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Bonded (Adhesive) Anchors
NCSEA Seminar – January 13, 2015
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• Three part presentation• Part 1 Cast‐in‐place and mechanical expansion anchors
• Part 2 Bonded (adhesive) anchors
• Part 3 Example problem showing application of design provisions
Overall Webinar Outline
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• Briefly review ACI history of anchoring to concrete
• Review failure modes for post‐installed adhesive anchors
• Review basic design models • Review relationship between ACI 318 and anchor qualification requirements
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Learning Objectives
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• Anchor types• Fundamentals of “structural” anchors• Code provisions in ACI 318• Anchor qualification procedures • Questions
Part 2 Outline
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5
Definitions
Embedment
Attachment Anchor
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6
Types of Anchors
Undercut Expansion
Cast ‐ in
Grouted
Post – InstalledMechanical
Post – InstalledBonded
Adhesive Grouted
Not included In ACI 318
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• Chemical bond Resin mixed with
activator• Rods are typically deformed
reinforcing bars or continuously threaded bars Qualification procedure
does not cover development length
Adhesive Anchors
Photograph courtesy of Hilti AG
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Anchor Systems
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BOSTON I‐90 TUNNEL Central Artery/Tunnel (CA/T)July 10, 2006Ceiling Panel Collapse
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Map of Downtown Boston
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Boston “Big Dig” Tunnel
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Failure Sequence
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Boston Big Dig Tunnel
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Boston Big Dig Tunnel
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Photos from NTSB Files
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16
FHWA Laboratory Testing
Fast Set
Standard Set
Fast set
Regular set
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• NTSB –Cause: poor creep resistance of fast‐set adhesive anchors subjected to sustained tension load
• NTSB –Conclusion: insufficient understanding by designers and lack of standards on adhesive anchors in sustained tension
• NTSB – Recommendation: prohibit use of adhesive anchors in sustained loading
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NTSB Report
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• FHWA – prohibit use• AASHTO – develop
standards for testing• DOTs – prohibit use• ICC – revise building
codes • Powers – revise
packaging to state for short‐term loading
• Sika – revise product literature
• ACI – use codes, forums, educational materials to inform
• ASCE – inform members of creep characteristics
• AGC – inform members of creep characteristics
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NTSB Comments to Organizations
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Cause – Poor Installation in combination with other factors
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Loadings and behavior models
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Photograph courtesy of Hilti AG
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• Tension load Sustained load – tension (creep)
• Compression load• Shear load
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General Loads on Adhesive Anchors
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Adhesive Anchors – Failure Modes in Tension
Concrete cone
failureBond failures
Adhesive/substrate Adhesive/threaded Mixed Anchor steel
failure
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Bond Models
hef
NN
Threaded Rod Mortar Concrete
hef
NN
Threaded Rod Mortar Concrete
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24
Uniform Bond Failure Model
Nbond
hef
da
Nbond = da hefda = diameter of anchor rodhef = embedment depth = average bond stress
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25
Uniform Bond Model and Tests
0
100
200
300
400
500
600
0 10000 20000 30000 40000 50000 60000
Load
(KN
)
measured loadsmean - uniform bond model5% fractile - uniform bond model
Bond Area (A b ) (mm2)
Only 17 of 891 data points below 5% fractile (1.9%)
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Uncracked Bond Stress at Ultimate –Adhesive Anchors
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• Hole cleanliness• Temperature• Sustained load• Moisture in hole• Cracking
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Key Variables Affecting Tension Strength
Adhesive anchors are affected by many factors, these are a few key factors and are product dependent – Prequalification of anchor necessary
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ACI 318 Code
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• Adhesive anchors must meet the assessment criteria in ACI 355.4 This is an ACI standard Similar to ACI 355.2 (qualification standard for mechanical [expansion] anchors)
• Can be used in seismic applications• Can be installed horizontally or upwardly inclined Installer certification required for these positions
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Adhesive Anchor Scope
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Anchor QualificationACI 355.4
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• Variable load‐displacement behavior, particularly in cracked concrete
• Behavior sensitive to installation• Must verify basic behavior and reliability using
standard tests • Based on reliability, assign anchor performance
category and corresponding strength reduction factor phi ()
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What is ACI 355.4 for Post‐installed Adhesive Anchors
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• Check conformance to product description Generic or trade name Anchor element Adhesive components Quality‐control requirements
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ACI 355.4 Identification Tests
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• Establish baseline performance of anchors for comparison with Reliability and Service‐Condition tests
• Test in Uncracked Concrete Tension tests in low‐strength and high‐strength concrete
• Test in Cracked Concrete: Tension tests in low‐strength concrete and high‐strength concrete with crack width 0.012 in. (0.3 mm)
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ACI 355.4 Reference Tests
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• Sensitivity to hole cleaning Dry installation Water‐saturated installation Water‐filled installation Submerged installation
• Sensitivity to mixing effort
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ACI 355.4 Reliability Tests
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• Sensitivity to cracking Installation in cracked concrete Crack width cycling with sustained load
• Sensitivity to freeze and thaw cycling• Sensitivity to sustained load• Sensitivity to installation direction
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ACI 355.4 Reliability Tests (continued)
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• Classify anchor based on ratio between performance in reliability and reference tests
% of Reference Capacity
• Category 1 80 or above • Category 2 70 ‐ 79• Category 3 60 ‐ 69
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ACI 355.4 Classification
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• Verify anchor behavior under Elevated temperature installation Curing time at low temperatures Resistance to alkalinity Resistance to sulfur Seismic tension Establish minimum spacing and edge distances to preclude splitting during installation (torqueing) and tension loading Establish shear capacity of anchor steel (may be calculated)
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ACI 355.4 Service‐Condition Tests
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Qualification Standard Report
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Scope of ACI 318 –Adhesive Anchor Provisions
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• Post‐installed adhesive anchors do not have a generically predictable pullout capacities
• Post‐installed adhesive anchors must be qualified by testing according to ACI 355.4
• High‐cycle fatigue and impact (blast) are excluded
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Scope
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• Seismic load effects covered Applicable to Seismic Design Categories (SDC) C, D, E, and F
• Anchors in plastic hinge zones excluded Post‐Installed Anchors shall be qualified for earthquake loading per ACI 355.2 or ACI 355.4 Simulated Seismic Tests
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Seismic Design Requirements
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• Adhesive anchors installed horizontally or upwardly inclined Must be qualified by ACI 355.4 Must be installed by certified installer when subjected to sustained load
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General Requirements
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• Lightweight concrete modification factor, a• Modification factor Cast‐in and undercut concrete failure: a = 1.0 Expansion and adhesive anchors concrete breakout
failure: a = 0.8 Adhesive anchor bond failure: a = 0.6
o 1.0 excluded• determined by Section 8.6.1
o = 1.0 normal weighto = 0.85 sand‐lightweighto = 0.75 all‐lightweight
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Lightweight Concrete
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• Code equations are valid for : fc 10,000 psi for cast‐in anchors fc 8,000 psi for post‐installed anchors
• Post‐installed anchors in concrete with fc > 8,000 psi must be tested according to ACI 355.4
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Concrete Strength
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• Limits of embedment depth for adhesive anchors4 da ≤ hef ≤ 20 da Design using bond model is satisfactory
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Embedment Depth Limitationsfor Adhesive Anchors
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ACI 318 Code Design
Design for Tension, Shear, or Combinations of Tension and Shear
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Photograph courtesy of Ambex
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• Designer must consider the following tension failure modes for adhesive anchors: Steel failure Concrete breakout failure Bond pullout failure
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Tension Design
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• Steel rupture
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Steel Failure Mode ‐Tension
Nsa = Ase futa (D – 2)futa < 1.9 fyfuta < 125,000 psi
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Concrete Failure Mode –Tension Cone Breakout
Courtesy of University of Stuttgart
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• Single anchor in tension in cracked concrete
Nb = kc a fc’ (hef )3/2 (D – 6) kc = 24 for cast ‐ in anchorskc = 17 for post ‐ installed anchorsa = Lightweight concrete modification factor
Note: an adhesive anchor should be considered like a post‐installed anchor even though there are no wedging forces developed at embedded end for concrete breakout
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Single Anchor Concrete Breakout
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User Friendly CCD Design Model for Concrete Breakout – Projected Area ANco
hef
1.5hef
1.5hef
1.5hef
Plan ViewElevation
ANco = 9hef
1.5hef
Nn1.5hef 1.5hef
2
350
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Concrete Breakout (Tension)
Nn Nn
Single anchor not near an edge
Single anchor Near an edge
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Concrete Breakout (Tension)
Anchor group with over lapping breakout cones
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Concrete Breakout with Groups and Edges ‐ Projected Area ANc
ca1 s1 1.5hef
1.5hef
Ca2
s2
ef1 h0.3s
efa2 h5.1c ef2 h0.3s
efa1 h5.1c
Limit
Corner
ANc ≤ nANco
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Concrete Breakout Strength of Anchor Group (Tension)
Accounts for post - installed anchor (splitting)
Accounts for cracking
Accounts for edge effectsAccounts for eccentricity
Accounts for projected area of failure surface Basic single anchor strength
Applies to only anchors in tension
Ncbg = (ANc /ANco ) ec,N ed,N c,N cp,N Nb (D-4)
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• Tension design Sustained loading
►0.55 Nba > Nua,sustained
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Bond ‐General Requirements
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• Single anchor in cracked concrete
Nba = a cr da hefcr = 5% fractile result in cracked concrete from
ACI 355.4a = Lightweight concrete modification factor for
adhesive anchors a,lightweight = 0.6 normal
[0.6 factor not applicable for normal‐weight concrete]
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Bond Pullout Strength
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User Friendly CCD Adhesive Anchor Design Model
hef
cNa
cNa
cNa
Plan ViewElevation
ANao = (2cNa)2
cNa
NncNa cNa
Bond failure/concrete breakout transition CCD model for edges and spacing
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• Cast‐in‐place and post‐installed mechanical anchors ACI 318
scritical = 2ccritical = 3.0 hef• Bonded anchors ACI 318
scritical = 2ccrticalwhere: ccritical = cNa = 10 da (uncr / 1100)1/2
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Critical Spacing and Edge Distance for Bond Failure
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Bond Failure of Adhesive Anchor GroupsProjected Area ANa
ca1 s1 cNa
cNa
Ca2
s2
1 2 s
a2 cNac 2 2 cNas
a1 cNac
Limit
cNa
ANa < n ANao
Corner
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Nag = (ANa /ANao) ec,Naed,Nacp,Na Nba
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• Steel failures Ductile steel = 0.75 Brittle steel = 0.65
• Concrete breakout and adhesive bond failures
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Phi Factors ‐Tension
Condition A Condition BCategory 1 0.75 0.65Category 2 0.65 0.55Category 3 0.55 0.45
Condition A – Supplemental steel Condition B – Plain concrete
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Environment Concrete moisture
Peak service temperature
cr uncr
Outdoor Dry to fully saturated
175o F 200 psi 650 psi
Indoor Dry 110o F 300 psi 1000 psi
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Code Guidance for Bond Stress Design
For sustained tension load, multiply table cr and uncr values by 0.4
For seismic design in SDC C, D, E, and F, multiply table cr value by 0.8 and uncr value by 0.4
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• No changes because anchor is adhesively bonded
• Choice of two equations; use smaller value for design
Vb = 7 a (e /da )0.2√da√fc’ (ca1 )1.5or
Vb = 9 fc’ (ca1)1.5
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Shear Design
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Tension ‐ Shear Interaction
Nu Nn
0.2Nn
0.2Vn VnVu
VV 3 0
Nu 3 1N
5
n
u
5
n
.
2.1VV
NN
n
u
n
u
Simplification
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Installer Certification
66Courtesy of Hilti AG
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• Anchors to be installed by qualified personnel• Installation in accordance with Manufacturer’s Printed Installation Instructions (MPII)
• Extensive installation, inspection, and proof load requirements
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Anchor Installation and Inspection
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• Training for installers Provided by manufacturers
• Written test – test baseline knowledge Environmental conditions Equipment / materials OSHA issues
• Performance test Downhand vertical into concrete Vertical overhead into a tube or pipe
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AAI Certification
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• Hole depth• Hole cleaning technique• Adhesive dispensing• Rod / bar installation
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Downhand Vertical
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Hole Drilling
• Hole normal to surface
• Correct depth• Removal of concrete dust spoil
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Express Initial Material
• Initial material is typically not well mixed
• Need to discard• Did the installer do this?
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Installation –All Thread
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Blind Overhead Vertical
• Install adhesive in a upside‐down hole
• Use insertion tube to feel depth of hole
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Overhead Vertical
Poor filling Close up
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• Written test• Performance exam• Program is live ! Administered by ACI
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Adhesive Anchor Installer Certification (AAI)
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• Three part presentation• Part 1 Cast‐in‐place and mechanical expansion anchors
• Part 2 Bonded (adhesive) anchors
• Part 3 Example problem showing application of design provisions
Overall Webinar Outline
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Questions and Answers
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