Understanding Bridge Galvanizing Specifications
Hot-dip galvanizing specifications for bridge and highway structures involve navigating the relationship between two important standards: ASTM A123 and AASHTO/NSBA S8.3. While both govern batch hot-dip galvanizing of fabricated steel products, AASHTO/NSBA S8.3 includes additional requirements and commentary specifically addressing the needs of bridge and transportation infrastructure projects. Understanding the similarities and key differences between these standards helps specifiers, fabricators, and galvanizers ensure successful project outcomes.
ASTM A123, Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products, serves as the primary standard throughout North America for general fabricated products galvanized using the batch process. AASHTO/NSBA S8.3, developed jointly by the American Association of State Highway and Transportation Officials and the National Steel Bridge Alliance, builds upon A123 foundations while incorporating bridge-specific requirements and guidance.
Development and Scope
AASHTO/NSBA S8.3 emerged from collaboration between transportation agencies and the steel bridge industry to address specific needs of bridge galvanizing that general-purpose standards might not fully address. The standard contains guide specification language and commentary relevant to owners, contractors, fabricators, galvanizers, and inspectors involved in bridge projects.
Not all content within AASHTO/NSBA S8.3 applies directly to galvanizing operations. Many sections address contractual obligations, project management responsibilities, or design considerations outside the galvanizer's direct control. In numerous instances, galvanizing according to ASTM A123 while ensuring design and fabrication comply with ASTM A385, A384, and A143 satisfies the galvanizing requirements of AASHTO/NSBA S8.3. However, specific additional requirements may apply depending on project specifications and contractual arrangements.
Scope Comparison
Both standards apply to batch hot-dip galvanizing of fabricated steel products. ASTM A123 addresses a broad range of fabricated items across all industries and applications. AASHTO/NSBA S8.3 focuses specifically on bridges and highway structures, incorporating bridge-specific considerations for member types, connection details, and service environments typical of transportation infrastructure.
Steel Material Considerations
AASHTO/NSBA S8.3 includes specific requirements regarding steel grades and heat treatment that are not explicitly addressed in ASTM A123. The bridge standard prohibits galvanizing steels that have been heat-treated in the temperature range of the galvanizing kettle, recognizes potential challenges with ASTM A709 HPS 70W and 100W grades, and requires preblasting when galvanizing weathering steel structures.
Regarding steel reactivity—the tendency of certain steel chemistries to develop thicker galvanized coatings—AASHTO/NSBA S8.3 recommends considering reactivity management practices but does not mandate specific requirements. The commentary references ASTM A385 for guidance on managing reactive steels. Neither A123 nor S8.3 contains hard requirements limiting steel reactivity, though both recognize that silicon and phosphorus content influence coating development.
Material Test Reports (MTRs) are not required by either standard to be provided to the galvanizer, though both AASHTO/NSBA S8.3 and ASTM A385 discuss the value of sharing MTRs for process planning purposes. Providing MTRs to galvanizers before processing enables proactive management of steel chemistry effects on coating characteristics.
Product Identification and Handling
Both standards address product identification requirements, though AASHTO/NSBA S8.3 adds specific restrictions. While both standards reference or contain guidance on satisfactory identification methods, the bridge standard specifically prohibits weld marking unless contract documents explicitly allow this practice. This restriction responds to concerns about weld defects or distortion that might result from identification welds on bridge components.
Shop Drawings and Design Communication
AASHTO/NSBA S8.3 includes a dedicated section on shop drawings and handling that has no direct counterpart in ASTM A123. The bridge standard requires shop drawings to show sufficient vent and drain holes per ASTM A385 and establishes a coordination process if holes prove inadequate. The standard also requires clear delineation of faying surface limits for slip-critical connections and mandates engineering approval for any additional holes or temporary weld attachments needed for galvanizing handling.
While ASTM A385 similarly requires adequate vent and drain holes, neither A123 nor A385 establishes formal procedures for resolving inadequate holes or approving handling attachments. The bridge standard's explicit coordination requirements reflect the critical nature of bridge components and the importance of maintaining structural integrity through all processing steps.
Coating Thickness Requirements
Both standards require galvanized coatings to meet ASTM A123 minimum thickness and surface criteria. However, AASHTO/NSBA S8.3 adds requirements addressing concerns about very low individual measurements (gauge readings) that might occur during coating inspection.
The bridge standard establishes a minimum single-spot reading requirement in addition to A123's average thickness requirements. This provision, detailed in S8.3 Section 6.2.1, references the dry film thickness inspection standard SSPC-PA 2 from AMPP and provides investigation and renovation procedures for low-thickness areas. This additional requirement aims to ensure adequate coating protection at all locations, not just on average.
Inspection of I-Shaped Members
For I-shaped primary members (rolled beams and plate girders), AASHTO/NSBA S8.3 specifies a sampling plan that differs from ASTM A123. Most notably, members longer than 65 feet require four inspection specimens rather than the three specified in A123. The bridge standard also mandates close visual inspection of thermally-cut edges for flaking—a requirement not explicitly stated in A123.
These enhanced inspection requirements reflect the critical nature of primary bridge members and the desire to ensure coating quality on long-span components where access for future maintenance may be limited or impossible.
Finish and Appearance Requirements
At minimum, AASHTO/NSBA S8.3 requires exposed galvanized surfaces to meet ASTM A123 appearance requirements. The bridge standard introduces terminology not found in A123, specifically defining \"prominences\" as localized elevated points that may be excessive zinc deposits, dross, or other debris covered by zinc.
Pedestrian Contact Areas
For areas subject to direct human contact or close observation (pedestrian contact areas), AASHTO/NSBA S8.3 establishes elevated aesthetic requirements that may necessitate additional galvanizer efforts including vent and drain hole optimization, extra handling care, prominence smoothing, or cosmetic touch-ups. These heightened standards recognize that bridge railings, walkways, and other pedestrian-accessible areas face greater aesthetic scrutiny than typical structural steel.
Faying Surfaces for Slip-Critical Connections
Slip-critical connections—common in bridge construction—require firm contact between connection plies during high-strength fastener tightening. AASHTO/NSBA S8.3 includes specific finish requirements for faying surfaces to ensure proper connection performance. While ASTM A123's \"intended use\" clause can address prominences affecting connection performance, the bridge standard makes these requirements explicit.
Importantly, S8.3 recognizes that achieving required faying surface conditions may involve post-galvanizing smoothing work by fabricators or erectors rather than sole reliance on galvanizing process control. Specifiers should carefully consider responsibility allocation for meeting faying surface requirements rather than placing complete responsibility on galvanizers when connection geometry or coating thickness variations may necessitate additional smoothing.
Adhesion and Embrittlement Testing
In certain circumstances, AASHTO/NSBA S8.3 requires inspection for edge flaking and adhesion testing. This requirement responds to observations that common bridge fabrications often combine thick and thin materials—such as thick flanges with thin webs or thick base plates with thin stiffeners—and that bridge steels tend toward reactive chemistries (ASTM A709 allows silicon up to 0.40 percent).
The reactive steel chemistry combined with thin material cross-sections can produce very thick coatings (20 mils or more). While such coatings are not inherently defective, they may require additional handling care during shipment and installation to prevent flaking. Should flaking occur, it can be repaired at the jobsite and typically does not continue after installation.
Fabrication and Surface Preparation
AASHTO/NSBA S8.3 contains detailed requirements and commentary for fabricators regarding field-applied shear studs, thermally cut edges, copes, sheared edges, progressive dipping, blast cleaning before galvanizing, surface preparation procedures, temporary bracing to minimize distortion, and acceptance criteria for distortion. ASTM A123 does not address these topics directly but references ASTM A385 for design and fabrication best practices and ASTM A384 for distortion control guidance.
For galvanizers, S8.3 assigns responsibility for inspecting copes in connection plates and girder webs for cracks after galvanizing—a specific requirement not found in A123. When abrasive blasting of built-up plate girders and rolled beams before galvanizing is specified, recognize that not all galvanizing facilities maintain blast equipment. Best practice involves establishing mutual agreement between fabricator and galvanizer regarding blast cleaning responsibility during project planning.
Handling, Storage, and Duplex Systems
ASTM A123 scope does not extend to handling and storage of galvanized products. AASHTO/NSBA S8.3, by contrast, places explicit requirements on galvanizers, fabricators, and erectors for proper handling and storage. Galvanized products must be stored off the ground and away from standing water. All handling must avoid coating damage including raising burrs at lift points.
For duplex systems (paint or powder coating over galvanizing), both standards align in practice by referencing industry best practices such as ASTM D6386 for painting preparation and ASTM D7803 for powder coating preparation. The S8.3 commentary recommends that galvanizers be responsible for surface smoothing while painters handle further preparation—a division of responsibility that should be clearly established through mutual agreement to prevent expectation misalignment.
Quality Management Systems
A significant difference between the standards involves quality management system (QMS) requirements. AASHTO/NSBA S8.3 requires implementation of a quality management system with detailed requirements listed in Section 11.2. ASTM A123, conversely, functions as a \"performance-based\" standard focused on final product quality rather than mandating specific procedural systems.
This distinction reflects different philosophical approaches: A123 allows galvanizers flexibility in how they achieve specified coating quality, while S8.3 requires documented quality systems typical of major infrastructure projects. Galvanizers serving bridge markets must be prepared to demonstrate QMS compliance as part of project qualification.
Practical Application Guidance
For many projects, galvanizing to ASTM A123 while ensuring design and fabrication follow ASTM A385 guidance will satisfy AASHTO/NSBA S8.3 requirements. The additional S8.3 provisions primarily address project management, enhanced inspection protocols, and quality system documentation rather than fundamentally different coating requirements.
Key areas where S8.3 imposes requirements beyond A123 include minimum single-spot thickness readings, enhanced primary member inspection, elevated aesthetic standards for pedestrian areas, explicit faying surface provisions, potential adhesion testing requirements, cope crack inspection, and quality management system implementation.
Early communication among all project participants—owner, engineer, fabricator, and galvanizer—ensures clear understanding of which standard requirements apply and how responsibilities are allocated. This coordination prevents disputes and supports successful project execution.
Frequently Asked Questions
Can I specify ASTM A123 for bridge projects instead of AASHTO/NSBA S8.3?
Yes, though many transportation agencies prefer or require S8.3 for bridge work. Consult with the project owner regarding specification preferences and requirements.
What are the most significant differences between the standards?
Enhanced inspection requirements for primary members, minimum single-spot thickness provisions, elevated pedestrian area aesthetics, faying surface requirements, and mandatory quality management systems represent the most impactful differences.
Do both standards result in the same coating quality?
Both require compliance with A123 minimum coating thickness and surface criteria. S8.3 adds inspection rigor and addresses bridge-specific concerns but does not fundamentally change coating quality standards.
Who should I contact with questions about specification interpretation?
The American Galvanizers Association provides technical support for both A123 and S8.3 interpretation. Your galvanizer can also help clarify how requirements apply to specific project circumstances.
Supporting Bridge Project Success
V&S Galvanizing maintains comprehensive familiarity with both ASTM A123 and AASHTO/NSBA S8.3 requirements. Our quality systems address the enhanced documentation and inspection protocols required for bridge projects, and our technical staff work collaboratively with bridge fabricators to ensure coating outcomes meet both performance and aesthetic requirements. We participate actively in standards development to help shape future revisions and ensure specifications reflect practical galvanizing capabilities. Whether your project specifies A123 or S8.3, our experience with transportation infrastructure ensures successful galvanizing outcomes.
For a detailed comparison of AASHTO/NSBA S8.3 and ASTM A123 requirements, refer to the original AGA resource on specification differences.
