How Rebar Is Designated
In the United States, reinforcing bars are designated by a bar number that represents the nominal diameter in eighths of an inch. A #5 bar has a nominal diameter of 5/8" = 0.625 inches. A #8 bar has a nominal diameter of 8/8" = 1.0 inch. This system runs from #3 (the smallest commonly used bar) through #18 (the largest).
Bar numbers correspond to nominal values — actual deformed bar geometry varies slightly between mills, but nominal diameter, area, and weight are standardized in ASTM specifications. All structural calculations use nominal dimensions.
Rebar Size Chart: #3 Through #11
The following table covers the bars used in the vast majority of structural concrete work. Bars #14 and #18 are used in heavy columns and caissons but are rarely encountered on typical projects.
| Bar Size | Nominal Diameter (in) | Nominal Area (in²) | Weight (lb/ft) |
|---|---|---|---|
| #3 | 0.375 | 0.11 | 0.376 |
| #4 | 0.500 | 0.20 | 0.668 |
| #5 | 0.625 | 0.31 | 1.043 |
| #6 | 0.750 | 0.44 | 1.502 |
| #7 | 0.875 | 0.60 | 2.044 |
| #8 | 1.000 | 0.79 | 2.670 |
| #9 | 1.128 | 1.00 | 3.400 |
| #10 | 1.270 | 1.27 | 4.303 |
| #11 | 1.410 | 1.56 | 5.313 |
Note: #9, #10, and #11 bars do not follow the eighths-of-an-inch rule exactly — they correspond to the old 1-inch, 1-1/8-inch, and 1-1/4-inch square bars they replaced, with areas equal to those square bars.
Deformed vs. Plain Bar
Nearly all structural reinforcing steel is deformed bar — rolled with raised ribs and lugs on the surface that create mechanical interlock with the concrete. This interlock is what allows the rebar to develop tension force in concrete members. Plain (smooth) bars are only permitted in ties, spirals, and certain non-structural applications under ACI 318.
ASTM A615 defines the standard specification for deformed and plain carbon-steel bars. It is by far the most commonly specified and produced rebar in the United States.
Rebar Grades and ASTM Standards
The grade of rebar refers to its minimum yield strength in ksi (thousands of pounds per square inch). Grade 60 is the default in virtually all U.S. structural concrete design per ACI 318.
- Grade 40 (fy = 40 ksi) — ASTM A615. Now rarely specified for new structural work; was common in construction before the 1970s. Still occasionally used for small stirrups and ties where ductility is important and cost is secondary.
- Grade 60 (fy = 60 ksi) — ASTM A615. The standard structural grade. Unless a project specifies otherwise, assume Grade 60 for all structural reinforcing. Available in bar sizes #3 through #18.
- Grade 75 (fy = 75 ksi) — ASTM A615. Used for heavily loaded columns to reduce bar congestion. ACI 318-19 permits Grade 75 in columns per Section 20.2.2.
- Grade 80 (fy = 80 ksi) — ASTM A706. Weldable, seismic-grade rebar (see below). Introduced in the 2015 edition of ASTM A706.
- Grade 100 (fy = 100 ksi) — ASTM A1035. High-strength bar for congestion reduction in high-rise and seismic applications; also available as Grade 120.
ASTM Standards for Rebar
- ASTM A615 — Standard specification for deformed and plain carbon-steel bars. The most common specification. Covers Grades 40, 60, and 75. Does not guarantee weldability — welding requires verifying carbon equivalent using mill certifications (per AWS D1.4).
- ASTM A706 — Low-alloy steel deformed and plain bars for concrete reinforcement. Specifically designed for welding and seismic applications. Chemistry is controlled to ensure predictable yield-to-tensile ratio and ductility. Covers Grades 60 and 80. Required by ACI 318-19 in Seismic Design Categories D, E, and F unless ACI 318 exceptions are met.
- ASTM A1035 — High-strength deformed and plain bars. Available in Grades 100 and 120. Used in heavily loaded columns and post-tensioned elements to reduce bar congestion. The higher yield strength requires using the same or larger development lengths due to the bond-dominated nature of concrete anchorage.
Corrosion-Resistant Rebar Options
Standard carbon-steel rebar corrodes when exposed to moisture, chlorides (road salt, seawater), or aggressive soils. Several corrosion-resistant options exist:
- Epoxy-coated rebar (ASTM A775 / A934) — standard Grade 60 bar with a fusion-bonded epoxy coating applied at the mill. The coating is green or gray. Widely used in bridge decks, parking structures, and coastal construction. Requires careful handling on site to avoid coating damage; damaged areas must be repaired with epoxy patching compound. Development and splice lengths must be increased per ACI 318 Table 25.4.2.4 (multiply by 1.2 or 1.5 depending on cover).
- Galvanized rebar (ASTM A767) — hot-dip galvanized zinc coating. Provides better handling durability than epoxy coating and is used in some marine and industrial floor applications. Less commonly specified than epoxy.
- Stainless steel rebar (ASTM A955) — the premium corrosion-resistant option. Type 316LN stainless provides exceptional corrosion resistance in marine splash zones, bridge piers exposed to deicers, and wastewater treatment facilities. Cost is 5–8× that of carbon steel, so it is reserved for high-consequence corrosion exposures where life-cycle cost justifies the premium.
- MMFX / ASTM A1035 (corrosion-resistant) — microcomposite steel with improved corrosion resistance compared to A615, sometimes marketed for bridge decks, though performance claims vary and its primary advantage is high yield strength.
Minimum Concrete Cover Requirements (ACI 318-19, Table 20.6.1)
Concrete cover is the distance from the face of the concrete to the nearest surface of the reinforcing bar. Cover protects rebar from corrosion and provides fire resistance. ACI 318-19 Table 20.6.1 specifies minimum cover for cast-in-place concrete:
| Condition | Bar Size | Min. Cover |
|---|---|---|
| Concrete cast against and permanently exposed to earth | All | 3 in |
| Concrete exposed to earth or weather — #6 through #18 | #6–#18 | 2 in |
| Concrete exposed to earth or weather — #5 and smaller | #5 and smaller | 1-1/2 in |
| Slabs, walls, joists — not exposed to weather — #14 and #18 | #14, #18 | 1-1/2 in |
| Slabs, walls, joists — not exposed to weather — #11 and smaller | #11 and smaller | 3/4 in |
| Beams, columns — primary reinforcement, ties, stirrups | All | 1-1/2 in |
| Shells and folded plate members — #6 and larger | #6 and larger | 3/4 in |
Epoxy-coated bar and galvanized bar may have modified cover requirements in some exposure conditions — verify with your project specifications and ACI 318-19 Section 20.6.1.
Development Length and Lap Splice Basics
Development length (ℓd) is the minimum embedment of a straight bar into concrete required to develop the bar's yield strength through bond. ACI 318-19 Section 25.4 provides the equations. Development length increases with bar size, decreases with cover and spacing, and is modified by factors for top bars (casting position), epoxy coating, and lightweight concrete.
As a rough reference for Grade 60, normal-weight concrete, with adequate cover and no special conditions: a #8 bar in 4,000 psi concrete requires roughly 38 to 48 bar diameters of development length (approximately 38–48 inches for a #8 bar). Always use the ACI 318 equations for design.
Lap splice length is the length over which two bars overlap to transfer force between them. Class A tension lap splices are 1.0× ℓd; Class B are 1.3× ℓd. The class depends on the percentage of bars spliced at the same location and the ratio of As,provided to As,required.
Bar Marking System
Every deformed bar has a series of raised marks on its surface that identify the producing mill, bar size, steel type, and grade:
- Mill mark — a letter or symbol identifying the producer.
- Bar size — the bar number (#3 through #18) in raised digits.
- Steel type — S for carbon steel (A615), W for low-alloy (A706), A for axle steel, R for rail steel.
- Grade mark — Grade 60 bars have one continuous line or the number 4 (in some systems); Grade 75 has two lines or the number 5. Grade 40 has no grade line. This system is standardized in ASTM A615.
If you receive rebar on site without visible mill markings, do not use it in structural applications without mill certifications confirming the grade and ASTM compliance.
When to Use Higher-Strength Grades
Higher rebar grades reduce bar congestion and total steel weight, but they come with trade-offs:
- Grade 75 or 80 makes sense in heavily reinforced columns where Grade 60 bars are too congested to allow concrete placement and vibration. The additional yield strength allows the use of fewer or smaller bars.
- Grade 100 (A1035) is cost-effective in high-rise columns and mat foundations where reducing bar weight and congestion has large downstream construction benefits. Its corrosion resistance is a secondary advantage in bridge decks.
- A706 (seismic grade) is required — not optional — in special moment frames and special structural walls in high seismic areas. The controlled yield-to-tensile ratio is essential for ductile behavior during earthquake loading.