Axial Load Testing of Hybrid Concrete Columns Consisting of UHPFRC Tube and Normal-Strength Concrete Core
International Journal of Concrete Structures and Materials
Axial Load Testing of Hybrid Concrete Columns Consisting of UHPFRC Tube and Normal-Strength Concrete Core
Thomas H.-K. Kang
Issa Brown Mpalla
To investigate the axial load behavior of hybrid concrete columns consisting of an ultra-high performance fiberreinforced concrete tube (20 mm thick, 92.6 MPa or 143.3 MPa) and normal-strength concrete core (28.2 MPa), concentric axial load tests were performed for five column specimens. The UHPFRC tube can function as a form during core-filling and as a cover having high performance at service and ultimate load conditions. Axial loading was applied to the core only, and the tube was indirectly loaded by bearing of transverse threaded bars. Test parameters included the volume fraction of steel fibers, volumetric ratio of transverse bars, and usage of wire-mesh in the tube. Test results showed that structural performance such as initial stiffness, peak load, displacement ductility, and energy absorption capacity varied greatly according to each test parameter. During the course of axial loading applied on the core only, the axial load behavior of the hybrid concrete columns generally corresponded to the full composite action at the initial loading stage and then changed to that of non-composite action (core only) after the failure of the threaded bars.
hybrid concrete column; UHPFRC tube; normal-strength concrete core; concentric axial load test
Ultra-high performance fiber-reinforced concrete
(UHPFRC) is a type of fiber-reinforced cementitious
composites having advantages of very high strength (both in
compression and tension), toughness, and durability
and Monteiro 2006)
. The use of UHPFRC has gained
widespread acceptance in the construction industry over the
past two decades. Although there are still competing
approaches to obtain an optimized mechanical behavior
(Mehta and Monteiro 2006)
, according to the French
Association for Civil Engineering (AFGC)
et al. 2000; Toutlemonde and Resplendino 2011)
with a compressive strength of 150 MPa or greater is now
commercially available by careful selection/control of
material compositions, mixing methods, equipment, placing
procedures, and curing process.
As a construction material, UHPFRC has a high potential
for newer applications, because of the following superior
performance under harsh environments: high impermeability
against moisture or other chemicals, high mechanical
resistance to corrosion and abrasion, and high physical resistance
(Mirmiran and Mohsen 1997; Foster and Mario
2001; Neves and Fernandes de Almeida 2005; Pimienta and
Chanvillard 2005; Mehta and Monteiro 2006; Fehling et al.
2008; Paultre et al. 2010; Toutlemonde and Resplendino
2011; Nematollahi et al. 2012; Park et al. 2016)
. This is
possible because of the dense microstructure associated with
steel fibers. Further advantages of UHPFRC include the low
probability of cracking, high modulus of elasticity, early
removal of concrete formwork, and high load-carrying and
energy absorption capacity.
One common application of UHPFRC is found in vertical
elements of tall buildings, because UHPFRC enables columns
to have a smaller cross-section. Using the hybrid application of
placing UHPFRC only in the perimeter of a cross-section is
relatively economical. Having different strength concretes for
the cover and core is structurally efficient and cost-effective.
Concrete cover provides the protection of structural steel and
reinforcement against corrosion and fire (ACI 318-14 2014),
but early spalling of the concrete cover may occur due to
shrinkage effects and weakness of planes between the concrete
cover and core (bounded by longitudinal and transverse bars)
(Collins et al. 1993; Cusson and Paultre 1994)
phenomenon becomes more obvious when higher strength
concrete and denser reinforcement are used
(Collins et al. 1993;
Cusson and Paultre 1994)
. In particular, the confinement effect
of transverse bars, which is critical in the design of columns, is
not fully developed until a column is subjected to sufficient
compression and deformation, and the large compression and
deformation lead to cover-spalling (ACI 318-14). However, in
the case of using UHPFRC for the cover, the cover-spalling
can be much retarded. Thus, the UHPFRC cover (or tube),
which can also act as a light and durable permanent form, can
improve the efficiencies of hybrid concrete columns (HCC).
Similar types of columns can be seen in the strengthening of
(Wang and Lee 2007; Tsonos 2009; Koo
et al. 2016)
and in the construction of concrete-filled hollow
precast concrete columns
(Kim et al. 2016, 2017)
(Fig. 1). The
perimeter tube section could be either loaded (directly or
indirectly) or unloaded (confinement only) depending on the
details of tube-core interfaces. Although there are several
existing studies on the behavior of UHPFRC columns, in most
of them UHPFRC is used for the whole section and only a few
studies are available for hybrid UHPFRC columns.
In the present study, a thin tube (20 mm) of UHPFRC was
used as the perimeter cover, which was indirectly loaded by
bearing of transverse threaded bars, and the core was filled
with normal-strength concrete (NSC), which was directly
loaded. To investigate the effect of the UHPFRC tube with
various parameters, concentric axial load tests were
performed for five HCC columns.
2. Test Plan
2.1 Test Specimens
To evaluate structural performance and applicability,
concentric axial load tests were performed for five HCC
specimens consisting of a UHPFRC tube and NSC core. Test
parameters included the volume fraction (Vf) of steel fibers,
volumetric ratio ðqstÞ of transverse bars, and usage of
wiremesh. Figure 2 shows the configuration and dimensions, and
Table 1 summarizes the test parameters.
All the column specimens had a square cross-section of
400 9 400 mm and a length of 1200 mm. The wall
thickness of the UHPFRC tube was 20 mm, and four 22
mmdiameter deformed bars were used for longitudinal
reinforcement (reinforcement ratio q ¼ 1%). The UHPFRC tube
of each specimen was built with a particular set of the test
parameters. In the control specimen HCC1, the fiber volume
fraction was Vf = 1.5%, 10 mm-diameter transverse
deformed bars were provided at a vertical spacing of
s = 100 mm (qst = 2.52%), and 8 mm-aperture wire-mesh
(woven by 0.8 mm-diameter wires of ordinary low carbon
steel Q235) was used to improve the structural integrity of
four sides of the UHPFRC tube. Next, Vf was increased to
2.0% in HCC2, while s was increased to 200 mm
(qst = 1.26%) in HCC3. In HCC4, both Vf and s were
increased to 2.0% and 200 mm, respectively, to investigate
the combined effect of the greater fiber fraction and lower
transverse bar ratio. HCC5 was the same as HCC1, except
that wire-mesh was not used. For all specimens, 16
mmdiameter threaded bars were placed at a spacing of 200 mm
before the core of the UHPFRC tube was filled with NSC.
Since the threaded bars can act as form-ties for the
thinwalled tube during core-filling, the spacing was determined
(Kim et al. 2016)
threaded bars (embedded through the UHPFRC tube and
NSC core) also can act as shear-keys for bond between the
UHPFRC tube and NSC core when subjected to load. The
transverse bars and threaded bars were more closely spaced
at column ends to prevent local failure that may occur
accidentally during testing.
2.2 Material Properties
2.2.1 Concrete Mixes and Mechanical Properties
For UHPFRC, Grade 42.5 ordinary Portland cement was
used. Silica fumes were used to partially replace cements,
and silica flours were used as fillers (Shanmugapriya and
: Old Concrete
: New Concrete
Hollow PC Column
UHPFRC (20 mm)
(200 mm for HCC2&4)
(None for HCC5)
Uma 2013). Washed river sands with fractions of 0–2 mm
were used as fine aggregates, and no coarse aggregates (or
gravel) were provided. The aspect ratio (‘f/df, where fiber
length ‘f = 13 mm and fiber diameter df = 0.2 mm) and
tensile strength (ffu) of straight steel fibers were 65 and
2500 MPa, respectively. To achieve workable mixes with
desired quality and strength, super-plasticizer and other
chemical admixtures (expansive agent and deforming agent)
were added in the form of aqueous solution with a small
amount of water.
Table 2 presents the weight of materials per unit volume
(kg/m3) for two UHPFRC mixes. The two UHPFRC mixes
differed in the steel fiber content, and all the materials of
both mixes were treated/controlled under the same
conditions. Since the UHPFRC contained more constituents and
finer particles than NSC and also included short and
highstrength steel fibers, careful mixing was required to achieve
proper workability, particle distribution, and packing density.
In the present study, according to the recommendations of
AFGC (2002) and FHWA-HRT-06-103
fine dry particles were mixed first before adding the water
and chemical admixtures: (1) the cements, silica fumes,
silica flours, and sands were dry-mixed for the first
5–10 min; (2) the water premixed with chemical admixtures
was added slowly and continuously, and then the wet
mixture was mixed again for another 5–10 min; and (3) when
the mortar mixture showed enough fluidity to be workable
and viscose for uniform fiber distribution, steel fibers were
carefully introduced into the mixture by hand.
To investigate the mechanical properties of UHPFRC,
according to GB/T 50081-2002 (2002), JSCE (2008), and
ASTM C109-11 (2011), compression tests for 100 mm
cubes and uniaxial tension tests for wire-meshed and
notched dog-bone specimens (thickness 9 width 9 length of
reduced section = 20 9 150 9 190 mm) were performed.
Figure 3(a) shows the compressive stress–strain relationship
of the cubes. As shown in the figure, the compressive stress
linearly increased up to peak stress. Even though the
postpeak behavior was not fully measured due to damage of
strain-gauges, it was not brittle. The linear-elastic ascending
and non-brittle descending behavior were attributed to finely
graded and tightly packed materials and steel fibers
(Graybeal and Hartmann 2003)
. The average 28 day cube strength
(fcu) and modulus of elasticity (Ec = secant modulus at
aW water, and B binders including cements and silica fumes (silica flours were considered as a filling material).
0.45fcu, ACI 318-14) were 92.6 MPa and 25.7 GPa for the
1.5% Vf UHPFRC, respectively, or 143.3 MPa and 36.5 GPa
for the 2.0% Vf UHPFRC. The strain (eco) at peak stress was
approximately 0.004. The UHPFRC also showed quite large
tensile strength and ductility. As shown in Fig. 3(b), the
tensile behavior was also nearly linear-elastic up to the first
cracking (fct,cr), followed by gradual strain-softening after
reaching peak stress (tensile strength fct = 4.56 MPa and
tensile strain ect = 0.00026 for the 1.5% Vf UHPFRC, or
fct = 6.23 MPa and ect = 0.00020 for the 2.0% Vf UHPFRC).
Such a desirable tensile behavior was achieved by the
interaction of steel fibers in the microscopic level and their
ability to sustain loads after the onset of cracking. It is noted
that the mechanical properties of UHPFRC used in the
present study were lower than those specified in AFGC
(Toutlemonde and Resplendino 2011)
: compressive strength
greater than 150 MPa, elastic modulus around 50 GPa, and
tensile strength higher than 7 MPa. The lower mechanical
properties may be attributed to insufficient mixing and initial
curing before thermal treatment. Further research is required
on the lower mechanical properties.
For the NSC core, ready-mixed concrete was used,
resulting in fcu = 28.2 MPa and Ec = 21.5 GPa.
Direct tension tests for reinforcing bars were carried out
according to ASTM E8-09 (2009). Table 3 summarizes the
mechanical properties of the reinforcement. The modulus of
elasticity (Es), yield strength (fy), and ultimate (tensile)
strength (fu) on average were 218 GPa, 382 MPa, and
537 MPa, respectively, for 10 mm-diameter transverse bars.
For 22 mm-diameter longitudinal bars, Es = 228 GPa, fy =
263 MPa, and fu = 417 MPa. For 16 mm-diameter
threaded bars, Es = 182 GPa, fy = 398 MPa, and fu = 508 MPa.
2.3 Production of Column Specimens
Because the wall thickness of the UHPFRC tube was very
thin, each UHPFRC tube was produced by sequential
concrete placements rotating the section
(Kim et al. 2016)
Figure 4 shows the production sequence: (1) a pair of
UHPFRC panels (Face1 and Face2) were prepared by
casting concrete on the ground (Fig. 4a: except HCC5, a
wiremesh was embedded in each panel, and the inner surface of
each panel was roughened before concrete-setting for better
bond with the third and fourth panels and concrete core); (2)
after rotating the two panels to the opposite faces of the
section (90 , Fig. 4b), concrete was poured for the third
panel (Face3) between the two side panels (Fig. 4c); and (3)
after a 180-degree rotation, concrete was poured for the
fourth panel (Face4) to form a UHPFRC tube (Fig. 4d).
After each production step, the UHPFRC panels were
steam-cured at an elevated temperature of 100 C (total
48 h), and then the completed UHPFRC tubes were kept at
room temperature. About 7 days after the completion, the
UHPFRC tubes were erected and then their cores were filled
with NSC. Transverse, longitudinal, and threaded bars were
placed prior to core-filling.
2.4 Test Setup and Instrumentation
Figure 5 shows the test setup and instrumentation.
Concentric axial load tests were performed using a 5 MN
compression testing machine, with loading applied only to the
NSC core. To this end, a set of rigid steel plates for loading
(same in size with the core) was positioned at column ends.
To avoid any eccentricity, each column specimen was
adjusted in such a way that the center line of axial loading
coincides with the longitudinal axis of the column, and to
ensure the complete contact with the loading plates, the top
and bottom end surfaces of each column specimen were
leveled with a layer of high-strength plaster powder.
The column specimens were tested under both load and
deformation controls: (1) compression loading was applied at
an average rate of 1 kN/sec up to the level of 70% of the
expected peak load; (2) at this point, the loading rate was
changed to 0.5 kN/s until the peak load was reached; (3) in the
softening branch, loading was switched to deformation control
with a rate of 0.2 mm/s; and (4) testing was terminated when
the post-peak load was decreased to 75% of the peak load.
Eight linear variable differential transducers (LVDTs) were
installed to measure relative displacements (during testing,
LVDTs were carefully monitored to prevent any accidental
eccentricity), with four between the top and bottom loading
plates and the other four at the mid height of each specimen.
To measure strains, sixteen strain-gauges were installed on
concrete and transverse bars (Fig. 5; four on mid-height
UHPFRC surfaces, two on UHPFRC joint surfaces, eight on
UHPFRC surfaces beneath embedded threaded bars, and two
on mid-height transverse bars).
3. Test Results
3.1 Failure Mode
Figure 6 shows the failure mode of the column specimens
at the end of testing. As the axial load increased, vertical
cracks developed along the UHPFRC tube joints
(jointcracks between the UHPFRC panels) and gradually grew
wider under further loading. On the other hand, cracks
developing within the UHPFRC panels were relatively
minor, because steel fibers restricted the growth of cracks.
Around the peak load, there were notably loud sounds
associated with localized debonding between the structural
components, followed by the failure of steel fibers and
wiremesh and crushing of concrete. Finally, wide vertical,
horizontal, and/or diagonal cracks were observed in the middle
of the UHPFRC panels (middle-cracks around embedded
threaded bars), and the UHPFRC tube was delaminated from
the NSC core at a large displacement. The middle-cracks
were more obvious at the upper and lower parts of the
column. The following three causes appeared to be responsible
for these joint-cracks and delamination: (1) load transfer at
the interface between the UHPFRC tube and NSC core; (2)
insufficient integrity between the UHPFRC panels; and (3)
buckling of longitudinal bars at the corner locations. On the
other hand, the bearing of embedded threaded bars was
responsible for the middle-cracks.
3.2 Crack Patterns
The crack patterns of each column specimen on four faces
are also presented in Fig. 6. To identify the development and
propagation of cracks, each crack is plotted with a sequence
number, and the wide cracks resulting from local failure are
plotted using bold lines. In Table 4, the cracking sequences
for all the column specimens are summarized depending on
the load level.
More specifically, in the control specimen HCC1 (Fig. 6a),
the first crack developed along a UHPFRC joint (on Face4)
under the axial load of 2190 kN, and similar vertical cracks
were observed on the opposite side (Face3). Hairline cracks
then appeared within the UHPFRC panels. Under further
loading, crushing cracks appeared around the embedded
threaded bars due to bearing. Subsequently, wide vertical
and horizontal cracks between the embedded threaded bars
were caused by the bearing and outward bulging of the NSC
In HCC2 with the greater fiber fraction (Fig. 6b), crack
patterns were generally similar to HCC1, but the first crack
occurred in a later phase (under 2416 kN) than in HCC1.
Cracks were well distributed over the column length. In
HCC3 with the lower transverse bar ratio (Fig. 6c), the first
crack occurred much earlier under 2084 kN, and the vertical
cracks in the middle of UHPFRC panels were more
pronounced compared to HCC1 with the higher transverse bar
ratio. In HCC4 with the greater fiber content and lower
transverse bar ratio (Fig. 6d), the first crack occurred under
2351 kN, similarly to HCC2. This is a later phase than in the
control specimen HCC1. In HCC5 without wire-mesh
(Fig. 6e), the first crack occurred under the smallest load of
2100 kN and the vertical cracks in the middle of UHPFRC
panels were most severe. Additionally, a wide diagonal crack
extending from the upper left corner to the mid-height of the
column was also observed.
The crack patterns showed that (1) the provided steel
fibers were effective in delaying the middle-cracks; and (2)
the wire-mesh embedded in the UHPFRC panels was
effective to keep the joint-cracks tightly closed. However, in
the case of using the sequential concrete placement method
(or rotation method), the vertical cracks along the UHPFRC
tube joints (or joint-cracks) were critical to the overall
performance. Thus, in order to ensure the better performance,
the UHPFRC tube needs to be monolithically constructed.
3.3 Axial Load–Displacement Relationship
Figure 7 shows the axial load–displacement curves (P–d)
of the column specimens. The axial displacement (d)
indicates the change in vertical length between column ends. In
the figure, vertical large-dashed lines represent the yield
displacement (dy, defined as the yield displacement of an
equivalent elastoplastic system with the secant stiffness at
75% of the peak load
; see Fig. 8) and ultimate
displacement (du, defined as the post-peak displacement
corresponding to 80% of the peak load
Fig. 8), and circle marks indicate the measured loads (Pcr
and Pp) at the first cracking and peak, respectively. All the
curves are plotted together in Fig. 7(f) for comparison, and
the test results are summarized in Tables 5 and 6. In the
figure, two bilinear small-dashed curves indicate the
predictions by ACI 318-14, and solid triangle marks with Pb
indicate the design bearing strength. The predictions by ACI
318-14 and design bearing strength will be discussed in the
In the control specimen of HCC1 (Vf = 1.5%,
s = 100 mm, and wire-mesh), the axial load increased
linearly up to the first cracking at Pcr = 2190 kN and dcr =
0.7 mm. After reaching the peak load (Pp) of 3369 kN at
dp = 2.6 mm, the axial load gradually decreased. The initial
stiffness (ki) was 3466 kN/mm, where the nominal elastic
stiffness or slope of the curve was defined as the secant
stiffness at 0.45Pp. The displacement ductility was estimated
as l = du/dy = 3.3, where the ultimate displacement (du) was
Fig. 6 Failure modes and crack patterns of column specimens: a HCC1 (control: Vf = 1.5%, s = 100 mm, wire-mesh), b HCC2
(Vf = 2.0%), c HCC3 (s = 200 mm), d HCC4 (Vf = 2.0%, s = 200 mm), and e HCC5 (no wire-mesh).
In the HCC2 specimen (Vf = 2.0%), the use of the greater
fiber fraction in the UHPFRC tube resulted in increases of
the initial stiffness (ki = 3776 kN/mm or 109% of HCC1)
and peak load (Pp = 3729 kN or 111% of HCC1), though
the displacement ductility was similar (l = 3.2, du = 4.0
mm). In the HCC3 specimen with the larger s (200 mm) but
with the same Vf (1.5%), the peak load was unexpectedly
higher (Pp = 3440 kN or 102% of HCC1) than in HCC1, but
the increase was marginal and the ductility was much lower
(l = 2.7, du = 2.9 mm). In the specimen HCC4 with the
larger s (200 mm) and Vf (2.0%), the initial stiffness was
higher with 3689 kN/mm (or by 6%) and the peak load was
higher with 3624 kN (or by 8%), but the ductility was lower
as 3.0 (or by 10%), compared to HCC1. Although the
UHPFRC tube was indirectly loaded by bearing of threaded
bars, the positive effect of the fiber fraction in the UHPFRC
tube on the axial load behavior of the hybrid concrete
column can be observed by comparing the performance
between HCC3 and HCC4 and between HCC1 and HCC2
(see Tables 5 and 6).
In HCC5 without wire-mesh, the initial stiffness and peak
load (ki = 3476 kN/mm and Pp = 3370 kN) were similar to
HCC1. However, the post-peak behavior was not so ductile
(l = 2.4, du = 2.4 mm). This is clearly presented in
Fig. 7(f), where all the axial load–displacement curves are
plotted for comparison. This was due to early buckling of
longitudinal bars at corners, where the structural integrity
between the UHPFRC panels was poor. Thus, the steel
mesh embedded in the UHPFRC tube is essential for the
ductile behavior of the developed hybrid concrete columns.
In conclusion, the axial load behavior of the column
specimens varied greatly with the test parameters. The
different behavior was partly attributed to the contribution of
the UHPFRC tube, which was not directly loaded but
indirectly. The contribution of the UHPFRC also can be
observed in Fig. 9, where the strain of transverse bars at the
= Energy Absorption
Yield Displ., δy
Ult. Displ., δu
Fig. 8 Definitions of yield displacement, ultimate
displacement, and energy absorption capacity.
mid-height is plotted (yield strain of the transverse bars
eyt = fyt/Est = 0.0018). As shown in the figure, the transverse
bars of HCC2 and HCC4 (Vf = 2.0%) yielded before the
peak load, indicating that the axial load increased even after
the yielding of transverse bars. In contrast, the transverse
bars of HCC1 or HCC3 (Vf = 1.5%) yielded after the peak
load or did not yield at all. This is because the peak load of
the HCC column was developed by the combined effect of
the confinement of transverse bars (exerting on the NSC
core) and the contribution of the UHPFRC tube (related to
the steel fiber content). In HCC5, strain-gauges were
P = load, d = displacement, and in parenthesis = displacement ductility (l = du/dy).
aFor test results, initial stiffness ki = P/d was defined at 0.45Pp.
bFor predictions by design code, elastic stiffness ke and axial strength Po were determined by Eqs. (1a) and (2a) using converted cylinder
damaged by concrete crushing and cracking, thus the
measured strains cannot be employed in this case.
Eurocode 4-04 (2004b), as given in Eq. (3a) for threaded bar
failure and Eq. (3b) for UHPFRC panel failure.
4.1 Contribution of UHPFRC Tube
Even though the UHPFRC tube was not directly loaded,
the contribution of the UHPFRC on the axial behavior was
substantial. The behavior was primarily dependent upon the
degree of composition, which was affected by the bearing
strength of the bar-tube interfaces. To better examine the
contribution of the UHPFRC tube, the measured initial
stiffness (ki) and peak load (Pp) were compared with the
predictions obtained from ACI 318-14. Under concentric
axial compression, the design elastic stiffness (ke) and
nominal axial strength (Po) can be predicted using Eqs. (1a)
and (2a), respectively, where the subscript 1 or 2 indicates
the non-composite (NSC core only) or full-composite
section (NSC core and UHPFRC tube).
Po;1 ¼ 0:85 fc0Ac NSCþ fyAs
Po;2 ¼ 0:85 fc0Ac NSCþ fc0Ac UHPFRC þ fyAs
where Ec = modulus of elasticity of concrete (NSC or
UHPFRC), Es = modulus of elasticity of longitudinal bars,
Ac = cross-sectional area of concrete (NSC or UHPFRC), As
= cross-sectional area of longitudinal bars, L = column
length, fc0 = specified compressive strength of concrete
(NSC or UHPFRC), and fy = specified yield strength of
It is noted that, to take into account the size effect
(Graybeal and Davis 2008; Neville 2011)
in the predictions,
the cube strength was converted to the cylinder strength
based on the strength class conformity of Eurocode 2-04
(2004a): specific cylinder-to-cube strength ratios for strength
classes up to C55/67 and a constant difference of 15 MPa for
higher strength classes (Tam et al. 2017). The converted
cylinder strength fc0 was 77.6, 128.3, or 23.2 MPa for the
1.5% Vf UHPFRC, 2.0% Vf UHPFRC, or NSC.
Equations (1a) and (2a) give the upper and lower bounds
of the actual behavior, that is, the axial load capacities for the
full-composite section (consisting of the UHPFRC tube and
in-filled NSC core) and for the non-composite section (NSC
core only), respectively. The two bounds are summarized in
Table 6 and also plotted using two bilinear curves in Fig. 7,
which include the information of ke and Po. As shown, the
axial load behavior generally corresponded to the upper
bound at the initial loading stage and to the lower bound
beyond the bearing failure of the embedded threaded bars.
The design bearing strength (Pb) can be predicted by
where fu = tensile strength of a threaded bar, d = diameter of
a threaded bar, fc0 = specified compressive strength of
UHPFRC, and Ec = modulus of elasticity of UHPFRC.
Because threaded bars were placing passing through all 4
faces and with 7 levels over the column height, the total
bearing strength was calculated by the smaller of the values
obtained from Eqs. (3a) and (3b) multiplied by 28. As
shown by the triangular marks in Fig. 7, the predicted value
by Eqs. (3a) and (3b) (Pb) was 2288 kN for the specimens
with both 1.5 and 2.0% fiber volume fractions, because it
was governed by the failure of threaded bars. This predicted
value agreed quite well with the actual onset of the observed
Even after the bearing failure at the bar-tube interfaces, the
larger peak load was obtained compared with the lower
bound prediction. This is mainly due to the confinement
provided by transverse bars. However, the UHPFRC tube
also had a contribution to the axial load capacity. Figure 10
demonstrates the contribution of UHPFRC tube. The thin
dotted curves in Fig. 10 was obtained for the confined NSC
core only using the numerical method developed by
et al. (2012)
which accounts for the confinement effect quite
accurately. As shown in the figure, the HCC specimens had
the higher strength than the analysis results for the confined
NSC core only, indicating the presence of the UHPFRC tube
contribution even after the onset of the bearing failure. The
higher strength was not caused by the confinement effect of
the UHPFRC tube: the confinement effect of the UHPFRC
tube cannot be expected to be as high as that of transverse
bars, because the tensile strain of UHPFRC was much lower
than the yield strain of transverse bars and also the UHPFRC
tube was subjected to axial load, even though the
geometrical effectiveness of the UHPFRC tube (continuous along
the column height) was better than transverse bars
(intermittently spaced). As shown in Fig. 6, the bearing failure did
Numerical Analysis for Confined NSC Core only
not occur at the whole bar-tube interfaces, because the NSC
core was compressed by the top and bottom loading plates
together. The threaded bars around the mid-height
underwent the smaller deformation than the upper and lower
threaded bars. Thus, the UHPFRC tube did not completely
lose its contribution to the axial load capacity (by bearing)
until its full delamination at a large deformation. Because the
same threaded bars were used, the difference in peak load
was attributed to the contribution of the UHPFRC tube,
which was related to the steel fiber content.
In this comparison, it is also observed that the initial
stiffness of all the test specimens was larger than the analysis
results for the NSC core only, demonstrating that the
composite action would be effective at the service loading stage.
4.2 Energy Absorption Capacity
For more quantitative assessment of the effect of the test
parameters, energy absorption capacities were compared.
The energy absorption capacity (EA) is defined as the area
under the axial load–displacement curve up to the ultimate
displacement (shaded area in Fig. 8), as expressed using the
Figure 11 shows the energy absorption capacity of the
column specimens. As shown in Fig. 11a, the accumulative
energy absorption capacity was increased with the axial
strain, due to the strength development and displacement
ductility. Figure 11b compares the accumulative energy
absorption capacity at the ultimate displacement. Compared
with the control specimen HCC1 (EA = 11,578 kN mm),
HCC2 with the greater fiber fraction (EA = 12,093 kN mm)
showed the higher energy dissipation by 4%. However, the
energy dissipation was markedly decreased by 33% in
HCC3 with the lower transverse bar ratio (EA = 7706
kN mm) or by 24% in HCC4 with the greater fiber fraction
and lower transverse bar ratio (EA = 8796 kN mm). On the
other hand, HCC5 without wire-mesh (EA = 6220 kN mm)
showed the lowest energy dissipation (decreased by 46%)
due to early buckling of longitudinal bars. The transverse bar
ratio is shown to be the most influential parameter among the
tested parameters in terms of the energy dissipation. On the
other hand, the indirectly loaded UHPFRC tube had a
limited influence, but the structural integrity of the UHPFRC
tube appeared to affect the restraining of bar-buckling, which
could result in a reduction of the energy dissipation.
To investigate the axial load behavior of HCC consisting
of a UHPFRC tube (20 mm thick, 92.6 or 143.3 MPa) and
NSC core (28.2 MPa), concentric axial load tests were
performed for five HCC. Axial loading was applied to the NSC
core only, and the UHPFRC tube was indirectly loaded by
bearing of embedded threaded bars. Test parameters
included the volume fraction of steel fibers, volumetric ratio of
transverse bars, and usage of wire-mesh in the UHPFRC
tube. The conclusions from the experimental investigation
are summarized as follows:
(1) In terms of sectional and cost efficiencies, the use of
UHPFRC only in the perimeter of a cross-section is
beneficial (i.e., hybrid concrete columns with different
strengths in the concrete cover and concrete core),
because the UHPFRC has high strength both in
compression and tension, toughness, and durability
and can also function as a permanent form. The
feasibility of the system discussed was demonstrated
by structural tests of axially loaded column specimens.
(2) The axial load behavior of the hybrid column
specimens, such as cracking, initial stiffness, peak load,
displacement ductility, and energy absorption capacity,
varied greatly with the test parameters. Generally, as
the steel fiber fraction increased or the transverse bar
ratio increased, the axial stiffness, strength, ductility,
and energy absorption capacity were favorable. The
presence of wire-mesh primarily affected bar-buckling
and post-peak behavior.
(3) The axial load behavior corresponded to the upper
bound of full-composite action between the UHPFRC
tube and NSC core at the stage of initial loading, and
then corresponded to the lower bound of
non-composite action of the NSC core only after the bearing failure
of embedded threaded bars. The onset of the bearing
failure agreed quite well with the design bearing
strength predicted by Eurocode 4.
In the present study, the thin UHPFRC tube was
produced by sequential concrete placements rotating
the section, and there were many vertical cracks
observed at the corner cold joints. In the future, to
improve the overall structural integrity, hollow-core
UHPFRC tubes may be made by monolithic casting.
Further research on the hybrid concrete columns with the
permanent UHPFRC tubing form should be continued,
particularly regarding the behavior of slender hybrid
concrete columns with the UHPFRC tube under eccentric
axial loads as well as cyclic loads.
This research was supported by grants from the National
Natural Science Foundation of China (Grant No. 51678196)
and the Fundamental Research Funds for the Central
Universities (Grant No. HIT. NSRIF. 2013112), and the
authors are grateful to the authorities for their supports.
All of the authors contributed critically to conception,
design, test, and analysis as well as drafting and revision of
Conflict of interest
The authors declare that they have no conflict of interest.
This article is distributed under the terms of the Creative
Commons Attribution 4.0 International License (http://
creativecommons.org/licenses/by/4.0/), which permits unre
stricted use, distribution, and reproduction in any medium,
provided you give appropriate credit to the original
author(s) and the source, provide a link to the Creative
Commons license, and indicate if changes were made.
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