hot-dip galvanized reinforcing steel a construction
TRANSCRIPT
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Hot-Dip Galvanized Reinforcing Steel
A Construction Perspective
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Purpose of the Seminar
The purpose of this seminar is to educate engineers,
DOT personnel, contractors, and other specifiers about
the benefits of using hot-dip galvanized reinforcing steel
in concrete projects..
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Why would contractors want to use HDG ASTM A767 C1
• Will not be damaged during transportation/installation. NO TOUCH UP
• Shorter lap lengths. LESS STEEL
• Field durability. CAN SIT IN THE SUN
• Meets 75-100 Design-Build Criteria. ACCEPTED BY NYSDOT
• Already in use in NY State. 25+ years of experience with NYSTA bridges.
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Learning Objectives
• Upon seminar completion, you will be able to:
• Discuss the hot-dip galvanizing process
• Identify reasons specifiers choose hot-dipped galvanized (HDG) reinforcing steel (corrosion protection, durability, longevity, availability, sustainability, and cost)
• Understand construction and design issues affecting HDG reinforcing steel performance in concrete
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Hot-Dip GalvanizingSurface Preparation, Galvanizing, Inspection
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Why Galvanize Steel for Corrosion
Protection?
•Durable Barrier Coating
•Cathodic Protection
•Long Life
•Maintenance Free
•Economical
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Hot-Dip Galvanizing Process Overview
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Barrier Protection
Barrier protection resists
corrosion by isolating the
steel from electrolytes in
the environment
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Barrier Protection
• Tenacious Bond• Metallurgically
bonded to steel (3600 psi.)
• Abrasion resistance• Zn-Fe Alloy layers
are harder than base steel
• Ductile Coating• Pure Zn layer bends
w/o cracking
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Cathodic ProtectionGalvanic Series of
Metals
ZINC = ANODE
STEEL = CATHODE
This arrangement of metals determines
what metal will be the anode and cathode
when the two are put in a electrolytic cell
(dependent on salt water as electrolyte).
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Zinc Patina
ZincZinc OxideZinc HydroxideZinc CarbonateZinc Carbonate
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Galvanizing Protection
• Barrier Protection from
external electrolytes
• Cathodic Protection from
Zinc protecting steel
• Zinc patina is passive and
impervious making it a
critical part of HDG’s long-
lasting protection
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Why Protect Rebar?
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Corrosion of Black BarFe
FeO
Fe3O4
Fe2O3
Fe(OH)2
Fe(OH)3
Fe(OH)3‧3 H2O
0 1 2 3 4 5 6
Volume (cm3)
Veteran’s Memorial Bridge
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Visible Signs of Rebar Corrosion
Staining
Spalling Compl
ete
Failure
Crack
ing
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STONEHAM ARCH BRIDGE
HGD REBAR IN DECK, ARCHES AND
SUBSTRUCTURE
Stoneham, QC • 2012
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I-85/I-385 GATEWAY
PROJECT
Greenville, SC • 2018
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Design & Specification
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Specifying HDG Reinforcing Steel
➢ ASTM A767
➢ 5.9 mils (150 µm) -Class 1
➢ 3.4 mils (86 µm) - Class 2
➢ Fabricated and straight lengths (60’)
➢ ASTM A123➢ 3.9 mils (100 µm)
➢ Connected to steel fabrications
➢ ASTM A1094 (CGR)➢ not batch HDG
➢ 2.0 mils (50 µm)
➢ Straight lengths (60’)
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Bending Before Galvanizing
• Large as possible bend radii
• Avoid embrittlement of steel by cold working
• Smooth bends and hooks
• Use table in ASTM A767
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Bending After Galvanizing
• ASTM A767 Class I
• No bend radius limit
• Bend slowly to minimize flaking or cracking
• Not cause for rejection
• Coating can be repaired according to ASTM A780
• ASTM A767 Class II, A123, A1094
• No bend radius limit
• Little/no coating damage during bending
• Coating can be repaired to ASTM A780 as needed
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Rebar Durability & Corrosion Resistance
• Abrasion Resistance
• No special handling
• Metallurgical bond (~3,600 psi)
• Uniform Protection, Complete Coverage
• No holidays, no UV degradation
• Sacrificial action – cathodic protection
Coating grows
perpendicular
to the surface
= uniform
coverage
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Development of Zinc Compounds
• Before Concrete Sets
• Calcium Zinc Hydroxyzincate
• Help fill voids at concrete/rebar interface
• Makes area less permeable
• Reduces penetrationn of aggressive chlorides
• After Concrete Sets• Zinc Oxide
• Loose, powdery minerals, less voluminous than rust
• Migrate away from rebar, preventing pressure buildup and eventual spalling
• Add to rebar’s bond strength in concrete
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JESUP BRIDGE
Jesup, IA • 2013
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Zinc’s Initial Reaction in Fresh Concrete
➢Reaction begins immediately upon contact with wet cement solution (highly alkaline)
▪ Hydrogen Evolution (H2)
▪ Calcium Hydroxyzincate (CHZ)
➢Ceases once concrete hardens
➢Surface film stabilizes the zinc
▪ Second level of barrier protection
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Reducing Hydrogen Evolution
➢Hydrogen from zinc/concrete reaction stops once concrete sets
➢ASTM A767 and A1094 require HDG chromate passivating of rebar after galvanizing
➢ASTM A123 does not require chromate passivation
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THE TIOGA BRIDGE
• PA Rte15 over Mill
Creek, NB • 1974
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Economic Advantages
• Initial Cost Benefits
• Overall material and application costs
• Time savings in construction
• Life-Cycle Cost Savings
• Total cost throughout project life
• Includes maintenance costs and time value of money (interest/inflation)
• Often HDG initial cost IS life-cycle cost
• Life-Cycle Costs for Rebar are Complicated
• Core studies (coating on bar, chloride level)
• Deck replacement or concrete failures
• User Costs (future impact to motorists for detours and lane closures) MUST be considered in today's focus on the economy and the environment
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Cost Comparison vs Black Bar
Source: NYSDOT Bridge Manual
In Place Cost Ratio
Ind
ex
to
Bla
ck
Bar
1.10 1.201.00
2.50
0.00
0.50
1.00
1.50
2.00
2.50
3.00
Black Epoxy HDG -1 Stainless
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Effective Service Life
20
50
100
125
0
20
40
60
80
100
120
140
Black Epoxy HDG-1 Stainless
Serv
ice L
ife i
n y
ears
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Effective Cost/Service Life
0.05
0.02
0.01
0.02
0.00
0.01
0.02
0.03
0.04
0.05
0.06
Black Epoxy HDG-1 Stainless
Co
st v
s Lif
e R
ati
o
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BB
Epoxy
HDG-1
SS
0
20
40
60
80
100
120
140
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Serv
ice L
ife in
years
Cost Index to Black Bar
Service Life to Cost
LIFE AND COST DIMINISHING RETURNS
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New York State Thruway Authority
• Authority was concerned about the durability of epoxy in the field during storage and placement
• Conducted an extensive study of epoxy reinforcing steel vs. hot dipped galvanized reinforcing steel.
• Switched to 100% galvanized reinforcing steel for bridges in 1995.
• Still require A767 Class 1 today.
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GOVERNOR MARIO M.
CUOMO BRIDGE
Tarrytown,
NY• 2017
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Installing Galvanized Rebar
• Same as black bar
• Overlap lengths
• Handling procedures
• Field bending possible
• Touch-up field cut ends
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Pennsylvania Route 66 Bridge
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In the FieldWelding• Use a slower welding rate
• Maintain proper ventilation
Overlap Lengths
• Identical to uncoated steel rebar
Storage, Handling, and Installation
• Stored outside with no coating degradation
• No special handling or installation required due to coating durability
Handling guide on galvanizedrebar.com
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Summary
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Assessment Question
•What is the specification for Hot Dipped Galvanized Rebar
1.ASTM A767
2.ASTM A123
3.ASTM A1094
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Assessment Question
•What are requirements for handling HDG rebar?
1.Same as Black
2.Same as Epoxy
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Assessment Question
• What are two products formed when highly alkaline porewater reacts with Zinc?
1.Calcium Hydrozincate
2.Hydrogen• Minimize by
• Chromate
• Limit Retarder
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Assessment Question
What is the required coating thickness of ASTM A767 Class 1 #4 bar?
1.50 microns
2.86 microns
3.129 microns
4.150 microns
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Assessment Question
• What is the Expected Functional Service Life of ASTM A767 Class1 rebar?
1.30 years
2.50 years
3.85 years
4.100 years