Cl-浓度对钢筋混凝土在土壤中腐蚀行为的影响
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Influence of Cl- Concentration on Corrosion Behavior of Reinforced Concrete in Soil
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通讯作者: 高宇宁,E-mail:1365331633@qq.com,研究方向为交通运输工程机场供电及材料腐蚀与防护
收稿日期: 2020-10-23 修回日期: 2020-11-27 网络出版日期: 2021-07-14
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Corresponding authors: GAO Yuning, E-mail:1365331633@qq.com
Received: 2020-10-23 Revised: 2020-11-27 Online: 2021-07-14
作者简介 About authors
丁清苗,女,1984年生,博士,副教授
针对环境中Cl-侵蚀钢筋混凝土构筑物造成混凝土及其内部钢筋结构发生破坏的问题,采用数值模拟的方式研究了Cl-对钢筋混凝土腐蚀行为的影响。结果表明,钢筋混凝土在受到Cl-侵蚀时,试件靠近侵蚀界面的位置Cl-浓度较大;随着实验的进行,试件内部Cl-含量不断增加,且钢筋表面Cl-浓度差逐渐增大。混凝土试件内部的钢筋腐蚀深度与Cl-含量相关,钢筋表面Cl-浓度大的位置腐蚀较为严重。此外Cl-浓度范围在100~600 mol/m3之间时,Cl-浓度与钢筋钝化时间T满足四次函数关系,与钢筋表面的电位E之间满足五次函数关系。
关键词:
The influence of Cl- on the corrosion behavior of reinforced concrete was studied by means of numerical simulation method, aiming at the troubles related with the chloride ion induced corrosion of reinforced concrete structure in the environment and the damages of the concrete, as well as the reinforced bars within the concrete. The results show that: when reinforced concrete is suffered from attack in a chloride ion containing environment, the chloride ion concentration is greater in the boundary layer near the concrete surface; as the experiment progresses, the chloride ion content inside the concrete increases, and the gradient of chloride ion concentration on the reinforced steel surface gradually increases. The corrosion depth of the steel bars in the concrete is related to the chloride ion content, and the corrosion is more serious in locations where the chloride ion concentration is high on the surface of the steel bars. In addition, when the Cl- concentration range is between 100 and 600 mol/m3, the relationship between Cl- concentration and passivation time T of the steel bar may accord with a quartic function, while a quintic function for relationship between the Cl- concentration and the potential E of the steel bar surface.
Keywords:
本文引用格式
丁清苗, 高宇宁, 侯文亮, 秦永祥.
DING Qingmiao, GAO Yuning, HOU Wenliang, QIN Yongxiang.
钢筋混凝土在各种建筑设施中广泛使用,公路、桥梁及房屋等重要基础设施都由钢筋混凝土构成。而钢筋混凝土在服役过程中其结构会受到不可避免的破坏,严重时可能导致不可挽回的事故和巨大的经济损失。因此,混凝土结构的腐蚀机理和耐久性研究受到了众多学者和专家的关注。Aslani等[1]提出,处于腐蚀环境中的混凝土结构在服役的不同阶段具有不同类型的不确定性,应使用适当概率模型的可靠性分析这些不确定性。洪乃丰等[2]指出,环境中的Cl-会导致钢筋混凝土中钢筋发生锈蚀而缩径以及混凝土结构强度降低等后果,直接对建筑物的稳定性和耐久性产生影响。文献[3-5]强调Cl-是影响钢筋混凝土结构使用寿命的主要因素之一。中外学者对钢筋混凝土结构物破坏及Cl-扩散等方面已经进行了相关研究[6-9]。研究表明,在土壤环境中,Cl-侵入混凝土的一种方式是盐渍土中的Cl-通过渗透扩散进入混凝土到达钢筋表面[10-13]。然而,对于环境中Cl-对混凝土内部钢筋的腐蚀行为及其被破坏程度的预测还不够全面。
针对环境中Cl-侵蚀钢筋混凝土构筑物造成混凝土及其内部钢筋结构发生破坏失效的问题,本文采用数值模拟的方法对含有Cl-的土壤环境中钢筋混凝土腐蚀行为进行研究,可以为钢筋混凝土的防腐方法及寿命预测提供理论指导。
1 仿真模型的建立
1.1 物理模型
图1
图1
钢筋混凝土物理模型
Fig.1
Three-dimensional physical model (a) and two-dimensional physical model (b) of reinforced concrete
图2
图2
钢筋混凝土网格划分模型
Fig.2
Reinforced concrete meshing model: (a) coarser, (b) coarse, (c) normal, (d) fine
表1 网格划分结果统计
Table 1
Grid specifications | Smallest size / mm | Number of triangles | Number of edge units | Number of vertex units |
---|---|---|---|---|
Coarser | 0.6 | 380 | 60 | 8 |
Coarse | 0.2 | 471 | 68 | 8 |
Normal | 0.03 | 782 | 88 | 8 |
Fine | 0.03 | 1072 | 104 | 8 |
图3
1.2 数学模型
电化学反应模块中,带电粒子在电解质溶液中的运动具体划分为对流、扩散和电迁移[13]。其中,电解质溶液中i离子在x方向上的对流流量的表达式为:
式中,πi1(x)为对流流量 (mol·m-2·s-1),ui(x)为流速 (m·s-1),Ci为i离子在电解质溶液中的浓度 (mol·m-3)。
扩散流量表达式为:
式中,πi2(x)为扩散流量(mol·m-2·s-1),Di为扩散系数 (m·s-1),
电迁移引起的传质速率表达式为:
式中,πi3(x) 为电迁移速率 (mol·m-2·s-1),ui0为离子淌度 (m2·s-1·V -1),
所以,离子电极表面总的流量表达式如下:
假设电解质溶液中离子浓度为定值,电解质主体溶液为不可压缩液体且呈电中性。电流密度根据Faraday定律进行计算:
式中,F为Faraday常数 (C·mol-1);
图4
假设该模型中含有Cl-的模拟土壤溶液均匀且静止,沿x轴方向流入微元体的电流和流出微元体的电流大小相等,即:
沿y轴方向:
由方程 (5~7) 可得:
即采用Laplace方程作为腐蚀场中电位分布的控制方程。
根据边界条件求解Laplace方程 (8),求得电极表面各节点处的电位和电流密度分布。
HRB400E不锈钢的密度为ρ=6170 kg/m3,其摩尔质量根据
2 仿真结果与分析
2.1 钢筋混凝土中Cl-分布规律
图5
图5
钢筋混凝土内部Cl-分布云图
Fig.5
Cl- distribution cloud map in reinforced concrete at 100 d (a), 300 d (b), 500 d (c), 700 d (d) and 1000 d (e)
图6
图7
图7
钢筋表面Cl-浓度和腐蚀深度的变化
Fig.7
Chamges of chloride ion concentration (a) and corrosion depth (b) on the surface of steel bars
2.2 Cl-浓度对其自身在钢筋混凝土中扩散的影响
图8
图8
1000 d钢筋混凝土内部Cl-分布云图
Fig.8
Chloride ion distribution cloud diagram in reinforced concrete with 100 mol/m3 (a), 200 mol/m3 (b), 300 mol/m3 (c), 400 mol/m3 (d), 500 mol/m3 (e) and 600 mol/m3 (f) chloride ion concentrationat 1000 d
图9
图9
钢筋最高点处Cl-浓度随时间变化
Fig.9
Relationship between chloride ion concentration and time at the highest point of steel bars
如图10a所示,试件内部钢筋的脱钝时间随着环境中Cl-浓度的增大而减小。根据软件拟合结果,当环境中Cl-浓度范围在100~600 mol/m3之间时,钢筋脱钝时间与Cl-浓度的函数关系为一元四次函数。其具体表达式如
式中,t为钢筋脱钝时间 (d),x为Cl-浓度 (mol/m3)。
图10
图10
钢筋脱钝时间和电极电位随Cl-浓度的变化关系
Fig.10
Chamges rebar deactivation time (a) and electrode potential (b) with chloride ion concentration
图10b所示为1000 d钢筋电极表面相对电极电位与土壤模拟溶液中Cl-浓度的关系,利用Origin软件对结果进行拟合得到相应的函数关系式。
从图10b中可以看出钢筋相对电极电位随着Cl-浓度的增大而降低。拟合结果如图中曲线所示,当环境中Cl-浓度范围在100~600 mol/m3之间时,钢筋表面相对电极电位与Cl-浓度的函数关系式为:
式中,E为钢筋表面相对电极电位 (V),x为Cl-浓度 (mol/m3)。
2.3 Cl-浓度对钢筋混凝土腐蚀的影响
图11
图11
不同浓度Cl- (mol/m3) 对钢筋表面腐蚀深度的影响
Fig.11
Effect of chloride ion concentration on the corrosion depth of steel bars
图12
图12
1000 d钢筋表面腐蚀深度
Fig.12
Corrosion depth of the steel bar surface with 100 mol/m3 (a), 200 mol/m3 (b), 300 mol/m3 (c), 400 mol/m3 (d), 500 mol/m3 (e) and 600 mol/m3 (f) chloride ion concentration in the environment at 1000 d
3 结论
(1) 钢筋混凝土试件内部Cl-呈对称分布;且浸泡时间和渗透深度对试件内部的Cl-浓度分布均有影响。
(2) Cl-浓度范围在100~600 mol/m3之间时,Cl-浓度与钢筋钝化时间T满足四次函数关系:T=3365-28.3x+0.1x2-1.7×10-4x3+1×10-7x4;Cl-浓度与钢筋表面的电位E之间满足五次函数关系:E=-0.46-0.00116x+7.7×10-6x2-2.42×10-8x3+3.33×10-11x4-1.6710-14x5。
(3) Cl-促进了混凝土内钢筋的腐蚀,且Cl-浓度越大,钢筋腐蚀程度越严重;随着时间的延长钢筋上侧 (Cl-浓度较高) 腐蚀程度比钢筋下侧 (Cl-浓度相对较低) 腐蚀程度严重。
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