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A detailed computer model of corrosion distribution in reinforced concrete has been used to predict the extent of cathodic protection provided for partially submerged piles by a combination of bulk sacrificial anodes placed below water and surface anodes above water. The model predictions are directly compared with the experimental response to cathodic protection in laboratory piles having active corrosion in progress.
No abstract is provided for this article.
A set of partially immersed reinforced concrete columns with mature corrosion patterns was used to demonstrate that corrosion macrocell currents provided “cathodic prevention” to parts of the steel assembly that remained passive even though exposed to high chloride (~ 4.7% of the cement weight) concrete. Depassivation of that steel was observed upon macrocell decoupling.
Abstract Dry-cast reinforced concrete pipes (D-RCPs) are sometimes used in Florida at places in proximity to the ocean or at locations with high chloride concentrations where the water table seasonally saturates the D-RCPs. Previous studies suggest that corrosion would initiate after sometime in service once the critical chloride threshold has been exceeded at the reinforcement. However, very little is known as to how corrosion would propagate under the environmental service conditions. Instrumented segments obtained from two different types of D-RCPs in which corrosion had already initiated were used to investigate the corrosion propagation stage. During the propagation stage in different exposures, reinforcement eventually reached negative potentials values (< -600 mVsce), which suggest mass transfer limitations. The specimens show no visual signs of corrosion such as cracks or corrosion products. Moreover, the apparent corrosion rate values measured via linear polarization resistance suggest high corrosion rate. No crack appearance so far, could be explained by the corrosion products filling the pore system and also due to the small diameter of the reinforcement or longer exposure is required. Although there might be mass transfer limitations present, the current demanded by the anode is being matched by a larger cathode area due to macrocell effects, since the high moisture conditions likely lowered the concrete electrical resistance and hence increased the throwing power. This trend might change and longer exposure is planned to determine if the corrosion rate might become lower as a result of oxygen depletion at the cathode. Additionally, selected specimens are planned to be terminated to verify corrosion extent.
No abstract is provided for this article.
A literature review was conducted with the goal of identifying alternative, low-cost, corrosion-resistant steel reinforcement materials. The most promising alternate reinforcing materials ...
Reinforced concrete bridge substructures in Florida coastal waters have historically experienced deterioration as a consequence of embedded steel corrosion and resultant concrete cracking and spalling. Ultimately, this deterioration leads to added maintenance costs, reduced service life, and unsafe conditions unless intervention measures are instituted. Galvanic anode cathodic protection (CP), as affected by thermally sprayed zinc for cast-in-place substructure components and zinc mesh jackets for precast ones, is being employed to control this corrosion and extend useful service life. For both types of systems, a submerged bulk zinc anode (SBA) is included to polarize the reinforcement below the waterline and thereby reduce current drain from the lower portion of the thermal spray or zinc mesh. The objective of this research was to determine the extent to which SBAs alone provide protection to the above waterline region of bridge substructures. If adequate or even partial protection is afforded by this means, then considerable cost savings could be realized because of the reduced expense and ease of installation of SBAs compared to zinc thermal spray and CP jackets. To investigate this, two substructure piers on the Bahia Honda Bridge and two on the Niles Channel Bridge, both in the Florida Keys, that were comprised of cast-in-place footers and columns were instrumented with SBAs alone and monitored to determine the level of protection that resulted. For the former bridge, zinc was the SBA type and for the latter magnesium. It was determined that the entire footer reinforcement received protection and that polarization extended up the columns to a certain degree, more so for magnesium than zinc anodes. In addition, the substructures and resultant polarization from the SBAs were modeled using Boundary Element Analysis and two numerical methods. The model results were compared with the field measurements, and the two data sets were determined to be in general agreement. It was concluded that SBAs have utility for extending the useful service life of reinforced concrete bridge substructure elements.
No abstract is provided for this article.
A set of partially immersed reinforced concrete columns with mature corrosion patterns was used to demonstrate that corrosion macrocell currents provided “cathodic prevention” to parts of the steel assembly that remained passive even though it was exposed to high chloride concentrations (up to ∼4.7% of the cement weight) by the end of nine years of testing. The potential of that steel increased following macrocell decoupling, and instances of depassivation were observed 15 days to 75 days after decoupling. The results are contrasted with previous observations of potential dependence of the chloride corrosion threshold in concrete, and an approximate functional relationship between the threshold value and the potential is proposed.
Abstract Simulated deck slab samples cast with six rebars, three in the top row and three on the second row, are being monitored after 19 years of exposure. The rebars are different corrosion resistant reinforcement: 2304, 304, 316, two clad rebars (316 with carbon steel core) and an alloy with 12% Cr. The samples were exposed outdoors to 15% NaCl by wt% for at least 10 years (weekly wet and dry cycles). The concrete cover over the first row in most samples was one inch (2.53 cm). A variety of geometry and rebar surface preparation took place. Rebar potential and electrochemical methods were used to characterize current conditions. Most rebars appear not to be corroding. A couple of samples, containing clad rebar type 2 (two rebars were attached to create a crevice and the rebar ends were not coated), show cracks on the concrete reservoir surface due to corrosion of the rebar carbon steel core. Samples with 12% Cr also show corrosion signs; in this case an artificial crack reduced the concrete cover and two rebars were attached to each other to create a crevice. There are a few other samples with rebars that have no cracks and appear to be corroding based on corrosion rates. The rebars in these samples were either pickled or wire brushed with a carbon steel metal brush (this likely caused small crevices).
No abstract is provided for this article.
Surface resistivity profiles as a function of elevation are being measured on recent and mature marine reinforced concrete substructures. In a later phase of the project the surface resistivity values measured will be correlated with chloride diffusion coefficients measured from the same structures. Previous lab work suggests that a good correlation exists on saturated concrete between these two parameters. The objective of this study is to assess whether a similar correlation can be obtained from field surface resistivity readings. Preliminary results suggest that a conditioning method needs to be applied on-site to approximate water saturation conditions at the elevations of interest.
Concrete specimens measuring 12 in by 5 in by 3 in were prepared with binary concrete mixtures, in which fly ash was replaced for 20% of the ordinary Portland cement. A water to cementitious ratio of 0.41 was used. The specimens had 0.3 in concrete cover and were reinforced with single rebar. For ponding, different size of solution reservoirs were used, ranging from 2.5 cm to 17.5 cm. NaCl solution (10% by weight) was poured into the solution reservoirs. Accelerated chloride transport technique was employed, and this was continued for a week to a few months depending on the values of the off-rebar potential observed for each embedded specimen. The effect of rebar length under the reservoir for fly ash containing binary blended concrete mixes was examined. In this study, two different types of electrochemical measurements named as EIS and LPR were performed to monitor the values of rebar potential and corrosion current. The corrosion propagation was observed for at least 400 days. Sometimes, corrosion initiated several weeks after the removal of accelerated chloride transport approach. According to the experimental findings, it was observed that the corrosion current value increases with the length of the solution reservoir, except for a rebar embedded in specimens with a solution reservoir of 7.5 cm.
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.
