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.     

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.     

Effect of prior damage on the performance of cement based coatings on rebar: macrocell corrosion studies

The performance of any rebar coating system depends upon its resistance to corrosion in the presence of chloride contaminated concrete. It also mainly depends upon its tolerance towards damage that may occur during handling, transporting and concreting. The severe corrosion of epoxy coated reinforcing steel in long key bridge and other structures in Florida gave raise to the concern over the influence of damages in the coating system [R.J. Kessler, P.G. Powers, Interim report on corrosion evaluation of substructure in Long key bridge, Corrosion Report No. 87-9A, Florida Department of Transportation, Florida, 1987]. The performance of rebar coating such as galvanizing and epoxy based coating with prior surface damage has been evaluated and reported. [J. Hartley, Concrete Jan/Feb (1994) 12–15; A. Sagues, Performance of galvanized rebars in marine substructure service, Project ZE-418, Part I, October, 1994]. To date, the performance of cement based coatings with prior damages has not been widely studied and reported. In the present investigation the effect of prior damage produced during concrete pouring has been studied on inhibited cement slurry coating. To simulate the marine substructure environment, macrocell corrosion has been created via a chloride ion concentration gradient. Test conditions and method of macrocell current measurement as described in ASTM G 109-92 have been followed. The above investigation revealed that the cement based coating appears to have better tolerance towards defects in chloride contaminated concrete as compared to epoxy based coating system. The performance of the coating is independent of the height of concrete pouring.     

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.     

Impact of crack width on bond: confined and unconfined rebar

This paper reports the results of a research project comparing the effect of surface crack width and degree of corrosion on the bond strength of confined and unconfined deformed 12 and 16 mm mild steel reinforcing bars. The corrosion was induced by chloride contamination of the concrete and an applied DC current. The principal parameters investigated were confinement of the reinforcement, the cover depth, bar diameter, degree of corrosion and the surface crack width. The results indicated that potential relationship between the crack width and the bond strength. The results also showed an increase in bond strength at the point where initial surface cracking was observed for bars with confining stirrups. No such increase was observed with unconfined specimens.     

Comparative studies of experimental and numerical techniques in measurement of corrosion rate and time-to-corrosion-initiation of rebar in concrete in marine environments

The evaluation of the corrosion process for estimating the service life of concrete structure is of great importance to civil engineers. In this paper, the effects of different exposure conditions (i.e., tidal and splash zones) on macrocell and microcell corrosion of rebar in concrete were examined on concrete specimens with different w/c ratios in the Persian Gulf region. Experimental techniques such as macrocell corrosion rate measurement, Galvanostatic pulse, electrical resistivity, half-cell potential measurement, and numerical techniques were used to determine the corrosion rate and time-to-corrosion-initiation of rebar. Results showed that corrosion rates in the splash zone were higher than the ones in the tidal zone. This indicates that the propagation of corrosion in the splash zone is faster than the one in the tidal zone. There was also a strong correlation between the experimental results and those obtained from a numerical model in both tidal and splash zones.     

环氧钢筋涂料的增韧探讨

混凝土结构中的钢筋受外力作用会发生变形,由此而导致其防腐蚀涂层开裂,进而使钢筋发生腐蚀.对于室温固化的普通环氧钢筋涂料,由于其树脂交联度高,固化后脆性较大,必须增加树脂的韧性.通过拉伸试验考察了使用柔性环氧、增韧剂、长链聚酰胺等几种环氧涂料常用的增韧途径对涂料韧性的影响.结果表明,长链聚酰胺的掺入量是影响断裂伸长率的最重要因素,而柔性环氧树脂是影响抗张强度的最重要因素.通过钢筋的弯曲试验,最终确定采用添加柔性环氧的增韧方式.在此基础上,对涂料进行各种性能测试及实际使用,结果证明涂料能满足使用要求.     

Interactive Effects of Fly Ash and CNI on Corrosion of Reinforced High-Performance Concrete

The interactive effects of fly ash and CNI in corrosion of reinforced concrete were investigated. A 3⁴ full factorial design was developed considering water to cement ratio, fly ash percent, CNI and cracked condition as factors. The response factors were the weight loss calculated from Linear Polarization Resistance measurements and the pit depth of the steel bars embedded in concrete. Small-scale concrete slabs containing steel reinforcement with a cover depth of 20 mm were cast for this purpose. The slabs were subjected to a simulated marine environment with two cycles of wetting and drying per day during one year; after the exposure, the slabs were broken, the bars were cleaned and the pith depth measured by using SEM. Under the studied conditions, it was found that CNI alone does not provide corrosion protection of the steel reinforcement even for uncracked silica fume concrete in a 0.45 w/c ratio; however, the combination of CNI and fly ash can be useful to overcome this problem. The results indicate that low w/c ratio concrete in its crack state creates conditions suitable for the development of pitting corrosion.     

基于粘结-滑移的FRP筋钢纤维轻骨料混凝土梁裂缝宽度计算方法

将纤维增强筋(FRP筋)混凝土梁裂缝的开展过程视作FRP筋由两侧混凝土中拔出的过程,建立了基于粘结-滑移的FRP筋轻骨料混凝土梁裂缝宽度微分方程。根据规范给出的裂缝宽度限值,合理确定滑移量上限,提出并引入了适用于梁正常使用阶段的FRP筋钢纤维轻骨料混凝土“低滑移”阶段粘结-滑移本构模型。进而,在明确最大裂缝间距l_max、裂缝宽度放大系数h₂/h₁与裂缝截面纤维混凝土残余应力σ_fib等特征参数的基础上,利用迭代算法建立了FRP筋钢纤维轻骨料混凝土梁最大裂缝宽度计算模型。基于正常使用阶段裂缝宽度实测数据对建议模型的适用性进行评估,结果表明:裂缝宽度限值0.5 mm内,建议模型能够准确预测FRP筋钢纤维轻骨料混凝土梁的最大裂缝宽度。     

Efficiency of coatings applied on rebars in concrete

The performance of three reinforcing steel bars (rebars) coatings is analyzed by means of electrochemical methods. The coatings are representative of those commonly used during the repair of concrete structures affected by corrosion in the coastal regions of Argentina: an epoxy rust conversion coating, a zinc-rich epoxy, and a sprayed zinc coating. Two exposure conditions were investigated: immersed in a saline solution (wet) and exposed to an indoors atmosphere (dry). The rebar corrosion potential (E_corr), the corrosion rate (CR) and the electrical resistance (R) between bars were measured during approximately 800 days. In the dry condition the three coatings presented a satisfactory performance characterized by passive E_corr values and CR values lower than 1 m/year. On the other hand, the performance of the coatings in the wet condition depended on their formulations. The rust conversion coating showed active E_corr values and CR values higher than those measured on the uncoated bars. The zinc-rich epoxy and the sprayed zinc coatings presented E_corr values typical of active zinc. This indicates a certain degree of cathodic protection provided to the reinforcing steel. Besides, R values showed that in this condition, coatings do not provide a barrier type of protection.     

玻璃纤维筋锚具研制及其力学性能试验研究

将纤维增强筋(FRP筋)混凝土梁裂缝的开展过程视作FRP筋由两侧混凝土中拔出的过程,建立了基于粘结-滑移的FRP筋轻骨料混凝土梁裂缝宽度微分方程。根据规范给出的裂缝宽度限值,合理确定滑移量上限,提出并引入了适用于梁正常使用阶段的FRP筋钢纤维轻骨料混凝土“低滑移”阶段粘结-滑移本构模型。进而,在明确最大裂缝间距l_max、裂缝宽度放大系数h₂/h₁与裂缝截面纤维混凝土残余应力σ_fib等特征参数的基础上,利用迭代算法建立了FRP筋钢纤维轻骨料混凝土梁最大裂缝宽度计算模型。基于正常使用阶段裂缝宽度实测数据对建议模型的适用性进行评估,结果表明：裂缝宽度限值0.5 mm内,建议模型能够准确预测FRP筋钢纤维轻骨料混凝土梁的最大裂缝宽度。     

含氯混凝土中钢筋宏电池腐蚀的研究

为了确定含氯混凝土中钢筋的宏电池腐蚀的特性 ,研究了钢筋腐蚀速度与混凝土电阻间的关系。在宏电池阴阳极之间串联可变电阻来模拟混凝土电阻的变化。实验结果显示钢筋腐蚀反应的自然腐蚀电位主要为阳极反应决定 ,而腐蚀反应速度为阴极反应控制。混凝土电阻对钢筋腐蚀反应速度影响很大。     

Theoretical and experimental study of microcell and macrocell corrosion in patch repairs of concrete structures

Patch repair is commonly used to rectify localized corrosion induced damage in concrete structures. However, inadequate durability in patch repair systems caused by new corrosion attack is prevalent. From the prevailing understanding, the mechanism is attributed to macrocell corrosion formed between repaired area (called patch) and its adjacent unrepaired area (called substrate), and thereby ensuring electrochemical compatibility between the two areas is deemed to be the key element to reduce the corrosion risk and thus to achieve a successful repair. This paper examined the corrosion mechanism and the concept of compatibility in patch repair systems from fundamental electrochemical principles and experimental verification. It was illustrated that both macrocell and microcell corrosion mechanisms could play significant roles, and the total corrosion could be underestimated if the latter is overlooked. Although the incompatibility serves as the driving force for the macrocell corrosion, in light of corrosion kinetics, it was shown that the corrosion magnitude depends more on the individual corrosion kinetics of the anode or cathode.     

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.     

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.     

Propagation of steel corrosion in concrete: Experimental and numerical investigations

This paper focuses on experimental and numerical investigations of the propagation phase of reinforcement corrosion to determine anodic and cathodic Tafel constants and exchange current densities, from corrosion current density and corrosion potential measurements. The experimental program included studies on RC specimens with various binder compositions, concrete cover thicknesses, and concrete cover crack widths. Modelling and fitting of experimental data using an electrochemical model allowed for the determination of parameters, which are key parameters for electrochemical modelling tools. The numerical model was, furthermore, used to identify electrochemical parameters, which are independent of concrete cover thickness and crack width and at the same time allow for determination of the corrosion current density and corrosion potential of concrete structures within an acceptable error.Very good comparisons between the experimentally measured and numerically simulated corrosion current densities and corrosion potentials were found for the various RC specimens. Anodic and cathodic Tafel constant between 0.01 and 0.369 V/dec and 0.01 and 0.233 V/dec, respectively, were found in the present study through numerical simulations of the experimental data. Anodic and cathodic exchange current densities ranged from 1.0E–12 to 1.0E–09 A/mm² and 1.0E–12 to 1.1E–09 A/mm², respectively.     

A study on the bond strength of tension lap splices in self compacting concrete

The primary objective of the research reported in this paper was to evaluate the effect of using self compacting concrete on the bond strength and mode of bond failure of tension lap splices anchored in normal strength concrete (NSC). Studies on the effect of transverse reinforcement on anchored reinforcement in self compacting concrete to prevent brittle mode of failure is limited. To meet this objective, full-scale NSC beam specimens were tested. Each beam was designed with bars spliced in a constant moment region at mid-span with various levels of stirrup confinement. The slip of the reinforcement with respect to concrete is also measured by providing notches at the end of the splices. Test results indicated that there is an increase in the bond strength when self compacting concrete is used in place of vibrated concrete. Ductility and splice strength increased as the confinement increased. When the stirrup spacing is less than 150 mm, the failure in the splice region was by yielding of steel. The influence of confinement on the crack formation of the beams is also reported.     

ANALYSIS OF FATIGUE CRACK GROWTH IN ROTARY BEND SPECIMENS AND RAILWAY AXLES

Fatigue crack growth tests have been carried out on pre-cracked specimens of steel AFNOR XC 38 under rotary bending conditions. The semi-elliptical surface crack shape evolution was determined and the results were analyzed by da/dN =f(ΔK) using a stress intensity factor solution for semi-elliptical surface cracks in round bars subjected to a bending stress modified to take into account the rotatary bending conditions. Results show a very good correlation with fatigue crack growth data obtained in standard centre cracked tension specimens of the same steel. An application to the sensitivity of design parameters to an analysis of cracked railway axles is proposed, allowing an improvement in maintenance procedures.     

Natural marine exposure results for reinforced concrete slabs with corrosion inhibitors

Reinforced concrete slabs were cast with a concrete cover of 20 mm. The water-to-cement ratio was 0.40, and the concrete slabs were either uncracked or precracked. A simulated crack 0.2 or 0.4 mm wide was formed transverse to the axis of the reinforcing bar during the casting. Two types of commercial corrosion inhibitors were added to concrete mixtures for corrosion protection. Slabs were placed about 1 m below high tide at the Treat Island, Maine, USA, natural marine exposure site. The specimens were visually inspected and the corrosion rates were measured annually using the linear polarization technique. Some of the concrete slabs were broken open after 12 months of exposure and corrosion damage was evaluated. Water-soluble chloride content analysis was performed at this time. After three years of exposure, it was found, that both corrosion inhibitors were effective in reducing the corrosion rate for uncracked concrete slabs, but relatively ineffective in preventing localised corrosion of reinforcing steel in the crack area for precracked concrete slabs.     

Mechanistic model and numerical analysis for corrosion damage of reinforced concrete structure

Through sampling, we have conducted systematically the analyses of changing characteristics of steel bar corrosion and crack width on reinforced concrete surfaces. Based on the analyses from the sampling, we have obtained the mathematical model for outer elliptical contour and the formulas to calculate thickness of corrosion layer at any associated point on steel bars. A stress model of discrete displacement on fictitious boundary for the corrosion layer and corresponding solutions for displacement and stress on the fictitious boundary have been advanced, which solve quantitatively the stress to the structure acted by corrosion layer. Finally, under the conditions of displacement on the fictitious boundary and by applying numerical manifold method, we had performed the simulation of track for crack propagation caused by corrosion and expansion of side steel bars as time elapses in existing reinforced concrete structures. The simulation results have conformed to those of the experiment.