Behavior Of Concrete Material Research Articles

Behaviour and design method of short square tubed-steel-reinforced-concrete columns under eccentric loading

The tubed-steel-reinforced-concrete (TSRC) column is a kind of special concrete-encased steel composite columns using the thin-walled steel tube instead of a reinforcement cage to confine the concrete core. The steel tube discontinues at the beam-column joints and thus does not carry any direct axial load. In this study, twelve short square TSRC columns were tested to investigate their load-carrying and deformation capacities under eccentric compression. The studied parameters are width-to-thickness ratio of the steel tube, eccentricity of the axial load, and the flange shear studs. The test results indicate that the encased steel shape works with the concrete compatibly due to the confinement from the steel tube, and the flange shear studs are not effective in enhancing the flexural stiffness and the axial resistance. To account for the inelastic behaviour of structural steel and concrete materials, a numerical model based on the fibre sectional analysis was developed, which predicts the results in general agreement with the test results. Parametric analysis is carried out based on the numerical model. Moreover, a new simplified method to estimate the capacity interaction diagram of a square TSRC section is proposed.

Analytical simulation of seismic collapse of RC frame buildings

Application of a fibre-element nonlinear modelling technique for seismic collapse capacity assessment of RC frame buildings in comparison with conventional lumped plasticity models is investigated in this paper. Constitutive material models of concrete and steel for fibre elements are adopted to enable simulation of the loss in vertical load carrying capacity of structural columns. Inclusion of the nonlinear second order P−Δ effects accelerated by degrading behaviour of steel and concrete materials in the fibre model allows prediction of the sidesway mode of collapse. The model is compared with nonlinear lumped plasticity models in which stiffness and strength degradation is replicated through degrading parameters in structural components. Static cyclic analyses of an example cantilever column and a portal frame indicate that the variation of axial loads in columns may result in accelerated degradation and failure of structural components which is not taken into account by lumped plasticity models. Moreover, incremental dynamic analysis of a ten-storey RC frame shows that the lumped plasticity model may overestimate building collapse capacity when vertical failure of structural components occurs prior to sidesway instability.

Modeling of impact on concrete plates by use of the microplane approach

A numerical approach to model 3D impact scenarios on concrete structures is presented and validated by experimental investigations. Special focus is given to dynamic contact modeling of an impactor loading a concrete plate at very high velocity. In this context, the main aspect is the elimination of non-physical oscillations of the kinematic fields by using an updated Newmark integration scheme. Two Lagrangian multipliers are used for this update which successfully reduces the unphysical oscillations of the above mentioned fields. In a further step, the material behavior of concrete is described by the microplane model introducing a new rate-dependent non-local damage formulation. Using the equivalent strain as a non-local field, the method is able to model, in an adequate manner so that the crack pattern in concrete is represented by the damage evolution in case of transient problems. The accuracy is validated and proved by the help of benchmark simulations as well as the observed phenomena to experimental investigations.

A competitive mechanism driven damage-plasticity model for fatigue behavior of concrete

In this paper, an innovative three-dimensional plastic-damage model is proposed to describe the behavior of concrete material under fatigue loading. In the proposed model, two damage variables are defined to describe the fatigue degradation of the mechanical properties of concrete under tension and shear, respectively. Meanwhile, by adopting the elastoplastic damage energy release rates, which are derived from the elastoplastic Helmholtz free energy potentials, a novel fatigue damage evolution law is developed. Specifically, the damage evolution process is governed by the competition between the damage healing effect and the damage driving effect induced by the propagation and closure of micro cracks under fatigue loading. The proposed model also allows the accumulation of fatigue damage within the classical damage surface by adopting the loading/unloading irreversibility concept. Moreover, the evolution of plastic strains is considered within the framework of the effective stress space plasticity. In the end, a set of numerical tests is presented, and the validity and effectiveness of the proposed model are illustrated according to the simulated results and the comparisons with the experimental data in literatures.

Composite behavior of concrete materials under high temperatures

This study examines the performance of concrete under elevated temperatures at the meso-scale level of observation where aggregate particles and the embedding hydrated cement paste form interacting continua. Decomposing concrete into these two constituents leads to mismatch of the thermal and hydraulic transport properties and hence to self-equilibrating internal stresses introducing progressive damage of the mechanical response behavior of concrete. Thereby the internal stresses are disregarded by the macro-scale level approach when the heterogeneities are replaced by equivalent effective material properties using homogenization. In other terms, the macroscopic approach eliminates the contrast among the individual constituents and consequently negates the development of stresses causing pervasive microcracking in concrete.The current study resolves concrete into its main components, the aggregate particles and the cement paste, bonded by a weak interface transition zone that reduces to some extent the mismatch between the two constituents. The study illustrates the magnitude of the stress state in representative concrete specimens and the resulting damage evolution under high temperatures.

Calibration of K&C Concrete Model for UHPC in LS-DYNA

Karagozian & Case Concrete (KCC) model in LS-DYNA is a parameter automatically generated material model for simulation of concrete. By giving the unconfined compressive strength of the concrete material, all other constitutive material parameters will be generated by the model. It is a valuable concrete model for users who have no laboratory data or conduct tentative simulations. However, the automatically generated parameters will induce error when the concrete material behavior deviates from the regular behavior. In this research, a calibration method is introduced to show a simple way for generating input material parameters of ultra-high performance concrete (UHPC). Empirical equations for triaxial compressive tests are used here for the calibration. The calibrated model is firstly checked by the simulation of uniaxial compression test with one element and the results reveal that the calibrated model shows a better agreement with the test data than the parameter automatically generated KCC model.

Machine learning in concrete strength simulations: Multi-nation data analytics

Machine learning (ML) techniques are increasingly used to simulate the behavior of concrete materials and have become an important research area. The compressive strength of high performance concrete (HPC) is a major civil engineering problem. However, the validity of reported relationships between concrete ingredients and mechanical strength is questionable. This paper provides a comprehensive study using advanced ML techniques to predict the compressive strength of HPC. Specifically, individual and ensemble learning classifiers are constructed from four different base learners, including multilayer perceptron (MLP) neural network, support vector machine (SVM), classification and regression tree (CART), and linear regression (LR). For ensemble models that integrate multiple classifiers, the voting, bagging, and stacking combination methods are considered. The behavior simulation capabilities of these techniques are investigated using concrete data from several countries. The comparison results show that ensemble learning techniques are better than learning techniques used individually to predict HPC compressive strength. Although the two single best learning models are SVM and MLP, the stacking-based ensemble model composed of MLP/CART, SVM, and LR in the first level and SVM in the second level often achieves the best performance measures. This study validates the applicability of ML, voting, bagging, and stacking techniques for simple and efficient simulations of concrete compressive strength.

Concrete Material Models in LS-DYNA for Impact Analysis of Reinforced Concrete Structures

Performance of three widely used concrete material models available in LS-DYNA is compared using experimental results of drop-weight impact on a reinforced concrete (RC) beam and high speed aircraft engine missile impact on an RC wall. An overview of these material models and typical concrete material behavior shown by these models using single element analysis are also presented. The study is useful for users who have limited experience on the selection of an appropriate material model for concrete in impact simulation of RC structures.

3D meso-scale modelling of concrete material in spall tests

Tensile strength is one of the key factors of concrete material that need be accurately defined in analysis of concrete structures subjected to high-speed impact loads. Dynamic tensile strength of concrete material is usually obtained by conducting laboratory tests such as direct tensile test, Brazilian splitting test and spall test. Concrete is a heterogeneous material with different components, but is conventionally assumed to be homogeneous, i.e., cement mortar only, in most previous experimental or numerical studies. The aggregates in concrete material are usually neglected owing to testing limitation and numerical simplification. It has been well acknowledged that neglecting coarse aggregates might not necessarily give accurate concrete dynamic material properties. In the present study, a 3D meso-scale model of concrete specimen with consideration of cement mortar and aggregates is developed to simulate spall tests and investigate the behaviour of concrete material under high strain rate. The commercial software LS-DYNA is used to perform the numerical simulations of spall tests. The mesh size sensitivity is examined by conducting mesh convergence tests. The reliability of the numerical model in simulating the spall tests is verified by comparing the numerical results with the experimental data from the literature. The influence of coarse aggregates on the experimental test results is studied. The wave attenuation in concrete specimen is analysed, and empirical equations are proposed for quick assessment of the test data to determine the true dynamic tensile strength of concrete material. The contributions of aggregates to dynamic strength in spall tests are quantified for modifying the test results based on mortar material in the literature.

Damage prediction of concrete gravity dams subjected to underwater explosion shock loading

The damage prediction of concrete gravity dams under blast loads has gained importance in recent years due to the great number of accidental events and terrorist bombing attacks that affected engineering safety. It has long been known that an underwater explosion can cause significantly more damage to the targets in water than the same amount of explosive in air. While the physical processes during an underwater explosion and the subsequent response of structures are extremely complex, which involve lots of complex issues such as the explosion, shock wave propagation, shock wave-structure interaction and structural response. Hence a sophisticated numerical model for the loading and material responses would be required to enable more realistic reproduction of the underlying physical processes. In this paper, a fully coupled numerical approach with combined Lagrangian and Eulerian methods, incorporating the explosion processes, is performed. The RHT (Riedel–Hiermaier–Thoma) model including the strain rate effect is employed to model the concrete material behavior subjected to blast loading. Detailed numerical simulation and analysis of a typical concrete gravity dam subjected to underwater explosion are presented in this study. In terms of different TNT charge weights, the structural response and damage characteristics of the dam at different standoff distances are investigated. Based on the numerical results, critical curves related to different damage levels are derived.

Numerical Computation of Structures with Quasi-brittle Materiel under Dynamic Loading: Case Study of Concrete Gravity Dam

The present paper deals with a numerical computation using finite element method for concrete structures subjected to dynamic loads, usually of seismic nature the consequences of which are often devastating. In the general equation of dynamic, a law of concrete material behavior taking into account models based on the damage mechanics has been implemented. From a phenomenological viewpoint, the chosen behavior model takes into account the damage caused by the opening of micro-cracks. For numerical application, two dimensional seismic analysis of Koyna gravity dam is performed by using the 1967 Koyna earthquake records. The effects of damage on earthquake response of the concrete gravity dam are studied.

Study on Vibrational Liquefaction Behavior of Air-Entrained Concrete

With respect to the liquefaction behavior of cement concrete material in a vibrational condition, the varying patterns of the vibrational viscosity coefficients, vibrational energy transfer and rheological mechanical behavior of air-entrained concrete and non-air-entrained concrete in a vibrational shear condition were compared by analyzing the vibrational liquefaction processes of concrete. The study has found that the vibrational viscosity coefficient of concrete with air entraining agent added increased notably so that air-entrained concrete can maintain good shuttering behavior during slip-form construction; presence of a large number of tiny bubbles in air-entrained concrete allowed more even transfer of vibrational energy in concrete; and the rheological mechanical behavior of concrete was improved after air entraining agents were added so that it became easier to liquefy by vibration and had better shaping effect upon completion of vibration. Keywords: air-entrained concrete, slip-form construction, cement concrete pavement, vibrational liquefaction

Studying the effect of a hydrostatic stress/strain reduction factor on damage mechanics of concrete materials

AbstractIn the non-linear finite element analysis (NFEA) of concrete materials, continuum damage mechanics (CDM) provides a powerful framework for the derivation of constitutive models capable of describing the mechanical behavior of such materials. The internal state variables of CDM can be introduced to the elastic analysis of concrete to form elastic-damage models (no inelastic strains), or to the elastic-plastic analysis in order to form coupled/uncoupled elastic-plastic-damage models. Experimental evidence that is well documented in literature shows that the susceptibility of concrete to damage and failure is distinguished under deviatoric loading from that corresponding to hydrostatic loading. A reduction factor is usually introduced into a CDM model to reduce the susceptibility of concrete to hydrostatic stresses/strains. In this work, the effect of a hydrostatic stress/strain reduction factor on the performances of two NFEA concrete models will be studied. These two (independently published) models did not provide any results showing such effect. One of these two models is an elastic-damage model, whereas the other is an uncoupled elastic-plastic-damage model. Simulations and comparisons are carried out between the performances of the two models under uniaxial tensile and compressive loading conditions. Simulations are also provided for the uncoupled elastic-plastic-damage model under the following additional loading conditions: biaxial tension and biaxial compression, uniaxial cyclic loading, and varying ratios of triaxial compressive loadings. These simulations clearly show the effect of the reduction factor on the numerically depicted behaviors of concrete materials. To have rational comparisons, the hydrostatic stress reduction factor applied to each model is chosen to be a function of the internal state variables common to both models. Therefore, once the two models are calibrated to simulate the experimental behaviors, their corresponding reduction factors are readily available at every increment of the iterative NFEA procedures.

ABAQUS Dynamic Elasto-Plastic Analysis in Earthquake-Resistant Structure Design

Since the Wenchuan earthquake in 2008, the earthquake-resistant consideration in structure design is highlighted in the Chinese code. This paper introduces a dynamic elasto-plastic method in structure modeling using ABAQUS finite element analysis. In the first stage, structural models are transferred into ABAQUS INP files. Then the user subroutines are given to describe the elastic and plastic behaviors of both concrete and steel materials. Finally, the models are analyzed in an ABAQUS implicit or explicit analysis. The result can suggest the weakness of the building and provide advisement in reinforcing the structure.

Microplane constitutive model M4L for concrete. I: Theory

The present, Part I, of this two-part study presents a new microplane constitutive model for simulating the fracture and damage behavior of concrete materials subjected to various loadings. The concept of stress–strain boundaries (strain-dependent yield limits) is used to describe the inelastic behavior of concrete. To facilitate the constitutive formulation, the relation between the microplane strain components and the macroscopic strain tensor is firstly identified. A new concept of confinement-adjusted effective microplane tangential moduli is introduced. Based on the material experiment observations and physical concepts, some novel microplane stress–strain boundaries are developed. A new numerical algorithm for calculating the microplane stress components is also presented. The validation of the new model against experimental data will be presented in the second part of this study.

Mesoscopic modeling and simulation of the dynamic tensile behavior of concrete

We present a two-dimensional mesoscopic finite element model for simulating the rate- and moisture-dependent material behavior of concrete. The idealized mesostructure consists of aggregate grains surrounded by an interfacial transition zone embedded in the bulk material. We examine the influence of the most significant constitutive model parameters on global and local response. Different distributions and shapes of the aggregate grains are tested. Three model parameter sets, corresponding to different moisture conditions, are employed in the analysis of two specimens in which the applied loading rate is significantly different. The results indicate that the loading rate has a stronger influence than the saturation level on fracture processes and global strength.

Analytical modeling of nonlinear behavior of composite stub-girders

This paper presents an inelastic analytical model for modeling the nonlinear behavior of partially composite stub-girder under midspan concentrated load with partially end joint stiffness. The method treats the stub-girder as a beam with three distinct layers, the top and bottom layers behave according to the normal beam bending theory, while the middle layer is a shear layer without any bending stiffness. At the interface between the steel and concrete elements, the headed stud shear connectors are modeled as linear elastic built-in cantilevers. The present method smears the effects of all the contributing shear elements, stubs and studs, along the span. The differential equation for the stub-girder is solved to obtain the general expression of elastic deflection for the case when one concentrated load is applied at the mid-span. Based on the present approach a computer program called “PZA” is developed. The program accounts for nonlinear behavior of concrete and steel materials. The accuracy and reliability of the program are demonstrated by the analysis of three stub girders and one composite beam over the entire loading range up to ultimate load and the results are compared with published experiments. A relatively good agreements with the experimental findings are found. Then a parametric study is carried out to discuss the effect of the initial stiffness of shear connector and the stub height on the behavior of stub-girder.

A Coarse Grid Correction to Domain Decomposition Based Preconditioners for Meso-Scale Simulation of Concrete

Meso-scale simulation is one of the important ways to study dynamic behaviors of concrete materials, while most of the simulation time is used to solve the sparse linear systems. Because the discrete grid is three dimensional and is of large scale, iterations are the best solutions. But the convergence depends on the distribution of the eigenvalues of the coefficient matrix, to make the eigenvalues distributed more closely each other, it is required to adopt preconditioning techniques. In this paper, with the characteristics of the sparse linear systems considered, there provides a coarse grid correction algorithm, which is based on domain decomposition preconditioners and aggregation of sub-domains, with each aggregated into a single super-node. A linear system with small scale size is formed, which contains the global information and the solution is used to correct the solution components of the original auxiliary linear system. For incomplete factorization preconditioner parallelized with block Jacobi, classic additive Schwarz, and factors combination techniques, the experiments show that the presented algorithm can improve the convergence rate and the efficiency.

Numerical simulation of impact tests on reinforced concrete beams

This paper focuses on numerical simulation of impact tests of reinforced concrete (RC) beams by the LS-DYNA finite element (FE) code. In the FE model, the elasto-plastic damage cap (EPDC) model, which is based on continuum damage mechanics in combination with plasticity theory, is used for concrete, and the reinforcement is assumed to be elasto-plastic. The numerical results compares well with the experimental values reported in the literature, in terms of impact force history, mid-span deflection history and crack patterns of RC beams. By comparing the numerical and experimental results, several important behavior of concrete material is investigated, which includes: damage variable to describe the strain softening section of stress–strain curve; the cap surface to describe the plastic volume change; the shape of the meridian and deviatoric plane to describe the yield surface as well as two methods of incorporating rebar into concrete mesh. This study gives a good example of using EPDC model and can be utilized for the development new constitutive models for concrete in future.

Fibre composite railway sleeper design by using FE approach and optimization techniques

This research work aims to develop an optimal design using Finite Element (FE) and Genetic Algorithm (GA) methods to replace the traditional concrete and timber material by a Synthetic Polyurethane fibre glass composite material in railway sleepers. The conventional timber railway sleeper technology is associated with several technical problems related to its durability and ability to resist cutting and abrading action of the bearing plate. The use of pre-stress concrete sleeper in railway industry has many disadvantages related to the concrete material behaviour to resist dynamic stress that may lead to a significant mechanical damage with feasible fissures and cracks. Scientific researchers have recently developed a new composite material such as Glass Fibre Reinforced Polyurethane (GFRP) foam to replace the conventional one. The mechanical properties of these materials are reliable enough to help solving structural problems such as durability, light weight, long life span (50-60 years), less water absorption, provide electric insulation, excellent resistance of fatigue and ability to recycle. This paper suggests appropriate sleeper design to reduce the volume of the material. The design optimization shows that the sleeper length is more sensitive to the loading type than the other parameters.