混凝土动力强度提高的机理探讨

土木工程中,混凝土结构都不可避免的会承受动力荷载,尤其是对高拱坝来说,地震荷载是非常重要的荷载。准确地认识混凝土材料的动态力学性能是进行准确的混凝土结构动力响应分析的基础。因此混凝土动力本构模型的研究是一个十分有意义的课题。而现有混凝土动力模型大多是宏观、唯像的模型,很难反映混凝土材料的细观特征。采用细观力学方法得到了混凝土的动力强度,与文献中的试验结果对比吻合得较好。     

Genetic algorithm model for shear capacity of RC beams reinforced with externally bonded FRP

A variety of on-site construction applications using FRP materials have been realized worldwide. However, this technology is currently at a stage where its future widespread implementation and competitiveness will depend on the development of reliable design guidelines based on sound engineering principles. This paper presents simple, yet improved, equations to calculate the shear capacity of FRP bonded-reinforced concrete beams based on the genetic algorithms (GAs) approach applied to 212 experimental data points available in the open literature. The performance of the proposed equations was compared to that of commonly used shear design methods, namely the ACI 440, Eurocode (EC2), the Matthys Model, Colotti model and the ISIS Canada guidelines. Results demonstrate that the proposed equations better agree with the available experimental data than the existing models investigated. Moreover, a sensitivity analysis was carried out to investigate the effect of the shear span-to-depth ratio on the shear capacity contributed by concrete, the ultimate effective strain in FRP sheets, and the ultimate effective stress in transverse rebars. Results indicate that the shear span-to-depth ratio has a significant effect on the shear behaviour of FRP bonded-reinforced concrete beams.     

Reliability of reinforced concrete structures under fatigue

This paper focuses on time-variant reliability assessment of deteriorating reinforced concrete structures under fatigue conditions. A strategy combining two time scales, namely the micro-scale of instantaneous structural dynamics (or statics) and the macro-scale of structural lifetime, is proposed. Non-linear response of reinforced concrete structures is simulated by means of the finite element method with adequate material model. A phenomenological fatigue damage model of reinforced concrete is developed and calibrated against experimental results available in the literature. Reliability estimates are obtained within the response surface method using the importance/adaptive sampling techniques and the time-integrated approach. The proposed assessment strategy is illustrated by an example of a concrete arch under fatigue loading. The obtained results show a general inapplicability of local and linear fatigue models to system level of structures.     

Models relating mixture composition to the density and strength of foam concrete using response surface methodology

There have been several investigations in the past on the influence of mixture composition on the properties of foam concrete. Conventionally strength is related to density alone and few models have been developed relating strength with density, porosity, gel–space ratio, etc. Very little work has been reported in the literature in predicting the properties of foam concrete from the knowledge of its mixture proportions. This paper discusses the development of empirical models for compressive strength and density of foam concrete through statistically designed experiments. The response surface plots helps in visually analysing the influence of factors on the responses. The relative influence of fly ash replacement on strength and density of foam concrete is studied by comparing it with mixes without fly ash and brought out that replacement of fine aggregate with fly ash will help in increase in the strength of foam concrete at lower densities allowing high strength to density ratio. Confirmatory tests have shown that the relation developed by statistical treatment of experimental results can act as a guideline in the mixture proportion of foam concrete.     

Plastic viscosity of fresh concrete – A critical review of predictions methods

Rheological properties of fresh concrete, namely plastic viscosity and yield stress, are critical for the concrete industry because they affect placement and workability. Moreover, these rheological properties influence the productivity and quality of concrete, including mechanical properties and durability. Therefore proper characterization of these properties is needed to control the quality of fresh concrete and ensure sustainability of concrete structures.Fundamental and phenomenological rheological models have been proposed in the literature for characterizing the behaviour of fresh concrete. Establishing a model for predicting the plastic viscosity of concrete based on its composition will be extremely valuable for the concrete industry. This paper provides a critical review of the most prevailing models in concrete technology as well as models proposed in the literature for predicting the plastic viscosity of dense suspensions to a total of eight models. Review has revealed that Mahmoodzadeh and Chidiac models based on the cell method provides a higher degree of correlation to the experimental data as well as a more consistent and reliable predictions in comparison to the models currently proposed in the literature for concrete and/or dense suspensions.     

Evaluating and proposing models of circular concrete columns confined with different FRP composites

Commonly used fiber-reinforced polymer (FRP) includes Carbon, Glass, and Aramid FRP composites. Meanwhile, some new FRPs such as PBO (Polypara-phenylene-Benzo-bis-Oxazole), PET (Polyethylene Terephthalate/Polyester), Dyneema, and Basalt have been gradually applied in recent years. Over the past 20 years, there has been extensive research on modeling of stress–strain response of confined concrete using the common types of FRP. In this study, most popular and recent models are investigated to evaluate their general practical application in predicting the response of FRP-confined concrete with strain-hardening performance without any restriction on the fiber used. The aim of the study is twofold. In case of different types of FRP composites, providing equivalent confinement modulus (lateral stiffness), five models are employed to find the FRP-confined concrete stress–strain relationship of three scale-model circular columns. Second ascending part (second stiffness) of the stress–strain relationship of FRP-confined concrete with strain-hardening performance is evaluated in the light of available database from the existing literature using these analytical models. The results showed that the examined models do not satisfy the fact that the slope of the second ascending branch of the stress–strain curve of FRP-confined concrete is independent of its types, provided the design confinement modulus is the same. A comparison of predicted values of the second stiffness with the collected test results of 257 cylinder specimens confined with the common types of FRP composites revealed the necessity for a more accurate model. Based on the discussion of the features and accuracy of these models, a model considering the effect of FRP lateral stiffness is proposed. Because of the variability observed in the test data, however, it appears impossible to develop simple empirical models based on the current database with less than approximately 27% mean absolute error for the second stiffness, and 16% mean absolute error for ultimate strength.     

Model for an engineering approach to the design of concrete structures in chemically aggressive environments

The paper presents a draft proposal for a procedure for designing concrete structures in chemically aggressive environments. Instead of empirical rules which are applied to produce durable concrete, a procedure has been proposed to analytically define the degree of environment agression. On the other hand, definitions have been given for concrete characteristics relevant for a durable concrete structure, as can be numerically set and checked during the construction and after the completion.     

Calculating the elastic moduli of steel-fiber reinforced concrete using a dedicated empirical formula

The equivalent inclusion method (EIM) is adopted to study the characteristics of the equivalent material properties of steel-fiber reinforced concrete as a function of the volume fraction and the length to diameter ratio of the fibers. It is found that the equivalent material moduli of concrete reinforce with randomly orientated and distributed fibers are insensitive to the length to diameter ratio of the steel fibers. A set of empirical formulae is then proposed for the purposes of engineering applications. The proposed empirical model can simplify the calculation of the equivalent material moduli. Verifications of the proposed empirical formulae with the EIM model and with experimental data are performed with two examples. The first is a compression test. The second is 4 point bending test. The empirical formulae, based on the equivalent inclusion method proposed in this study, represent an alternative means of quickly calculating the effective elastic modulus of steel-fiber reinforced concrete materials.     

Relevance of a mesoscopic modeling for the coupling between creep and damage in concrete

In its service-life concrete is loaded and delayed strains appear due to creep phenomenon. Some theories suggest that micro-cracks nucleate and grow when concrete is submitted to a high sustained loading, thereby contributing to the weakening of concrete. Thus, it is important to understand the interaction between the viscoelastic deformation and damage in order to design reliable civil engineering structures. Several creep-damage theoretical models have been proposed in the literature. However, most of these models are based on empirical relations applied at the macroscopic scale. Coupling between creep and damage is mostly realized by adding some parameters to take into account the microstructure effects. In the authors’ opinion, the microstructure effects can be modeled by taking into account the effective interactions between the concrete matrix and the inclusions. In this paper, a viscoelastic model is combined with an isotropic damage model. The material volume is modeled by a Digital Concrete Model which takes into account the “real” aggregate size distribution of concrete. The results show that stresses are induced by strain incompatibilities between the matrix and aggregates at mesoscale under creep and lead to cracking.     

Energy absorption evaluation of reinforced concrete beams under various loading rates based on particle swarm optimization technique

This study proposes an energy absorption model for predicting the effect of loading rates, concrete compressive strength, shear span-to-depth ratio, and longitudinal and transverse reinforcement ratio of reinforced concrete (RC) beams using the particle swarm optimization (PSO) technique. This technique avoids the exhaustive traditional trial-and-error procedure for obtaining the coefficient of the proposed model. Fifty-six RC slender and deep beams are collected from the literature and used to build the proposed model. Three performance measures, namely, mean absolute error, mean absolute percentage error and root mean square error, are investigated in the proposed model to increase its accuracy. The design procedure and accuracy of the proposed model are illustrated and analysed via simulation tests in a MATLAB/Simulink environment. The results indicate the minimal effect of swarm size on the convergence of the PSO algorithm, as well as the ability of PSO to search for an optimum set of coefficients from within the solution space.     

Nonlinear formulation based on FEM,Mazars damage criterion and Fick’s law applied to failure assessment of reinforced concrete structures subjected to chloride ingress and reinforcements corrosion

Structural durability is an important design criterion, which must be assessed for every type of structure. In this regard, especial attention must be addressed to the durability of reinforced concrete (RC) structures. When RC structures are located in aggressive environments, its durability is strongly reduced by physical/chemical/mechanical processes that trigger the corrosion of reinforcements. Among these processes, the diffusion of chlorides is recognized as one of major responsible of corrosion phenomenon start. To accurate modelling the corrosion of reinforcements and to assess the durability of RC structures, a mechanical model that accounts realistically for both concrete and steel mechanical behaviour must be considered. In this context, this study presents a numerical nonlinear formulation based on the finite element method applied to structural analysis of RC structures subjected to chloride penetration and reinforcements corrosion. The physical nonlinearity of concrete is described by Mazars damage model whereas for reinforcements elastoplastic criteria are adopted. The steel loss along time due to corrosion is modelled using an empirical approach presented in literature and the chloride concentration growth along structural cover is represented by Fick’s law. The proposed model is applied to analysis of bended structures. The results obtained by the proposed numerical approach are compared to responses available in literature in order to illustrate the evolution of structural resistant load after corrosion start.     

Analysis-oriented stress–strain model of CRFP-confined circular concrete columns with applied preload

The compressive behavior of FRP-confined concrete is a current issue in the field of structural retrofitting. The available models well predict the stress–strain behavior under monotonic and cyclic loads. However, in the practical applications, columns that need an increasing of bearing capacity are often strengthened under serviceability load conditions, with a stress and strain state that could change the response of the reinforced systems with respect to the case of the unloaded state. In this paper, the compressive behavior of circular FRP-confined concrete columns with preload is analyzed with the introduction of a modified analysis-oriented model. Differently from the classical formulations in which stress–strain model is aimed to the evaluation of the confinement capacity of non-preloaded elements in monotonic regime, the proposed model is also suitable for the determination of the combined response of unconfined and confined concrete subjected to an established stress/strain state in the unconfined state. The proposed model is also compared with the experimental results available in the literature under different assigned preloading levels.     

Bond between FRP composites and concrete: Assessment of design procedures and analytical models

The use of fibre reinforced polymers (FRPs) to strengthen reinforced concrete (RC) structures has gained a wide popularity in the last decades. Although many experimental and analytical studies are available in literature, some issues are still under discussion in the research communities. Since the typical failure mode of FRP–concrete joints is reported to be debonding of the composite from the concrete substrate [1], the estimation of the bond strength between FRP and concrete substrate represents a key issue for the proper use of this technology. For this reason, several analytical models for the evaluation of the FRP–concrete bond strength and few models for the estimation of the effective bond length were proposed (some of them are included in design codes/recommendations/guidelines); however they were not assessed by means of an appropriate experimental database.This work shows an assessment of twenty analytical models for the evaluation of the FRP–concrete bond strength. The assessment is based on the analysis of a wide experimental database collected from the literature. The results are provided distinguishing between the test setup adopted (single or double shear test, bending test) and the material used (post impregnated sheets or pre impregnated laminates). The accuracy of each model was evaluated by means of a simplified statistical analysis. The influence of the test setup and basic material on the accuracy of the model used was analysed as well. Lastly, the accuracy of twelve available models in providing an estimation of the effective bond length was also assessed.     

Concrete cover rip-off of R/C beams strengthened with FRP composites

In this paper the authors consider four FRP strengthened R/C beams brought to failure for concrete cover rip-off under uniform load conditions. Based on the available experimental results, a comparative study of different models for crack spacing evaluation is presented, accounting the influence of the FRP strengthening on the crack pattern development and stabilization. Among the considered models, the authors select a simple and efficient expression suitable to be proposed as a design tool. Thanks to the selected crack spacing expression, the authors work out a simple model that can predict the rip-off failure load of R/C beams externally strengthened with FRP with an acceptable accuracy. The model is calibrated making use of the four available experimental beams and is validated accounting for 23 experimental beams derived from the literature.     

Distribution-free high-dimensional two-sample tests based on discriminating hyperplanes

In this article, we propose a general procedure for multivariate generalizations of univariate distribution-free tests involving two independent samples as well as matched pair data. This proposed procedure is based on ranks of real-valued linear functions of multivariate observations. The linear function used to rank the observations is obtained by solving a classification problem between the two multivariate distributions from which the observations are generated. Our proposed tests retain the distribution-free property of their univariate analogs, and they perform well for high-dimensional data even when the dimension exceeds the sample size. Asymptotic results on their power properties are derived when the dimension grows to infinity and the sample size may or may not grow with the dimension. We analyze several high-dimensional simulated and real data sets to compare the empirical performance of our proposed tests with several other tests available in the literature.     

Analytical prediction of pressure loss through a sudden-expansion in two-phase pneumatic conveying lines

Estimation of separation or minor pressure losses for pipe fittings of a pneumatic conveying system at design stage is critical as much as determination of frictional pressure losses through it. The flow in many pneumatic conveying systems is a two-phase flow; it is so complex and difficult to be investigated by experimental techniques. The static pressure recovery and the minor loss coefficient through an axis-symmetric, circular cross-section, sudden-expansion fitting of a horizontal pneumatic conveying line with air–solid particle flow are analytically studied. The theoretical models proposed in the literature are scarce, and do not confirm the experimental studies. The well-known homogeneous and separated flow models proposed in the literature are initially applied to the case by means of mass and momentum conservation laws. The predictions of both the models on the pressure recovery were compared with the experimental and the numerical data in the literature and a bad agreement was observed between them; therefore, a new original analytical model is proposed by the present study. The new model is called as the slip flow model, which takes into account the slip velocity between gas and solid phases evaluated by coupling the well-known separated flow model with the empirical slip ratio predictions in the literature. The predictions of the proposed slip flow model on both the pressure recovery and minor loss coefficient are found in good agreement with the corresponding data in the literature.     

A 3D Computational Model of RC Beam Using Lower Order Elements with Enhanced Strain Approach in the Elastic Range

A procedure has been described to carry out three-dimensional elastic analysis of reinforced concrete beam employing finite element technique, which uses lower order elements. The proposed procedure utilizes 8-noded isometric solid /hexahedral elements HCiS18 with enhanced assumed strain (EAS) formulation, recently developed in the literature, to predict load-deformation and internal stresses produced in case of a simply supported RC beams in the elastic regime. It models the composite behaviour of concrete and reinforcements in rigid /perfect bond situation and their mutual interaction in bond-slip condition considering continuous interface elements at the material level. Although, bond-slip relation are very much non-linear in behaviour even at the beginning of the loading condition, predictions from the proposed model /procedure are found to be very close to the experimental observations as far as accuracy is concerned in the elastic range. The sole purpose of this paper is to demonstrate the general applicability and to explore the potentiality of using lower order solid elements in the 3D finite element analysis with an aim of developing a general analytical method for the study of reinforced concrete beam in the elastic range.     

A practical method for uniaxial tension test of concrete

A test method for uniaxial tension test of concrete to obtain tension softening curves has not been established yet, because there are several difficulties in performing the test. In the past 40 years, many different test conditions, sometimes even completely opposite conditions have been adopted. In order to show which conditions should be selected and why the conditions should be selected, theoretical and experimental investigation was carried out. For the experiment, prismatic specimens with notches and an originally designed gear system preventing secondary flexure were adopted. It was shown that the tension softening curves were successfully monitored by the present procedure with a high success ratio. A practical test procedure of uniaxial tension test of concrete was proposed based on the investigation.