Stainless-Steel Rebars

Stainless steel is an extended family of steel types with a wide variety of characteristics with regard to physical and mechanical properties, cost, and corrosion resistance. They have a much higher corrosion resistance than carbon steel, which derives from a chromium-rich passive film present on their surface. Stainless-steel bars can be used as a preventative technique for structures exposed to aggressive environments, especially in the presence of chlorides. They can also be selectively used in those parts of structures where corrosion is most likely to occur and in the repair of corroding structures. Dilferent available types of stainless steel allow the engineers to select the most suitable in terms of strength, corrosion resistance and cost. 

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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 ... 

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. 

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. 

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. 

Stainless steel rebar has been applied in special circumstances but it is a very expensive option. A clad stainless steel with a mild steel core and stainless... 

Stainless steel rebar has been applied in special circumstances but it is a very expensive option, A duplex steel with a mild steel core and stainless outer 1-2 mm has also been offered, which is cheaper, but still relatively expensive. Stainless steels can be susceptible to pitting attack, so the correct grade of stainless steel must be used. 

Because of the tremendous cost involved in the repair of the existing concrete bridges it is necessary to develop designs that will permit longer life of these structures. One such method is the use of stainless steel rebars that offer the following benefits ... 

Ease of handling and shipping. Stainless steel rebar is easier to work with during shipment and while on site than coated rebar. The inherent protective oxide layer of the stainless steel rebar is resistant... 

Greater strength (lighter structure). The yield and tensile strengths of either duplex stainless steel or austenitic stainless steel rebar are superior to those of carbon steel, permitting smaller diameter stainless steel rebars to be used. This reduces the overall weight of the structure. 

The concept of using stainless steel rebars is relatively new. At the present time no design standards or specifications are available however, the Canadian Standards Association expects to have a stainless steel design specification available by the end of 1998 for use in Canada. Since working codes and allowable bridge loadings vary in different political subdivisions, design specifications will have to be developed by each individual political subdivision. 

The only real long term testing of types AISI 302, 315 and 316 stainless steels in chloride contaminated concrete is reported by Cox etal. (1996). Both industrial and marine splash conditions were tested with stainless steels embedded in prisms or beams of concrete with w/c ratio 0.75 (high permeability) and w/c 0.6. In the marine test site chlorides penetrated into the concrete and the chloride concentration after 22 years was found to be 0.5-0.75% C17cement at 30 mm depth (position of the stainless steel rebars). Examination of the steels after 22 years showed no signs of corrosion. At the industrial site beams with chlorides added to the mix (up to 3.2% with respect to cement content) were tested. Carbonation did not reach the steels and no corrosion was observed. 

Stainless steel rebar. The use of stainless steel rebar is as yet not widespread and is still a novelty in the construction industry. It may thus appear surprising that some industrial applications of stainless rebar... 

TABLE 2.33 Examples of Stainless Steel Rebar Applications... 

Naturally, the initial cost of a structure with stainless steel rebar will be higher than that of a conventional structure. However, the overall construction cost increase may actually be relatively modest. The case for stainless steel rebar can be strengthened when a life-cycle cost approach is followed. This approach helps to focus attention on total costs over the lifetime of a structure, including the frequency and cost of future maintenance and replacement work. In such an analysis performed for a bridge, the cost benefits of austenitic stainless steels over carbon steel were clearly apparent after a time period of 18 to 23 years, at which time major repair costs would be incurred for the conventionally reinforced structure. ... 

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