Abstract As part of a recent experimental campaign several samples prepared in 1994 with various concrete compositions were terminated. These samples were exposed outdoors to seawater wet/dry cycles from 1994 till 2019, and since then to outdoor marine exposure. The remaining samples were monitored since 2021 with galvanostatic pulse measurements and more recently using a commercial device. The terminated samples showed cracks on the concrete surface, a negative rebar potential and a high corrosion rate. One or more rebars were removed from the selected samples. In some cases, the three top rebars were removed, in other cases only one rebar or two rebars were removed. The rebar surface condition upon exposure was recorded by photographing the sample, then the rebars were cleaned by sandblasting the rebars with walnut sand. The cross-section loss as a function of length was measured by using a caliper with conical tip. The pitting factor was calculated using only the sections that showed corrosion, the non-corroding sections were not included to calculate the pitting factor. The percentage mass loss (η) values were estimated by using the mass of the rebar section rebar embedded with respect to the mass a non-corroding rebar of the same length.
Sea water induced reinforcing steel corrosion often results in high maintenance costs and can be service life limiting for concrete bridge substructure elements in marine environments. In the present research, a novel piling type specimen assembly and test protocol were developed to simulate performance of actual substructure elements undergoing marine exposure. Specimen mix design was based on the mortar component of a Florida Department of Transportation (FDOT) Class V high performance concrete, both with and without fly ash. The relatively low diffusion coefficient for such mixes and the long time that normally would be required for corrosion initiation was offset by employing covers of 12 mm for the non-fly ash mortar and 8 mm for the fly ash one. Exposures involved partial submergence in 15 wt% NaCl and in some cases periodic spraying of the above waterline zone to simulate splash. The rebar of some sprayed specimens was connected to submerged bare steel such that the lower portion of the simulated piling rebar was cathodically polarized, and it is demonstrated that the resultant potential profile was similar to that of actual marine pilings for which reinforcement below the waterline also exhibits a relatively negative potential because of oxygen concentration polarization. Consequently, the above waterline (splash) zone of the present specimens was cathodically polarized similar to what occurs in actual structures. Times to corrosion for the specimens were approximately the same for sprayed and unsprayed specimens but were greater for sprayed and polarized ones. In some cases, corrosion initiated above the waterline for sprayed and sprayed and polarized specimens, as typically occurs in actual bridge substructure elements. Threshold chloride ion concentrations to initiate corrosion, C sub T, were measured on the rebar mortar trace subsequent to corrosion initiation and specimen dissection using Energy Dispersive X-Ray Analysis and reported for various test conditions. Values for C sub T for corrosion initiation in both the submerged and above waterline zones are projected for both mix designs; however, these were based in some cases on limited data. It is recommended that the specimen design and test protocol investigated here be studied further.
Two separate binary blended concrete mixes were prepared, one of which contains cement replacement of 50% slag (referred to as SL) and the other of which contains cement replacement of 20% fly ash (termed as FA). Concrete specimens were made using a water to cementitious ratio of 0.41. Different size reservoir lengths ranging from 2.5 cm to 17.5 cm were installed on the top surface of each specimen, and these reservoirs were ponded with 10 percent NaCl solution. Chloride transport was accelerated by electromigration, using an applied potential of 9V initially, and then reduced to 3V after around a week. Steel corrosion parameters were monitored using rebar potential measurements and galvanostatic pulse measurements, over a period of approximately 400 days. Corrosion current values obtained from experimental measurements were then converted to mass loss. Based on the results of experimental findings, the corrosion current values as well as the mass loss values were found to be significantly influenced by the length of the solution reservoirs. This study provides a guideline for subsequent research on accelerated steel corrosion in concrete.
The effective throwing power of a common metallic cladding was investigated by analyses of samples of scribed Alclad, AA2024-T3 (UNS A92024), which were exposed outdoors at Daytona Beach, ...
This investigation addressed the prognosis for 75-year durability of the substructure of Florida Department of Transportation (FDOT) marine bridges constructed with promising concrete formulations. Thirteen bridges, most built with improved concrete formulations, were investigated to determine rate of chloride ion penetration and how it may be affected by preexisting stress cracks. Sound concrete made per recent FDOT specifications for high cement factor, low water cement ratio (w/c) and pozzolanic cement replacement exhibited very slow chloride penetration in aggressive marine bridge substructure service. The best performing concrete, having >752 lb/cu yd (446 kg/cu m) cementitious content, 20% fly ash cement replacement, and w/c ~0.32 showed an average chloride diffusivity ~0.01 sq in./y (~2 x 10 to the -9 power sq cm/sec) at age 11 years in the tidal and low elevation region. Thin (typical ~0.15 mm) stress cracks were found in many of the substructures examined. Many of these cracks in footers or piles reached down to the waterline and extended to at least the rebar depth. Crack incidences in the order of one crack every several meters of waterline perimeter were not uncommon. Even though the cracks were thin, there was substantial preferential chloride penetration immediately around the crack compared with the surrounding sound concrete in the splash evaporation zone. However, no clear indications of corrosion were observed in any of the crack locations examined. Numerical modeling indicates that even very thin preexisting cracks could substantially increase chloride penetration in the immediately surrounding concrete, and that corrosion if initiated could be locally severe. Experiments revealed that the amount of critical corrosion penetration needed to cause damage was greater when corrosion was localized than when corrosion was more uniform. An integrated corrosion initiation and propagation model for sound concrete was created that takes into account the concrete mixture proportions, rebar cover and size, and system geometry. Additional modeling revealed that rebar itself can act as an obstruction to the diffusional chloride flow, causing a local increase in concentration and considerable relative reduction in the projected time to corrosion initiation when the rebar cover is low or the critical chloride concentration high. Derating factors to account for this effect were computed and proposed for use. Further modeling indicated that the region immediately above high tide may be amenable to cathodic prevention of the passive steel with sacrificial anodes.
Cathodic polarization to elevate the corrosion threshold (“cathodic prevention”) was implemented with commercial Zn bulk anodes on laboratory columns to simulate a reinforced concrete marine system. Both a detailed computational model and another simplified, less computational intensive model of the polarization distribution in the laboratory columns were formulated and yielded results in reasonable agreement with the experiment. The simplified model was applied to predict cathodic prevention throwing power on field scale structures for a variety of plausible conditions. The calculations suggest that with an immersed anode useful levels of cathodic prevention may be reasonably expected, even under conservative assumptions, in the area immediately above high tide where conditions are otherwise very severe.
The ability of a SS 316L surface wetted with a thin electrolyte layer to serve as an effective cathode for an active localized corrosion site was studied computationally. The dependence of the total net cathodic current, I net, supplied at the repassivation potential E rp (of the anodic crevice) on relevant physical parameters including water layer thickness (WL), chloride concentration ([Cl−]) and length of cathode (Lc) were investigated using a three-level, full factorial design. The effects of kinetic parameters including the exchange current density (i o,c) and Tafel slope (β c) of oxygen reduction, the anodic passive current density (i p) (on the cathodic surface), and E rp were studied as well using three-level full factorial designs of [Cl−] and Lc with a fixed WL of 25μm. The study found that all the three parameters WL, [Cl−] and Lc as well as the interactions of Lc×WL and Lc×[Cl−] had significant impact on I net. A five-factor regression equation was obtained which fits the computation results reasonably well, but demonstrated that interactions are more complicated than can be explained with a simple linear model. Significant effects on I net were found upon varying either i o,c, β c, or E rp, whereas i p in the studied range was found to have little impact. It was observed that I net asymptotically approached maximum values (I max) when Lc increased to critical minimum values. I max can be used to determine the stability of coupled localized corrosion and the critical Lc provides important information for experimental design and corrosion protection.
Over the last few decades flexible fillers such as greases or waxes have been used in Europe as an alternative to cementitious grout. The Florida Department of Transportation (FDOT) is considering flexible fillers as alternative filler materials for post-tensioned (PT) tendons. In this study, the corrosion resistance of PT tendons is investigated experimentally as they are coated with flexible filler materials and contaminated with fungi. Three types of fungi were considered including Fusarium oxysporum (FO), Penicillium chrysogenum (PC), and Aspergillus flavus (AF). The flexible fillers investigated are commercially available microcrystalline waxes, and five different filler types were investigated. Both exposure and electrochemical corrosion tests were designed and performed. As for the exposure tests, two exposure conditions were considered including direct outdoor exposure test and indoor exposure test as the greases were contaminated with fungi. Electrochemical corrosion test showed that most of the PT tendons had impedances higher than 1 Gohm and galvanic corrosion currents lower than 1 nA. However, for PT tendons coated with filler type 3, an increase in the galvanic corrosion current and a decrease in the impedance were observed after two weeks of tests, indicating corrosion initiation. Direct outdoor exposure tests showed that rust was present on PT single wires coated with type 1, type 2, type 3, type 4, type 5 filler after 20 days, 43 days, 49 days, 114 days and 126 days of exposure, respectively. Indoor exposure tests showed that severe corrosion was present on PT single wires as their surrounding greases were contaminated by a fungi mixture. Different from the general corrosion observed on wires subjected to direct outdoor exposure tests, localized corrosion was present on indoor wires which were coated with flexible filler and contaminated with fungi.
Dry-cast Reinforced concrete pipes (RCP) are frequently used as drainage pipes by several state departments of transportation (DOTs) across the United States. Corrosion has been found on culverts at locations close to the ocean where they experience wet/drying cycles of chloride rich solution. However, modest or no corrosion of RCPs have been observed on RCPs placed in soils and exposed at sites with known high chloride concentration and high moisture content. It is frequently assumed that this good performance is due to oxygen concentration limitations at the reinforcement. Another factor contributing to this observation is that on those dry-cast RCPs in which corrosion had initiated the high porosity of dry-cast RCP allows the corrosion products to move through the pore structure without causing cracks or spalls. This paper presents preliminary results of an investigation being carried out to better understand and characterize on-going corrosion on RCP exposed to relevant conditions (concrete in a high moisture content state and/or exposed to low oxygen concentration at the rebar). The results presented here correspond to the methodology developed to accelerate chloride transport as to initiate corrosion within a short period of time (a few days to a few months instead of a several years).
Specimens with binary and ternary concrete mixtures were prepared. The specimens were reinforced with a #3 rebar and have 0.75 cm concrete cover. Electromigration was used to accelerate chloride transport, this lasted anywhere between a week to a few months. The effect of rebar length under the reservoir and concrete composition was investigated. This paper presents the monitoring via linear polarization resistance and galvanostatic pulse to determine the corrosion current. The solution resistance and rebar potential were monitored for over 600 days. Corrosion in some cases initiated several weeks after removing the electromigration. Selected samples were terminated, only small corrosion spots were found.
Simulated reinforced concrete substructure element (piling) specimens were fabricated, cured for twelve months and exposed partially submerged in natural sea water. The specimens were made of mortar with 20% Fly Ash and a low water to cement ratio, simulating a FDOT Type V concrete. Exposure involved, first, intermittent spraying of the above waterline zone to simulate splash and spray and, second, electrically connecting the embedded reinforcement to submerged bare steel bars to affect a potential gradient from relatively negative in the submerged zone to positive in the atmospheric as occurs in actual members. Three different polarization levels were achieved by inserting resistors. On-potentials and 24 hrs depolarization tests at various elevations were used to better understand the influence of these polarizations on potential vs. elevation and how it affects time to activation. Chloride analysis and visual inspection was performed on selected terminated specimens. The proposed specimen geometry coupled to a submerged rebar provides a good alternative to investigate chloride threshold on partially immersed substructures.
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This investigation was performed to gain insight and assist in determining the long-term durability of reinforced concrete structures where the external chloride concentrations are different than those typically observed at the permanently immersed, tidal, and splash zones of partially immersed bridges. Insight into the diffusion properties of the concrete at these locations may lead to changes in the state’s policy on corrosion control of reinforced concrete structures. Structures at locations where the external chloride concentrations may have spatial, let alone time, variation are of interest. Additionally, at elevations above the splash zone, the amount of chlorides deposited would be lower, and the concrete close to the surface not saturated. Similarly, the section of structures that extend inland would be subjected to seawater spray and the concrete close to the surface would usually not be water saturated. The performance in regard to chloride penetration of specimens made with three base compositions (the supplementary cementitious materials were: 20% fly ash; 20% fly ash + 8% silica fume; and 50% slag), and water-to-cementitious ratios of 0.35, 0.41, or 0.47 were investigated here. Experiments investigated the diffusion of chloride ions into concrete samples that were exposed in scenarios that simulated the splash, tidal, and immersed portions of a marine structure, with the solution ranging from brackish water to 10% seawater, to seawater. Bulk diffusion experiments were conducted in solutions that contained 0.6%, 3%, and 16.5% NaCl. Rapid migration tests and resistivity measurements were also performed several times over two years, and the non-saturated migration coefficient vs. resistivity values were correlated. The apparent diffusion values from the bulk diffusion tests were correlated to corresponding equivalent resistivity values. Samples exposed for over 18 years, simulating tidal exposure were also part of this investigation. The field component investigated the chloride concentration as a function of elevation. Other experiments were conducted which involved controlling the degree of saturation and exposing a designated surface to finely ground salt. These apparent diffusion results were compared to the apparent diffusion results from specimens in which the chloride transport was due to natural marine atmosphere.
Damage of structural significance from crevice corrosion of corrosion resistant alloys requires that at least a portion of the creviced area remain active over a sufficiently long period. Stifling results shen the aggressive chemistry required inside the crevice to keep the material depassivated, i.e., actively corroding, cannot be maintained. This loss of critical chemistry occurs when the rate of mass transport out of the crevice exceeds the rate of dissolution and subsequent hydrolysis of metal ions inside the crevice. For the treatment considered here, the mass transport conditions are constant for a given geometry and potential. What then controls the stability of the internal chemistry is the interaction between the electrochemical kinetics at the interface and the crevice chemistry composition. This work focuses on the parameters that control the stability of crevice corrosion by modeling the evolution of the chemical and electrochemical conditions within a crevice open only at one end (e.g. the mouth) in which the entire crevice is initially filled with the Critical Chemistry Solution (CCS) or filled with chemistries slightly less or more aggressive than the CCS. The crevice mouth is in contact with a weak acid solution (pH 3) that provides the boundary conditions at the crevice mouth. The potential at the mouth was held constant at +0.1 V{sub sce} in most instances with selected cases held at 0.0 V{sub sce}. The material selected was Ni-22Cr-6Mo alloy. The electrochemical kinetics at the pH values of interest have been recently characterized via potentiodynamic polarization. Figure 1 shows the polarization curves for Ni-22Cr-6Mo samples tested at room temperature in various HCl solutions. These data were used in all calculations. That is as the pH changed, a new polarization curve was applied to the position in the crevice. E, pH was calculated at each position and from this data, current at each position was determined. The effects of the crevice gap and the crevice length on stabilization were studied by conducting calculations on samples of various gaps and lengths. In addition, the importance of the increase in the activity coefficient for hydronium ion with high ionic strengths is shown to be critical for stabilizing crevices of the investigated Ni-22Cr-6Mo.
