Steel is a conventional reinforcing material used in construction for imparting tensile strength and ductility to concrete structures. Steel and concrete possess similar thermal characteristics that make them to expand and contract at similar rates. Both of them, hence, work together as a single unit in concrete structures.
One of the biggest problems with steel reinforcement is corrosion, where the rust has a volume five times greater than the amount of the original steel. This results in the damage of concrete structures. To overcome this issue, few reinforcement types have been developed, namely:
- Stainless steel reinforcement
- Fiber-reinforced polymer steel
- Epoxy-coated carbon steel
This article explains the essential features of different reinforcing steel available in the construction market. This material guide can help in choosing the best reinforcing material as per the requirement.
1. Steel Reinforcement Bars
The selection of steel reinforcement bars is a simple process. The steel is specified in terms of its diameter and grade. The standard steel specifications are provided in ASTM A615/A615M-20 "Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement".
A Grade 40 steel means a steel bar of minimum yield strength of 40,000 psi as per ASTM standards and specifications. The bar sizes are designated in inch-pound bar sizes or metric bar sizes.
Steel reinforcement bars are a good option when corrosion is not a concern for the structure. Being the conventional reinforcing option, it is readily available. For concrete structures that require higher tensile strength, a higher grade reinforcing steel can be employed.
2. Fiber Reinforced Polymer Reinforcement
The fiber reinforcement polymers (FRP) are made of fiber reinforcement, fillers, additives, and resin. Compared to steel reinforcement, FRP reinforcement is highly corrosion-resistant. It is also light in weight and tougher.
The use of fiber reinforcement in concrete structures increase the stiffness and tensile strength.
The resin employed in FRP reinforcement possesses high compressive strength that can bind the whole concrete mix into a more firm mass.
The commonly used FRP reinforcement fibers are aramid, carbon, and fibers. The fiberglass is less expensive, while carbon fiber is one of the costliest fibers.
3. Stainless Steel Reinforcement Bars
The stainless steel forms a low-carbon steel, with a total carbon content of less than 1%. It contains a minimum of 10.5% chromium, which creates a layer on its surface to make it corrosion-resistant. This makes the steel stainless.
Compared to standard steel bars, stainless steel bars are expensive. But they provide high strength and resistance to corrosion attacks, especially in harsh marine environments, attacks from road salts, etc.
Based on this property, stainless steel is best used for columns, piers, parking garages, retaining walls, jetties, and moorings.
The ASTM A955/A955M "Standard Specification for Deformed and Plain Stainless Steel Bars for Concrete Reinforcement" provides the standard recommendations for stainless steel reinforcement.
4. Epoxy-Coated Reinforcement
Epoxy-coated rebars or green bars replace conventional steel reinforcement to increase concrete strength and corrosion resistance. The steel reinforcement bars are coated with epoxy before their transportation to the construction site. The ASTM A775 specifies the specifications and recommendations for epoxy-coated reinforcement bars.
They are used to construct pavements, bridges, parking structures, and structures exposed to marine water and deicing salts.
A properly manufactured and handled epoxy-coated rebar can play a significant role in delaying the corrosion of reinforcement. One challenge faced by epoxy-coated rebar is the quality of the coating. The more delicate the coating, the lesser is the corrosion resistance. This is one reason where stainless steel scores more over epoxy-coated rebar. In terms of cost, epoxy-coated rebars are eight times less costly than stainless steel.
The main types of reinforcement used in construction are:
1. Standard steel reinforcement
2. Stainless steel reinforcement
3. Epoxy-coated steel reinforcement
Good quality manufactured epoxy-coated steel reinforcement bars have a significant role in delaying the rate of corrosion. The more delicate the coating, the lesser is the corrosion resistance. This is one reason where stainless steel scores more over epoxy coated rebar.
Stainless steel provides excellent corrosion resistance and guaranteed durability, unlike epoxy-coated rebars. But stainless steel is eight times more expensive than epoxy-coated bars.
So, a choice is made depending on the durability required and the budget of the project.
The commonly used FRP reinforcement fibers are aramid, carbon, and fibers. Among the three, fiberglass is less expensive, while carbon fiber is one of the costliest fibers.