Probabilistic evaluation of initiation time in RC bridge beams with load-induced cracks exposed to de-icing salts

In this study, a reliability-based method for predicting the initiation time of reinforced concrete bridge beams with load-induced cracks exposed to de-icing salts is presented. A practical model for predicting the diffusion coefficient of chloride ingress into load-induced cracked concrete is proposed. Probabilistic information about uncertainties related to the surface chloride content and the threshold chloride concentration has been estimated from a wide review of previous experimental or statistical studies. Probabilistic analysis to estimate the time to corrosion initiation with/without considering the effect of the load-induced cracks on the chloride ingress into concrete has been carried out. Results of the analysis demonstrate the importance of considering the effect of the load-induced cracks for correct prediction of corrosion initiation in RC bridge beams exposed to chlorides.     

Capillary transport of water through textile-reinforced concrete applied in repairing and/or strengthening cracked RC structures

The use of textile-reinforced concrete (TRC) has great potential for innovative solutions in repairing, protecting, and strengthening concrete and RC structures. The article at hand reports on an investigation on composite concrete specimens made of cracked ordinary concrete as substrate and textile-reinforced concrete (TRC) as a cover layer for its strengthening and repair. The TRC cover layer was assessed with regard to its effectiveness as a protective layer against the ingress of water through capillary action. Since in real applications such TRC layers may be cracked or presumed to be so, thereby activating the load-carrying function of the textile reinforcement, the TRC layer was cracked for purposes of this study. The water transport in the cracked ordinary concrete specimens without the TRC layer was used as a reference. Gravimetric measurements and neutron radiography served as the testing techniques. In ordinary concrete quick and deep ingress of water through relatively wide macro-cracks of approximately 100 μm width, followed by transport through the capillary pore system, caused saturation of large areas in a rather short time. TRC applied to the RC surface reduced the ingress of water to a large extent. Its small crack widths of 15 to 20 μm changed suction behaviour fundamentally. In the cracked substrate of ordinary concrete, capillary suction was prevented, and transport through the pore system of the matrix became the prevailing transport mechanism of capillary action. Not only was the mechanism altered, but the transport of water deep into inner regions was markedly retarded as well.     

Capillary suction and diffusion model for chloride ingress into concrete

A numerical approach, named TransChlor, is proposed to simulate transport phenomena of various substances in concrete. This approach is a theoretical model based on finite elements and finite differences methods. The model consists of coupled nonlinear partial differential equations based on Fick's diffusion law and on kinematics equations. Simulation results from a parametrical study highlight the influence of microclimatic conditions, exposure to deicing salts and concrete cover permeability and thickness on chloride ingress in concrete. The results show that the chloride ion concentration increases quickly in concrete cover when a structure is exposed to deicing salts at a mountainous location; whereas permeability of concrete cover is an insignificant parameter when the concrete is in direct or splash water contact.     

Modeling of chloride diffusivity coupled with non-linear binding capacity in sound and cracked concrete

The objective of this research is to establish a model that can predict chloride transport phenomena in sound and cracked concrete. The chloride diffusivity is formulated based on computed micro-pore structure, which considers tortuosity and constrictivity of porous network as reduction factors in terms of complex micro-pore structure and electric interaction of chloride ions and pore wall. In the real environment, concrete structures are not always crack-free, therefore chloride transport in cracked concrete is also simulated by section large void spaces in a control volume to represent the crack and by proposing a model of chloride diffusivity through the cracked region The proposed models are implemented into a finite-element computational program DuCOM, which simulates the early-age development process of cementitious materials. The calculated concentration profiles of total chloride ions are verified through a comparison with experiments results.     

混凝土中氯离子迁移特征的表征

混凝土中氯离子的迁移是个非常复杂的过程.一般将测量和计算硬化水泥浆体和混凝土中氯离子的迁移的方法分为非稳态和稳态两大类.对混凝土中氯离子迁移的测试方法进行了评述,并分析了其优点和缺点.北欧标准NTBuild492是目前非稳态条件下快速测试氯离子扩散性最好的方法,但实际的氯离子浓度分布与其计算公式用的模型的模拟仍存在一定的差异.北欧标准NT Build 355是目前稳态条件下快速测试氯离子扩散性最好的方法,但理论基础过于简化.尽管ASTM C1202或AASHTO T277在世界上被广泛应用,但实际上它是一个测量混凝土电导的方法,它可用于混凝土的质量控制,如其结果没有与混凝土的氯离子渗透性试验结果建立相关性时,不能直接用于评价混凝土的氯离子渗透性.     

Microstructural modifications in Portland cement concrete due to forced ionic migration tests. Study by impedance spectroscopy

The forced migration test has been used during the last decades as an accelerated and useful method for determining the resistance of concrete to chloride ingress. Furthermore it allows to determine transport parameters of chloride ions through porous materials. The application of electric fields seems to introduce variations in the microstructure of concrete. In this work these variations have been studied using the impedance spectroscopy technique in the high frequency region (1 kHz-1 MHz). The results show that the dielectric response of concrete is also strongly modified during the migration experiments. The variations of the dielectric parameters during the migration tests can be explained in terms of modifications of the microstructure of concrete. The observed modifications are in good agreement with the results of mercury porosimetry analysis.     

沿海混凝土结构氯离子对流区深度计算模型

沿海干湿交替环境下的氯离子传输方式和影响机理等问题是混凝土耐久性研究的主要内容,描述混凝土内部氯离子传输行为的计算模型通常由偏微分方程组成,求解方法较为复杂。为了简单而相对精确地分析氯离子的传输过程,本文提出了计算修正Fick定律模型中对流区深度参数Δx的新途径,通过综合考虑混凝土细观尺度组成、环境温度场变化、水分迁移和氯离子传输等因素,建立起干湿交替区域氯离子传输的计算模型,依据该模型可以较为准确地得到Δx的数值,进而对不同服役环境中的氯离子传输展开分析,根据计算结果给出考虑服役时间、日平均气温和干燥湿润时间比等参数的对流区深度Δx时变公式。这些公式提高了预测干湿交替环境中氯离子传输的修正Fick定律模型的适用性与准确性,可为干湿交替环境中的混凝土结构耐久性设计提供了简单有效的计算方法。     

Hysteretic moisture behavior of concrete: Modeling and analysis

Durability of reinforced concrete is crucial for civil and building infrastructures. Deterioration related to corrosion of the reinforcement can be caused by chloride diffusion or carbonation due to atmospheric CO₂ ingress. Chlorides and CO₂ transport coefficients are dependent on the saturation degree of the porous material. Thus, the durability of concrete elements submitted to cyclic atmospheric conditions could depend on the hysteretic moisture behavior of concrete. To assess this influence, an analytical model has been developed describing hysteretic moisture behavior based on the independent domain theory. This model is incorporated in a finite element code for coupled heat and mass transfer, allowing to calculate the concrete response to changing temperature and relative humidity conditions. A comparison is made between simulations using the hysteresis model and using either the main ad- or desorption curve. The results show that durability risks may be underestimated when omitting moisture hysteresis in the outer concrete layer.     

干湿交替对混凝土中氯离子分布影响的多相耦合数值分析

钢筋混凝土结构的耐久性会因为氯盐侵蚀而受到严重影响.为研究环境湿度对混凝土中氯离子分布的影响,建立了多相耦合的氯离子侵蚀模型,利用数值模拟方法探讨了氯离子侵蚀过程随环境湿度变化的规律.结果表明,环境湿度的改变不仅使混凝土中含水率产生变化,而且对混凝土中氯离子的分布产生显著影响,尤其是会引起孔隙中盐分结晶,这是混凝土性能...     

Approximate migration coefficient of interfacial transition zone and the effect of aggregate content on the migration coefficient of mortar

In this study, the electrochemical technique is applied to accelerate chloride ion migration in mortar to estimate its transport properties. In order to investigate the effect of aggregate content on the chloride migration coefficient of mortar, specimens with different fine aggregate volume fractions were cast and tested. The chloride migration coefficient of mortar was determined experimentally as a function of the volume fraction of aggregate. The chloride migration coefficient of mortar is used to assess the dilution, tortuosity and interfacial transition zone (ITZ) effects of aggregate in the cement-based composites. A model modified from the Bruggeman theory for the migration coefficient of mortar is used, and the regression analysis is used to determine the approximate chloride migration coefficient of ITZ. Based on the experimental and regression analytical results, the approximate ITZ migration coefficient is 2.83, 1.76 and 1.55 times of the matrix migration coefficient for the ITZ with the thickness of 20, 40 and 50 μm, respectively.     

Crack closure of reinforced concrete by electrodeposition technique

The electrodeposition method for rehabilitation of cracked reinforced concrete, based on the electrochemical technique, is presented here. Investigations on reinforced concrete beams of size 15×15×125 cm were conducted in this study. Specimens cracked by chloride attack for 18 months were immersed in a ZnSO₄ solution, and were applied with a constant current for 8 weeks. The development of crack closure due to the precipitation of electrodeposits (ZnO) was evaluated. Water permeability, chloride ion concentration, polarization curve and half-cell potential were measured in order to assess the effectiveness of this method in the improvement of concrete properties. The results indicate that electrodeposits formed on the concrete surface were able to close the concrete cracks and to decrease the concrete permeability. In addition, the investigation shows that the application of electrodeposition have effects on the desalination of concrete and re-passivity of the reinforcing steel in concrete.     

海洋环境条件下碱骨料反应对海洋结构物性能影响的研究现状、存在问题及其发展趋势

海洋环境中有害物质侵蚀是造成海洋结构物性能劣化的主要原因。本文从影响碱骨料反应膨胀因素、氯离子与碱骨料反应共同作用对碱骨料反应膨胀行为影响、氯离子扩散与结合行为影响和力学性能损伤及损伤模型等方面出发,对海洋环境下碱骨料反应对结构物性能影响研究现状进行总结。分析此研究领域存在问题。提出了需引入能更准确反映碱骨料反应下材料损伤程度的变量参数;需加强氯离子与碱骨料共同作用下多维应力状态下混凝土长期力学性能演化规律研究;建立三轴应力状态下混凝土损伤的本构模型等新的研究思路。     

Effect of the non-linear membrane potential on the migration of ionic species in concrete

In a concrete migration test the ionic species are present either in the boundary layer or in the pore solution and they flow as is indicated by the Nernst-Planck equation. This equation supposes that the flux of each ion is independent of every other one. However, due to ion-ion interactions there are ionic fields that affect the final flux producing an additional voltage known as the membrane potential. In order to understand this phenomenon, this paper describes a theoretical and experimental investigation of the implications of the membrane potential on the migration of chlorides in concrete. A new electrochemical test has been carried out and its results were compared with a numerical simulation. From the results it has been proved that the voltage drop during a migration test is not linear and changes during the test. Thus the Nernst-Planck equation may be applied to the simulation of migration of any ionic species through a saturated porous medium by using the non-linear voltage to represent all the microscopic interactions in a macroscopic way. The results may be used to improve the reliability of the ASTM C1202 chloride migration test.     

On the mathematics of time-dependent apparent chloride diffusion coefficient in concrete

This paper provides an improved mathematical analysis of chloride penetration into concrete employing a time-dependent diffusion coefficient for the solution of Fick's second law of diffusion. In the paper the possible errors caused by the application of oversimplified mathematical expressions used in some models for the evaluation of service life of reinforced concrete structures are discussed. The results from this mathematical analysis demonstrate that some models based on the oversimplified error function complement (ERFC) solutions may easily overestimate the service life by orders of magnitude, especially when the age factor is high. Some chloride profiles after up to 10 years' field exposure were used to compare the oversimplified with the improved models. The results show that both the oversimplified and the improved models fairly well predict the 10 years' chloride ingress in Portland cement concrete, but the oversimplified ERFC model significantly underestimates the chloride ingress in concrete with fly ash.     

基于房室模型的氯离子在混凝土中的分布规律

为研究干湿循环作用下氯离子在混凝土中的分布规律,本文引入医用生物数学中的房室模型。该模型能动态地计算不同时间氯离子分别在混凝土对流区和扩散区的浓度。结合实际试验40 d、80 d、120 d和160 d的数据,用最小二乘法计算出该模型中相关系数的最小二乘解,进而可以得到该模型的解。模型解的形式并不复杂,便于在实际工程中推广应用。本文提供的模型为沿海工程海水干湿交替区域氯离子监测和耐久性设计提供参考。     

Effects of material and environmental parameters on chloride penetration profiles in concrete structures

On the basis of the transport mechanism of chloride ion, a prediction model of chloride penetration into concrete structures has been developed. The model includes the diffusion of chloride and its dependences on temperature, age, relative humidity, chloride binding and chloride convection by moisture transport. The experimental program has been set up to verify the model developed in the present study. Several series of concrete specimens were immersed in 3.5% chloride solutions for 15 weeks, and the chloride profiles of the specimens were measured and compared to the predicted chloride profiles. In addition, field measurements have been also conducted. From 10-year-old bridge piers, the chloride profiles in concrete under tidal zone were measured and compared with the predicted chloride profiles. The effects of chloride binding, relative humidity, temperature, exposure condition, and age-dependence on the chloride penetration in concrete were clarified from the present analyses. It was found from the present study that all these variables affect greatly the chloride penetration profiles in concrete. The comparison of the laboratory and field test data with the present theory confirms that the proposed model can be realistically used to predict the penetration of chloride ions into concrete structures under sea environments. Further, these results may be efficiently used for the realistic assessment and design for durability of concrete structures.     

Corrosion effects on pullout behavior of hooked steel fibers in self-compacting concrete

Fiber reinforced concrete structures are subjected to chloride and carbonation penetration that could initiate corrosion of steel fibers, with eventual pernicious consequences in terms of structural and durability performance. Cracks in concrete are known to hasten initiation of steel corrosion in reinforced concrete structures. The investigation of the impact of cracks on the corrosion initiation and the associated interfacial damage between concrete and steel fibers is important for understanding the mechanical behavior of steel fiber reinforced concrete.In the present work, with the aim of studying the corrosion action on the mechanical behavior of cracked Steel Fiber Reinforced Self-Compacting Concrete (SFRSCC), an experimental program was performed to characterize the corrosion of hooked-end steel fibers and to assess the fiber pullout behavior in cracked concrete, previously subject to the action of corrosion by exposure to aggressive chloride environment.     

Enhancement of barrier properties of cement mortar with graphene nanoplatelet

The transport properties of cement mortar with graphene nanoplatelet (GNP) are investigated experimentally in this study. GNP, a low cost carbon-based nano-sheet, was added to mortar at contents of 0, 2.5, 5.0 and 7.5%, by weight of cement. The water penetration depth, chloride diffusion coefficient and chloride migration were determined for cement mortar with GNP and compared with plain cement mortar specimens. Test results showed that the addition of 2.5% GNP can cause significant decrease of 64%, 70% and 31% for water penetration depth, chloride diffusion coefficient and chloride migration coefficients respectively. The reduced water and ions ingress can be partially attributed to a reduction in the critical pore diameter of about 30%. This refinement of the microstructure by the GNP is validated by the mercury intrusion porosimetry (MIP) results. The impermeable GNP also contributes to the reduced permeability due to the increased tortuosity against water and aggressive ions ingress.     

A strain-softening model for steel-concrete bond

A new analytical model is proposed to describe the bond strength between a ribbed reinforcement bar and concrete. The model is based on the partly cracked solution of a thick-walled cylinder exposed to radial internal pressure. Instead of considering the minimum crack number in previous solutions, smeared cracking and average stress-strain of concrete in tension are used in radial direction to describe the softening behavior of concrete. The results are compared with Tepfers' classic solutions and other previous solutions.     

氯离子扩散系数快速测试方法综述

氯离子扩散系数是评价混凝土结构抗氯离子渗透性能和使用寿命的重要指标,选择一种快速、准确的测试方法至关重要.本文以非稳态电迁移实验(Rapid Chloride Migration,RCM)为基准,与国内外常用的几种氯离子扩散系数快速测试法比较,总结每种方法的特点及适用范围,建立RCM法与其他测试方法的相关性.结果表明:...