Load-Carrying Performance and Hydrostatic Tests of Encapsulated Anchor Systems for Unbonded Post-Tensioning Single-Strands
International Journal of Concrete Structures and Materials
Load-Carrying Performance and Hydrostatic Tests of Encapsulated Anchor Systems for Unbonded Post-Tensioning Single-Strands
Ah Sir Cho 0
Thomas H.-K. Kang 0
0 Department of Architecture & Architectural Engineering, Seoul National University , Seoul , South Korea
ACI 318-14 and 423.7-14 require the use of encapsulation system for an unbonded single-strand tendon for the purpose of corrosion protection and enhanced durability. In this study, load-carrying performance and hydrostatic tests were conducted for a newly developed, unbonded post-tensioning (PT) anchor with encapsulation, following the guidelines put forth in KCI-PS101 and ACI 423.7-14. The static load and fatigue tests as part of this study were used to evaluate the anchorage system behavior under static and cyclic loads, respectively. The hydrostatic test was used to demonstrate that the encapsulation system withstood at least 9.3 kPa of water pressure in a 24 h period according to ACI 423.7-14, and thus has been proven to promote the durability of unbonded PT single-strand tendons. Additionally, the push-in and jacking tests were used to assess the compressive and tensile strengths of the anchor as used in the field. Although these tests were not performed to failure for safety reasons, the anchor endured at least 1.18 and 1.02 times the nominal ultimate strength of a strand in each test, respectively. The authors' previous load transfer tests were also reported in this paper, showing that test specimens with only two horizontal backup bars were capable of resisting at least 1.7 times the nominal ultimate strength of a strand. Based on the favorable results from the loadcarrying performance and hydrostatic tests, the developed encapsulated anchor systems are being applied in actual practice as an economically feasible option in Korean PT industry and are expected to improve the corrosion resistance and durability of posttensioned structures.
post-tensioned anchor; encapsulation; corrosion; durability; unbonded; performance test; static load test; fatigue test; hydrostatic test
The post-tensioning (PT) method provides a
pre-compressive force in concrete by jacking high-strength steel
strands after the concrete hardens
(Kang et al. 2009b; Yang
and Kang 2011; Lee et al. 2016)
. This method is mainly used
for floor slabs/beams in buildings, bridge girders and
containment structures, because it enables reduction in the
member volume, providing greater control over deflections
and providing essential confinement. Given both the
structural and regulatory limitations on slab thickness, a
singlestrand PT tendon system is commonly used for office and
residential buildings. An unbonded single-strand tendon
consists of a seven wire strand that is coated with grease and
high-density polyethylene (HDPE), post-tensioned anchors
at both ends, and encapsulated systems and other
accessories. Note that the sheathing of extruded single-strand
tendons should be made of high-density polyethylene
(HDPE) or polypropylene (HDPP); however, the material for
plastic coating to protect the anchors from corrosion is not
specified in ACI 423.7-14. For the post-tensioned anchors
used in this study, linear low-density polyethylene (LLDPE)
coating with 2 mm thickness on average is applied. The
benefits of using such unbonded PT systems include easier
installation of strands and no need for grouting.
Since the adoption of ACI 423.7-14
Committee 423 2014)
in ACI 318-14
(ACI Committee 318
, the encapsulation of a single-strand PT tendon is
required for elevated floors in the United States. Given the
condition that PT strands are not bonded to or covered by
concrete, the use of encapsulated anchor systems is much
needed for the durability of the strand. In this paper, both
load-carrying performance and hydrostatic tests of unbonded
PT anchors are conducted in accordance with existing
specifications or industry/manufacturer standards. The tested
encapsulated single-strand anchor was developed by the
authors for the first time in Korea, which would provide cost
effectiveness and practical feasibility compared to imported
anchors. Load-carrying capacity, sustainability, and
durability of concrete structures with a variety of materials/
products are important considerations and should be
(Kang et al. 2009a, 2011, 2014; Yang
et al. 2014)
. The durability design requirements for
reinforced and post-tensioned concrete structures are becoming
more stringent in the 21st century than a few past decades.
The current experimental study is expected to contribute to
PT concrete industry in this aspect.
2. Encapsulated Anchor System
The authors conducted static load and load transfer tests
for their developed post-tensioned single-strand anchor
et al. 2015)
. The authors also performed the load transfer
tests for three different anchorages with respect to anchor
geometry and reinforcement detailing in the anchorage zone
(Cho and Kang 2017)
conducted fatigue testing
of post-tensioned anchors and couplers for 12.7 mm
diameter single-strand tendons, and
Walsh and Kurama (2009)
performed static load and fatigue tests of bare anchors and
barrel anchors for both 12.7 and 15.2 mm diameter strands.
Both institutions followed the testing procedure and
acceptance criteria of AC303
, and their test results
met the criteria. However, these research programs tested a
bare (or barrel) anchor for all performance tests and did not
include hydrostatic tests, as it was not a corrosion-protective
encapsulated system. Previously,
Hayes Industries Ltd.
) performed hydrostatic tests of encapsulated anchor
systems under 11.0 kPa for 24 h using a PVC hydrostatic
test chamber in accordance with Section 2.2.6 of PTI
, which is similar to
Sections 9.3(a) and 9.6 of ACI 423.7-14.
similar hydrostatic test but under more severe condition, that
is, under 68.8 kPa for 24 h seemingly in accordance with
Sections 9.3(b) and 9.6 of ACI 423.7-14 (though the report
indicated that it was tested according to Section 2.6.8 of ACI
(Joint ACI-ASCE Committee 423 2007)
has only one hydrostatic pressure of 8.6 kPa). Both
hydrostatic tests revealed that the encapsulation systems satisfied
the criteria with no water leak.
To prevent contact with water/moisture, a bare anchor is
coated and covered by coating of LLDPE plastic materials
(i.e., encapsulated). Figure 1 shows an encapsulated anchor
that has been developed by the authors in Korea, and is
being manufactured and supplied for use in several building
projects. The sleeve is plastic-molded together with the
plastic cover of an anchor as an integrated encapsulation.
The tightness of an end cap should also be checked against
moisture intrusion. In this study, both performance and
hydrostatic tests are carried out for the developed
encapsulated anchor systems with 15.2 mm diameter 7-wire strands
to evaluate their corrosion resistance and load-carrying
3. Test Methods
3.1 Static Load Test
A series of static load tests were carried out to assess the
strength of an anchor and evaluate its behavior, as well as
that of the wedge, strand, and strand-wedge connection. The
test was conducted according to the Korean specification,
KCI-PS 101, which is a part of the Appendix of
This is similar to ETAG 013
. After pre-jacking
at about 10 to 15% of the minimum ultimate tensile strength
(MUTS), displacement gauges were installed. Then, tensile
loading was increased up to 80% of the MUTS of the strand.
Three specimens (SLT1 * 3) were tested. To control both
load and displacement, the live-end anchor was set onto a
steel frame, which was moved by an actuator, rather than a
jacking device (Fig. 2a). LVDT 01 * 04 (Fig. 2b)
measured the displacement of an anchor plate, core (king) wire,
one of the six helical (perimeter) wires, and one set of the
wedges. The free length of the strand was 3500 mm. For
SLT3, LVDT 05 and LVDT 06 (Fig. 2c) were added to
measure the elongation in the middle of the strand, thereby
satisfying stipulations in ACI 423.7
Committee 423 2014)
After pre-jacking, displacement measurements confirmed
that the load increased at a rate of 100 MPa/min. up to 80%
of MUTS. During the first half hour of testing under the 80%
of MUTS, the wedge and strand were settled into the anchor
and stabilized. For safety, the loading was stopped just
before the fracture of the strand. Only the test for SLT3 was
carried out until the strand fractured.
3.2 Fatigue Test
A fatigue test was performed to evaluate the anchorage
performance during fatigue or under cyclic loading (Fig. 3).
Unlike in bridges, the fatigue in buildings has a negligible
effect on a tendon
; nonetheless, fatigue testing is
required by KCI-PS101
for unbonded tendons.
The testing methodology in this study followed in
accordance with KCI-PS101. Because it was not easy to develop a
jig to hold the plastic-coated anchor, it was tested using bare
The strand was inserted into the anchor with a wedge, and
each anchor was fixed to the steel plates. Both ends were
fixed with hinges so that the longitudinal axis was
maintained parallel to the ground plane when the strand was
elongated. The initial free length of the specimen was
1000 mm. One end was fixed, with repeated loading applied
using an actuator at the other end. The repeated loads
between 160 and 171 kN were applied at 3 Hz for 2,000,000
3.3 Hydrostatic Test
Hydrostatic tests were performed to affirm watertight
efficacy for encapsulation systems. This testing
methodology was applied, exactly meeting the specifications of
Section 9.3 of ACI 423.7-14. Instead of a wedge, a white tissue
was placed inside the anchor cavity, which was covered by
an end cap. The end of the sleeve was sealed with tape as
used in the field. A dye was added to the water to examine
whether the water entered into the anchor cavity after the
experiment. For 24 h, four encapsulated anchor specimens
(Fig. 4a) were placed in a hydrostatic pressure chamber
(Fig. 4b), with a hydrostatic pressure of over 8.6 kPa.
3.4 Push-in Test
Similar to the wedge plate test specified by Sections 4.1.1
and 4.1.2 of PTI M50.1-98
, push-in tests were
performed to evaluate the strength of an anchor when a
twopiece wedge was pushed in, with and without an offset or
gap (Dg) between the two wedge pieces, as shown in Fig. 5.
To simulate the actual prestressing force of a tendon, a strand
was inserted into an anchor and gripped by a two-piece
wedge Then the load was applied to the strand, not on the
wedge. This test can be easily carried out without jacking or
pulling, and is suitable for measuring the maximum capacity
of an anchor system itself with actual wedge pieces seated
A total of six specimens were tested, including one
specimen of a non-encapsulated bare anchor. A wooden
board was used to support the bare anchor against a steel
block, due to its uneven front surface of the bearing plate.
Three of the encapsulated anchors were tested with a gap Dg
between the two pieces of the wedge, akin to a poor
condition that may occur in the actual construction field
(Fig. 5a). The test setup is shown in Fig. 5b. The load was
applied until the anchor fractured, which was not easy to
perform during the load transfer test as the concrete test
block failed first.
3.5 Jacking Test
A jacking test is a simple experiment which physically
simulates the post-tensioning process. As shown in Fig. 6,
an anchor was fixed to a steel block, and the strand was
jacked from the other side. The jacking force was measured
by a load cell installed between the live-end anchor and the
jack. This setup is similar to that used in precast plants.
4. Test Results and Discussion
4.1 Static Load Test
For all specimens, the load of 80% of the MUTS was
maintained for 1 h, after which the load was increased up to
over 260 kN (Fig. 7). Only the SLT3 specimen was tested up
to strand fracture; however, the LVDTs were removed after
1 hour to adhere to equipment safety guidelines. The
measured maximum loads (268.5, 266.6 and 270.5 kN) exceeded
100% (260 kN) of the MUTS for all specimens, indicating
that the load-carrying capacity has been confirmed.
The anchor slip displacements (suction) of the wedge, core
wire, and helical wire were calibrated by subtracting the
displacement of the anchor plate from the total measured
value. At 80% of the MUTS, the anchorage slip
displacement was only approximately 3 mm, which was maintained
with little variation. According to the EOTA document
, the slip displacement at a fixed end (i.e.,
wedge draw-in) should be in the range of 3–5 mm. This
result demonstrated the stable performance of the tendon
The partial elongation in the middle of the SLT3 specimen
was obtained using the difference between the displacements
of LVDT 05 and LVDT 06. The measured portion was much
close to the fixed end location. The partial elongation in the
middle was very small because the measured original length
was only a portion (26%) of the total free length.
The relationship between the elongation percentage
(strain) and stress is plotted in Fig. 8. The elongation
percentage was increased rapidly to over 3% beyond
1800 MPa. In the STL3 specimen, fracture occurred at about
4% elongation. The elongation percentage results
demonstrate the sufficient ductility of the tested strands prior to
facture and satisfatory performance of the tested tendon
assemblies. The partial elongation for the STL3 specimen
was approximately 75% of the total elongation, as the stress–
strain curve in Fig. 8 was located lower than the rest of the
Fig. 8 Relationship between elongation percentage and
curves. This is a new finding that the tensile strain increased
more at the jacking end than at the fixed end. See
Fig. 2(a) where the measured portion between the LVDT 05
and LVDT 06 was located much closer to the fixed end, and
note that the LVDT 05 and LVDT 06 were removed after one
hour holding period.
4.2 Fatigue Test
The fatigue load test, also called the high cycle dynamic
tensile test, was performed over more than 7 days to
generate 2,000,000 loading cycles. The KCI-PS101
specifications, which are the same as those of ETAG 013, were used
as a testing protocol. The applied cyclic loads are shown in
Fig. 9. Since the actuator was operated by a hydraulic pump,
there were slight variations in load amplitude. Despite that,
the overall loading range was steadily maintained as
specified by KCI-PS101 or ETAG 013 throughout 7 days and
beyond. The data from the day four to seven were not
obtained due to the equipment’s recording storage problem.
No cracking of steel was observed in the anchors and
wedges after the completion of the test (Fig. 10). No fracture
of steel also has occurred, demonstrating the satisfactory
fatigue behavior of the tested tendon assemblies. This was
based on the third party’s visual observation in accordance
with KCI-PS101 or ETAG 013. It is noted that the fatigue
test was conducted in an independent commercial testing
laboratory. The long-term service life is related to the fatigue
behavior. Therefore, the data herein will help increase
practical applications of the developed encapsulated anchor
4.3 Hydrostatic Test
The hydraulic pressure in the equipment was maintained
between 9.3 and 9.8 kPa for 24 h, as shown in Fig. 11a,
which exceeded the minimum ACI 423-specified pressure of
8.6 kPa. Even though the pressure was attempted to remain
constant, there was a modest variation during 24 h. After the
hydrostatic test for 24 h, all the moisture on the outside
surface of the anchor was removed, and the fastened end cap
was opened (unfastened by hand). No paint or water was
observed as the white tissues placed inside the cavity were
not affected by the paint or internal moisture at all
(Fig. 11b). Next, plastic accessories were disassembled, and
it was re-confirmed that there was no water intrusion in the
anchorage system. Because the use of encapsulated PT
systems is now required in the U.S. for all elevated
unbonded PT floors and would possibly be required even for
slabson-ground in the U.S. or for PT construction in Korea soon,
the test results in this study will help provide viable options
for those in need and promote application of more durable
PT systems. Because of the superior watertightness of
encapsulation systems and low probability of damaging the
coating during transportation, plastic encapsulation is only
permitted by ACI 423.7-14 (epoxy coating is no longer
4.4 Push-in Test
The results of the push-in tests are shown in Fig. 12 and
Table 1. Some of the specimens were not pushed towards
failure due to safety concerns. Even so, the measured
maximum values exceeded the nominal ultimate strength of a
strand (260 kN) by far. It was possible because this test had
nothing to do with strand performance, as the pushing force
was applied up to or almost up to failure of the cast-iron
anchor itself. The large slip in the bare anchor was due to the
squeeze put on the wooden board. The encapsulated anchors
(P1 * 5) also exhibited small compressive slips due to the
squeezed plastic cover.
Specimens P3, P4, and P5 had staggered wedges, and two
pieces of the wedges had an average step height Dg of
7.32 mm. After the test, the intentionally given offset Dg
was decreased to an average of 6.28 mm. This reduction of
1.04 mm on average was due to additional slip of one wedge
piece while the wedges were set inside the cavity.
4.5 Jacking Test
The results of jacking tests are summarized in Table 2.
Because of limitations in the jacking device, the strand was
not pulled up to fracture and stopped after exceeding the
MUTS. The average of all maximum loads was 264.8 kN,
approximately 2.6% greater than the nominal tensile force
(260 kN) of a 15.2 mm diameter seven wire strand. On
average, the pre-jacking load was 27.8 kN, about 10.5% of
Fig. 10 Bare anchor and wedge after fatigue testing in accordance with KCI-P101 and ETAG 013. a Bare single-strand anchor
and b two-piece wedge.
the maximum load strength or 10.8% of the nominal tensile
force of the strand. This was done to remove any slack prior
to full jacking. The jacking test results re-confirmed that the
developed encapsulated anchor itself has sufficient
5. Load Transfer Test
Cho et al. (2015)
previously conducted load transfer tests
of the same bare anchors according to KCI-PS101.
Horizontal backup bars (HB) and hairpin bars (HP) were used as
independent variables. Table 3 summarizes the load transfer
test results, with more details provided in the reference
et al. 2015)
. The values indicate the maximum compressive
force divided by the nominal tensile strength of strands.
With basic reinforcement (provided for all specimens as
specified by KCI-PS101), the specimen without HB or HP
could register 1.64 times its MUTS. In the case of
specimens with two horizontal backup reinforcement, the ratio
ranges from 1.7 to 1.89, with a concrete compressive
strength of 20 to 21.5 MPa. The concrete test block
specimen included the single-strand anchor, duct and anchorage
zone reinforcement (or not), as well as basic reinforcement
that simulates the continuity of actual members, such that
the load transfer and interaction between the anchor and
concrete/anchorage zone reinforcement is verified. The
authors’ previously reported results were consistent with the
, proving that the bearing
plate size and shape were adequate even without anchorage
6. Applications to Real Buildings
The developed encapsulated post-tensioned anchor
systems have been used for several real building projects
including residential, office and church buildings in Korea,
which have been recently completed or currently under
construction. Figure 13 shows an actual building project,
where the developed systems were applied for the first time
in Korea. To the authors’ knowledge, the developed bare
anchor was also the first Korean anchor for a single-strand
tendon, as other anchors were either imported products or
under other countries’ license. Not to mention that it is the
first Korean encapsulated system. The ACI 423.7-14
requirement for the use of encapsulated anchor systems is
not yet required in Korea or by Korean Building Code
(Architectural Institute of Korea 2016)
. However, because
there is a growing demand to improve the durability and
sustainability of PT structures with unbonded single-strand
tendons, such corrosion-protective encapsulated anchor
systems would be much needed in the near future in the
region of East Asia.
In this study, load-carrying performance and hydrostatic
tests of the developed encapsulation anchor were
undertaken according to the Korean and U.S. testing standards.
Static load and fatigue tests were used to evaluate the
loadcarrying performance of the tendon assembly and
mechanical interactions between the wedge and strand or
anchor. The stable behavior was verified under static load
and cyclic load. The anchor slip was only approximately
3 mm, which was about half the typical wedge slip of 6 to
9 mm. The fracture of the strand occurred at 4%
elongation, which is considered to be quite large. The
waterproofness of the encapsulation system was confirmed via a
In addition, a push-in test served as a compressive test of
the anchor casting itself, while a jacking test functioned as a
tensile test as used in the field. In the push-in test, the
loading procedure was stopped for some specimens due to
safety concerns, but all the anchors endured at least 1.18
times the nominal ultimate strength of the strand before the
test was stopped or the anchor failed. The jacking test
reflected the real-world condition, but there was no strand
fracture under the maximum loading force of the jacking
device, which was larger than the minimum ultimate tensile
In the previous load transfer tests, the developed anchor
could endure at least 1.7 times the nominal ultimate strength
of the strand, with only two horizontal backup bars, at the
specified concrete strength for jacking of about 21 MPa.
Furthermore, the specimen without anchorage zone
reinforcement resisted at least 1.64 times the nominal ultimate
strength of the strand.
The test results in this study demonstrated both the
loadcarrying and waterproofness performance of the developed
encapsulated anchor system and would be expected to
promote the durability of unbonded post-tensioning
The work was funded by the National Research Foundation
of Korea (2015-055508) and the Institute of Construction
and Environmental Engineering of Seoul National
University. Partial support from Samsung E&T for push-in and
jacking tests of KFA (Korea’s First Anchor) is also
acknowledged. Particularly, Y. W. Cho, B. K. Jeon, Y.
N. Kim and J. K. Choi are sincerely appreciated for their
assistance. The views expressed are those of authors, and do
not necessarily represent those of the sponsors.
This article is distributed under the terms of the Creative
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author(s) and the source, provide a link to the Creative
Commons license, and indicate if changes were made.
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