Concrete Repair: Bridges and Tunnels--Epoxies
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Transcript of Concrete Repair: Bridges and Tunnels--Epoxies
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Concrete Repair: Bridges and Tunnels
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Civil uses
Major roles:• Repair and Protection
• Infrastructure Renovation– Bridges & Tunnels– Airports– Rail
• Structural Rehabilitation– Commercial, Industrial & Residential– Public & Private Sectors
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Corrosion = spalling
Iron oxide (corrosion product commonly called rust) occupies about 7 times the volume of a similar mass of steel. The formation of rust generates high internal forces leading to spalling.
Spalling is more common in applications with insufficient cover (depth of concrete over the steel reinforcement).
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Salt: tough on steel and concrete
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Monitor crack movement (strain gauge)
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History of Epoxies
1936 First epoxy patents
1940s-50s Bakelight introduces commercial applications (electrical and jewelry)
1948 First use as construction adhesive (hardened-to-hardened concrete bonding)
1954 Caltrans uses bonding agent for traffic markers
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History of Epoxies
late 1950s Patching, bolt grouting, crack injection and overlays more common.
1959 U.S. Army Corp of Engineers issues specification
1959 PG&E dam successfully repaired using meter/mix pump and structural epoxy from Adhesive Engineering
1978 ASTM* C881 spec. published *American Society of Testing and Materials
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Specification for civil engineering
ASTM C881Standard U.S. specification for epoxy bonding systems for concrete
AASHTO* M-235 used by U.S. transportation design engineers as specification for epoxy bonding
AASHTO M235 = ASTM C881
*American Association of State Highway and Transportation Officials
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Properties: Viscositymeasure of a material’s resistance to flow—common unit is centipoise (cps)
Water = 1 cps
Motor Oil = 100 - 500 cps(similar to some unfilled epoxies)
Peanut Butter = 250,000 cps(similar to some epoxy pastes)
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Properties: Modulus of ElasticityRatio of stress to strain
High modulus rigid or stiff• Structural uses
– High creep resistance
Low modulus flexible• Impact resistant, less brittle
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Properties: Coefficient of Thermal Expansion(Thermal Compatibility {ASTM D648})
Materials change in size at varying rates upon being heated or cooledNeat epoxies expand/contract at a higher rate than steel or concrete
Filled epoxies behave more like concrete or steel
• Aggregate gradation is key criteria for filled epoxies (gap-grading is helpful)
Important factor for thick layers and bulk applications
EFFECT of BOND LINE STRESSES
50
45
40
35
30
25
20
15
10
5
0
CONCRETE 10x6-6
1 2 3 4 5 6 7 8 9
NEAT EPOXY (SAND FREE)
MAXIMUM EPOXY SAND LOAD
AGGREGATE/BINDER RATIO EFFECT ON THE THERMAL COEFFICIENT OF EXPANSION FOR AN EPOXY SYSTEM.
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Properties: Heat deflection temperature (HDT)
Temperature an adhesive polymer deflects under 3 point load
120ºF is minimum value for ASTM C881 load bearing structural use
Cold temperature curing may undermine HDT performance Special adhesives available offering higher HDTs for high
temperature use Creep is potential problem if use temp > HDT
VariableHot Oil
BathEPOXY BEAM 1/2”x1/2”x5”
CONSTANT LOAD 264 psi
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Properties: Reactions are exothermic
Epoxies produce heat during cure Temperature dependent Mass dependent Can undermine product performance in bulk
applications Exotherm may increase apparent shrinkage
upon cure• Thick cross-sections and bulk uses should
consider special low exotherm systems
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Properties: Cure rate vs. temperature
CURE RATE OF EPOXY ADHESIVES
020406080
100120140160
30 40 50 60 70 80 90 100
Cure T empera ture Deg. F
Typical epoxy resinSlowerFaster
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Advantages
Low shrinkage Tolerates moisture after cure Mechanical and chemical bond
• Very high bond strength Cost effective repair vs. complete removal /
replacement Chemically inert Long storage life of A and B components Repair is considered permanent
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Limitations
Thermal incompatibility• Minimal concern with thin glue line
Creep• Minimal concern with thin glue line
Vapor barrier Proper mixing and ratio is critical Short open time for some formulations Clean-up requires solvents Cure and viscosity is temperature sensitive
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Surface Preparation
Clean and free of contaminants (remove loose substrate, laitance, dirt, dust, grease, oils, chemical spills) Metal shot blasting machines, sand blasting, “dust-free” diamond grinding, hydroblast, scarifiers, scabblersDry--no standing water
• Some epoxies are designed for wet or underwater use
Prepare surface profile for good mechanical / chemical bonding
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Surface prep
Shotblasting
Hydroblasting
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ASTM C-881 (or AASHTO M-235)
Crack Repair – pressure injection or gravity feed
(Types I & IV)
Concrete Bonding (Types II & V)
Low mod overlays and spall/joint repair (Type III)
Segmental Bridge Adhesives (Types VI & VII)
ASTM Bond Types: I, II, III, IV,V, VI, VII
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Non-ASTM Specification Uses
Miscellaneous Epoxy Products• Base plate grouts• Mortars for patching• Coatings • Control joints and wire loops
– Airport runway lighting (see right)– Traffic control
• High strength grouts• Polymer concrete• Handicapped access warning strip adhesives
Adhesive Anchoring (ICC-ES/AC308) External Structural Strengthening
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ASTM C-881 used in crack repair
Type I – Non-load bearing, bond hardened concrete to hardened concrete
Type IV – Load bearing, bond hardened concrete to hardened concrete
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Why Repair Cracks?
Extend structural lifespanRepair settling from poor soilsEarthquake damage repair Cracked member unable to transfer design load
Fix spalls due to freeze/thaw cycles, unrepaired cracks, steel corrosionStop leakage in fluid containing structures (see above)Cracks are unsightly in architectural concrete
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Crack Treatment Considerations
Some moving cracks may re-appear after injection
Cracks with hydrostatic pressure (i.e., running water) are frequently epoxy treated, but may also be candidates for urethane grouting if non-structural
Vertical / Overhead applications –Injection technique
Horizontal applications – Gravity-feed or injection
Voids and honeycombed concrete may require special low exotherm epoxy
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Crack Treatment Considerations(for reinforced concrete used in structural applications)
0.100.004Water retaining structures (excluding pressure pipes)
0.150.006Seawater and seawater spray: Wetting and drying
0.180.007De-icing chemicals
0.300.012Humidity, moist air, soil
0.410.016Dry air or protective membrane
Tolerable crack width (mm)Tolerable crack width (in.)Exposure condition
ACI 224 guidelines for crack width
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Restore monolithic strength
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Structural Crack Repairs
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Injection: beams and columns
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Injection--tunnels
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Injection--Rail
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Injection--airports
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Injection--bridges
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Injection: pile caps
Mass pours (large volume single pours) can develop severe cracks associated with high heat buildup during cure
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Bridge--Gravity Feed Crack Repair
For concrete decks with high amounts of small crack networksGravity feedVery low viscosity systems use epoxies and HMWM (methacrylates)Open to traffic in 5 hrsRestore monolithic integrity to deckProtect reinforcing steel from corrosionAlso available in special long potlife version
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Spray Applied – Gravity feed
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QC Check: Taking Cores
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Visual Inspection
Can penetrate cracks assmall as 0.002” (0.05 mm)
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ASTM Standard C-881 (continued)
Type II – Non-load bearing, bond freshly mixed concrete to hardened concrete
Type V – Load bearing, bonding freshly mixed concrete to hardened concrete
Typical Applications:• Introduction of newly poured concrete for
large area repairs.• Primers for overlays (Type III) & patch repairs
with epoxy and cementitious mortars.
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Concrete Bonding
Requirements for “fresh to hardened”• Achieve monolithic structural bond of new
material to old concrete• Epoxy must have proper viscosity to maintain
uniform film thickness of 20 mils• Epoxy must be tacky when new concrete or
new material is placed
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Bonding: Hardened to Hardened
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Trowel Applied
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Set Precast Panel
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Spray Applied Concrete Bonder
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Fresh Concrete Pour (over epoxy)
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Bridge deck overlay delamination repair
Modification of structural crack injection practicesObserved more frequently in climates with high cold-to-hot temp ranges (Canada and parts of China)Fill voids between overlay and structural slabConsiderable savings compared to replacement of deck overlay
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ASTM Standard C881 (cont.)
Type III – Bond skid resistant materials to hardened concrete (low modulus)• Binders in epoxy
mortars used on traffic bearing surfaces (overlays)
• Impact resistant joint nosing repair on bridge decks
• Handicapped access warning strips
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What is an Overlay?• A resurfacing of an existing horizontal surface
(bridge deck or parking structure)
Function of overlay• Protects structure• Provides non-skid surface• Extends life of the structure
Properties• Low modulus.• Medium viscosity.• Excellent adhesive properties.• Long term abrasion resistance.
Polymer Overlays
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Broadcast overlay
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Seed with aggregate to excess
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Completed thin overlay
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ASTM Standard C-881 (continued)Segmental bridges
Type VI – Bonding / sealing segmental precast elements with internal tendons (temporary post tensioning applied)• Normal Set
Type VII – For use as a nonstress carrying sealer for segmental precast elements (temporary post tensioning not applied)• Slow Set
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Segmental Bridges
Technology first developed in France and Switzerland in the 1940’s
Early projects left segment joints dry Epoxies used since early 1970’s for
sealing segments ASBI (American Segmental
Bridge Institute)• Developed a standard epoxy
specification
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Functions of Epoxy Resin (Segmental use)
Lubricant - allows segments to slide easily into place during construction
Distributes compressive/shear stresses across each joint
Provides water-tight seal between segments Prevents loss of cementitious grout at joints when
pumped through cable ducts Specific temperature ranges often used
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Apply by Hand
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Place Segments
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Epoxy Squeeze-Out
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Early applications used steel plate Change in use Used in seismic upgrades Steel and fiberglass forms used on columns
Steel saddles on beams
Epoxy resin and hand-applied composite fiber wrapping technologies are popular today
Fabrics are glass or carbon fiber
External strengthening
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Why add Strengthening?
Insufficient reinforcement Corrosion of reinforcement Change in use Excessive deflections Structural/Fire damage Seismic upgrade
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Failed columns
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Beam damage
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External fixes: Stitching
Mid 1960s drawing from US Army Corp of Engineers
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Plate strengthening used with epoxy injection
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Steel saddle (3-sides) over concrete beamused with epoxy injection
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Column grouting Steel (or rigid fiberglass) jackets
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Composite Systems
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Bridge Column Upgrade
Applying fairing epoxy to corner
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Test substrate adhesion (after surface prep)
ASTM D4541(Elcometer)
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Prime Substrate
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Saturate Fabric – Wet Lay-Up
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Apply Saturated Fabric
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Dry Lay-Up
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Reinforce Tension Zone with CF strips
Fiberglass bars inset in pre-cut channels used in a “stitching”technique are an alternative to strips