裂缝对混凝土内氯离子扩散和钢筋锈蚀的影响

为了研究受力裂缝对混凝土内氯离子扩散的影响,对8根开裂(预制裂缝和受弯裂缝)的钢筋混凝土梁试件进行了盐溶液干湿循环试验;并采用半电池电位法和RCT(快速氯离子含量检测)法,分别对梁内钢筋的锈蚀状态以及各裂缝处氯离子含量进行了检测;最后采用ADINA有限元软件对开裂混凝土内氯离子的二维扩散行为进行模拟分析。试验和模拟结果表明:1)裂缝加速了氯离子的侵入,导致钢筋过早的开始锈蚀,且为局部氯离子的二维扩散提供了条件;2)裂缝宽度越大或间距越小,其对氯离子侵入引起的钢筋锈蚀影响程度越大;3)有限元模型的计算与试验结果吻合较好。     

Laboratory studies on influence of transverse cracking on chloride-induced corrosion rate in concrete

To clarify the corrosion mechanism of steel induced by transverse crack, a study on the influence of crack widths and epoxy coating on corrosion of steel bars in cracked concrete is presented here. Microcell and macrocell corrosions of bars were investigated on single crack specimens with crack widths of 0.08, 0.26, 0.38 and 0.94 mm. The entire study was carried out in an artificially created chloride ion-induced corrosion environment. The results show that the steel in cracks was activated once the transverse crack occurred on concrete element, and the macrocell corrosion must co-exist with microcell corrosion of reinforcements in test specimens with transverse crack. The macrocell current of steel elements were separated from the crack width, and the wider the transverse crack is, the higher corroded area and the greater microcell current of the rebar is. Oxygen and water go into concrete through crack instead of through concrete cover. The epoxy coating cannot prevent the occurrence and propagation of crack, so it was not effective to prevent corrosion of steel bars in cracked concrete.     

Permissible crack widths in steel fibre reinforced marine concrete

The paper presents some results from a continuing study of the marine durability of steel fibre reinforced concrete. The overall aim of the investigation is to develop the material for marine applications. The results reported here pertain to pre-cracked specimens of steel fibre reinforced concrete which were exposed to wet-dry cycles of marine spray in the laboratory simulating tidal zone conditions of exposure. Two types of concrete mixes were used in the investigation—one with standard concrete constituents and OPC and the second replacing about 26% of cement with pfa. The cement content of the mixes was 590 and 435 kg m⁻³, respectively. Fibre reinforcement was provided by means of low carbon steel fibres and melt extract steel fibres at a v _f ℓ/d ratio of 100 and 147. Prism specimens were manufactured and these were precracked to induce cracks of width ranging between 0.03 and 1.73 mm. After cracking, both sealed and unsealed specimens were exposed to laboratory marine spray cycles using sea water. Some control specimens were cured in the laboratory air throughout. Tests were carried out after 650 marine cycles (450 days) and 1450 marine cycles (900 days). Based on data on flexural strength, energy absorption capacity, stiffness and state of corrosion of the fibres, recommendations are made regarding suitable permissible crack widths for the design of steel fibre reinforced concrete for marine applications. The results indicate that a permissible crack width of 0.2 mm is satisfactory for concrete reinforced with melt extract fibres. A smaller value is recommended for concrete reinforced with low carbon steel fibres. Complete healing of open cracks of small widths is observed under exposure to marine cycles.     

Effect of crack opening on carbon dioxide penetration in cracked mortar samples

The paper addresses the effect of crack opening on the ability of carbon dioxide to diffuse along a crack. The experimental tests were carried out on mortar samples. A mechanical expansive core was used to generate cracks of constant width across the thickness of the sample. Cracked specimens with crack openings ranging from 9 to 400 μm were exposed to accelerated carbonation for 65 days. Then they were removed to determine the depth of carbonation perpendicular to the crack path. Theses depths were compared to the measured ones on the reference samples. The results show that crack opening significantly influences the ability of carbon dioxide to diffuse along the crack. Indeed, the carbonation depth perpendicular to the crack wall indicates a lower capacity to diffuse in cracks less than 41 μm in width. For crack openings ranging from 9 to 41 μm, there was still diffusion along the crack path. Moreover, carbonation of the interface between steel and mortar was observed inducing a depassivation of the reinforcement. For the duration of the experiments, there was no diffusion in crack openings of less than 9 μm. The effect of interlocking phenomena between the fracture surfaces on the ability of carbon dioxide to diffuse along the crack, was also studied. The results showed that interlocking phenomena in cracks is the main factor limiting the diffusion of carbon dioxide in fine cracks.     

Evaluating ion diffusivity of cracked cement paste using electrical impedance spectroscopy

Cracking can significantly accelerate mass transport in concrete and as such, impact its durability. This paper is aimed at quantifying the effect of saturated cracks on ion diffusion. Electrical conductivity, measured by electrical impedance spectroscopy (EIS), was used to characterize the diffusion coefficient of fiber-reinforced cement paste disks that contained one or two through-thickness cracks. Crack widths in the range 20–100 μm were generated by controlled indirect tension test. Crack profiles were digitized and quantified by image analysis to determine crack volume fraction and average crack width. Crack connectivity (e.g., inverse of tortuosity) was calculated from the conductivity results measured by EIS. The results suggest that the diffusion coefficient of cracked samples is strongly and linearly related to the crack volume fraction; but is not directly dependent on crack width. Crack tortuosity does reduce the ion diffusion through cracks, but its impact is not very significant. Overall, the most important parameter governing ion diffusion in saturated cracked concrete is the volume fraction of cracks. Theoretical justifications of these observations are also provided.     

氯离子在饱和混凝土裂缝中的扩散系数分析

裂缝的存在加速混凝土中氯离子的侵蚀速度,进而加快钢筋混凝土结构力学性能的劣化速度,因此钢筋混凝土结构耐久性设计中需要考虑裂缝的影响。现有文献中关于裂缝对氯离子扩散行为定量影响的试验结果具有很大的离散性,给出的拟合公式带有局限性。对此,该文建立了一个统一的裂缝中氯离子扩散系数D_cr与裂缝宽度w_cr之间的定量关系式。该关系式各参数具有明确的物理意义,对应的曲线圆滑且不存在拐点。与已有试验数据吻合良好,验证了公式的合理性。数值模拟了开裂混凝土中氯离子扩散行为,数值结果与文献结果吻合良好,证明了方法的有效性。     

Simulation studies of methods to delay corrosion and increase service life for cracked concrete exposed to chlorides

The ingress of chlorides in reinforced concrete leads to the onset of steel reinforcement corrosion and eventually compromises a structure’s integrity. To extend its service life and improve safety, it is crucial to develop sound repair strategies for our nation’s infrastructure. In this paper, results are presented for numerical simulations to study the effectiveness of fillers for repair of cracks in concrete, so as to delay the onset of corrosion in reinforcing steel. Concretes without cracks and with either a 50 μm or 500 μm wide crack located directly above the steel reinforcement are simulated, with the addition of silica fume, a corrosion inhibitor, or epoxy-coated reinforcement being considered as additional scenarios. The effectiveness of the crack filler depends not only on its inherent diffusivity with respect to chloride ions, but also on its ability to penetrate and fill the damaged zone or interface between the open crack region and the bulk concrete. Additional simulations indicate that using continuum models instead of models that include details of the rebar placement can lead to underestimating the chloride concentration and overestimating the service life. Experiments are needed to study the ingress of chlorides in damaged (interfacial) regions adjacent to the crack or at the reinforcement surface, as the local transport properties of these regions can significantly influence service life predictions.     

Influence of corrosion rate on the mechanical interaction of reinforcing steel,oxide and concrete

This study offers an experimental and numerical analysis of the influence of the current density on oxide formation and the resulting cracking of reinforced concrete subjected to accelerated corrosion. The specimens were idealized reinforced concrete prisms in which a calibrated steel tube replaced the standard ribbed bar reported in most published works. This allowed the evolution of the inner diameter and volume of the tube to be recorded, providing relevant information on the mechanical interactions of the steel–oxide–concrete system. In addition, the information recorded during the tests also included the evolution of the corrosion depth and width of the main crack that grew across the concrete cover. Furthermore, the crack pattern after the corrosion process was analyzed by using slices of the specimens impregnated with fluorescent resin. Experiments were conducted for several current densities. The results show that decreasing the corrosion current density results in an increase in the corrosion depth necessary for crack initiation, a smaller effective volumetric expansion and a more irregular crack pattern. When combined with finite-element simulations carried out by using a model that reproduces the expansive behavior of the oxide and the cohesive fracture of concrete, the results show that the effective expansion factor of the oxide becomes smaller as the density of current is reduced. This would suggest that the corrosion rate affects both the pressure build-up in the growing oxide layer and, closely linked to this, the diffusion of oxide within the pores and cracks in the surrounding concrete.     

Damage model for simulating chloride concentration in reinforced concrete with internal cracks

We present a method to simulate, in three dimensions, the concentration of chloride ions that penetrate into concrete with internal cracks. The method comprises the crack-propagation analysis of concrete and the diffusion analysis of chloride ions. A finite-element model with a damage model that is based on fracture mechanics for concrete was applied in the crack-propagation analysis, and we were able to reproduce the three-dimensional geometry of the internal cracks. Chloride-ion transfer through internal cracks was simulated by diffusion analysis with the simultaneous consideration of damage, and a diffusion coefficient that was expressed as a function of the damage variable obtained from crack-propagation analysis. We present a formulation of crack-propagation analysis by using the damage model and unsteady-diffusion analysis in consideration of damage. We also present a verification analysis of internal cracking in concrete to demonstrate that the crack width and the chloride concentration can be evaluated without mesh dependency. This is followed by a validation analysis. A comparison between the numerical and experimental results shows that the proposed method enables the high-accuracy simulation of chloride penetration into concrete with internal cracks.     

Modeling of chloride-induced corrosion in reinforced concrete structures

A numerical procedure is presented in this paper for the prediction of chloride induced steel corrosion in reinforced concrete structures. Finite element analysis is introduced for the mechanical analysis of crack initiation and propagation due to the accumulation of corrosion products around the reinforcement, while the alternating direction implicit method is used to solve the transport equations of temperature, humidity, chloride ions and oxygen in concrete. Based on the assumption of a uniform distribution of corrosion products, a self-adaptation process for the variation of boundary conditions is proposed through a series of diffusion analyses together with crack propagation in concrete. Therefore, the interaction between the corrosion rate and the propagation of cracks in concrete is taken into account. Furthermore, a numerical program is developed and a case study involving bridge deck exposed to a marine environment in Hong Kong is investigated. The results show that interactive behavior has a significant effect on the corrosion rate of the reinforcement, and the non-cracking model significantly overestimates the service life of structures.     

Non-steady state chloride diffusion in concrete with different crack densities

With an increasing concern about long-term durability, chloride transport and service-life prediction of cracked concrete are now a hot area in concrete research. However, only few researches up to now investigated the effect of crack density on chloride transport in cracked concrete. Nevertheless, in real concrete structures, multiple cracks instead of a single crack are most commonly observed. The present study investigates non-steady state chloride diffusion in concrete samples with different crack densities prepared by a non-destructive notch method. The relationship between diffusion coefficient and crack density is discussed. Water and acid soluble chloride contents increase significantly with increasing crack density up to a certain value. A linear function is found for the relationship between diffusion coefficient determined by acid soluble chloride and crack density. A piecewise function including linear and exponential function is proposed to describe the relationship between diffusion coefficient determined by water soluble chloride and the crack density.     

Effects of crack properties and water-cement ratio on the chloride proofing performance of cracked SHCC suffering from chloride attack

This study aims at developing an SHCC mixture that offers improved workability and high rebar corrosion proofing performance while ensuring moderate tensile ductility. Specimens were made from several mixtures with part of the cement replaced with limestone powder, and with a reduced fiber content. Mechanical properties and corrosion proofing performance under tensile stress were examined to determine the effects of chloride in a simulated situation under uniaxial tensile strain. The results indicated that the fiber content in the mixture had no influence on the crack properties of steel reinforced SHCC under tensile load. It was found that the chloride proofing performance of SHCC with multiple cracks was affected by the crack properties such as the number of cracks and the accumulated crack width. Reducing the water-cement ratio was effective to enhance the chloride proofing performance of SHCC.     

Serviceability of corrosion-affected RC beams after patch repairs and FRPs under load

Substantial resources continue to be used worldwide to repair corroding reinforced concrete (RC) structures with the intent to meet or extend their design service life. Experimental observations have shown that the rate of steel corrosion as well as the extent of corrosion damage on RC structures is very much influenced by the level of the sustained load during the corrosion process. It is therefore expected that the effectiveness of interventions to maintain the serviceability of corroded structures will also be influenced by the level of the sustained load. This paper presents experimental results and a discussion on the effectiveness of patch repairs and fibre reinforced polymer (FRP) bonded plates to restore the serviceability state of quasi-full-scale RC beams (153 × 254 × 3,000 mm) that were corroded and repaired whilst under sustained service loads. Steel corrosion was induced using two sequential corrosion processes; firstly accelerated corrosion by impressing an anodic current followed by natural corrosion. It was found that under natural steel corrosion; (1) no additional widening of corrosion cracks was observed when the load applied on test beams was reduced without repairs. However, increasing the load increased the rate of corrosion crack widening; (2) despite having a significantly lower rate of steel corrosion, the rate of widening of corrosion cracks was at times comparable to the corresponding rate under the accelerated corrosion process; and (3) strengthening with FRPs without patch repairs increased the load-bearing capacity of the beams but worsened their serviceability state in terms of crack widths. It was concluded that strengthening of corrosion-affected RC structures with FRPs should be done subsequent to patch repairs.     

Analysis of crack propagation due to rebar corrosion using RBSM

Cracking behavior due to rebar corrosion in concrete specimens having a single rebar is evaluated experimentally and analytically. In the experiments, in which corrosion was induced electronically, the propagation of cracks (including internal crack patterns and surface crack widths) was monitored. In addition, deformation of the specimen surface was measured using a laser displacement meter. In the analysis, a three-dimensional Rigid-Body-Spring Method (RBSM), combined with a three-phase material corrosion–expansion model, is proposed to simulate crack propagation due to rebar corrosion. The effects of the properties of corrosion products such as elastic modulus, penetration of corrosion products into cracks, and local corrosion after cracking of the concrete are investigated. Cracking behavior due to rebar corrosion is simulated reasonably well. The simulations using RBSM provide insight into the mechanisms of crack initiation and propagation due to rebar corrosion.     

Crack width for thick offshore plates

Using thick concrete covers in offshore and nuclear containment applications is increasing because it is a durability issue. Most crack width models indicate that increasing concrete covers results in increased crack spacing and hence increased crack width this means that thick concrete covers are detrimental to crack control. In this paper, tests were conducted on two groups of thick plates. Group I, included five specimens that had two concrete covers, 60 and 70 mm. Group II, included four thick heavy reinforced specimens; all specimens in this group had a clear concrete cover of 70 mm. Using thick concrete covers is a common practice in offshore and containment structures for nuclear power generation. The objective of testing both groups is to measure flexural crack widths under different load levels and, most importantly, under service loads. Group I was intended primarily to investigate the effect of increasing the cover and bar spacing on the crack width. Group II represents a unique experimental investigation in assessing the magnitude of crack width in full-scale thick plates under service loads. An analytical investigation is presented in this work. The main focus of this study is to evaluate the available codes’ models for estimating the crack width of thick concrete plates having thick concrete covers used for offshore and nuclear containment structure applications. It was concluded that crack control can still be achieved by limiting the spacing of the reinforcing steel despite using thick concrete covers.     

基于分形理论分析裂缝形态对纤维/混凝土渗透性的影响

分形维数可以表征裂缝形态,能够用来分析混凝土裂缝断面的粗糙程度。裂缝形态对开裂混凝土的渗透性有重要影响,为研究这种影响,利用劈裂试验获得不同宽度的裂缝,使用不同的纤维种类,并设置多种纤维掺量,得到粗糙程度不同的裂缝断面,通过水渗透试验测量不同裂缝宽度时混凝土的渗透系数。采用激光扫描仪扫描裂缝断面并重构3D断面几何形态,采用立方体覆盖法计算断面分形维数。采用分形维数将实测裂缝宽度和有效裂缝宽度联系起来,联立达西定律和泊肃叶定律建立开裂混凝土渗透系数和分形维数的函数关系。结果表明:使用相同的网格划分法,分形维数随着纤维掺量的增加而增大;渗透系数随着纤维掺量的增加而减小;函数关系式中分形维数的指数绝对值和修正系数都随裂缝宽度增加而减小。      

聚丙烯纤维对混凝土损伤渗透特性的影响

为研究损伤开裂后聚丙烯纤维/混凝土(PPF/PC)的渗透特性,通过圆盘劈裂试验预制了不同宽度的裂缝(100~400 μm),比较了PPF掺量对裂缝曲折度的影响。利用自行设计的渗透试验装置对混凝土损伤后的渗透率进行了研究,分析了不同PPF掺量、不同水压力下混凝土的损伤渗透率的变化规律。通过研究发现,加入PPF后,混凝土脆性较低,内部裂缝更易控制,裂缝曲折度相对于PC更高,且与PPF掺量成正比;相同水压力,相同有效裂缝宽度条件下,随着PPF掺量的增加,混凝土损伤渗透率降低,PPF的存在能够有效提高混凝土损伤后的抗渗性能;相同水压力下,相同PPF掺量的混凝土试件损伤渗透率整体上与有效裂缝宽度成正比;不同水压力下,相同PPF掺量的PPF/PC损伤渗透率在同一有效裂缝宽度情况下,随水压力增加而减小;修正后的泊肃叶渗流模型可以更好地评价PPF/PC损伤渗透特性。      

Corrosion Process and Mechanisms of Corrosion‐Induced Cracks in Reinforced Concrete identified by AE Analysis

Abstract: Concrete structures could suffer from the corrosion of reinforcing steel bars (rebars) because of the penetration of chloride ions. For crack detection and damage evaluation in concrete, acoustic emission (AE) techniques have been extensively applied to concrete and concrete structures. In the corrosion process of reinforced concrete, it is demonstrated that continuous AE monitoring is available to identify the onset of corrosion and the nucleation of concrete cracking because of the expansion of corrosion products. At the latter stage, the expansion of corrosion products generates corrosion‐induced cracks in concrete. The generating mechanisms of these cracks are studied in accelerated corrosion tests of reinforced concrete beams. Kinematics of microcracks are identified by SiGMA (Simplified Green’s functions for Moment tensor Analysis) analysis of AE. It is demonstrated that AE activity at the onset of corrosion and at the nucleation of corrosion‐induced cracks is in remarkable agreement with the phenomenological model of the corrosion process in steel. Then, mechanisms of corrosion‐induced cracks are visually and quantitatively investigated by the SiGMA analysis.     

Towards incorporating the influence of cover cracking on steel corrosion in RC design codes: the concept of performance-based crack width limits

This paper advocates for the adoption of performance-based limiting crack widths with respect to steel corrosion in reinforced concrete structures. The authors argue that, from both durability and sustainability viewpoints, the practice of adopting a universal limiting crack width for a wide range of in-service exposure conditions and concrete cover conditions and quality is not valid. As new performance-based concrete design codes are being developed and/or improved, the influence of cover cracking on steel corrosion needs to be incorporated in these codes. An experimental set-up was designed to investigate the influence of cover cracking, cover depth and concrete quality on chloride-induced corrosion. Beam specimens (120?×?130?×?375?mm) were cast using five concretes made using two w/b ratios (0.40 and 0.55) and three binders (100?% CEM I 42.5?N (PC), 50/50 PC/GGBS and 70/30 PC/FA). Other variables in the experiments included cover depth (20 and 40?mm), crack width (0, 0.4 and 0.7?mm). A total of 105 beam specimens were cast and exposed to cyclic 3-days wetting (with 5?% NaCl solution) and 4-days air-drying in the laboratory (23?°C, 50?% relative humidity). Corrosion rate was monitored bi-weekly in the specimens. The results relevant to this paper are presented and discussed. For a given concrete quality and cover depth, corrosion rate increased with increasing crack width. If crack width and cover depth are kept constant, corrosion rate increases with decreasing concrete quality, and vice versa. A model framework that can be used to objectively select cover depth, concrete quality and crack width is proposed. Such a model can be improved into, for example, a nomograph and used in the design process for RC structures prone to corrosion. Performance-based crack width limits should be adopted in the design of RC structures prone to steel corrosion. These crack width limits should be dependent on a complex interaction of, inter alia, concrete quality, cover depth, crack characteristics and prevailing exposure conditions. This study showed the inter-relationship between crack width, cover depth and concrete quality in affecting chloride-induced corrosion rate. Accurate corrosion rate prediction models incorporating the influence of cover cracking on corrosion are a pre-requisite to implementing the influence of cover cracking in future concrete design codes.