混凝土模拟孔隙液中钢筋腐蚀临界氯离子浓度试验研究

钢筋腐蚀的临界氯离子浓度是混凝土耐久性研究的重要参数之一.影响临界氯离子浓度的参数非常多,其中,钢筋表面的电位被认为是反映各参数的综合的控制性因素.采用恒电位方法,考虑pH值的影响,研究了不同电位下混凝土模拟孔隙液中氯离子浓度对钢筋腐蚀状态的影响,并用统计处理的方法得到了不同的pH值和不同控制电位下具有95%保证率的临界氯离子浓度值.试验结果发现:pH值对临界氯离子浓度有比较大的影响,钢筋开始腐蚀时的氯离子浓度均值与模拟液的pH值呈指数关系;钢筋开始腐蚀时的氯离子浓度均值的对数与钢筋电位呈线性关系.     

利用极化电阻测试混凝土模拟孔隙溶液中钢筋锈蚀临界氯离子浓度

从钝化膜破坏标志钢筋锈蚀开始的本质出发,通过建立极化电阻与钢筋钝化膜间的对应关系,得到了钢筋钝化膜破坏时的极化电阻变化规律,研究了不同pH值下钢筋钝化膜破坏的临界氯离子浓度值,得到了pH值与临界氯离子摩尔浓度的关系,分析了pH值对钢筋钝化膜的影响。结果表明:当钢筋极化电阻值低于2.7×105Ω.cm2并持续下降时钢筋钝化膜已经破坏,在pH值为12.5~13.4之间,混凝土模拟孔隙溶液中临界氯离子浓度为[Cl-]/[OH-]比值0.1,pH值与临界氯离子摩尔浓度呈对数关系,pH值为11.7时临界氯离子浓度值接近为零,高pH值对维持钢筋钝化膜稳定效果明显。     

Durability assessment of concrete structures: reinforcement depassivation due to carbonation

This paper deals with durability design and comments on the assessment of reinforced concrete (RC) structures in relation to existing codes. Enhancement is provided by advanced approaches incorporating probabilistic simulations, mathematical models of the carbonation process in concretes made from CEM I through to CEM V, and the reliability issue. This paper presents practical software tools used for service life prediction, concrete cover evaluation and assessment of relevant reliability levels. A partial verification using test results is also presented.     

Influence of CaCl2 and NaCl from different sources on chloride threshold value for the corrosion of steel reinforcement in concrete

The influence of CaCl₂ and NaCl from internal and external sources on the chloride threshold value for the corrosion of steel reinforcement in concrete has been investigated. The onset of active corrosion for the steel reinforcement has been detected by half-cell potential and linear polarization methods. Also, the threshold value with different indices, such as free chlorides, [Cl^?]/[OH^?], total chlorides and [Cl^?]/[H⁺] has been measured. It has been found that the chloride threshold values for the corrosion of the specimens with CaCl₂ and NaCl from different sources depend on the indices. Especially, the critical [Cl^?]/[H⁺], i.e. the ratio of acid soluble chlorides to acid neutralisation capacity irrespective of the type and different sources of chloride salt approximately keeps constant at 0.02.     

Corrosion propagation of prestressing steel strands in concrete subject to chloride attack

A review of research literature suggests that steel corrosion in concrete can accelerate if the corroding steel is under high levels of stress but this important phenomenon has not been investigated thoroughly. Both laboratory tests and field surveys indicate that the effects of corrosion on structural behaviour in reinforced and prestressed concrete structures are different, with the latter having less concrete cracking but more serious structural collapses. The intention of this paper is to investigate the corrosion propagation of prestressing steel strands in chloride-laden environment. A long term experimental program is presented in which the corrosion of twelve prestressing steel strands embedded in concrete was examined. The effects of the level of stresses in the steel and type of steels on corrosion rate are studied. It is found that the corrosion rate of prestressing steel strands in concrete increases with the increase of the level of stresses applied and that the pitting corrosion is the main form of prestressing steel corrosion in chloride-laden environment. The paper concludes that the corrosion of prestressing steel in concrete structures poses higher risk to the structure than that of reinforcing steel in terms of structural collapse.     

混凝土氯离子提取液中氯离子含量测定方法研究

分析了常用的混凝土氯离子提取液中氯离子含量的测定方法,对普通佛尔哈德化学滴定法进行了改进,并将该方法与普通佛尔哈德化学滴定法和电位法进行对比.研究发现,电位法操作简单、用时较短,可以用于工程中氯离子的快速测定,但由于其精度较低,不适合用于对混凝土氯离子提取液中氯离子含量的精确分析;利用改进佛尔哈德化学滴定法测定标准溶液中氯离子含量在10 mg/ml以下时的误差小于1%,与普通的佛尔哈德化学滴定法相比,改进的佛尔哈德化学滴定法具有更高的测定精度,适用于混凝土氯离子提取液中氯离子含量的精确分析.     

随机有限元法分析混凝土中氯离子随机扩散规律

氯离子随机扩散分析是海洋混凝土结构可靠性和服役寿命的基础。利用局部平均法,将混凝土的氯离子扩散系数随机场离散为一组随机变量,利用Taylor级数将氯离子分布矩阵、扩散矩阵和节点浓度向量进行摄动展开,建立了氯离子扩散分析的随机有限元法递归方程,据此分析氯离子随机浓度的均值和变异系数,并研究了氯离子在混凝土中随机扩散的规律。通过算例比较分析,表明了所建立的摄动随机有限元方法具有较高的计算精度,而且计算结果对随机场的相关结构不敏感。验证了氯离子扩散系数的变异性对氯离子扩散过程具有显著影响。     

混凝土氯离子扩散性评价的交流阻抗法

如何延长氯盐环境中钢筋混凝土结构的使用寿命已经成为混凝土耐久性设计的主要目标。通过交流阻抗法评价配合比、掺合料和表面涂层对混凝土氯离子扩散性的影响,基于试验结果,定量分析了混凝土交流电阻随水灰比、最大骨料直径、骨料级配、粉煤灰掺量、耐蚀剂和表面涂层的变化特性,并给出了相应的解释,发现水灰比、骨料级配、粉煤灰和表面涂层对交流电阻的影响较大,这些结论可为钢筋混凝土结构的耐久性设计提供理论依据。     

The importance of chloride content at the concrete surface in assessing the time to corrosion of steel in concrete structures

A literature review in the present study has shown that the chloride ions on the concrete surface exposed to a marine environment including tidal, splash and aerated zones can be accumulated then to increase with time. In order to take this effect into account in a model prediction of the rate of chloride ingresses in concrete, the surface chloride was expressed as a function of time such that (1) constant, (2) linear build-up, (3) square root build-up and (4) square root build-up with an initial set of the surface chloride (a refined model), at a given diffusion coefficient (2 × 10^?12 m²/s). As a result, the constant surface chloride model produced the greatest level of chloride penetration, as an initial set of the surface chloride content was overestimated. The rate of chloride ingresses derived from the highest surface chloride led to the greatest risk of chloride-induced corrosion of steel in concrete, whereas the linear and square root build-up models indicated 2–3 times longer duration of corrosion-free service life. The refinement of a surface chloride build-up suggested a more realistic prediction of chloride profile and corrosion risk, thereby imposing the rate of chloride ingresses between the constant surface chloride, and linear or square root build-up.     

Influence of detection methods on chloride threshold value for the corrosion of steel reinforcement

The aim of this study is to evaluate the influence of depassivation detection methods on chloride threshold value for the corrosion of steel reinforcement in a concrete. The detection methods of depasstivating steel reinforcement have been examined including electrochemical impedance spectra (EIS), potentiodynamic polarization (PP) in a simulated concrete pore solution and half-cell potential, linear polarization (LP) and electrochemical impedance spectra (EIS) in the concrete contaminated by chloride ions. The results have indicated that in a saturated Ca(OH)₂ solution, the chloride threshold value for the corrosion of steel reinforcement concluded by the potentiodynamic polarization is 0.02 mol/L, which is identical with that decided by EIS. However, in the chloride contaminated concrete, no agreement among the chloride threshold values determined by the three detection methods mentioned above can be found. The range of the values decided by half-cell potential is relatively lower than those by the others, besides, the highest values have been obtained by EIS.     

钢筋阻锈剂对临界氯离子浓度的影响

采用半电池电位和线性极化法监测钢筋在掺有两种常用阻锈剂的模拟孔溶液和硬化砂浆中的锈蚀时间,研究钢筋阻锈剂对临界氯离子浓度的影响。试验结果表明:掺入阻锈剂后,提高了钢筋在砂浆中的临界氯离子浓度,延长了钢筋发生锈蚀的时间。通过阻锈剂在混凝土中的抗氯离子渗透性研究,探讨了阻锈剂的作用机理。     

水泥混凝土板硬化温度翘曲正向计算方法研究

硬化翘曲的存在使得水泥混凝土路面（机场道面）的开裂模式向多样化发展,对路面长期服役行为产生深远影响。不同于现有的反算法和直接确定法,提出了计算水泥混凝土板硬化温度翘曲的正向计算方法。编写了基于ABAQUS的水泥混凝土路面温度场计算子程序和能够考虑拉压徐变差异的本构子程序,模型能够预测混凝土路面自浇筑后的温度场和应力场变化历史。研究表明,硬化温度翘曲以残余应力和残余变形的方式存在于混凝土板内,其分布和位置有关,且二者在混凝土浇筑30d后趋于稳定值。通过提出采用复位法将残余变形转化为残余应力,进一步以等效线性温度梯度表征,便于水泥混凝土路面设计采用。最后通过足尺试验路数据评估了正向计算方法的可行性,指出当存在相邻板的约束时,硬化温度翘曲主要以残余变形的状态存在,峰值出现在接缝附近。     

A method to determine water retainability of porous fine aggregate for design and quality control of fresh concrete

Water retainability of a porous aggregate is defined, including absorption as well as adsorption. This moisture property is important for mix proportion and quality control of fresh concrete. An alternative test method to determine water retainability of porous fine aggregates is developed. By the conventional test method for obtaining absorption, it is not possible to reach the saturated surface dryness by collapsed-cone criterion due to the high inter-particle friction of the porous aggregate. The test method was designed, based on the concept of gravitational removal of excess water from the porous aggregate sample. Two procedures i.e. static and accelerated methods were studied to achieve the acceptable reliability within a reasonable time of testing. The comparison between test results of water absorption according to ASTM C 128 and those of water retainability by the proposed method indicated that this method had a better testing precision for all porous fine aggregates, as proved by the smaller standard deviations within the test results given by different individuals. Feasibility of the method was also validated by comparing the properties of conventional concrete (CVC) and self-compacting concrete (SCC) made of normal and porous fine aggregates. The water content of mixtures incorporating expanded clay and two types of bottom ash as river sand replacement of 0, 10, and 20% were adjusted, on the basis of the water absorption from the ASTM C 128 method and the water retainability from the proposed method. From the comparison, it was shown that the proposed test method was effectively applied as an alternative test for water retainability of porous fine aggregates in the quality control of fresh concrete.     

Incremental collapse threshold for pushout resistance of circular concrete filled steel tubular columns

This paper presents the results of a series of pushout tests using static loading (SL) and variable repeated loading (VRL) on concrete filled steel tubular (CFT) circular stub columns. The main parameters examined in this paper were the strength and age of concrete and the loading protocol. Under SL tests, the interface bond strength in CFT columns filled with normal strength concrete was found to be higher than that with high strength concrete. The SL test results showed that the interface bond strength varied from 0.41 to 0.85 MPa but from 0.33 to 0.66 MPa under VRL tests. A lower bound for the incremental collapse threshold of the pushout resistance of 70% of the static collapse load was empirically derived. Also an expression of the average growth of slip per loading cycle was empirically derived and recommended for design purposes. A comparison between the bond strength of the columns obtained from the present and previous test results, and available design codes is presented. Two newly derived bond strength limits were experimentally obtained and proposed for the design of structures subjected to either predominantly static or predominantly cyclic loading.     

钢纤维混凝土单轴拉伸实验方法

在介绍了混凝土单轴拉伸试验中试件的形状及受力方式的基础上,对其中的内埋式试验方法进行改进,通过对比分析普通混凝土和钢纤维混凝土的拉伸试验数据,得出在加入钢纤维后,混凝土的抗拉强度和拉伸变形能力都有所提高。     

氯盐环境条件下预应力混凝土氯离子侵蚀模型研究

文章总结了国内外有关混凝土中氯离子侵蚀模型的研究现状,并对国内外几种典型的扩散方程进行了分析评价;针对氯盐侵蚀的预应力混凝土结构,讨论了氯离子扩散系数的计算。氯离子扩散系数对于预应力混凝土来说是一个重要的因素,因此合理确定氯离子扩散系数,对预测氯离子环境条件下预应力结构的耐久性具有重要意义。对于预应力混凝土,计算氯离子扩散系数除了要考虑混凝土与氯离子之间的结合能力、环境温度、湿度影响外,还要考虑混凝土的应力水平,氯离子有效扩散系数是变化的,且氯离子扩散系数对PC结构耐久性的影响很大。本文对预应力混凝土氯离子侵蚀模型进行了改进,并根据试验结果对改进后模型进行了评价。     

氯盐对水泥混凝土道面的损害及防治

针对道面混凝土所处的环境、结构类型和使用特点,分析了氯盐在混凝土中存在的方式、氯离子的来源及对混凝土道面结构所产生的危害,如表面剥落、黑色斑块及孔洞,导致道面使用性能的下降.根据危害所产生的原因,提出了相应的措施予以防治.     

基于混合惩罚函数法的型钢混凝土框架柱优化设计

基于型钢混凝土结构构件的粘结滑移理论和受力性能研究,提出了型钢混凝土框架结构柱单目标离散型变量优化设计方法。设计变量取结构构件几何尺寸、截面尺寸、材料强度和型钢截面尺寸,目标函数取构件的造价,约束条件为型钢混凝土组合结构基于粘结滑移理论的设计计算理论以及现行设计规范中所规定的合理的设计基本要求、计算要点和构造措施。为型钢混凝土框架结构的优化设计提供了实用方法与途径,亦可为其他型钢混凝土结构的优化设计提供参考。     

边界元法研究混凝土中氯离子的扩散及分布规律

有限元法是分析氯离子在混凝土中扩散常用的方法,但它需要在时间域上做细密的离散网格,增加了计算时间及模型的复杂性.首次采用边界元法建立二维混凝土结构中氯离子一维扩散分析模型,提出了计算长度的概念及其表达式,使得计算模型中避免了对时间域的离散.同时由于边界元法可以将二维问题降低为一维离散问题,所以该计算模型可以大大简化计算过程,提高计算精度.算例分析验证了该方法的计算效率和可行性.     

钢纤维混凝土受弯构件正截面的设计方法

钢纤维混凝土是在普通混凝土中掺入乱向分布的短钢纤维所形成的一种新型的多相复合材料。本文介绍了钢纤维混凝土受弯构件的正截面设计方法．并与普通混凝土构件进行比较．以此说明钢纤维混凝土在受弯构件中应用的优势。