Steel rebar

Galvanized steel rebar in concrete reduces the corrosion rate to acceptable levels (i.e., <0.5 pm/year in most of the cases) as shown by the data in Table 4.81. 

Corrosion mitigation of steel rebar in concrete can be achieved by ... 

The first mode of corrosion mitigation involves selection of material such as stainless steel rebar and the merits and demerits of this mode are detailed below. 

The standards for stainless steel rebar are available as follows ... 

Carbon steel (rebar, piling) HSLA (incl. line pipe), castings, forgings... 

Often the use of stainless-steel reinforcement is hmited to the outer part of the structure (skin reinforcement) or to its most critical parts for economical reasons. Furthermore, when stainless-steel bars are used in the rehabihtation of corroding structures, they are usually connected to the original carbon-steel rebars. Concern has been expressed with regard to the risk of galvanic corrosion of carbon steel induced by coupHng with stainless-steel bars. Actually, the galvanic corrosion that can arise when stainless steel is used in partial substitution of carbon steel has to be compared with that which takes place in the absence of stainless steels [30]. 

Galvanized-steel rebars can be used as a preventative measure to control corrosion in reinforced concrete structures exposed to carbonation or mild contamination with chlorides, such as chimneys, bridge substructures, tunnels and coastal buildings. 

Methods based on the principle of repassivation of rebars must insure that the reinforcement becomes passive and further depassivation is prevented during the remaining service hfe of the structure. For structures suffering carbonation-in-duced corrosion, this means that alkaline conditions have to be restored around the steel rebars. Therefore, the reinforcement will be protected even if the moisture content of the concrete should be high. 

The most promising corrosion-resistant rebars are galvanized (zinc-coated) rebars, stainless steel-clad rebars, and solid stainless steel rebars. Titanium has also been considered as a rebar metal, but its cost is prohibitive although it is highly corrosion-resistant. 

Stainless Steel Rebars Research in stainless steel rebars consists of clad stainless steel over a carbon steel substrate and solid stainless steel rebar. The primary concerns of cladding when stainless steel alloy is chosen are the following ... 

McDonald et al. studied the performance of solid stainless steel rebars (types 304 and 316) and found that they performed well while ferritic stainless steels (types 405 and 430) developed pitting (15). Studies by McDonald et al. reported investigations on a 10-year exposure of 304 stainless steel in Michigan and Type 304 stainless steel clad rebar in a bridge deck in New Jersey and found no corrosion (15). In a study by Virmani and Clemena, the type 316 stainless steel-clad rebar extended the estimated time to the cracking of the concrete beyond 50 years, but not as much as solid types 304 and 316 stainless steels (100 years) (16). 

In addition, McDonald et al. (15) reported on two highway structures constructed with stainless steel rebar. No corrosion was observed for solid 304 stainless steel rebar in a bridge deck in Michigan as well as in New Jersey. The chloride levels in both bridge decks were below or at the threshold level for corrosion initiation in black steel rebars. It is estimated that the use of solid stainless steel rebar provides an expected life of 75-100 years (15, 16). McDonald et al. estimated the costs, at three installations, of the use of solid stainless steel and found the overall cost to be 6-16% higher than black steel (17). 

Stainless steel rebars have been reported to be used in several projects in the United States, including Michigan and Oregon (17). The expected life of structures using stainless steel rebars was stated to be 120 years. 

Fluctuation in the cost of raw materials used in the production of stainless steel impacts on the economic viability of the use of stainless steel rebars in concrete decks. The rebar cost also depends on the grade of stainless steel used. 

From the point of view of cost, it is preferable to use stainless steel-clad rebar instead of stainless steel rebar. It is estimated that the use of stainless steel-clad rebar provides an expected life of 50 years. The cost of stainless steel cladding can vary depending on the raw material market prices just like solid stainless steel, but also depends on the cladding manufacturer, cladding thickness, and the chosen grade of stainless steel. With proper quality control, stainless steel-clad rebar promises to be an effective means of control for bridge deterioration because of corrosion of reinforcing steel. 

An FHWA study by Thompson and Lankard (19, 20) reviewed the effect on the corrosion of steel in the concrete of several variables, including cement type, mineral admixtures, water-to-cement ratio, and aggregate type. This study showed silica fume to be the most effective mineral admixture in the mitigation of corrosion of steel rebar. It also suggested that careful selection of the concrete mix components could extend the life of a concrete bridge member. It is estimated that use of a silica fume admixture provides an increase of expected life of 10 years beyond that provided by black steel rebar in conventional concrete. 

To provide longer service life to the concrete decks of the order of 75-120 years without the need to repair corrosion-induced concrete damage, a number of solid and clad corrosion-resistant 304 and 316 stainless steel rebars have been developed. Both alloys provide excellent corrosion protection but at higher cost. Type 316 stainless rebar requires more detailed studies. 

There are 543,019 concrete and steel bridges of which 78,448 are structurally deficient, leaving 464,571 bridges to be maintained for estimating purposes it is assumed that all these bridges have a conventionally reinforced concrete deck. The annualized life-cycle direct cost of original construction, routine maintenance, patching and rehabilitation for a black steel rebar deck costs between 18,000 and 22,000. These costs are both corrosion- and non-corrosion related. 

Performance of Stainless Steel Rebar in Concrete Corrosion protection of steel rebar can be achieved by (i) selection of corrosion-resistant steel (ii) use of coatings (iii) addition of corrosion inhibitors such as calcium nitrite to concrete mix (iv) addition of concrete sealers (iv) use of membranes (v) use of thicker concrete overlay (vi) cathodic protection. 

The potential of the corroding surface can be monitored periodically by means of a reference electrode. One such example is the corrosion potential measurement of reinforced steel rebar in concrete structures. Corrosion of the steel in reinforced concrete is a major factor in the deterioration of highway and bridge infrastructure. A survey of the condition ofa reinforced concrete structure is the first step toward its rehabilitation. A rapid, cost-effective, and nondestructive condition survey offers key information to evaluate the corrosion, aids in quality assurance of concrete repair and rehabilitation. 

Fig. 2.12 Corrosion potential measurement of reinforced steel rebar in concrete.

The extent of galvanic corrosion has been measured between steel rebars and chromium containing steel rebars in concrete reinforcement [29]. The corrosion rates were estimated in both chlorinated and carbonated environments. 

Electrochemical corrosion of steel rebar occurs in the local pore solution with the presence of oxygen. 

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