What is Rebars (Steel Reinforcement Bars ) | Types and Grades of Steel Reinforcement | Structural Design
What are Rebars (Steel Reinforcement Bars ) | Types and Grades of Steel Reinforcement
What are Rebars ?
Rebars also know as Steel reinforcement bars are specific bars that are used to improve the tensile strength of the concrete as concrete alone is very weak in tension, but is strong in compression.
Steel bars has the best reinforcement properties for concrete structurally than any other type of reinforcement material. It is because steel offers significant strength of its own.
Steel bars can bend without breaking under stress or pressure. More so, the steel thermal coefficient which is, steel bar rate of expansion and contraction when subjected to heat or cold , is almost the same to that of concrete.
Once steel is embedded, reinforcement will expand and contract together so that the concrete maintains cohesion.
A reinforcement material with different thermal coefficient would result in broken concrete rather quickly.
Types of Rebar
What are the different types of rebar?
Rebar comes in a variety of lengths and diameters, and is designed to strengthen solid concrete structures and roadways. While it’s important to know what size of rebar you need, it’s equally important to choose the right type of rebar.
These are the common types of rebars used in construction for the reinforcement of concrete:
- Threaded rebar
- Tempered steel rebar
- Basalt rebar
- Stainless steel rebar
- Epoxy coated rebar
- Fiberglass rebar
- Welded Wire Fabric
Carbon steel rebar/Tempered steel rebar
Carbon steel rebar is the backbone of concrete work. It’s used in the majority of concrete pours. Why? It has a lower cost and provides solid strength. Tempered steel rebar, often known as carbon steel rebar or black bar, is the most frequently used type of rebar. It is less expensive than other varieties of rebar but corrodes more easily.
Glass fiber reinforced polymer
Glass fiber reinforced polymer GFRP is an alternative to traditional steel rebar. Using fiberglass, the rebar is created in sticks to provide reinforcement. It’s stronger in terms of tensile strength.
Basalt rebar is a product made of inert volcanic rock that has various advantages over regular steel rebar. Basalt rebar is 2-3 times stronger than steel rebar and weighs around a quarter of the weight of comparable diameter steel rebar. Additionally, basalt rebar is electrically and thermally inert, non-hygroscopic, and corrosion resistant.
Galvanized rebars are used in situations where corrosion can be a problem. The rebar is coated with zinc using several different processes. These can include cold plating, hot plating or electroplating. The zinc provides a protective barrier over the steel.
Threaded rebar, also known as jumbo rebar, has one or two threaded ends that allow for the use of conventional UNC fasteners to secure items in place, comparable to an anchor bolt. Threads must be cut either by machining down the rebar to create a turned-down piece equal in diameter to the rebar deformation at its lowest point, or by rolling or cutting the threads.
Welded Wire Fabric
While not commonly referred to as rebar, welded wire fabric is a product made of fusion welded low carbon steel or stainless steel wire that is woven into a square grid pattern or mesh in standard sizes and used for concrete slab support.
It functions similarly to other forms of rebar in terms of increasing the tensile strength of concrete slabs.
Stainless steel rebars
Stainless steel rebars are used in corrosive environments where zinc can’t be used due to its galvanizing effects. It tends to be more expensive than galvanized rebar. For this reason, it’s only used when absolutely necessary.
Epoxy coated rebar
Epoxy coated rebar is a modern, economical option for corrosive environments. The rebar is coated in epoxy to protect it from corrosion. However, it may not be the best bet in environments where shifting subgrade, cracks or mechanical motion may damage the protective coating, allowing corrosive materials access to the steel.
European rebars are simply measured in different formats than what we see on American markets. They still provide solid reinforcement of your structural concrete.
Uses of Rebar in Concrete and Masonry Structures
Concrete is an extremely strong material in compression but is relatively weak in tension. To compensate for this inconsistency in the behavior of concrete, rebar is cast into it to carry tensile loads. Although the majority of steel reinforcement is classified as primary or secondary reinforcement, there are a few more minor applications:
The term “primary reinforcement” refers to the steel that is used to ensure that the structure as a whole has the resistance necessary to support the design loads.
Secondary reinforcement, also known as distribution or thermal reinforcement, is used for durability and aesthetic purposes, since it provides sufficient localized resistance to prevent cracking and to withstand stresses generated by temperature fluctuations and shrinkage.
Additionally, rebar is used to provide resistance to concentrated loads by providing sufficient localized resistance and stiffness to spread a load across a larger region.
Additionally, rebar can be used to secure other steel bars in the proper position to support their loads.
Masonry constructions and the mortar that holds them together share many of the same qualities as concrete, but have a limited capacity to support tensile loads.
Certain typical masonry pieces, such as blocks and bricks, provide voids for the placement of rebar, which is then fastened in place with grout. This is referred to as reinforced masonry.
Placement of Rebar in a Concrete Slab
Rebar cages are often made on or off-site using hydraulic benders and shears. For simple or specialized jobs, however, a tool called a Hickey, or hand rebar bender, is suffice.
Steel fixers “rodbusters” or concrete reinforcing iron workers lay the rebar, using bar supports and concrete or plastic rebar spacers to establish concrete cover and assure appropriate embedment.
The cages’ rebar are joined through spot welding, steel wire tying, and occasionally an electric rebar tier, or mechanical connections.
Typically, epoxy coated or galvanized wire is used to tie epoxy coated or galvanized rebar.
Stirrups in Placement of Rebar
Stirrups define the perimeter of a rebar cage. Stirrups are typically rectangular in shape in beams and circular in shape in piers and are installed at regular intervals along a column or beam to retain the structural rebar and prevent it from sliding during concrete application.
Stirrups or ties are primarily used to increase the shear capacity of the reinforced concrete component they are attached to.
Welding in Placement of Rebar
The American Welding Society (AWS) provides welding standards for rebar in the United States. Without additional consideration, the only ready-to-weld rebar is W grade (Low-alloy — A706).
Without calculating the “carbon-equivalent,” rebar that is not manufactured to the ASTM A706 criteria is generally unsuitable for welding. Welding is permitted on materials having a carbon equivalent of less than 0.55.
Normally, rebar cages are connected using wire, although spot welding has been the norm in Europe for many years and is becoming more prevalent in the United States. Welding is not possible with high strength steels used in prestressed concrete.
Placement of reinforcements in rolls
Roll reinforcement is an incredibly fast and cost-effective method for quickly installing large amounts of reinforcement. Typically, roll reinforcement is prepared off-site and unrolled on-site.
Roll reinforcement has been effectively installed in slabs (decks, foundations), wind energy mast foundations, walls, and ramps, among other applications.
Mechanical connections, also known as “mechanical couplers” or “mechanical splices,” are used to join reinforcing bars together.
Mechanical couplers are an efficient method of alleviating rebar congestion in heavily reinforced portions of cast-in-place concrete construction. Additionally, these couplers are used at the junctions between members in precast concrete construction.
Reinforcement Bar Types, Grades and Sizes
Different bar sizes and grades of steel are available to provide different levels of strength.
Rebar is produced in a variety of grades and standards based on yield strength, ultimate tensile strength, chemical composition, and elongation percentage.
By itself, a grade merely defines the lowest permitted yield strength; it must be used in combination with a material specification to completely explain the rebar product specifications.
Material standards provide grade criteria as well as other features such as chemical composition, minimum elongation, and physical tolerances.
When inspected and tested, fabricated rebar must exceed the grade’s minimum yield strength and any other material specification criteria.
The grade designation in the United States is equal to the bar’s minimum yield strength in ksi (1000 psi), for example, grade 60 rebar has a minimum yield strength of 60 ksi.
Rebar is most frequently manufactured in grades 40, 60, and 75, with greater strength grades 80, 100, 120, and 150 commonly available. Grade 60 (420 MPa) rebar is the most frequently used rebar grade in modern construction in the United States. The historic grades 30, 33, 35, 36, 50, and 55 are no longer in use.
Steel Bar Sizes in MM
Metric bar designations represent the nominal bar diameter in Millimetres
For building slabs 8 mm,10mm,12mm diameter are common. For beams 12mm to 25 mm depending upon the span and size.
Slabs are generally constructed using 8mm to 12mm diameters. Beams and columns are constructed using 12mm to 25mm steel bars that too depending on the span and size.
Bars whose diameter ranges from 25 mm to 36 mm are commonly used for hydraulic structures like dams, bridges etc.
Deformed Steel Bar Size and Weight
The deformed Reinforcing Steel Bar is supplied in length of 9m or 12 m as common sizes. The steel bar diameter applied is different and accordingly the weight varies.
Comparison Table of Deformed Steel Bar Size in MM and Weight in KG:
When should you use rebar for 4-inch slabs?
Rebar is a steel bar or rod that is used to reinforce concrete and masonry structures. The rebar typically runs parallel with the length of the slab in order to hold it together as well as keep it from cracking.
It Should be necessary for you to add rebar if your slab has a thickness of 5 inches or more. Rebars are generally available at most hardware stores and can be cut down into any size needed.
So, do you need rebar for 4-inch slab? Reinforcing concrete slabs with rebar is an excellent way to prevent slab failure, as long as the slab is not over 4 inches thick. If the slab is 4 inches or less than it should not be reinforced. If the slab is thicker, then re-bar should be used.
Rebar Spacing for Concrete Slab: What is the Correct Rebar Grid Spacing For Concrete Slab?
The spacing of the rebar (reinforcement bars) on concrete slap may vary from 12 inches to 24 inches grid spacing. Closer mat spacing provides more strength to compensate for use of smaller-diameter rebar. Rebar should be placed evenly throughout the project.
For a flat slab, such as a driveway, consider placing the rebar in an 18-inch grid spacing, keeping the edge rebar at the same distance from each side. For a patio you can use a grid spacing of 24 inches grid spacing.
What Size Concrete Slab Need Rebar?
Rebar is recommended for concrete that measures 5-6 inches in depth. The type of and intended use of concrete impacts the need for rebar reinforcement. Rebar must be placed at the center of or slightly above the center of the concrete slab—hence why it should be a certain thickness for best results.
What Size Rebar for Concrete Slab?
Patios, basement floors, footings, and driveways may use rebar ranging in size from 3 to 6. Contractors often follow the “1/8 Rule,” which states that the rebar should be 1/8 the thickness of the slab.
For instance, a 6-inch-thick slab may include rebar labelled as size 6 or 3/4-inch.
Again, majority of residential slabs are made of #3 (3/8″ diameter) or #4 (1/2″ diameter). #5 or 5/8” diameter slabs may be used for residential slabs that bear heavy weights.
In a 4” slab of 3,000 PSI concrete, the addition of #3 rebar typically strengthens the pad to 6,600 PSI, while the addition of 1/8” #4 rebar increases the strength yield to 11,000 PSI.
The rebar should be no thicker than 1/8 the thickness of the slab, which means that a 4” slab should have no steel thicker than #4 or 1/2′′ bar. It is prudent to consult a structural engineer to determine if your slab will bridge or cantilever.
#3 rebar is often used in 4″ concrete slabs used for driveways and patios. Driveways that are commonly used to accommodate everyday traffic by heavy vehicles are frequently constructed with 1/2′′ or #4 steel bar. Pads that span culverts, ditches, or cantilever may require #5 rebar or a more condensed grid pattern for #4.
Does Rebar Rust in Concrete?
Reinforcement bars covered in concrete is relatively impervious to corrosion due to concrete’s considerable impermeability and alkaline composition.
Concrete’s high pH value contributes in the protection of the steel by forming a thin oxide layer. However, chloride ions found in coastal salt and road salts can infiltrate concrete and destroy the steel.
Corrosion will occur if rebar is not completely encased in concrete or is exposed due to cracking or delamination, thus ensure that repairs are made to preserve the steel.
Rebar or mesh will oxidize and stretch along the threads, causing the concrete to crack internally. Rust stains on the surface of concrete, unless they are caused by anything being placed on the concrete, frequently indicate reinforcing steel rusting.
Before using, proper preparation and installation of steel is just as critical as preserving the location throughout the pour.
Steel that is too close to the concrete’s edges, top, or bottom is susceptible to corrosive pressures. Utilizing pebbles or pieces of wood to support rebar exposes it to corrosion caused by groundwater and salt.
Rusted Rebar in Concrete: Can You Use Rusted Rebar in Concrete?
Yes. Any rusty rebar can be used. This is because concrete has an alkaline reaction due to its typical pH value of 12.8. With a 2610-6mm passivation film on the surface, reinforcing bars are resistant to corrosion.
Additionally, rust steel bars do not continue to corrode inside concrete. Again, slightly rusty rebar bonds to concrete more effectively than uncorroded rebar.
In the case of corroded steel reinforcement, corrosion improves bond strength by approximately 1% of steel volume. Even with extensive corrosion, rust has a negligible effect on bond strength, unless it is loose rust.
The reason for this is that the bonding of steel reinforcement is determined by the impacts of nodes, not by the surface’s adhesiveness. However, free rust can impair the bond between reinforcing bars and concrete.
Consequently, the use of rust-resistant steel reinforcement has no negative impact on the overall of concrete structures.