The American Society of Civil Engineers recently marked the nation's structures at 'D-plus' due to their poor condition and it estimates that to repair or replace these will cost US$1.3 billion. Similarly, national television reports have claimed that approximately 25 % of US bridges are in need of replacement, mainly because of damage due to corrosion, and must be regularly inspected for safety.
But the new reinforcing bar being promoted by MMFX Steel Corporation is claimed to be highly resistant to corrosion, in addition to being almost twice as strong as standard rebar. The product was formally launched earlier this year at the World of Concrete event in Las Vegas.
It has already been extensively tested in commercial laboratories and is reported to have a minimum 758MPa yield strength, which is almost twice the strength of the current construction grade ASTM A615 steel, an ultimate tensile strength of 1172MPa and exceptional ductility.
In composition terms, the steel has a low carbon content, less than 1%, and contains around 8-10% chrome. The fact that it has a negligible amount of nickel makes it economical to produce, the company claims. What's more, says MMFX, the combination of chemistry and the proprietary production process gives the steel a 100% lath martensite structure that exhibits all the key properties mentioned above.
MMFX has developed two products, although only the latter is currently in production because of US capacity limitations:
Dual Phase Steel is a microcomposite ferritic/martensitic low carbon steel that is rolled and quenched in a controlled manner. It has superior corrosion resistance in reinforced concrete applications, as well as superior mechanical properties when compared to the regular A615 steels it replaces. It also offers a low cost alternative to epoxy coated steels for meeting corrosion requirements; the new steel does not require coatings, allows field bending of shapes, and does not require either special handling, or any added design considerations for bond/epoxy shear.
Microcomposite Steel has similar microstructures to Dual Phase steels but without ferrite. It differs from Dual Phase in its material composition and does not require quenching to produce the prerequisite microstructure for its corrosion resistance and mechanical properties. It is superior in strength to Dual Phase, and has additional features which improve its corrosion resistance and other properties.
Introduction of the new reinforcing bar material is particularly timely since most of the reinforced concrete structures built during the 1930s are now approaching the end of their anticipated 70 year service life. Many others - in particular those in northern states - have suffered from the heavy use of de-icing salts, while those on the country's seaboards and states along the gulf of Mexico have been exposed to harsh marine environments.
Repair of reinforced concrete already compromised by chloride attack on the steel reinforcement is impractical, hence most of the structures will eventually require replacement.
The new nano structure steel was designed from first principles with the aim of developing a cost-effective material with the requisite properties, especially higher strength and greater resistance to corrosion. It competes well on cost and performance with other available options - epoxy-coated steels for example - as well as the much more expensive option of stainless steel rebar.
But because it is not coated, it does not require any special handling and is not susceptible to construction site or transport damage. There is no requirement for surface coating integrity inspection or repair and there is no need to worry about construction site storage. It can be delivered from the mill directly to the site or fabricated without the need for any additional coating operation. Unlike clad stainless steel bar, there is no problem bending or cutting it to the desired shapes and lengths and except for the heavier sections, most operations may be safely made on site with standard field tools.
Its ductility means it can withstand much tighter bends and shapes than standard or coated or clad steels, making erection of the steel cages easier and cheaper. Once the full range of material strengths are incorporated into the engineering of reinforced concrete structures, there is the potential for using less steel for the same loads and hence achieving a substantial reduction in the amount of steel required. This should eventually lead to reductions in the labour needed to assemble steel, and a much simpler and faster filling of forms with concrete.
The new steel has already been produced to a commercial quantity and quality and is now ready for use as a direct, one-for-one replacement of A615 steel, but with the added benefit that it will have a low corrosion index measurable as 'chloride threshold'. This is a standard measurement used by the Federal Highways Administration to determine a reinforced structure's service life.
MMFX has developed an Accelerated Corrosion Testing protocol invented by David Trejo, assistant professor at Texas A&M University, that can measure, quantitatively, the chloride threshold value for any steel, by means of a two or three week test. Both the test cell and the testing protocol are being evaluated by a panel of experts and are expected to become available as a quick way of screening corrosion resistant steels.
This process is much faster and cheaper than the tests that are currently used. The cell and the protocol will be made available to any client wishing to test the new steel or indeed any steel it wants to compare with the standard A615 product. The test kit has been tested and is in the process of being validated by comparing the chloride threshold index of A615 obtained from the longer (one to two year) tests with those from the new ACT cell. Results from both tests are expected to validate the chloride threshold value being measured for the MMFX Microcomposite Steel.
A total of five bridges are being planned as part of the FHWA innovative bridge construction programme in several states and there are a number of other projects under consideration for this year. Meanwhile MMFX has completed a draft of a new ASTM specification for the advanced performance steel and is in the process of completing all the testing necessary to support both the material properties claims and the corrosion resistance performance of its new steels.
