Fire Protection Requirements for Galvanized Steel
Structural steel in commercial and institutional buildings requires passive fire protection systems maintaining load-bearing capacity during fire events, with building codes establishing fire resistance ratings—expressed as hourly durations (1-hour, 2-hour, 3-hour, 4-hour)—based on occupancy classification, building height, and structural function. Hot-dip galvanized steel members specified for corrosion protection in parking structures, manufacturing facilities, chemical plants, and other corrosive environments simultaneously require fireproofing compliance with code-mandated ratings, creating the technical challenge of achieving adequate bond strength between applied fireproofing materials and zinc-coated surfaces exhibiting different adhesion characteristics compared to bare or painted steel.
Passive fireproofing systems fall into two distinct categories regarding galvanized surface preparation: mechanically fastened systems (rigid board, flexible blankets) requiring no surface modification beyond standard cleanliness, and adhesion-dependent systems (spray-applied fire-resistant materials and intumescent coatings) demanding specific surface preparation protocols ensuring minimum cohesive and adhesive bond strength compliance. The absence of industry-standardized guidelines specifically addressing fireproofing application over hot-dip galvanizing necessitates reliance on manufacturer technical data sheets, testing laboratory directory listings (Underwriters Laboratories, Intertek, FM Global), and adaptation of surface preparation standards developed for painting galvanized steel—primarily ASTM D6386—to fireproofing applications.
Passive Fireproofing System Categories
Mechanically Fastened Systems
Rigid Board Products:
Materials:
- Mineral fiber boards (calcium silicate, ceramic fiber)
- Gypsum-based boards
- Cement-based boards
- Thickness: 1/2 inch to 4+ inches depending on rating
Installation Method:
- Mechanical fasteners (pins, clips, screws) penetrating fireproofing and attaching to steel
- Wire ties wrapping structural members
- Banding systems for columns and beams
Surface Preparation: None required beyond standard cleanliness:
- Remove loose debris and dust
- No zinc surface modification needed
- Mechanical attachment independent of surface chemistry
Advantages:
- No adhesion concerns with galvanized surfaces
- Predictable installation on any substrate
- Factory-controlled product quality
Limitations:
- Higher material costs than spray-applied systems
- Labor-intensive installation
- Difficulty conforming to complex geometries
Flexible Blanket Systems:
Materials:
- Ceramic fiber blankets
- Mineral wool blankets
- Encased in protective fabric or mesh
Installation:
- Wrapped around members
- Secured with banding, wire ties, or clips
- Pre-fabricated to member dimensions
HDG Compatibility: Excellent—no surface preparation concerns
Adhesion-Dependent Systems
These systems rely on chemical or mechanical bonding to steel surfaces, creating challenges with galvanized substrates:
Spray-Applied Fire-Resistant Materials (SFRM):
Applied by spray equipment creating thick insulating layer:
- Cement-based SFRM
- Gypsum-based SFRM
- Sprayed fiber SFRM
- Applied thickness: 1/2 inch to 4+ inches
Intumescent Fire-Resistant Materials (IFRM):
Multi-layer paint-like system:
- Primer/tie coat layer
- Intumescent base coats (expand when heated)
- Sealer/topcoat
- Total dry film thickness: 20-100+ mils
Surface Preparation Challenges with Galvanized Steel
Zinc Surface Chemistry
Adhesion Barriers:
Zinc Oxide/Hydroxide Formation: Atmospheric exposure creates loosely-bonded surface layers:
- Zinc oxide (ZnO)
- Zinc hydroxide (Zn(OH)₂)
- Zinc carbonate (ZnCO₃)
These layers provide weak boundary for adhesive bonding—fireproofing may bond to oxide layer, which then separates from metallic zinc.
Low Surface Energy: Zinc oxide surface energy (200-300 dynes/cm) substantially lower than bare steel (1,000+ dynes/cm), reducing wettability and adhesive bond potential.
Smooth Surface Texture: As-galvanized surfaces often smoother than mill scale steel:
- Limited mechanical interlocking
- Reduced anchor profile for adhesive grip
Comparison to Bare/Painted Steel
Bare Steel Advantages:
- Mill scale provides rough texture
- High surface energy
- Direct chemical bonding possible
Primed Steel Advantages:
- Primer designed for fireproofing adhesion
- Controlled surface properties
- Manufacturer-tested substrate
Galvanized Steel:
- Surface chemistry different from design basis
- Requires adaptation of application procedures
- May need enhanced surface preparation
ASTM D6386 Surface Preparation Standard
Standard Overview
Full Title: "Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting"
Scope: Establishes methods preparing galvanized surfaces for paint application—directly applicable to fireproofing tie coats, primers, and adhesion-dependent systems.
Key Methods:
Solvent Cleaning (SSPC-SP 1): Removes oil, grease, and soluble contaminants:
- Isopropyl alcohol wipe-down
- Acetone cleaning
- Commercial degreasers
Sweep Blasting: Light abrasive blast cleaning:
- Aluminum oxide or garnet media
- Low pressure (40-60 PSI)
- Removes 0.1-0.3 mils zinc surface
- Creates anchor profile without excessive zinc removal
Chemical Treatment: Proprietary surface conditioners:
- Phosphate conversion coatings
- Acidic etch primers
- Improve adhesion characteristics
Application to Fireproofing
When D6386 Preparation Needed:
For adhesion-dependent fireproofing systems:
- SFRM requiring bonding agents or primers
- All intumescent coating systems
- Any application specifying minimum bond strength
Typical Preparation Sequence:
- Solvent cleaning removing contaminants
- Light abrasion (if specified) creating profile
- Primer/tie coat application (if required)
- Fireproofing application per manufacturer specs
Spray-Applied Fire-Resistant Materials (SFRM)
Material Types
Cement-Based SFRM:
Composition:
- Portland cement binder
- Lightweight aggregates (vermiculite, perlite)
- Reinforcing fibers
Characteristics:
- Hard, durable finish
- Good impact resistance
- Typical density: 15-22 lbs/ft³
Gypsum-Based SFRM:
Composition:
- Gypsum binder
- Lightweight aggregates
- Reinforcing fibers
Characteristics:
- Softer finish than cement-based
- Lower density: 12-18 lbs/ft³
- Good thermal performance
Sprayed Fiber SFRM:
Composition:
- Mineral wool or ceramic fibers
- Inorganic binders
- Adhesive agents
Characteristics:
- Fibrous texture
- Very lightweight: 10-15 lbs/ft³
- Conformability to complex shapes
Surface Preparation Requirements
Minimum Preparation (Most Products):
Solvent Cleaning:
- Remove oils, greases, dirt, debris per SSPC-SP 1
- Allow complete drying before fireproofing application
- Typically sufficient for many SFRM products
Enhanced Preparation Options:
Some manufacturers specify additional measures:
Metal Lath:
Application:
- Expanded metal lath mechanically fastened to steel
- Provides mechanical key for SFRM adhesion
- Compensates for limited zinc surface bonding
Installation:
- 1/4" to 3/8" diamond mesh typical
- Fastened at 12-16 inch spacing
- Laps minimum 1 inch at joints
Advantages:
- Reliable mechanical attachment independent of surface chemistry
- Proven performance on galvanized substrates
Bonding Agents:
Materials:
- Proprietary latex or acrylic emulsions
- Spray or brush applied to galvanized surface before SFRM
Function:
- Creates adhesive interface between zinc and SFRM
- Improves bond strength
- Reduces dusting and friability
Application:
- Coverage: 200-400 ft²/gallon typical
- Dry time: 2-24 hours before SFRM application
- Follow manufacturer specifications exactly
Primer Tie Coats:
Materials:
- Acrylic, vinyl, or epoxy primers compatible with both zinc and SFRM
- Applied per ASTM D6386 recommendations
Purpose:
- Provides designed adhesion substrate
- Seals zinc surface
- Manufacturer-tested system component
Application:
- Surface preparation per D6386 (typically solvent clean + light sweep blast)
- Primer application per manufacturer DFT requirements
- Cure per manufacturer specifications
- SFRM application within recoat window
Application Requirements
Spray Equipment:
Specialized SFRM spray equipment:
- High-volume pumps
- Large-orifice spray nozzles
- Consistent delivery pressure and flow
Application Technique:
Build-Up Process:
- Multiple passes achieving specified thickness
- Allow partial setting between passes
- Final thickness per fire rating requirements
Quality Control:
Thickness Verification:
- Measure at specified intervals
- Probe or template gauge method
- Verify minimum thickness at all locations
Density Verification:
- Weigh representative samples per ASTM E605
- Verify within manufacturer specification range
- Critical to fire performance
Intumescent Fire-Resistant Materials (IFRM)
System Components
Multi-Layer System:
Layer 1: Primer/Tie Coat
- Directly applied to galvanized steel
- Acrylic, vinyl, or epoxy chemistry
- Designed for zinc substrate adhesion
- DFT: 3-5 mils typical
Layer 2: Intumescent Base Coat
- Multiple coats achieving specified thickness
- Contains expansion agents (char-forming chemicals)
- When heated, expands to insulating char layer (up to 50× thickness)
- DFT per coat: 5-20 mils
- Total intumescent DFT: 20-100+ mils depending on rating
Layer 3: Sealer/Topcoat
- Protective finish coat
- Weather resistance (if exposed)
- Aesthetic appearance
- DFT: 2-4 mils typical
Surface Preparation for IFRM
ASTM D6386 Compliance Required:
Intumescent systems demand proper surface preparation ensuring reliable primer adhesion:
Standard Preparation Sequence:
Step 1: Solvent Cleaning
- Remove all oils, greases, handling marks
- SSPC-SP 1 solvent cleaning
- Clean, lint-free cloths
- Allow complete evaporation
Step 2: Light Sweep Blasting (typically required)
- Aluminum oxide 80-120 grit
- 40-60 PSI pressure
- Anchor profile: 0.5-1.5 mils
- Removes zinc oxide/hydroxide surface layers
- Creates profile for mechanical interlocking
Step 3: Post-Blast Cleaning
- Remove all blast dust
- Solvent wipe-down
- Verify clean, dry surface
Alternative: Chemical Profiling Some systems allow chemical surface conditioners instead of blasting:
- Phosphate conversion coating
- Acidic etch primers
- Apply per manufacturer instructions
Application Procedures
Primer/Tie Coat Application:
Critical First Layer:
- Apply within specified time after surface preparation (typically <4 hours)
- Airless spray preferred for uniform coverage
- Achieve specified DFT (verify with wet film gauge)
- Inspect for holidays, thin spots
- Cure per manufacturer requirements
Intumescent Base Coat Application:
Multiple Coat Build-Up:
- Apply specified number of coats
- DFT per coat within manufacturer limits
- Recoat windows: Minimum and maximum times specified
- Cross-coat (perpendicular spray patterns) for uniformity
- Avoid sags, runs, excessive thickness
Quality Verification:
- Measure total dry film thickness
- Multiple locations per member
- Document thickness measurements
Sealer/Topcoat Application:
Final Protection Layer:
- Apply per manufacturer specifications
- Aesthetic and durability finish
- May have color options
Bond Strength Testing
ASTM E736 Standard
Full Title: "Standard Test Method for Cohesion/Adhesion of Sprayed Fire-Resistive Materials Applied to Structural Members"
Purpose: Verify fireproofing exhibits adequate bond strength preventing delamination or failure.
Test Method:
Pull-Off Test:
- Dolly Attachment:
- Circular metal disc (2" diameter typical)
- Bonded to fireproofing surface with strong epoxy adhesive
- Cure per adhesive specifications
- Cutting:
- Cut around dolly through fireproofing to substrate
- Isolates test area
- Pull-Off:
- Mechanical or hydraulic pull tester
- Apply increasing tensile force perpendicular to surface
- Record failure load
- Calculation:
- Bond strength (PSI) = Failure load / Dolly area
- Failure Mode Assessment: Critical determination:
- Cohesive failure within fireproofing: Acceptable if strength adequate
- Adhesive failure at fireproofing-substrate interface: May indicate preparation issue
- Failure within substrate (zinc coating): Zinc cohesive strength governs
Acceptance Criteria
Minimum Bond Strength Values:
Vary by product, application, and testing laboratory:
Typical Requirements:
- SFRM: 150-300 PSI minimum
- Intumescent coatings: 200-400 PSI minimum
Sources for Criteria:
- Manufacturer product data sheets
- Testing laboratory directory listings (UL, Intertek, FM)
- Project specifications
- Building code requirements
Testing Frequency:
Initial Application Testing:
- Test panels before production application
- Verify surface preparation and application procedures
- Establish baseline performance
Production Testing:
- Field testing per project specifications
- Typical: 1 test per 10,000 ft² or similar frequency
- Additional tests if conditions change or concerns arise
Manufacturer Pre-Approval and Directory Listings
HDG Surface Listings
Pre-Approved Substrates:
Many fireproofing products tested and listed for application over hot-dip galvanized steel:
- Testing laboratory conducted fire resistance testing with galvanized substrate
- Specific surface preparation documented in directory listing
- Designer specification can directly reference listing
Directory Resources:
Underwriters Laboratories (UL):
- Online Product iQ directory
- Search by design number
- Verify substrate compatibility
Intertek:
- Directory of Listed Products
- Fire resistance designs
FM Approvals:
- Approval Guide
- Fire protection systems
When HDG Not Pre-Approved
Options:
Contact Manufacturer:
- Request technical guidance for galvanized application
- Manufacturer may provide field-tested procedures
- Document correspondence for project records
Independent Testing:
- Commission testing per ASTM E736
- Establish project-specific procedures and acceptance criteria
- More expensive but provides definitive data
Alternative Fireproofing Selection:
- Select product with established HDG compatibility
- Simplifies approval and reduces risk
Specification and Coordination
Design Phase Considerations
Early Integration:
Coordinate fireproofing and galvanizing decisions early:
- Identify fireproofing products with HDG compatibility
- Understand surface preparation requirements
- Budget for preparation labor and materials
- Schedule implications (preparation time before fireproofing)
Specification Language:
Recommended Approach:
"Passive fireproofing shall be applied to hot-dip galvanized structural steel. Fireproofing products shall have documented compatibility with galvanized substrates through testing laboratory listings or manufacturer technical data. Surface preparation shall be per ASTM D6386 as modified by fireproofing manufacturer requirements. Bond strength shall be verified per ASTM E736 meeting minimum [specify] PSI."
Contractor Coordination
Key Interfaces:
Galvanizer:
- Coordinate timing of galvanizing with project schedule
- Ensure adequate storage preventing contamination before fireproofing
Fireproofing Contractor:
- Review manufacturer requirements for HDG application
- Verify equipment and materials for specified preparation
- Conduct test panels before production application
- Document surface preparation and bond test results
Passive fireproofing application on hot-dip galvanized structural steel requires careful material selection and surface preparation addressing zinc coating adhesion characteristics that differ substantially from bare or primed steel substrates for which most fireproofing systems were originally developed. Mechanically fastened systems including rigid boards and flexible blankets provide reliable fire protection on galvanized members requiring no surface modification beyond cleanliness, while adhesion-dependent systems—spray-applied fire-resistant materials (SFRM) and intumescent fire-resistant materials (IFRM)—demand specific preparation protocols following ASTM D6386 guidance including solvent cleaning, optional light sweep blasting, and application of bonding agents, metal lath, or primer tie coats per manufacturer specifications. SFRM application typically requires minimum solvent cleaning with enhanced options including mechanically fastened metal lath providing reliable mechanical key independent of surface chemistry, spray-applied bonding agents creating adhesive interface between zinc and fireproofing, or primer tie coats establishing designed adhesion substrate, while intumescent systems universally require comprehensive D6386 preparation including solvent cleaning followed by light sweep blasting (0.5-1.5 mils anchor profile) and primer application within specified time windows. Bond strength verification per ASTM E736 pull-off testing at specified frequencies confirms minimum adhesion/cohesion values ranging from 150-400 PSI depending on product type and testing laboratory requirements, with failure mode assessment distinguishing between acceptable cohesive failure within fireproofing and problematic adhesive interface failure indicating inadequate surface preparation. The absence of industry-standardized guidelines specifically addressing fireproofing over galvanized steel necessitates reliance on manufacturer technical data sheets, testing laboratory directory listings (UL, Intertek, FM Global) identifying products with pre-approved HDG substrates and documented preparation procedures, and early design-phase coordination between specifiers, galvanizers, and fireproofing contractors establishing compatible material selections, realistic surface preparation budgets, and appropriate quality verification protocols ensuring code-compliant fire resistance ratings while preserving galvanizing's long-term corrosion protection benefits. To view the original AGA knowledge base article on this topic, click this link.
