Engineering Structures 252 (2022) 113754
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Engineering Structures
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Bonded anchors with post-installed supplementary reinforcement under
tension loading – Experimental investigations
Norbert Vita a, *, Akanshu Sharma b, Jan Hofmann c
a
fischerwerke GmbH & Co. KG, Klaus-Fischer-Straße 1, 72178 Waldachtal, Germany
Lyles School of Civil Engineering, Purdue University, 4118 Delon and Elizabeth Hampton Hall, 550 Stadium Mall Drive, West Lafayette, IN 47907-2051, USA
c
Institute of Construction Materials, Department of Fastening and Strengthening Methods, University of Stuttgart, Pfaffenwaldring 4G, 70569 Stuttgart, Germany
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Bonded anchors
Anchor groups
Post-installed reinforcement
Strengthening
Tension loading
Reinforcement yielding
Strut failure
This paper presents the results of comprehensive and systematic experimental investigations carried out on
anchorages (single anchors and groups) of bonded anchors supplemented with post-installed reinforcement
subjected to tension loading. Until now, the use of supplementary reinforcement is allowed in the codes only for
anchorages with cast-in headed anchors supplemented with cast-in reinforcement. In this work, tension tests are
performed on single anchors and anchor groups with different amounts and arrangements of post-installed
reinforcement. Epoxy based mortar was used for installation of both anchors and the reinforcement. It was
found that the post-installed supplementary reinforcement can significantly increase the capacity of anchorages
undergoing concrete cone breakout failure. Due to high bond strength of the mortar, even relatively short bond
lengths within the breakout body were sufficient to activate the reinforcement and contribute significantly to­
wards the resistance. The ultimate load was limited either by reinforcement yielding (for low amounts of rein­
forcement) or strut failure (for moderate to high amounts of reinforcement). Not just the amount but also the
placement and arrangement of reinforcement plays an important role in the tension behavior of anchorages with
post-installed supplementary reinforcement. At peak load, both the concrete as well as reinforcement contribute
significantly to the resistance of the anchorage.
1. Introduction
Post-installed fastening systems (anchorages) are often used because
of their high flexibility in many applications and because of the
improved performance that has been achieved through continuous
optimization of these systems in the last decades. In particular, bonded
anchors (or adhesive anchors) offer the highest flexibility in design as
they can be tailored to a whole range of sizes. The design of anchorages
using post-installed anchors is regulated in standards such as ACI 318
[1], EN1992-4 [2] and guidelines such as fib Bulletin 58 [3]. The
development of high strength mortar systems for post-installed bonded
anchors has led to the fact that often concrete cone failure (under ten­
sion loading) or concrete edge breakout (under shear loading) governs
the design.
With the new EAD [4] (for bonded fasteners for use in concrete) it is
possible to design bonded anchors with a life time of 100 years. In this
relatively long service life, the load to be resisted by the anchorage
might increase for various reasons. In such cases, the fastening must be
recalculated and, if required, replaced with a stronger anchorage.
However, it may not be possible to replace the anchorage due to func­
tional requirements and therefore strengthening of an anchorage might
be required.
The influence of long time loading for bonded anchors has been
studied in e.g. [5,6], while differences in the design between cast-in and
post-installed are summarized in [7]. Recently, analytical models to
predict the capacity of cast-in and bonded anchors under combined
tension and shear loads considering dowel action are presented in [8].
Concrete cone resistance of anchorages loaded in tension can be
increased at the design stage either by increasing the concrete strength
(base material) or by increasing the effective embedment depth of the
anchors [9]. By using different base material e.g. steel fiber reinforced
concrete, due to the enhanced properties (tensile strength, fracture en­
ergy and steel fiber bridging mechanism) higher failure loads can be
obtained [10–12].
Ordinary surface reinforcement may also induce an increased ca­
pacity compared to unreinforced concrete in tension, see e.g. [13,14].
* Corresponding author.
E-mail addresses: norbert.vita@fischer.de (N. Vita), akanshu@purdue.edu (A. Sharma), jan.hofmann@iwb.uni-stuttgart.de (J. Hofmann).
https://doi.org/10.1016/j.engstruct.2021.113754
Received 2 July 2021; Received in revised form 18 November 2021; Accepted 7 December 2021
Available online 17 December 2021
0141-0296/© 2021 Elsevier Ltd. All rights reserved.
N. Vita et al.
Engineering Structures 252 (2022) 113754
For anchorages with headed studs, adding supplementary rein­
forcement also known as anchor reinforcement in the form of surface
reinforcement and stirrups is a well-known method to enhance the
tension resistance of the anchorages and the method is also included in
codes and standards [1–3]. In case of anchorages with supplementary
reinforcement under tension load, once the concrete cone crack in­
tercepts the stirrups, the reinforcement takes up the tension forces, while
the compression is transferred by a network of concrete struts, see Fig. 1.
The effectiveness of anchor reinforcement in increasing the resistance of
headed-studs have been shown by various researchers in the past
[15–17]. Furthermore, Sharma et al. [18–21] studied the load-bearing
behavior of anchor groups consisting headed-studs with supplemen­
tary reinforcement in detail. A comprehensive research campaign on
headed stud anchorages with supplementary reinforcement under ten­
sion and shear towards the edge was studied and on the basis of the
results a new model for predict the load-carrying capacities of anchor­
ages with supplementary reinforcement was developed [22–26].
Furthermore, the beneficial influence of supplementary reinforcement
on the behavior and resistance of cast-in anchorages has been demon­
strated by [27–29].
Randl and Kunz [30] investigated the behavior of post-installed
bonded and expansion anchors loaded in shear towards the edge rein­
forced with cast-in hairpin reinforcement. The U-shaped hairpin rein­
forcement were cast-in with an inclination of 5–10 degress to the
concrete surface. Due to knowing the exact position of the reinforce­
ment, it was possible to install the post-installed anchorages at the right
position to the reinforcement. They reported significant increase in the
load-carrying capacity (up to 3-times the reference value without rein­
forcement for anchorages with small edge distance) and ductility of
anchorages with hairpin reinforcement. On the basis of 76 test results, a
model equation is given to predict the load for anchorages under shear
loads towards the edge with hairpin reinforcement.
So far, most of the research for anchorages with supplementary
reinforcement has been carried out with cast-in supplementary rein­
forcement. The studies focusing on the behavior of anchorages with
post-installed anchors strengthened using post-installed supplementary
reinforcement are absent. However, due to recent advancements in
product developments and design methods [31–34] it is possible to use
post-installed rebar also as supplementary reinforcement for
anchorages.
For the first time, in [35] Vita et al. reported the results of tension
tests on single post-installed bonded anchors reinforced with different
amount and arrangement of post-installed bars as supplementary rein­
forcement. The main objective was to investigate the feasibility and
efficacy of using post-installed reinforcement on the load capacity and
the load–displacement behavior of post-installed bonded anchors. For
both, anchors and reinforcement, high strength epoxy-based injection
mortar was used.
A significant increase in the ultimate loads and an improved
displacement behavior was obtained for the anchors with additional
post-installed supplementary reinforcement, provided the failure of
anchors without reinforcement occurs due to concrete cone breakout.
Moreover, it was shown that a preloading of the anchors up to the
characteristic failure load prior to the rebar installation has no negative
influence on the load capacity of the anchors. Therefore, it was proposed
that this method is suitable also for applications requiring strengthening
of existing anchorages. The work reported by Vita et al. [35] served as
the pilot project for the work reported in this paper and therefore, some
of the results reported in [35] are also included in this paper.
Furthermore, Vita el al. [36] investigated the efficacy of postinstalled supplementary reinforcement used for bonded anchors
loaded in shear towards to the edge to enhance the concrete edge
breakout resistance. Due to the use of high strength mortar, it was shown
that even with relatively small anchorage length of the reinforcement in
the theoretical concrete edge breakout body, high increase in the ulti­
mate load can be reached.
In this work, systematic investigations were performed on anchor­
ages (single anchors and groups) using post-installed (bonded) anchors
and strengthened with post-installed supplementary reinforcement
under short-term tension loading. This paper provides details of the
experimental campaign carried out to investigate the influence of postinstalled supplementary reinforcement on the behavior of anchorages
with bonded anchors loaded in tension. The tests were performed on
single bonded anchors and 1x2 anchor groups with different amount and
arrangement of post-installed supplementary reinforcement. The test
program, test setup and results are discussed in detail. The test program
is part of a greater test campaign which also includes strengthening of
anchorages with bonded anchors under shear loading. The results of the
shear tests will be presented in detail in a future paper.
1.1. Failure mechanics of anchorages with supplementary reinforcement
Supplementary or anchor reinforcement has a significantly influence
on the load-bearing capacity of anchorages loaded in tension or shear
failing due to concrete cone or edge breakout. The behavior of anchor­
ages with supplementary reinforcement can be described with a strutand-tie model acc. to [2], given in Fig. 1. When the anchorage is
loaded in tension, after the concrete cracks, the stirrups get activated.
The applied tension load on the anchorage is transferred from the anchor
by a network of concrete struts taking up the compression forces while
the stirrups and surface reinforcement resist the tension forces. In
principle this strut-and-tie model has three major components, the
concrete struts, the tension ties and the nodes [19] which provide the
possible mechanisms for the failure of anchorages with supplementary
Fig. 1. Fastening with supplementary reinforcement under tension load acc. to EN1992-4 [2], (a) details, (b) corresponding strut and tie model.
2
N. Vita et al.
Engineering Structures 252 (2022) 113754
reinforcement.
The possible failure mechanisms of the supplementary reinforcement
can be the bond failure or yielding of the reinforcement according to [2].
Bond failure of the stirrups occurs, if the anchorage length of the rebars
(l1) within the theoretical concrete breakout body is not sufficient to
develop the reinforcement. If the stirrups could be activated to the point
of yielding, it would lead to significantly enhanced load and deforma­
tion capacity of the anchorage.
In principle, the resistance of the anchorage can be increased by
increasing the amount of the supplementary reinforcement that can be
activated. However, increase in strength of the anchorage is not pro­
portional to the amount of reinforcement only and beyond a certain
level of the reinforcement, failure of the concrete struts limit the ca­
pacity of the anchorage [15,19].
A similar behavior can be expected in case of post-installed anchors
with post-installed supplementary reinforcement where the anchors
without reinforcement fail by concrete cone breakout. However, with
post-installed supplementary reinforcement it is relatively easy to
realize many different arrangements of reinforcement, which could lead
to different patterns of struts from the anchor. This aspect will be shown
and discussed in details with the help of test results in later sections.
depth or spacing the anchorages) were selected in such a way that in the
reference test series without reinforcement, concrete cone breakout
governed the failure of the anchorages. Furthermore, in the tension tests
with supplementary reinforcement, the aim was to reach yielding of the
reinforcement or strut (compression) failure.
For the bonded anchor as well as for the post-installed reinforcement,
a high-strength epoxy-based injection mortar FIS EM Plus (mean ulti­
mate bond strength of approx. 35 N/mm2) from the company fischer was
used. The epoxy mortar is approved for use with bonded anchors [37] as
well as with post-installed reinforcement [38].
The test program with detailed test parameters is summarized in
Table 1. All tension tests were performed without edge influence with an
edge distance higher than the critical edge distance (c > ccr,N = 1,5∙hef).
For bonded anchors, diameter, ds = 24 mm threaded rods with steel
grade of 12.9 (to avoid steel failure) were used, except the test series TS_1, in which 16 mm threaded rods (steel grade 12.9) were installed. The
effective embedment depth of anchors (hef) in all the tests was kept as
either 100 mm or 140 mm. In case of test series denoted as T-G (see
Table 1) one by two group anchorages (1 × 2) were tested. The anchor
spacing was kept constant in all the group tests with s = 140 mm which
was smaller than the critical anchor spacing according to [9,37] (s <
scrit = 3,0∙hef) in order to have group effect.
The first two single test series, T-S_1 and T-S_2 served as the pilot test
series in which, different installation conditions for the post-installed
reinforcement were investigated as depicted in Fig. 2. The different
installation configurations included (compare Fig. 2):
2. Experimental investigations
2.1. Test program
Unconfined tension tests on single anchors and anchor groups con­
sisting of bonded anchors without and with post-installed reinforcement
were carried out in normal-strength concrete to investigate the influence
of post-installed supplementary reinforcement on the behavior of an­
chorages undergoing concrete cone breakout. Therefore, a comprehen­
sive test program was compiled to study the influence of various
parameters such as total cross-section area of the reinforcement,
arrangement of the reinforcement and or their distance to the anchor.
The installation parameters of the anchorages (anchor size, embedment
(a) rebars installed vertically without steel plates at the free end on
concrete surface (Fig. 2 a). This was the standard configuration of
reinforcement used in this work.
(b) rebars installed vertically but with steel plate at the free end on
the concrete surface (Fig. 2 b),
(c) same as case (a) but with specified preloading of the anchor prior
to the installation of reinforcement (Fig. 2 c),
Table 1
Test Program for bonded anchor with post-installed supplementary reinforcement.
Test Series
T-S_1.0
T-S_1.1
T-S_1.2
T-S_2.0
T-S_2.1
T-S_2.2
T-S_2.3
T-S_3.0
T-S_3.1
T-S_3.2
T-S_3.3a
T-S_3.3b
T-S_3.4
T-S_3.5
T-S_3.6a
T-S_3.6b
T-S_3.7
T-S_3.8
T-S_3.9
T-G_0
T-G_1
T-G_2
T-G_3
T-G_4
T-G_5
T-G_6
T-G_7
ANCHOR
REINFORCEMENT
Remarks
Geometry
[-]
Size
[-]
hef
[mm]
Number
[-]
Diameter
[mm]
Area
[mm2]
Distance (a)
[mm]
hef,R.
[mm]
Single
Single
Single
Single
Single
Single
Single
Single
Single
Single
Single
Single
Single
Single
Single
Single
Single
Single
Single
1x2
1x2
1x2
1x2
1x2
1x2
1x2
1x2
M16
M16
M16
M24
M24
M24
M24
M24
M24
M24
M24
M24
M24
M24
M24
M24
M24
M24
M24
M24
M24
M24
M24
M24
M24
M24
M24
100
100
100
100
100
100
100
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
–
2
2
–
2
2
2
–
1
2
1
1
1
4
2
2
3
4
6
–
2
4
2
6
4
6
6
–
10
10
–
12
12
12
–
8
8
12
12
16
8
12
12
12
12
12
–
8
8
12
8
12
10
12
–
157
157
–
226
226
226
–
50
101
113
113
201
201
226
226
339
452
679
–
101
201
226
302
452
471
679
–
50
50
–
50
50
70
–
50
50
50
100
50
50
50
100
50
50
50
–
50
50
50
50
50
50
50
–
175
175
–
210
210
210
–
210
210
210
210
210
210
210
210
210
210
210
–
210
210
210
210
210
210
210
Note: By Test Series T-G on 1x2 anchor groups, the spacing was kept constant with s = 140 mm.
3
Reference
without steel plate
with steel plate
Reference
without preloading
with preloading
reinf. diagonal
Reference
Reference
N. Vita et al.
Engineering Structures 252 (2022) 113754
Fig. 2. Different installation configurations of the post-installed supplementary reinforcement (not scaled); (a) standard, (b) with steel plates, (c) with preloading of
the anchor and (d) rebars inclined.
(d) rebars installed inclined to the concrete surface at an angle of 55
degrees (perpendicular to the theoretical concrete cone crack) as
shown in Fig. 2 d.
The concrete mix of the used (three) concrete batches for tests are
given in Table 2. From each concrete batch, four concrete slabs were
cast. The compressive strength of each concrete mixtures used for testing
were measured according to DIN EN 12390–3 [39] on standard 150 mm
cubes (3 cubes per concrete mix). The measured compressive strength of
the concrete were obtained in the range fcc,150 = 34,0 to 39,4 MPa, with
an average strength of fcc,150,m = 36,7 MPa.
For the post-installed supplementary reinforcement standard
straight, ribbed reinforcing steel bars with a specified characteristic
yield stress of fyk = 500 MPa were used.
The distance a between the anchors (threaded rods) and the postinstalled rebars was maintained at 50 mm or 100 mm in all cases,
which corresponded to 0.36∙hef and 0.71∙hef respectively (for anchors
with hef = 140 mm).
Note that according to EN 1992–4 [2], the reinforcement may be
considered as active if the distance between the anchor and the rebar is
smaller than a ≤ 0,75∙hef provided it has sufficient anchorage length
within the breakout body. However, according to fib Bulletin 58 [3],
only reinforcing bars with distance to the anchorages a ≤ 0,5∙hef are
considered to take up the tension load from the anchor. The reinforce­
ment diameter (ds,re = 8, 10, 12, 16 mm) and the number of the rein­
forcement bars (nre = 1, 2, 3, 4, 6) were varied in the different test series
which resulted in different total cross-section areas of the post-installed
reinforcement (As,re = 50 mm2 up to 679 mm2).
The embedment depth of the post-installed supplementary rein­
forcement (hef,re) was primarily kept as 210 mm (1.5 times the embed­
ment depth of the anchor with hef = 140 mm). In this way, the anchorage
length of the reinforcement within the concrete breakout body (l1) was
almost the same as its anchorage length outside the breakout body (lbd)
for the reinforcement bars placed with a distance of a = 50 mm from the
anchor (Fig. 2 a). Given the relatively high bond strength of the epoxy
mortar, the length of the bar was sufficient to yield the rebar prior to
bond failure.
In each test series, three test were planned to perform. However, in
some test series only two test could be carried out.
2.3. Anchor and reinforcement installation
As mentioned before, an epoxy-based injection mortar, FIS EM Plus
from the company fischer was chosen for the installation of the postinstalled bonded anchors for the tensile tests without and with postinstalled supplementary reinforcement. FIS EM Plus is not only
approved as bonded anchor use in concrete (ETA-17/0979 [37]) but also
suitable and approved for use for post-installed reinforcement (ETA-17/
1056 [38]). Therefore, it was possible to use the same injection mortar
for the post-installed anchorages as well as for the post-installed rein­
forcement. The installation of bonded anchors, as well as the postinstalled reinforcement, was carried out according to the manufac­
turer’s specifications [37,38].
In general, bonded anchor and post-installed reinforcement were
installed in single work step, which involves drilling the bore holes for
both, anchors and reinforcement, cleaning, injecting the mortar and
installing the threaded rods and reinforcing bar. After the minimum
specified curing time (10 h at 20 degreesC [38]), the tension test were
carried out. This installation procedure was used for saving time during
the tests.
However, in the test series T-S_2.2, it was first confirmed that
simultaneous installation and consecutive installation (rebar installation
for a pre-loaded anchor) are equivalent and lead to similar increase in
load-bearing capacity of the bonded anchorages. To simulate an existing
2.2. Material properties and test specimens
All tension tests were carried out in unreinforced concrete slabs
(dimensions: L*W*H = 190*190*40 cm3) made of normal strength
concrete of grade C20/25.
4
N. Vita et al.
Engineering Structures 252 (2022) 113754
Table 2
Concrete Mix composition.
Concrete Batch
ID
Concrete strength
class
Mean cube comp.strength fc,
Cement
type
Cement
cube,act.
[-]
[-]
[N/mm2]
[-]
1
Normal
34.0
2
Normal
38.6
3
Normal
39.4
CEM I
32.5R
CEM I
32.5R
CEM I
32.5R
and loaded anchorage and to investigate the influence of post-installed
supplementary reinforcement on the load-bearing behavior of the pre­
loaded anchors, different installation procedure was used in T-S_2.2. In
this case, first only the bonded anchor was installed (drilling, cleaning,
injection and installing the threaded rods). After curing, it was loaded
with 75% of the mean ultimate resistance of the anchor obtained from
the reference series. The load of 75% was chosen as it corresponds to the
characteristic concrete cone breakout resistance of the anchor assuming
15% coefficient of variation [9].
After unloading, the bonded single anchor was strengthened with the
post-installed reinforcing bars, i.e. the reinforcement was installed
(drilled, cleaned injected and set). After the required curing time of the
reinforcement bonding agent, the tension tests were carried out until
failure. A detailed description of the different installation procedures are
also given in [35]. The obtained test results without and with preloading
(new and existing anchorages) will be discussed in Section 3.1.2.
Aggregate
Water
w/c
Superplasticizer
[-]
[M% of Cem.]
0.68
2.3%
0–2
mm
2–8
mm
8–16
mm
[kg/
m3]
252.4
[kg/
m3]
778.0
[kg/
m3]
488.0
[kg/m3]
736.0
[kg/
m3]
172.5
254.0
812.0
485.0
725.0
170.0
0.67
2.0%
256.0
774.0
488.0
726.0
185.0
0.72
1.8%
distance from anchor to support ≥ 2hef). The load was applied to the
single anchor through a steel fixture with specified clearance hole or by
anchor group through a stiff anchor base plate (L*W*H = 200*60*60
mm3). To transfer the load from the hydraulic cylinder into the steel
fixture or anchor base plate, a high-strength threaded rod was used. In
case of group anchorages, a hinge was placed between anchor base plate
and threaded rod to enable rotation free uniform load distribution
among the two bonded anchors in the group.
The used calibrated load cells had an accuracy class of two and their
measuring range was chosen based on the expected ultimate load as
40–200 kN or 100–500 kN. The vertical anchor displacement was
measured at the top of the single anchor or on both anchors of the group
using a calibrated Linear Variable Displacement Transformer (LVDT).
Furthermore, in a few selected tests, strain gauges were installed on the
post-installed reinforcing bars to obtain the information about the force
taken up by the reinforcement and to separate the concrete and rein­
forcement configuration. The strain gauges were applied on the bars at
the position where the theoretical concrete cone crack was expected to
intercept the rebar under tension test.
The applied load, the anchor displacements and the strains in the
post-installed reinforcement (for tests with strain gauges) were recorded
at a frequency of 5 Hz by using the commercial data acquisition software
DiAdem. All performed unconfined tension tests on single or group an­
chorages without or with post-installed supplementary reinforcement
were carried out in oil pressure control using quasi-static loading rate.
According to [40] the peak loads were reached within 1 to 3 min.
2.4. Test setup and instrumentation
Unconfined tension tests on single bonded anchors and on 1x2 an­
chor groups were carried out in accordance with the ETAG 001, Annex A
[40]. Fig. 3 shows the typical test setups. This consisted of a testing rig, a
hydraulic cylinder, high strength threaded rod for loading, a calotte, a
calibrated load cell, fixture plate and displacement transducers (LVDT)
with holder. The testing rig had a suitable wide support with sufficient
support distance to enable unrestricted concrete cone breakout (free
Fig. 3. Test setups.
5
N. Vita et al.
Engineering Structures 252 (2022) 113754
Note: In anchor technology, oil pressure controlled tests are quite
common due to their simpler set-up. This is well-accepted since until
peak load, the oil-pressure controlled tests are known to provide reliable
response. However, the post-peak response is best obtained by per­
forming a displacement controlled test using a hydraulic actuator. In
these tests, a standard hydraulic cylinder was used and the measuring
frequency was kept as 5 Hz, which provided sufficient data points
especially for the tests with supplementary reinforcement. The proced­
ure used for the tests in this work was identical to the procedure used in
the previous tests reported by Sharma et al. [19,26]. However, the au­
thors highly recommend that in future, further tests on anchors with
post-installed supplementary reinforcement should be carried out in
displacement control.
Fig. 4. Typical concrete cone breakout bodies obtained in reference tests
(T-S_2.0).
3. Test results
3.1. Tests on single anchors
third test series (T-S_3.0) different reinforcement areas and arrangement
were investigated. In the next sections, typical test results using
load–displacement curves will be described and discussed.
The results of the tension test performed on single bonded anchors
without and with post-installed supplementary reinforcement are sum­
marized in Table 3. This table enlists test results in terms of individual
failure loads (Nu,i), mean value of the ultimate (failure) loads (Nu,m), the
corresponding coefficient variation of the ultimate loads (CoV), the
mean displacements at the ultimate load (δu,m) and the observed failure
mode. It may be noted that due to the limited number of tests (2) in some
test series, the CoV may not be representative.
In all reference tension tests, concrete cone breakout was observed as
the failure mode. Typical concrete cone breakout body obtained in the
reference tests are showing in Fig. 4. As expected, the concrete cone
surface started almost from the bottom of the threaded rods and
developed at an angle of approx. 35 degree to the concrete surface which
agrees well with the assumption given in [9].
As seen from Table 3, due to the introduction of post-installed sup­
plementary reinforcement a significant increase in the load-bearing
capacity of single bonded anchors could be achieved. The highest in­
crease was obtained in case of the test Series T-S_3.9 (6xd12) with more
than 100% increase in the peak load corresponding to the mean ultimate
load obtained in the reference tests.
Moreover, different conditions of the post-installed reinforcement
were tested in the first two test series (T-S_1.0 and T-S_2.0) while in the
3.1.1. Influence of end plate (Test series T-S_1)
A comparison of the load–displacement curves obtained in test series
T-S_1 on single anchor M16, hef = 100 mm without and with postinstalled supplementary reinforcement with two bars of diameter, ds,
re = 10 mm installed without and with steel end plate is depicted in
Fig. 5. The distance of the rebars to the anchor was 50 mm (0.5∙hef).
Although the amount of supplementary reinforcement in these cases
is relatively small (As,re = 157 mm2), the failure loads obtained in tests
with supplementary reinforcement are significantly higher (approx.
40% higher) than the failure loads obtained for the corresponding single
anchor without reinforcement. Moreover, no significant difference was
observed in the load displacement curves obtained for the test without
and with steel plate connected to the rebars. This suggests that even with
a relatively short bond length within the breakout body (l1 = 6,5∙ds,re) of
the reinforcement the reinforcement could be activated without any end
anchorage or the hook. This is attributed to the high mean bond strength
of the used epoxy-mortar. However, in none of the cases the reinforce­
ment yielding was observed. Since the pullout resistance of the bonded
anchor is rather high, the failure mode in both test series was identified
as strut failure as shown in Fig. 5.
Table 3
Summary of results of tests on single anchors.
Test
Series
Test ID
T-S_1.0
T-S_1.1
16-100_0
16-100_2xd10_no steel
plat
16-100_2xd10_steel plate
24-100_0
24-100_2xd12_no preload
24-100_2xd12_preload
24-100_2xd12_diagonal
24-140_0
24-140_1xd8
24-140_2xd8
24-140_1xd12
24-140_1xd12_a = 100
24-140_1xd16
24-140_4xd8
24-140_2xd12
24-140_2xd12_a = 100
24-140_3xd12
24-140_4xd12
24-140_6xd12
T-S_1.2
T-S_2.0
T-S_2.1
T-S_2.2
T-S_2.3
T-S_3.0
T-S_3.1
T-S_3.2
T-S_3.3a
T-S_3.3b
T-S_3.4
T-S_3.5
T-S_3.6a
T-S_3.6b
T-S_3.7
T-S_3.8
T-S_3.9
1)
Ultimate load Nu,i
Mean ultimate
load
Nu,m
CoV
Mean displ. at
Nu,m
δu,m
Relative increase in load
capacity
Nu,m/Nu,m,Ref
Failure
mode1)
[kN]
–
103.9
[kN]
77.2
111.3
[%]
3.4
7.4
[mm]
1.10
1.38
[-]
1.0
1.44
[-]
CC
Strut
–
83.0
156.8
146.2
115.5
129.3
152.3
191.0
200.6
–
193.9
238.3
233.1
–
259.6
272.9
299.1
107.6
83.4
155.7
150.0
117.9
130.8
148,1
181.5
186.1
164.0
197.9
235.8
230.3
181.4
253.9
249.8
284.5
1.6
2.0
3.4
3.8
3.2
6.4
2.8
5.0
6.9
6.0
2.3
4.4
4.2
0.7
7.3
8.5
4.7
1.54
0.48
1.32
1.30
0.93
0.78
1.25
1.19
1.43
1.10
1.29
1.68
1.63
1.15
2.03
1.82
2.42
1.39
1.0
1.87
1.80
1.41
1.00
1.13
1.39
1.42
1.25
1.51
1.80
1.76
1.39
1.94
1.91
2.18
Strut
CC
Strut
Strut
Strut
CC
Yield
Yield
Yield/Strut
Strut
Strut
Strut
Strut
Strut
Strut
Strut
Strut
Test
No.1
Test
No.2
Test
No.3
[kN]
79.0
120.2
[kN]
75.3
109.7
108.8
82.0
160.4
156.6
122.3
139.9
144.6
173.1
181.2
171.0
202.9
244.7
219.6
180.4
268.9
245.7
272.8
106.3
85.3
149.9
147.1
115.9
123.3
147.1
180.4
176.5
157.0
196.9
224.4
238.2
182.3
233.3
230.9
281.7
Note: CC = concrete cone breakout failure, Yield = Yielding of the reinforcement, Strut = failure of concrete strut
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Fig. 5. Left: Load-displacement curves obtained in tension tests on single anchor without (black) and with post-installed rebars, ds,re = 10 mm, without (blue) and
with steel plate (orange) and corresponding failure modes.
3.1.2. Influence of pre-loading of the anchor (Test series T-S_2.2)
Test series T-S_2 were carried out with M24 bonded anchors with an
embedment depth of hef = 100 mm. Two test series with the same
condition (2 × ds,re = 12 with distance a = 0,5∙hef) as well as reference
test without reinforcement were performed. In the case of the Series TS_2.1, the bonded anchors and the post-installed reinforcing bars were
installed in one work step. For the series T-S_2.2 the single anchor were
installed first and loaded up to a load of 5%-fractile value of the mean
ultimate load obtained from reference tests (=0.75∙Num,ref). The refer­
ence tests were performed by simply installing the anchors according to
manufacturer’s instructions and testing them after observing the
required curing time. The mean value of the reference test was NRm,
c = 83,4 kN which results to a pre-load value of NRk,c = 62,6 kN. Certain
fine radial cracks (See Fig. 6) were noticed on the concrete surface
initiating from the anchor as a result of the pre-loading.
A comparison of the load–displacement curves obtained in these
three test series (T-S_2.0, T-S_2.1 and T-S_2.2) is given in Fig. 7. The
observed mean failure load was 1,87-times and 1,80-times higher than
the corresponding mean failure load in the reference tests for series TS_2.1 (without pre-load) and for T-S_2.2 (with preload), respectively.
The initial stiffness obtained for reference test series as well as test series
without pre-load is rather similar, which indicates that the presence of
reinforcement does not influence the initial stiffness of the anchorage.
This can be explained by the fact that the reinforcement gets activated
only after the concrete cone crack intercepts the reinforcing bars and
therefore until the initiation of concrete crack, the load–displacement
curves without and with reinforcement follow the same path. However
in the series S1.3, where the tests were performed after the application of
a preload and subsequent unloading, the initial stiffness becomes
slightly smaller, which can be attributed to the initiation of local damage
in the concrete due to the applied pre-load. Nevertheless, there is only an
insignificant difference in the load–displacement behavior, failure loads
as well as the failure modes (see Fig. 7 b-d) observed for series T-S_2.1
and T-S_2.2. While in the reference test (T-S_2.0) the typical concrete
cone breakout can be seen, in the series with reinforcement (T-S_2.1 and
T-S_2.2) the concrete breakout was found to be impeded by the rein­
forcement and strut failure were observed.
3.1.3. Influence of angle of the reinforcement (Test series T-S_2.3)
In test series T-S_2.3 the influence of inclined supplementary rein­
forcement on the load-bearing behavior of bonded single anchor was
also investigated. According to [9], the effectiveness of the reinforce­
ment is greatest when the crack crosses perpendicular the reinforce­
ment. Therefore, based on the assumption of the CCD method [41] that
the crack is propagated at an angle of 35 degrees to the concrete surface,
the post-installed reinforcements were installed at 55 degrees to the
concrete surface according to the schematic shown in Fig. 2 (d). The
load–displacement curves obtained from these test carried out with in­
clined supplementary reinforcement as well as from the corresponding
reference test are shown in Fig. 8 (a), while the obtained failure mode is
showing in Fig. 8 (b). The relative increase in the load capacity was 1,41
which significantly lower than in case of perpendicular installed sup­
plementary reinforcement. The failure mode was identified as strut
failure. The lower resistance compared to the vertically installed rebar
can be explained due to the fact that although the rebar is perpendicular
to the crack, it is inclined to the applied load and therefore, only the
vertical component of the force taken up by the reinforcement resists the
applied load.
3.1.4. Test series T-S_3
The results of test series T-S_1 and T-S_2 showed that the influence of
end plate as well as of the pre-loading on the anchor is negligible.
Furthermore, test series T-S_2.3 showed that the vertically installed
reinforcement is significantly more efficient compared to the inclined
reinforcement. Therefore for simplicity, the tests in test series T-S_3 were
performed as follows:
i. anchorages and post-installed rebar’s were installed in one work
step, without preloading the anchor
ii. the rebars were installed without steel end plate
iii. the rebars were installed vertically parallel to the load direction
Fig. 6. Bonded single anchor after preloading with fine radial cracks
(red arrows).
In test series 3, the influence of amount and arrangement of post7
N. Vita et al.
Engineering Structures 252 (2022) 113754
Fig. 7. (a) Load-displacement curves obtained from tension tests on single bonded anchor without and with post-installed supplementary reinforcement and (b-d)
corresponding failure modes.
Fig. 8. (a) Load-displacement curves obtained from tension tests on single anchor without and with post-installed supplementary reinforcement installed inclined
and (b) corresponding failure mode.
installed supplementary reinforcement on the load carrying capacity of
single anchors was investigated. Furthermore, the influence of the dis­
tance between anchors and post-installed supplementary reinforcement
was also studied. The results of test series 3 are discussed in the
following sub-sections.
Fig. 9. Load-displ. curves obtained from tension tests on single anchor without and with post-installed supplementary reinforcement (ds,re = 12 mm) with different
distance between anchor and rebars (Left: single rebar, Right: two rebars).
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4. Influence of the distance between anchor and reinforcement
were 1,76 and 1,39 for the distance of a = 50 mm and a = 100 mm,
respectively.
This behavior is understandable by observing the failure pictures of
the performed test series given in Fig. 10.
In this case, even in the tests series with a = 100 mm distance (TS_3.3b and T-S_3.6b), with a relatively small bond length (~70 mm) in
the theoretical concrete cone body, no pull-out of the reinforcement was
observed. This is attributed to the high mean bond strength of the epoxy
based mortar used to install the reinforcement. The observed failure
mode showed truncated concrete cone breakout body indicating strut
failure (compare Fig. 21) in all the cases (see Fig. 10). This explains the
lower load carrying capacities for reinforcement installed at higher
distance due to flatter struts.
The distance between anchorage and post-installed supplementary
reinforcement was kept constant in all test series at a = 50 mm except for
two test series (T-S_3.3b and T-S_3.6b) where the distance was increased
to a = 100 mm, to investigate the influence of the load-carrying capacity
of the anchorage with supplementary reinforcement. Note that acc. to
EN 1994–2 [2], the reinforcement with a maximum distance of a =
0.75∙hef can be considered as active (provided it has sufficient
anchorage length) and taken into account in the calculation of the ca­
pacity for the anchorage. Thus, acc. to EN 1994–2 in all performed test
series the post-installed reinforcement was sufficient distance to get
activated (a = 0.5∙hef for T-S_1 and T-S_2; a = 0.36–0.71∙hef for T-S_3
series). However, the fib guideline Bulletin 58 [3] give smaller value for
the distance with a = 0.5∙hef and therefore in the test series T-S_3.3b and
T-S_3.6b with a = 100 mm = 0.71∙hef the reinforcement would not be
considered active.
In Fig. 9 the load–displacement curves obtained for test series with
same amount of reinforcement (left: 1xd12, right: 2xd12) and different
distance from the anchor (a = 50 or 100 mm) are compared. Further­
more, in Fig. 9 corresponding reference test results are also shown. As
expected, the peak load decreases with increasing distance between the
anchor and the reinforcement independent of the amount of the rebars
in both cases. This is attributed to the fact that with increasing distance,
the bond length within the breakout body reduces and the strut becomes
flatter. The relative increase in the load carrying capacity compared to
the corresponding mean reference load were obtained in case of single
rebar (1xd12) as 1,42 for a = 50 mm and 1,25 for a = 100 mm. The
corresponding relative increase in the capacity for two rebars (2xd12)
5. Influence of amount and placement of supplementary
reinforcement (Test series T-S_3)
Fig. 11 shows the load displacement curves obtained for the test
series with 1xd8 (T-S_3.1), 2xd8 (T-S_3.2) and 4xd8 (T-S_3.5) postinstalled rebars, as well as the curves from reference test series (TS_3.0). The pictorial legends in Fig. 11 indicate the respective configu­
ration of the reinforcement used for the single anchors. The relative
increase in the load carrying capacity with respect to the reference test
series were 1.11, 1.39 and 1,80-times for tests with 1x, 2x and 4x d8
rebars, respectively. This shows that even a relatively small amount of
supplementary reinforcement (2xd8, As = 101 mm2) can significantly
enhance the load carrying capacity of the anchorage. Addition of only
one 8 mm bar resulted in only a marginal increase in the failure load.
This is attributed to the fact that the presence of only one reinforcement
Fig. 10. Typical failure modes obtained from tension tests on anchor with post-installed supplementary rebars (ds,re = 12 mm), with small distance (a + c) and with
greater distance (b + d).
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Engineering Structures 252 (2022) 113754
positively, if the amount of reinforcement is increased. The initial
stiffness of the load–displacement curves is quite similar for the series
with and without reinforcement.
As can be seen, even a relatively small amount of reinforcement leads
to a significant increase in the failure load of the anchors. Even with a
single 12 mm diameter reinforcement, the capacity of the anchor
increased by more than 40%. In general, the peak load increased with
increasing amount of reinforcement but this increase in the peak loads is
not proportional to the cross-sectional area of the supplementary rein­
forcement. Except in the case of test with single 12 mm reinforcement,
the failure was governed by strut failure in all the tests. Note that in all
the cases, the reinforcement was placed in a symmetric pattern (in plan)
around the anchor so that a stable strut mechanism is possible. It can be
seen that beyond a certain amount of reinforcement, the increase in
failure load with the amount of reinforcement is relatively low, since the
strut failure limits the maximum achievable increase in the load carrying
capacity of the anchorage. Similar behavior was shown valid for cast-in
anchorages with cast-in supplementary reinforcement [15,24]. With the
highest amount of reinforcement of 6x12 (As,re = 678 mm2), the mean
failure load reached was 2.18 times the mean failure load of the refer­
ence test without reinforcement.
Typical failure pictures of these series are given in Fig. 13. While in
reference test (Fig. 13 (a)) ordinary concrete cone breakout was dis­
played, in cases with supplementary reinforcement of 12 mm diameter
bars 1x, 2x, 3x, 4x and 6x (Fig. 13 b, c, d, e and f respectively) concrete
cone was restricted by the reinforcement. With one bar (Fig. 13 b), the
failure cone is truncated only on the side of the bar, while for the 2x
reinforcement configuration (Fig. 13 c), a dog bone shaped breakout
body is visible. For the cases with 3x, 4x and 6x reinforcement, see
Fig. 13 (d, e, f), due to steep and more than two struts, the failure of the
anchors is similar to the pull-out failure with confined test setup.
Fig. 11. Load-displacement curves from tension tests on single bonded anchors
without and with post-installed supplementary reinforcement, ds,re = 8 mm.
results in unsymmetric strut formation, which does not result in a stable
strut mechanism. However, two symmetrically placed reinforcing bars
result in stable strut mechanism resulting in higher capacity of the
anchorage.
In the test with 1x and 2x d8 rebars, yielding of the reinforcement
was reached which is confirmed by the fact that in both cases, unre­
stricted concrete cone breakout bodies were formed. On contrary, for the
tests with 4 × d8 rebar’s, strut failure was observed with only a small
cloverleaf shaped concrete breakout area.
Fig. 12 summarizes the load–displacement curves obtained from the
tension tests on single anchors reinforced with ds,re = 12 mm rebars
placed at a distance of a = 50 mm from the anchor with different number
(n = 1, 2, 4, 3, 6) of the bars. To enable a direct comparison the results,
the reference curves obtained in unreinforced tests (Series T-S_3.0) are
also plotted in each case in Fig. 12. Depending on the number of 12 mm
bars (1x, 2x, 3x, 4x, 6x) following values of mean peak loads were
measured: 186,1 kN; 197,9 kN; 230,3 kN; 253,9 kN; 249,8 kN; 284,5 kN
respectively.
The results of the tests on anchors reinforced with ds,re = 12 mm
diameter reinforcing bars shows, in general, similar behaviour to the
results with 8 mm diameter rebars, but as expected, the effect of 12 mm
bars is stronger than of 8 mm bars. Not only the failure load, but the
displacement at the failure load of the anchorages are influenced
6. Influence of geometric arrangement of the reinforcement
(Test series T-S_3.4 þ 3.5)
According to current standards [1–3], the maximum diameter that
can be used for supplementary reinforcement is limited to ds,re = 16 mm.
This is partly due to indirectly avoiding strut failure and partly due to
the fact that the mandrel diameter of bars bigger than 16 mm diameter
becomes rather large according to EN1992-1–1. However, it has been
shown by the previous works by Sharma et al. [19,24] as well as in this
work that the strut failure may be reached even with smaller bar di­
ameters depending on the configuration (placement) and amount of
supplementary reinforcement. In test Series T-S_3.4, only one ds,re = 16
Fig. 12. Load-displacement curves from tension tests on single bonded anchor without and with post-installed supplementary reinforcement, ds,re = 12 mm.
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Engineering Structures 252 (2022) 113754
Fig. 13. Typical failure mode obtained from tension test on anchor without and with post-installed supplementary rebars, ds,re = 12 mm.
mm (As,re = 201 mm2) rebar was used as supplementary reinforcement.
The aim was to investigate the influence of the asymmetric arrangement
of the reinforcement.
In Fig. 14, a comparison the load–displacement curves obtained for
the anchor reinforced with one 16 mm rebar (T-S_3.4) and obtained with
four 8 mm rebars placed symmetric around the anchor (T-S_3.5) along
with the reference test results without reinforcement are given. In both
reinforced test series the total cross-section area of the reinforcement is
the same (As,re = 200 mm2). It can be clearly seen that the anchor
reinforced with 4–8 mm rebars results in a higher increase in the load
carrying capacity. The relative increase in the load capacity was 1,51-,
and 1.80-times the mean reference failure obtained for the tests with
1x16 and 4xd8 rebar’s, respectively. This can be explained with the fact
that in case of 1x16, only one strut can be formed which is relatively
unstable. However, in case of 4x8 rebars, the symmetric and stable struts
form which leads to higher load increase. It may be argued that the
higher increase in 4x8 bar case might be due to higher bond area
compared to 1x16 bar. However, as seen in Fig. 14, comparing the
failure pictures for ds,re = 16 mm and ds,re = 8 mm, in both test series
strut failure was the decisive failure mode due to strong bond resistance
of the epoxy mortar. Whereas with 1x16 bar case, the concrete failure
was only slightly restricted on one side, with 4x8 bars, a rather small
concrete break-out body formed.
6.1. Tests on 1x2 anchor groups
Additional to the tension tests on single anchors, unconfined tension
tests were performed on 1 × 2 group anchorages consisting of bonded
anchors with 24 mm threaded rods without and with post-installed
supplementary reinforcement. In Table 4 the results of tests on anchor
groups are summarized. Individual test results in terms of failure loads
(Nu,i), mean value of the ultimate failure loads (Nu,m), the corresponding
coefficient variation of the ultimate loads (CoV), the mean displace­
ments at the ultimate load (δu,m) are reported. Furthermore Table 4
Fig. 14. (a) Load-displacement curves from tension tests on single bonded anchor without (black) and with post-installed supplementary reinforcement, one rebar ds,
re = 16 mm (blue) and four rebars ds,re = 8 mm (orange and (b) typical failure mode obtained from tension test on single anchor with post-installed supplementary
rebars ds,re = 16 mm and (c) with ds,re = 8 mm.
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Table 4
Test Results by 1x2 anchor groups.
Test
Series
T-G_0
T-G_1
T-G_2
T-G_3
T-G_4
T-G_5
T-G_6
T-G_7
1)
Test ID
M24-140-140_0
M24-140-140_2xd8
M24-140-140_4xd8
M24-140140_2xd12
M24-140-140_6xd8
M24-140140_4xd12
M24-140140_6xd10
M24-140140_6xd12
Ultimate load Nu,i
Mean ultimate
load
Nu,m
CoV
Mean displ. at Nu,
Test No.1
Test No.2
Test No.3
δu,m
Rel. increase in load
capacitiy
Nu,m/Nu,m,Ref
[kN]
180.9
248.2
300.4
298.6
[kN]
167.3
264.6
269.3
278.1
[kN]
195.7
234.1
271.0
265.2
[kN]
181.3
249.0
280.2
280.6
[%]
7.9
6.1
6.2
6.0
[mm]
0.55
0.93
1.28
1.51
[-]
1.0
1.37
1.55
1.55
[-]
CC
Yield
Yield
Yield
341.8
325.9
318.2
326.6
358.2
298.2
339.4
316.9
5.9
5.1
1.59
1.45
1.87
1.75
Strut
Strut
321.6
322.1
380.8
341.5
10.0
1.23
1.88
Strut
315.5
346.3
379.9
347.2
9.3
1.51
1.92
Strut
m
Failure
mode1)
Note: CC = concrete cone breakout failure, Yield = Yielding of the reinforcement, Strut = failure of concrete strut
includes the obtained failure mode as well as the relative increase in the
mean load carrying capacity with respect to the mean ultimate load
obtained in the reference tests without reinforcement
In the reference tests series T-G_0, ordinary concrete cone breakout
for the anchor group was observed.
The obtained mean ultimate breakout load of 181.3 kN corresponds
well to the calculated load 187,7 kN according to the CCD method.
The behavior of anchor groups of bonded anchors loaded with postinstalled supplementary reinforcement loaded under tension is similar
to that observed in case of single anchors. Even with a relatively small
amount of reinforcement in case of test Series T-G_1 (2xd8 = As,re = 101
mm2), a significant increase of 37% in the ultimate load was obtained.
With increasing amount of supplementary reinforcement, the capacity
of anchorage increases but not directly proportional to the amount of
reinforcement. Beyond a certain amount of supplementary reinforce­
ment, the capacity of anchorage seems to saturate, at around twice the
failure load of the anchorage without reinforcement, due to attainment
of strut failure.
as 1.37, 1.55 and 1.87-times for the test Series with 2x (T-G_1), 4x (TG_2), and 6x (T-G_4) reinforcement, respectively. Nevertheless, the
stiffness of the anchor groups without or with supplementary rein­
forcement is similar to as observed in case of tests on single anchors.
Fig. 16 shows typical failure pictures of these anchor groups tests with
ds,re = 8 mm supplementary reinforcement.
To investigate the influence of the amount of supplementary rein­
forcement, test series with ds,re = 12 mm rebars were performed with
different number (n = 2, 4, 6) of the bars while the distance to the an­
chor was kept constant at 50 mm (0.36∙hef). The rebars were installed
symmetric around the anchor group. The load–displacement curves
obtained from the tension tests on anchor groups with supplementary
reinforcement (ds,re = 12 mm) along with the corresponding reference
curves are summarized in Fig. 15 on the right. The ultimate breakout
loads as well as the displacement at the ultimate loads increase with
increasing amount of the reinforcement while the initial stiffness of the
anchor groups are not affected by the amount of reinforcement. While
the mean failure load for the reference tests was 181.3 kN, the mean
failure loads obtained for tests with 2x, 4x and 6xd12 rebars were 280.6,
316.9 and 347.2 kN respectively, which leads to 1.51, 1.75 and 1.92times increase in the failure load relative to the reference test series.
Whilst in case of 2xd12 rebars (T-G_3) yielding of the reinforcement was
achieved, with 4xd12 and 6xd12 rebars strut failure governed the failure
load.
6.1.1. Influence of amount of supplementary reinforcement
Fig. 15 summarizes the load–displacement curves obtained on 1x2
anchor groups loaded in tension with different number (n =
0, 2x, 4x, 6x) of ds,re = 8 mm rebars (left) as supplementary reinforce­
ment. The relative increase in the ultimate breakout load was obtained
Fig. 15. Load-displacement curves from tension tests on 1x2 anchor groups without and with post-installed supplementary reinforcement with ds,re = 8 mm (left) and
with ds,re = 12 mm (right).
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Engineering Structures 252 (2022) 113754
Fig. 16. Typical failure mode obtained from tension test on 1x2 anchor groups with post-installed supplementary reinforcement, ds,re = 8 mm.
6.1.2. Influence of the arrangement of reinforcement
To investigate the arrangement of supplementary reinforcement in
case of 1x2 anchor groups, tension tests were performed on the groups
with 4xd12 (T-G_5) and with 6xd10 (T-G_6) rebars resulting in almost
equal cross-section area of As,re = 452 and 471 mm2, respectively.
Typical load–displacement curves obtained from both test series and
corresponding reference test series without reinforcement are plotted in
Fig. 17. In both test series the anchor groups failed by strut failure with a
typical small concrete breakout body within the reinforcement. The
mean load capacities were obtained as 1.75-, and 1.88-times of the
corresponding reference mean load for T-G_5 (4xd12) and T-G_6
(6xd10), respectively. Due to the almost same amount of the rein­
forcement, the difference can be only explained that with six rebars
more struts can form resulting in slightly higher capacity. This behavior
highlights two aspects: (i) not only the amount of the reinforcement, but
also their arrangement have an influence of the load-carrying capacity of
the anchorages failing due to strut failure, and (ii) the strut failure de­
pends not only on the geometry of the anchorage and reinforcement but
also on the possible number of struts that can be activated to transfer the
applied load from the anchorage.
analyzed on the basis of the test results with constant distance between
anchor and post-installed supplementary reinforcement. The normal­
ized increase factors in the load carrying capacity for the anchor groups
tested are plotted in Fig. 18 as a function of the cross-section area (As,re)
of the post-installed reinforcement for single anchor tests (left) and for
anchor groups (right). The data plotted includes only the tests with 24
mm threaded rods with hef = 140 mm the distance between anchor and
reinforcement as a = 50 mm.
In both the diagrams it can be clearly seen that initially (for low
amount of reinforcement), the rate of increase of the failure load is
stronger than for higher amounts of reinforcement. Until a certain
amount of reinforcement, the failure is governed by yielding of the postinstalled reinforcement and therefore with increasing reinforcement, a
strong increase in the capacity of the anchorage is obtained. However,
beyond this certain amount of reinforcement, further increase in the
capacity is rather low because from this point onward, the failure is
governed by the strut failure. As shown earlier, the strut failure depends
not on the amount of the reinforcement, but rather on the position of the
reinforcement to the anchor (a, hef), which define the angle of the strut.
It is interesting to note that in the single anchor tests (Fig. 18 left
side), even after reaching the strut failure (As,re ~ 200 mm2), the ca­
pacity of the anchorage further increased with increasing amount of
reinforcement. This increase is attributed to not the cross-sectional area
of the reinforcement, but to the larger number of bars that result in more
struts being formed. In case of anchor groups, the limit of the change in
the failure mode from yielding of the reinforcement to strut failure was
7. Discussion
7.1. Influence of the amount of supplementary reinforcement
The influence of the amount of supplementary reinforcement is
Fig. 17. Load-displacement curves from tension tests on 1x2 anchor groups without and with post-installed supplementary reinforcement, ds,re = 10 mm T-G_6 (1x2M24 + 6xd10)and typical failure picture.
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Engineering Structures 252 (2022) 113754
Fig. 18. Load increase factor depending on the cross-section area of the reinforcement for constant distance between anchor and reinforcement.
reached at approx. As,re = 300 mm2 with 6xd8 reinforcing bars where
the increase in the load capacity 1.76-times the peak load obtained in
the reference test series on anchor groups. On further increasing the
amount of reinforcement with 4xd12, 6xd10 and 6xd12 bars (As,
2
re = 452, 471 and 679 mm respectively), led to 1.70, 1.79 and 1.80times higher capacity than the corresponding reference load. This in­
dicates that if the failure mode is governed by strut failure, the increase
in capacity does not depend directly on the amount of the reinforcement
but on the number of possible struts and the angle of the strut. This will
be further discussed with more examples in the next section.
As mentioned earlier, the presence of the post-installed supplemen­
tary reinforcement results in an increase of not only the ultimate load
but also the displacement at the ultimate load. This can be clearly seen
from Fig. 19, where the relatively increase of the mean displacements at
the ultimate load obtained for the tests with reinforcement is given with
respect to the mean displacement at ultimate load obtained from the
reference tests, as a function of the amount of the reinforcement are
given. With a relatively moderate amount of reinforcement (2xd12) it is
possible to achieve 2,5-times higher displacements than without
reinforcement.
7.2. Influence of the number and arrangement of rebar
The influence of the number of the rebars or struts on the increase in
the load carrying capacity of the anchors can be clearly seen in Fig. 20
where the relative increase factor is plotted as a function of the number
of reinforcing bars. To compare, the results of the test Series T-S_3.4, TS_3.5 and T-S_3.6a where the amount of the reinforcement used was
1xd16, 4xd8 and 2xd12 rebars, respectively are plotted. Note that in all
these three test series, the total cross-section area of the reinforcement
can be considered as practically equal. However, as the number of bars
(and hence the number of struts) increase, the strut failure load also
increased.
As shown in Fig. 21, if the anchor is reinforced with one reinforcing
bar each on both sides arranged symmetrically outside the anchorage
(Type 1), the struts will be active only in the plane of the reinforcing bars
(two struts). These struts impede the spread of the concrete cone
breakout body and in such case, the possible failure mode in Section A-A
(with supplementary reinforcement) and in Section B-B (rectangular
without suppl. reinforcement) would be different resulting on the con­
crete surface in a dog bone shape breakout body, see Top View in Fig. 21,
Type 1.
If additional post-installed rebars are installed in the B-B section, (see
Fig. 21, Type 2), a symmetric and more stable strut mechanism can be
mobilized compared to that for Type 1. In this case, the struts are active
Fig. 19. Relative increase in the mean displacement at peak load for tests with
reinforcement in respect to the mean displacement at peak load for reference
tests depending on the cross-section area of the reinforcement for constant
distance between anchor and reinforcement.
Fig. 20. Increase factors as a function of the number of rebars.
14
N. Vita et al.
Engineering Structures 252 (2022) 113754
Fig. 21. Consideration of strut formation in case of anchors with supplementary reinforcement.
in both orthogonal planes Section A-A and B-B, resulting in four struts
(two in each orthogonal direction). In this case, the struts will restrict
the formation of the concrete cone breakout body in both the directions
and no unrestricted concrete cone breakout is possible. In this case, a
smaller, circular concrete breakout can form on the concrete surface
only inside from the reinforcements (as shown in Fig. 21, Type 2, top
view).
In general, if the number of post installed reinforcing bars is
increased, more struts can form. Since the angle of the strut depends on
the ratio of the distance of the rebar from the anchor and the embedment
depth of the anchor, the bars placed closer to the anchor would result in
higher resistance to the applied load by the strut action. In principle, a
symmetric strut formation (symmetric arrangement of rebars) is
considered to provide higher resistance compared to unsymmetric strut
formation for otherwise same number of struts.
contribution of concrete against the applied tension force was obtained
by deducting the total tension force taken up by all the rebars from the
total applied load.
Fig. 22 shows the evaluated results of tension tests on single anchors
with 2xd8 and 2xd12 rebars (Series T-S_3.2 Test No.3 and T-S_3.6a Test
No.3) to show the separate contribution of concrete and reinforcement
towards tension resistance. The figure plots the total applied load on the
anchor, the tensile forces in the two rebars, the total force taken up by
the reinforcement and the calculated contribution for concrete, as a
function of the anchor displacement. In both the tests, at the beginning,
the curve of the applied load and the curve of the force carried by
concrete are identical, because the reinforcing bars are not activated and
therefore the total load is taken up by concrete. With increasing the
applied load, once the applied load reaches almost the value of the
reference failure load (without reinforcement), the concrete cracks cross
the rebars, which starts to take up the load significantly while the
contribution of concrete gradually drops down. In case of 2xd8 rebars
(Fig. 22, left), the reinforcing bars reached the yield at the peak of
applied load, while for with higher amount of reinforcement (2xd12)
(Fig. 22, right), at the peak load, the reinforcing bars did not yield and
the failure was governed by strut failure.
Nevertheless, in both the cases it can be seen that at peak load, both
the reinforcement as well as concrete contribute significantly towards
the tension resistance of the anchor. This observation is similar to the
behavior observed and reported by Sharma et al. [23–25] for anchorages
with cast-in headed studs reinforced with cast-in supplementary rein­
forcement. This aspect will be discussed and used for the development of
an analytical model to calculate the tension resistance of post-installed
anchors with post-installed supplementary reinforcement, which will
be presented in another paper.
7.3. Individual contribution of concrete and reinforcement towards
tension resistance of the anchorage
To separate the contribution of the concrete and reinforcement in the
load-bearing behavior and load transfer mechanism for post-installed
anchorages with post-installed supplementary reinforcement, in
certain tests strain gauges were installed on the post-installed supple­
mentary reinforcement to measure the strains developing in the rebars.
Two strain gauges were attached (glued) on the reinforcement, where
the theoretical crack should intersect the rebars during the tension tests.
The strain measured in the reinforcement were converted into the cor­
responding stress in the rebar assuming an elastic-perfectly plastic
behavior with modulus of elasticity of steel as 200 GPa and mean yield
stress as 550 MPa (1.1 times the specified characteristic yield stress).
The stress was converted into tensile force taken up by the rebar by
multiplying the stress with the cross-section area of the rebars. The
15
N. Vita et al.
Engineering Structures 252 (2022) 113754
Fig. 22. Separated contribution of reinforcement and concrete to tension resistance for tests on single anchors with 2xd8 (left) and 2xd12 (right).
8. Conclusions and further work
well as the concrete contribute significantly towards the resistance of
the anchorage with reinforcement.
7) Based on the detailed evaluation of the test results, an analytical
model is developed by the authors for the calculation of resistance of
post-installed bonded anchors with post-installed supplementary
reinforcement. This model will be presented later in another paper.
In this work, the influence of post-installed supplementary rein­
forcement on the behavior of anchorages with bonded anchors under
tension loads was investigated. The test program included unconfined
tension tests on single anchors and on 1x2 anchor groups without (as
reference) and with post-installed reinforcement as a supplementary
reinforcement. Based on the test results, the following conclusions can
be summarized:
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
1) The test results on bonded anchorages (single and group) shows
clearly the positive influence of the post-installed reinforcing bars
installed relatively close to the anchors. Even with a relatively small
amount of reinforcement, a strong enhancement in the load-bearing
behavior of anchorages is obtained both in terms of failure load as
well as a relatively ductile behavior.
2) The pilot tests showed that the influence of pre-loading, if any, up to
75% of the ultimate capacity of the anchorage without reinforcement
is negligible on the performance of the anchorage strengthened with
supplementary reinforcement. Therefore, post-installed supplemen­
tary reinforcement can be used also for strengthening of existing
anchorages in the same way as in the design of a new anchorage.
3) By increasing the amount of the supplementary reinforcement, the
relative increase in the ultimate load increases proportionally with
the amount of the reinforcement up to a certain amount of rein­
forcement that results in yielding of reinforcement. Once the failure
mode starts being governed by the strut failure, the increase in the
failure load with increasing amount of reinforcement is not so sig­
nificant and only a moderate increase with increasing number of
struts is observed.
4) Tests with supplementary reinforcement placed at a relatively large
distance to the anchor (a = 0.75∙hef) have shown that even with a
relatively small bond length of the post-installed reinforcing bars in
the concrete cone breakout body, strut failure can be reached instead
of pull-out of the reinforcement. This is due to the high bond strength
of the epoxy mortar used for post-installed rebar. Due to this, even
smaller bond lengths can be used for post-installed rebar compared
to the cast-in rebar.
5) In case of strut failure, besides the influence of the relative position of
the reinforcement to the anchorages (a / hef) that define the angle of
the strut, the number of rebars and thus the number of possible
compression struts have an influence on the capacity of anchorages
with supplementary reinforcement.
6) The evaluation of tension tests performed with strain gauges showed
that when the ultimate load is reached, both the reinforcement as
Acknowledgement
The experimental investigations presented in this paper were
financially supported by fischerwerke GmbH & Co. KG. The support
received from fischerwerke is greatly acknowledged. The opinions,
findings, and conclusions or recommendations expressed in this publi­
cation are those of the authors and do not necessarily correspond with
those of the sponsoring organization.
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