Corrosion of reinforcing bars

C. Andrade, V. Feliu, Corrosion of reinforcing bars in carbonated concrete , British Corrosion Journal, 1980, 15, 135. 

Latex-modified mortar and concrete have an excellent water impermeability as described in Sec. 2.4. Such good water impermeability also provides the high resistance of the latex-modified mortar and concrete to chloride ion (O ) penetration, which is one of the most important factors affecting the corrosion of reinforcing bars in reinforced concrete structures. Table gives the apparent chloride ion diffusion coefficient of latex-... 

Latex-modified mortar and concrete also have an excellent oxygen diffusion resistance, since they have an impermeable structure as stated above. Figure 4.64 represents the effect ofthe polymer-cement ratio on the oxygen diffusion coefficient of latex-modified mortars. Such excellent oxygen diffusion resistance is found to contribute to the prevention of the rust formation in the corrosion of reinforcing bars in reinforced concrete structures. 

Virmani, Y.P., Clear, K.C. and Pasko, T.J. (1983). Time to Corrosion of Reinforced Steel in Concrete Slabs, Vol. 5, Calcium Nitrite Admixtures or Epoxy Coated Reinforced Bars as Corrosion Protective Systems, FHWA-RD-83-012, FHWA, US Department of Transportation, 71. 

Epoxy coating of reinforcing bars is a protective technique developed in the 1970s in North America. Laboratory results confirmed the effectiveness of the epoxy-coated bars, in many cases, in preventing corrosion of reinforcement in carbonated or chloride-contaminated concrete [46]. Recently, however, doubts have arisen about their long-term durability in very aggressive environments, doubts borne out above all by negative experience reported on structures in tropical environments [47-49]. 

D. McDonald, D. Pfeifer, P. Virmani, Corrosion resistant reinforcing bars - findings of a 5-year study , Int. Conf on Corrosion and Rehabilitation of Reinforced Concrete Structures, Federal Highway Administration, Orlando, 7-11 December 1998 (CD ROM). 

Figure 4.65 Effects of polymer-cement ratio and calcium nitrite content on corrosion rate of reinforcing bars embedded in latex-modiHed mortars containing calcium nitrite.

In many cases, the consequences of corrosion are not even attributed to corrosion. When a building collapses because of the corrosion of reinforcing steel bar in concrete, it is attributed to faulty materials instead of corrosion. 

Covering of reinforcing bars with resin, apphed as an anti-corrosion protection, can lower their adhesion to cement paste. Worsening of adhesion occurs in the case of vinyl polychloride, but it does not occirr if the epoxy resins are used. However, some epoxy resins reveal the creep effect [56]. 

By applying the moment tensor analysis, kinematics of cracks can be analyzed (Ouyang, Landis et al. 1992). In the expansion test, which simulates crack propagation due to corrosion of reinforcing steel-bar, the moment tensor analysis was performed to identify cracking mechanisms. Here, crack modes of micro-cracks are classified into a tensile crack, shear crack and the mix-mode as illustrated in Fig. 10.27. 

One method of conducting these tests is to have two mats of reinforcing bars with or without admixed chlorides in the top mat and to pond with chloride. The macrocell current between the top and bottom mats as well as the corrosion potentials are measured according to ASTM C 876. This procediue is described in Ref 29. 

Corrosion with resulting expansion of reinforcing bars... 

Figure 6.33. Iron is a base metal that decomposes by electrochemical corrosion in a humid environment. If iron (or steel) is embedded in concrete which is strongly alkaline, however, it forms a passivating surface layer of dense iron oxides. This passivation is decisive of the corrosion resistance of reinforcing bars.

In practice, electrochemical passivation often determines the corrosion resistance of base metals. Passivation of steel by the reaction in eqn. (6.57), for example, is decisive of the durability of reinforcement bars in concrete. As will be shown later in a calculation example, the strongly alkaline environment in concrete with pH 12.5 furthers the formation of a passivating oxide layer of Fe203 on the embedded reinforcement. 

Nickel, cadmium and zinc have all been shown to be capable of delapng and, in some cases, preventing the corrosion of reinforcing steel in concrete, but only zinc-coated (galvanized) bars are commonly available. Field studies of galvanized bars in service for many years in either a marine environment or exposed to deicing salts have failed to show any deficiencies in the concrete. ... 

Corrosion of metal reinforcement bars has caused concrete to fall off the corners of cooling towers. 

Corrosion usually results in a leak or failure of a support because a vessel or support gets too thin. It is then not strong enough to withstand the pressure or load. However, rust can cause failure in another way. It occupies about seven times the volume of the steel from which it was formed. V/hen rust occurs between two plates that have been bolted or riveted together, a high pressure develops. This can force the plates apart or even break the bolts or rivets (see Section 9.1.2 g). Corrosion of the reinforcement bars in concrete can cause the concrete to crack and break away. 

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