No abstract is provided for this article.
The stability of localized corrosion sites on SS 316L exposed to atmospheric conditions was studied computationally by considering the wetted cathode and the crevice anode separately and linking them via a constant potential boundary condition at the mouth of the crevice. The limitations on the ability of the cathode that are inherent due to the restricted geometry were assessed in terms of the dependence on physical and electrochemical parameters (temperature, electrolyte layer thickness, solution conductivity, and the size of the cathode). An analysis of variance showed that the solution conductivity and the length of the cathode were the most important parameters in determining the total cathodic current capacity of the external surface. A semi-analytical equation was derived for the total current from a restricted geometry held at a constant potential at one end.
Corrosion of steel reinforcement is one of the main causes of premature deterioration in reinforced concrete (RC) structures. It causes concrete cover cracking, degrades the steel/concrete bond strength, reduces the cross section of steel bars, and consequently reduces the carrying capacity of RC structures. In general, steel corrosion in concrete structures can be divided into two stages: corrosion initiation and corrosion propagation. In this study, the effect of high performance concrete on the corrosion behavior of steel bar in RC specimens are experimentally investigated, particularly during the propagation stage. Concrete specimens with the addition of fly ash, silica fume, and calcium nitrite were fabricated and tested for over 23 years. The corrosion evolution over time was monitored with open circuit potential. During the propagation stage, the corrosion potential, concrete resistivity, and corrosion rate were also measured. Some selected concrete specimens were terminated and visual observation was conducted. Concrete powder was collected above the rebar trace and concrete core samples were drilled and sliced on selected specimens. The chloride concentration was determined. In addition, the steel bars were cleaned and the corrosion morphology was examined. The maximum corrosion pit depth was measured and the cross sectional loss of steel bars was also estimated. Results showed that during the propagation stage, a more negative corrosion potential is related to a higher corrosion rate and a lower concrete resistivity. Localized corrosion was observed on most of the steel bars, with average mass loss ranging from 1.66% to 9.86%, and a pitting factor varying from 1.12 to 2.65, respectively. Addition of fly ash, silica fume, and calcium nitrite reduced the corrosion rate, compared with ordinary Portland cement (OPC). Use of fly ash and silica fume also reduced the chloride concentration at the steel bar surface.
This report describes the findings of concrete resistivity characterization using the surface resistivity (SR) test method. This characterization was conducted on Florida's new and old coastal bridge structures. The testing took place on partially immersed substructures on more than 60 bridges. The SR measurements were performed in areas above the mean tide line (marine growth). The SR profiles at several locations as a function of elevation were performed using a commercial Wenner probe. This Wenner probe is the same instrument used in Florida Department of Transportation's FM5-578 test method. A resistivity gradient from low to high proceeding from the submerged to atmospheric zone is usually observed. This gradient results from the elevation dependence of moisture saturation and pore solution composition. A conditioning procedure was designed and implemented as part of the field testing to increase the moisture content of the concrete being examined (tidal and splash zone). The characteristic SR after one or two days of monitoring is called herein the SRfield. This value was usually the minimum resistivity value observed at the second or higher examined elevation. During the field trips nominal 2-in. concrete cores were obtained in the vicinity of where the SR profiles were made. These cores were used either to perform wet SR measurements in the laboratory or to obtain chloride profiles. The chloride profiles were then used to estimate the apparent chloride diffusivity. The wet SR was achieved after a few weeks of exposure in a high humidity chamber, and is named herein the SRwet. A correlation between SRfield and SRwet was performed and it was found that SRfield was about 3 times that of SRwet; i.e., no full saturation was obtained with the conditioning method. A correlation was computed between chloride diffusivity (average per component) vs. SRwet. However, due to the field nature of the data there is some scatter, the R-squared varied between 0.62 and 0.69 depending on whether all bridges visited were included or if only a subset was used in the fitting. A second component of this project consisted of experiments on laboratory samples aimed at understanding how the resistivity varies over time depending upon the concrete mix and environmental exposure, as well as in the presence - and absence - of reinforcing steel. Numerical modeling was performed to obtain K (cell constants). The K values were used to normalize the SR measurements performed on laboratory samples. Additionally, numerical modeling was performed to investigate additional scenarios (different concrete cover thicknesses, multiple rebar presence, dimensions of the concrete block interrogated, presence of multiple concrete layers). Very useful insight was obtained from this research effort.
The electrical resistivity is an important parameter currently used to characterize the durability of reinforced concrete. Concrete resistivity measurements have been correlated to the corrosion rate of depassivated steel reinforcements and to the chloride penetration resistance of concrete. The resistivity of concrete is influenced by a number of factors such as pore structure, ion composition in pore water, cement content and degree of saturation. Concrete resistivity is temperature dependent and thus an increase in temperature leads to a decrease in resistivity. An investigation was carried out to study the influence of temperature on resistivity measured on mature saturated high performance concrete cylinders (fully hydrated or close-to). Measurements were performed on 10 cm diameter ×20 cm concrete cylinders under temperatures ranging from 10°C to 45°C. Results show that temperature has different effects depending on the intrinsic resistivity of concrete. It is concluded that an accurate method can be developed to normalize concrete resistivity measured at different temperatures and converted these resistivity values to a reference temperature.
The use of reinforced concrete is foundational to modern infrastructure. Acknowledging this, it is imperative that health monitoring techniques be in place to study corrosion within these structures. By using a non-destructive method for detecting the early formation of cracks within reinforced concrete, the method presented in this paper seeks to improve upon traditional techniques of monitoring corrosion, within reinforced concrete structures. In this paper, the authors present a method to evaluate the physical characteristics of reinforced concrete subject to corrosion using a poro-elastic acoustic model inversion technique applied to a set of ultrasonic measurements, which constitutes a novel approach to the problem of observing the impact of corroding rebars and resulting concrete damage. A non-contact ultrasonic transducer is operated at a carrier frequency of 500 [kHz], with a layer of saltwater separating the sensor from the concrete surface. Following this non-contact measurement collection of the surface and rebar echo responses, a poro-elastic model is used to model the sound propagation, through an adapted version of the Biot-Stoll model. At first, a set of default parameters, obtained from the physical characteristics of the reinforced concrete, are used to match experimental and simulated acoustic signature of the sample. Performing statistical averaging along the corroding rebar within three samples over a period of nearly nine months, a small but monotonous increase in the distance between the concrete surface and the top of the rebar, indicating gradual corrosion of the rebar. Next, a non-linear optimization algorithm is used to optimize the match between measured and simulated echoes. Through the implementation of this model parameter optimization, the root mean square error between measured and simulated responses was reduced by 63.7% for the full signal, and 62.6% for the rebar echo.
20% fly ash and 50% slag (termed as T1), while the other contains cement replacement of 20% fly ash and 8% silica fume (termed as T2). Five (T1) or six (T2) single rebar specimens were prepared per concrete mix with a 0.75 cm concrete cover (0.3 in). The water to cementitious ratio of T1 and T2 concrete mixes were 0.41 and 0.37 respectively. Embedded single rebar sections had a radius of 0.47 cm. On the top surface of the specimens, different size reservoirs were attached, ranging from 5 cm to 15 cm. The reservoirs were filled with NaCl solution with 10% concentration. Corrosion initiation occurred by the application of accelerated chloride transport method, and it lasted anywhere from a week to a few months. For ternary blended concrete mixes, the influence of rebar length under the reservoir was analyzed. Subsequently, the electrochemical behavior of the specimens was assessed using electrochemical impedance spectroscopy and linear polarization resistance techniques. The corrosion related parameters such as rebar potential, solution resistance and corrosion current were monitored for around 500 days. Based on the experimental findings, it was observed that the corrosion current values for T1 concrete mixes were found to be larger than those of corrosion current values for T2 concrete mixes.
This research was conducted to better understand the corrosion propagation stage of steel rebar embedded in marine structures prepared with binary blended concrete exposed to high humidity environment. Specimens with binary (Slag (SL)) concrete mix (30.5 cm x 12.7 cm x 7.6 cm) were prepared. A w/cm ratio of 0.41 was used to make this concrete mixture. The specimens were reinforced with #3 rebar and have a 0.75 cm concrete cover. Different size reservoirs were installed on the top surface of the specimens. The reservoir was filled with NaCl solution (10% by weight). Electromigration, which accelerated chloride transport and lasted for a week to a few months, was used. The influence of rebar length under the reservoir for slag containing binary concrete mixes was investigated. This study describes corrosion current monitoring via linear polarization resistance and electrochemical impedance spectroscopy measurements. The solution resistance and rebar potential were monitored for over 600 days. In some instances, corrosion initiated several weeks after the electromigration was removed. Based on the results of the experiments, it was observed that the corrosion current values were considerably influenced by the length of the solution reservoirs.
The stability of localized corrosion sites on SS 316L exposed to atmospheric conditions was studied computationally. The localized corrosion system was decoupled computationally by considering the wetted cathode and the crevice anode separately and linking them via a constant potential boundary condition at the mouth of the crevice. The potential of interest for stability was the repassivation potential. The limitations on the ability of the cathode that are inherent due to the restricted geometry were assessed in terms of the dependence on physical and electrochemical parameters. Physical parameters studied include temperature, electrolyte layer thickness, solution conductivity, and the size of the cathode, as well as the crevice gap for the anode. The current demand of the crevice was determined considering a constant crevice solution composition that simulates the critical crevice solution as described in the literature. An analysis of variance showed that the solution conductivity and the length of the cathode were the most important parameters in determining the total cathodic current capacity of the external surface. A semi-analytical equation was derived for the total current from a restricted geometry held at a constant potential at one end. The equation was able to reproduce all the model computation results both for the wetted external cathode and the crevice and give good explanation on the effects of physicochemical and kinetic parameters.
Close-packed coupled multielectrode arrays simulating a planar electrode were used to monitor the anodic current evolution as a function of position during initiation and propagation of crevice corrosion of AISI 316 stainless steel (UNS S31600) and Ni–Cr–Mo alloy 625 (UNS N06625). Scaling laws vs and vs derived from polarization data in simulated crevice solutions guided the implementation of rescaled crevices with greater spatial resolution. and are the distances from the mouth to the location where the potential reaches two different critical values, and is the crevice gap. Scaling laws were also used along with anodic polarization data in simulated crevice solution to predict crevice corrosion behavior of alloy 22 (UNS N06022). Crevice corrosion of AISI 316 stainless steel in NaCl at readily initiated close to the crevice mouth (i.e., ) at modest applied potentials (e.g., ) and spread both inward and outside the crevice with time. Crevice corrosion initiated farther inside the crevice (i.e., is large) and required higher applied potentials (e.g., ) in the case of alloy 625. The local crevice current density increased dramatically over a short period of time to reach a limiting value in the case of AISI 316; while metastable dissolution behavior over a large area was observed for alloy 625. The ramification of the larger critical depth for Ni–Cr–Mo alloys toward crevice corrosion susceptibility in the case of crevice formers of finite length is discussed. Crevice corrosion shifts to the mouth of the crevice for the less corrosion-resistant materials in crevice solutions saturated in metal salts but remains confined at a distance for alloy 625 under the conditions tested.
There are indications that submerged bulk anodes alone provide some protection in the splash zone, albeit not as much as when combined with either thermal-spray or zinc jackets. Experimental results as well as calculations were used to evaluate the extent of cathodic protection that may be achieved in marine substructures by means of submerged magnesium bulk anodes (SMgBA). To investigate the extent of any contribution of SMgBA in protecting the above-waterline zone, two substructure piles were instrumented with SMgBAs. In addition, a substructure with a SMgBAs was modeled using 1D and 2D finite difference models, the results compared with the field measurements, and the utility of cathodic protection using SMgBAs alone projected.
Close packed coupled multi-electrodes arrays (MEA) simulating a planar electrode were used to measure the current evolution as a function of position during initiation and propagation of crevice corrosion of AISI 316 stainless steel. Scaling laws derived from polarization data enabled the use of rescaled crevices providing spatial resolution. Crevice corrosion of AISI 316 stainless steel in 0.6 M NaCl at 50 C was found to initiate close to the crevice mouth and to spread inwards with time. The local crevice current density increased dramatically over a short period to reach a limiting value.
The Florida Department of Transportation (FDOT) has been using high performance concrete since the late 1980s early 1990s. Some of the older bridge structures (assuming 1990 as the completion year) are reaching 35 year of age, and depending on the concrete cover thickness (those with thinner covers), and the diffusivity of the concrete, corrosion of the reinforcement might initiate in a few years on some of these structures. The corrosion propagation stage of carbon steel rebar in high performance concrete might last longer than the typically five years usually attributed for carbon steel rebar in concrete with type I/II Portland cement as the only cementitious material. It is well known that reinforcement corrosion typically initiates at a later time on specimens in which the concrete contains supplementary cementitious materials such as fly ash, silica fume, slag or a combination of these materials when compared to reinforced concrete with no cementitious materials of the same w/cm ratio, cementitious content, and concrete cover. There are some reports [1-2] that indicate that the chloride threshold is lower on concrete with supplementary cementitious materials, particularly for concrete with high fly ash content (i.e., 50%); thus, this needs to be considered when modeling service life. There are several reports [3-4] that indicate that the corrosion rate of carbon steel rebar embedded in high performance concrete is lower when compared to rebar corroding on concrete with no supplementary cementitious materials. This observation in part can be attributed to the higher concrete electrical resistivity for concrete with supplementary cementitious materials when comparing concretes with similar moisture content. It is possible that there are reduced macrocell effects. The higher resistivity is in part due to pore refinement and higher tortuosity that develop with time on concrete structures with supplementary cementitious materials. In this investigation chlorides were driven into reinforced concrete specimens via an electro-migration method, as a way to accelerate chloride transport and allow corrosion to initiate after a short period of time. Mature (samples prepared in 2008) and recently prepared (April/2016) reinforced concrete samples were used in this investigation to gain additional insight on the corrosion propagation stage. The cementitious in the concrete of the older samples: 1) ordinary portland cement (OPC), 2) OPC and 20% fly ash, and 3) OPC, 20% fly ash and 8% silica-fume. Recent concrete samples contained 1) OPC and 50% slag or 2)OPC and 20% fly ash composition. All specimens had a w/cm ratio of .41 and 390 kg/m 3 of cementitious material. The older specimens had either a single rebar embedded or four rebars (# 5 rebar/2 inch cover). Recently prepared samples had one rebar (#3 rebar/0.75 inch cover). Solution reservoirs ranged from one inch to 4 inches in length, as a way to vary the anode length. Rebar potential was used to determine if corrosion had initiated. The corrosion propagation was monitored via linear polarization measurements, solution resistance measurements, and rebar potentials. The corrosion propagation monitoring ranged from 300 days to several years. 1. M.D.A. Thomas and J.D. Matthews, “Chloride penetration and reinforcement corrosion in marine-exposed fly ash concretes”. In: Malhotra VM, editor, Third CANMET/ACI International Conference on Concrete in a Marine Environment, ACI SP-164, Detroit: American Concrete Institute; p. 317-38 1996 2. F. Presuel-Moreno, M. Paredes, “16 Years’ Exposure Of Fly Ash And Silica Fume Concretes On Salt Induced Reinforcing Steel Corrosion: Corrosion Potential, Resistivity And Diffusivity”, International Conference in Durability of Concrete, Trondheim, Norway, June 18-21, 2012 (Proceeding published on USB/Electronic form) 3. C Andrade, C Alonso “Test methods for on-site corrosion rate measurement of steel reinforcement in concrete by means of the polarization resistance method” Materials and Structures, v37 p623, 2004 4. W. Morris, A. Vico, M. Vázquez, “Chloride induced corrosion of reinforcing steel evaluated by concrete resistivity measurements” Electrochemica Acta, V 49 pp4447–4453 2004
No abstract is provided for this article.
Close packed coupled multielectrode arrays simulating a planar electrode were used to measure the current evolution as a function of position during initiation and propagation of crevice corrosion of AISI 316 stainless steel. Scaling laws derived from polarization data guided the implementation of rescaled crevices providing spatial resolution. Crevice corrosion of AISI 316 stainless steel in NaCl at was found to initiate close to the crevice mouth and to spread inward and outward with time. The local crevice current density increased dramatically over a short period to reach a limiting value.
Results from laboratory specimens and finite element modeling are being used to better understand the apparent surface resistivity values obtained from field measurements. Concrete specimens were prepared with no rebar and also specimens with two different rebar arrangements. Most of the specimens were moist cured for 60 days, and then transferred to lab environment for the following 60 days; thereafter, exposure was outdoor cyclic ponding with sea water. The last two exposures are expected to produce multi-layered concrete resistivity close to the surface. An analytical method was used to study multi-layered resistivity close to concrete surface. Finite element methods were used to assess rebar presence, multi-layered resistivity close to the surface, and the combined effect of these two parameters. It was found that the finite geometry also affects the resistivity measurements.
Chloride-induced corrosion of carbon steel has been widely recognized as one of the main causes of premature failure on the reinforced concrete structures. Various strategies and measures such as employing stainless steel reinforcements have been developed to address this problem. Past studies have been concerned with the identification and characterization of chloride threshold since corrosion would not initiate as long as the chloride concentration values at the reinforcing stainless steel depth remains below this threshold value. It is therefore a critical parameter for the design of new stainless steel reinforced concrete structures and the assessment of existing concrete structures. This study presents the finding on the chloride threshold of stainless steel UNS32304 embedded in mortar with two different mixes. Reinforced mortar specimens were subjected to ponding exposure and wet/dry cycle exposure with a sodium chloride solution. The specimens were monitored by using the measurements of the open circuit potential, electrochemical impedance spectroscopy, and linear polarization resistance. The paper also discusses the chloride threshold values of such stainless steel embedded in mortar and concrete with other mixes reported by other researchers and the factors that may affect these values.
