Stainless Steel Reinforcing Bars


Many countries today are facing the problems brought by rotting reinforced concrete infrastructures. This often calls for expensive repairs and replacement of these structures – in some cases, after a very short period of installment. Due to these problems, government bodies are not demanding a greater attention and emphasis on life cycle costs for new construction projects to be commenced, in contrary to the trend of only taking into account the original capital cost. This new approach contributes to focus awareness on total cost, considering the price and occurrence of maintenance and replacement works in future.

In United States of America, National Highway System (NHS) Designation Act came into action in 1995 AD. It states that every NHS project with capital of more than $25 million should be a subject to life cycle cost analysis.

Different cost analyses for concrete highway bridges have shown that selective usage of stainless rebar augments the original capital project cost by 1% to maximum of 15%, keeping in account the size and complexity of the highway bridges. This ‘humble’ initial cost raise can be compensated by decreased maintenance cost.

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 case “disruption costs” are included to maintenance cost, the proposal in favor of the stainless steels becomes even stronger. In case of elevated highways or repairment of strategic bridges, these indirect costs consists wasted fuel, mislaid productivity, setbacks in deliveries etc.  These costs may be hard to evaluate but it can be very high for big highways. In case of toll tunnels/bridges or harbor facilities, loss of revenue, profit and productivity can be calculated easily.

Above mentioned negative effects can be summarize with a fresh statement released by a famous North American politician: “”Roads are arteries of our economic prosperity; clogged arteries are bad for your economic health.”

Following are some possible advantages cited for stainless steel rebar:

  • Inbuilt good corrosion resistance
  • Decreased life cycle expenditure for concrete structures
  • Good strength
  • Proper weld-ability for regular rebar grades
  • Ductility for regular rebar grades ( able of 3D U-bends)
  • Ability to withstand shipping, handling, and bending
  • No coatings to crack, chip or degrade
  • No coating dent to repair
  • No exposed cut ends to cover or coat
  • Non magnetic or magnetic, depending on alloy cited
  • Good low and high temperature mechanical properties of regular rebar grades

Distinctive 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

 

Currently, there are 2 standards that deal with stainless steel rebar; ASTM A995/A995M (2004) and BS 6444 (2001).

British Standard BS 6744:2001 “Stainless steel bars for the reinforcement and use in concrete – Requirements and test methods” came into action on September 2001 AD. This standard takes over from BS 6744:1986, which is now not in action.

This updated revision contains new strength levels and new grades not in 1986 standard. Usages of Informative annexes have been carried out to monitor magnetic properties, grade selection, third party certifications and coefficients of thermal growth.

This British Standard specifies different 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 above mentioned steel designation numbers are in agreement with the new BS EN 10088-1 standard.

The size of the bars in all grades varies from 3mm to 50mm (nominal). Mass per meter length and nominal cross sectional areas are offered in this standard for different types of alloy grades.

3 strength levels are covered, Grades 200, 500 and 650, matching to minimum 0.2% Proof Strength levels of 200 MPa, 500 MPa and 650 MPa correspondingly.

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

 

The ASTM Standard which was revised in 2004 covers rebars in a extensive range of alloys, consisting 316 LN (UNS S31653), 304 (UNS S30400), 316L (UNS S31603), and UNS S31803. Additional in depth specifications and heat treatment circumstances for these alloys are illustrated in the ASTM Standard A276. The designer therefore has a wide range of mechanical properties and corrosion resistance from which to choose a appropriate rebar material.

Broad range of available alloys

Nominal size: 9.5 to 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)

 

Note- other mechanical properties can be asked from the suppliers

Magnetic permeability testing is covered by Supplementary Requirements section.

Degradation of reinforced concrete due to corrosion of carbon steel reinforcing bars (rebars) is a widespread problem throughout the world. The corrosion product i.e. rust takes up a bigger volume than original steel bar and this generates a pressure due to which cracking and later spalling of the surrounding concrete occurs.

Deterioration of carbon steel rebars is very much speeded up when chlorides are there in the concrete along with necessary moisture and oxygen levels to carry on the corrosion reactions. In many parts of the world, chlorides are included into the initial mix due to their inevitable occurrence in the sand, aggregate and water. Most of the times, chlorides breaks through the cover when exterior surfaces of the concrete are open to the elements of seawater, de-icing salts and nautical environment

Numerous methods are at present being implied in an effort to lessen the decomposition of carbon steel rebars. These comprise of rebar coatings, amplified concrete cover; diminished water/cement ratios; corrosion inhibitors added to concrete mix and concrete sealers.

Nonetheless, there is growing awareness in usage of reinforcing materials that have naturally better corrosion resistance, therefore diminishing the need for preservation and examination of the structure. Stainless steels are such materials and have been extensively used as concrete rebars in all continents.