridging force becoming negative upon reaching a certain degree of unloading, with its maximum value governed by the compressive buckling critical. Distinctly, the fiber-bridging behavior of flexible PE fiber exhibits a nonlinear rapid decrease to zero during unloading and tends to return along a similar path during reloading. Further, an integrated cyclic fiber-bridging model encompassing the buckling characteristics of steel and PE fibers is proposed. The introduction of the elastic modulus reducer factor greatly enhances the accuracy of fiber-bridging model prediction. The proposed unloading law that introduces the modified compression buckling criterion could accurately calculate the unloading-reloading behavior, critical buckling load, and post-peak behavior of steel fiber when unloading at any slip deformation. Meanwhile, a simple formulation can quickly and effectively calculate the unloading-reloading process by simply changing the parameter values is proposed for flexible fibers. Finally, the simulated cycling fiber-bridging relationship for both steel and PE fibers agree well with their actual cyclic fiber pull-out behavior.