Galvanic Corrosion of Steel in Contact with Carbon-Polymer Composites. I: Experiments in Mortar

Galvanic corrosion is a possible form of deterioration when carbon fiber-reinforced polymer (CFRP) composites and steel are in direct contact in concrete, as in stirrups tied to prestressing CFRP tendons. Laboratory experiments were performed to determine the magnitude of galvanic currents that may take place as a consequence. Preliminary results show little galvanic corrosion of steel (between 10?6 to 0.08 μA/cm2 of steel current density) when coupled with CFRP composites in mortar free of chlorides with a 0.5 water-to-cement ratio (w/c=0.5). However, significant galvanic action (steel current densities as high as 1.5 μA/cm2) was observed in chloride contaminated mortar. The study suggests that steel corrosion caused by deicing salts or marine exposure may be aggravated by the direct contact between steel and CFRP.     

Galvanic Corrosion of Carbon and Steel in Aggressive Environments

The demand for the use of carbon-fiber-reinforced plastics (CFRP) in rehabilitation of deteriorating infrastructure is increasing worldwide. The design characteristics of reinforced concrete or steel members can be enhanced significantly by epoxy bonding CFRP laminates to the critically stressed tension areas. There is, however, a concern regarding possible galvanic corrosion when carbon and steel are bonded together. This paper presents the result of a study on the galvanic corrosion between CFRP laminates and steel. A total of 38 specimens made of steel and carbon fibers were prepared and tested. Two simulated aggressive environments and three different amounts of epoxy coating were used in addition to samples with no coating at all. Furthermore, the effect of the sizing agent on the galvanic corrosion rate was investigated, and three different solvents were used to remove the sizing agents from the surface of the carbon fibers. Potentiodynamic polarization and galvanic corrosion tests were conducted. The results of the experiments showed the existence of galvanic corrosion; however, the rate of such corrosion could be decreased significantly by epoxy coating.     

Effect of Fiber-Reinforced Polymer Wraps on Corrosion Activity and Concrete Cracking in Chloride-Contaminated Concrete Cylinders

This paper presents the results of an experimental study designed to investigate the effect of fiber-reinforced polymer (FRP) wraps on corrosion activity and concrete cracking in chloride-contaminated concrete cylinders. Thirty-five concrete cylinders, each having 102?mm diameter and 204?mm height, concentrically reinforced with one steel reinforcing bar, were subjected to accelerated corrosion exposure for 80?days. Test parameters included level of applied potential, presence of FRP wraps, and bar diameter. The corresponding current and concrete expansion were continuously monitored throughout the corrosion exposure. At the end of the test, the steel bars were extracted, cleaned of rust, and weighed to determine the actual steel mass loss. The results showed that, for the same applied fixed potential, FRP wraps effectively reduced the corresponding current, the concrete expansion, and the steel mass loss. For the same applied potential, the current density increased as the bar diameter decreased. For the same corrosion depth, the circumferential expansion of the cylinder caused by corrosion decreased as the concrete cover-to-bar diameter ratio (c/d) increased.     

Fatigue Flexural Behavior of Corroded Reinforced Concrete Beams Repaired with CFRP Sheets

This study investigated the flexural behavior of corroded steel reinforced concrete beams repaired with carbon-fiber-reinforced polymer (CFRP) sheets under repeated loading. Thirty beams (152×254×2,000?mm) were constructed and tested. Fatigue flexural failure occurred in 29 of these beams. The study showed that pitting of the steel reinforcement due to corrosion occurred only after about a 7% actual mass loss which coincided with a decrease in the fatigue performance of the beam. The controlling factor for the fatigue strength of the beams is the fatigue strength of the steel bars. Repairing with CFRP sheets increased the fatigue capacity of the beams with corroded steel reinforcement beyond that of the control unrepaired beams with uncorroded steel reinforcement. Beams repaired with CFRP at a medium corrosion level and then further corroded to a high corrosion level before testing had a comparable fatigue performance to those that were repaired and tested after corroding directly to a high corrosion level.     

Structural Performance of CFRP-Strengthened RC Slabs in a Corrosive Environment

This study was undertaken to address the effect of the main steel corrosion on the structural performance of RC slabs strengthened with carbon-fiber-reinforced polymer (CFRP) strips and exposed to a corrosive environment. A total of eight specimens (500×100×1,500?mm) were constructed and tested under monotonic static loading. Three specimens were CFRP-strengthened and corroded, three specimens were CFRP-strengthened and kept at room temperature, one specimen was unstrengthened and corroded, and one specimen was neither strengthened nor corroded. Three different strengthening schemes were applied: (1) externally bonded CFRP strips; (2) externally bonded CFRP strips provided with CFRP anchors; and (3) near surface mounted (NSM) CFRP strips. During the corrosion process, the specimens were placed in a small tank filled with sodium chloride (NaCl) solution concentration (3%) which covered only the slabs’ bottom third, and corrosion was induced by means of an impressed current. The corrosion process lasted for 20 days, and the average mass loss of the main steel reinforcement due to corrosion was 9%. Following corrosion, the specimens were tested under four-point bending. The experimental results showed that the increase in flexural capacity achieved using the three strengthening schemes were significantly reduced due to corrosion of the main steel. The recorded reductions in flexural strength gains for the CFRP-strengthened corroded slabs relative to the gains for the strengthened uncorroded slabs were about 55, 38, and 41% for the externally bonded CFRP system without anchors, externally bonded CFRP with anchors, and NSM-CFRP system, respectively.     

Effects of Corrosion of Steel Reinforcement on RC Columns Wrapped with FRP Sheets

This study investigated the effectiveness of carbon fiber-reinforced polymer (CFRP) sheets in protecting reinforced concrete (RC) columns from corrosion of steel reinforcement. Thirty small-scale RC columns and four midscale RC columns were used in this study. The small-scale columns were used for a comprehensive parametric study, whereas the midscale columns were used to evaluate design guidelines proposed based on the results of the small-scale column tests. The test columns were conditioned under an accelerated corrosion process and then tested under uniaxial compression up to failure. The test results showed that although CFRP sheet wrapping decreased the corrosion rate, the corrosion of steel reinforcement could continue to occur, eventually showing a decrease in ultimate axial compression capacity. Design guidelines were proposed based on the small-scale RC column tests and evaluated through a comparison with the test results of midscale RC columns. The proposed design guidelines introduced a concept of effective area to account for the corrosion damage, such as internal cracking and cross-sectional loss of steel reinforcement.     

Repair of Cracked Steel Girders Connected to Concrete Slabs Using Carbon-Fiber-Reinforced Polymer Sheets

This paper presents the results of an experimental study on the repair of artificially damaged steel–concrete composite beams repaired using adhesively bonded carbon-fiber-reinforced polymer (CFRP) sheets. Eleven, 2 m long, beams composed of W150×22 steel sections with 465×75?mm concrete slabs were tested in four-point bending. Severe damage was first introduced in ten beams by saw cutting the tension flange completely at mid span, to simulate a fatigue crack or a localized severe corrosion. Standard modulus (SM) and high modulus (HM) CFRP sheets were then used to repair nine damaged beams. The length and number of CFRP layers applied to the cracked flange on the underside, or on both sides, were varied. Results showed that the damage had reduced flexural strength and stiffness by 60 and 54%, respectively. Nevertheless, CFRP-repaired beams achieved various levels of recovery, and in some cases, exceeded the original capacities. The strength of beams repaired with sheets, ranging in length from 8 to 97% of the span, varied from 46–116% of the original undamaged strength, whereas the stiffness range was 86–126% of original stiffness. SM-CFRP failed by debonding whereas HM-CFRP was ruptured. Bonding the sheets to both sides of the flange was not very advantageous over bonding to the underside only.     

Carbon-Fiber-Reinforced Polymer Repair to Extend Service Life of Corroded Reinforced Concrete Beams

This paper presents the results of an experimental study designed to investigate the viability of using externally bonded carbon-fiber-reinforced polymer (CFRP) laminates to extend the service life of corroded reinforced concrete (RC) beams. A total of 14 beams, 152×254×3,200?mm each, were tested. Three beams were not corroded; two of them were strengthened by CFRP laminates, while one specimen was kept as a virgin. The remaining 11 beams were subjected to different levels of corrosion damage up to a 31% steel mass loss using an impressed current technique. Six of the corroded beams were repaired with CFRP laminates, whereas the remaining five beams were not repaired. Eventually, all specimens were tested to failure under four-point bending. Corrosion of the steel reinforcement significantly reduced the load-carrying capacity of RC beams. At all levels of corrosion damage, CFRP repair increased the ultimate strengths of the corroded beams to levels higher than the strength of the virgin beam but significantly reduced the deflection capacity.     

Performance of Coated Reinforcing Bars in Cracked Bridge Decks

The presence of cracks in bridge decks that are reinforced with epoxy-coated reinforcing (ECR) bars has raised some concerns among bridge and maintenance engineers in the state of Iowa. To study the effects of deck cracking on the performance of ECR bars, several concrete cores that contained reinforcing bars were collected from 80 bridges that are located in different counties throughout the state of Iowa. These samples were collected from cracked and uncracked areas of the bridge decks. Concrete powder samples were collected from these cores and were analyzed in the laboratory to determine the diffusion of the chloride in the bridge decks. This study revealed that no sign of corrosion was detected for the ECR rebars that were taken at the uncracked bridge deck locations. In addition, no delamination or spalling was observed for the bridge decks where bars in the core samples, which were taken at the cracked bridge deck locations, exhibited signs of corrosion. The collected ECR rebars samples were rated according to the degree of the corrosion that was observed on each bar. These ratings were used to develop condition/age relationships that were utilized to estimate the functional service life of bridge decks that are reinforced with ECR bars.     

Control of Corrosion-Induced Damage in Reinforced Concrete Beams Using Carbon Fiber-Reinforced Polymer Laminates

This paper presents the results of an experimental study conducted to investigate the effect of carbon fiber-reinforced polymer (CFRP) confinement on the cracking damage induced by impressed current-accelerated corrosion of reinforced concrete beams. The beams were 254?mm deep by 152?mm wide by 3,200?mm long. Two different corrosion configurations, namely uniform and shear-span corrosion, were investigated in eight specimens at three different degrees of corrosion (5, 10, and 15% theoretical mass loss). Uniform corrosion along the whole length of the beams (3,000?mm) and shear-span corrosion (900?mm from each beam end) were considered. The different degrees of corrosion were induced using an accelerated corrosion technique with an impressed current. Based on the results, it was concluded that CFRP laminate confinement reduces corrosion expansion by up to 70% and slows the rate of corrosion through decreasing the corrosion mass loss by up to 35%.     

Effect of Severe Environmental Exposures on CFRP Wrapped Concrete Columns

Deterioration of concrete structures caused by corrosion of reinforcing steel, aging, and weathering is a major problem in harsh environments such as coastal areas and cold regions. In addition, a hot environment, such as in the Arabian Gulf, is recognized as one of the most severe and aggressive environments that affects concrete durability. The purpose of this study is to investigate the effectiveness of strengthening plain concrete cylinders, subjected to extreme temperature variations, by wrapping with two layers of unidirectional carbon fiber-reinforced polymer (CFRP) sheets. Thirty-six plain concrete cylinders (150×300?mm) were tested. Nine specimens served as unstrengthened controls and the remaining cylinders were strengthened with two layers of CFRP sheets. Cylinders were subjected to high temperatures (45°C), to heating and cooling cycles (23 to 45°C), and to prolonged heat exposure (45°C). Some of the cylinders that were subjected to heating and cooling, were later subjected to freezing and thawing cycles, while others were submerged in fresh water or salt water. The specimens were loaded to failure under uniaxial compressive load and the axial and lateral deformations were monitored. High temperature exposure was not found to decrease the strength of the wrapped concrete cylinders.     

Bond Strength of Fiber Reinforced Polymer Rebars in Normal Strength Concrete

The bond behavior of reinforcing bars in concrete is a critical issue in the design of reinforced concrete structures. This study focuses on the bond strength of fiber reinforced polymer (FRP) rebars in normal strength concrete. Four different types of rebars were tested using the pullout method: aramid FRP (AFRP); carbon FRP (CFRP); glass FRP (GFRP), and steel. This involved a total of 151 specimens containing 6, 8, 10, 16, and 19?mm rebars embedded in a 203?mm concrete cube. The test embedment lengths were five, seven, and nine times the rebar diameter (db). For each rebar, the test results include the bond stress–slip response and the mode of failure. The test results showed that the bond strength of an FRP rebar is, on average, 40–100% the bond strength on a steel rebar for pullout failure mode. Based on this research, a proposal for the average bond strength of straight FRP rebars in normal strength concrete is made, which verifies an existing bond strength relationship (GFRP) and extends its application to AFRP and CFRP. It is an expression that is a function of the rebar diameter, and the concrete compressive strength.     

CFRP Tendons for the Repair of Posttensioned, Unbonded Concrete Buildings

The deterioration attributable to corrosion of concrete structures reinforced with unbonded, posttensioned tendons is a costly problem. Recent research has shown composite materials such as fiber-reinforced polymers (FRP) to be suitable alternatives to steel because they provide similar strength without susceptibility to electrochemical corrosion. Carbon-FRP (CFRP) in particular has great promise for prestressed applications because it shows resistance to corrosion in environments that might be encountered in concrete and experiences less relaxation than steel. This paper outlines the testing and implementation of a posttensioned system that uses CFRP tendons to replace corroded, unbonded posttensioned steel tendons. This system was then implemented in a parking garage in downtown Toronto. To the writers’ knowledge, this is the first example of an unbonded, posttensioned tendon replacement using FRP tendons. The system used split-wedge anchors designed specifically for CFRP tendons. The dead end was anchored by directly bonding the tendon to the concrete slab. The CFRP tendon was successfully inserted in the opening created by the removal of the corroded tendon and stressed. Although the system was shown to be feasible, the current anchorage configuration results in load losses of up to 60% during the transfer. Changing the orientation of the anchor was found to reduce the load loss to an acceptable range of 1–9%.     

Long-Term Durability of State Street Bridge on Interstate 80

Destructive and nondestructive techniques were employed to evaluate the long-term durability of the carbon fiber reinforced polymer (CFRP) composite and externally CFRP-reinforced concrete of the State Street Bridge. Nondestructive evaluation was conducted through strain gauges, tiltmeters, thermocouples, and humidity sensors installed on the bridge bents for real-time health monitoring. Destructive tests were performed to determine the ultimate tensile strength, hoop strength, concrete confinement enhancement, and bond-to-concrete capacity of the CFRP composite for 3 years of exposure. Thermographic imaging was used for detection of voids between CFRP composite and concrete. Although environmental conditions were found to have an effect on the durability of the CFRP composite and CFRP-reinforced concrete substrate, no evidence of steel reinforcement corrosion was observed, and the CFRP composite retrofit is still effective after 3 years.     

In-situ Corrosion Characterization of API X80 Steel and Its Corresponding HAZ Microstructures in an Acidic Environment

During heat treatment processing,microstructures of heat affected zone(HAZ)were formed in X80 pipeline steel.After observation by optical microscopy,scanning electron microscopy and transmission electron microscopy,microstructure of the as-received X80 steel was confirmed to be acicular ferrite,while the microstructures of quenched,normalized and annealed X80 steels were lath bainite,granular ferrite and quasi-polygonal ferrite,respectively.After immersion in the simulated acidic soil solution for 48 h,corrosion rates of these four steels were determined by mass loss measurements and corrosion products were examined by Raman spectroscopy and X-ray photoelectron spectroscopy.Scanning vibrating electrode technique was used to characterize the micro-galvanic corrosion behaviors of the synthetic bimetallic electrodes which were formed by coupling each of the simulated HAZ microstructures with the as-received steel in direct physical and electric contact.It is demonstrated that the as-received steel acts as cathode in the as-received/quenched and as-received/normalized couples,while the annealed steel acts as cathode when coupling with the as-received steel.The distinction of current density between the galvanic couples reduces with prolonging the immersion time.     

Mechanism of Bond Behavior of Concrete Beams Strengthened with Near-Surface-Mounted CFRP Rods

Bond tests were conducted on concrete beams strengthened with near-surface-mounted (NSM) nonprestressed and prestressed carbon fiber-reinforced polymer (CFRP) rods under static loading. In the NSM technique, the CFRP rods are placed inside precut grooves and bonded to the concrete with epoxy adhesive. Six concrete beams were tested. The test variables included presence of internal tension steel reinforcement (unreinforced and reinforced), use of NSM CFRP strengthening (nonprestressed and prestressed), and type of CFRP rod (spirally wound and sand blasted). The beams were tested statically in four-point bending. Based on the test results, the transfer length for the prestressed CFRP rod in epoxied groove was 150 and 210 mm for the sand blasted and spirally wound rods, respectively. The main failure mode was debonding between the CFRP rod and the epoxy that starts at sections close to the midspan then, as the load increases, it propagates toward the supports. At failure, the beams strengthened with a given rod type showed the same CFRP strain at sections close to the support (29% of ultimate strain for spirally wound bars and 39% of ultimate strain for sand blasted bars). A cracked section analysis was carried out and compared well with the measured results.     

Flexural Behavior of R/C Beams Strengthened with Carbon Fiber Reinforced Polymer Sheets or Fabric

Their resistance to electro-chemical corrosion, high strength-to-weight ratio, larger creep strain, fatigue resistance, and nonmagnetic and nonmetallic properties make carbon fiber reinforced polymer (CFRP) composites a viable alternative to bonding of steel plates in repair and rehabilitation of reinforced concrete structures. The objective of this investigation is to study the effectiveness of externally bonded CFRP sheets or carbon fiber fabric in increasing the flexural strength of concrete beams. Four-point bending flexural tests were conducted up to failure on nine concrete beams strengthened with different layouts of CFRP sheets and carbon fiber fabric and on three beams with different layouts of anchored CFRP sheets. An analytical procedure, based on compatibility of deformations and equilibrium of forces, was presented to predict the flexural behavior of beams strengthened with CFRP sheets and carbon fiber fabric. Comparisons were made between the test results and the analytical calculations. The flexural strength was increased up to 58% on concrete beams strengthened with anchored CFRP sheets.     

Flexural Strengthening of Steel Bridges with High Modulus CFRP Strips

Acceptance of carbon fiber-reinforced polymer (CFRP) materials for strengthening concrete structures, together with the recent availability of higher modulus CFRP strips, has resulted in the possibility to also strengthen steel structures. Steel bridge girders and building frames may require strengthening due to corrosion induced cross-section losses or changes in use. An experimental study investigating the feasibility of different strengthening approaches was conducted. Large-scale steel-concrete composite beams, typical of bridge structures, were used to consider the effect of CFRP modulus, prestressing of the CFRP strips, and splicing finite lengths of CFRP strips. All of the techniques examined were effective in utilizing the full capacity of the CFRP material, and increasing the elastic stiffness and ultimate strength of the beams. Results of the experimental program were compared to an analytical model that requires only the beam geometry and the constitutive properties of the CFRP, steel, and concrete. This model was used to investigate the importance of several key parameters. Finally, an approach for design is proposed that considers the bilinear behavior of a typical strengthened beam to the elastic-plastic behavior of the same beam before strengthening.     

Flexural Strengthening of RC Beams with Prestressed CFRP Sheets: Development of Nonmetallic Anchor Systems

This paper presents a novel anchoring technique for strengthening reinforced concrete beams with prestressed carbon fiber- reinforced polymer (CFRP) sheets. Permanent steel anchors are commonly used for the application of prestressed CFRP sheets. The steel anchors are, however, susceptible to corrosion and may not blend into the aesthetics of the original structure. As a result, it may be preferable to remove the steel anchors after transferring the required prestress to the structure with minimal losses of sustained prestress. A technique for replacing the steel anchors with nonmetallic anchors is investigated and reported herein. Nine doubly reinforced concrete beams are tested with various types of nonmetallic anchor systems such as nonanchored U-wraps, mechanically anchored U-wraps, and CFRP sheet-anchored U-wraps. The developed nonmetallic anchorages successfully transfer the sustained prestress in the CFRP sheets with insignificant prestress losses. A closed-form solution for the transfer of prestress is developed and compared to the experimental results.