Stainless steel reinforcement: the role of nickel

Nickel-containing stainless steel reinforcement (rebar) is increasingly used to ensure that the infrastructure such as bridges, flyovers (overpasses) or tunnels are able to pass the test of time, even in the harshest of environments.

Stainless Steel Reinforcement

When one or more lanes of a bridge, overpass or elevated roadway are closed for repairs, which may take months to complete, traffic congestion and chaos result. As our infrastructure ages, concrete deterioration has become one of the most costly and widespread problems. The reason, according to Dr Karen Scrivener, Head of the Construction Laboratory at the Swiss Federal Institute for Technology, is that corrosion attack on reinforcement bars is “the cause of over 90 percent of problems with concrete durability”. This is why specifying nickel-containing stainless steels for critical infrastructure represent the cost-effective choice, offering added benefits for both citizens and the environment.

Often, the repairs are needed because the carbon steel rebar - whether epoxy-coated or not - corrodes, causing the concrete to crack. Salt, whether for keeping roads free of ice in winter or entering via the atmosphere from marine environments, penetrates the concrete and causes the corrosion. As the rust produced has a greater volume than the steel, it causes the concrete to crack which in turn allows more salt-laden water to penetrate faster and deeper into the concrete structure. To prevent the eventual catastrophic failure of the structure, expensive and intensive refurbishment becomes vital, usually highly-disruptive in cities and major roads.

Why is stainless steel reinforcement superior?

  • Stainless steel rebar substantially prolong the useful lifespan of those reinforced concrete structures exposed to corrosive conditions, such as road de-icing salt or marine environments

  • Some stainless steel reinforcement applications date back more than 30 years, such as the I-696 Bridge in Michigan; one marine application – the Progreso Pier in Mexico - has been in service for more than 75 years

  • Highway bridges, local bridges, flyovers, underpasses and ramps have all been built or repaired using stainless steel rebar, as have coastal facilities, tunnels and parking garages

  • Historical structures have been strengthened and repaired using stainless steel rebar

  • Stainless steel rebar combines strength, ductility and toughness over a wide temperature range, including very low temperatures

  • Stainless steel rebar and associated products, such as dowels, connectors, tie wire are readily available

  • Stainless steel rebar is supplied in accordance with standards ASTM A955 and BS 6744, which ensure an alloy’s good corrosion resistance and mechanical properties

  • Stainless steel rebar can be supplied in non-magnetic condition - essential for certain military, medical and laboratory applications.

History of stainless steel rebar

Progreso Pier, Mexico

The Progreso Pier on the Yucatan Peninsula in Mexico is an outstanding example of the long service that stainless steel rebar can deliver. This 2.1 km long pier, completed in 1941 (and subsequently extended to 6.5 km in 1985), has withstood constant exposure to the tropical marine environment of the Gulf of Mexico without any significant maintenance or repairs. The reinforcement used in the pier is similar to the current Type 304 stainless steel.

Background: Progreso Pier, built with stainless steel reinforcement in 1941. Foreground: remains of a pier built with carbon steel reinforcement around 20 years later.

There have been several articles on the Progreso Pier. A life cycle cost analysis also clearly demonstrated the cost-effective performance of the stainless steel rebar.

I-696 Bridge, Michigan, USA

The oldest-known use of stainless steel rebar (Type 304) in the USA is in the deck of a highway bridge on Interstate Highway I-696 near Detroit, Michigan. The bridge was built in 1983 and the deck is de-iced with salt every winter. Recent inspections by the Michigan Department of Transport found the bridge to be in excellent condition.

Bridges in the U.K. and Europe

Several bridges in continental Europe and the UK were built or repaired using stainless steel rebar in the period late 1960s to early 1990s. As they have provided virtually trouble-free service, the exact construction dates and details of several of these structures have now become difficult to determine.

Advantages of stainless steel rebar

  • Inherent corrosion resistance, particularly to chlorides

  • Prolonged service life and reduced life cycle cost for reinforced concrete structures

  • Excellent strength, ductility and toughness (down to sub-zero temperatures)

  • Ease of cutting and bending (capable of tight bends)

  • Easily shipped, handled, joined and placed

  • Good weldability (for where the cage construction is required)

  • No coatings that could chip, crack or degrade

  • No exposed cut ends requiring covering or coating

  • No need for ‘High-Performance Concrete’ (although compatible with)

  • No need for cathodic protection

  • No need to add or apply corrosion inhibitors to the concrete

  • No need for concrete sealers or membranes

  • No need to increase concrete cover depth

  • Excellent fire resistance

  • Properties remain the same over the centuries

  • Certain stainless steels are non-magnetic, making them ideal for applications such as MRI facilities

  • Stainless steel has a modulus of elasticity similar to that of carbon steel

  • The coefficients of thermal expansion for Alloys 2304 and 2205 are similar to that of carbon steel, ensuring continued bond strength when the concrete is heated or cooled

Selective substitution

Where a structure needs to withstand a corrosive environment, stainless steel rebar can replace carbon steel rebar in locations where corrosive agents may penetrate through the concrete cover. This selective substitution can greatly extend the service life of the structure.
Deeper within the concrete structure, where corrosive agents are unlikely to penetrate, stainless steel rebar is not required. The total cost of a new bridge with selective use of stainless steel is typically only a couple percent more than one with all carbon steel rebar.

Although some designers and civil engineers may not be overly familiar with stainless steel, they can be reassured that stainless steel is still a type of steel. A simple substitution of carbon steel rebar with stainless steel will mean:

  • Identical rebar design and placement

  • Identical development length

  • Similar rebar pull-out strength

Studies have shown that connecting stainless steel rebar to carbon steel rebar in concrete does not create an active galvanic couple and thus does not accelerate corrosion of the carbon steel rebar.

Life-cycle cost

In addition to the initial construction cost, a life cycle analysis takes into account future maintenance and rehabilitation/replacement costs. The selective use of stainless steel rebar will greatly reduce maintenance costs and postpone the need for any major rehabilitation or replacement of the structure. When ‘disruption’ costs are added to the maintenance costs, the benefits of using stainless steel rebar become increasingly compelling. For example, in the event of major repairs to strategic bridges or elevated highways, disruption costs can include:

  • Wasted fuel, when vehicles are idling or are forced to take lengthy detours)

  • Employees late for work

  • Delayed deliveries of goods and freight

  • Lower productivity

  • Loss of revenue, if traffic on toll bridges or highways is reduced or stopped

Some of these costs are difficult to measure, but are likely to have major economic, environmental and social impacts.

The diagram on the right illustrates the life cycle cost approach. It is based partially on repair work that took place on a bridge in Sweden.

The bridge was built using carbon steel rebar. Following around 18 years in service, extensive rebar corrosion had taken place due to chloride penetration of the concrete cover. This led to significant deterioration to the concrete, necessitating major rehabilitation work. Carbon steel rebar was used in the rehabilitation and it was hoped that another 20+ years of maintenance-free service could be expected as a result. However, the amount of damage suffered by the bridge towards the end of this second 20+ year period remains unclear; it is possible that further significant rehabilitation may be needed.

Selective substitution with stainless steel rebar would have greatly reduced future maintenance costs and would have extended the service life of the bridge to 75-100 years. These significant savings could have been achieved with only a relatively modest increase in the initial project cost. The additional costs of selectively using stainless steel rebar depend on the size and complexity of the structure and the grade of stainless steel selected. For some bridges, the extra cost has been found to add between 1 percent and 3 percent to the total project cost.

For some bridges, the extra cost of not using stainless steel rebar has been found to add between 1% and 3% to the total project cost

Alloys

Older applications usually used Type 304 stainless steel reinforcement. Since then, other rebar alloys have become commercially available which possess enhanced corrosion resistance and improved mechanical properties. These alloys include Type 316LN, Alloy 2205 and Alloy 2304 (the latest grade to be introduced). Their chemical compositions are shown in Table 1. All demonstrate excellent strength, ductility and toughness, even at sub-zero temperatures.

Duplex alloys (2205 and 2304) are magnetic. Austenitic alloys (Types 304, 316, 316L and 316LN) can be supplied in a non-magnetic condition.

TABLE 1: ALLOY CHEMICAL COMPOSITIONS (balance iron)

ALLOY UNS % Cr % Ni % Mo % N PREN* (aver.)
304 S30400 18-20 8-10.5 - - 19
316LN S31653 16-18 10-14 2-3 0.10-0.16 27
316/316L S31600/S31603 16-18 10-14 2-3 - 25
2205 S31803 21-23 4.5-6.5 2.5-3.5 0.08-0.20 34
2205+ S32205 22-23 4.5-6.5 3.0-3.5 0.14-0.20 36
2304 S32304 21.5-24.5 3.0-5.5 0.05-0.60 0.05-0.20 26

*PREN = Pitting Resistance Equivalence Number = % Cr+3.3x % Mo+16x % N

The PREN has been shown to be a good indicator of the relative resistance of an alloy to pitting in chloride-containing environments. The higher the PREN, the greater the resistance of the alloy.

In the mid-1990s, it was generally believed that Type 304 rebar (PREN = 19) may lack sufficient resistance to chloride-containing environments. Therefore, many later applications used Type 316LN (PREN = 27) and Alloy 2205 (PREN = 34) rebar. Today, Alloy 2304 is readily available as well as cost effective, offering an excellent combination of corrosion resistance and strength suitable for many bridge applications.

Standards

The alloys listed in Table 1 are supplied in accordance with standards ASTM A955 and BS 6744. ASTM A955 covers nominal rebar sizes from No. 3 (10 mm) to No. 18 (57 mm). BS 6744 covers nominal rebar sizes from 6 mm to 50 mm.

The mechanical properties required by ASTM A955 are presented in Table 2:

TABLE 2: MECHANICAL PROPERTIES REQUIRED BY ASTM A955

Property Grade 60 (420) Grade 75 (520)
Yield strength (min.) 60,000 psi / 420 MPa 75,000 psi / 520 MPa
Tensile strength (min.) 90,000 psi / MPa 100,000 psi / MPa
Elongation in 8 in. (200mm), min. 20% 20%

Bend tests on rebar specimens are also required by ASTM A955 and BS6744.

Applications

Stainless steel rebar has been used in the following applications:

  • Highway bridges

  • Local bridges

  • Highway ramps

  • Elevated highways

  • Flyovers / overpasses

  • Underpasses

  • Barrier and retaining walls

  • Tunnels

  • Marine structures; piers, docks, moorings, sea walls, coastal buildings

  • Parking garages

  • Waste water treatment lagoons

  • MRI facilities in hospitals, clinics, laboratories (non-magnetic rebar, e.g. Type 316LN)

  • Military facilities, data storage facilities (non-magnetic rebar)

  • Historical structures (strengthened and repaired using stainless steel rebar)

  • LNG storage facilities (low temperature service).

Bridges

A large number of bridges have been built or rehabilitated using stainless steel rebar in several countries, including the USA, Canada, the UK, Ireland, Switzerland, Denmark, United Arab Emirates, Bahrain and Hong Kong. Table 3 sets a few of the bridge projects using stainless steel rebar:

TABLE 3

LOCATION COUNTRY DATE ALLOY TONS (approx.)
Bridge on I-696 near Detroit, Michigan USA 1983 304 33
Schaffhausen Bridge Switzerland 1995 304+2205 13
Bridge on Hwy. 407, north of Toronto, Ontario Canada 1996 316LN 11
Bush Creek Bridge, Oregon USA 1998 316LN 75
Smith River Bridge, Oregon USA 1998 316LN 125
Highnam Bridge, A48 UK 1998 316 11
Church Rd. Bridge, Hwy. 401, Ajax, ON Canada 1999 316LN 150
Haynes Inlet Slough Bridge, Oregon USA 2003 2205 400
Broadmeadows Bridge Ireland 2003 316 186
Driscoll Bridge, New Jersey USA 2005 2205 1,300
Shenzhen Western Corridor Bridge Hong Kong 2007 316+2205 1,300
Woodrow Wilson Bridge, Virginia - Maryland USA 2008 316LN+2205 1,100
Stonecutters Bridge Hong Kong 2008 304 3,000
Hastings Bridge, Minnesota USA 2010 2304 367
Sakonnet River Bridge, Rhode Island USA 2010 2205 800
Shaikh Zayed Bridge, Abu Dhabi UAE 2010 2304 900

Sitra Causeway Bridge

Bahrain 2010 2205+2304 6,400
Lafayette Bridge, Minnesota USA 2011 2304 1,963
S. Saskatchewan River Br., Medecine Hat, Alberta Canada 2011 2304 190
Steinhauser Bridge, Hwy 63, Ft. McMurray, Alberta Canada 2011 2304 478
Allt Chonoglais Bridge, A82, Scotland UK 2013 2304 67
Daniel Hoan Bridge, Milwaukee, Wisconsin USA 2014 2304 5,000
Kenaston Overpass, Manitoba Canada 2014 2304 200
Temburong Bridge Brunei 2016- 304 3,500
New Champlain Bridge, Montreal, Quebec Canada 2016- 2304 17,000
Hong Kong-Zhuhai-Macau Bridge Hong Kong 2016- 2304 11,000

 

Parking garages

In regions where salt is used to de-ice the roads in winter, vehicles bring the salt into parking garages on their tyres and bodywork. The salt can penetrate the concrete floors, entrance and exit ramps and inter-floor ramps, causing corrosion of the carbon steel reinforcement. It can also migrate to the supporting columns and beams.

In order to prevent such corrosion, stainless steel rebar has been used during the building or rehabilitation of several parking garages. Some examples are set out in Table 4:

Table 4

Parking Garage Location Stainless Steel Rebar involved (tons)

USA

 

Albany, New York

15

Albany, New York

14

Brighton, Massachusetts

20

Boston, Massachusetts

9

Pittsfield, Massachusetts

15

Middlebury, Vermont

4

Hartford, Connecticut

12

Exeter, New Hampshire

9

Bloomington, Illinois

23

Canada

 

Kelowna, British Colombia

55

Tunnels and underpasses

Stainless steel rebar was used during rehabilitation of the Thorold Tunnel under the Welland Canal in Ontario, Canada.

Type 316 stainless steel rebar (240 tons) was used in the road slab of an underpass at Cradlewell near Newcastle-upon-Tyne, England in 1995.

Associated products

In addition to rebar, the following stainless steel products are available:

  • Tie-wire
  • Rebar couplers
  • Welded wire mesh (produced to ASTM A1022)
  • Dowel bars
  • Anchor bolts

Links

The International Stainless Steel Federation (www.worldstainless.org) has a site devoted to stainless steel rebar: www.stainlesssteelrebar.org

Information on standards from ASTM International is available on their website www.astm.org

Information on standards from the British Standards Institution is available on the website www.bsigroup.com

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