Influence of crack width,cover depth and concrete quality on corrosion of steel in HPC containing corrosion inhibiting admixtures and fly ash

In this study the influence of crack width, cover depth and concrete quality on corrosion of steel in high performance concrete was investigated. Three mixtures, one control and two more containing corrosion inhibiting admixtures, Calcium Nitrite and Disodium Tetrapropenyl Succinate, in combination with 20% fly ash replacement with respect to the cement weight were prepared. Specimens were concrete cylinders measuring 100 mm in diameter and 65 and 105 mm in length, with a 16-mm steel bar centrally placed at two concrete covers of 25 and 45 mm. Before being exposed to a simulated marine environment, the specimens were pre-cracked under a controlled splitting test with crack widths ranging from 90 to 330 μm formed perpendicularly to the reinforcing bars. During a 16-month exposure, the corrosion risk of the reinforcing bars was evaluated by half-cell corrosion potentials and the corrosion rate by linear polarization method. Also, the total integrated corrosion current was estimated. Results show that, albeit to different degrees, cracking was found to be an influencing factor in promoting corrosion of the steel in concrete with either 25 mm or 45 mm concrete cover; nevertheless, the effectiveness of the concrete cover depended greatly on the crack thickness. Results also revealed that corrosion inhibitors and fly ash were effective in delaying corrosion even in cracked concrete.     

钢筋混凝土腐蚀监测技术及其应用

混凝土中的钢筋腐蚀是影响混凝土结构耐久性的主要因素.对钢筋腐蚀进行监测,是钢筋混凝土安全服役的重要保障手段之一.叙述了钢筋腐蚀监测几种常用的电化学测量和物理方法测量技术.分析了各种方法的原理及特点,介绍了各种方法技术在钢筋混凝土腐蚀监测上的应用.并对今后的发展趋势进行了展望.     

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.     

Effect of steel manufacturing process and atmospheric corrosion on the corrosion-resistance of steel bars in concrete

This paper presents results of a study conducted to evaluate the effect of steel manufacturing process and the surface condition of reinforcing steel on their corrosion-resistance when embedded in concrete. Steel bars produced by water quenching and air-cooling were utilized. The corrosion-resistance of fresh bars, i.e., those that were clean and shiny, and those exposed to atmosphere and accelerated salt spray, when embedded in concrete, was evaluated. The corrosion-resistance of the clean and corroded reinforcing steel bars was assessed by measuring corrosion potentials and corrosion current density. Accelerated impressed current technique was also utilized to evaluate the corrosion-resistance of clean and corroded reinforcing steel bars in concrete. A longer time-to-initiation and lower rate of reinforcement corrosion was noted in the concrete specimens prepared with water-quenched steel bars compared to similar bars manufactured by the hot-rolling process. Similarly, the rate of reinforcement corrosion in the concrete specimens prepared with corroded steel bars, exposed to atmosphere for 12 months and salt spray, was less than that on the unexposed bars. The data developed in this study also indicate that the surface layer formed on the water-quenched steel bars, due to the cooling process, provides protection to the metal substrate as against the loose mill scale formed on the steel bars produced by the air-cooling process.     

Regression models to predict SCC pressure exerted on formworks containing vertical and transverse reinforcing bars

The use of self-consolidating concrete (SCC) containing recycled concrete aggregate (RCA) considerably increased in sustainable structural applications and civil engineering works. However, current literature and construction practices are not clear regarding the influence of RCA additions and presence of steel reinforcement on formwork pressure exerted by the plastic concrete. This paper reports experimental data obtained from 32 SCC mixtures possessing different stability levels and cast in 1.6-m high formwork containing various combinations of vertical and transverse steel bars. Test results have shown that mixtures incorporating recycled aggregates exhibited reduced initial maximum pressure, given the higher RCA surface roughness that promotes internal friction and material build-up at rest. The decrease in pressure was particularly accentuated in presence of steel bars, suggesting that the reinforcement cage confines the plastic concrete and carries part of its load. The transverse steel was around 1.5-times more influential than vertical steel in reducing the formwork pressure. The rates of pressure drop over time were not altered because of steel, implying that pressure decay is governed by the concrete intrinsic properties such as thixotropy, RCA friction, and cement hydration. Special emphasis was placed to develop regression models and examine suitability of existing ones to predict lateral pressure of RCA-modified SCC cast in formworks containing reinforcing bars.     

Corrosion of steel reinforcement due to atmospheric pollution

The aim of this study is to examine the performance of four different sets of reinforcing steel rebars, S220, S400, S500s Tempcore and S500s Vanadus, exposed to the Greek atmosphere, before their installation into the concrete. The performance against atmospheric corrosion of the aforementioned steel rebars was evaluated by means of microscopy techniques and corrosion rate measurements. In particular, we studied the influence of corrosion products of a set of reinforcement steels on the bond strength between concrete and steel bars, during the hydration process of cement. Furthermore, microscopy techniques and visual observation were used to identify the rust strains and the corrosion product morphology of steel reinforcements before their installation into the concrete. The experimental results shown that the steel type, which exhibits the higher resistance, as far as atmospheric corrosion concerns, was S220 reinforcing steel. In contrary, S500s Tempcore has the least corrosion resistance. The bond strength between concrete and the steel rebars was found to decrease with increasing weathering from 45 to 122 days, due to the morphology and to the thickness of the rust layers formed on steel surface, as observed by the ESEM.     

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.     

Bond and interfacial properties of reinforcement in self-compacting concrete

This paper reports a study carried out to assess the impact of the use of self-compacting concrete (SCC) on bond and interfacial properties around steel reinforcement in practical concrete element. The pull-out tests were carried out to determine bond strength between reinforcing steel bar and concrete, and the depth-sensing nano-indentation technique was used to evaluate the elastic modulus and micro-strength of the interfacial transition zone (ITZ) around steel reinforcement. The bond and interfacial properties around deformed steel bars in different SCC mixes with strength grades of 35 MPa and 60 MPa (C35, C60) were examined together with those in conventional vibrated reference concrete with the same strength grades.The results showed that the maximum bond strength decreased when the diameter of the steel bar increased from 12 to 20 mm. The normalised bond strengths of the SCC mixes were found to be about 10–40% higher than those of the reference mixes for both bar diameters (12 and 20 mm). The study of the interfacial properties revealed that the elastic modulus and the micro-strength of the ITZ were lower on the bottom side of a horizontal steel bar than on the top side, particularly for the vibrated reference concrete. The difference of ITZ properties between top and bottom side of the horizontal steel bar appeared to be less pronounced for the SCC mixes than for the corresponding reference mixes.     

Repairing corroded RC beam with near-surface mounted CFRP rods

This experimental program investigates the possibility of using Carbon-Fiber-Reinforced Polymer (CFRP) rods to repair RC beams damaged by steel corrosion. The 6-mm-diameter carbon-epoxy pultruded FRP rods were implemented in undamaged areas of the concrete cover by using the Near Surface Mounted reinforcement (NSM) technique. The corroded beam tested was stored in a chloride environment under service loading for 23 years, which was representative of real structural conditions. The corroded beam showed longitudinal corrosion cracks more than 3 mm in width along the tension reinforcing bars. The repaired corroded beam was tested in three-point flexure up to failure. Then, concrete was removed around the reinforcing bars in order to measure the real local steel cross-section distribution. The maximum reduction in steel cross-section measured in the tension area was about 36% and was located at mid-span. In spite of this heavy corrosion, the bearing capacity measured on the repaired beam was equal to that of the uncorroded control beam. In our experimental conditions, the NSM technique was applicable for repairing corroded concrete structural members.     

The effect of ribs on the mechanical behavior of corroded reinforcing steel bars S500s under low-cycle fatigue

An experimental study has been carried out to assess the effects of gradually accumulating corrosion damage on the low-cycle fatigue behavior of smoothed concrete reinforcing steel bars S500s. Low-cycle fatigue loading may simulate in a satisfactory way the loading applied to the reinforcement of structures during seismic actions. Profiled (ribbed) concrete reinforcing steel bars have been tested after the removal of the ribs by means of filing. The fatigue behavior of the smoothed bars is compared against that of profiled steel bars, which are widely used as concrete reinforcement. The low-cycle fatigue tests performed indicate that the presence of ribs greatly reduces the performance of the steel bars under cyclic loading. A significant increase of the number of cycles to failure, as well as of the dissipated energy, has been observed, whereas there was only a minor reduction of the load-carrying ability of the bars due to the reduction of the ribs. The results indicate that the use of non-profiled steel bars could be considered in certain areas of reinforced concrete frames, especially for structures located in seismically active areas, which suffer the most from earthquake loadings.     

锈蚀RC桥梁弯曲抗力时变概率模型与试验研究

在退化RC结构的时变性能评估、剩余寿命预测以及维修加固决策分析中,其抗力模型的建立至关重要。钢筋锈蚀导致钢筋的屈服强度降低、改变钢筋力学行为和影响钢筋与混凝土之间的粘结行为。钢筋腐蚀损失量影响钢筋的失效模式,即从延性破坏到脆性破坏变化。该文发展了退化RC 桥梁的概率抗力时变模型,可以定量考虑腐蚀对钢筋力学行为、倒塌机理和粘结性能的影响。建立退化结构的抗力概率模型可以抓住退化结构的退化机理、点蚀分布、结构尺寸参数、粘结行为、腐蚀扩展、钢筋失效模式和计算模型的随机性和变异性。RC 梁的加速腐蚀试验和旧桥的破坏性试验验证了该文模型的正确性和可靠性。     

A critical discussion on rebar electrical continuity and usual interpretation thresholds in the field of half-cell potential measurements in steel reinforced concrete

Corrosion of steel in reinforced concrete structures is a recurrent problem affecting civil engineering structures and costing the world billions of dollars per year. This physical phenomenon mainly results from chloride ingress or concrete carbonation. Corrosion can be diagnosed through a nondestructive method such as half-cell potential measurements. The present paper studies this method on a reinforced concrete wall containing eighteen unconnected steel bars and subjected to chloride-induced macrocell corrosion. Three corrosion systems with different configurations of connections between the steel bars are generated, involving three different anode-to-cathode surface ratios. Then, half-cell potential variations are observed versus macrocell corrosion current. The results lead to a critical discussion regarding the physical relevance of the usual potential threshold method to detect corroding rebars in reinforced concrete structures. In addition, the experiments demonstrate that electrical continuity between reinforcing steel bars is not necessary to get meaningful information about the macrocell corrosion system. At last, the paper show that the electric field (potential gradient) relative to a macrocell corrosion system may be measured by connecting the measurement system (reference electrode?+?voltmeter) to any electrochemical system in electrolytic contact with the concrete.     

萘系减水剂对硬化水泥浆体及孔隙液中钢筋腐蚀的影响

为了研究外加剂对混凝土中钢筋腐蚀的影响,模拟了混凝土孔隙液及制备了硬化水泥浆体,研究了添加不同含量的萘系减水剂对钢筋腐蚀的影响.结果表明:在模拟混凝土孔隙液中加入萘系减水剂对钢筋腐蚀有轻微的促进作用,当减水剂含量达到一定值时腐蚀增大的趋势就会消失;在硬化水泥浆体中加入萘系减水剂可以减缓钢筋的腐蚀,添加0.5%萘系减水剂阻锈效果最明显,且随着时间的延长,萘系减水剂对硬化水泥浆体中钢筋的阻锈效果增加;加入减水剂增强了硬化水泥浆体的密实性,从而减缓了钢筋的腐蚀.     

微锈蚀钢筋混凝土高温后粘结锚固性能试验研究

为研究高温对锈蚀钢筋混凝土结构粘结锚固性能的影响,对锈蚀试件(锈蚀率为1.08%)与非锈蚀试件先进行高温试验(20℃、200℃、400℃、600℃、800℃),再进行中心拔出试验。试验结果表明:随温度的升高,锈蚀试件与非锈蚀试件粘结强度均呈下降趋势,与非锈蚀试件相比,锈蚀试件在温度不超过400℃时,其粘结强度下降趋势较为平缓。分析了高温作用后混凝土抗压强度、钢筋极限强度、钢筋与混凝土间粘结强度三者之间的关系,并建立了考虑不同温度、不同锈蚀率等因素影响下钢筋与混凝土的粘结-滑移关系式。     

Use of polymers for cement-based structural materials

Cement-based materials are widely used in the civil infrastructure. Polymers as admixtures can improve the properties, particularly in relation to water absorption reduction, toughness enhancement, vibration damping and increase of the bond strength of cement to reinforcements. Polymeric admixtures include particles, short fibers and organic liquids. Latex in the form of an aqueous particle dispersion is most common. Other than being used as admixtures, polymers are used as partial replacement of fine aggregate, for coating, sealing and repairing concrete and for coating steel reinforcing bars for corrosion protection.     

Bond strength of reinforcing steel embedded in fly ash-based geopolymer concrete

Geopolymer concrete (GPC) is an emerging construction material that uses a by-product material such as fly ash as a complete substitute for cement. This paper evaluates the bond strength of fly ash based geopolymer concrete with reinforcing steel. Pull-out test in accordance with the ASTM A944 Standard was carried out on 24 geopolymer concrete and 24 ordinary Portland cement (OPC) concrete beam-end specimens, and the bond strengths of the two types of concrete were compared. The compressive strength of geopolymer concrete varied from 25 to 39 MPa. The other test parameters were concrete cover and bar diameter. The reinforcing steel was 20 mm and 24 mm diameter 500 MPa steel deformed bars. The concrete cover to bar diameter ratio varied from 1.71 to 3.62. Failure occurred with the splitting of concrete in the region bonded with the steel bar, in both geopolymer and OPC concrete specimens. Comparison of the test results shows that geopolymer concrete has higher bond strength than OPC concrete. This is because of the higher splitting tensile strength of geopolymer concrete than of OPC concrete of the same compressive strength. A comparison between the splitting tensile strengths of OPC and geopolymer concrete of compressive strengths ranging from 25 to 89 MPa shows that geopolymer concrete has higher splitting tensile strength than OPC concrete. This suggests that the existing analytical expressions for bond strength of OPC concrete can be conservatively used for calculation of bond strength of geopolymer concrete with reinforcing steel.     

Effects of chromium(VI) reducing agents in cement on corrosion of reinforcing steel

During the production of cement it is necessary to add a reducing agent that converts soluble hexavalent chromium into trivalent chromium. This paper explores effects of iron(II) sulphate reducing agents, namely monosulphate and heptahydrate, as cement admixtures, on corrosion of concrete reinforcement. Accelerated corrosion tests of reinforcing steel have been performed in pore solutions simulating concrete prepared using either CEM I or CEM II type cement, with addition of various concentrations of iron(II) sulphate reducing agent. All of the test results indicate corresponding tendencies and point towards the potential for iron(II) sulphate to foster or accelerate corrosion of the reinforcement. The results of this study indicate an immediate need for a more detailed research, especially in concrete as a more realistic corrosion environment, of the application of iron(II) sulphate as a reducing agent in cement.     

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.     

Corrosion products that form on steel within cement paste

Most research on chloride-induced corrosion of reinforcing steel is conducted either in simulated pore solution or concrete. The former has the disadvantage that the experiments cannot replicate the environmental variations on the surface of the steel induced by the presence of the cementitious cover. The difficulty with the latter is that it is impossible to clearly assess the impact of many different parameters that influence the corrosion in commercial concrete. Consequently, the present study involves steel plates embedded in cement paste to minimize the number of active corrosion sites relative to steel in pore solution, thus simplifying an investigation into the location and accumulation of corrosion products in the cement paste and the influence of cracks on the corrosion process.Corrosion monitoring techniques (i.e., open circuit potential mapping, linear polarization resistance) were correlated with chemical and microstructural analyses, including Raman spectroscopy of the corrosion products, to study the specimens. The results indicate the following: a range of corrosion products forms, primarily within shrinkage cracks; their specific volumes are typically in the range of 2 to 3; cracks had a greater impact on the corrosion rate of steel in white cement than in Type 10 cement; and that the surface sealing of the cracks using epoxy did not prevent corrosion initiation from occurring under the epoxy.     

Bar-concrete bond in mixes containing calcium carbide residue,fly ash and recycled concrete aggregate

A mixture of calcium carbide residue and fly ash (CRFA) is an innovative new binder for concrete instead of using ordinary Portland cement (OPC). Therefore, this study aims at investigating the bond interaction between common steel reinforcing bars and the aforementioned concrete. To this end, both CRFA and OPC concretes using crushed limestone and recycled concrete aggregate (RCA) as a coarse aggregate were prepared to investigate the bond strength of smooth and deformed bars by pull-out tests. The bond stress−slip relationships were also identified to determine the effects of CRFA binder and RCA on the bond strength behavior. The results indicate that the values the of bond-slip behavior and bond strengths of steel bar in CRFA concretes are similar to those embedded in OPC concrete. Moreover, the bond strength was significantly affected by RCA and the types of steel bar. Although the concretes had the same compressive strengths, the deformed bar embedded in CRFA concrete with RCA had a lower bond strength than the one with crushed limestone. However, the reduction in bond strength of the CRFA concrete with RCA was still less than that of OPC concrete with RCA. For the CRFA concretes, the bond strengths of the deformed bars were approximately 1.7–3.6 times higher than that of smooth bars.