Microwave reflection and dielectric properties of mortar subjected to compression force and cyclically exposed to water and sodium chloride solution

Corrosion of the reinforcing steel is a major cause of damage and deterioration in reinforced concrete structures such as concrete bridge decks and columns. Chloride intrusion into concrete can lead to depassivation of the steel and initiation of corrosion. Thus, it is very important to be able to nondestructively detect and evaluate the free chloride content in concrete. Near-field microwave nondestructive testing techniques, using open-ended rectangular waveguide probes, have shown great potential for evaluating various properties of concrete, including the successful detection of sodium chloride added to mortar mixing water. In this study, several mortar samples are cyclically soaked in distilled and salt water while also experiencing compression force. Compression force, simulating in-service loading, causes microcracking, which results in increased microcracking and permeability, promoting chloride ingress. The daily microwave reflection and dielectric properties of these samples were measured at 3 GHz. The results show the capability of these microwave measurements for detecting the increased level of chloride permeation and loading as a function of the increasing number of soaking cycles. The influence of salt ingress is shown to be more prominent in the loss factor, while the effect of loading is more evident in the permittivity of the samples.     

配筋超高性能混凝土用作免拆模板对短柱力学性能影响的实验研究

为进一步提高在海洋浪溅区等严重氯盐腐蚀环境下混凝土结构的耐久性,采用以配筋超高性能混凝土(UHPC)用作免拆模板加素填芯混凝土的技术思路,通过轴压实验研究了配筋UHPC用作免拆模板对短柱轴压力学性能的影响。结果表明,与破坏时海工混凝土柱出现大量的混凝土剥落和钢筋裸露不同,UHPC免拆模板柱的整体性良好,表面的UHPC仅出现少量剥落且钢筋不外露。同时,在等截面和相同配筋的情况下,UHPC免拆模板方柱和圆柱的开裂荷载明显高于高强海工混凝土方柱和圆柱的极限荷载,且分别达到其自身极限荷载的93%和88%。考虑到混凝土对钢筋的有效保护是影响海工混凝土结构耐久性的重要因素,配筋UHPC用作免拆模板这一思路在提高柱体的极限承载力、延性、刚度的同时还可以提高结构的开裂荷载、抗裂性能,有效地提高结构耐久性,适用于沿海工程。其中圆柱的力学性能提高幅度要远高于方柱,从配筋UHPC免拆模板的使用效果和施工便利性考虑,优选UHPC免拆模板圆柱。     

考虑氯离子侵蚀与混凝土碳化的公路桥梁时变可靠度分析

基于国内外对氯离子侵蚀和混凝土碳化环境下钢筋锈蚀速率的最新研究成果,建立了混凝土构件在氯离子侵蚀下考虑坑蚀和在混凝土碳化下考虑平均锈蚀的弯曲抗力退化模型,用Monte Carlo方法和统计回归法编制了退化钢筋混凝土构件及系统的时变可靠度计算程序。以北京地区一座公路桥为算例,结果表明:在氯离子侵蚀下考虑坑蚀的桥梁承载能力时变可靠度在30年左右即下降到设计目标可靠度,在混凝土碳化下考虑平均锈蚀的桥梁承载能力时变可靠度在50年左右即下降到设计目标可靠度,从而需要补强,这表明考虑主筋锈蚀后我国钢筋混凝土公路桥一般达不到100年设计使用期;对因主筋锈蚀导致混凝土保护层胀裂而言,构件达到抗裂正常使用极限状态远远早于达到承载能力极限状态,建议将混凝土保护层开裂时间作为桥梁检查/维修参考点。     

Detection of chlorides and moisture in concrete structures with ground penetrating radar

Corrosion of re-bar within reinforced concrete is a major problem in countries where salt is applied to roads for de-icing. Concrete structures are periodically inspected in order to monitor possible damage caused by chloride induced corrosion of the reinforcement. However, bridge decks covered with asphalt pavements are not accessible for visual inspection and probing is limited. As a result, the planning of rehabilitation of bridge decks is usually based on a small number of probes. Consequently, the condition of bridge decks can only be assessed with low certainty. Therefore, a method enabling to study the conditions of concrete bridge decks covered by asphalt pavements is desirable. This paper describes a laboratory experiment aiming at the investigation of the effects of moisture and chloride content on the amplitudes of radar signals. It can be shown that both, moisture and chloride content have a measurable influence on signal amplitudes. This may enable the future use of ground penetrating radar for the planning of probing campaigns or for the extrapolation of results obtained at single probing points.     

Benefits of internal curing on service life and life-cycle cost of high-performance concrete bridge decks – A case study

This paper investigates the impact of internal curing on the service life of high-performance concrete (HPC) bridge decks by using analytical models to predict the times to onset of corrosion, onset of corrosion-induced damage, and failure of decks. Three bridge deck design options were compared: (i) normal concrete deck; (ii) HPC deck with supplementary cementing materials (SCM); and (iii) HPC deck with SCM and internal curing. It was found that the use of internal curing can extend the service life of high-performance concrete bridge decks by more than 20 years, which is mainly due to a significant reduction in the rate of penetration of chlorides in concrete as a result of reduced early-age shrinkage cracking and reduced chloride diffusion. Compared to normal concrete, HPC with SCM and internal curing was predicted to add more than 40 years to the service life of bridge decks in severe environmental conditions. Life-cycle cost reductions of 40% and 63% were estimated when conventional HPC and internally-cured HPC were used in bridge decks instead of normal concrete, respectively, despite the fact that the in-place unit cost of internally-cured HPC can be 4% higher than that of conventionally-cured HPC, which in turn can be up to 33% higher than that of normal concrete. This is due to a longer service life and less frequent maintenance activities offered by low-permeability HPC bridge decks.     

Damaging effects of deicing chemicals on concrete materials

The damaging impact of various deicing chemicals and exposure conditions on concrete materials was investigated. Five deicing chemicals (sodium chloride, calcium chloride with and without a corrosion inhibitor, potassium acetate, and an agricultural product) were studied. Freezing–thawing (F–T) and wetting–drying (W–D) exposure conditions were considered. Mass loss, scaling, compressive strength, chemical penetration, and micro-structure of the paste and concrete subjected to these deicing chemicals and exposure conditions were evaluated. Results indicated that the various deicing chemicals penetrated at different rates into a given paste and concrete, resulting in different degrees of damage. Among the deicing chemicals tested, two calcium chloride solutions caused the most damage. Addition of a corrosion inhibitor into the calcium chloride solution delayed the onset of damage, but it did not reduce the ultimate damage. Chloride-related deicing chemicals often brought about leaching of calcium hydroxide, as well as chemical alterations in concrete. Potassium acetate caused minor scaling, associated with alkali carbonation of the surface layer of concrete. Although producing a considerable number of micro-pores on the surface of the samples, the agricultural deicing product resulted in the least chemical penetration and scaling damage of paste and concrete.     

Studies on corrosion inhibition of steel reinforcement by phosphate and nitrite

This work deals with the effectiveness of sodium phosphates and nitrites used as inhibitors against steel corrosion in concrete reinforcement. First, concrete pore water was simulated with several alkaline solutions. Then, reinforced concrete specimens with inhibitors added in fresh concrete mix are immersed in chloride solution. The effectiveness of inhibitors was assessed by applying electrochemical techniques, namely Electrochemical Impedance Spectroscopy (EIS), Polarisation Curves (PC), and Open Circuit Potential (OCP) measurements. The final concrete condition was analysed with Scanning Electronic Microscopy (SEM), X-Ray Diffraction (XRD) and Infrared Spectroscopy (FT-IR).In solutions simulating concrete contaminated with chloride, the influence of the inhibitors on the steel corrosion was assessed by (PC) and (EIS). The results obtained show that phosphate prevents pitting corrosion when its content is equal to chloride concentration, and that nitrite only contributes to increase the value of pitting potential. Corrosion rate is reduced with both inhibitors at the different contents tested.For reinforced concrete specimens immersed in chloride solution, their conditions were assessed by applying EIS. The results indicate that after 1 year of immersion with the two inhibitors the corrosion rate decreased. Then, after 3 years of immersion no influence of inhibitors on the corrosion rate was observed. However a significant increase in concrete electrical resistance was observed when inhibitors were present. Visual examinations showed that all steel bars were corroded in the presence of chlorides.Results from analytical tests done on the concrete after 3 years of immersion show that as far as the final concrete condition is concerned, the tested inhibitors did not change the type of compounds in concrete. But the final free chloride content remained higher than the critical chloride threshold. The results of FT-IR technique showed that nitrites are likely washed out of concrete during immersion in chloride solution and phosphates interfere with the equilibrium between CO₃ ^2? and HCO₃ ^? in concrete.The main conclusion of this study is: the efficiency of the tested inhibitors decreases with time, after two years of immersion in chloride solution.     

Nonlinear static cyclic pushover analysis for flexural failure of reinforced concrete bridge columns with combined damage mechanisms

In a fixed connection of a reinforced concrete bridge column, experiments have shown that the longitudinal reinforcing bars slip at the interface of the connection under cyclic seismic loading. The bond-slip (or strain penetration) of the longitudinal reinforcing bars causes a pinching effect in the column’s hysteresis curve. The bond-slip (or strain penetration) reduces the column’s stiffness and increases its deformations during an earthquake event, significantly affecting the performance of the column. Significant strength degradation has also been observed after the column reaches its ultimate strength. This study is to model a reinforced concrete column’s performance under cyclic pushover analysis with combined damage mechanisms including concrete cracking, concrete strength degradation due to concrete spalling, longitudinal reinforcing bars buckling, and bond-slip between longitudinal reinforcing bars and concrete. Two multi-scale nonlinear finite element models with and without the bond-slip (or strain penetration) of a reinforced concrete bridge column are proposed. The simulated column’s hysteresis curves under nonlinear cyclic pushover are compared with available experimental data. The results show that the proposed models with bond-slip together with combined damage mechanisms can effectively predict the seismically induced flexural failure behavior of the reinforced concrete bridge columns.     

Effect of corrosion-inhibitor-added deicing salts and salt substitutes on reinforcing steels: II. Influence of temperature and oxygen content

The effect of corrosion-inhibitor-added deicing salts and salt substitutes on reinforcing steels (rebars) was investigated at various temperatures and oxygen contents using galvanic cells and optical microscopy. Temperatures and oxygen contents were varied by applying stirring, oxygen-bubbling, high temperature, freeze-thaw cycle, and dry-wet cycle conditions to simulated concrete solutions mixed with 6% corrosion-inhibitor-added deicing salts, salt substitutes, and plain sodium chloride. Corrosion of reinforcing steel varied with oxygen contents and temperatures of the corrosion-inhibitor-added deicing salt and salt substitute solutions. The reinforcing steels under the freeze-thaw cycle condition showed the least corrosion, whereas those under the dry-wet cycle condition showed the most severe corrosion. The effectiveness of corrosion-inhibitor-added deicing salts and salt substitutes was found to vary with temperatures and dissolved oxygen contents.     

Fatigue behaviour of concrete bridge deck slabs reinforced with GFRP bars

GFRP bars are often used for the internal reinforcement of concrete bridge deck slabs as an alternative to traditional steel reinforcements with excellent results in terms of corrosion resistance. Several experiments on bridge decks were conducted to evaluate their structural behaviour but their fatigue performance still needs an adequate experimental investigation. This paper presents the results of an experimental campaign on four full scale concrete bridge deck specimens reinforced with GFRP bars that were designed, constructed and tested to resist cyclic moving loads. Two hydraulic jacks were used to simulate moving concentrated loads. After the cycles, the load was increased to the static failure. The slabs reinforced with GFRP bars showed a better fatigue performance compared to the requests of the European codes.     

Effect of corrosion-inhibitor-added deicing salts and salt substitutes on reinforcing steels: I. Influence of concentration

Corrosion of reinforcing steel (rebar) samples was investigated in concrete-saturated solutions containing corrosion-inhibitor-added deicing salts and salt substitutes as well as in plain sodium chloride solutions. Galvanic cells were used to determine the effects of corrosion-inhibitor-added deicing salts and salt substitutes on rebar corrosion. The reinforcing steel samples were galvanically coupled and were recovered after 240 days. Pit depths and area percentages of corrosion were determined on the reinforcing steels with an optical microscope. Optimum concentrations were found to exist for the corrosion-inhibitor-added deicing salts and salt substitutes in reducing the rebar corrosion. Dramatic changes of pH values were noted in the concrete-saturated solutions containing corrosion-inhibitor-added deicing salts and salt substitutes. The formation of precipitates caused by chemical reactions between a concrete-saturated solution and corrosion-inhibitor-added deicing salts and salt substitutes was observed.     

FRP confined concrete columns: Behaviour under extreme conditions

The behaviour of concrete columns wrapped with fibre reinforced polymer (FRP) materials when exposed to several extreme conditions is evaluated. Cold regions environments, FRP repair of corroding reinforced concrete columns, and fire resistance are all considered. For the cold regions exposure, FRP wrapped cylinders (152 × 305 mm) are exposed to temperatures as low as −40 °C or to up to 300 cycles of freeze-thaw (−18 °C to +15 °C). The combination of freeze-thaw exposure with sustained loading is also examined. For FRP wrapping of corroding reinforced concrete columns, the results of tests on cylinders and larger-scale circular columns (300 × 1200 mm) are presented. The specimens are corroded and then wrapped with FRP sheets. The rate of corrosion is monitored both before and after wrapping. The final extreme condition that is considered is fire exposure. Tests on full-scale reinforced concrete columns (400 × 3800 mm) exposed to a standard fire are described and discussed. Overall, the results demonstrate that FRP confined concrete columns tested in concentric axial compression have adequate performance under several extreme conditions such as low temperature, freeze-thaw action, corrosion of internal reinforcement, and fire exposure.     

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.     

Service life prediction of concrete bridge decks repaired with bonded concrete overlays

Service life predictions with respect to chloride initiated corrosion of repaired concrete bridge decks are of major concern in order to develop cost-efficient repair strategies. A service life prediction incorporates a service life criterion, a concrete cover and a method to predict the chloride ingress rate. All three topics are discussed and quantified in this paper on old concrete bridge decks repaired by water jetting and bonded steel fibre reinforced concrete overlays. The service lives for two bridge decks are estimated. The parameters used are based on comprehensive field studies of repaired decks that have been in service between five and ten years after the repairs. The proposed method is capable of taking the distribution of covers and transport coefficients into consideration and any probabilistic level could be used. All calculations can be made by hand and no subjective decisions are needed. The serice life with respect to chloride initiated corrosion was found to be more than 100 years for the repaired concrete bridge decks. Bonded concrete overlays constitute a durable repair alternative for deteriorated concrete bridge decks.

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Résumé En ce qui concerne la corrosion due aux chlorures, les prédictions de durée de vie en service, après rénovation, des tabliers de ponts en béton sont de première importance pour développer une stratégie de rénovation financièrement efficace. Une telle prévision est constituée d'un critère de durée de vie, d'une couverture en béton et d'une méthode pour prédire le taux de pénétration des chlorures. Ces trois aspects sont discutés et quantifiés dans cet article sur les vieux tabliers de ponts en béton, réparés par un giclage d'eau et une couverture en béton armé avec des fibres d'acier. Ensuite, les durées de vie en service de deux tabliers de ponts sont estimées. Les paramètres utilisés sont determinés à partir d'une étude d'ensemble de tabliers qui ont été en service entre cinq et dix aus après rénovation. La méthode proposée est capable de prendre en considération la distribution de la couverture et les coefficients de transports; n'importe quel niveau de probabilité peut être utilisé. Cette méthode est facile à employer et à comprendre. Tous les calculs peuvent être faits à la main et aucune décision subjective n'est nécessaire, ce qui rend la méthode indépendante de la personne qui réalise la prédiction de durée de vie. En ce qui concerne la corrosion due aux chlorures, la durée de vie en service a été estimée à plus de 100 ans pour les tabliers réparés. Fixer une converture de béton constitue une alternative durable pour la rénovation des tabliers de ponts en béton qui sont détériorés.

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Editorial Note The Royal Institute of Technology is a RILEM Titular Member.     

Behaviour of fiber reinforced concrete columns in fire

This paper presents the results of a research program on the behaviour of fiber reinforced concrete columns in fire. Several fire resistance tests on fiber concrete columns with restrained thermal elongation were carried out. The percentage of steel reinforcing bars on the testing columns varied in function of the percentage of steel fibers being always the total amount of steel (steel fibers + steel reinforcement) similar. The aim of this research was to study the possibility of replacing the longitudinal reinforcement bars on the concrete columns by steel fibers. Furthermore, polypropylene fibers were also used on the concrete in order to enhance the fire behaviour of the columns and avoid the concrete spalling. Polypropylene fibers under fire action will create a network of micro-channels for the escape of the water vapour.     

用电阻探针法研究水环境中钢筋混凝土的腐蚀行为

混凝土中钢筋腐蚀十分严重,为了对混凝土内钢筋腐蚀进行在线监测,设计了箔栅电阻探针试样及钢筋腐蚀监测仪.介绍了电阻探针监测系统的原理及组成,采用电阻探针法测量了10 mm和20 mm两种混凝土保护层厚度的试样浸泡在青岛海水全浸区、潮差区和长江港淡水环境以及实验室内3%氯化钠溶液中的钢丝电阻腐蚀情况,由电阻值变化可计算出钢丝腐蚀率.试验结果表明,电阻探针法能实时地监测混凝土中钢筋腐蚀的行为.     

考虑锈蚀黏结退化的钢筋混凝土桥墩抗震性能分析

受氯离子腐蚀作用影响,钢筋混凝土结构的抗震性能在其服役期内会发生退化。以钢筋混凝土桥墩为例,采用OpenSees有限元软件模拟了非锈蚀足尺桥墩的振动台试验以及锈蚀缩尺桥墩的拟静力试验结果,后者引入了氯离子腐蚀作用模型,同时考虑了桥墩的纵筋力学性能衰减以及粘结退化问题。随后基于概率地震需求模型对足尺桥墩的三种纵筋腐蚀工况进行了易损性分析。研究结果表明:采用非线性梁柱单元与零长度截面单元串联方式建立的数值模型能够有效模拟钢筋混凝土桥墩的足尺振动台试验结果和缩尺锈蚀拟静力试验结果;当纵筋腐蚀率较小时,纵筋的性能退化对墩顶最大位移影响不大,但基底剪力和基底弯矩下降明显;轻微破坏状态下,桥墩的抗震性能受纵筋锈蚀的影响不明显,随着破坏程度的加深,纵筋锈蚀对桥墩抗震性能的影响逐渐显著,较小的腐蚀率能导致桥墩的破坏概率发生较为明显的提高。研究内容可为考虑全寿命周期内性能退化的钢筋混凝土结构抗震设计提供参考。     

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.     

Corrosion resistance of self-consolidating concrete in full-scale reinforced beams

The corrosion of steel reinforcement embedded in full-scale self-consolidating concrete (SCC) beams was investigated compared to normal concrete (NC). 400 mm width × 363 mm depth × 2340 mm length beams containing epoxy- and non-epoxy-coated stirrups were monitored under an accelerated corrosion test. The corrosion performance of NC/SCC beams was evaluated based on the results of current measurement, half-cell potential tests, chloride ion content, mass loss and bar diameter degradation. The investigation also included the effect of admixture type and the size of specimen on corrosion performance.In general, SCC beams showed superior performance compared to their NC counterparts in terms of corrosion cracking, corrosion development rate, half-cell potential values, rebar mass loss and rebar diameter reduction. However, SCC beams showed localized corrosion with concrete spalling due to non-uniform concrete properties along the length, which was a result of the casting technique. The results also showed that the difference between SCC and NC mixes in terms of corrosion was more pronounced in large-scale beams, and that types of admixture used in SCC have no influence on corrosion performance.     

Increasing concrete durability with high-reactivity metakaolin

High-reactivity metakaolin (HRM) is a manufactured pozzolan produced by thermal processing of purified kaolinitic clay. Field performance and laboratory research of concrete containing HRM have demonstrated its value for bridge decks, bridge deck overlays, industrial flooring, high-strength concrete and masonry products. This paper discusses laboratory evaluations to assess the long-term performance of concrete containing HRM produced in North America for resistance to chloride penetration and reduction in expansion due to alkali-silica reactivity. Bulk diffusion testing indicated that HRM substantially reduced chloride ion penetration in concrete with w/cm of 0.30 or 0.40. Reductions in diffusion coefficients compared to control specimens were of the order of 50% and 60% for concrete with 8% and 12% HRM, respectively. Also, the performance of the concrete containing 8% or 12% cement replacement with HRM showed improved performance versus merely reducing the w/c from 0.4 to 0.3. Such reductions can be expected to have a substantial impact on the service life of reinforced concrete in chloride environments. Expansion tests on concrete prisms containing reactive aggregates showed that 15% HRM can prevent deleterious expansion due to alkali-silica reactivity (ASR). The mechanism of control is likely linked to the substantial reduction in pore solution alkalinity seen in pastes containing 20% HRM in comparison to the control specimen which contained no supplementary cementing materials. However, the reduction was not large enough to depassivate steel reinforcement.