Upload
others
View
7
Download
0
Embed Size (px)
Citation preview
Hot-Dip Galvanized Reinforcing Steel
A Construction Perspective
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..
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.
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
Hot-Dip GalvanizingSurface Preparation, Galvanizing, Inspection
Why Galvanize Steel for Corrosion
Protection?
•Durable Barrier Coating
•Cathodic Protection
•Long Life
•Maintenance Free
•Economical
Hot-Dip Galvanizing Process Overview
Barrier Protection
Barrier protection resists
corrosion by isolating the
steel from electrolytes in
the environment
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
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).
Zinc Patina
ZincZinc OxideZinc HydroxideZinc CarbonateZinc Carbonate
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
Why Protect Rebar?
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
Visible Signs of Rebar Corrosion
Staining
Spalling Compl
ete
Failure
Crack
ing
STONEHAM ARCH BRIDGE
HGD REBAR IN DECK, ARCHES AND
SUBSTRUCTURE
Stoneham, QC • 2012
I-85/I-385 GATEWAY
PROJECT
Greenville, SC • 2018
Design & Specification
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’)
Bending Before Galvanizing
• Large as possible bend radii
• Avoid embrittlement of steel by cold working
• Smooth bends and hooks
• Use table in ASTM A767
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
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
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
JESUP BRIDGE
Jesup, IA • 2013
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
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
THE TIOGA BRIDGE
• PA Rte15 over Mill
Creek, NB • 1974
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
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
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
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
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
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.
GOVERNOR MARIO M.
CUOMO BRIDGE
Tarrytown,
NY• 2017
Installing Galvanized Rebar
• Same as black bar
• Overlap lengths
• Handling procedures
• Field bending possible
• Touch-up field cut ends
Pennsylvania Route 66 Bridge
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
Summary
Assessment Question
•What is the specification for Hot Dipped Galvanized Rebar
1.ASTM A767
2.ASTM A123
3.ASTM A1094
Assessment Question
•What are requirements for handling HDG rebar?
1.Same as Black
2.Same as Epoxy
Assessment Question
• What are two products formed when highly alkaline porewater reacts with Zinc?
1.Calcium Hydrozincate
2.Hydrogen• Minimize by
• Chromate
• Limit Retarder
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
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