Numerical Concrete Model Research Articles

Simulation of Frost-Heave Failure of Air-Entrained Concrete Based on Thermal-Hydraulic-Mechanical Coupling Model.

The internal pore structural characteristics and microbubble distribution features of concrete have a significant impact on its frost resistance, but their size is relatively small compared to aggregates, making them difficult to visually represent in the mesoscopic numerical model of concrete. Therefore, based on the ice-crystal phase transition mechanism of pore water and the theory of fine-scale inclusions, this paper establishes an estimation model for effective thermal conductivity and permeability coefficients that can reflect the distribution characteristics of the internal pore size and the content of microbubbles in porous media and explores the evolution mechanism of effective thermal conductivity and permeability coefficients during the freezing process. The segmented Gaussian integration method is adopted for the calculation of integrals involving pore size distribution curves. In addition, based on the concept that the fracture phase represents continuous damage, a switching model for the permeability coefficient is proposed to address the fundamental impact of frost cracking on permeability. Finally, the proposed estimation models for thermal conductivity and permeability are applied to the cement mortar and the interface transition zone (ITZ), and a thermal-hydraulic-mechanical coupling finite element model of concrete specimens at the mesoscale based on the fracture phase-field method is established. After that, the frost-cracking mechanism in ordinary concrete samples during the freezing process is explored, as well as the mechanism of microbubbles in relieving pore pressure and the adverse effect of accelerated cooling on frost cracking. The results show that the cracks first occurred near the aggregate on the concrete sample surface and then extended inward along the interface transition zone, which is consistent with the frost-cracking scenario of concrete structures in cold regions.

Numerical prediction for life of damaged concrete under the action of fatigue loads

High-speed railways have become a critical component of modern transportation, and ensuring the stability and durability of high-speed railway concrete under environmental and train fatigue loads is essential for ensuring project safety. This paper employs a numerical simulation method to predict the performance and lifespan of high-speed railroad concrete under fatigue loading with environmental damage. The study analyzes the effects of stress level (0.5–0.8), stress ratio (0.1–0.9), and load frequency (5 Hz–40 Hz) on the fatigue performance and lifespan of railway concrete during bending fatigue loading. A four-point bending numerical model of concrete is developed in this paper, which considers elastic modulus decay and surface cracking, in order to quantify the degradation of concrete properties in response to environmental and load-induced cracks. The numerical model is well-validated with experimental results, and the study establishes a correlation between concrete degradation and environmental and load-induced cracks, as well as the fatigue resistance of high-speed railroad concrete. The bending fatigue frequency of concrete should not exceed 25 Hz at high stress levels of 0.7. This study also shows that environment-induced initial damage will greatly reduce the fatigue resistance of concrete, and fatigue life degradation and elastic modulus decay have a nearly linear relationship. This research provides a crucial foundation for the fatigue design of concrete materials for high-speed railroads.

Numerical Analysis of Hybrid Fibre-Reinforced Concrete Beam-Column Joint

Beam-column joint is the most vulnerable location of a moment-resisting reinforced concrete frame structure. The joint region experiences the maximum shear stress both in vertically and horizontally which is generated due to the shear transfer mechanism from the adjoining beams and columns. The shear capacity and bond stress capacity are the two major factors affecting the strength of a joint core in RC structure. An important discovery recently is the ductile behaviour of the whole structure under repeated loading. The behaviour of the concrete beyond elastic limit which is in the concrete hardening zone can drastically influence the ductility of the concrete. The non-linear stress-strain behaviour after the onset of the initial crack and up to ultimate compressive strength plays an important role in improving ductility. Beyond the ultimate compressive strength, concrete will undergo softening which is neglected in this study as once concrete reaches ultimate stress it is unsafe for service. This material ductility can be fulfilled with the application of high-strength fibres with ductile behaviour. However, the hybridization of two or more fibres can incorporate two different characteristics of the fibre used. The use of ordinary-grade of concrete moreover reduces the shear-resisting capacity of the joint. A hybrid mix of hooked-end steel fibre with basalt fibre and crimpled steel fibre with polypropylene fibre are used with a volume fraction of 1% to 1.4% of the concrete. In this study, ordinary M25 grade concrete and fibre mixed M25 grade concrete is employed under static and cyclic loading. The laboratory tests are also conducted to evaluate the compressive strength, split-tensile strength, and flexural strength of the hybrid mix fibre-reinforced concrete at the age of 28th days. Five full-scale models of the beam-column joint are designed as per the Bureau of Indian Standards. Numerical models of concrete and steel reinforcement are developed. Numerical analysis is carried out using finite element software ANSYS-v21. The behaviours of the beam-column joint are observed under static as well as cyclic loading. Crack patterns, first crack load, initial displacement, ultimate load, and ultimate displacement are observed under static conditions. And under cyclic loading, hysteresis load vs displacement, energy dissipation, and stiffness degradation are observed. The hybridization of hooked steel with basalt fibre gives better results in mechanical strengths and the hybrid effect of crimpled steel with polypropylene fibre gives better results in mechanical strengths. And also under numerical study, the above specimens show an improvement in energy dissipation capacity. Keywords beam-column joint, hybrid fibre reinforced concrete, numerical concrete model, ANSYS, static, reverse cyclic, energy dissipation, stiffness, crack

Research on compressive damage mechanism of concrete based on material heterogeneity

To reveal the influence of material heterogeneity on the mechanical properties of concrete, computed tomography (CT) tests of concrete under uniaxial compression were carried out. A CT scan test of concrete under uniaxial compressive stress was carried out, and CT scan images of a loading stage were obtained. Based on the theory of damage evolution, a mapping relationship between elastic modulus and CT number was constructed, andthe “numerical concrete model of material heterogeneity” was established. The equation for damage evolution and the constitutive equation for subsection damage based on porosity were constructed. Then, the influence of heterogeneity on the damage mechanical properties of concrete under uniaxial compressive stress was studied. The results show that material heterogeneity has a more significant effect on the mechanical properties of concrete due to damage. In addition, the elastic modulus of the material heterogeneity samples is smaller than that of the structural heterogeneity samples, and the peak strength of the material heterogeneity samples is 10.7% lower than that of the structural heterogeneity samples. The numerical concrete model of material heterogeneity can effectively reflect the initial defects of the concrete. Furthermore, the crack propagation of the sample is orderly. The crack is evident around the aggregate and has good directivity. The shear crack characteristics are also noticeable. Meanwhile, the main crack is roughly at a 45-degree angle with the sample, which is closer to the actual physical CT test.

Three-dimension mesoscale model of concrete by random placement algorithm and its failure mechanism analysis

This contribution presents the development and application of a 3D numerical concrete model for investigating the fracture behavior of concrete at the mesoscale. The reconstruction of actual geometric models of pebble and crushed aggregates is carried out using a 3D laser scanner. An aggregate and pore library were then constructed, and a 3D random placement method is developed to generate the realistic mesostructure of concrete. Additionally, the cohesive zone model-based numerical computation approach was employed to simulate the fracture problem in concrete. Then, the numerical concrete model is applied to investigate the mechanical response and fracture behavior of cylindrical concrete specimens under uniaxial compression and concrete beams under three-point bending. Results show that the 3D mesoscale concrete model effectively captures the complex fracture process and nonlinear mechanical behavior. This methodology can be used for investigating the effect of mesostructure on macroscopic behavior of concrete and also has potential applications in studying the behavior of particulate composites under various conditions.

A Research on the Macroscopic and Mesoscopic Parameters of Concrete Based on an Experimental Design Method.

Concrete is a composite material that has complex mechanical properties. The mechanical properties of each of its components are different at the mesoscopic scale. Studying the relationship between the macroscopic and mesoscopic parameters of concrete can help better understand its mechanical properties at these levels. When using the discrete element method to model the macro-mesoscopic parameters of concrete, their calibration is the first challenge. This paper proposes a numerical model of concrete using the particle discrete element software particle flow code (PFC). The mesoscopic parameters required by the model need to be set within a certain range for an orthogonal experimental design. We used the proposed model to perform numerical simulations as well as response surface design and analysis. This involved fitting a set of mapping relationships between the macro–micro parameters of concrete. An optimization model was established in the MATLAB environment. The program used to calibrate the mesoscopic parameters of concrete was written using the genetic algorithm, and its macro-micro parameters were inverted. The following three conclusions can be drawn from the orthogonal test: First, the tensile strength and shear strength of the parallel bond between the particles of mortar had a significant influence on the peak compressive strength of concrete, whereas the influence of the other parameters was not significant. Second, the elastic modulus of the parallel bonding between particles of mortar, their stiffness ratio and friction coefficient, and the elastic modulus and stiffness ratio of contact bonding in the interfacial transition zone had a significant influence on the elastic modulus, whereas the influence of the other parameters was not significant. Third, the elastic modulus, stiffness ratio, and friction coefficient of the particles of mortar as well as the ratio of the contact adhesive stiffness in their interfacial transition zone had a significant influence on Poisson’s ratio, whereas the influence of the other parameters was not significant. The fitting effect of the response surface design was good.

Analysis of the wave propagation paths in numerical reinforced concrete models

The paper aims to study the characteristics of the wave propagation paths in reinforced concrete and to improve the existing velocity models. Therefore, coefficients of reflection and transmission with regard to energy were analyzed for each component of the reinforced concrete (cement matrix, aggregate grains, air voids and reinforcement bars) in case of a free- and joint-material boundary, respectively. The findings were employed to investigate the wave propagation paths in cracked concrete and reinforced concrete. Three-dimensional numerical models to approximate the heterogeneous concrete (numerical concrete model, NCM) and reinforced concrete (numerical reinforced concrete model, NRCM) were implemented in a rotated staggered-grid finite-differences scheme. Numerical simulations of the elastic wave propagation were performed to illustrate the interaction of waves with boundaries. Both velocity models, NCM and NRCM, were applied to localize acoustic emissions (AE). An established source localization estimation method (FastWay) served as a tool to validate the heterogeneous velocity models. FastWay was applied to a synthetic cracked reinforced concrete beam to localize six randomly located AE sources. The influence of non-straight wave propagation paths on the performance of the localization method was discussed.

Numerical Reconstruction Model and Simulation Study of Concrete Based on Damaged Partition Theory and CT Number.

The applicability of mesoscopic models plays an important role in studying the mesoscopic mechanical properties of concrete. In this study, the computerized tomography (CT) test of concrete under uniaxial compression conditions is conducted using a portable dynamic loading equipment developed by Xi’an University of Technology and a medical Marconi M8000 spiral CT scanner. On the basis of damage partition theory, a probabilistic statistical method for determining threshold values is proposed, and a CT test images is obtained and divided into aggregate, hardened cement and hole-crack areas. A ‘structural random numerical concrete model’ is also established on the basis of the coordinates of each pixel unit in CT images. Uniaxial static compression and tensile numerical simulation tests are conducted. Results show that the structural random numerical concrete model can not only reflect the microscopic composition of concrete but also the interfacial transition zone (ITZ) between aggregate and mortar. The ITZ thickness is approximately 0.04 mm, which is close to the real concrete sample ITZ thickness (approximately 10–50 μm). In the two tests, the specimen damage starts from the initial defects, and the damage crack expands through the weaker ITZ around the aggregate. No matter under the action of static tension or compression load, the damage cracks of the sample almost never pass through the aggregate. Most of the many cracks in uniaxial compression are shear cracks. However, many cracks form at the beginning of uniaxial tension, and only one main crack, which is roughly perpendicular to the loading direction, exists in the end.

Three-Dimension Random Aggregate Generation of Numerical Concrete Model

The aggregate generation of concrete is one of the important problems in concrete mesoscopic mechanics. Firstly, the mesoscopic numerical model with spherical aggregates is obtained by the method of excluding the occupied space, and fully-graded concrete model of high aggregate content can be quickly generated. Then, based on the spherical aggregate model, the generation method of random convex polyhedral aggregates is proposed. Finally, a full-graded concrete model with spherical aggregates is shown in Case 1 and a cylindrical concrete model with random convex polyhedral aggregates is shown in Case 2. The result shows that the aggregates are equally distributed in the concrete models which can be used in the study of mesoscopic numerical calculation.

Numerical Simulation of Recycled Concrete Using Convex Aggregate Model and Base Force Element Method

By using the Base Force Element Method (BFEM) on potential energy principle, a new numerical concrete model, random convex aggregate model, is presented in this paper to simulate the experiment under uniaxial compression for recycled aggregate concrete (RAC) which can also be referred to as recycled concrete. This model is considered as a heterogeneous composite which is composed of five mediums, including natural coarse aggregate, old mortar, new mortar, new interfacial transition zone (ITZ), and old ITZ. In order to simulate the damage processes of RAC, a curve damage model was adopted as the damage constitutive model and the strength theory of maximum tensile strain was used as the failure criterion in the BFEM on mesomechanics. The numerical results obtained in this paper which contained the uniaxial compressive strengths, size effects on strength, and damage processes of RAC are in agreement with experimental observations. The research works show that the random convex aggregate model and the BFEM with the curve damage model can be used for simulating the relationship between microstructure and mechanical properties of RAC.

Time reverse characterization of sources in heterogeneous media

Time reverse modeling of wave propagation in elastic media is applied to localize and characterize acoustic emission using a numerical concrete model. The aim is to transform a specific method within exploration geophysics to non-destructive testing. In contrast to previous standard localization techniques, no single event or first onset time identification is applied. The method is described and illustrated with synthetic data. Time reverse localization and characterization with limited knowledge of boundary values are feasible using the rotated staggered finite-difference method. Several imaging conditions are applied and compared to each other. We demonstrate the localization of two synchronous acoustic emissions with a limited number of sensors and using effective elastic properties. The imaging condition based on the maximum energy density is most indicative. Source characteristics can also be recovered. The goal is to apply our method to acoustic emissions measured during experiments carried out on concrete and reinforced concrete specimen. Several aspects of the described methodology can also be transferred back to exploration geophysics.

Modeling of internal curing in maturing mortar

Prediction of the influence of internal curing both on the material behavior during maturing and on its final physical properties is an important issue in concrete research. A novel, mechanistic-type numerical model of concrete is applied for modeling the chemo-physical phenomena taking place during internal curing. The model takes into account full coupling between chemo-thermal, hygral and mechanical phenomena in porous media (i.e., maturing cement paste or mortar). In this work, the recent development of the model for description of internal curing is presented along with the results of simulations. The model allows for description of the influence of internal curing on such phenomena as hydration, self-desiccation or autogenous shrinkage. The results of the simulations are compared with the experimental data concerning autogenous phenomena in low water-to-cement ratio mortars containing superabsorbent polymers.

Application of time reverse modeling on ultrasonic non-destructive testing of concrete

Time reverse modeling (TRM) is applied to localize and characterize acoustic emission using a numerical concrete model. Aim is to transform a method within exploration geophysics to non-destructive testing. In contrast to previous time reverse applications, no single event or first onset time identification is applied. The method is described from a mathematical point of view. So-called source TRM with limited knowledge of boundary values is compared with so-called full TRM where a complete set of boundary conditions is used. The resulting localization accuracy of both approaches is similar. With a known three-dimensional analytical solution we demonstrate the applicability and the limitations of the two-dimensional wave propagation method solving the elastodynamic wave equation. With the help of CT images we are able to digitalize a concrete specimen and to verify a used numerical concrete model. TRM localization using this highly scattering material is feasible using the rotated staggered finite-difference method. We demonstrate the localization of acoustic emission with a limited number of sensors and using effective elastic properties. Source characteristics can also be recovered. Goal is to apply our method to acoustic emissions measured during experiments carried out on concrete and reinforced concrete specimen.

Elastic wave propagation in a segmented X-ray computed tomography model of a concrete specimen

The paper aims to demonstrate the synergy between X-ray computed tomography (CT) data for a concrete specimen and numerical simulations of elastic wave propagation in a purely numerical concrete model (NCM). Uncracked and cracked concrete specimens are scanned using X-ray tomography, and in a subsequent step, are post-processed to analyze the influence of concrete constituents on wave propagation behavior. Numerical modeling of uncracked concrete as a heterogeneous randomly-distributed medium is introduced. Homogenized material properties, the so-called effective elastic properties, are determined for the segmented X-ray CT images and the NCM. The results of both approaches are compared and discussed. Numerical simulations of wave propagation in the segmented X-ray CT model and the numerical concrete model are performed and compared. The influence of concrete constituents such as aggregate grains and air voids on wave propagation behavior is studied.

A numerical model for elastic modulus of concrete considering interfacial transition zone

The effect of interfacial transition zone on mechanical properties of concrete has been found to be significant, thus the interfacial transition zone should be considered in the analysis for better estimation of elastic modulus of concrete. However, it is difficult to estimate elastic modulus of concrete practically using simple models proposed so far. In this study, a numerical concrete model that adopts three-phase model and finite element with material discontinuity was proposed to analyze concrete with complex interface in three dimensions. The validity of the proposed model was verified by comparing the calculated elastic moduli of concrete with those obtained from experiments. The effect of interfacial transition zone on elastic modulus of concrete with either low or high w/c was also investigated. The analysis results suggest that careful selection of characteristics for interfacial transition zone should be made for the accurate estimation of elastic modulus of concrete.