AWS D1.4 — Structural Welding Code for Reinforcing Steel
AWS D1.4 is the structural welding code for reinforcing steel used in concrete construction. It governs rebar-to-rebar splices, rebar-to-structural steel connections, and embed plate attachments using carbon-equivalent-based preheat requirements from Table 7.2 applied to ASTM A615, A706, and A996 rebar specifications.
Preheat lookup: D1.4 preheat is driven by carbon equivalent and bar size, not steel grade tables. Use the D1.4 Rebar Preheat Calculator to look up the minimum preheat from Table 7.2 based on your rebar CE and bar size. For rebar-to-structural connections, compare with D1.1 preheat and use the higher value.
What Is AWS D1.4?
AWS D1.4 governs structural welding of reinforcing steel (rebar). It covers ASTM A615, A706, and A996 rebar grades and addresses direct butt splices, indirect butt splices, and lap splices. Preheat is determined by carbon equivalent from Table 7.2, not by steel grade category as in D1.1.
AWS D1.4/D1.4M — Structural Welding Code — Reinforcing Steel — covers the welding of reinforcing bars (rebar) used in concrete construction. The current edition is AWS D1.4:2018. The standard applies to welded splices between reinforcing bars, welded connections between rebar and structural steel members (embed plates, brackets, base plates), and welded connections between rebar and steel pipe or tubing used as structural members in concrete-filled applications.
Rebar welding presents unique challenges compared to structural steel welding under D1.1. Reinforcing steel is manufactured to meet mechanical property specifications (yield strength and tensile strength), but the chemical composition can vary significantly between heats and even between bars from the same heat. ASTM A615, the most commonly specified rebar standard, does not tightly control carbon or manganese content. This means the carbon equivalent — and therefore the susceptibility to hydrogen-induced cracking — varies widely from bar to bar. D1.4 addresses this variability by requiring preheat based on the actual carbon equivalent of each heat rather than a fixed table based on grade, as D1.1 does for structural steel.
The standard is referenced by the American Concrete Institute (ACI) 318 Building Code, ACI 349 for nuclear safety-related concrete structures, and various state departments of transportation for infrastructure projects. When ACI 318 Section 26.6.4 requires welded reinforcing steel, the welding must comply with D1.4.
Preheat Requirements (Table 7.2)
D1.4 Table 7.2 determines preheat from the carbon equivalent (CE) of the rebar and the bar size. Six CE ranges span from 0.40 or less to over 0.65. Larger bars and higher CE values require higher preheat temperatures. CE is calculated from the mill test report using the IIW formula.
The preheat system in D1.4 is fundamentally different from D1.1. Instead of grouping steels by ASTM specification and assigning preheat by category (as D1.1 Table 5.11 does), D1.4 uses the actual carbon equivalent (CE) calculated from the mill test report (MTR) chemistry for each heat of rebar. This approach accounts for the wide composition variation inherent in reinforcing steel production.
D1.4 Clause 1.5.4 defines two carbon equivalent formulas. For all bars except ASTM A706/A706M (Eq. 1): CE = %C + %Mn/6. For ASTM A706/A706M bars (Eq. 2): CE = %C + %Mn/6 + %Cu/40 + %Ni/20 + %Cr/10 − %Mo/50 − %V/10. The A706 formula accounts for the additional alloying elements controlled in that specification. The mill test report must provide the chemical analysis for the calculation.
Table 7.2 cross-references the CE value against bar size to determine the minimum preheat temperature. The table is organized into CE ranges (typically 0.40 or less, 0.41 to 0.45, 0.46 to 0.55, 0.56 to 0.65, and 0.66 to 0.75) and bar size groups. Larger bars require higher preheat at the same CE level because the greater mass creates faster cooling rates in the heat-affected zone. A number 11 bar at CE 0.55 requires significantly more preheat than a number 4 bar at the same CE.
For rebar welding, preheat is critical because the high carbon equivalent values common in A615 rebar (CE values of 0.50 to 0.75 are typical) create a hardened heat-affected zone that is highly susceptible to hydrogen-induced cracking if the cooling rate is not controlled. Low-hydrogen welding processes and electrodes are mandatory for all D1.4 welding for this reason.
Rebar Specifications
D1.4 covers three primary rebar specifications: ASTM A615 (carbon steel, most common), A706 (low-alloy, specifically designed for welding with maximum CE of 0.55), and A996 (rail steel and axle steel). A706 is the preferred grade for welded connections because its controlled chemistry produces lower and more predictable preheat requirements.
ASTM A615 (Standard Rebar)
ASTM A615 is the most widely used reinforcing bar specification in North America. It covers deformed and plain carbon steel bars in Grades 40, 60, 75, 80, and 100 (yield strength in ksi). A615 does not restrict chemical composition beyond requiring the bars to meet the specified mechanical properties. This means A615 bars can have carbon content ranging from 0.20% to over 0.50% and manganese up to 1.50%, resulting in CE values from 0.35 to well over 0.70. The wide CE range means that preheat requirements vary dramatically between different heats of A615 rebar, and each heat must be evaluated individually using the MTR chemistry.
ASTM A706 (Weldable Rebar)
ASTM A706 was specifically developed for applications where welding is required. It restricts carbon to 0.30% maximum and carbon equivalent to 0.55% maximum. These composition limits ensure that A706 bars have consistently lower preheat requirements than A615 bars of the same grade. When the designer knows that rebar splices will be welded rather than mechanically connected, specifying A706 reduces fabrication costs by reducing preheat requirements and improving weldability. A706 is available in Grades 60 and 80.
ASTM A996 (Rail and Axle Rebar)
ASTM A996 covers reinforcing bars manufactured from rail steel (Type R) and axle steel (Type A). Rail steel rebar can have very high carbon content (up to 0.50% typical) and correspondingly high carbon equivalent values. Welding of A996 rebar requires careful evaluation of the MTR chemistry because the CE values often fall in the highest preheat ranges of Table 7.2. Type R (rail) bars are particularly challenging due to potential for inclusions from the rail manufacturing process.
Splice Types and Joint Details
D1.4 defines three splice types: direct butt splice (bars aligned end-to-end with CJP groove weld), indirect butt splice (splice plate or angle connecting two bars), and lap splice (parallel bars overlapping with fillet welds). Each splice type has specific joint detail requirements for groove angle, root opening, and weld length.
Direct Butt Splices
Direct butt splices join two rebar ends together in a groove weld configuration. The bars are aligned end-to-end with a specified root opening, and a complete joint penetration groove weld is made. This splice type provides the most efficient load transfer but requires the most demanding welding technique. The bar ends must be prepared (typically by sawing or grinding) to produce flat, square surfaces. Back-up material (a backing bar or copper backing shoe) is typically used to support the root pass.
Indirect Butt Splices (Lap Splices)
Lap splices overlap two parallel bars and connect them with fillet welds or flare bevel groove welds along the overlap length. The required weld length depends on the bar size and the required splice strength. Lap splices are easier to fit up and weld than direct butt splices but require more material (the overlap length) and create an eccentric load path. They are the most common splice type in field applications because they tolerate greater fit-up variation.
Rebar-to-Structural Steel Connections
Connections between rebar and structural steel members (embed plates, brackets, base plates, structural shapes) must satisfy both D1.4 and D1.1 requirements. The critical rule is that the preheat must be the higher of the two code requirements. If D1.4 Table 7.2 requires 200°F based on the rebar CE and bar size, but D1.1 Table 5.11 requires 300°F based on the structural steel grade and thickness, then 300°F governs. The filler metal must be compatible with both the rebar chemistry and the structural steel grade. Low-hydrogen electrodes (E7018 minimum) are required for all connections.
Procedure and Welder Qualification
D1.4 provides prequalified WPS options for standard rebar joints. When the procedure falls outside prequalified limits, qualification by testing per Clause 6 is required. Welder qualification requires bend tests on rebar specimens. For rebar-to-structural steel connections, the welder must be qualified under both D1.4 and D1.1.
AWS D1.4 generally requires welding procedures to be qualified by testing per Clause 8.2. However, Clause 8.1.2.1 provides one exception: fillet weld WPSs are considered prequalified and exempt from testing, unless performed with GTAW. All other joint types — direct butt splices, flare bevel groove welds, and lap splices using groove welds — require full WPS qualification testing that demonstrates the welds meet the specified mechanical properties. The qualification testing typically includes tensile tests and macroetch examination of test coupons welded using the WPS parameters.
Essential variables in D1.4 include welding process, filler metal classification, base metal bar size range, CE range, preheat temperature, joint type (direct butt or lap), position, and shielding gas composition (for GMAW and FCAW). A change in any essential variable beyond the qualified range requires re-qualification.
Welder qualification requires each welder to demonstrate the ability to produce sound welds on rebar using a qualified WPS. The welder performance test includes producing a test coupon in the applicable position that passes visual inspection and either bend testing or radiographic examination. Welders must be qualified specifically for D1.4 rebar welding — a D1.1 structural steel qualification does not automatically qualify a welder for rebar welding under D1.4.
How D1.4 Compares to Other AWS Structural Codes
D1.4 uses carbon equivalent-based preheat (Table 7.2); D1.1 uses steel grade and process-based preheat (Table 5.11). For rebar-to-structural steel connections, the higher preheat from either D1.4 or D1.1 governs. D1.4 covers rebar splice types not addressed in D1.1. Both share the same prequalified WPS framework.
D1.4 vs D1.1 (Structural Steel)
D1.1 covers structural steel members where the composition is tightly controlled by the ASTM specification. D1.1 groups steels into preheat categories (Table 5.11) based on the specification and assigns preheat by thickness and process. D1.4 uses individual CE calculations because rebar composition varies too widely to be grouped by specification. D1.1 provides a broad prequalified WPS path for CJP and PJP groove welds and fillets; D1.4 only prequalifies fillet welds (per Clause 8.1.2.1, except GTAW) and requires testing for all other joints. When rebar connects to structural steel, both codes apply simultaneously, and the higher preheat requirement governs.
D1.4 vs D1.8 (Seismic Supplement)
D1.8 supplements D1.1 for seismic applications but does not directly address rebar welding. In seismic zones, welded rebar connections in special moment frames and shear walls must meet both D1.4 requirements and any additional requirements imposed by ACI 318 Chapter 18 for seismic detailing. The engineer of record must specify the required splice strength as a percentage of the bar yield strength (typically 100% or 125% for seismic applications).
| Aspect | D1.4 (Rebar) | D1.1 (Structural) |
|---|---|---|
| Base metals | A615, A706, A996 rebar | A36, A572, A992 structural steel |
| Preheat method | Table 7.2 (CE-based) | Table 5.11 (category-based) |
| Preheat input | Carbon equivalent + bar size | Steel grade + thickness + process |
| Rebar-to-steel joints | Higher of D1.4 and D1.1 preheat | Not covered |
| Splice types | Direct butt, indirect butt, lap | Not applicable |
| Prequalified WPS? | Yes | Yes (Clause 5) |
Related Standards Guides
Frequently Asked Questions
AWS D1.4 uses carbon equivalent (CE) to determine preheat requirements through Table 7.2. D1.4 Clause 1.5.4 defines two CE formulas: for most bars, CE = C + Mn/6 (Eq. 1); for ASTM A706 bars, CE = C + Mn/6 + Cu/40 + Ni/20 + Cr/10 - Mo/50 - V/10 (Eq. 2). Table 7.2 cross-references the CE value against bar size to determine the minimum preheat temperature. Higher CE values and larger bar sizes require higher preheat. For rebar-to-structural steel connections, the preheat must be the higher of the D1.4 Table 7.2 requirement and the D1.1 Table 5.11 requirement.
ASTM A615 is the standard specification for deformed and plain carbon-steel bars for concrete reinforcement. It does not restrict chemical composition tightly, so A615 bars can have high carbon equivalent values that require significant preheat. ASTM A706 is specifically designed for welding — it restricts carbon to 0.30% maximum and carbon equivalent to 0.55% maximum, which reduces preheat requirements. When welding is anticipated, A706 is the preferred specification because its controlled chemistry produces consistently lower preheat temperatures and better weldability.
When rebar is welded to structural steel members, both D1.4 and D1.1 requirements apply. The preheat temperature must be the higher of the two code requirements — D1.4 Table 7.2 based on the rebar CE and bar size, and D1.1 Table 5.11 based on the structural steel grade, process, and thickness. The filler metal must be compatible with both the rebar and the structural steel. The WPS must be qualified under D1.4, and the welder must hold D1.4 qualification for the rebar side of the connection.
AWS D1.4 permits SMAW (shielded metal arc welding), GMAW (gas metal arc welding), FCAW (flux-cored arc welding), and GTAW (gas tungsten arc welding). SMAW with low-hydrogen electrodes (E7018 or E8018 series) is the most common field process for rebar welding. Low-hydrogen electrodes are required because rebar typically has higher carbon equivalent than structural steel grades, making the heat-affected zone more susceptible to hydrogen-induced cracking.
D1.4 generally requires WPS qualification by testing per Clause 8.2, but Clause 8.1.2.1 provides one exception: fillet weld WPSs are considered prequalified and exempt from testing, unless performed with GTAW. All other joint types (direct butt splices, flare bevel groove welds, lap splices using groove welds) require full procedure qualification. This is because rebar chemistry varies widely between heats, and the CE-based preheat system requires verification that the procedure accounts for the actual chemistry.