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.
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.
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.
A literature review was conducted with the goal of identifying alternative low-cost corrosion resistant steel reinforcement materials. The most promising alternate reinforcing materials seen to date that are less expensive than 300 series stainless steels include low-nickel austenitic stainless steels, and a variety of ferritic or martensitic 12-15 weight percent chromium steels. Steels with 2.5-10 weight percent chromium may also be of interest because they offer a marginal gain in corrosion performance at a very low cost. Several steel types that should undergo further evaluation are 201LN, 216, S41003, S24100 and S24000, HSS2, S41425, S41426, and S42300.
Abstract Understanding the performance of modern reinforced concrete structures exposed to marine environments is important to determine its reliability. The splash zone it is usually where corrosion initiates first. However, in Florida, a few older bridges have shown deterioration due to corrosion at the atmospheric zone when exposed to ocean spray for a prolonged period of time. The focus of this study will be on conditions relevant to reinforced concrete bridges exposed to marine environment, for regions >12 ft above the mean high tide (AMHT). Concrete cores were obtained from two bridges in Florida that present corrosion deterioration at the atmospheric zones. To calculate the chloride concentration as a function of depth chloride profiles were obtained. Wet candles were deployed at relevant heights. With these results, we have been able to observe a trend in the chloride deposition at the atmospheric zone in concrete bridges exposed to marine environment. Therefore, one of the purposes of this study is to investigate chloride deposition on concrete bridges exposed to a close proximity to the ocean and no significant obstacles to marine aerosols.
Crevice corrosion is currently mostly studied using either one of two techniques depending on the information desired. The first method involves two multicrevice formers or washers fastened on both sides of a sample plate. This technique provides exposure information regarding the severity of crevice corrosion (depth, position, frequency of attack) but delivers little or no electrochemical information. The second method involves the potentiodynamic or potentiostatic study of an uncreviced sample in a model crevice solution or under a crevice former in aggressive solution where crevice corrosion may initiate and propagate and global current is recorded. However, crevice corrosion initiation and propagation behavior is highly dependent on exact position in the crevice over time. The distance from the crevice mouth will affect the solution composition, the pH, the ohmic potential drop and the true potential in the crevice. Coupled multi-electrode arrays (MEA) were used to study crevice corrosion in order to take in account spatial and temporal evolution of electrochemistry simultaneously. Scaling laws were used to rescale the crevice geometry while keeping the corrosion electrochemical properties equivalent to that of a natural crevice at a smaller length scale. one of the advantages was to be able to use commercial alloys available as wires electrode and, in the case of MEA, to spread the crevice corrosion over many individual electrodes so each one of them will have a near homogeneous electrochemical behavior. The initial step was to obtain anodic polarization curves for the relevant material in acid chloride solution which simulated the crevice electrolyte. using the software Crevicer{trademark}, the potential distribution inside the crevice as a function of the distance from the crevice mouth was determined for various crevice gaps and applied potentials, assuming constant chemistry throughout the crevice. The crevice corrosion initiation location x{sub crit} is the position where the potential drops to E{sub Flade}. Figure 1 illustrates the resulting x{sub crit} vs. G scaling laws for 316 Stainless Steel in 1 M HCl at 50 C. The coupled multi-wire array is composed of one hundred identical 316 Stainless Steel wires in a five by twenty formation inserted in a groove of a 316 Stainless Steel rod such that the ends of the wires are flush mounted with the rod. The 100 wires are coupled electrically through in-line zero resistance ammeters. The diameter of the wires (250 {micro}m) was chosen so that x{sub crit} (critical initiation distance from the crevice mouth) and the expected zone of crevice corrosion (predicted from the scaling law) would be larger than the radius of a single wire. The array created a flush mounted planar electrode with the surface/volume ratio obtained in planar crevices. The observation of the current evolution as a function of position inside and outside the crevice as function of time was made possible as illustrated in Figure 2 in 0.6 M NaCl at 50 C.
No abstract is provided for this article.
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 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.
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.
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.
