Long-Term Performance of Epoxy-Coated Reinforcing Steel in Heavy Salt- Contaminated Concrete
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Contents:
These aggressive species have to disrupt the physical barrier of the CHZ film to initiate corrosion of galvanized reinforcement.
Carbonation lowers pH from highly alkaline to around neutrality pH 7 , where the rate of Zn corrosion is very low. As a result, galvanized reinforcement does not generally corrode in carbonated concrete. Chlorides are the more aggressive ions for reinforced concrete and are the most frequent cause of reinforcement distress. Chloride ions come from the raw construction materials, marine environments, or deicing salts.
Zinc is attacked by chloride ions but has a higher threshold value for corrosion to initiate than bare steel. That is, the concentration of chloride ions needed to start corrosion of zinc is up to four times higher than the concentration required to start corrosion of black steel. The overall behavior depends on the source of the chloride ions, the state of the galvanized surface including protection afforded by Zn corrosion products , and the degree of protection provided by the concrete cover.
The corrosion of steel in concrete can be viewed as developing through a two-stage mechanism: initiation and propagation.
Mitigation of corrosion in concrete bridges_图文_百度文库
Efforts to achieve long-term durability of reinforced concrete have been mostly directed at delaying initiation of corrosion of the reinforcement, i. The presence of a pure zinc layer on the surface of the steel reinforcement is the best way to delay the onset of corrosion of the reinforcement. The tenacious passivating film of CHZ is the first line of defense. The pure zinc layer will then corrode uniformly at less than one-tenth the corrosion rate of the base steel, thereby extending the onset of corrosion of the steel reinforcement.
It should also be noted that the zinc corrosion products migrate away from the corrosion site and help densify the concrete surrounding the reinforcement, further delaying the onset of corrosion and also increasing bond strength. Recent research from the National Research Council of Canada reviewed how zinc-coated reinforcing steels perform favorably compared to uncoated reinforcing steels.
In heavily chloride-contaminated concrete, galvanized steel was found to have times lower corrosion rates than carbon steel, depending on environmental exposures. It also explores the corrosion mitigation strategies for further improving its corrosion resistance to extend the service life of concrete structures.
Fusion Bonded Epoxy Coating
The three-year study included a comprehensive program to obtain corrosion data that represent a broad spectrum of corrosion states of galvanized steel in concrete structures. The corrosion rates spanned over five orders of magnitude, from a passive state to low, moderate, and high corrosion rates, by combining the key corrosion parameters including environmental exposure and chloride content as well as concrete type.
Another recent University of Waterloo thesis, Evaluation of the Corrosion Behaviour of Continuously Galvanized Rebar , reviews autopsied concrete slabs that contained several varieties of black and galvanized reinforcement after days of exposure. Electrochemical test measurements were made throughout the life of the project. For the corrosion potentials, ASTM C which characterizes black steel corrosion in concrete was referenced. These were taken bi-weekly over the day period for three types of concrete samples: sound non-cracked , transverse cracked, and longitudinally cracked samples.
The electrochemical readings were interpreted using the NRC report which gave the threshold values for corrosion of zinc in concrete. All standards require that manufacturers of epoxy-coated steel take steps to properly prepare the bars prior to coating, ensure that contaminants are not present, and confirm that the coatings are fully cross-linked and bonded to the bar.
Just like any material used on a jobsite, appropriate handling of epoxy-coated reinforcing steel is required. Handling and storage requirements for epoxy-coated reinforcing steel may be included in contract documents by referencing ACI or ASTM D or within individual agency specifications. Coating damage will affect the long-term performance of the coated reinforcing bar; however, even steel with damaged coatings provide better protection than uncoated bars.
During coated bar manufacture, the holes that cannot be seen with the eye, termed holidays, are monitored and must be less than one per foot, and all visible damage must be repaired. General handling and storage requirements are outlined below. During sagging, steel bars may rub on each other, causing coating damage.
Nylon or padded slings should be used, and at no time should bare chains or cables be permitted. Steel should be unloaded as close as possible to the point of concrete placement to minimize rehandling, and at no time should coated steel bars be dragged, as this may result in coating damage on bar ribs.
Coated and uncoated steel should be stored separately. If the steel bars are to be exposed outdoors for more than 30 days, they should be covered with a suitable opaque material that minimizes condensation.
Long-Term Performance of Epoxy-Coated Reinforcing Steel in Heavy Salt-Contaminated Concrete
The coated steel should be tied using a coated tie wire. This wire is typically When used with epoxy-coated reinforcing bars it is typically coated with PVC. Figure 3. The impedance spectra obtained for the steel rebar exposed to blank solution in presence and absence of the PAMA1 and the PAMA2 are shown in Figure 4 a and b and the corresponding calculated impedance parameters, such as R ct and C dl , values are shown in Table 2.
The analysis of impedance plots reveals that the rebar steel — electrolyte interface response shows only one loop as capacitive arcs. The characteristics of impedance spectra for the rebar in blank and in the presence of azomethine polyamide inhibitors are following similar trend as seen in their corresponding figures. The simple Randle circuit is used to fit the impedance data insets in Figure 4 a, b , where R s indicates the solution resistance and the nanometric adsorptive film resistance contributed by the polymeric inhibitors, R ct , indicates the charge transfer resistance and C dl refers the double layer capacitance.
Table 2. As the Nyquist plots are not perfect semicircles, the difference attributes to the frequency dispersion 43, It is important to notice that the radii of the semi-circles mainly depend on the additive or inhibitor concentrations. The results show that the R ct value increases with increasing polymers concentration, supports the formation of surface film with increasing concentration. Also, increase in R ct and decrease in C dl with increase in polymer concentration during corrosion of rebar refer the higher surface coverage facilitated by the inhibitor.
Further, the protection efficiencies of polymers during the rebar corrosion confirms the interaction role played by the polymeric functional groups with the active sites of the steel rebar. The surface coverages of the PAMA1 and PAMA2 with respect to the corresponding inhibitor efficiency on the steel rebar during the corrosion in chloride contaminated pore solution are plotted in Figure 5 a, b.
The variation of inhibition efficiency as a function of inhibitor concentrations is shown. The corresponding adsorption isotherms are constructed by using surface coverage values as shown in Figure 6 a, b. This provides the basic information about the surface interaction of the metal with the inhibitor molecules. This would help in arriving adsorption mechanism of the polymer molecules and their nature.
The famous approach as accepted by majority is that adsorption of organic molecules follow mainly two types of adsorption i. Analysis of these plots reveals that the azomethine polyamides in chloride contaminated pore solutions follow the Langmuir adsorption. Since adsorption isotherms follows Langmuir type, the organic molecule attached to the metal surface as a monolayer by physisorption. However, sometimes electron density in the polymers also responsible for the interaction with metal surface and hence this can be argued that at a given situation or response, the polymers would have adsorbed on metal surface via chemisorption as well.
It is well reported that chemisorption involves charge sharing or charge transfer from the inhibitor molecules to the metal surface which resulted in the formation of a coordinate-type bond 45 thereby uniform and high surface coverage occurs. The mechanism of adsorption of polymers on steel rebar surface during corrosion is elaborated in section 3.
Figure 6. The microstructure of the polished rebar specimen has no pit holes or cavities which ensures the surface uniformity of the specimen after polishing. However, the presence of cavities in the microstructure of rebar exposed to blank solution as shown in Figure 7 b confirms the corrosion effect by Cl - ions.
The SEM images as shown in Figure 7 c and d show the existence of highly dense and uniform protective layer over the steel surfaces.
UBC Theses and Dissertations
The results indicate that the presence of inhibitors in pore solution has an auto-repairing effect on the defective areas of adsorptive layer during the corrosion of rebar under vulnerable environment. There is also a possible scenario that the protective film might have been formed due to the formation of complex between the corrosion products and the solution components of chlorides and inhibitors, which have protected the metal from further attack.
However, component levels studies are required to prove that mechanism, which can also be helpful in understanding the long term performance of the inhibitors on strength and durability of aspects the concrete structures. Figure 7. AFM was used to probe the surface of steel rebar in the presence and absence of adsorbed polymer film. The 2D topography image of the plain steel rebar after polishing and the steel rebar after corrosion in the presence of the blank and the PAMA2 inhibitor are shown in Figure 8 a-c , respectively.
Precipitation of corrosion products in the concrete pores may give rise to expansive stresses and lead to concrete cracking and spalling. Galvanized reinforcement as with other galvanized products may be satisfactorily welded by all common welding techniques. Although inorganic based corrosion inhibitors, mainly nitrite 17, 18 , and other organic based corrosion inhibitors are used for the corrosion protection of rebar, their negative impact on setting time and strength reduction are yet to be studied in detail 10, The so-called linear polarization resistance method LPR makes use of the inverse proportionality between R p and the instantaneous corrosion rate, known as the Stern—Geary equation [ ], which was derived under several assumptions, one of them being the uniform corrosion morphology [ ]. Abstract: In this experiment, three beams are chosen from a used hollow slab bridge, and one of them is reinforced by planted steel bars in old pavement and others are left intact or cleared away from old pavement for comparison with the first one through static lastage pressing experiments. Data of the properties of rebar steel brands in Lagos, Nigerian market used in reinforced concrete applications.
As seen in Figure 8 a , the polished rebar shows no corrosion pit. Pit holes due to the corrosion attack by Cl - ions on the steel rebar surface in the absence of inhibitors are clearly visible in Figure 8 b. It is evident from this study that the addition of the PAMA2 changes the topography of the steel rebar through film adsorption on its surface. Hence, the AFM result of the rebar in presence of the PAMA2 during corrosion process indicates that the as-formed protective layer contributes to the protection of the steel rebar against chloride attack in the simulated pore solution.
Figure 8. AFM topography of a polished steel rebar b steel rebar after corrosion in blank c steel rebar after corrosion in presence of PAMA2. The bonding of the inhibitor molecules on the surface of metal is mainly influenced by the functional group present in it 8, It is a well-known phenomenon that more is the functional or branches in the structure, higher would be the adsorption of inhibitor resulting in higher inhibition. Displacement of Cl - ions and water molecules from the rebar steel surface by the polyamide molecules.
Formation of a gel like adduct between the displaced Cl - ions and polymer molecules. Prevention of the ingress of both the aggressive Cl - ions as well as water molecules towards the metal surface. During the adsorption, the aromatic polyamide molecules orient themselves in near flat orientation with reference to the metal surface. The polymer molecules arrange themselves parallel to each other on the metal surface forming an organic monolayer that covers the entire metal surface and effectively block the ingress of Cl - ions.
TABLE OF CONTENTS
The general adsorption behavior of aromatic polymer on the metal surface during chloride induced corrosion is represented in Figure 9 a. The results also emphasized that the orientation of the molecules, nature of each functional groups interactions and their electron donating and withdrawing nature with respect to time dependent exchange process etc have made the as synthesized functional polymers as unique and frontier inhibitor for the protection of steel rebar from corrosion 8. Figure 9.
Mechanism of inhibition through a adsorption of aromatic ring present in the polymers on metal steel rebar sites b adsorption of PAMA1 polymer on steel surface and c adsorption of PAMA2 polymer on steel surface. The above studies conclude that the two azomethine polyamides, namely PAMA1 and PAMA2, were successfully synthesized and they act as novel inhibitor for the corrosion protection of the steel rebar from corrosion in chloride contaminated pore solution.
The maximum efficiency of Also, results indicate that appreciable and remarkable inhibition efficiencies of the inhibitors towards the corrosion protection of rebar have been achieved at low concentration level itself.