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Numerous countries are experiencing problems created by a decaying reinforced concrete infrastructure. This often necessitates the costly repair or replacement of important structures – some of them after only a relatively short lifetime. Faced with these problems, governing authorities are increasingly demanding a greater emphasis on life cycle costs for new construction projects, as opposed to considering only the initial capital costs. This approach helps to focus attention on total costs, including the frequency and cost of future maintenance and replacement work.

In the USA, the National Highway System (NHS) Designation Act came into effect in November 1995. It requires every NHS project over $25 million to be subject to a life cycle cost analysis.

Cost analyses for concrete highway bridges have revealed that the selective use of stainless rebar increases the initial capital project costs by 1% to 15%, depending on the size and complexity of the bridge. This "modest" initial cost increase can be offset by reduced maintenance costs, as illustrated by the following example:

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Adapted from: S.R. Kilworth and J. Fallon "Stainless Steels for Reinforcement", 2nd Regional Conference on Concrete Durability in the Arabian Gulf, Bahrain Society of Engineers, 1995.

In the above example, the initial capital cost increase amounts to 4% for Type 304 and 8% for Type 316.

If "disruption costs" are added to maintenance costs, the argument in favor of stainless steels becomes even more compelling. In the case of repairs to strategic bridges or elevated highways these indirect costs include lost productivity, wasted fuel, delays in deliveries etc. These costs may be difficult to estimate but can be very high for major highways. In the case of toll bridges/tunnels or harbour facilities, the loss of productivity and revenue can be determined more easily.

The above negative effects can be summed up with a recent statement by a North American political leader: "Roads are arteries of our economic prosperity; clogged arteries are bad for your economic health."

 

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The following potential advantages can be cited for stainless steel rebar:

         Inherently good corrosion resistance.

         Reduced life cycle cost for concrete structures.

         Good strength.

         Good weldability for common rebar grades.

         Good ductility for common rebar grades (capable of 3d U-bends).

         No coatings to chip, crack or degrade.

         No coating damage to repair.

         No "exposed" cut ends to coat or cover.

         Capable of withstanding shipping, handling, bending.

         Magnetic or non-magnetic, depending on the alloy specified.

         Good high- and low-temperature mechanical properties of common rebar grades.

Typical Mechanical Properties of Stainless Steels used for Rebars

Type

Yield Strength (MPa)

Tensile Strength (MPa)

Elongation (%)

Magnetic

Austenitic

205-500

550-860

30-60

No*

Duplex

330-560

570-1000

25-40

Yes

 

 

There are currently two standards that deal with stainless steel rebar; BS 6744 (2001) and ASTM A955/A995M (2004).

British Standard BS 6744:2001 "Stainless steel bars for the reinforcement and use in concrete – Requirements and test methods" came into effect on 15 September 2001. This standard supersedes BS 6744:1986, which is now withdrawn.

The revision contains new grades and strength levels not in the 1986 standard. Informative annexes have been used to give guidance on grade selection, magnetic properties, coefficients of thermal expansion and third party certification.

This British Standard specifies various alloys, listed below:

BS EN 10088 Alloy Identifier

AISI or UNS Alloy Identifier

1.4301

AISI 304N

1.4436

AISI 316

1.4429

AISI 316LN

1.4462

UNS S 31803

1.4529

UNS N 08367

1.4501

UNS S 32750


The steel designation numbers are in accordance with the (new) BS EN 10088-1 standard.

Sizes of bars in all grades range from 3 mm to 50 mm (nominal). Nominal cross sectional areas and mass per meter length are provided in this standard for the different alloy grades.

Three strength levels are covered, Grades 200, 500 and 650, corresponding to minimum 0.2% Proof Strength levels of 200 MPa, 500 MPa and 650 MPa respectively (see standard for further details).

ASTM A955/A995M: 2004 "Deformed and Plain Stainless Steel Bars for Concrete Reinforcement"

This ASTM Standard, revised in 2004, covers rebars in a wide range of alloys, including 304 (UNS S30400), 316L (UNS S31603), 316 LN (UNS S31653), and UNS S31803. Further detailed specifications and heat treatment conditions for these alloys are described in the ASTM Standard A276. The designer or specifier thus has a broad spectrum of mechanical properties and corrosion resistance from which to select a suitable rebar material.

  • Wide range of available alloys

  • Nominal sizes: 9.5-57.3 mm diameter

Grade

Minimum Yield Strength*

40 [300]

40 ksi [300 MPa]

60 [420]

60 ksi [420 MPa]

75 [520]

75 ksi [520 MPa]

* Other mechanical properties can be requested from suppliers

Magnetic permeability testing is covered in the Supplementary Requirements section.

Deterioration of reinforced concrete caused by corrosion of the carbon steel reinforcing bars (rebars) is a worldwide problem. The corrosion product ("rust") occupies a greater volume than the original steel bar and this creates a pressure which causes cracking and subsequent spalling of the surrounding concrete.

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Corrosion of carbon steel rebars is greatly accelerated when chlorides are present in the concrete (along with the requisite moisture and oxygen levels to sustain the corrosion reactions). In some parts of the world, chlorides may be incorporated into the original mix due to their presence in the sand, aggregate or water. Most often, chlorides penetrate through the "cover" when the external surfaces of the concrete are exposed to seawater, marine atmospheres or de-icing salts.

Several methods are currently employed in an attempt to reduce the corrosion of carbon steel rebars. These include: rebar coatings (fusion-bonded epoxy, galvanizing); increased concrete cover; reduced water/cement ratios; corrosion inhibitors added to the concrete mix, concrete sealers, cathodic protection; and concrete coatings or membranes.

However, there is increasing interest in the use of reinforcing materials that have inherently good corrosion resistance, thus minimizing the need for maintenance and monitoring of the structure. Stainless steels are such materials and they have been used as concrete rebars in Europe, North America, Asia, Australia and Africa.

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