混凝土碳化深度数值模型的分析研究

发布时间：2019-06-01 18:23

【摘要】：混凝土碳化现象普遍存在于混凝土结构工程，诱发混凝土结构中的钢筋锈蚀膨胀，导致混凝土保护层开裂甚至剥落，是一项影响混凝土结构耐久性的重要因素。碳化深度是判断混凝土碳化程度的量性指标。混凝土碳化过程中受物理化学共同作用，通过二氧化碳气体的物理扩散，在混凝土孔隙液相中与混凝土本身的碱性物发生化学反应。目前，国内外学者从不同角度对混凝土碳化机理进行分析，提出了相应的碳化深度预测模型，所得结论基本一致，混凝土的碳化深度与碳化时间的平方根成正比。碳化过程的扩散系数可以通过理论推导或对实测数据的拟合获得。在数值模型中，如采用理论模型的扩散系数，以扩散系数值为常量，进行模拟，造成预测结果偏差较大；而采取经验模型中由拟合而获得的系数，缺乏相应理论的支持。因此，有必要探寻建立数值模型的理论依据，，以及适应于其计算碳化过程扩散系数。本文总结分析国内外混凝土碳化预测模型的理论推导过程。通过对混凝土碳化机理的分析，采用菲克第二定律为描述碳化过程数值模型的控制方程。并针对普通混凝土表层采取分区分层法来确定扩散系数，即将混凝土分为水泥硬化浆体与骨料两部分，沿深度方向从不同层面确定扩散系数。为解决二氧化碳在混凝土中的扩散问题提供了一种新方法。主要内容如下：（1）通过分析已有的碳化深度预测模型的建立过程，拟定本文的数值碳化模型的控制方程。由于混凝土碳化是物理化学共同作用，但就发生速率而言，化学反应远远高于物理扩散，因此可认为其控制方程为仅考虑物理扩散作用，拟定以二氧化碳的扩散深度表示整体的碳化深度。由于气体在混凝土孔结构中，多以菲克扩散形式为主，因此应用菲克第二扩散定律，拟定数值模型的控制方程。（2）为实现其数值模型的建立，以普通混凝土为例，拟定普通混凝土碳化模型中的扩散系数。在能反映普通混凝土扩散性的四个指标中（水灰比、水泥用量、砂率以及抗压强度），运用数理统计中通径分析方法找到水灰比为其主要影响因素。（3）以水灰比为主要因素，考虑骨料在混凝土中的分布特点，提出了分区分层法拟定扩散系数，即将混凝土分为水泥硬化浆体与骨料两部分，沿深度方向从不同层面确定扩散系数。（4）结合试验，对所建立的普通混凝土数值模型进行验证。由于分析模式同热传导瞬态问题相似，故采用有限元软件ANSYS进行模拟。模拟数据与试验数据具有一定联系，一定程度上证实本文的数值模型的可行性。
[Abstract]:The carbonation of concrete generally exists in concrete structure engineering, which leads to the corrosion and expansion of steel bar in concrete structure, which leads to the cracking and even spalling of concrete protective layer, which is an important factor affecting the durability of concrete structure. Carbonation depth is a quantitative index to judge the carbonation degree of concrete. In the carbonation process of concrete, through the physical diffusion of carbon dioxide gas, the chemical reaction with the alkaline substance of concrete itself takes place in the pore liquid phase of concrete. At present, scholars at home and abroad analyze the carbonation mechanism of concrete from different angles, and put forward the corresponding carbonation depth prediction model. The conclusions are basically the same. The carbonation depth of concrete is proportional to the square root of carbonation time. The diffusion coefficient of carbonation process can be obtained by theoretical derivation or fitting of measured data. In the numerical model, if the diffusion coefficient of the theoretical model is used and the diffusion coefficient value is taken as the constant, the deviation of the prediction results is large, while the coefficient obtained by fitting in the empirical model is not supported by the corresponding theory. Therefore, it is necessary to explore the theoretical basis of establishing numerical model and to calculate the diffusion coefficient of carbonation process. In this paper, the theoretical derivation process of concrete carbonation prediction model at home and abroad is summarized and analyzed. Based on the analysis of carbonation mechanism of concrete, Fick's second law is used as the governing equation to describe the numerical model of carbonation process. The diffusion coefficient is determined by zonal delamination method for the surface layer of ordinary concrete, that is, the concrete is divided into two parts: cement hardened paste and aggregate, and the diffusion coefficient is determined from different levels along the depth direction. It provides a new method to solve the problem of carbon dioxide diffusion in concrete. The main contents are as follows: (1) by analyzing the establishment process of the existing carbonation depth prediction model, the governing equation of the numerical carbonation model in this paper is worked out. Because concrete carbonation is a physical-chemical interaction, but as far as the occurrence rate is concerned, the chemical reaction is much higher than the physical diffusion, so it can be considered that the governing equation is to consider only the physical diffusion. The overall carbonation depth is expressed by the diffusion depth of carbon dioxide. Because the gas is mainly in the form of Fick diffusion in the concrete pore structure, the governing equation of the numerical model is drawn up by using the Fick's second diffusion law. (2) in order to establish the numerical model, the diffusion coefficient in the carbonation model of ordinary concrete is worked out by taking ordinary concrete as an example. Among the four indexes that can reflect the diffusivity of ordinary concrete (water-cement ratio, cement dosage, sand ratio and compressive strength), the path analysis method in mathematical statistics is used to find that the water-cement ratio is the main influencing factor. (3) taking the water-cement ratio as the main factor and considering the distribution characteristics of aggregate in concrete, a zonal delamination method is proposed to draw up the diffusion coefficient, that is, the concrete is divided into two parts: cement hardened paste and aggregate. The diffusion coefficient is determined from different levels along the depth direction. (4) combined with the test, the numerical model of ordinary concrete is verified. Because the analysis mode is similar to the transient problem of heat conduction, the finite element software ANSYS is used to simulate the problem. The simulation data are related to the experimental data, which proves the feasibility of the numerical model in this paper to a certain extent.
【学位授予单位】：西北农林科技大学
【学位级别】：硕士
【学位授予年份】：2014
【分类号】：TU528

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