Stainless Steel Rebar for Concrete Reinforcement: An Update and Selection Guide
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Use of high strength, corrosion resistant stainless steel rebar for concrete reinforcement in bridges, highways, buildings and other construction projects has been on the rise recently - especially when the life cycle costs of this material upgrade are appropriately weighed against the initially lower costs and perceived savings of carbon steel. The trend to stainless has been particularly evident in coastal areas of the United States, and in Canada and Europe.
Increasingly, the higher up-front costs of solid, spiral ribbed stainless steel rebar can be justified when compared with the initial costs, lifetime maintenance costs, replacement costs and operating costs incurred when using carbon steel rebar, with and without cladding or coating.
In practice, stainless steel rebar has been used in many concrete structures to provide high strength and long term resistance to the corrosive attack of chlorides from road salt and harsh marine environments, as well as chlorides formed by concrete in which the rebar is buried.
Possible applications for corrosion resistant stainless rebar could include a host of marine structures such as bridge decks, sidewalks, ramps, parapets, pilings, barriers, retaining walls, anchoring systems, parking garages, sea walls, columns, piers, jetties and moorings. Stainless rebar might be considered also for the infrastructure of chemical and other process plants where corrosion resistance may be important.
Stainless steel rebar, offering a good combination of high strength, toughness, ductility and fatigue resistance, along with corrosion resistance, has been used for construction of bridges and other structures in areas of high seismicity. Of paramount concern here is the need for high strength to preserve the structural integrity of any bridge subject to a seismic disturbance, and the safety of motorists using it.
The seismic retrofit of bridges, it should be noted, is one of six major categories earmarked for funding by the Federal Highway Administration (FHWA). Other FHWA infrastructure projects include innovative bridge research and construction, value pricing projects, and ferry boats and terminals. Any or all three of these projects may require a re-evaluation of rebar materials.
There are also an increasing number of rebar applications, requiring controlled magnetic permeability, where carbon steel cannot be considered an option. Non-magnetic stainless steel rebar has been used successfully in electric motor foundations, and in the construction of buildings housing MRI and similar equipment.
In addition, the same non-magnetic stainless alloys have been used in constructing "deperming" piers, where the proper function of instrumentation is restored in docked ships before they return to sea. Designers or materials specifiers who want a stainless steel with low permeability should specify that the material be checked in accordance with ASTM test method A 342.
Cladding and Coating
In an effort to improve corrosion resistance at less initial cost than stainless steel, some construction projects over the years have tried cladding with stainless Type 316 or epoxy coating carbon steel rebar, with mixed to poor results long term.
Cladding rebar is a relatively new process yet to be proven for long term use. Without an adherent and uniform clad thickness, the rebar will be susceptible to corrosive attack. Furthermore, cladding of carbon rebar is generally done on relatively short lengths. Thus, a large number of bars have to be individually capped and sealed at both ends. This operation is performed either by the bar fabricator or at the construction site, where work is labor intensive. After adding the hazards of transportation, the loading and unloading of stock, and fabrication of the rebar grid, the prospects of having unblemished clad bar have greatly diminished. Without a perfect cladding, carbon steel will be exposed.
Liquefied epoxy coatings, applied to carbon steel rebar by dipping or spraying, have lasted up to 20 years in mild corrosive environments. However, the coating must adhere perfectly to the bar surface and remain free of scratches or damage after application. Similar to cladding, the smallest spot that does not adhere or that suffers surface damage will expose the base metal to corrosion attack.
Since the defect usually cannot be seen or found in the concrete, the bar can seriously deteriorate, shortening the life of the structure in which it is used. Even without a surface flaw, coatings have been known to deteriorate prematurely due to variations in coverage and film thickness.
The problems associated with cladding and coating have caused many materials specifiers to re-think the merits and cost efficiency of stainless steel rebar, particularly for use in severe corrosive environments.
Stainless Properties
Although ASTM designation A 276 lists a good number of stainless alloys that are suitable for use in concrete reinforcement, any one of four major stainless steels can be considered for most applications. These are stainless (S), stainless Type 316LN (S), 18Cr-3Ni-12Mn stainless (S) and stainless Type 304LN (S). See Fig. 1 for their nominal chemical compositions.
For rebar applications, the alloy selection process should start with an evaluation of each alloy&#;s mechanical properties. ASTM specification A955 covering deformed and plain stainless steel bars for concrete reinforcement lists the standard property requirements. This standard allows stainless steel rebar to be produced at three strength levels.
However, Carpenter can achieve a yield strength of 75 ksi (518 MPa) or higher for all four alloys to be considered, and a tensile strength of 100 ksi minimum (690 MPa). These values represent the highest of the three strength levels listed by ASTM A955. The highest strength level can be reached in all standard bar diameters from No. 3 to No. 11 &#; or 0.375" dia. (10 mm) to 1.375" (35 mm). Strength levels, in fact, can be tailored to bar size by modifying hot rolling parameters.
All four stainless steels offer exceptional ductility, which allows the rebar to be easily formed and fabricated. Their elongation properties are in the range of 20 to 30 percent, which is two or even three times the 7 to 12 percent minimum elongation established by ASTM specification A955 for the same alloys at the 75 ksi yield strength level. Elongation is a key property of fabricators who perform numerous bending operations. (Photo 1) In addition, all four alloys have good toughness and fatigue resistance.
This unique combination of mechanical properties makes all four stainless steels candidates for construction projects in areas of active seismicity. Their high strength levels allow designers to use less material and conserve weight. Their good ductility permits structures to flex, without breaking, during any seismic disturbance.
The stainless is a duplex stainless steel with a microstructure consisting of austenite and ferrite phases. This duplex structure, along with the chemical composition, give the alloy an excellent combination of strength and corrosion resistance. In the annealed and hot rolled condition, stainless is ferromagnetic.
Stainless Type 316LN is a nitrogen-strengthened version of stainless Type 316L. It has a significantly higher yield and tensile strength than stainless Type 316L, without significantly affecting ductility, corrosion resistance or non-magnetic properties.
18Cr-3Ni-12Mn stainless is a high-manganese, nitrogen-strengthened austenitic stainless steel that provides substantially higher yield and tensile strengths than stainless Type 304. It can be considered for applications where the strength or magnetic permeability of stainless Type 304 is unsuitable.
Stainless Type 304LN is a nitrogen-strengthened version of stainless Type 304L available in the hot rolled condition. This grade has a much higher yield and tensile strength than Type 304L, without any loss in ductility, corrosion resistance or non-magnetic properties.
Corrosion Resistance
Selection of the best candidate stainless steel for a rebar application may depend on the amount of corrosion resistance required, particularly in view of the similarities in the alloys&#; key mechanical properties. Of the four rebar grades discussed, stainless offers the best overall corrosion resistance.
Compared with conventional stainless steels like Type 304 and Type 316, stainless has superior chloride pitting and crevice corrosion resistance due to higher chromium, molybdenum and nitrogen content. It also has superior resistance to chloride stress corrosion cracking because of its duplex microstructure. Under test conditions, its pitting resistance equivalent number was approximately 50 percent higher than that of the other three alloys.
In general, the corrosion resistance of stainless Type 316LN is similar to that of stainless Type 316L. The higher nitrogen content enhances its resistance to chloride pitting and crevice corrosion. Due to its low carbon content, stainless Type 316LN has good resistance to intergranular corrosion in the as-welded condition. It can be considered for use in severe coastal marine environments.
The 18Cr-3Ni-12Mn stainless provides general corrosion resistance between that of stainless Types 430 and 304. It can be considered for rebar applications where corrosion resistance approaching stainless Type 304 is adequate, but where the strength or magnetic permeability of stainless Type 304 is unsuitable. It also offers good resistance to atmospheric corrosion.
Stainless Type 304LN has corrosion resistance similar to that of the 18Cr-3Ni-12Mn alloy. It offers good resistance to atmospheric corrosion, and can be useful in other less severe environments. This grade also offers useful resistance to road salt environments and the chlorides in concrete.
Magnetic Properties
Three of the four alloys discussed may be considered for those rebar applications where controlled magnetic permeability is most important. Stainless Type 316LN offers low magnetic permeability, which is essential for rebar that could be used in structures close to sensitive electronic devices or magnetic resonance medical equipment.
Like the stainless Type 316LN alloy, the 18Cr-3Ni-12Mn stainless is also non-magnetic in the annealed and hot rolled conditions. It has been used successfully in a Norfolk, Va., "deperming" pier, and also may be considered for use near sensitive electronic devices and medical resonance imaging equipment.
Like the previous two alloys, stainless Type 304LN is also non-magnetic. It therefore may be a candidate for rebar applications close to sensitive electronic devices and MRI machines, so long as its mechanical properties and corrosion resistance are also suitable.
Rebar Length
Steel bars used to reinforce concrete must be connected to each other for maximum strength. They are joined usually by splicing them together with wire, or by means of mechanical couplers or connectors. A customary overlap of about 4 feet of bar at both ends, for splicing, reduces the effective bar length. The percentage of bar lost to overlap is smaller in a long bar than that lost in shorter bar lengths.
Sleeves also must be used to protect unions of dissimilar metals to prevent galvanic corrosion. In summary, the more splicing, connecting, overlapping and sleeving required in a reinforced structure, the more expensive it becomes.
This makes bar length important. Long stainless steel bars, currently made up to 40 feet in length, require less time and expense to join than a larger number of smaller bars. In addition to the time and labor savings, long bars with fewer connections also save weight and space without any loss of strength.
Classic Example
A coastal replacement bridge currently under construction at North Bend, Ore., graphically demonstrates the benefits of using stainless steel rebar instead of carbon steel rebar for critical structural elements in a harsh marine environment. Oregon Department of Transportation (ODOT), which has chosen to use duplex stainless from Carpenter Technology Corporation&#;s Talley Metals subsidiary in Hartsville, S.C., expects the new bridge to provide maintenance-free service for an amazing 120 years. That is 2.5 times the service life of the bridge it is replacing!
When finished by the end of , the bridge will cost approximately $12.5 million. The stainless rebar accounts for only 13 percent of the total bridge cost. For that small increase, ODOT will save the cost of normal bridge replacement in 50 years. That is an amount likely to be $25 million, or at least twice the cost of bridge construction today. In terms of life cycle costs, that is a real accomplishment.
The rebar had to have superior corrosion resistance to withstand the attack of salt-laden air and fog from the Pacific Ocean, and the chloride-containing moisture that used to initiate corrosion underneath the structures.
Extra high strength was required of the stainless rebar to facilitate design of the new bridge, and to deal with the potentially devastating seismic activity in this area. ODOT required a strength level of 75 ksi (520 MPa). This was a strength level new to bridge building, and substantially higher than that of the stainless rebar it used previously in the replacement of two other coastal bridges.
Along with such high strength, the rebar also had to provide good ductility (25 percent elongation) so it could be effectively fabricated. With a higher strength stainless alloy like stainless, ODOT is also enjoying an economic advantage of less stainless rebar weight than would have been required using an alloy of less strength.
It should be noted that stainless steel rebar in this project, the Haynes Inlet Slough Bridge, has been used selectively where its properties were most needed. A much larger volume of uncoated carbon steel rebar is being used in the new bridge for substructure elements where corrosion is less of a problem.
Bar Delivery
The issue of bar delivery, always important, has to be negotiated with the metals supplier. It should be noted, in this regard, that the number of American producers who melt and roll large volumes of stainless steel rebar in the U.S. is limited to a few at the present time. Carpenter Technology, a major supplier, melts and manufactures stainless steel rebar in all grades and sizes entirely in the U.S. These rebar products are marketed by Talley Metals Technology, Inc., a Carpenter subsidiary located in Hartsville, S.C.
Nominal Chemical Compositions of Stainless Steel Rebar
Stainless Steel
UNS No.
Cr%
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Ni%
C%
N%
Mo%
Mn%
Si%
Duplex
S
21.5
5.0
0.02
0.17
2.7
1.7
0.5
Type 316LN
S
17.5
10.5
0.02
0.12
2.1
1.5
0.8
18Cr-3Ni-12Mn
S
17.7
3.5
0.04
0.35
0.3
12.0
0.5
Type 304LN
S
18.5
8.5
0.02
0.12
0.3
1.5
0.5
***
Published in Advanced Materials & Processes, October
By John H. Magee and Raymond E. Schnell
Carpenter Technology Corporation
Reading, PA
USA
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