混凝土结构的耐久性外文翻译.doc

蜂窝式无线通讯系统自动控制方法来分析在恶劣环下混凝土结构的耐久性摘要：这篇文章描述了一种解决在外部荷载作用下混凝土结构耐久性分析和寿命评估问题的新颖的方法。这个被提到的假说主要用于梁和框架，但是它也很容易扩展到其它结构类型。通过使用蜂窝式无线通讯系统自动控制来模仿这个散乱的过程。通过采用合适的材料降解法来评价散乱的机械损伤。由于质量扩散的速度通常取决于应力状态，已坏结构的扩散过程和力学特性也通过建立一个合适的质量传递中的随机效应模型来考虑。为了这个目的，在这段时间的非线性结构分析在 有限元框架 中 通过一个不断恶化的钢筋混凝土梁单元的方法来完成。在处理复杂的几何和力学边界条件方面，所提到的一套方法的效果被证明是有用的。首先，钢筋混凝土箱形梁横截面被考虑，所造成的破坏性进程通过相应的弯矩曲率图和轴力弯矩抵抗域来描述。其次，钢筋混凝土连续t - 梁的耐久性分析被发展了。最后，所提到的方法应用于已建拱桥的分析和它的重要构件的鉴定。 令人满意的结构特性通常被描述成参照特定的把结构的理想状态与不理想状态分开的极限状态。在这方面，结构设计的主要目的是在结构的整个使用寿命过程中，对于每个指定的极限状态保证有足够的结构性能水平。一般来说，作用效应s小于或等于结构抗力r时，结构是安全的。然而，对于混凝土结构，结构性能必须被认为是不定常的，主要是因为 材料力学性能的逐步恶化，这使结构系统不足以承担施加的荷载。因此，所需的作用效应s和结构抗力r可能随时间而变，并且导致实际寿命的可靠评估的结构耐久性分析 ta 应该能够 需要这种 变异。如此，设计者能够解决概念设计过程或者设计结构修复以使结构寿命达到规定的设计值。接下来，注意力应该主要集中在破坏过程，包括环境侵略性攻击扩散剂，例如能够导致混凝土恶化和钢筋腐蚀的硫酸盐和氯化物。这种过程包括一些因素，例如温度和湿度。它的动态受是由热度，湿度和各种化学物质组成。另外，破坏包括由机械载荷与环境因素的相互作用，加速恶了化过程。基于先前的考虑，在恶劣的环境下混凝土结构的耐久性分析应该能够包括扩散过程和相应的机械损伤，以及在扩散、破坏、结构状态之间的耦合效应。然而，关于环境因素和材料特性的可用信息通常是非常有限的，并且在详细和复杂的模型中不可避免的不确定性可能导致虚构的结果。基于这些原因，结构寿命的评估可以通过宏观模型来进行而变得更可靠，模型是用来开发扩散基本规律的影响力和通用性而用于定量预测损坏结构体系的时变反应。这篇文章描述了一个在环境侵袭下混凝土结构耐久性分析的新颖方法。 这个被提到的假说主要用于梁和框架，但是它也很容易扩展到其它结构类型。扩散过程的分析通过使用一类被称作细胞自动机的特殊进化算法来进行，这种方法把实际系统数学理想化，在这种方法中，空间和时间是彼此分离的，物质的量来源于一个有限集分离的价值。原则上，任何满足不同平衡的物理系统通过引入离散坐标系和变量，以及离散的时间步骤可近似为一个细胞自动机。然而，值得指出的是基于细胞自动机的模型提供了一个物理模型而不是一个近似可供选择的方法。值得注意的是， 在混凝土中，典型物理过程的元胞自动机模型的例子在水泥复合材料领域可以发现。事实上，它们表述了一个复杂的性能，类似于与微分方程相关联，但是由于它们简单的公式化的表述更有潜力适用于更复杂，更完整的系统，提供给整个系统一些突发的性质，只有通过它的本身动态自我包括。基于这个演化模型，耦合扩散的机械损伤 通过引入混凝土和钢筋有效抵抗区的降级理论，依据合适的损伤指数来评估的。由于扩散的比率通常取决于应力状态，损坏结构的扩散过程和机械性能之间的相互作用通常也通过一个合适的质量传递随机效应的模型来考虑。为了这个目的，在这段时间的非线性结构分析在有限元框架中通过一个不断恶化的钢筋混凝土梁单元的方法来完成。在处理复杂的几何和力学边界条件方面，所提到的一套方法的效果被证明是有用的。首先，钢筋混凝土箱形梁横截面被考虑，所造成的破坏性进程通过相应的弯矩曲率图和轴力弯矩抵抗域来描述。其次，钢筋混凝土连续t - 梁的耐久性分析被发展了。最后，所提到的方法应用于已建拱桥的分析和它的重要构件的鉴定。扩散过程模型扩散过程和细胞自动机固体中化学成分扩散的动力学过程通常通过把大规模扩散率与造成网状系统质量传递原因的浓度梯度联系起来的数学关系来表述(glicksman 2000)。 最简单的模型是由fick第一定律来描述的，这个定律假定质量转移与扩散梯度之间是线性关系。fick的模型与质量守恒定律的结合产生了fick第二定律，这个定律在各向同性介质中单个组合的情况下可以写成一下形式：其中：c=c（x, t）=该组件的质量浓度d=（x, t）=扩散系数x=(x, y , z)时间 tc=grad c. 导致这个简单模型修改的复杂性可能产生于各向异性，多组分扩散，化学反应，外部的应力场，内存和随机效应。例如，在混凝土结构而言，扩散系数取决于几个参数，如相对湿度，温度和机械应力,ficks方程,必须与热和水分的流动方程,以及力学问题构成原理相结合。然而，像所提到的，由于这些模型校准的不确定性，用宏观的方法评估结构寿命更容易进行，它利用ficks定律的力量和通用性预测进经受扩散的系统的定量反应。尤其，如果扩散系数d被假定为一个常数，二阶非线性偏微分方程（1）被简化成以下线性形式，其中，尽管方程是线性的，但是这种方程的解析解只存在于一些有限的简单经典问题中。因此，处理复杂几何和力学边界条件的一般方法通常需要使用数值方法。在这项研究中，通过使用一类特殊的进化算法称为细胞自动机的方法有效地解决了扩散方程。细胞自动机首次是在19481950被neumann and ulam首次引进的，接下来其他的研究人员在许多科学领域应用它。起初，涉及到对图灵机的自我复制问题的研究，细胞自动机在20世纪70年代离开实验室，并在学术界随着conway发明的著名游戏流行 。基本上，他们代表了简单的物质系统数学理想化，在这一系统，空间和时间是不分离的，物理量取自一个离散值的有限集合。事实上，如前所述， 通过引入离散坐标和变量，以及离散时间的步骤，任何满足微分方程的物理系统可近似作为细胞自动机。因此，合理地说，基于细胞自动机的模型提供了一个可供选择和更广泛的物理模型而不是近似值的方法 。他们表现出复杂的行为，类似于复杂的微分方程，但是，在这种情况下，根据简单的规则从简单的实体之间的相互作用出现8cellular automata approach to durability analysis of concrete structures in aggressive environmentsabstract: this paper presents a novel approach to the problem of durability analysis and lifetime assessment of concrete structures（under the diffusive attack from external aggressive agents.the proposed formulation mainly refers to beams and frames, but it can be easily extended also to other types of structures. the diffusion process is modeled by using cellular automata. the mechanical damage coupled to diffusion is evaluated by introducing suitable material degradation laws. since the rate of mass diffusion usually depends on the stress state, the interaction between the diffusion process and the mechanical behavior of the damaged structure is also taken into account by a proper modeling of the stochastic effects in the mass transfer to this aim , the nonlinear structural analyses during time are performed within the framework of the finite element method by means of a deteriorating reinforced concrete beam element . the effectiveness of the proposed methodology in handling complex geometrical and mechanical boundary conditions is demonstrated through some applications. firstly , a reinforced concrete box girder cross section is considered and the damaging process is described by the corresponding evolution of both bending moment-curvature diagrams and axial force-bending moment resistance domains . secondly, the durability analysis of a reinforced concrete continuous t - beam is developed. finally, the proposed approach is applied to the analysis of an existing arch bridge and to the identification of its critical members. introduction satisfactory structural performance is usually described with reference to a specified set of limit states, which separate desired states of the structure from the undesired ones. in this context, the main objective of the structural design is to assure an adequate level of structural performance for each specified limit state during the whole service life of the structure. from a general point of view, a structure is safe when the effects of the applied actions s are no larger than the corresponding resistance r. however , for concrete structures the structural performance must be considered as time dependent , mainly because of the progressive deterioration of the mechanical properties of materials which makes the structural system less able to withstand the applied actions . as a consequence, both the demand s and the resistance r may vary during time and a durability analysis leading to a reliable assessment of the actual structural lifetime ta should be able to account for such variability (sa1ja and vesikari 1996; enright and frangopol 1998a, 1998b). in this way, the designer can address the conceptual design process or plan the rehabilitation of the structure in order to achieve a prescribed design value td of the structural lifetime.in the following , the attention will be mainly focused on the damaging process induced by the diffusive attack of environmental aggressive agents , like sulfate and chloride , which may lead to deterioration of concrete and corrosion of reinforcement ( ceb 1992 ) . such process involves several factors, including temperature and humidity . its dynamics is governed by coupled diffusion process of heat, moisture, and various chemical substances. in addition, damage induced by mechanical loading interacts with the environmental factors and accelerates the deterioration process( saetta et al. 1993 , xi and bazant 1999 ; xi et al . 2000 ; kong et al . 2002 ) . based on the previous considerations, a durability analysis of concrete structures in aggressive environments should be capable to account for both the diffusion process and the corresponding mechanical damage, as well as for the coupling effects between diffusion, damage and structural behavior. however, the available information about environmental factors and material characteristics is often very limited and the unavoidable uncertainties involved in a detailed and complex modeling may lead to fictitious results. for these reasons , the assessment of the structural lifetime can be more reliably carried out by means of macroscopic models which exploit the power and generality of the basic laws of diffusion to predict the quantitative time-variant response of damaged structural systems . this paper presents a novel approach to the durability analysis of concrete structures under the environmental attack of aggressive agentsthe proposed formulation mainly refers to beams and frames, but it can be easily extended also to other types of structures. the analysis of the diffusion process is developed by using a special class of evolutionary algorithms called cellular automata, which are mathematical idealizations of physical systems in which space and time are discrete and physical quantities are taken from a finite set of discrete values.in principle, any physical system satisfying differential equations may be approximated as a cellular automaton by introducing discrete coordinates and variables, as well as discrete time steps.however, it is worth noting that models based on cellular automata provide an alternative approach to physical modeling rather than an approximation.in fact, they show a complex behavior analogous to that associated with differential equations, but by virtue of their simple formulation are potentially adaptable to a more detailed and complete analysis, giving to the whole system some emergent properties, self-induced only by its local dynamics (von neumann 1966; margolus and toffoli 1987; wolfram 1994, 2002; adami1998).noteworthy examples of cellular automata modeling of typical physical processes in concrete can be found in the eld of cement composites (bentz and garboczi 1992; bentz et al. 1992,1994). based on such an evolutionary model, the mechanical damage coupled to diffusion is then evaluated by introducing a degradation law of the effective resistant area of both the concrete matrix and steel bars in terms of suitable damage indices.since the rate of mass diffusion usually depends on the stress state, the interaction between the diffusion process and the mechanical behavior of the damaged structure is also taken into account by a proper modeling of the stochastic effects in the mass transfer.to this aim, the nonlinear structural analyses during time are performed within the framework of the finite element method by means of a deteriorating reinforced concrete beam element (bontempi et al. 1995;malerba 1998; biondini 2000). the effectiveness of the proposed methodology in handlingcomplex geometrical and mechanical boundary conditions isdemonstrated through some applications. firstly, a reinforcedconcrete box girder cross-section is considered and the damaging process is described by the corresponding evolution of both bending momentcurvature diagrams and axial force-bending moment resistance domains. secondly, the durability analysis of a rein-forced concrete continuous t-beam is developed. finally, the proposed approach is applied to the analysis of an existing arch bridge and to the identification of its critical members.diffusion processes and cellular automatamodeling of diffusion processesthe kinetic process of diffusion of chemical components in solids is usually described by mathematical relationships that relate the rate of mass diffusion to the concentration gradients responsible for the net mass transfer (glicksman 2000). the simplest model is represented by the ficks first law, which assumes a linear relationship between the mass ux and the diffusion gradient. the combination of the ficks model with the mass conservation principle leads to ficks second law which, in the case of a single component diffusion in isotropic media, can be written as follows:where c=c（x, t）=mass concentration of the component and d=（x, t）=diffusivity coefficient, both evaluated at pointx=(x, y , z) and time t, and where c=grad c. complexities leading to modifications of this simple model may arise from anisotropy, multicomponents diffusion, chemical reactions, external stress fields, memory and stochastic effects. in the case of concrete structures, for example, the diffusivity coefficient depends on several parameters, such as relative humidity,temperature, and mechanical stress, and the ficks equations must be coupled with the governing equations of both heat and moisture flows, as well as with the constitutive laws of the mechanical problem (ceb 1992; saetta et al. 1993; xi and baant 1999; xi etal. 2000). however, as mentioned, due to the uncertainties involved in the calibration of such complex models, the structural lifetime can be more conveniently assessed by using a macroscopic approach which exploits the power and generality of the basic ficks laws to predict the quantitative response of systems undergoing diffusion. in particular, if the diffusivity coefficient d is assumed to be a constant, the second order partial differential nonlinear eq. (1) is simplied in the following linear form:where despite of its linearity, analytical solutions of such an equation exist only for a limited number of simple classical problems. thus, a general approach dealing with complex geometrical and mechanical boundary conditions usually requires the use of numerical methods. in this study, the diffusion equation is effectively solved by using a special class of evolutionary algorithms called cellular automata.short history, formal definition, and emergingproperties of cellular automatacellular automata were firstly introduced by von neumann and ulam in 19481950 (von neumann 1966) and subsequently developed by other researchers in many fields of science (see for reviews: margolus and toffoli 1987; adami 1998; wolfram 2002).originally related to the study of self-replication problems on the turings machine, cellular automata left laboratories in the 1970s and became popular in the academic circles with the now famous game of life invented by conway (gardner 1970). basically, they represent simple mathematical idealizations of physical systems in which space and time are discrete, and physical quantities are taken from a finite set