Crack closure of reinforced concrete by electrodeposition technique

The electrodeposition method for rehabilitation of cracked reinforced concrete, based on the electrochemical technique, is presented here. Investigations on reinforced concrete beams of size 15×15×125 cm were conducted in this study. Specimens cracked by chloride attack for 18 months were immersed in a ZnSO₄ solution, and were applied with a constant current for 8 weeks. The development of crack closure due to the precipitation of electrodeposits (ZnO) was evaluated. Water permeability, chloride ion concentration, polarization curve and half-cell potential were measured in order to assess the effectiveness of this method in the improvement of concrete properties. The results indicate that electrodeposits formed on the concrete surface were able to close the concrete cracks and to decrease the concrete permeability. In addition, the investigation shows that the application of electrodeposition have effects on the desalination of concrete and re-passivity of the reinforcing steel in concrete.     

Experimental study on bond strength of fiber reinforced polymer rebars in normal strength concrete

The bond behavior of reinforcing bars is an important issue in the design of reinforced concrete structures and the use of fiber reinforced polymer (FRP) rebars is a promising solution to handle the problems of steel reinforcement corrosion. This study investigates the bond characteristics of carbon and aramid FRP (CFRP and AFRP) bars embedded in normal strength concrete. A pullout test was performed on 63 normal strength concrete specimens reinforced with FRP and steel rebars with different embedment lengths and bar diameters. The average bond stress versus slip curve is plotted for all specimens and their failure modes are identified. The effects of the embedment length and diameter of an FRP rebar on its bond strength are examined in this work. The bond strengths obtained from the test results are compared with the predictions by the bond strength equation proposed by Okelo and Yuan (2005), and its validity is evaluated.     

Application of gamma-ray radiography and gravimetric measurements after accelerated corrosion tests of steel embedded in mortar

The accelerated corrosion by the impressed current technique is widely used in studies of concrete durability since it has the advantage that tests can be carried out within reasonable periods of time. In the present work the relationship between the applied current density and the resulting damage on the reinforcing steel, by applying optical microscopy, scanning electron microscopy, gamma-ray radiography and gravimetric measurements, was studied by means of the implementation of accelerated corrosion tests on reinforced mortar. The results show that the efficiency of the applied current is between 1 and 77%, regardless of the applied current density, the water/cement ratio and the mortar cover depth of the specimens. The results show the applicability of the gamma-ray radiography technique to detect localized corrosion of steel rebars in laboratory specimens.     

Bond performance of fiber reinforced polymer rebars in different casting positions

In the last two decades, fiber reinforced polymer (FRP) rebars have become attractive to civil engineers, due to their corrosion resistance and high strength‐to‐weight ratio. To take advantage of such properties, many research studies have been carried out on the mechanical and physical properties of FRP rebars in concrete, and critical issues have been discovered regarding the bond behavior of FRP rebars. In this study, an effort was made to investigate the effects of FRP rebar types, locations and concrete casting positions on the bond performance of FRP rebars embedded in concrete members. Experimental investigations were conducted using a total of 63 specimens, with seven types of reinforcing bars, and three different casting positions. According to the test results, the bond performance of FRP reinforced concrete was greatly influenced by FRP type and surface condition; while vertical casting showed higher bond strength than horizontal casting. In addition, the bond strength reduction ratio of FRP rebars cast in the horizontal direction was smaller than that of steel bars. POLYM. COMPOS., 37:2098–2108, 2016. © 2015 Society of Plastics Engineers     

Effectiveness of a conductive cementitious mortar anode for cathodic protection of steel in concrete

The paper reports a study on the behaviour of a cementitious conductive overlay anode used for cathodic protection (CP) of steel in concrete. The anode is made of nickel-coated carbon fibres in a cementitious mortar. Tests were carried out on concrete specimens with two layers of rebars that simulated reinforced concrete slabs. Anodic current densities in the range 10-100 mA/m² with respect to the anode surface were imposed. Steel and anode potentials, as well as feeding voltage, were monitored. Four-hour decay and the distribution of current and potential were regularly measured. Galvanostatic polarisation tests were also carried out on the anode material immersed in saturated calcium hydroxide solutions. The maximum anode current output was evaluated. The effectiveness of patch repair of the anode, on areas damaged by excessive current output, is also discussed.     

基于不同胶凝材料钢筋混凝土耐腐蚀性试验研究

针对混凝土结构中钢筋的锈蚀问题,以普通硅酸盐水泥和镁水泥作为胶凝材料,制作了钢筋裸露和涂有环氧树脂涂层的4种钢筋混凝土试件,将其浸泡在氯盐、硫酸盐及镁盐的耦合溶液中,利用电化学方法得到极化曲线和交流阻抗谱,通过塔菲尔外推法进行非线性最小二乘法拟合计算得到腐蚀电流密度、腐蚀电位、腐蚀速率等相关评价指标来衡量混凝土中钢筋锈蚀情况.结果表明:长期浸泡在耦合盐溶液中,普通硅酸盐水泥和镁水泥钢筋混凝土试件都发生了较严重的腐蚀,普通硅酸盐水泥的抗腐蚀性优于镁水泥;在钢筋表面涂有GHT涂层可极大地降低钢筋的腐蚀速率;镁水泥GHT涂层混凝土试件的抗腐蚀性能劣于硅酸盐GHT涂层混凝土试件但优于硅酸盐裸露钢筋混凝土.     

Corrosion behavior of pre-rusted rebars after placement in concrete

This paper presents the results of a preliminary study in which the effect of the initial rusting on the corrosion behavior of rebars embedded in concrete has been investigated. Concrete specimens were made with pre-rusted and rust free rebars of different compositions. Two concrete mixes, one with a sodium chloride content of 2 kg/m³ of concrete and the other without any sodium chloride were used. The reinforced concrete specimens, immersed in potable water and then transferred to 5% NaCl solution, were subjected to corrosion monitoring for a period of 10 months. The test results indicate that the initial rusting does not have an adverse effect on the corrosion resistance of rebars embedded in concrete.     

Effect of temperature on the corrosion behaviour of low-nickel duplex stainless steel bars in concrete

Stainless steel reinforcing bars can be a means for prolonging the service life of reinforced concrete structures exposed to tropical climates. To select a suitable grade of stainless steel according to exposure conditions and design service life, the definition of the chloride threshold for pitting corrosion initiation is required. This paper investigates the effect of temperature in the range 20–60 °C on the resistance to chloride-induced corrosion of low-nickel duplex stainless steel rebars and, for comparison, of traditional austenitic stainless steel rebars. Tests in concrete and in solutions simulating the concrete pore liquid were performed and an attempt to evaluate the chloride threshold levels for corrosion initiation was carried out. Results showed lower corrosion resistance and higher sensitivity to increase in temperature for low-nickel duplex stainless steel bars compared to traditional austenitic stainless steels.     

Development of a galvanic sensor system for detecting the corrosion damage of the steel embedded in concrete structure: Part 2. Laboratory electrochemical testing of sensors in concrete

The correlation between sensor output and the corrosion rate of steel bar was confirmed in concrete environment. Open-circuit potential, linear polarization resistance (LPR) measurement and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion behavior of steel bar embedded in concrete. Also, galvanic current measurements of designed sensors were conducted to obtain the charge of sensor embedded in concrete.In this study, the results of corrosion behavior of reinforcing steel showed a consistence among the data obtained by open-circuit potential monitoring, LPR and EIS measurements. Steel/copper sensor showed a good correlation in concrete environment between sensor output and corrosion rate of steel bar. However, there was no relationship between steel/stainless steel sensor output and corrosion rate of steel bar due to the low galvanic current output. Through the relationship between the steel/copper sensor output and the corrosion rate of reinforcing steel, the real corrosion damage of the reinforcing steel can be detected. Consequently, this confirms that the galvanic sensor system is a good method for detection of corrosion in reinforced concrete.     

Bond performance of FRP rebars with various surface deformations in reinforced concrete

One of the methods used to overcome the corrosion problem associated with steel rebars in reinforced concrete is the application of reinforced polymer (FRP) rebars. Bonding between the rebar and concrete is one of the major factors affecting the mechanical behavior of the structures. In this study, FRP rebars with four different surface configurations and geometries were constructed and their bonding to concrete was examined. These include a pultruded rod with a smooth surface (RO), externally wound FRP rib on the pultruded rod (RT), pultruded rod with sand sprayed on the surface (RS), and a combination of the last two configurations (RTS). Bonding strength of the rebars to concrete was assessed using pull‐out test at the embedment lengths of 12 and 18 cm. Results showed that the increase in surface roughness of the rebars and hence a greater inter‐mechanical locking, plays a major role in their bonding strength to concrete. RO rebar showed the lowest bonding strength to the concrete followed by the RT rebar. Bonding strength to concrete in the latter type was entirely controlled by the adhesion strength of the externally wound FRP rib to the pultruded core. RTS rebars with embedment length of 12 cm showed greater adhesion to the concrete, where as, in samples with embedment length of 18 cm, RTS and RS rebars showed close results. In all the tests conducted, delaminating of the surface texture was the major failure mode. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Electrochemical behavior of coupled carbon fiber-reinforced polymer (CFRP) rods in concrete

This paper presents results on the electrochemical behavior of carbon fiber-reinforced polymer (CFRP) composite rods in contact with steel or epoxy coated steel bars in chloride-contaminated concrete. Twelve concrete prisms reinforced by CFRP rods electrically connected to plain or epoxy coated rebars were exposed to 80% humidity for 345 days. Four identical specimens that were not electrically connected served as controls. Measured galvanic currents densities were found to be as much as 100 A cm^–2, raising concerns about the degradation of both CFRP and steel. Electrochemical impedance spectroscopy (EIS) tests were performed to detect possible changes in the electrochemical parameters of CFRP due to galvanic interaction with active steel. Equivalent circuit simulations of the pre- and post-galvanic interaction of CFRP rods with active steel were also evaluated. EIS data indicated that the composite surface was altered so as to have porous electrode characteristics. Optical microscopy provided visible evidence of interface changes on the composite surface, supporting EIS data. The preliminary findings suggest that it would be unwise to permit CFRP to be directly in contact with steel in reinforcing or prestressing applications.     

A migrating corrosion inhibitor evaluated in concrete containing various contents of admixed chlorides

The performance of a surface-applied migrating corrosion inhibitor (MCI) based on an alkylaminoalcohol was evaluated on concrete specimens containing reinforcing steel bar (rebar) segments. Two water/cement ratios (w/c's), various chloride contents and two exposure conditions were investigated. The inhibiting efficiency was followed over a period of 1000 days by means of parameters such as corrosion potential, corrosion current and electrical resistance. Results show that when concrete is exposed to the marine environment, the inhibitor is able to reduce the corrosion rate (CR) only when the initial chloride content is below 0.16 wt.% relative to cement content. Efficiency increases as the w/c increases. There is no beneficial effect when the initial chloride content is greater than 0.43%. When concrete is immersed in a saline solution, no beneficial effect associated to the use of the inhibitor could be appreciated, regardless of w/c or initial chloride content in concrete.     

Effects of ammonium chloride salt added to mixing water on concrete and reinforced concrete subject to atmospheric corrosion

Studied in this paper is the effect of SO₂ as a pollutant gas on concrete and reinforced concrete specimens at various relative humidity (RH) rates prepared with ideal mixing water (IMW) and IMW+80 mg NH₄⁺ (NH₄Cl) mixing water. The potential values of reinforcing steel were measured throughout 28 days and compressive strength values of concrete specimens in the same medium were determined at the end of the 28th day. SO₂ gas was observed to have a harmful effect at higher RHs on compressive strength of concrete and to make more negative the potential values of reinforcing steel.     

Electrochemical behavior of mild steel in concrete: Influence of pH and carbonate content of concrete pore solution

The increase of the rebars corrosion rate due to the concrete carbonation is the major cause of reinforced concrete degradation. The aim of this study was to investigate the transition from passive to active corrosion of mild steel rebars in carbonated concrete. For this purpose, electrochemical techniques (polarization curves, free corrosion potential measurements) and surface analyses (EDS, XRD, XPS) were used. Five different electrolytes, with pH ranging from 13 to 8.3, were chosen to simulate the interstitial concrete pore water at various degrees of carbonation. The results indicate that the transition pH is between 10 and 9.4. XPS results indicate a passivation of mild steel for pH values ranging from 13 to 10 due to the formation of a thin iron III oxide layer. Immersion tests highlight the importance of the buffering effect of the carbonate content. At the free corrosion potential in an aerated solution, a decrease of the carbonate content increases the corrosion rate. On the opposite, at low electrode potential, the kinetics of oxidation increases with the carbonate content.     

Corrosion by chlorides in reinforced concrete: Determination of chloride concentration threshold by impedance spectroscopy

Chloride penetration in reinforced concrete induces depassivation of the steel rebars and initiation of the corrosion process leading to degradation of the structure. The coupling of “low-frequency” impedance response with SEM observations and multielementary analyses emphasized that the strong decrease of the capacitive part is related to the corrosion initiation. This experimentally determined incubation period is used in an electrodiffusion model based on Fick's second law to quantify the chloride concentration threshold responsible for corrosion initiation on the reinforcing steel surface. This work thus allowed quantifying the incubation period and the critical chloride concentration, referred to in Tuutti's diagram [K. Tuutti, Corrosion of steel in concrete, CBI Research Report no. 4.82, Swedish Cement and Concrete Research Institute, Stockholm, Sweden, 1982].     

Corrosion behaviour of new stainless steels reinforcing bars embedded in concrete

The activities concerned with the evaluation, repair and restoration of structures are estimated to amount to 35% of the total volume of the work in the building sector and this continues to increase. The corrosion of rebars in the reinforced concrete structures (RCS) is the main reason for their degradation, so the use of reinforcing stainless steels seems to be one of the possible solutions with most probabilities of solving this problem. In this work, in order to demonstrate the advantages of using reinforcing stainless steels, the corrosion behaviour of AISI 304 and 316 reinforcements embedded in concrete slabs (C35/45 and C60/70 concrete) with two chloride contents are compared with three low-cost and low-Ni austenoferritic stainless steels and with the conventional carbon steel. The lower chloride contamination selected in this research, was enough to cause the corrosion in the active state of the carbon steel reinforcements, whereas the highest one exceeded the expected contamination in the natural environments, including sea media. The metallic materials remaining in the passive state can be considered, from the point of view of corrosion resistance, adequate as reinforcements in the RCS.     

The effect of SCMs on the corrosion of rebar embedded in mortars subjected to an acetic acid attack

Agricultural effluents are transformed under bacteria effect into organic acids which constitute severe chemical and electrochemical attacks for the reinforced concrete of agricultural structures. Among supplementary cementing materials (SCMs), blast-furnace slag (GBFS) and metakaolin (MK) are classified chemically resistant to the aggressiveness of acidic media and especially organic acids. The objective of this research was to evaluate the effect of GBFS and MK on the corrosion performance of reinforced mortars. Here, electrochemical measurements allow determining the time needed to initiate the corrosion. Mortar cylinders were made with three cement types including ordinary Portland cement (OPC), GBFS and MK cements at a fixed water/cementitious material (w/cm) ratio of 0.65. Corrosion of rebars was simulated by subjecting cylinder specimens to cyclic loading with acetic acid solution (pH 4, 0.5 M) and drying. Concrete resistivity and reinforcing steel potentials were measured up to 429 days of age. At the end of the experiment, all specimens were saw cut split open, and visually inspected. It was found that the drop in the linear polarization resistance and corrosion potential curves reflect the time needed to initiate the corrosion. Blending the cement with 20% of MK is beneficial with regard to delaying the onset of the corrosion by a factor of more than two. However, using high percentage of GBFS (80%) decreases the time to initiate the corrosion for specimen subjected to acetic acid attack.     

光圆钢筋与混凝土界面粘结滑移本构模型研究

对配置光圆钢筋的既有混凝土结构进行承载性能评估时,钢筋与混凝土界面粘结性能优劣是评价该类混凝土结构的重要内容.为此,制作了4组光圆钢筋的中心拉拔试件和2组变形钢筋对比试件,选取粘结长度、钢筋表面形式为主要变量对粘结性能进行试验研究,并通过4组铝合金拉拔试件对比分析光圆筋与混凝土界面的粘结机理.研究结果表明:光圆钢筋的粘结强度显著小于变形钢筋的粘结强度,平均约为变形钢筋粘结强度的18.3%;光圆钢筋的粘结强度主要由化学粘着力和界面摩擦力构成,而铝合金的粘结强度则由化学粘着力构成,平均约为光圆钢筋粘结强度的10%;增加粘结长度后,光圆钢筋的粘结强度随之增大,但铝合金筋的粘结强度无明显变化.进一步基于试验结果及理论分析,建立了光圆钢筋与混凝土界面的粘结-滑移本构模型,并通过已有试验数据对建议模型进行了验证,吻合较好.     

Modes and mechanisms for the degradation of fusion-bonded epoxy-coated steel in a marine concrete environment

Fusion-bonded epoxy is used extensively to protect steel reinforcing bars from corrosion. This coating has proven to be a cost-effective material for increasing the service life of reinforced concrete structures. However, field observations have reported premature corrosion of fusion-bonded epoxy-coated reinforcing bars used in marine concrete environments, leading to severe cracking and spalling of the reinforced concretes. This study was initiated to provide a better understanding of the modes and mechanisms of the corrosion of fusion-bonded epoxy-coated steel exposed in a marine concrete environment. Grit-blasted steel panels, were coated with two commercial fusion-bonded powder epoxy coatings at two thicknesses. Half of the coated panels were scribed, the other half were non-scribed. The panels were immersed in a saturated calcium hydroxide aqueous solution containing 0.6 mole/liter sodium chloride maintained at either 35°C or 50°C. Degradation was characterized and followed by infrared thermography, wet adhesion, and microscopic and analytical chemical techniques. Unscribed panels exhibited only water-induced adhesion loss, most of which was recovered after drying. However, in addition to the water-induced adhesion loss, scribed panels exhibited two modes of failure: anodic blistering near the scribe mark and cathodic delamination around the anodic blisters. Anodic blistering was attributed to localized crevice corrosion under the coating followed by blistering via an osmotic pressure mechanism. Cathodic delamination was probably induced by the alkaline cathodic reaction products, and water-induced adhesion loss was due to the presence of a water layer at the coating/steel interface. Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology, November 12–14, in Philadelphia, PA.