钢筋混凝土楼板火灾反应数值计算模型

基于退化壳原理,考虑火灾下板壳截面的不均匀温度场分布而引入分层模型,同时在每分层上考虑材料在不同温度下的热力弹塑性本构关系,建立了火灾下钢筋混凝土板壳结构的有限元数值计算模型。另外,通过全拉格朗日方法考虑了大位移的几何非线性影响。最后通过一钢筋混凝土板在高温下的试验进行了验证,并分析了配筋率和保护层厚度的影响。结果表明:提出的火灾下钢筋混凝土壳单元数值计算模型的计算结果与试验结果吻合较好,可以用来分析火灾下钢筋混凝土框架结构楼板的反应。     

Effects of thermal creep of prestressed steel on post‐tensioned concrete slabs in and after fire

The effects of thermal creep of prestressed steel on post‐tensioned concrete slabs in and after fire were investigated based on an existing thermal creep model and calibrated parameters in this paper. A nonlinear finite element model was built up employing ABAQUS package, taking into account frictionless contact behaviour between prestressed steel tendons and surrounding concrete. The nonlinear material behaviour of concrete and prestressed steel at elevated temperatures was taken into account, where three material models for prestressed steel were adopted with or without considering thermal creep, and based on the model from EN 1992‐1‐2. The finite element model developed was verified against experimental results from the literature, showing that the model considering thermal creep was more accurate. Then the fire resistance period and responses of post‐tensioned concrete slabs in and after fire were investigated based on the verified model. Ignoring thermal creep underestimated the fire resistance period but overestimated the residual tendon stresses. The model from EN 1992‐1‐2 achieved nearly the same effects as the model considering thermal creep in fire but might yield inaccurate evaluation of residual tendon stresses. The model considering thermal creep worked well under fire and in the post‐fire conditions yielding reasonable predictions.     

钢筋混凝土轴心受压柱的抗火设计方法研究

该文以四面受火钢筋混凝土轴心受压柱为研究对象,在利用自编钢筋混凝土温度场有限元分析程序进行大量数值分析计算的基础上,通过引入材料平均强度折减系数的概念,建立了截面材料性能随受火时间变化的规律,从而提出了一种类似常温下钢筋混凝土轴心受压柱设计计算的抗火设计新方法,并编制了相应的分析程序。这种设计方法不直接依赖于具体的温度场分析,仅取决于受火时间的长短,为设计人员的抗火设计工作提供了很大便利。最后,利用大量试验数据与该文提出的方法进行了对比研究。结果表明:该文提出的钢筋混凝土轴心受压柱抗火设计方法是安全而有效的。     

Internal relative humidity and drying stress gradients in concrete

A moisture gradient develops as concrete dries, inducing a drying shrinkage stress gradient that can lead to early-age cracking in restrained concrete. A new internal relative humidity measurement system was used to quantify the moisture gradient in early-age concrete exposed to drying. A simple model was developed to estimate the stress gradient associated with drying in both free and fully restrained concrete specimens. The model predicts that the stresses in the surface layer of restrained concrete exceed the tensile strength of the material prior to complete specimen failure. Comparison of the model and experimental results indicate that the fully restrained specimens with the most severe drying stress gradients failed at the earliest ages.     

Comprehensive study of concrete creep,shrinkage, and water content evolution under sealed and drying conditions

A comprehensive study of the time‐dependent behaviour of concrete, enhanced by measuring the evolution of the respective mass water content, is presented. Compressive creep as well as shrinkage are investigated on sealed and unsealed specimens in terms of the concrete age at loading of 2, 7, and 28 days, respectively, at a loading level of 30% of the compressive strength at loading. In addition, on the basis of the collected measurement data, the impact of load application on the moisture content is studied and the Pickett effect is re‐examined. The obtained set of material data consists of basic and drying creep strains for different concrete ages at loading, autogenous shrinkage strains as well as combined autogenous and drying shrinkage strains, the evolution of the mass water content, the water desorption isotherm, and the time‐dependent evolution of Young's modulus and the compressive strength. It provides a consistent set of material data for a particular concrete grade and will be available for calibrating and validating concrete models.     

Drying of saturated lightweight concrete: an experimental investigation

The changes of moisture content during drying are experimentally investigated in the present work. Particular emphasis is placed on the initial stage of drying of saturated concrete, when moisture contents are high, since the resistance of the material to several deterioration processes is reduced at high moisture content levels, and experimental data for this stage of drying is lacking. The experimental investigation is performed for concrete cylinders of different lengths with one end exposed to drying. In this way, moisture flow is forced to be unidirectional. The gravimetric method is employed to obtain moisture content distribution in the material at different times of drying. The cylinders are made of lightweight concrete with varying water-to-cement ratios and moist curing times, and the influence of these variables upon the drying process is assessed. Higher initial water content and more rapid changes of water content occur in lightweight concrete with a higher w/c ratio. An increased moist-curing period results in a decrease of drying rates throughout the drying process.     

A viscoelastic approach for the assessment of the drying shrinkage behaviour of cementitious materials

A new model for the drying shrinkage of concrete is presented. In this model, drying shrinkage strains are regarded as being a spherical elastic and creep response of the material under rising pore pressures during the drying process. Therefore, a basic creep model which allows to incorporate these pore pressures is developed on the basis of microscopic considerations of the role of water in the creep mechanism. Then, the model response is compared to experimental results performed on a cement paste specimen subjected to drying. The developed model is able for describing the main features of the shrinkage behaviour of cement based materials.     

Experimental Investigation of Strain Behaviour of Heated Cement Paste and Concrete

The material degradation of concrete subjected to fire events has a severe influence on the load‐carrying capacity of support structures. Spalling of concrete layers, exposing the reinforcement bars and degradation of the material properties (Young's modulus, compressive strength) may lead to significant damage of the reduced cross‐section and, therefore, cause failure of the structure. In order to understand the stress build‐up at the heated surface caused by thermal expansion due to fire loading, finally leading to damage and spalling of concrete, the strain behaviour of cement paste and concrete exposed to combined thermo‐mechanical loading is the focus of this work. Hereby, the evolution of thermal strains, Young's modulus and Poisson's ratio with increasing temperature are investigated experimentally. For this purpose, the specimens are loaded uniaxially while the temperature is increased up to 800 °C. The obtained results provide the proper basis for the development of realistic material models, allowing more sophisticated simulations of structures exposed to fire.     

Thermal-stress analysis of RC beams reinforced with GFRP bars

This paper aims to develop a 3D nonlinear finite element (FE) model that is capable of accurately predicting the performance of reinforced concrete (RC) beams reinforced with internal Glass Fiber-Reinforced Polymer (GFRP) bars when exposed to fire loading. The developed FE model is based on tested experimental data collected from the open literature. The model accounts for the variation in the thermal and mechanical constituent materials with temperature associated with the RC beam. To study the heat transfer mechanism and mechanical behavior of the RC beam, transient thermal-stress finite element analysis is performed using the ANSYS. It was shown that the FE predicted temperature and mid-span deflection results are in a good agreement with that of the measured experimental data. The validated FE model is used to conduct a parametric study to investigate the effect of the different parameters on the flexural performance of the reinforced beam specimens. The parametric study consisted of varying the concrete cover thickness as well as exposing the FE model to different fire curves. It is concluded that successful FE modeling of this structure would provide an economical and alternative solution to expensive and time consuming experimental testing. Other observations and recommendations are also discussed.     

Nonlinear analysis of concrete shells including effects of normal and transverse shear stresses

The previously developed numerical model of the authors for the analysis of conventional reinforced and prestressed concrete shells under short‐term and long‐term loading was improved by including the effects of transverse shear stresses on the shell failure. The 9‐node degenerated shell element with the layered material model through the thickness of the shell was used. The reinforcement was modelled as a separate layer. To include the effect of transverse shear stresses on the shell failure, the failure criterion for concrete and longitudinal reinforcement was defined by a relation of transverse shear stresses and normal stresses in two mutually perpendicular vertical planes. The total transverse shear bearing capacity of the shell cross‐section is obtained by summing up the concrete and reinforcement contributions. The developed numerical model and appropriate software were verified based on experimental tests.     

Parametric Study of Hydroforming of Paper Materials Using the Explicit Finite Element Method with a Moisture‐dependent and Temperature‐dependent Constitutive Model

A moisture‐dependent and temperature‐dependent constitutive model for paper materials was proposed and implemented into a finite element model of the paper hydroforming process. Experimental hydroforming was conducted at temperatures of 23°C and 110 °C and moisture contents of 6.9 and 10.6 (respectively corresponding to 50 and 80% relative humidity). The proposed model, which also included the effects of drying, captured the extent of forming of all experimental results within reasonable accuracy. For the moisture content and temperature conditions in this study, the phenomenon of drying was found to be the reason why the application of temperature had a much greater effect on the degree of forming than hydroforming at various moisture contents. A simulation‐based parametric study was conducted in order to identify the importance of various process and material parameters. This parametric study confirmed many previous empirical findings and was capable of quantifying the extent to which these process and material parameters affect the three‐dimensional formability of paper. The coefficient of friction was identified as one of the most important factors when determining the extent of forming. Copyright © 2016 John Wiley & Sons, Ltd.     

Thermal dilation and internal relative humidity of hardened cement paste

The coefficient of thermal dilation (CTD) of hardened cement paste and concrete is a function of the state of internal moisture in the pore system. It has been theorized that changes in the pore fluid pressure induced by temperature change causes additional dilation when the material is partially saturated. Drying shrinkage stresses in early-age concrete also evolve from changes in the pore fluid pressure. The Kelvin-Laplace equation relates changes in the pore fluid pressure to the measured internal relative humidity (RH). This research investigated the role of pore pressure changes on the CTD through internal RH measurements. A maximum change in humidity (ΔRH) due to temperature change (ΔT) was measured when the initial humidity was at an intermediate value. Likewise, the maximum CTD was also measured at an intermediate initial RH. Based on these findings, the additional thermal strain caused by changes in pore fluid pressure was modelled using internal RH measurements as a primary parameter.     

Bond characteristics of CFRP plated concrete members under elevated temperatures

Fire related structural degradation is a challenge to the safe design of structural members. Carbon fibre reinforced polymer (CFRP) strengthened concrete members show degradation of bond properties with temperature. Therefore, a detailed investigation of heat effects on composites is required before further application of this system can be effective. This paper focuses on a 3D model developed to predict the behaviour of CFRP–concrete composites under fire. Heat transfer analysis was employed to predict the temperature within the adhesive layer of epoxy, which is the most critical part of the member at high temperature. The model showed that the epoxy reached the failure point within a short time under standard fire. The model was also used to predict the required insulation thickness requirement for two-hour and three-hour fire resistance levels. The effects of rate of temperature increase on bond strength of composite structures are demonstrated through numerical analysis. The model results were validated with experimental data.     

考虑水浸温度影响的复合材料吸湿动力学模型EI北大核心CSCD

研究了在恒温水浸吸湿实验中,水浸温度对复合材料吸湿参数的影响。通过对国产碳纤维/双马复合材料在60,70,80℃恒温水浸中进行的吸湿实验,得到了不同水浸温度下的吸湿曲线。由吸湿曲线分别求出了各水浸温度下的扩散系数和平衡吸湿率以及它们与水浸温度的关系。结合Arrhenius关系和Fick定律,得到了反映此复合材料在任意水浸温度下吸湿行为的吸湿模型。该吸湿模型能较为准确地预测此复合材料在95℃恒温水浸中任意时刻的吸湿量及预估达到特定吸湿量所需要的时间。     

Stress measurements on chrome-tanned leather

A sensitive method has been developed for the measurement of mechanical stresses in compliant planar samples originating from changes of their solvent content or temperature. The samples are mechanically conjoined with a silicon wafer. Thus, an alteration of the sample stress causes a spatial displacement of the wafer which is detected capacitively. The method is applied to the measurement of stresses in both acetone-dried and air-dried chrome-tanned leather which has been exposed to a stepwise modulation of ambient humidity. The stress development in the sample can be well characterized by a first-order exponential decay. The mean moisture content exhibits a similar time dependence. However, the relation between stress development and moisture content differs for swelling and shrinkage. This behavior is explained on the basis of a novel two-capillary model. Moreover, the response of the sample to the alteration of ambient humidity is found to be related to the structure of its collagen fiber network. The tight structure of air-dried leather with a poor isolation of fibrils yields much higher stresses than acetone-dried leather.     

早龄期混凝土湿度应力计算与开裂风险评估

该文以现浇混凝土圆柱为例,建立了自收缩与干燥收缩统一的早龄期混凝土收缩应力解析计算方法,并采用松弛系数法对混凝土徐变的影响作了修正,同时对混凝土早期开裂风险进行了分析。应用所建模型,对不同环境湿度条件、不同尺寸混凝土圆柱的湿度应力进行了计算与分析。结果表明:混凝土内部湿度随时间的变化幅度是控制湿度应力的主要因素,混凝土内部湿度与环境湿度差值越小,干缩应力越小;徐变参数的取值对混凝土湿度应力的计算影响较大;构件尺寸对干缩应力也有一定的影响,柱半径越大,相同条件下其外表面拉应力越大,越容易开裂。     

钢筋混凝土双向板火灾行为的数值分析

根据傅里叶热传导理论,提出蒸发阶段水分修正函数,建立改进温度场模型;结合非线性板壳理论和热弹塑性本构关系,建立了火灾下钢筋混凝土双向板的数值模型,发展现有计算程序,通过计算结果与试验结果对比,验证了模型及程序的可靠性。在此基础上,分析了火灾工况、约束作用、骨料类型、板厚和保护层厚度等参数对混凝土双向板变形行为和耐火极限的影响规律。结果表明:火灾工况、骨料类型、板厚和保护层厚度对双向板火灾行为影响较大;双向板变形行为和破坏模式对约束作用非常敏感,力学机理较为复杂。     

Effect of temperature and humidity on transient hygrothermal stresses during moisture desorption in laminated composite plates

Polymer matrix composites are relevant materials for future supersonic aircraft due to their high specific properties. However, in such aeronautical applications, the material is exposed to severe environmental conditions. The present paper aims at assessing an approximate model to evaluate hygrothermoelastic stress in composite laminated plates during moisture desorption taking into account the change of mechanical characteristics induced by the variation of temperature and moisture. The developed method permits us to calculate such stresses during desorption phase without the computation of the moisture concentration, through laminated plates. It observed through this study that the variation of elasticity modulus due to the temperature causes a stress relaxation. These stresses have to be taken into account for the design of composite structures submitted to a moist environment.

Through the presented study, we hope to contribute to the understanding of hygrothermal behaviour of composite laminated plates.     

Determination of moisture distributions in porous building materials by neutron transmission analysis

The movement of moisture inside building structures can affect them in important ways, causing physical and chemical damage. Therefore the study of moisture transport in porous building materials is highly relevant to a better understanding of the durability of building structures made of porous materials. The moisture transport can be described phenomenologically by a diffusion equation using a moisture content dependent moisture transfer coefficient. To determine the transfer coefficient in a given material, the experimental quantitative measurement of the time and space dependent moisture distribution in the material is necessary. Neutron radiography provides a highly sensitive non-destructive method for the detection of the presence of water and provides high spatial resolution. A new neutron transmission analysis technique has been developed and optimized for the study of moisture in building materials to extract the quantitative information from the experimental data. Typically, moisture contents down to a few mg/cm³ can be detected at a spatial resolution of 1 mm. As an application example, the determination of the time and space dependent moisture distribution in a brick sample and the subsequent determination of the moisture transfer coefficient are presented.