Rebars in Construction | Types & Grades of Reinforcement Bars

Rebars are reinforcing bars that are used in imparting tensile strength to concrete structures. Plain concrete is strong in compression and weak in tension. Rebars in R.C.C concrete together resist tension and compression under the action of load. 

Figure-1

Rebars used for concrete can be plain or deformed. Compared to plain rebars, deformed patterns on the reinforcement bars help the concrete adhere or bond to the reinforcing steel surface without slipping and together sustain tensile stress.

Plain and deformed rebars can be made of steel or fiber. The type of rebar is selected based on the material and properties suitable for the construction application. 

This article explains in detail the various types and grades of rebars that are used in the construction industry based on Indian Standard (IS) and American Standard (ASTM) codes.

Plain and Deformed Reinforcement Bars

Reinforcing rebars are manufactured as plain or as deformed bars. Plain rebars are reinforcing bars that are plain with threadless circular rods in fixed lengths as shown in Figure 2. Such rebars possess less friction, less bond strength with concrete, good and amount of slippage. Plain rebars can be classified as mild steel bars, carbon steel rebars or fibre bars.

Figure-2: Plain and Deformed Rebars

Deformed rebars have ribs, lugs, and indentations on the surface of the bar to obtain a proper bond with the concrete to avoid slippage. Usually, the pattern used in rebars is not specified. But, the spacing and the height of the "bumps" on the bars are regulated.

Figure-3: Deformed Rebars

The tensile properties of deformed bars are higher compared to the other rebars. Deformed steel bars can be either mild steel ribbers bars, carbon steel bars, HYSD bars, low-alloy steel bars, stainless steel bars, rail, and axle steel bars, galvanized steel bars, low-carbon chromium bars, epoxy-coated bars, and FRP bars.

1. Mild Steel Rebars

Mild steel rebars are a type of carbon steel reinforcement that contains a very less amount of carbon, around 0.16 to 0.29% by weight. Steel is very cheap and most commonly used, as it is malleable and ductile.

 

Mild steel is less strong and harder than other steel types. It is used for special purposes like contraction joints in roads and runways or as expansion joints.

Mild steel bars can be plain or deformed. The permissible stresses for deformed mild steel bars are the same as plain mild steel bars. However, deformed bars provide 40 percent higher permissible bonding stress than plain mild steel rebars. 

Table-1. Mechanical Properties of Mild Steel Rebars (Minimum Values) IS: 432 (Part 1)-1982

Types of nominal size of bars	Ultimate Tensile Stress N/mm2 minimum	Yield Stress N/mm2	Elongation Percent minimum
Mild Steel Grade I or Grade 60
For bars up to 20mm	410	250	23
For bars above 20mm upto 50 mm	410	240	23
Mild Steel Grade-II or Grade 40
For barbars to 20mm	370	225	23
For bars above 20mm up to 50 mm	370	215	23
Medium Tensile Steel Grade-75
for bars up to 16mm	540	350	20
for bars above 16 mm up to 32 mm	540	340	20
for bars above 32 mm up to 50 mm	510	330	20

The minimum specified yield strength of mild steel bars is 250 MPa conforming to IS:432 (Part 1). It is the most ductile steel type. They can be Mild steel Grade 1, Grade 2, and medium tensile steel as shown in Table-1.

2. Carbon Steel Rebars 

Carbon steel plain rebars are made of carbon steel and are commonly called as tempered steel rebar or black rebars, due to their carbon color. It contains 0.05 to 1.7 % carbon by weight. They can be plain or deformed bars. 

ASTMA615/A615M-06, “Standard Specification for Deformed and Plain Carbon-Steel for Concrete Reinforcement,” American Standard Testing Material, International, West Conshohocken, PA, (2004) is the standard code of carbon steel rebars.  

The code covers four grades, Grades 40, 60, 75, and 80. Their minimum yield strength levels in psi and MPa are given in Table-2. These bars are represented with the letter S (for type of steel). These are commonly used for reinforcing bars in the United States. 

Table-2: Grades of Deformed and Plain Carbon-Steel Reinforcing Bars; ASTM A 615

Grade	GRADE 40	GRADE 60	GRADE 75	GRADE 80
Minimum Yield  Strength (psi)	40,000	60,000	75,000	80,000
Minimum Yield  Strength (MPa)	280	420	520	550

Figure-4: ASTM-A615-Gr-60-SD500-SD400-Reinforcing-Steel-Rebar

Based on the carbon content, carbon steel can be low, medium, and high. Lower carbon content rebars are more weldable.  Carbon steel rebars possess High tensile strength, are Bendable and versatile, and Least expensive. The main disadvantage of carbon steel rebars is that they are susceptible to corrosion and are heavy.

3. Low-Alloy Steel Rebars

A low-alloy steel is composed of steel and another metal of desired properties. It contains 1%-5% of alloying elements. They are manufactured for precise chemical composition for better mechanical properties and better corrosion resistance. 

Low-alloy deformed and plain bars conform to the requirements provided by ASTM A706/A706M). 

The bars of two minimum yield strength levels are designated as Grade 60 and Grade 80, whose strength values are provided in Table-2.

Table-2: Minimum Yield Strength Values of Low-Alloy Steel Rebars

Grade	GRADE 60	GRADE 80
Minimum Yield Strength (psi)	60,000	80,000
Minimum Yield Strength (MPa)	420	550

Low-alloy steel bars are used in construction applications where a higher rate of weldability and ductility is demanded.

4. HYSD Rebars

High-yield strength deformed (HYSD) steel rebars that have lugs, ribs, or deformations on the surface. These bars are manufactured by hot rolling and cold twisting of mild steel bars and have a higher percentage of carbon as compared to mild steel. They are available in diameters between 5 mm to 40 mm.

As per IS 1786: 2008, High Strength Deformed Bars and Wires for Concrete Reinforcement, HYSD deformed steel bars and wires as reinforcement in concrete are available in the following grades:

a) Fe 415, Fe 415D, Fe 415S; 

b) Fe 500, Fe 500D, Fe 500S; 

c) Fe 550, Fe 550D; and 

d) Fe 600.

NOTES: The figures following the symbol Fe indicated the specified minimum 0.2 percent proof stress or yield stress in N/mm2 The letter D following the strength grade indicates the category with the same specified minimum 0.2 percent proof stress/yield stress but with enhanced specified minimum percentage elongation.

Table-3: Mechanical Properties of High-Strength Deformed Bars and Wires
As per Clause 8.1 (Table 3) of IS: 1786-2008

Property	Fe 415	Fe 415D	Fe 500	Fe 500D	Fe 550	Fe 550D	Fe 600
Yield Stress N/mm2	415	415	500	500	550	550	600
Ultimate Tensile Stress N/mm2	10 percent more than the actual 0.2 percent proof stress/ yield stress but not less than 485.0 N/mm2	12 percent more than the actual 0.2 percent proof stress/ yield stress but not less than 500 N/mm2	8 percent more than the actual 0.2 percent proof stress/ yield stress but not less than 545 N/mm2	10 percent more than the actual 0.2 percent proof stress/ yield stress but not less than 565 N/mm2	6 percent more than the actual 0.2 percent proof stress/ yield stress but not less than 585 N/mm2	8 percent more than the actual 0.2 percent proof stress/ yield stress but not less than 600 N/mm2	6 percent more than the actual 0.2 percent proof stress/ yield stress but not less than 660 N/mm2
Elongation (%)	14.5	18	12	16	10	14.5	10

HYSD bars can be classified into cold twisted deformed bars (CTD bars), thermo mechanically twisted bars (TMT bars), and corrosion-resistant bars (CRS).

4.1. Cold Twisted Deformed (CTD) Bars

CTD bars are high-strength reinforcement steel twisted bars that involve the process of cold twisting. As CTD derives strength from hardening, they possess decreased ductility. 

TOR is the brand name – Toristeg Steel Corporation of Luxembourg. Their name (TOR) became synonymous with Cold Twisted Deformed (CTD) steel bars due to their popularity.

4.2. Thermo Mechanically Treated (TMT) Bars

TMT bars are high-strength reinforcement steel bars that are manufactured using hot treatment technology. TMT bars possess better bending and re-bending ability compared to CTD bars. 

4.3. Corrosion Resistant Bars (CRS)

CRS is the more advanced version of TMT bars, with additional corrosion resistance. The advanced properties are developed by adding copper, chromium, and phosphorus. 

5. Stainless Steel Rebars (SSR)

Stainless steel is the name given to a group of corrosion-resistant steel alloys that contain a minimum of 10.5% chromium. Stainless steel reinforcement is made of unalloyed steel which won't show any kind of corrosion and rust forming that is noticeable in normal environmental conditions. There are two main codes and standards today that incorporate the SSR. They are:

• The British Standard BS6744
• The American Standard ASTM A955

Table-4: Tensile Requirements for Stainless Steel Rebars ATM A955

Property	GRADE 40	GRADE 60	GRADE 75
Tensile Strength psi[MPa]	70000 [500]	90,000[620]	100,00 [690]
Yield Strength psi [MPa]	40,000[280]	60,000[420]	75,000[520]

Stainless Steel Rebars

The ASTM A955/A955M Standard Specification for Deformed and Plain Stainless-Steel Bars for Concrete Reinforcement. The bars are of three minimum yield strength levels namely Grade 40, Grade 60, and Grade 75, whose properties are given in Table-4. It is the main Standard for stainless steel rebar in the USA and Canada.

6. Epoxy-Coated Rebars

Epoxy-coated rebars also called green bars are steel bars with epoxy coating to ensure corrosion resistance. They can be plain or deformed bars. It is used in the construction of bridges, marine structures, repair works, parking structures, and pavement where they are exposed to deicing salts or marine environments.

Figure-5: Epoxy Coated Steel Rebar

ASTM A775/A775M-22, Standard Specification for Epoxy-Coated Steel Reinforcing Bars, is the standard code for epoxy-coated rebars that are coated using the electrostatic spray method. 

This code provides the guidelines when the steel bars are fabricated with epoxy coating. ASTM A934/A934M-07 Standard Specifications for Epoxy-Coated Prefabricated Steel Reinforcement Bars provide guidelines for steel bars that are epoxy coated after fabrication.

 

7. Galvanized Rebars

Galvanized rebars are reinforcing steel that is coated with a protective layer of zinc metal. The zinc coating on the rebars serves as a barrier to corrosive elements. 

ASTM A767/A767M-19, Standard Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement. This specification covers steel reinforcing bars with protective zinc coatings applied by dipping the properly prepared reinforcing bars into a molten bath of zinc. 

Hot-dip galvanizing is a process of protecting reinforcing steel from corrosion by immersing them in molten zinc.

 

8. Low-Carbon Chromium Bars

Low-carbon, chromium steel reinforcing bars is specified according to ASTM A1035/A1035M, Standard Specification for Deformed and Plain LowCarbon, Chromium, Steel Bars for Concrete Reinforcement. The code specifies three alloy types, namely CS, CM, and CL with two yield strengths (100 ksi and 120 ksi). Each possesses different chemical composition and each type is selected depending on the required corrosion resistance. 

Table-5: Tensile Requirements of Low-Carbon Chromium Bars 

As per A1035/A1035M

Type	A1035 CL		A1035 CM		A1035 CS
Grade	Grade 100 [690]	Grade 120 [830]	Grade 100 [690]	Grade 120 [830]	Grade 100 [690]	Grade 120 [830]
Tensile Strength, min, psi	150,000	150,000	150,000	150,000	150,000	150,000
[MPa]	[1030]	[1030]	[1030]	[1030]	[1030]	[1030]
Yield Strength (0.2% offset), min, psi [MPa]	100,000 [690]	120,000 [830]	100,000  [690]	120,000  [830]	100,000 [690]	120,000  [830]
Elongation in 8 in.[200 mm], min
Bar Designation No.3 through 11 [10 through 36]14, 18, 20 [43, 57, 64]
	7	7	7	7	7	7
	6	6	6	6	6	6

9. Rail and Axle Steel Bars

ASTM A996/A996M-16, is the Standard Specification for Rail-Steel and Axle-Steel Deformed Bars for Concrete Reinforcement. Three types of products are included, designated with a “rail symbol,” an “R” for bars made of rail steel, and an “A” for bars made of axle steel.

Table-5: Tensile Requirements of Rails and Axle Steel Bars

As Per ASTM A996/A996M-16

Property	Type R Rail Steel Bars		Type A Axle Steel Bars
	Grade 50	Grade 60	Grade 40	Grade 60
Tensile Strength, Min, psi [MPa]	80,000 [550]	90,000 [620]	70,000[500]	90,000 [620]
Yield Strength, Min, psi [MPa]	50,000 [350]	60,000 [420]	40,000[280]	60,000 [420]

The bars shall be rolled from standard section Tee rails or from carbon steel axles for railway cars and locomotives.

10. Fiber-Reinforcing Polymer (FRP) Rebars

FRP has been used as an alternative for steel reinforcement for concrete structures. FRP rebars can be made out of aramid fiber (AFRP), carbon fiber (CFRP), or glass fiber (GFRP) that is embedded in a resin matrix. 

Glass FRP Rebars

ACI 440.1R-15, Design and Construction of Structural Concrete Reinforced with FRP Bars provide the guidelines for using FRP bars in concrete. ACI 440.3R provides a comprehensive list of test methods and material specifications to support design and construction guidelines. ACI 440.5 provides specification details for construction with FRP reinforcing bars. Material specifications for FRP bars are found in ACI 440.6.

Table-6: Tensile Properties of Reinforcing Bars 

As per 440.1R-15

	Steel	GFRP	CFRP	AFRP
Nominal yield stress, ksi (MPa)	40 to 75 (276 to 517)	NA	NA	NA
Tensile strength, ksi (MPa)	70 to 100 (483 to 1600)	70 to 230 (483 to 690)	87 to 535 (600 to 3690)	250 to 368 (1720 to 2540)
Elastic modulus, x 103 ksi (GPa)	29.0 (200.0)	5.1 to 7.4 (35.0 to 51.0)	15.9 to 84.0 (120.0 to 580.0)	6.0 to 18.2 (41.0 to 125.0)
Yield strain, percent	0.14 to 0.25	NA	NA	NA
Rupture strain, percent	6.0 to 12.0	1.2 to 3.1	0.5 to 1.7	1.9 to 4.4

FRP materials are nonmagnetic and noncorrosive, the problems of electromagnetic interference and steel corrosion can be avoided with FRP reinforcement. Additionally, FRP materials exhibit several properties, such as high tensile strength, that make them suitable for use as structural reinforcement.

Read More: Carbon Fiber Fabric for Structural Strengthening