Steel-reinforced concrete is widely used in construction. The corrosion of the steel reinforcing bars (rebars) in concrete limits the life of concrete structures. It is one of the main causes for the deterioration of the civil infrastructure. Corrosion occurs in the steel regardless of the inherent capacity of concrete to protect the steel from corrosion; accelerated corrosion results from the loss of alkalinity in the concrete or the penetration of aggressive ions (such as chloride ions). Methods of corrosion control of steel-reinforced concrete include cathodic protection, surface treatments of the rebars (epoxy coating, galvanizing, copper cladding, protective rust growth, surface oxidation, and sandblasting ), the use of admixtures (organic and inorganic corrosion inhibitors, silica fume, fly ash, slag and latex ) and the use of surface coating.
1. Steel Surface Treatment
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| Sandblasting |
Steel rebars are made of mild steel because of low cost. (Stainless steel is excellent in corrosion resistance,100 but its high cost makes it impractical for use in concrete.) The coating of a steel rebar with epoxy is commonly used to improve corrosion resistance, but it degrades the bond between rebar and concrete, and the tendency of the epoxy coating to debond is a problem. Furthermore, the cut ends of the rebar and areas of the rebar where the epoxy coating is damaged are not protected from corrosion. On the other hand, galvanized steel attains corrosion protection by its zinc coating, which acts as a sacrificial anode. Galvanized steel bonds to concrete better than epoxy-coated steel, and the tendency of the coating to debond is also less. Areas of the rebar where the zinc coating is damaged are still protected; the exposed areas, such as the cut ends, are protected provided they are less than 8 mm from the zinc coating.Steel surface treatments that improve both corrosion resistance and bond strength are attractive. They include sandblasting and surface oxidation.
Sandblasting involves discharging ceramic particles (typically alumina particles around 250 μm) under pressure (about 80 psi or 0.6 MPa). It results in roughening as well as cleaning the surface of the steel rebar. Cleaning relates to the removal of rust and other contaminants on the surface, as a steel rebar is usually covered by rust and other contaminants. Cleaning causes the surface of the rebar to be more uniform in composition, which improves corrosion resistance. Roughening enhances the mechanical interlocking between rebar and concrete, thus increasing bond strength.
Water immersion means total immersion of the rebar in water at room temperature for two days. It causes the formation of a black oxide layer on the surface of the rebar, thus enhancing the composition uniformity of the surface and improving corrosion resistance. In addition, the oxide layer enhances the adhesion between
rebar and concrete, thereby increasing bond strength. Water immersion times that are less than or greater than two days yield less desirable effects on both bond strength and corrosion resistance. Steel rebars can also be coated with a corrosion-inhibiting cement slurry or a cement-polymer composite for corrosion protection.
rebar and concrete, thereby increasing bond strength. Water immersion times that are less than or greater than two days yield less desirable effects on both bond strength and corrosion resistance. Steel rebars can also be coated with a corrosion-inhibiting cement slurry or a cement-polymer composite for corrosion protection.
Of all the methods described for treating the surface of steel rebar, the most widely used are epoxy coating and galvanizing because of their long history of usage.
2. Corrosion Protection by Using Admixtures in Concrete
Admixtures are solids or liquids that are added to a concrete mix to improve the properties of the resulting concrete. Admixtures that enhance the corrosion resistance of steel reinforced concrete include those that are primarily for corrosion inhibition and those that are primarily for improving the structural properties. The latter are attractive because of multifunctionality. The former are mostly inorganic chemicals (such as calcium nitrite, copper oxide, zinc oxide, sodium thiocyanate and alkaline earth silicate) that increase the alkalinity of the concrete, although they can be organic chemicals such as banana juice. Admixtures primarily for structural property improvement can be solid particles such as silica fume, fly ash and slag, and solid particle dispersions such as latex.
Silica fume as an admixture is particularly effective for improving the corrosion resistance of steel-reinforced concrete due to the decrease in the water absorptivity, and not so much because of the increase in electrical resistivity. Latex improves corrosion resistance because it decreases water absorptivity and increases electrical resistivity. Methylcellulose improves corrosion resistance only slightly. Carbon fibers decrease corrosion resistance due to a decrease in electrical resistivity. However, the negative effect of the carbon fibers can be compensated by adding either silica fume or latex, which reduce water absorptivity. The corrosion resistance of carbon fiber-reinforced concrete, which typically contains silica fume for improving fiber dispersion, is superior to that of plain concrete.Table 1 shows the effects of silica fume, latex, methylcellulose, and short carbon fibers as admixtures on the corrosion potential (Ecorr, measured according to ASTM C876 using a high-impedance voltmeter and a saturated calomel electrode placed on the concrete surface; Ecorr that is more negative than –270 mV suggests 90% probability of active corrosion) and the corrosion current density (Icorr, determined by measuring the polarization resistance at a low scan rate of 0.167 mV/s) of steel-reinforced concrete in both saturated Ca(OH)2 and 0.5 N NaCl solutions. The saturated Ca(OH)2 solution simulates the ordinary concrete environment; the NaCl solution represents a high-chloride environment. Silica fume improves the corrosion resistance of rebars in concrete in both saturated Ca(OH)2 and NaCl solutions more effectively than any of the other admixtures, although latex is effective.
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| Table 1: Effect of carbon fibers, methylcellulose,silica fume and latex on corrosion resistance of steel rebars in concrete |
Methylcellulose slightly improves the corrosion resistance of rebar in concrete in Ca(OH)2 solution. Carbon fibers decrease the corrosion resistance of rebars in concrete, mainly because they decrease the electrical resistivity of concrete. The negative effect of fibers can be compensated by either silica fume or latex. Instead of using a corrosion-inhibiting admixture in the entire volume of concrete, one may use the admixture to modify the cement slurry that is used as a coating on the steel rebar. Compared to the use of rebars that have been either epoxy coated or galvanized, this method suffers from its labor-intensive site-oriented process. On the other hand, the use of a shop-coating based on a cement-polymer composite is an emerging alternative. Of all the admixtures described for improving the corrosion resistance of steel reinforced concrete, the most widely used are calcium nitrite, silica fume, and latex.
3. Surface Coating on Concrete
Coatings (such as acrylic rubber) can be applied to the concrete surface for the purpose of corrosion control through improving impermeability. However, this method suffers from the poor durability of the coating, and the loss of corrosion protection in areas where the coating is damaged.
4. Cathodic Protection
Cathodic protection is an effective method for corrosion control of steel-reinforced concrete. It involves the application of a voltage to force electrons to go to the steel rebar, thereby making the steel a cathode. As the voltage needs to be constantly applied, the electrical energy consumption is substantial. This can be alleviated by the use of carbon fiber-reinforced concrete.
As the steel rebar is embedded in concrete, the electrons need to go through the concrete in order to reach the rebar. However, concrete is not very conducting electrically. The use of carbon fiber-reinforced concrete for embedding the rebar facilitates cathodic protection, as the short carbon fibers enhance the conductivity of the concrete.For directing electrons to the steel-reinforced concrete, an electrical contact that is connected to the voltage supply is needed on the concrete. One choice of an electrical contact material is zinc, a coating deposited on the concrete by thermal spraying. It has a very low volume resistivity (thus requiring no metal mesh embedment), but it suffers from poor wear and corrosion resistance, the tendency to oxidize, high thermal expansion coefficient, and high material and processing costs. Another choice is a conductor-filled polymer, that can be applied as a coating without heating, but it suffers from poor wear resistance, higher thermal expansion coefficient, and high material cost. Yet another choice is a metal (e.g., titanium) strip or wire embedded at one end in cement mortar that is in the form of a coating on the steel-reinforced concrete. The use of carbon fiber-reinforced mortar for this coating facilitates cathodic protection, as it is advantageous to enhance its conductivity.
Due to the decrease in volume electrical resistivity associated with carbon fiber addition (0.35 vol. %) to concrete, concrete containing carbon fibers and silica fume reduces the driving voltage required for cathodic protection by 18% compared to plain concrete, and by 28% compared to concrete with silica fume. Because of the decrease in resistivity associated with carbon fiber addition (1.1 vol. %) to mortar, overlay (embedding titanium wires for electrical contacts to steel-reinforced concrete) in the form of mortar containing carbon fibers and latex reduces the driving voltage required for cathodic protection by 10% compared to plain mortar overlay.
In spite of the low resistivity of mortar overlay with carbon fibers, cathodic protection requires multiple metal electrical contacts embedded in the mortar at a spacing of 11 cm or less.
5. Steel Replacement
The replacement of steel rebars by fiber-reinforced polymer rebars is an emerging technology that is attractive because of the corrosion resistance of fiber-reinforced polymer. However, this technology suffers from high cost, the poor bonding between concrete and the fiber-reinforced polymer rebar, and the low ductility of the fiber-reinforced polymer.
6. Conclusion
Methods of corrosion control of steel-reinforced concrete include steel surface treatment, the use of admixtures in concrete, surface coating on concrete, and cathodic protection.