Why stainless?

What are the benefits?

  • Because it is highly resistant to corrosion from chloride ion
  • It does not rely on the high alkalinity of concrete for protection
  • Concrete cover can be reduced
  • Concrete sealant, such as Silane, can be eliminated
  • Concrete mix can be simplified to suit concrete design needs not for rebar protection needs
  • It improves durability
  • It reduces maintenance and repair
  • It can be used selectively for high risk elements cost-effectively
  • It will eventually be recycled

Stainless steels vs. other corrosion protection solutions [14] [15]

 

Advantages

Drawbacks

Epoxy coating

Lower initial costs

  • Cannot be bent without cracking
  • Requires careful handling to avoid damaging it  during installation

Galvanizing

Lower initial costs

  • Cannot be bent without cracking
  • Zn coating corrodes faster than iron, no longer effective when zinc has corroded

Fiber-reinforced Polymers

Lower initial costs
Light

  • Low modulus (low stifffness)
  • Cannot be bent without cracking
  • No heat resistance and poor impact resistance in harsh winters
  • Properties over time (Aging)?
  • Degradation by water & temperature?
  • Disposal at end of life must be taken care of

Cathodic protection

Lower initial costs

  • Requires careful design for overall protection
  • Requires careful installation to maintain proper electrical contacts
  • Requires a permanent source of current (which must be monitored and maintained) or sacrificial anodes that require monitoring & replacement

Membranes

Lower initial costs?

  • Require careful installation
  • Performance over time debatable
  • Limited to horizontal surfaces

STAINLESS STEEL

Low Life Cycle cost:

  • Design similar to C-steels
  • Mixed C-steel/stainless reinforcements work well
  • Tolerates poor workmanship (insufficient concrete cover, poor compaction, inadequate curing…)
  • Insensitive to poor concrete quality
  • Easy installation
  • No maintenance
  • No life limit
  • Allows a thinner concrete cover
  • Better fire resistance
  • 100% Recycled

Removes corrosion, hence corrosion-induced concrete cracking

  • Higher initial cost….but limited to a few percent of the cost of the structure

Grade selection

Recommendations of the Technical Center on Consulting for Cement and Concrete, Switzerland [16]:

Material of the reinforcement

Pitting
Resistance

1)

Corrosion
Resistance
Class

Carbonation of concrete

No

Yes

No

Yes

Yes

2)

Yes

2)

Chloride content 3)

Zero

Zero

Low

Low

Mid

High

Ordinary black rebar

0

0

+

-

+/-

(-)

-

-

Epoxy-coated rebar

0

?

4)

Galvanised rebar

0

0/1

+

+

(+)

-

-

-

Chromium-Steels             5)

10-16

1

+

+

+

(+/-)

(+/-)

-

Chromium-Nickel-Steels and Chromium-Nickel-Molybdenum-Steels

 

17-22

2

+

+

+

+

+

(+)

Chromium-Nickel-Molybdenum-Steels

 

23-30

3

+

+

+

+

+

+

Chromium-Nickel-Molybdenum-Steels

 

>31

4

For special cases e.g.:

  • Very high chloride content
  • High chloride content and further unfavourable circumstances

1)  WS: Pitting resistance equivalent calculated according to: WS=%Cr+3.3%Mo+0%N.
The minimum content of chromium and molybdenum according to EN 10088 and Stahlschlüssel (Germany) was used for the calculation. The nitrogen content was not taken into account.

2)  The influence of the chloride content dominates. Carbonation is of minor importance since the rate of carbonation is low or the concrete cover is high

3)  Chloride content:

low:

≤0.6 M.% in respect to cement content

 

middle:

≥0.6, but ≤1.5M.% in respect to the cement content

 

high:

≥1.5, but ≤5 M.% in respect to the cement content

 

very high:

>5 M.% in respect to the cement content

4)  The classification is uncertain/controversial. Comparative considerations and judgement: see Chapter 4.7.

5)  The susceptibility to pitting corrosion of chromium steels with a low chromium content increases rapidly with decreasing pH. Depending on the concrete cover the carbonation of the concrete is, therefore, more or less important

Recommendations from:

BSSA (British Stainless Steel Association) Special report (2003) [17]

Selection of material Grade for given exposure condition

Exposure condition

Material Grade

Stainless steel embedded in concrete with normal exposure to chlorides in soffits, edge beams, diaphragm walls, joints and structures

1.4301

As above but where design for durability requirements are relaxed in accordance with table 3.7.

1.4301

As above but where additional relaxation of design for durability is required for specific reasons on a given structure or component i.e. where waterproofing integrity cannot be guaranteed over the whole life of the structure

1.4436

Direct exposure to chlorides and chloride bearing waters for example dowel bars, holding down bolts and other components protruding from the concrete

1.4429

1.4436

Specific structural requirements for the use of higher strength reinforcement and suitable for all exposure conditions

1.4462

1.4429

Please note, however, that since then lean Duplex grades gained acceptance and are now widely used (please go to stainless properties)

Is Stainless Steel Cost-effective?

Example 1 [18]

ECONOMIC EXAMPLE:*
Total construction costs of a road bridge

Solution

Total bridge cost indications

100%

Carbon Steel Rebar

100

10%

Stainless Steel Rebar

103

25%

Stainless Steel Rebar

107.5

50%

Stainless Steel Rebar

115

100%

Stainless Steel Rebar

130

(*) The calculations are taken from a publication by the French information centre for cementeous materials, CIMbéton. The stainless steel used is grade 304. The numbers will vary with market prices and grade selection. Nevertheless, the indications represent typical road bridge construction experience.

Example 2 [19]
The Progreso pier was built in 1941, using stainless steel rebar. The LCA /LCC comparisons were made with an hypothetical alternative of C-steel rebar. They show that stainless rebar results in a lower Life Cycle Cost and a lower Global Warming Potential.
    
Example 3 [20]
Ministry of Transportation of Ontario, Canada
“The advantage of stainless steel is the extremely slow rate at which it corrodes in a concrete/chloride environment. Minimal corrosion damage is anticipated during a service life of 75 years. Although it is much more expensive to purchase than black or epoxy-coated reinforcing steel, it is more cost-effective over the long term because the corrosion-induced damage typically seen with black steel and coated steel will not occur.” 
Example 4 [21]
Comparison of initial cost and life cycle costs of new bridges with various types of deck reinforcing

Reinforcing Type

ECR, Galvanized

MMFX 2

FRP

Solid Stainless

EnduraMet© 32 Stainless

Deck cost (compared to total initial cost of “base” structure)

38.00%

39.14%

39.90%

42.18%

39.90%

Steel cost (compared to total initial cost of “base” structure)

31.00%

31.00%

30.50%

30.50%

30.50%

Foundation cost (compared to total initial cost of “base” structure)

25.00%

25.00%

24.00%

24.00%

24.00%

Earthwork, etc. cost (compared to total initial cost of “base” structure)

6.00%

6.00%

6.00%

6.00%

6.00%

Total initial cost of structure

100.00%

101.14%

100.40%

102.68%

100.40%

Estimated Life (Years)

40

50

65

100

100

Present worth of deck replacement at end of life

9.89%

6.88%

3.93%

1.05%

1.00%

100-year life cycle cost as a percentage of initial cost of “base” structure

111.48%

108.02%

103.88%

103.74%

101.40%

 
Example 5 [22] [23] [24]

The Schaffhausen bridge across the river Rhine was completed in 1995. The bridge is required to resist to atmospheric corrosion and de-icing salts. The bridge is expected to last 80 years without loss of structural integrity and without significant maintenance. 

Three options were considered for the rebar reinforcement:

  • Carbon steel: expensive repairs would have been required every 25 yars and caused heavy traffic disruption
  • Epoxy coated carbon steel: less extensive repairs would nevertheless have to be carried out every 25 years with reduced traffic disruption
  • Carbon steel with stainless steel in the critical areas: meets the design life requirements

The comparative costs of the three alternatives are shown below.

Proper use of stainless steel leads to an extra cost of only 0.5%, paid back 30 times by the savings achieved over the lifetime of the bridge!

LIFE CYCLE COST SUMMARY OF A RIVER CROSSING HIGHWAY BRIDGE:

Description

Carbon Steel

Epoxy C.S.

Stainless Steel

Material costs

Fabrication costs

Other installation costs

8,197

0

15,611,354

31,420

0

15,611,354

88,646

0

15,611,354

Initial cost (CHF)

15,619,551

15,642,774

15,700,000

Maintenance

Replacement

Lost production

Material related

0

256,239

2,218,524

0

0

76,872

2,218,524

0

0

-141

0

0

Operating costs (CHF)

2,474,763

2,295,396

-141

Total LCC (CHF)

18,094,314

17,938,170

15,699,859

 

Example 6 [Reference 25 Chapter 11]
It describes the repair of the facade of an 18-storey apartment building in Alicante, Spain.(see front view, below) The 30 year-old building is located close to the sea. Cracks in the façade panels were observed and required replacement by new ones reinforced with stainless steel.

 
Example 7 [Reference 25 Chapter 12]
Compares the costs of two concrete structures ( i.e. apartment building and underground car park)  reinforced with C-steel or with stainless steel. The use of stainless steel on the most exposed areas only leads to an increase of the construction cost of  about 9€/m2 (about 2,5%)  for the apartment building. A full replacement of C-steel by stainless steel would increase the cost by about 6%.  This is not much in view of the longer life of the construction without major maintenance.

 

Fire Resistance [25 Chapter 11]

Building codes demand that structures should meet a proper level of fire resistance, usually 2 hours minimum, before collapse. One of the key parameters for reinforced concrete beams is the concrete cover (over the reinforcing bar).

Increasing the concrete cover will be efficient, however, only if the concrete does not exhibit large cracks or is deteriorated. This may occur over time as a result of corrosion.

Another problem, for thick concrete covers is that of "spalling", i.e. concrete cover cracking and flaking in case of fire. Ways to avoid this are enforced, but they add to the cost.

An efficient way to improve the fire resistance of reinforced concrete is to use stainless steel rebar. Stainless steels retain their mechanical properties better than carbon steel at elevated temperatures.

Note: EN 1.4311 (AISI 304N): stainless rebar data from ISSF, EN 1.4301 (AISI 304) Eurocode data

Computer simulations show that the fire resistance of Stainless reinforced beams is much better than that of carbon steel reinforced ones.

The curve below obtained by computer simulation illustrates how a concrete beam reinforced with either C-steel or stainless steel (grade EN 1.4311 - AISI 304N) bends under load in case of fire (ISO 834 standard - cellulosic fire). Beam deflection increases with time (and beam temperature) until it cannot anymore bear the load.

Stainless steel almost doubles the time before the beam collapses. In addition, the high elongation provides a progressive onset of beam failure and therefore warns of imminent danger.

 

 

The increase in time before collapse brought by stainless steel reinforcement is an attractive alternative to the increase of concrete cover:

  • it does not increase the weight of the structure, hence the load on the members
  • it maintains the fire resistance in corrosion-prone structures