1. Scope

This specification covers deformed and plain steel bars in cut lengths for concrete reinforcement intended for applications where additional mechanical properties are required for compatibility with exposure to cryogenic temperatures, down to -320ºF [-196ºC].

Keywords

21.1 concrete reinforcement; deformations (protrusions); steel bars; low temperature service; cryogenic design temperature

Rationale

Rationale:

This standard specification will cover deformed and plain steel bars in cut lengths for concrete reinforcement intended for applications where additional mechanical properties are required for compatibility with exposure to cryogenic temperatures, down to -320F [-196C].

This standard is intended to be an ASTM standard specifying the properties and testing required for steel to be used in Cryogenic storage tanks, such as liquid natural gas. There is market demand for the material, and the product is available as an alternative to stainless steel rebar with 9% nickel in it. Currently when specifying and certifying this type of reinforcing steel, the purchaser and producer must include specific language, such as “Also conforms to ACI 376 and EN 14620-3 Annex A3 requirements for reinforcing steel”. There is actually no official grade or standard for the reinforcing steel to be used under cryogenic conditions. The performance requirements for this use of reinforcing steel have been in place int he above referenced design and construction codes for year. The required tests in both codes are identical.

Test 1 is a “Notch Sensitivity Ratio”, which quite simply means that you test two coupons at cryogenic design temperature – one has a notch similar to a charpy V notch and the other is un-notched. The tensile strength of the notched bar must exceed the yield strength of the un-notched bar.

Test 2 is that the strain at ultimate stress must be greater than or equal to 3% at cryogenic design temperature. These two tests are required in both design and building codes and must be in this draft/new standard in order for the material to be compatible with the codes

Primary use is for concrete reinforcement of liquefied gas storage tanks. Tanks are typically an inner liner made of a cryogenic duty plate steel. The inner tank is then "reinforced" with reinforced concrete for secondary containment. The primary purpose of the concrete is containment in the event the inner lining ruptures. The material must be guaranteed to perform at the lowest temperature to which it will be exposed. The Alternative to this material is to either use fully austenitic stainless steel reinforcement or to use grade 40 steel and over build the structure to compensate for lack of performance guarantee with raw mass. The standard being proposed is the most desired and economical solution for the design and build of these types of storage tanks.

There is a domestic source for this grade of reinforcing steel where none previously existed, and the intention behind the specification is to combine the dimensional, chemical, and mechanical requirements of room temperature with the requirements for cryogenic design temperature performance into a single standard. The proposed standard also defines the testing required to certify cryogenic performance of the material. The proposed ASTM standard could simply be called out in the ACI 376 building code, which simplifies the code and removes the burden from the building code to maintain material specifications and testing requirements. Additionally, when certifying the steel, it can simply be certified to one standard instead of listing modified versions of other standards and what those modifications are on the mill certificates. Provisions are also made to allow for “dual certifying” the steel with A615 so that overages could be used as A615 reinforcing steel, as long as the manufacturer also meets those requirements. Dimensionally, the bar specification is equivalent to A615– the only differences being chemical and mechanical properties.

The title and scope are in draft form and are under development within this ASTM Committee.