混凝土碳化深度數(shù)值模型的分析研究
發(fā)布時間：2019-06-01 18:23
【摘要】：混凝土碳化現(xiàn)象普遍存在于混凝土結(jié)構(gòu)工程，誘發(fā)混凝土結(jié)構(gòu)中的鋼筋銹蝕膨脹，導(dǎo)致混凝土保護層開裂甚至剝落，是一項影響混凝土結(jié)構(gòu)耐久性的重要因素。碳化深度是判斷混凝土碳化程度的量性指標。 混凝土碳化過程中受物理化學(xué)共同作用，通過二氧化碳氣體的物理擴散，在混凝土孔隙液相中與混凝土本身的堿性物發(fā)生化學(xué)反應(yīng)。目前，國內(nèi)外學(xué)者從不同角度對混凝土碳化機理進行分析，提出了相應(yīng)的碳化深度預(yù)測模型，所得結(jié)論基本一致，混凝土的碳化深度與碳化時間的平方根成正比。碳化過程的擴散系數(shù)可以通過理論推導(dǎo)或?qū)崪y數(shù)據(jù)的擬合獲得。在數(shù)值模型中，如采用理論模型的擴散系數(shù)，以擴散系數(shù)值為常量，進行模擬，造成預(yù)測結(jié)果偏差較大；而采取經(jīng)驗?zāi)Ｐ椭杏蓴M合而獲得的系數(shù)，缺乏相應(yīng)理論的支持。因此，有必要探尋建立數(shù)值模型的理論依據(jù)，，以及適應(yīng)于其計算碳化過程擴散系數(shù)。 本文總結(jié)分析國內(nèi)外混凝土碳化預(yù)測模型的理論推導(dǎo)過程。通過對混凝土碳化機理的分析，采用菲克第二定律為描述碳化過程數(shù)值模型的控制方程。并針對普通混凝土表層采取分區(qū)分層法來確定擴散系數(shù)，即將混凝土分為水泥硬化漿體與骨料兩部分，沿深度方向從不同層面確定擴散系數(shù)。為解決二氧化碳在混凝土中的擴散問題提供了一種新方法。主要內(nèi)容如下： （1）通過分析已有的碳化深度預(yù)測模型的建立過程，擬定本文的數(shù)值碳化模型的控制方程。由于混凝土碳化是物理化學(xué)共同作用，但就發(fā)生速率而言，化學(xué)反應(yīng)遠遠高于物理擴散，因此可認為其控制方程為僅考慮物理擴散作用，擬定以二氧化碳的擴散深度表示整體的碳化深度。由于氣體在混凝土孔結(jié)構(gòu)中，多以菲克擴散形式為主，因此應(yīng)用菲克第二擴散定律，擬定數(shù)值模型的控制方程。 （2）為實現(xiàn)其數(shù)值模型的建立，以普通混凝土為例，擬定普通混凝土碳化模型中的擴散系數(shù)。在能反映普通混凝土擴散性的四個指標中（水灰比、水泥用量、砂率以及抗壓強度），運用數(shù)理統(tǒng)計中通徑分析方法找到水灰比為其主要影響因素。 （3）以水灰比為主要因素，考慮骨料在混凝土中的分布特點，提出了分區(qū)分層法擬定擴散系數(shù)，即將混凝土分為水泥硬化漿體與骨料兩部分，沿深度方向從不同層面確定擴散系數(shù)。 （4）結(jié)合試驗，對所建立的普通混凝土數(shù)值模型進行驗證。由于分析模式同熱傳導(dǎo)瞬態(tài)問題相似，故采用有限元軟件ANSYS進行模擬。模擬數(shù)據(jù)與試驗數(shù)據(jù)具有一定聯(lián)系，一定程度上證實本文的數(shù)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.
【學(xué)位授予單位】：西北農(nóng)林科技大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2014
【分類號】：TU528
本文編號：2490486
[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.
【學(xué)位授予單位】：西北農(nóng)林科技大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2014
【分類號】：TU528
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