Welding of Reinforcing Bars

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Introduction to Reinforcing Bars in Concrete Construction

Deformed steel reinforcing bars (rebar) serve as essential components in modern reinforced concrete construction, finding applications in buildings, bridges, highways, locks, dams, docks, and piers. Their primary use extends to reinforcing columns, girders, beams, slabs, pavements, and both precast and prestressed concrete structures.

Concrete, while exceptionally strong in compression and shear, exhibits weakness in tension. The integration of deformed steel reinforcing bars within concrete addresses this limitation by accommodating tensile stresses, creating a composite material that combines the compression strength of concrete with the tensile strength of steel.

Understanding Rebar Specifications and Identification

The effectiveness of reinforced concrete depends on the strong bond between concrete and steel, achieved through deformations rolled into the bars. These deformations prevent the bars from slipping through the concrete, ensuring structural integrity. Reinforcing bars are manufactured in various sizes, designated by numbers based on eighths of an inch in nominal diameter.

The American Society for Testing and Materials (ASTM) has established three primary specifications for reinforcing bars: A 615 for plain billet steel bars, A 616 for rail steel reinforcing bars, and A 617 for axle steel reinforcement bars. Each type serves specific purposes in construction applications.

Bar Identification and Grade Systems

All U.S.-manufactured reinforcing bars feature rolled markings that provide essential identification information. These markings include the manufacturer's code in raised letters, followed by the steel mill identification letter and bar size number. The type of steel is indicated by specific symbols: N for new billet steel, A for axle steel, and a railroad rail cross-section symbol for rerolled railroad rails.

Grade designation is indicated by numbers representing the minimum yield strength in thousand pounds per square inch. Additional grade indicators include longitudinal line markings, where a single line indicates middle-strength grade and a double line denotes the highest-strength grade.

Material Properties and Chemical Composition

While ASTM specifications don't mandate specific chemical requirements, mills typically provide chemical analysis reports upon request. Rail steel bars, produced from recycled railroad rails, contain relatively high amounts of carbon and manganese. Similarly, axle steel bars, made from salvaged railroad car axles, feature high carbon and manganese content, placing them in the hard-to-weld category.

Welding Considerations and Procedures

The American Welding Society's "Reinforcing Steel Welding Code" provides comprehensive guidance for welding operations. The code includes carbon equivalent tables relating to bar size and recommends appropriate preheat and interpass temperatures. This information is crucial for determining proper welding procedures and ensuring weld quality.

Splice Types and Joint Design

Three primary splice types are commonly used in rebar welding:

• Direct Butt Splices: End-to-end splices of bars with aligned axes and similar sizes
• Indirect Butt Splices: Utilizing intermediary pieces such as steel plates or angles
• Lap Welded Splices: Overlapping bars welded together

For horizontal butt splices, single groove welds with 45° to 60° included angles are most common. Vertical axis welding typically employs single or double bevel groove welds.

Filler Metal Selection and Quality Control

Filler metal selection depends on the grade number of the steel. For shielded metal arc welding, specific AWS electrode classifications are recommended: E-7018 for Grade 40, E-8018 for Grade 50, E-9018 for Grade 60 and low-alloy A706, and E-10018 for Grade 75. Gas metal arc welding typically uses E-70S electrodes, while flux-cored arc welding employs E70T type electrodes for Grade 40 bars.

Preheating Requirements and Procedure Qualification

Minimum preheat and interpass temperatures are determined based on the carbon equivalent of the reinforcing bars and bar size. For large bars with unknown carbon equivalent, a 500° preheat is recommended. Welding procedures must be established considering the welding process, filler metal specifications, and welding technique.

Conclusion

Welding has proven to be the superior method for splicing reinforcing bars, consistently producing joints that exceed both the strength requirements of reinforced concrete and the performance of traditional lap splices. When properly executed according to established procedures and specifications, welded splices provide reliable, high-strength connections essential for modern construction applications.