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The Assessment and Design of Adhesive Anchors in Concrete for
Sustained Loading R. Eligehausen1, J. Silva2 7-Jan-08
1.0 Foreword On July 10, 2007 the National Transportation Board
(NTSB) issued its final report on the partial collapse of the
ceiling system in the I-90 Seaport Portal Tunnel on July 10, 2006.
The collapse of the concrete ceiling panels resulted in one
fatality and significant traffic disruption in the Boston Central
Artery/Ted Williams Tunnel system over an extended period. The
cause of the collapse was identified as creep failure of adhesive
anchors installed overhead and subjected to sustained tension
loading. The NTSB report3 specified the following safety issues
relative to the ongoing use of adhesive anchors in
construction:
Insufficient understanding on the part of designers and builders
regarding the nature of adhesive anchoring systems; and
Lack of standards for the testing of adhesive anchors in
sustained tensile load applications.
The veracity of these statements notwithstanding (standards for
the creep testing of adhesive anchors have existed since 1993), the
NTSB report has generated legitimate concerns in the design and
constructions communities regarding the qualification, design and
use of adhesive anchors for safety-related applications in
construction. The report also makes the following recommendation to
the Federal Highway Administration:
Prohibit the use of adhesive anchors in sustained tensile-load
overhead highway applications where failure of the adhesive would
result in a risk to the public until testing standards and
protocols have been developed and implemented that ensure the
safety of these applications.4,5
What follows is an overview of the assessment and design of
adhesive anchors in the U.S. The following points are
emphasized:
a. Testing of adhesive anchors under sustained loading
conditions has been ongoing for over a quarter of a century.
1 Prof. Dr.-Ing. Rolf Eligehausen, Institut fr Werkstoffe im
Bauwesen, University of Stuttgart. 2 John Silva, S.E., Director of
Codes & Standards, Hilti North America. 3 National
Transportation Safety Board, Accident Report No. NTSB/HAR_07/02
Ceiling Collapse in the Interstate 90 Connector Tunnel, Boston,
Massachusetts, July 10, 2006, Executive Summary July 10, 2007, p.
ix. 4 Ibid., p. 109. 5 This recommendation has as of this writing
been adopted by the FHWA and by at least one state highway agency.
The use of adhesive anchors for sustained tensile-load
overheadapplications in highway construction is likely limited in
any case. Historically, typical applications include the anchorage
of guardrails, lighting standards and post-installed reinforcing
bars.
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b. Standards for the assessment of adhesive anchors to address
this condition have been in place for over a decade.
c. The use of adhesive anchors overhead to resist tension loads
in safety-related applications is admissible provided that the
system has been properly qualified, designed and installed.
The findings of the aforementioned NTSB report with regard to
cause of failure or responsibility are not discussed, and the
information presented herein should not be construed as an opinion
on the part of the authors or Hilti with respect to the NTSB
investigation and subsequent proceedings.
2.0 Background Adhesive anchors are widely used around the world
to address a variety of structural and non-structural fastening
problems in both new construction and structural renovation of
concrete structures. [As used here, the term adhesive anchor refers
to anchorages comprised of a steel anchor element, usually threaded
rod or reinforcing bar, installed in a drilled hole and bonded to
the surrounding concrete with a polymer-based adhesive filling an
annular gap of no more than 1-1/2 times the anchor element
diameter.6 Anchorages based on larger annular gaps are typically
referred to as grouted anchors and are generally executed with
cementitious grouts. These are not addressed further here.] The
widespread use of adhesive anchors can be attributed to several
factors:
Thixotropic adhesives (gels) are generally suitable for all
orientations of installation, provided that issues of void-free
installation and creep under sustained load have been adequately
addressed. Cementitious-based grouts are usually suitable for
down-hole applications only.
Adhesive anchors provide the designer with a wide range of
possible embedment depths to accommodate the specific geometry and
material parameters of the anchorage. Most mechanical anchor
systems provide only limited options for varying the embedment
depth.
Adhesive anchors accommodate a wide variety of anchor element
types (threaded rod of any grade, reinforcing bar, internally
threaded inserts)
Adhesive anchors do not generate the large expansion forces upon
installation that are associated with most mechanical anchor
systems. This makes them more suitable for near-edge applications
where splitting of the concrete is a concern.
Similarly, the use of adhesive anchor systems for the
installation of reinforcing in hardened concrete, usually for the
purpose of shear transfer between new and existing concrete
elements, but sometimes also for flexural and direct tension
applications, is common practice. Again, the utility, flexibility
and reliability of injectable adhesive anchoring systems makes them
preferable to other solutions (drypack, poured grout) for these
applications.
6 The limit of 1-1/2 times the anchor element diameter is based
on current practice. Many adhesive anchor systems specify thin bond
lines (on the order of 1/16-inch or 1.5 mm) in order to limit
shrinkage and maximize the value proposition for the system.
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The use of adhesive anchor systems in construction is predicated
on several points:
The availability of systems that provide for consistent mixing
of the adhesive components in the correct proportion and the
efficient delivery of the mixed adhesive into the drilled hole.
Relative insensitivity of the adhesive anchor system to minor
variations in the installation procedure.
Predictable response of the installed adhesive anchor to loading
at service and ultimate load levels.
Stable behavior of the cured adhesive over time frames
consistent with the lifespan of the built environment and under
conditions as might be anticipated to occur over the life of the
anchorage (temperature variations, etc.)
The verification of these critical characteristics is the domain
of the assessment procedure used to provide the building official
or other authority having jurisdiction with the requisite assurance
of code compliance as well as the information required for design.
Prior to the mid-1990s, assessment of adhesive anchor systems was
performed on an ad hoc basis by the International Conference of
Building Officials Evaluation Service (ICBO-ES) and other
evaluation agencies in the U.S. and Canada. The development of
standardized test procedures specifically for adhesive anchor
systems was first incorporated in ASTM 1512-93, and a complete set
of testing requirements and assessment criteria on the basis of
that standard was issued by ICBO-ES as AC58, Acceptance Criteria
for Adhesive Anchors in Concrete and Masonry Elements in 1995. This
document has since been revised to address adhesive anchors in
masonry only, and as of January 1, 2007 the assessment of adhesive
anchors in concrete is covered exclusively (in the context of
IBC/IRC jurisdictions) by AC308, Acceptance Criteria for
Post-Installed Adhesive Anchors in Concrete Elements.7 These two
criteria, AC58 and AC308, are briefly compared and contrasted with
respect to their treatment of both service load and suitability
assessment procedures in the following.
7 AC308 is based substantially on Part 5 Bonded Anchors, of the
European Technical Approval Guideline (ETAG) 001. As of October of
2007, 73 adhesive anchor systems had been assessed and 140 European
Technical Approvals issued under this guideline.
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3.0 Assessment of adhesive anchors for service conditions
3.1 Service condition testing under AC58
Under AC58, assessment for service loads was conducted using an
allowable stress design (ASD) format, whereby a global safety
factor (see Fig. 1) was applied to the mean of five replicates to
develop allowable loads for comparison with unfactored load
combinations. Group and near-edge effects were assessed on the
basis of replicate tests with groups and near-edge anchors at
specific anchor spacings and edge distances, and these results were
then extended to other cases, usually by linear interpolation.
Typically, testing of all diameters was required to establish
allowable loads for single anchors whereas values for edge
distances and spacings less than the value required for full
capacity were based on testing of small, intermediate and large
diameters. Where
multiple embedment depths were associated with a single
diameter, tests were required at each embedment depth for which
recognition was desired. Testing was typically conducted in three
concrete strengths.
Fig. 1 Global factors of safety for various codes and test
conditions as reproduced from AC588
3.2 Service condition testing under AC308 AC308 was developed
for use in conjunction with the limit state (LRFD) design format
established in ACI 318 Appendix D. It is based largely on
procedures developed by the European Organization for Technical
Approvals for the assessment of adhesive anchors.9 Assessment for
service conditions in AC308 primarily consists of testing to
establish the characteristic bond strength associated with the
adhesive anchor system.10 In this context,
8 ICC-ES, AC58 Acceptance Criteria for Adhesive Anchors in
Concrete and Masonry Elements, as approved June 2005, p. 11. 9 See
European Organization for Technical Approvals (EOTA), European
Technical Approval Guideline 001, Part 5, Bonded Anchors, Brussels,
March 2002. 10 AC308, in accordance with ACI 318 Appendix D, makes
a general distinction between anchors to be used in concrete that
may develop cracks in the anchor vicinity over the service life of
the anchor (cracked
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the term adhesive anchor system is understood to comprise the
adhesive, injection system, installation procedures and anchor
element. The bond strength k is suitable for use in a uniform bond
stress equation (see Fig. 2) to predict the anchor resistance as
governed by bond failure11 for comparison with the calculated
strength associated with the other applicable failure modes. The
assessment is potentially valid for embedment depths ranging from 4
to 20 anchor diameters. This represents a significant departure
from past anchor testing criteria.
4.0 Assessment of adhesive anchors for suitability
4.1 Suitability testing under AC58 In addition to service load
testing, AC58 contained suitability tests for in-service
temperature (required), response to sustained tension loading
(optional), dampness (optional), freezing and thawing (optional)
and seismic (optional) with a table linking the successful
performance of the creep and seismic tests to the global safety
factor (see Fig. 1) and permissible load cases (seismic, sustained
loads).13 For example, a product that either was not tested for
creep or did not satisfy the acceptance criteria for creep testing
in AC58 would be limited to applications involving wind or
earthquake loading only (no dead or live loads), and would carry a
safety factor of 5.33 instead of 4. In particular, the test
procedures for response to sustained tension loads (creep testing)
have come under close scrutiny recently, and this is discussed in
further detail below.
hD.5.3.9 The basic strength of a single adhesive anchor in
tension in cracked concrete shall not exceed ,a0 k cr efN d =
where
,k cr = characteristic bond stress in cracked concrete; d =
anchor diameter
efh = anchor embedment
Fig. 2 Bond stress equation for cracked concrete as expressed in
AC30812
concrete applications) and those that will not (uncracked
concrete applications) and contains test procedures for both cases.
AC58 was originally formulated for testing in uncracked concrete
only, and as such was only suitable to qualify anchors for the
uncracked concrete applications. This was clearly stated in
evaluation reports issued by ICC-ES on the basis of AC58 testing
for use with IBC/IRC codes. 11 The value of Na0 corresponds to the
resistance of a single anchor far from edges or adjacent loaded
anchors. See Eligehausen, R., Cook, R., and Appl, J., Behavior and
Design of Adhesive Bonded Anchors, ACI Structural Journal Vol. 103,
No. 6, December 2006, pp. 822-831 for additional information. 12
ICC-ES, AC308 Acceptance Criteria for Post-installed Adhesive
Anchors in Concrete Elements, as approved February 2007, p. 19. A
similar expression addresses uncracked concrete applications. 13
Fire tests were permitted as an option in AC58; however, guidance
was lacking with respect to how the resulting design values should
be applied in an ASD design environment.
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4.2 Suitability testing under AC308 AC308 likewise requires
suitability testing. In addition to tests for the conditions
covered in AC58, AC308 specifies tests for sensitivity to reduced
cleaning effort in dry, wet and underwater conditions, sensitivity
to installation direction and sensitivity to mixing effort. In
contrast to the requirements of AC58, tests for sensitivity to
sustained tension load are not optional in AC308.
5.0 Assessment for response to sustained tension loading
5.1 Assessment for sustained loading under AC58 Testing for
response to sustained tension loads in AC58 (designated as creep
testing) consists of subjecting 1/2-inch diameter anchors installed
to an embedment of 4-1/2 inches in concrete blocks at a constant
elevated temperature of 110 F (43.3 C) to 40% of the mean ultimate
tension strength in tension as measured in tests at room
temperature (see Fig. 3).14 The load is maintained over a period of
at least 42 days (1,008 hours) with the displacement measured at
roughly 24-hour intervals. The resulting displacement measurements
are then extrapolated to 600 days using a logarithmic function of
the form y = c ln(x) + b and the mean displacement at 600 days is
compared with the mean displacement at peak load as measured on
anchors tested in tension to failure at 110 F (43.3 C) . Criterion
for acceptance (passing) is that the mean extrapolated displacement
should not exceed the mean displacement corresponding to peak load
in short-term tests at elevated temperature or 0.12 inches (3 mm),
whichever is less. No requirements are set on the residual strength
of the anchor on the assumption that any anchor fulfilling the
displacement criteria will not exhibit appreciable strength
loss.
14 AC58 permits the use of either unrestrained tests (in
accordance with E 488) or restrained tests (in accordance with E
1512 Section 7.1.2) for the establishment of the mean ultimate
tension strength; however, where unrestrained tests are used to
establish the sustained load (40% of the mean ultimate strength),
the sustained load test is to be performed with wide support
spacing as well. The following observations are relevant: 1) Since
the ultimate strength associated with a restrained test, wherein
the support spacing is purposely restricted in order to preclude
concrete breakout, is generally elevated (from 10-35% in uncracked
concrete, depending on the bond characteristics of the adhesive)
over that associated with unrestrained testing, the restrained
testing option likely represents a more severe standard. It is not
known how many product assessments under AC58 are based on
sustained loads determined from restrained vs. unrestrained
testing, however, in the authors experience many creep tests were
performed using a semi-restrained test setup (see Fig. 3a) 2) There
is a question regarding the impact of close support spacing (see
Fig. 3b) on the displacements recorded in a sustained test; it is
not known to what degree the reduction in displacements associated
with restrained creep testing offset the increased sustained load
associated with restrained reference tests.
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spring dashpot
LVDT ea. side
temperature-controlled chamber
1.5 do
do
spring dashpot
LVDT ea. side
temperature-controlled chamber
1.5 do
do
spring dashpot
LVDT ea. side
temperature-controlled chamber
spring dashpot
LVDT ea. side
temperature-controlled chamber
a) Semi-restrained test configuration b) Restrained test
configuration
Fig. 3 Typical test setups for sustained loading
Duration of load t [hours]
Dis
plac
emen
t [m
m]
2000
600 days
1000 4000
1000 Duration of load t [hours]
2
1
2
Dis
plac
emen
t [m
m]
Detail A
Detail A
data points used for extrapolation
log function extrapolation
600
Duration of load t [hours]
Dis
plac
emen
t [m
m]
2000
600 days
1000 4000
1000 Duration of load t [hours]
2
1
2
Dis
plac
emen
t [m
m]
Detail A
Detail A
data points used for extrapolation
log function extrapolation
600
Fig. 4 Extrapolation of sustained load displacements per
AC58
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5.2 Assessment for sustained loading under AC308 The fundamental
premise used for evaluating the long-term performance of adhesive
anchors under AC308 is the same as that used in AC58; namely, that
relatively short-term test results may be extrapolated to predict
long-term behavior. However, the parameters specified in AC308 for
the load level, the extrapolation range, displacement limits, and
requirements on residual capacity vary from AC58. While the
requirements on the minimum duration of the test (42 days) and on
the number of data points used for the extrapolation (minimum last
twenty) are the same in both criteria, AC308 uses an extrapolation
equation commonly referred to variously as the Findley Power Law or
Findley Creep Law. Originally developed for plastics, it has the
general form
where 0 nt += + 0 , + , and are functions of material (plastic).
This equation has been found to provide very satisfactory
predictions for plastic laminates, polyethylene and
polyvinylchloride over long time (10+ years) spans.
n
15 It yields superior predictions to those provided by linear
viscoelastic models involving combinations of dashpots and springs.
According to Findley, et al.
This is due in part to the fact that creep of plastics, concrete
and some metals under moderate stresses starts out at a very rapid
rate immediately after loading and progresses at a continuously
decreasing rate.
Furthermore, AC308 requires that sustained load tests be
conducted at two concrete temperatures: standard temperature
(essentially room temperature) and the maximum long-term elevated
concrete temperature16 established for the adhesive anchor system.
The displacements obtained by extrapolating the data from these two
sets of tests to 50 years (standard temperature) and 10 years
(maximum long-term elevated temperature) are not compared with
values corresponding to peak load in static short-term tests as in
AC58, but rather with the mean displacement associated with loss of
adhesion in tension tests conducted at the respective temperatures
(see Fig. 5).
Displacement
Tension load N
Load at loss of adhesion Nadh
N
Peak load Nult
adh ult Displacement
Tension load N
Load at loss of adhesion Nadh
NN
Peak load Nult
adh ult
Fig. 5 Establishment of displacement corresponding to loss of
adhesion The use of the displacement at
loss of adhesion as a marker
15 Findley, W., Lai, J., and Onaran, K., Creep and Relaxation of
Nonlinear Viscoelastic Materials, Dover, N.Y., N.Y., 1976, p. 14.
16 The long-term elevated concrete temperature is assumed to be
roughly constant over significant periods relative to the life of
the anchorage. AC308 also defines short-term elevated concrete
temperatures as those that occur over brief intervals, e.g., as a
result of diurnal cycling.
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for creep behavior is well established.17
The sustained load level used in AC308 is 55% of the mean
ultimate load established from short-term tension tests to failure
at standard and maximum long-term temperature, respectively.
Comparing the sustained load used in AC58 (assuming unrestrained
tests18) to that specified in AC308 as a function of the design
load:
AC58: = =sust udesign u
N 0.4N 1.6N 0.25N
(1)
AC308: =
sust u
design u
N 0.55N 1.6N 0.65 0.75N
1.4
(2)
whereby in the second instance the ratio of characteristic
strength to mean ultimate strength is assumed to be 0.75, the
strength reduction factor is conservatively taken as 0.65 (Category
1 anchor) and the load factor for sustained load is taken as
1.4.
A summary comparison of the creep test parameters defined in
AC58 and AC308 is provided in Table 1.
17 Eligehausen, R., Mallee, R., and Silva, J., Anchorage in
Concrete Construction, Ernst & Sohn, Berlin, 2006, p. 201. 18
It should be noted that an additional margin of safety, anywhere
from 10 to 25%, would be present if restrained reference tests were
used to establish the sustained load (see Footnote 14).
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Table 1 Summary comparison of creep test parameters in AC58 and
AC308
Test condition AC58 AC308
Static tension load
u,std temp0.40 N u0.55 N *
Temperature(s) during creep test
110F (43.3C) standard (room) temp. max. long-term elevated
temp.
Duration of test min. 42 days min. 42 days
Extrapolation period
600 days (elevated temp.) 50 years (room temp.)
10 years (elevated temp.)
Extrapolation method
Logarithmic
( ) = + +( t ) a ln t b0 Findley Power Law
( ) = + b( t ) a t0 Residual capacity
No test required Test anchors in tension to failure following
application of
sustained load
Acceptance criteria ( )
u ,elevated temp( 600 days ) min0.12in. 3mm
lim,roomtemp
lim,elevated temp
( 50 yrs )
(10 yrs )**
Residual load: req=0.90 * The mean ultimate loads associated
with standard temperature and elevated temperature conditions are
used for the sustained load tests at room temperature and elevated
temperature, respectively. **The calculated estimated displacement
service for any one test may not exceed 1.2lim
6.0 Validity of current methods for predicting creep behavior
The methodology used for determining the response to sustained
tension load in both AC58 and AC308 fundamentally assumes that
relatively short-term testing (typically in the range of 1,000
hours) can be extrapolated to long-term behavior. This is an
admissible assumption assuming that the adhesive behaves like a
visco-elastic material and it has been applied to other cases where
adhesives are used in thin bond lines (e.g. externally-applied
carbon fiber reinforcing19). It further assumes that the behavior
of the tested 19 Triantafillou, T., Fardis, M., Strengthening of
historic masonry structures with composite materials, Materials and
Structures, Vol. 30, No. 8, Springer Netherlands, November 2006,
pp. 486-496.
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anchor diameter and embedment is representative of the entire
anchor diameter and embedment range, and that all other factors
investigated in the assessment of the anchor system for short-term
strength, such as incomplete hole cleaning, affect the long-term
behavior to the same degree. These assumptions are less well
supported by systematic investigation. There is no evidence to
indicate that they are incorrect, however.
Current experience with long-term testing of adhesive anchors is
extensive owing to the number of manufacturers engaged in the
development and marketing of adhesive anchor systems over the past
30 years.20 It may be observed from Fig. 6, Fig. 7, and Fig. 8 that
the displacement curves exhibit increasing stability over time.
This is generally true of systems that exhibit stiff response up to
ultimate in short-term testing to failure. However, systems that
exhibit a large ratio between peak load and the load at loss of
adhesion (as evidenced by a sharp change in the load-displacement
response) have a greater tendency to show increased displacements
over time when the sustained load exceeds the load corresponding to
loss of adhesion. It has further been noted that sustained loading
does not appear to impair short-term strength provided that
long-term failure is not imminent.21 Finally, it may be stated that
the predicted anchor displacements associated with sustained
tension loading as yielded by the logarithmic function (AC58) are
in accordance with current experience and that those associated
with the Findley Power Law (AC308) are generally conservative.
y = 0.1629Ln(x) - 0.7079
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 50000 100000 150000 200000 250000
Dis
plac
emen
t [m
m]
Duration of load [hrs]
AC58 logarithmic projection
Measured displacements
Spring re-tension
27 years
y = 0.1629Ln(x) - 0.7079
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 50000 100000 150000 200000 250000
Dis
plac
emen
t [m
m]
Duration of load [hrs]
AC58 logarithmic projection
Measured displacements
Spring re-tension
27 years
Fig. 6 Long-term testing of 16 mm capsule anchors in 2,900 psi
(19/20 MPa) concrete subjected to Nsust 0.36 Nu,m comparison with
logarithmic projection
20 Mszrs, J., Tragverhalten von chemischen Befestigungen unter
zentrischer Belastung, doctoral thesis, University of Stuttgart
Institut fr Werkstoffe im Bauwesen, April 2002, p. 18. 21
Eligehausen, R., et al., op. cit., p. 201.
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6.1 Adequacy of Adhesive Anchoring Systems for Sustained
Load
6.1.1 Systems qualified under AC58 Ongoing changes to
qualification methods for adhesive anchors with respect to
sustained load naturally raises questions concerning the adequacy
of existing installations based on earlier qualification methods
(i.e. AC58). In this regard, the following may be said:
Where adhesive anchor systems qualified under AC58 for sustained
loading have been designed properly and installed correctly (e.g.,
without voids and with proper hole cleaning) 22, the likelihood of
premature failure under sustained load is extremely low.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 20000 40000 60000 80000 100000 120000
Dis
plac
emen
t [m
m]
Duration of load [hrs]
AC308 (Findley) projection
AC58 logarithmic projectionMeasured displacements
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 20000 40000 60000 80000 100000 120000
Dis
plac
emen
t [m
m]
Duration of load [hrs]
AC308 (Findley) projection
AC58 logarithmic projectionMeasured displacements
Fig. 7 Long-term testing of 12 mm injection anchors in 4,200 psi
(29 MPa) concrete subjected to Nsust 0.41 Nu,m comparison with
logarithmic and Findley extrapolations This is in part due to the
relatively large safety factor associated with the ASD design
paradigm. It is also due to the fact that the conditions imposed
under AC58 (elevated temperature) represents an extreme usually not
seen in practice, i.e., service conditions generally do not produce
a constant 110F (43.3C) in situ temperature in the concrete. (The
extrapolation to 600 days in the AC58 criteria is based on outdoor
field tests and was intended to represent the number of high
temperature days that an anchor might experience over its service
life.) Additionally, experience shows that the practical
22 It should be noted that tests to verify the effectiveness of
overhead installation procedures were not included in AC58, and
where specific instruction for overhead installations (either in
writing or on the jobsite) were not provided, the quality of the
installation may be questionable.
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considerations associated with running creep tests dictate that
the displacement criteria established for passing the test
(non-exceedence of the lesser of failure displacement or 3 mm) are
rarely if ever fulfilled without an additional margin of safety,
and design conditions (group effects, near edges, steel capacity,
practical constraints on minimum bolt diameter) often dictate a
lower bond stress than that corresponding to marginal long-term
behavior.23
Finally, it should also be noted that some manufacturers (e.g.
Hilti) have conducted extensive testing of adhesive anchor systems
over many years to verify their performance under conditions and
load levels that exceed the requirements of AC58.
Where there is doubt about the correct installation of adhesive
anchors subjected to sustained tension loading and where the level
of sustained load is high relative to the anchor design bond
strength, it is advisable to investigate their behavior via on site
proof load testing, regular displacement monitoring or both. In
specific cases, testing to failure of a sample of the installed
anchors may also be warranted to ascertain the quality of the
installation.
y = 0.1629Ln(x) - 0.7079
y = 0.018x0.4565
0
1
2
3
4
5
6
0 50000 100000 150000 200000 250000
Dis
plac
emen
t [m
m]
Duration of load [hrs]
AC308 (Findley) projection
AC58 logarithmic projection
Measured displacements
y = 0.1629Ln(x) - 0.7079
y = 0.018x0.4565
0
1
2
3
4
5
6
0 50000 100000 150000 200000 250000
Dis
plac
emen
t [m
m]
Duration of load [hrs]
AC308 (Findley) projection
AC58 logarithmic projection
Measured displacements
Fig. 8 Long-term testing of 16 mm capsule anchors in 2,900 psi
(19/20 MPa) concrete subjected to Nsust 0.36 Nu,m data plotted
against AC58 logarithmic projection and AC308 Findley Power Law
(compare to Fig. 6.)
23 As stated earlier, an additional margin of safety would also
be present where restrained reference tests were used to establish
the sustained load (see Footnotes 14 and 18).
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6.1.2 Systems qualified under AC308 Under the LRFD design
paradigm associated with AC308, the global safety factor can be
less than that mandated by AC58. Assuming a strength reduction
factor on concrete-related failure modes of 0.65 (optimum anchor
reliability) and a relationship of characteristic to mean strength
of 0.75, the global safety factor resulting from ACI 318 Appendix D
for a sustained permanent (dead) load would be:
u
design
N 1.4 2.9N 0.65 0.75
=
(3)
This reduced safety factor (for superior systems as determined
through reliability tests) is justified by the increased robustness
of the assessment and design processes embodied in AC308 and ACI
318 Appendix D. With respect to creep behavior, the reduced safety
factor is partly offset by the fact that anchors qualified under
AC308 for sustained load must meet a substantially stricter
standard due to the increased time of extrapolation, the use of the
more conservative Findley expression for predicting creep
displacements (see Fig. 8) and the limitation on displacement
corresponding to loss of adhesion vs. peak load (see Table 1 and
Fig. 5).
6.2 Further Considerations
6.2.1 Overhead installation The installation of adhesive anchors
in the overhead position presents particular challenges.24 These
may be summarized as follows:
a. Void-free injection of adhesive into the hole. b. Avoidance
of adhesive run and attendant fouling of anchor rod25. c. Securing
of the anchor rod in the hole prior to cure of the adhesive,
particularly
for larger diameters.
Anecdotal evidence indicates that the presence of entrained air
bubbles and voids caused by adhesive run are particularly relevant
to long-term behavior and can aggravate the creep response of
anchors subjected to sustained tension. This may be ascribed to two
effects: 1) the effect of the limited oxygen in the voids on the
curing process of the adhesive and 2) the loss of bond area;
whereas the impact on cure is dependent on the adhesive
formulation, loss of bond area will always result in a lowering of
the load associated with loss of adhesion. It is therefore
particularly important that measures be taken to prevent adhesive
loss and air entrainment. These may include use of specialized
24 AC308 mandates testing of anchors installed in the overhead
position for anchors that are intended for this application. 25
Protective wear appropriate to the hazard level of the adhesive
being used should always be used. Refer to manufacturer
instructions and MSDS.
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injection equipment such as stoppers fitted to the end of the
injection tube. One method for assessing the effectiveness of
procedures intended to ensure void-free installation is to perform
the injection in a Plexiglas tube of corresponding diameter and
length. This method is particularly effective when performed blind
by the installer (see Fig. 9).
6.2.2 Long-term strength and factor of safety
The foregoing discusses methods for qualifying adhesive anchor
systems for applications involving sustained tension load. For
practical reasons, these are conceived as pass-fail tests with
pre-defined levels of acceptable displacement based on short-term
behavior.
Clear plastic tube (e.g., Plexiglas)
support system for tube
shield to block line of sight beyond concrete surface
Clear plastic tube (e.g., Plexiglas)
support system for tube
shield to block line of sight beyond concrete surface
Fig. 9 Use of a Plexiglas tube to verify the installation
procedures and equipment for an adhesive anchor system
To extract more complete information regarding the response of
an adhesive anchor system to long-term loading it would be
necessary to conduct tests at various levels of sustained tension
load (see Fig. 10). Testing at higher sustained loads (i.e.,
approaching the anchor ultimate capacity) would necessarily result
in failure after a short time. Lower levels of load would provide
correspondingly longer time periods prior to failure. Ultimately,
sufficient tests could be conducted to develop a sustained load
strength curve with the sustained load plotted against the time to
failure. Such a curve would show, at some level of sustained load,
runout behavior whereby failure does not occur for any reasonably
anticipated time duration. (This is analogous to the runout portion
of s-n fatigue curve.)
Page 15
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tension load
time t from onset of loading
sustained load strength curveNsust,1
Nsust,2
tfail,1 tfail,2
Npermissible
Nu,m
margin against failure under short-term load
margin against failure under sustained load
Nsust,3
mean tension capacity under short-term load
2
scatter associated with sustained load behavior
tension load
time t from onset of loading
sustained load strength curveNsust,1
Nsust,2
tfail,1 tfail,2
Npermissible
Nu,m
margin against failure under short-term load
margin against failure under sustained load
Nsust,3
mean tension capacity under short-term load
2
scatter associated with sustained load behavior
Fig. 10 Concept of a sustained load strength curve
Considering such a process, it can be observed that the margin
between the design or permissible load and the failure load as
described by the sustained load strength curve necessarily
decreases over time and that at some load level Nsust corresponding
to runout, the margin between long-term strength and applied load
is defined. Such a curve would be associated with some scatter, and
this is represented in the form of a Gaussian distribution.
Similarly, the applied loading is associated with some uncertainty,
and the usual relationship between load and resistance can be
drawn.
This approach is outlined in ASTM D 468026,27 which provides
guidelines for testing creep performance of glued wood joints in
static shear (see Fig. 11):
To establish a curve of stress versus time to failure, a common
practice is to load specimens at four or more evenly spaced
intervals of stress beginning at 90%. Stress is expressed as a
percentage of the average short-term ultimate shear strength of
adhesive bonds. It is desirable that at least one data set at each
stress level fall within each base-10 log of time cycle.
26 ASTM D 4680-98, Standard Test Method for Creep and Time to
Failure of Adhesives in Static Shear by Compression Loading
(Wood-to-Wood), Annual Book of ASTM Standards Vol. 15.06, pp.
392-393. 27 ASTM D 2990-01, Standard Test Method for Tensile,
Compressive, and Flexural Creep and Creep-Rupture of Plastics,
contains a similar procedure.
Page 16
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Fig. 11 Stress versus log of time to failure curve excerpted
from ASTM D 4680
Development of a long-term strength curve for an adhesive anchor
system would naturally lead to the application of the safety factor
directly to the predicted long-term strength. While appealing for
its simplicity, this approach assumes that the necessary long-term
strength data can be generated in a reliable and consistent manner.
This has yet to be verified experimentally.
The relationship of the methodologies embodied in AC58 and AC308
to such an approach is unclear at present.
In view of the above, and considering the difficulties
associated with overhead installations and the possibility that the
creep behavior could be negatively affected by poor installation,
it is reasonable to perform an additional design check for overhead
installations subjected to sustained tension loading (e.g. hanger
installations), whereby a reduced resistance is compared with only
those portions of the load that is sustained. This supplemental
design proof thus takes the form of:
R, S ,N N (4) where
R,N is the resistance associated with a reduced bond value k
where k is the bond strength generated by the AC308 qualification
process.
S ,N is the tension component of the sustained load (usually,
dead load plus some portion of the live load that is assumed to be
sustained)
This supplemental proof has been implemented by ICC-ES in AC308
on an ad hoc basis for overhead applications involving direct
tension whereby a value of 0.75 has been adopted for . Extension of
this proof to other design conditions such as cantilever
Page 17
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beams where anchors may be subject to sustained tension loading
is warranted for specific cases (e.g., where the ratio of live to
dead loads is small).
7.0 Conclusions The use of adhesive anchors for safety-related
applications involving sustained tension loading is supported by
extensive experience both in the field and in the laboratory. The
following points are relevant:
1. Standards for the assessment of adhesive anchor systems to
address sustained loading have been in place for over a decade.
2. Assuming proper installation and design in all other
respects, allowable stress designs of adhesive anchors qualified to
resist sustained tension loading in safety-related applications
under AC58 provide the requisite level of safety against creep
failure.28
3. The qualification and design of adhesive anchors for
sustained tension load applications in accordance with the
provisions of AC308 and ACI 318 Appendix D provide a level of
safety in accordance with current standards and procedures for
reinforced concrete design. Pending further research on the
relationship of creep testing to long-term strength, the additional
check on long-term strength as discussed in Section 6.2.2 should be
included in the design.
4. Particular care should be exercised in the installation of
adhesive anchors in overhead conditions due to the potential for
degraded creep response associated with inadequate hole cleaning
and injection techniques.
5. Ongoing research into creep phenomenon in adhesives used to
transfer sustained loads is warranted. This research has
applicability to a wide range of applications, including the use of
surface-applied strengthening materials (carbon fiber, other),
repair of reinforced concrete elements with crack injection,
etc.
6. In light of the standards now in place for the assessment of
adhesive anchor systems for sustained loading, the authors are of
the opinion that the NTSB recommendation to prohibit the use of
adhesive anchors in sustained tensile-load overhead highway
applicationsuntil testing standards and protocols have been
developed and implemented... is excessively broad in scope. A
prohibition on the use of systems that have not been assessed in
accordance with AC308 would be appropriate.
28 Where anchors have been installed in the overhead position to
resist sustained tension loads and there are questions regarding
the quality of the installation or the procedures used to assure
good bond and a void-free installation, it may be prudent to
perform periodic field checks as discussed in Section 6.1.
Page 18
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Postscript: Discussion of Federal Highway Administration
report29 on creep behavior of adhesive anchors in connection with
the I-90 Seaport Portal Tunnel partial ceiling collapse of July 10,
2006.
Background:
The partial collapse of the ceiling system in the I-90 Seaport
Portal Tunnel resulted in an extensive investigation by the
National Traffic Safety Board (NTSB) and the Federal Highway
Administration (FHWA). As part of that investigation, tests were
conducted at the FHWAs Turner-Fairbank Highway Research Center on
the anchors used in the tunnel, including tests specifically
intended to investigate creep behavior.
The apparent purpose of the FHWA investigation into creep
behavior was to establish whether the products used in that
application were subject to creep failure and whether this could
have been anticipated with current screening methodologies. The
focus of the investigation, however, was on realistic re-creation
of in-situ conditions in the I-90 tunnel, not laboratory testing
and qualification procedures.
Synopsis of the FHWA testing and analysis:
Twelve anchor specimens were installed in the overhead position
and individually loaded with dead weights placed by forklift. The
anchors were installed in cored holes and the ambient laboratory
temperature during the test varied between approximately 75 and 90
degree Fahrenheit30. The measured displacements and residual
failure loads were recorded and analyzed. Subsequent inspection
revealed the existence of substantial air voids in at least 8 of
the 12 test specimens, particularly in the anchorages executed with
the fast-cure adhesive. A power function was used to extrapolate
the displacements out to various time periods. These were then
compared to the predicted displacements using the natural log
function specified in AC58. A limiting displacement of 0.20 in.
(5.1 mm) was taken for predicting the life of the anchors under
sustained load, and a conclusion was reached that the time to
failure could best be predicted with the power function developed
using regression analysis and that the log function specified in
AC58 is inadequate for this task.
Analysis:
A preliminary review of the FHWA findings contained in the
report indicates the following:
1. That practical difficulties associated with installing the
anchors overhead (as reported in their findings) resulted in
significant voids in the adhesive mass with attendant significant
decreases in the bond strength of the anchors.
29 Federal Highway Administration Turner-Fairbanks Highway
Research Center, Report I-90 Seaport Portal Tunnel Partial Ceiling
Collapse Investigation: Sustained Load Behavior of Powers Fasteners
Power-Fast+ Adhesive Anchors, July 2006, as made available on the
Boston Globe website www.bostonglobe.com. 30 Ibid., p. 16, p. 74,
The report notes that the increase in ambient temperature towards
the end of the testing correlated with increased anchor
displacements.
Page 19
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2. That the fast-cure product in question, had it been subjected
to the complete criteria contained in AC58 for creep testing
(constant elevated temperature of 110 degrees Fahrenheit,
displacement limit derived from static tests with maximum
displacement 0.12 inches, etc.), would not have been qualified for
long-term loading, not even at the reduced design capacity
associated with the application in question (2.6 kips per anchor
corresponding to a mean ultimate of 10.4 kips and a sustained load
requirement of 4.2 kips.31).
Conclusion:
In several respects, the FHWA testing of the subject product did
not correspond to an AC58 assessment. Had such an assessment been
conducted by the FHWA, they would have found that the fast-cure
adhesive in question would be precluded for long-term loading and,
in fact, it was not rated for such loading in the ICBO-ES
evaluation report32 issued for the product after the anchors had
been installed. That evaluation report was based on an assessment
using AC58 criteria.
AC308, the current ICC-ES acceptance criteria for assessing
adhesive anchors, requires extrapolation of measured displacements
under sustained loading to 50 years at room temperature and 10
years at elevated temperature (mandatory requirement). The
establishment of a time window of 75 to 100 years for highway
construction (as opposed to 50 years for buildings) does not
materially affect the outcome since the incremental increase in
displacement at these long time frames is marginal.
It is therefore the opinion of the authors that the conclusion
stated in the FHWA report that the continued use of adhesive
anchors subject to sustained tension loads should be very limited
if not eliminated for life safety applications33 is not supported
by these tests and is unwarranted.
31 National Transportation Safety Board, op cit., p. 37. 32
ER-4514, Chem-Stud and Power-Fast Adhesive Anchor Systems, ICBO
Evaluation Service, Inc., Whittier, CA, re-issued February 1, 2000,
p. 2. 33 Federal Highway Administration Turner-Fairbanks Highway
Research Center, op cit., p. 48.
Page 20
1.0 Foreword2.0 Background3.0 Assessment of adhesive anchors for
service conditions4.0 Assessment of adhesive anchors for
suitability5.0 Assessment for response to sustained tension
loading6.0 Validity of current methods for predicting creep
behavior6.1.1 Systems qualified under AC586.1.2 Systems qualified
under AC3086.2.1 Overhead installation6.2.2 Long-term strength and
factor of safety
7.0 Conclusions
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