高温条件下混凝土模拟孔隙液中钢筋腐蚀
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摘要
自从波特兰水泥问世以来,混凝土结构已经成为基础建设工程中最为常用的建筑形式之一。众所周知,钢筋混凝土结构结合了混凝土的优点,造价较低,是土木工程结构设计中的首选形式,其应用范围非常广泛。但很多时候人们只关注混凝土结构的承载力计算而忽略耐久性设计,导致钢混结构因耐久性不足而提前失效。可见,耐久性研究是钢筋混凝土结构研究中很迫切且重要的问题。
氯离子渗入而引起的钢筋腐蚀是导致钢筋混凝土结构材料劣化的常见原因。如海洋环境中或冬天撒除冰盐的情况下,当钢筋表面的混凝土孔隙溶液中的游离氯离子浓度超过一定值时,即使溶液碱度较高,氯离子也能破坏钢筋表面的钝化膜,形成点蚀现象。
国际混凝土组织提出基于性能的钢筋混凝土结构耐久性设计方法,目标是设计钢筋混凝土结构在设计寿命里有足够的可靠性,保证结构的承载力和适用性。此种设计方法需要确定钢筋腐蚀的初始时间,这就需要获得准确的氯离子渗透速度和临界氯离子浓度。关于氯离子渗透已经有很多的研究得出其与混凝土材料的关系。但是对临界氯离子浓度的研究,特别是与温度的关系,目前研究比较匮乏。
前人已经对临界氯离子浓度作了较深入的研究,得到的临界氯离子浓度值因表达方式、试验环境以及试验方法的不同而出现非常大的离散性。为确保混凝土结构的可靠性,各国规范常采用保守的下限取值。
本文紧紧围绕钢筋腐蚀的机理——“钢筋腐蚀电位超越点蚀电位导致钢筋脱钝”这条主线,考虑高温(50℃)的影响,在不同pH值的混凝土模拟孔隙液中,采用恒电位方法进行试验,得到了不同电位、pH值的钢筋腐蚀时的氯离子浓度均值、变异系数及具有95%保证率的临界氯离子浓度值。并与陈卿在常温(25℃)和低温(5℃)条件下的得到的试验结果进行比较,发现:当pH值小于11.6时温度对钢筋腐蚀时临界氯离子浓度的影响不明显,而pH值大于11.6时温度影响显著;当钢筋电位大于-300mV SCE时温度对临界氯离子浓度的影响不明显,而钢筋电位小于-300mV SCE时温度的影响显著。当温度从5℃上升到50℃,钢筋腐蚀的临界氯离子浓度下降了88%。作者分析了pH值、电位以及温度对钢筋腐蚀临界氯离子浓度影响的机理。
考虑到实际情况温度并不是恒定不变,而是随时间变化的,本文设计了试验研究钢筋腐蚀情况与温度波动的关系,旨在考察钢筋在经历温度升高出现腐蚀后回到原温度是否保持脱钝状态。试验发现,在一定氯离子浓度下,尽管钢筋在常温下保持钝态,但在不管加热时间维持在2小时还是24小时,经历了温度升高后的钢筋脱钝是不可逆的。这说明钢筋腐蚀的临界氯离子浓度并不是与环境条件唯一对应的,而是与环境变化有关。这对临界氯离子浓度值的选取有重要的现实意义。
Since the invention of Portland cement, concrete structure has been the most used structure form in infrastructures. As widely known, reinforced concrete (RC) structure embraces the advantages of concrete, costs less, and it has become the primary choice of structure design in civil engineering, with its wide range of usage. However, most of the time people focus on calculation of load capacity and ignore the durability of concrete structures, leading to early functional loss of structures. Research on durability of RC structure is very urgent and important.
Corrosion of steel bars induced by penetration of chloride in the concrete cover is a frequent cause of degradation of RC structures exposed to marine environments or to the use of de-icing salt. Chloride lead to a local breakdown of the protective oxide film on the reinforcement in alkaline concrete, so that a subsequent localized corrosion attack takes place.
Performance based approaches to the durability of RC structures have recently been proposed by the International Federation of Concrete, which are aimed at the design of concrete structures in order to guarantee the stability and serviceability requirements throughout the design service life with an adequate level of reliability. These approaches require the estimation of the initiation time of corrosion. This can be done only if an accurate estimation of both the chloride penetration in time and the chloride threshold level (CTL) is available. A great amount of studies are published regarding the penetration of chloride ions as a function of concrete composition. Conversely, there is a lack of knowledge regarding the chloride threshold level, especially under temperature’s influence.
Many studies have focused on the CTL, but with different presentations, conditions, and detection methods, CTL varies largely. To ensure the reliability of RC structures, standards of many countries adopt a quite conservative value.
In this dissertation, author mainly cling to the idea that --corrosion potential exceeding pitting corrosion is the sufficient and essential condition for depassivation of reinforcement. Experiments are done under high temperature (50℃), in synthetic pore solution with different pH values, using potentiostatic method. Mean values and variation coefficients of Cl- content at the reinforcement corrosion initiation and CTL values with 95% acceptance under different pHs, different potentials are suggested. Comparison has been done between CTL under high temperature and results suggested by Chen Qing under low temperature (5℃) and normal temperature (25℃). It shows that: when pH is less than 11.6, the influence of temperature to CTL is less evident, while pH is greater than 11.6, temperature plays an important part in CTL; when potential is greater than -300mV SCE, temperature’s influence is less evident, on the other hand, when potential is less than -300mV SCE, temperature’s influence on CTL is more obvious. If temperature rises from 5℃to 50℃, CTL will decrease by 88%.
Since temperature is not a constant value, it is time- dependant, this dissertation has also designed an experiment to study the relationship between steel corrosion and temperature fluctuation, which aims to investigate how steel reacts after staying in high temperature for some time and corrodes. Experiment shows that, in a certain Cl- content, even steel keep passive under normal temperature, after staying in high temperature for no matter 2 hours or 24 hours, depassivation of steels is irreversible. It illustrates that CTL does not correspond to the only one environment condition; it is relevant to the history of environment conditions, which has real important meaning in deciding CTL values.
氯离子渗入而引起的钢筋腐蚀是导致钢筋混凝土结构材料劣化的常见原因。如海洋环境中或冬天撒除冰盐的情况下,当钢筋表面的混凝土孔隙溶液中的游离氯离子浓度超过一定值时,即使溶液碱度较高,氯离子也能破坏钢筋表面的钝化膜,形成点蚀现象。
国际混凝土组织提出基于性能的钢筋混凝土结构耐久性设计方法,目标是设计钢筋混凝土结构在设计寿命里有足够的可靠性,保证结构的承载力和适用性。此种设计方法需要确定钢筋腐蚀的初始时间,这就需要获得准确的氯离子渗透速度和临界氯离子浓度。关于氯离子渗透已经有很多的研究得出其与混凝土材料的关系。但是对临界氯离子浓度的研究,特别是与温度的关系,目前研究比较匮乏。
前人已经对临界氯离子浓度作了较深入的研究,得到的临界氯离子浓度值因表达方式、试验环境以及试验方法的不同而出现非常大的离散性。为确保混凝土结构的可靠性,各国规范常采用保守的下限取值。
本文紧紧围绕钢筋腐蚀的机理——“钢筋腐蚀电位超越点蚀电位导致钢筋脱钝”这条主线,考虑高温(50℃)的影响,在不同pH值的混凝土模拟孔隙液中,采用恒电位方法进行试验,得到了不同电位、pH值的钢筋腐蚀时的氯离子浓度均值、变异系数及具有95%保证率的临界氯离子浓度值。并与陈卿在常温(25℃)和低温(5℃)条件下的得到的试验结果进行比较,发现:当pH值小于11.6时温度对钢筋腐蚀时临界氯离子浓度的影响不明显,而pH值大于11.6时温度影响显著;当钢筋电位大于-300mV SCE时温度对临界氯离子浓度的影响不明显,而钢筋电位小于-300mV SCE时温度的影响显著。当温度从5℃上升到50℃,钢筋腐蚀的临界氯离子浓度下降了88%。作者分析了pH值、电位以及温度对钢筋腐蚀临界氯离子浓度影响的机理。
考虑到实际情况温度并不是恒定不变,而是随时间变化的,本文设计了试验研究钢筋腐蚀情况与温度波动的关系,旨在考察钢筋在经历温度升高出现腐蚀后回到原温度是否保持脱钝状态。试验发现,在一定氯离子浓度下,尽管钢筋在常温下保持钝态,但在不管加热时间维持在2小时还是24小时,经历了温度升高后的钢筋脱钝是不可逆的。这说明钢筋腐蚀的临界氯离子浓度并不是与环境条件唯一对应的,而是与环境变化有关。这对临界氯离子浓度值的选取有重要的现实意义。
Since the invention of Portland cement, concrete structure has been the most used structure form in infrastructures. As widely known, reinforced concrete (RC) structure embraces the advantages of concrete, costs less, and it has become the primary choice of structure design in civil engineering, with its wide range of usage. However, most of the time people focus on calculation of load capacity and ignore the durability of concrete structures, leading to early functional loss of structures. Research on durability of RC structure is very urgent and important.
Corrosion of steel bars induced by penetration of chloride in the concrete cover is a frequent cause of degradation of RC structures exposed to marine environments or to the use of de-icing salt. Chloride lead to a local breakdown of the protective oxide film on the reinforcement in alkaline concrete, so that a subsequent localized corrosion attack takes place.
Performance based approaches to the durability of RC structures have recently been proposed by the International Federation of Concrete, which are aimed at the design of concrete structures in order to guarantee the stability and serviceability requirements throughout the design service life with an adequate level of reliability. These approaches require the estimation of the initiation time of corrosion. This can be done only if an accurate estimation of both the chloride penetration in time and the chloride threshold level (CTL) is available. A great amount of studies are published regarding the penetration of chloride ions as a function of concrete composition. Conversely, there is a lack of knowledge regarding the chloride threshold level, especially under temperature’s influence.
Many studies have focused on the CTL, but with different presentations, conditions, and detection methods, CTL varies largely. To ensure the reliability of RC structures, standards of many countries adopt a quite conservative value.
In this dissertation, author mainly cling to the idea that --corrosion potential exceeding pitting corrosion is the sufficient and essential condition for depassivation of reinforcement. Experiments are done under high temperature (50℃), in synthetic pore solution with different pH values, using potentiostatic method. Mean values and variation coefficients of Cl- content at the reinforcement corrosion initiation and CTL values with 95% acceptance under different pHs, different potentials are suggested. Comparison has been done between CTL under high temperature and results suggested by Chen Qing under low temperature (5℃) and normal temperature (25℃). It shows that: when pH is less than 11.6, the influence of temperature to CTL is less evident, while pH is greater than 11.6, temperature plays an important part in CTL; when potential is greater than -300mV SCE, temperature’s influence is less evident, on the other hand, when potential is less than -300mV SCE, temperature’s influence on CTL is more obvious. If temperature rises from 5℃to 50℃, CTL will decrease by 88%.
Since temperature is not a constant value, it is time- dependant, this dissertation has also designed an experiment to study the relationship between steel corrosion and temperature fluctuation, which aims to investigate how steel reacts after staying in high temperature for some time and corrodes. Experiment shows that, in a certain Cl- content, even steel keep passive under normal temperature, after staying in high temperature for no matter 2 hours or 24 hours, depassivation of steels is irreversible. It illustrates that CTL does not correspond to the only one environment condition; it is relevant to the history of environment conditions, which has real important meaning in deciding CTL values.
引文
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[2]张誉,蒋利学,张伟平等.混凝土结构耐久性概论[M].上海:上海科学技术出版社, 2003
[3]罗福午.建筑结构缺陷事故的分析与防止[M].北京:清华大学出版社, 1996
[4] O’Conner J.P., Cutts J.M. et al. Evaluation of historic concrete structures [J]. Concrete International, 1997, 19(8): 57-61
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[6]洪定海.混凝土中钢筋的腐蚀与保护[M].北京:中国铁道出版社, 1998
[7]柯伟.中国腐蚀调查报告[M].北京:化学工业出版社工业装备与信息出版中心, 2003
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[10] Schessl P., Raupach M., Chloridinduziertc Korrosion von Dtahl im Beton, Brton- Informationen, 1990, 30: 43-55
[11] Vargova M., Significance of the ambient temperature and the steel material in the process of concrete reinforcement corrosion [J]. Construction and Materials, 11(2): 99-103
[12]钱磊,氯环境下温度对钢筋混凝土结构中钢筋腐蚀的影响[学位论文],上海:上海交通大学, 2004
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[2]张誉,蒋利学,张伟平等.混凝土结构耐久性概论[M].上海:上海科学技术出版社, 2003
[3]罗福午.建筑结构缺陷事故的分析与防止[M].北京:清华大学出版社, 1996
[4] O’Conner J.P., Cutts J.M. et al. Evaluation of historic concrete structures [J]. Concrete International, 1997, 19(8): 57-61
[5]林玉珍,杨德钧.腐蚀和腐蚀控制原理[M].北京:中国石化出版社, 2007
[6]洪定海.混凝土中钢筋的腐蚀与保护[M].北京:中国铁道出版社, 1998
[7]柯伟.中国腐蚀调查报告[M].北京:化学工业出版社工业装备与信息出版中心, 2003
[8] Tuutti K., Service life of structures with regard to corrosion of embedded steel. RILEM Quality Control of Concrete Structures. Stockholm, Sweden, 1979: 293-299
[9] J.P. Broomfield. Corrosion of Steel in Concrete: Understanding, Investigation and Repair [M]. E & FN Spon, London, 1997
[10] Schessl P., Raupach M., Chloridinduziertc Korrosion von Dtahl im Beton, Brton- Informationen, 1990, 30: 43-55
[11] Vargova M., Significance of the ambient temperature and the steel material in the process of concrete reinforcement corrosion [J]. Construction and Materials, 11(2): 99-103
[12]钱磊,氯环境下温度对钢筋混凝土结构中钢筋腐蚀的影响[学位论文],上海:上海交通大学, 2004
[13] Sagoe- Crentsil K. K., Glasser F. P., Steel in concrete: Part I. A review of the electrochemical and thermodynamic aspects [J]. Concrete Research. 1989, 41(189): 205-212
[14] Valcarce M. B., Vazquez M., Carbon steel passivity examined in alkaline solutions: The effect of chloride and nitrite ions [J]. Electrochimica Acta, 2008,53: 5007-5015
[15] FIB, Model code for service life design, Federation International du Beton, Bulletin No.34, February 2006
[16] Arya C., Newman J.B., An assessment of four methods of determining the free chloridecontent of concrete [J]. Materials and Structures, 1990, 23: 319-330
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