performance of protective coatings on small bridges
TRANSCRIPT
Performance of Protective
Coatings on Small Bridges
Subject to BushfiresWilliam McLean, Peter Golding, Ann Sheehan,
Galvanizers Association of
Australia
Established in 1963, the GAA represents the hot dip
galvanizing industry throughout Australia
We conduct research and compile case studies in various
areas dealing with corrosion protection and hot dip
galvanizing
We provide technical and design advice on hot dip
galvanizing and corrosion protection to industry
We offer free CPD presentations in offices throughout
Australia
Outline
Increasing number of enquiries asking about condition of
protective coatings after building fires and bushfires
Common coating options for small bridges
Durability to corrosion
Performance in bushfires
Properties of bushfires and the effect they have on steel
Recommended Inspection
Repair Options
Examples
Protective Coatings
Batch hot dip galvanizing to
AS/NZS 4680
Metallurgical bond, Batch HDG
Hardness
Corrosivity in Australia described in AS 4312
Category Generic examples Specific examples
CXShore-line
severe surfSurf beach shoreline regions with very high salt deposition Some Newcastle beaches
C5Sea-shore
surf
Within 200 m of rough seas & surf beaches.
May be extended inland by prevailing winds & local conditions
More than 500 m from the coast in some areas
of Newcastle
C4Sea-shore
calm
From 200 m to 1 km inland in areas with rough seas & surf.
May be extended inland by prevailing winds & local conditionsAll coasts
From the shoreline to 50 m inland around sheltered bays
In the immediate vicinity of calm salt water such as harbour foreshores
C3 Coastal
From 1 km to 10 km inland along ocean front areas with breaking surf &
significant salt spray
May be extended inland to 50 km by prevailing winds & local conditions
Metropolitan areas of Perth, Wollongong,
Sydney, Brisbane, Newcastle, & the Gold Coast
From 100 m to 3 – 6 km inland for a less sheltered bay or gulf Adelaide & environs
From 50 m to 1 km inland around sheltered baysPort Philip Bay & in urban & industrial areas
with low pollution levels
C2Arid
Urban
Inland
Most areas of Australia at least 50 kilometres from the coastCanberra, Ballarat, Toowoomba and Alice
Springs
Inland 3 – 6 km for a less sheltered bay or gulf Adelaide & environs
Can extend to within 1 km from quiet, sheltered seas Suburbs of Brisbane, Melbourne, Hobart
C1 Dry indoors Inside heated or air conditioned buildings with clean atmospheres Commercial buildings
AS/NZS 2312.2 Table 6.2: Life to first maintenance (part)
System Reference Standard
Minimum thickness
Selected corrosivity category &
Calculated life (minimum-maximum, years)
g/m2 µm C2 C3 C4 C5 CX
Batch
HDGAS/NZS 4680
HDG390 390 55 78->100 26-78 13-26 6-13 2-6
HDG500 500 70>100
33-100 16-33 8-16 2-8
HDG600 600 85 40->100 20-40 10-20 3-10
HDG
sheetAS 1397
Z350 140 20 29->100 10-29 5-10 2-5 1-2
Z450 180 25 36->100 12-36 6-12 3-6 1-3
HDG
tube
AS/NZS 4792ZB140/140 140 20 29->100 10-29 5-10 2-5 1-2
HDG300 300 42 60->100 20-60 10-20 5-10 1-5
AS 4750 ZE50 50 7 10-70 3-10 2-3 1-2 0-1
AS/NZS 2312.1 Table 6.3: Life to first
maintenance (part)
SYSTEM DETAILS Estimated Life to First Maintenance
System
Des.
Surface
Prep.1st Coat 2nd Coat 3rd Coat
Total
DFTC2 C3 C4 C5-M T
ACC5 Sa 2½75 µm zinc
rich primer
125 µm
high build
epoxy
50 µm acrylic
2-pack250 µm 25+ 15-25 10-15 5-10 15-25
IZS1 Sa 2½
75 µm
inorganic
zinc silicate
- - 75 µm 25+ 15-25 10-15 5-10 15-25
PUR4 Sa 2½75 µm zinc
rich primer
125 µm
high build
epoxy
50 µm poly-
urethane gloss250 µm 25+ 15-25 10-15 5-10 15-25
Bushfires
Bushfires
Quite different temperature profile to building fires
Typically an intense heat that moves through quickly
Assumed flame temperature of 1090K (820ºC) by AS 3959
Temperature curves in AS/NZS 1530.8.2 for large flame testing
Intensity varies based on
Fuel
Vegetation management
Weather conditions
Slope of ground
11
Steel in Bushfires
Surface Temperature of Steel
Steel surface unlikely to reach same
temperature as the flame
Heat from fire can be reflected,
conducted or radiated, thus
temperature lags behind fire
temperature
Behaviour of steel has been tested
extensively for building fires and
ISO834 fire curve
Graph shows 100mm diameter,
200mm long steel rod in a furnace
Steel Thickness
Galvanized steel tested to building fire curve, similar behaviour to bare steel
10, 20 and 30mm plates
Thicker plates can absorb more heat
Zinc metallurgically bonded to steel and acts as single part, heat can be conducted
Paint coatings may limit this or have different expansion rates to steel on heating
Steel Strength
Graph based on Eurocode 3 for building fires
Yield strength and Young’s Modulus greatly reduced at raised temperatures
Can cause small bridge to deform in fire (example of thin steel later)
Burning material can keep steel at higher temperature for longer (e.g. timber slats)
Oftentimes structural integrity of small bridges maintained during and after a fire
Protective Coatings in Bushfires
Galvanized Coatings in Bushfires
Solid state reaction occurs for some coatings, converting pure zinc to alloy layers
If this doesn’t occur, pure zinc will melt and can drip off
Pure zinc vaporises at 907°C, unlikely to occur during fire
Testing performed on utility poles in past
≈ 420°C
≈ 650°C
≈ 1,500°C
Melting Temperature
Bushfire Testing of Galvanizing
Tested in 2006 by BlueScope in conjunction with
Bushfire CRC
Grid of liquid propane burners
Simulates from radiant heat through to flame
immersion
High heat flux achieved during testing
A maximum air temperature of 675°C was recorded
and the maximum surface temperature measured
was 520°C.
The galvanized coating remained intact even at the
highest temperatures
Appearance After Testing
Surface staining on poles after flame
immersion
Could be soot from liquid propane
burners
More likely burning of protective
sleeves on poles to protect from
aggressive soils
Paint Coatings in Bushfires
Paint manufacturers usually list upper temperature limit on their technical data sheets (TDS)
This is for corrosion protection to continue, aesthetics not needed to be maintained
Rarely list performance in flame exposures, dry heat only
Failure modes include peeling or crazing of the paint, ignition and burning, or melting.
Particular care if paints are combustible, can keep steel at higher temperatures for longer
Paint Type Zinc rich
epoxy
Inorganic Zinc
Silicate
Epoxy Acrylic Polyurethane
Temperature 120°C Up to 400°C 120°C 120°C 120°C
Appearance After Bushfire Testing of
Powder Coating
Recommended Inspection of Small
Bridges After Bushfire
1. Verify Structural Integrity
a) Engineer should perform this
b) Temperature steel reached is important (condition
of coating can give hints)
c) Typically highly annealed steel in Australia, verify no
phase changes
d) Check if distortion occurred when steel had lower
yield strength
2. Clean Steel and Visually Inspect
a) Remove any soot or burnt products
b) Look for bare areas where steel is exposed
c) Check for blistering, peeling, crazing or delamination
3. Measure Thickness of Remaining
Coating
a) Devices such as Elcometer 456 and Positector 6000 use a
magnetic method to determine distance from probe tip
to steel
b) For zinc coatings remaining durability can be calculated
based on AS/NZS 2312.2 corrosion rates
c) For paint coatings thickness typically doesn’t change
over time, if original specification is known damage can
be identified
Repair of Protective Coatings After
Bushfires
Localised Repair (Small Areas)
Taking steel off site unlikely to be economical
Paint repair most frequently used, with paint selected:
Having similar durability to remaining undamaged coating
Suitable for surface preparation possible (e.g. epoxy mastics are usually more surface tolerant and power tool cleaning without blasting may be adequate)
If remaining coating is galvanized with high estimated durability, thermal zinc spray could be considered
Full Repair (Large Areas)
If steel can be taken off site it can be galvanized if the
design is suitable
Blasting and painting is the most common repair option
when steel isn’t removed from site
Paint systems from previous table usually suitable
Achieving required cleanliness over large areas difficult with
power tool cleaning
Thermal zinc spraying another possible repair option
Examples
Images
Images from the Western Australian Department of Parks
and Recreation (DPaW)
Had a lot of infrastructure including small bridges affected
by bushfires
Elcometer 456 used to measure coating thicknesses
Original coating system used can be difficult to determine
after a fire, making keeping accurate records of
specifications important
Comments made based on images (often remote areas),
not by physical inspection on site
Continuously Galvanized
Sections with Paint Repair
Coating Measurements After Fire
Cold Formed
Sections
Continuously Galvanized Section
Hot Dip Galvanized Sections
Thank You
More information:
w: www.gaa.com.au
t: 03 9654 1266