Stainless Steel Reinforcement in Construction - Designation And Standards
Stainless steel is the name given to a group of corrosion-resistant steel alloys that contain a minimum of 10.5% chromium. When we refer to "stainless steel" it won't simply refer to a single material type. But this will belong to a group of corrosion-resistant highly alloyed steel. This type is in contrast to the unalloyed steel which won't show any kind of corrosion and rust forming that is noticeable in a normal environmental condition.
Stainless Steel Reinforcement ( Rebar)
In order to facilitate protection against corrosion, the stainless steel reinforcement does not seek the help of concrete. The application of stainless steel is a straightforward solution when the concrete elements are subjected to chloride penetration either from the marine environment or due to de- icing salts.
The stainless steel can be used in the design of long-life buildings like monuments and temples, areas where the equipment is very much sensitive to the magnetic fields. This will eliminate the need for magnetic reinforcement.
Corrosion Resistance of Stainless Steel Reinforcement
In order to undergo the basic reaction mentioned above i.e. corrosion, it is necessary to have steel in minimum concentration on a specific allowing element along with the existence of an oxidizing agent in the medium that is surrounding (oxygen). This will result in the passivation of the surface.
Passivity can be defined as a condition that will produce a strong inhibition of the reaction that involves the resolving of iron after the formation of a positive layer on the surface. Chromium is a particular type of element, that tend to undergo passivation.
A Chromium Oxide layer which is self-formed will help the material from corroding. When this protective layer gets damaged, the metal will behave self-healed under the action of oxygen. This property is gained in steel by the process of alloying. The general corrosion will decrease in corrosion promoting media that is contrary to the chromium content. This is shown in figure-2.
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Fig.2: The Rate of Corrosion of Chromium Steels in Industrial Air |
The passivity that is caused by the presence of chromium in steel is based on the attacking agent. The content of chromium in water and in the atmosphere must be at least 12 % by mass.
Chromium. nitrogen and molybdenum are the three important elements that are related to pitting corrosion. In an acid medium, the nickel will help in increasing the corrosion resistance.
Different Stainless Steel Types Used in Construction
The above-mentioned alloys, when changed in their respective composition, will influence the structure and other properties of the steel other than the mechanical, physical and the corrosion behavior.Hence the members of stainless steel family are placed in groups that will have same metallurgical groups. These are:
- Ferritic
- Austenitic
- Martensitic
- Ferritic - Austenitic (Duplex)
These alloys are used as reinforcing steel in concrete due to the following factors:
Higher resistance to corrosion
Increased corrosion resistance in chloride contained medium :The increase in the content of alloy will increase the material cost. Hence the selection of steel type with an alloy content must be chosen such a way that it has:
- Sufficient mechanical properties
- Sufficient corrosion resistance
- Sufficient Weldability properties
How to Specify Stainless Steel Bars
Specifying of Steel Bars or SSR will involve three steps:
- Identification of where to specify SSR
- Determine the designation to specify SSR
- How to specify SSR
For having service that is greater than 50 years, a predictive modeling has to be carried out for structures.
Designation of Rebars in Construction
The designation for rebar is given based on the Critical Chloride Threshold Level (CCTL). This is the level at which the chlorides will travel through the concrete by diffusion process and will initiate the steel reinforcing bar. This will eventually reach a concentration sufficient to facilitate pitting corrosion in the parental metal.
The generally used value for the resistance of carbon steel rebar is 0.4% chlorides by mass of the cement. The stainless steel gains corrosion resistance in the order of one magnitude or greater.
Predictive modeling of chloride penetration is carried out by the designers to understand, where comes the need for replacement of carbon steel bar and stainless steel rebar. This also determines at what level of corrosion resistance will the stainless steel require having the design life.
Standards for Stainless Steel Reinforcement
There are two main code and standards around the world today that incorporate the SSR. They are:
- The British Standard BS6744
- The American Standard ASTM A955
The fundamental difference between the codes are the profiles and the grades will follow their respective standards for carbon steel.
