混凝土设计参数对氯盐环境下钢筋锈蚀的影响研究

研究了混凝土设计参数包括水胶比、胶凝材料组成、保护层厚度和混凝土氯离子含量等对氯盐环境下混凝土中钢筋锈蚀的影响。通过模拟氯盐环境下混凝土中钢筋所处的锈蚀环境,并以一定的方法加速混凝土中钢筋的锈蚀,采用电化学测试手段(钢筋腐蚀电位和钢筋腐蚀电流密度)来评价各设计参数对钢筋锈蚀的影响。结果表明,水胶比越小、保护层厚度越大、混凝土氯离子含量越小、使用矿物掺合料能有效延缓钢筋开始锈蚀时间,并在不同程度上减小钢筋的锈蚀速率。试件在试验一段时间后被破损,将钢筋周围砂浆制样并进行SEM扫描电镜元素分析试验,进一步验证电化学测试方法的准确性及钢筋的锈蚀程度。     

盐雾环境下受荷混凝土中氯离子扩散试验

针对海洋盐雾环境,开展不同荷载水平作用下混凝土中氯离子扩散试验,测试混凝土中不同深度的氯离子含量,总结混凝土中氯离子扩散规律,拟合氯离子扩散荷载影响系数与构件应力状态之间的关系式。结果表明,拉应力作用下混凝土中氯离子含量增加,氯离子扩散系数增大;压应力作用下混凝土中氯离子含量降低,氯离子扩散系数减小;氯离子扩散系数随着时间增加逐渐减小,表面氯离子浓度随着时间的增加逐渐增大。考虑荷载影响的氯离子扩散模型可为预测实际工程受荷混凝土结构的使用寿命提供参考。     

考虑温度时变效应氯离子侵蚀混凝土的格子Boltzmann数值模型

对于长期暴露在氯盐环境下的混凝土结构,在外界温度影响下氯离子的侵蚀作用非常复杂,会影响钢筋锈蚀进程及其耐久性.为此,基于格子Boltzmann方法,采用双分布函数分别描述混凝土温度场和氯离子浓度场的演化过程,考虑外界温度的时变效应,建立氯离子侵蚀混凝土的数值模型.在此基础上,讨论水灰比、饱和度和昼夜温差等因素对氯离子侵蚀机制以及混凝土服役寿命的影响.研究表明:随着水灰比的增大,氯离子扩散系数逐渐增大,从而加剧了氯离子的侵蚀作用.混凝土饱和度越大,氯离子的扩散速率越快,从而导致混凝土的服役寿命缩短,当饱和度超过75%时,混凝土的服役寿命变化趋势逐渐趋于稳定.此外,昼夜温差越大,其对扩散系数的影响越显著,但总体而言,昼夜温差的变化对混凝土服役寿命的影响并不突出.     

氯盐侵蚀下铜矿渣混凝土高温后内部钢筋锈蚀规律

为探究高温及铜矿渣细骨料对混凝土中钢筋锈蚀模式的影响规律,对不同铜矿渣置换率的混凝土试件进行高温试验,然后采用干湿循环浸泡法对试件进行人工加速氯离子侵蚀试验,并利用电化学方法测量自然电位值以监测混凝土内部钢筋的锈蚀情况,最后测量混凝土内部氯离子含量及钢筋锈蚀率。结果表明：自然电位法可以较好地反映试件内部钢筋的实际锈蚀情况;高温破坏了混凝土抗氯离子侵蚀性能,从而导致混凝土试件中的钢筋锈蚀程度随经历温度的升高而增大;此外,高温下铜矿渣自身较大的膨胀变形及冷却后与水泥净浆间不协调收缩的综合作用进一步破坏了混凝土微结构,使钢筋锈蚀率随着铜矿渣置换率的提高而增大;最后建立了氯盐侵蚀下铜矿渣混凝土高温后内部钢筋锈蚀深度拟合公式。     

弯曲荷载与氯盐侵蚀共同作用下的预应力混凝土梁耐久性能研究

当前在进行海工混凝土结构受氯盐侵蚀的耐久性研究时往往忽略了混凝土力学损伤的影响,这可能会导致高估结构的耐久性能。该文提出了在数值分析中引入损伤因子实现综合考虑结构损伤影响及氯盐侵蚀共同作用的分析方法,通过干湿循环的氯盐侵蚀试验与数值模拟分析,研究遭受氯盐腐蚀且受到长期荷载作用的预应力混凝土梁的耐久性能。结果表明,数值分析结果和试验结果吻合良好。参数分析结果表明:0.2mm宏观裂缝的出现对氯离子浓度有着较为显著的影响,未出现宏观裂缝时,拉应力对氯离子输运的影响较为有限;荷载比的大小影响结构的损伤范围和程度,进而影响氯离子在混凝土内的传输;梁构件施加预应力后,在15%和30%两种低荷载比状态下延缓其力学损伤（裂缝）的开展,减小荷载作用对氯离子扩散系数的增大效应,有效提高构件的抗氯盐侵蚀性能,当荷载比增加到45%以上,普通钢筋混凝土梁相对于预应力梁的氯离子浓度增比降至10%以内,这表明较高应力比下预应力对梁抗氯盐侵蚀能力的影响减弱。     

混凝土内氯离子扩散影响因素的研究综述

随着建筑行业的迅速发展,钢筋混凝土结构得到了广泛应用.但钢筋混凝土结构长期处在氯盐环境中易发生钢筋腐蚀现象,从而降低结构的耐久性.因此,全面系统地认识混凝土内氯离子扩散的影响有助于解决结构氯盐侵蚀问题.氯离子在混凝土中的传输机制极其复杂,其中扩散是氯离子传输的主要机制.文章梳理了国内外近年来关于混凝土内氯离子扩散的研究进展,重点评述了混凝土内氯离子扩散的影响因素,笔者认为低水灰比、加入掺和料、适当增加保护层厚度、限制裂缝宽度等基本措施可以有效减缓钢筋腐蚀.若结构处于恶劣环境中,在采取基本措施的基础上,还需要进一步采取附加措施,主要包括添加缓蚀剂、进行表面涂层、电化学处理等.这些措施得到了一系列的理论及试验验证,为工程应用提供了可行性参考.     

海洋环境下混凝土中钢筋锈蚀机理及监测技术概述

2014年我国由于腐蚀产生的损失约为21 278亿元,占国家GDP的3.34%。海洋环境中氯离子渗透至混凝土内部导致钢筋锈蚀是海洋环境下混凝土结构损伤的重要原因。海洋不同腐蚀区域的离子浓度、氧气浓度及海水干湿循环作用时间各不相同,这使得海洋不同腐蚀区域中暴露混凝土的氯离子传输速度和分布规律、钢筋锈蚀产物及锈蚀模式、钢筋锈蚀速率存在显著差异。其中海洋浪溅区和高潮位区域因氧气充足、海水干湿循环作用剧烈及浪溅作用导致钢筋混凝土更易破坏。钢筋锈蚀产物体积是原始体积的2~6倍,持续增加的锈蚀产物将导致混凝土开裂、保护层剥落并进一步加速钢筋锈蚀;考虑钢筋非均匀锈蚀、锈蚀产物填充效应、钢筋及混凝土性能的钢筋混凝土锈胀开裂模型将更加精确。根据海洋不同腐蚀区带特点、钢筋混凝土性能及受荷情况,建立不同腐蚀区域中混凝土的氯离子传输模型,钢筋锈蚀速率模型和混凝土锈胀开裂模型有助于准确预测海洋环境下钢筋混凝土的服役寿命。 通过对混凝土中钢筋锈蚀的检测与监测有助于实时了解混凝土的服役状态。采用线性极化、电化学噪声和电化学阻抗谱等电化学方法可以较好地检测钢筋锈蚀状态、获得混凝土中钢筋的锈蚀速率。基于电化学原理开发的阳极梯和环形电极、基于钢筋锈胀应力测试的光纤监测技术以及基于数字图像技术获得混凝土中钢筋锈蚀应力应变场,有助于实现对混凝土中钢筋锈蚀的监测,并且部分已应用于海洋工程。相比于普通钢筋,锈蚀钢筋的导电率和导磁率均显著降低,采用电磁感应原理开发钢筋锈蚀装置实现了暴露在海水中的普通钢筋和耐蚀钢筋磁通量变化值与钢筋质量损失线性关系的建立。这也为更精确监测混凝土中钢筋锈蚀全过程、实现混凝土中钢筋锈蚀源定位及损伤程度识别提供可能。因此,综合利用氯离子、pH微电极等实现混凝土内部微环境监测,开发先进的钢筋锈蚀监测传感器实现混凝土中钢筋锈蚀源和锈蚀速率监测,通过图像监测技术实现钢筋锈蚀诱导混凝土开裂过程监测。综合上述措施将实现对钢筋混凝土结构腐蚀的全过程监测,并为海洋钢筋混凝土服役寿命预测模型的验证与修正提供依据,同时为海洋环境混凝土的耐久性评估系统提供预警机制。     

荷载作用下开裂混凝土中多离子传输的数值研究

临海地区的混凝土结构因氯离子侵蚀会导致严重的耐久性问题,尤其是在同时承受荷载作用的情况下。服役中的混凝土往往存在初始缺陷,荷载的持续作用会使得混凝土内部的孔隙结构进一步粗化并引起开裂,进而加剧氯盐侵蚀。同时,混凝土孔隙液中存在的多种离子也会影响到氯离子的传输。为了深入研究荷载-氯盐影响下的混凝土结构耐久性劣化,本工作针对前述机理开展多相数值研究,通过全面考虑荷载引起的混凝土开裂和孔隙损伤,以及异种离子间的电化学耦合效应的共同影响,提出了经过第三方试验验证的荷载-多离子传输耦合模型。研究发现,荷载大小和荷载施加方式的改变不仅会影响孔隙结构和裂缝形态,也会使静电势的分布产生差异,进而共同影响离子浓度的变化。荷载作用下的多离子电化学耦合效应会显著影响离子的传输规律,仅考虑单一离子会导致氯离子摄入量的预测偏低。荷载的作用同时还会放大环境条件(如盐溶液浓度)对氯传输的影响。     

纳米氧化铝提升海洋环境高速铁路桥梁混凝土结构服役寿命研究

氯离子传输并诱导内部钢筋锈蚀是海洋环境高速铁路桥梁混凝土结构耐久性失效的主要原因之一,增大混凝土氯离子传输阻力是提升混凝土结构服役寿命的根本途径。纳米技术与纳米材料的发展为混凝土材料高性能化提供了新的可能。本工作研究了纳米氧化铝(NA)对砂浆氯离子电迁移系数(D_RCM)与自然扩散系数的影响,基于氯离子等温吸附曲线,建立了考虑氯离子结合参数的氯盐传输模型,分析了NA对氯盐侵蚀环境下混凝土结构服役寿命的影响。结果表明：适宜掺量的NA可降低砂浆氯离子传输性能;承台桩基础混凝土钢筋保护层厚度为60 mm时,在C45混凝土中掺入2%(质量分数)NA后,仅考虑氯离子侵蚀时混凝土预期服役寿命由68年提升至107年。NA为提升海洋环境下高速铁路混凝土结构服役寿命提供了新的技术途径。     

海洋环境下锈蚀混凝土的新型电化学除氯工艺参数研究

电化学除氯(ECE)是一种针对锈蚀混凝土的无损处理技术,可以降低钢筋的腐蚀风险,提高混凝土耐久性,但对电化学除氯关键参数缺乏深入研究.为进一步探索电化学除氯技术的关键参数,以不锈钢网为阳极材料,研究了电解液、电流密度和钢筋类型对电化学除氯效率的影响.分别对含有耐蚀钢筋和普通碳素钢筋的混凝土进行了电化学除氯处理,以研究不同种类钢筋对混凝土的除氯作用.采用扫描电子显微镜(SEM)分析了电化学除氯前后混凝土的微观形貌演变.试验结果表明:电化学除氯效率随电流密度的增加而提高,综合考虑后确定最佳的电流密度为2 A/m2.Ca(OH)2溶液作为电解质溶液时,除氯效率最高.同时,试验发现钢筋类型对混凝土的除氯效率没有明显的影响.     

Comparative study of different test methods for reinforced concrete durability assessment in marine environment

There are many different test methods to assess reinforced concrete durability. As in marine environment reinforcement corrosion due to chloride attack is the most important degradation process, chloride penetration rate has been compared with different durability tests results (concrete strength, porosity, water absorption, water penetration depth under pressure, capillarity, water and oxygen permeability) carried out on concrete cores obtained from the caissons of seven Spanish wharves. Data have been studied separately, depending on concrete location (chloride penetration rate is faster in submerged concretes than in tidal zone concretes) and cement type (mineral admixtures reduce permeation rate due to pore size refinement). Results show that it is advisable to control concrete water tightness through water penetration under pressure test; additionally, in order to make sure a slow corrosion rate, it should be advisable to control oxygen permeability in tidal zone concretes.     

Nonlinear formulation based on FEM,Mazars damage criterion and Fick’s law applied to failure assessment of reinforced concrete structures subjected to chloride ingress and reinforcements corrosion

Structural durability is an important design criterion, which must be assessed for every type of structure. In this regard, especial attention must be addressed to the durability of reinforced concrete (RC) structures. When RC structures are located in aggressive environments, its durability is strongly reduced by physical/chemical/mechanical processes that trigger the corrosion of reinforcements. Among these processes, the diffusion of chlorides is recognized as one of major responsible of corrosion phenomenon start. To accurate modelling the corrosion of reinforcements and to assess the durability of RC structures, a mechanical model that accounts realistically for both concrete and steel mechanical behaviour must be considered. In this context, this study presents a numerical nonlinear formulation based on the finite element method applied to structural analysis of RC structures subjected to chloride penetration and reinforcements corrosion. The physical nonlinearity of concrete is described by Mazars damage model whereas for reinforcements elastoplastic criteria are adopted. The steel loss along time due to corrosion is modelled using an empirical approach presented in literature and the chloride concentration growth along structural cover is represented by Fick’s law. The proposed model is applied to analysis of bended structures. The results obtained by the proposed numerical approach are compared to responses available in literature in order to illustrate the evolution of structural resistant load after corrosion start.     

氯离子及迷流共同作用下持荷盾构管片钢筋锈层形态

针对盾构隧道结构在服役期间可能出现钢筋锈蚀的问题,考虑杂散电流、氯离子及外部荷载共同作用,建立了电-化-力三场耦合的三维管片数值模型,分析了多因素作用下拱腰部位管片钢筋的锈蚀率变化规律及锈层分布形态。研究表明: 1）靠近管片外侧钢筋的锈蚀率比内侧的大,不同区域的钢筋出现最大锈蚀率的位置与连心线的夹角在0°~52°之间。2）在荷载作用下管片钢筋的锈蚀率与体积应变有关,管片中部的锈蚀率大于两端的锈蚀率。3）在钢筋脱钝情况下,管片钢筋的锈蚀率随阴阳极电势差增大呈线性增加,随氯离子含量增大呈对数增加。4）在三种因素共同作用下管片钢筋的锈层分布呈偏心圆形态,且偏心圆圆心坐标及半径的大小与钢筋不均匀锈蚀系数及最大腐蚀电流密度有关。     

Probabilistic models applied to safety assessment of reinforced concrete structures subjected to chloride ingress

This paper addresses the analysis of probabilistic corrosion time initiation in reinforced concrete structures exposed to chloride penetration. Structural durability is an important criterion that must be evaluated for every type of structure. Considering reinforced concrete members, chloride diffusion process is widely used to evaluate durability, especially when these structures are constructed in aggressive atmospheres. The chloride ingress triggers the corrosion of reinforcements; therefore, by modelling this phenomenon, the corrosion process can be better evaluated as well as the structural durability. The corrosion begins when a threshold level of chloride concentration is reached at the steel bars of reinforcements. Despite the robustness of several models proposed in the literature, deterministic approaches fail to predict accurately the corrosion time initiation due the inherent randomness observed in this process. In this regard, structural durability can be more realistically represented using probabilistic approaches. A probabilistic model capable to model chloride ingress is presented in this paper. The chloride ingress is modelled using the Fick’s diffusion law. This law simulates the chloride diffusion process considering time-dependent effects. The probability of failure is calculated using Monte Carlo simulation and the first order reliability method with a direct coupling approach. Some examples are considered in order to study these phenomena. Moreover, a simplified method is proposed to determine optimal values for concrete cover.     

Utilization of fly ash concrete in marine environment for long term design life analysis

This paper presents the performance of 7-year fly ash concrete exposed to hot and high humidity climate in marine conditions. Control concrete and fly-ash concrete cube specimens of 200 mm were cast and steel bars of 12 mm in diameter and 50 mm in length were embedded at various cover depths. The concrete specimens were exposed to tidal zone of marine environment in the Gulf of Thailand. The concrete specimens were tested for chloride penetration profile, chloride content at the position of embedded steel bar, and corrosion of embedded steel bar after being exposed to tidal zone of sea water up to 7 years. Consequently, these experimental data were used to generate the empirical equation for predicting long term required cover depth of cement and fly ash concretes to protect against the initial corrosion of reinforcing steel in a marine environment.     

A model for a long-term diffusion of multispecies in concrete based on ion–cement-hydrate interaction

A model for simulating a long-term diffusion in concrete under submerged conditions has been developed. The model focuses on the interaction between cement hydrates and electrolyte solution adopting the physical properties of concrete as alternative parameters for estimating long-term diffusion. The model was verified by the large variety of long-term experimental data involving verification of cement hydrates, porosity properties, pH value, element distribution, and chloride penetration for sulphate-resistant Portland cement (SRPC) concretes. Evaluating impacts of hundreds of years’ exposure on concrete durability, a simulation was also performed for an exposure time of 500 years. The results confirmed the importance of ion–cement-hydrate interaction in the evaluation of a long-term diffusion of harmful substances such as chlorides into concrete. The simulation results show also that the solid phases of the SRPC concrete mix that was used are stable in the long-term. The threshold concentration of chlorides which may initiate reinforcement corrosion defined as Cl^?/OH^? ratio could be exceeded in concrete after moderate exposure period under the conditions investigated with typical protective concrete layers. Using of case-specific values for a threshold chloride content and evaluating the method used for estimating the initiation time of reinforcement corrosion are recommended in the structures with a long-design service life.     

Comparative performance of chloride attenuating and corrosion inhibiting systems for reinforced concrete

This paper reports a laboratory-based study carried out to compare the performance of various proprietary concrete protection systems, designed to reduce chloride ingress and reinforcement corrosion. These include: controlled permeability formwork (CPF), a silane/siloxane hydrophobic surface treatment (S/S), an integral liquid waterproofing admixture (WP) and a corrosion-inhibiting chemical admixture (CI). Tests were carried out on a Portland cement (PC) concrete (40 N/mm² design strength) for chloride diffusion index (using a two-cell compartment accelerated test) and, under cyclic wetting and drying conditions, total chloride content at cover depth (25 mm) and corrosion of carbon steel in reinforced concrete specimens (using half-cell potential and corrosion current density (polarisation resistance) measurements). The results indicate that for all protection systems, resistance to chloride ingress was improved, with the greatest benefits noted for the S/S and CPF concretes. Corrosion levels occurring for these generally followed the ranking of chloride ingress rates. The CI was found to reduce the rate of chloride ingress and to give lowest corrosion current densities in relation to chloride contents. This system appeared to provide best overall performance. The practical implications of the results are considered in terms of equivalence of the systems to an increase in design strength or cover depth,i.e. parameters used for specifying concrete durability in standards, and the wider issues relating to their selection and use are reviewed.     

Convolutional Neural Network-Based Regression for Predicting the Chloride Ion Diffusion Coefficient of Concrete

The durability performance of reinforced concrete (RC) building structures is significantly affected by the corrosion of the steel reinforcement due to chloride penetration, thus, the chloride ion diffusion coefficient should be investigated through experiments or theoretical equations to assess the durability of an RC structure. This study aims to predict the chloride ion diffusion coefficient of concrete, a heterogeneous material. A convolutional neural network (CNN)-based regression model that learns the condition of the concrete surface through deep learning, is developed to efficiently obtain the chloride ion diffusion coefficient. For the model implementation to determine the chloride ion diffusion coefficient, concrete mixes with w/c ratios of 0.33, 0.40, 0.46, 0.50, 0.62, and 0.68, are cured for 28 days; subsequently, the surface image data of the specimens are collected. Finally, the proposed model predicts the chloride ion diffusion coefficient using the concrete surface image data and exhibits an error of approximately 1.5E−12 /s. The results suggest the applicability of proposed model to the field of facility maintenance for estimating the chloride ion diffusion coefficient of concrete using images.     

Durability of lightweight OPS concrete under different curing conditions

The use of waste materials and by products from different industries for building construction has been gaining increased attention due to the rapid depletion of natural resources. It has been found that oil palm shell (OPS), which is a waste from the agricultural sector, can be used as coarse aggregate for the manufacture of structural lightweight concrete. However, for OPS concrete to be used in practical applications, its durability needs to be investigated. Therefore, this paper presents the durability performance of OPS concrete under four curing regimes. The durability properties investigated include the volume of permeable voids (VPVs), sorptivity, water permeability, chloride diffusion coefficient and time to corrosion initiation from the 90-day salt ponding test, and Rapid Chloride Penetrability Test (RCPT). Results showed that the durability properties of OPS concrete were comparable to that of other conventional lightweight concretes and proper curing is essential for OPS concrete to achieve better durability especially at the later ages.     

Specifying concrete for chloride environments using controlled permeability formwork

The paper describes a study carried out to explore how controlled permeability formwork (CPF) can be used within existing concrete durability specifications (mix limitations) for chloride environments. Tests were carried out to consider (i) chloride diffusion rates and, under wetting and drying conditions, (ii) rates of chloride contamination build up at cover depth and (iii) reinforcement corrosion. The effects of CPF were measured against design strength, cover depth and cement type of concrete cast against ply-wood formwork (impermeable formwork—IMF). The use of CPF liner on formwork was found to significantly enhance chloride and corrosion resistance of concrete. Moreover, the results demonstrated that CPF could be used within the BS 5328 durability framework for chloride environments to allow either a 20 mm cover reduction (50 to 30 mm) at fixed design strength (40 N/mm²), or a reduction in design strength of 10 N/mm² (50 to 40 N/mm²) at fixed cover depth (30 mm). It was additionally found for Portland cement (PC) concrete that the use of CPF gave equivalent performance to concretes containing PFA and GGBS as constituents of cement and a ternary cement comprising both materials, cast against ply-wood formwork. This suggests that the ‘trade offs’ within BS 8500 for PC/PFA and PC/GGBS cements in chloride environments, could also be permitted for CPF concrete containing PC.