performance of newly developed cfrp precast prestressed decked bulb t-beams
TRANSCRIPT
Performance of newly Developed CFRP Precast Prestressed Decked Bulb T Beams
Nabil Grace
Dean, College of Engineering, Lawrence Technological University, Southfield, MI, U.S.A.
Tsuyoshi Enomoto
Manager, Tokyo Rope Manufacturing Co. Ltd. Tokyo, Japan
Prince Baah
Graduate Student, Center for Innovative Materials Research (CIMR), Lawrence Technological University,
Southfield, MI, U.S.A.
Mena Bebawy
Post-Doctoral Research Fellow, Center for Innovative Materials Research (CIMR), Lawrence
Technological University, Southfield, MI, U.S.A.
ABSTRACT
This study introduces an innovative scheme of bridge superstructure for expedited
construction, improved inspection, serviceability, and extended lifespan. The new bridge
superstructure is assembled from precast prestressed decked bulb T beams reinforced and prestressed with corrosion-free fiber reinforced polymer (FRP) materials. An experimental
investigation accompanied by a numerical simulation was developed to evaluate the
performance of the newly developed beams. Through the experimental investigation, three single decked bulb T beams were constructed and tested to failure. The first beam served as
a control beam and was prestressed and reinforced with conventional steel strands and
reinforcing bars. The second and third beams were prestressed and reinforced with carbon fiber composite cables (CFCC) strands and carbon fiber reinforced polymer (CFRP)
tendons, respectively. The investigation revealed that the performance of beams reinforced
with CFCC strands or CFRP tendons is comparable with the performance of the control
beam at both service and ultimate limit states. All three beams exhibited high load carrying
capacity with large corresponding deflection and fair amount of absorbed energy before
failure. The study showed that the corrosion-free FRP-reinforced decked bulb T beams can
be safely deployed in construction to enhance the performance and extend the lifespan of bridge superstructures.
KEYWORD
carbon fiber reinforced polymer, prestressed, precast concrete, flexural, T beams
F3A02
1. INTRODUCTION
The use of decked bulb T beams in the
construction of bridge superstructures has
emerged rapidly during the last few decades. Several design agencies have implemented bulb
T beams in their design guidelines with some
differences in dimensions and construction techniques. For example, Utah Department of
transportation (UDOT) categorizes bulb T beam
bridges according to construction technique into
three classes: Bulb T beams with concrete deck,
decked bulb T beams without concrete deck, and
post-tensioned bulb T beam with concrete deck
and post-tensioning strands. Likewise,
Washington State Department of Transportation
(WSDOT) provides details for both bulb T
beams with deck and decked bulb T beams without decks [1].
Examples for construction of decked bulb T
beams in the U.S.A. can be traced back to 1986
with the construction of a six-span prestressed
concrete decked bulb T beam bridge in
Minnesota [2]. The bridge superstructure
comprised five decked bulb beams with a depth
of 1000 mm and top flange width of 1800 mm. The end spans had a length of 21.3 m, while the
interior spans had a length of 25.9 m. Steel bars
of 25.4 mm in diameter were used to transversely post-tension the top flange. Another
decked bulb T beam bridge was constructed in
Kittitas County, WA to replace a deteriorated bridge in 2009. The beams were interconnected
using welded steel joints. In addition, to
overcome the problem of longitudinal joint
leakage, the new bridge was provided with a
waterproof membrane in addition to an asphalt
emulsion to hold the membrane in place.
A bridge superstructure constructed using
adjacent decked bulb T beams has several advantages over bridge superstructures
constructed using different adjacent beams. For
instance, adjacent decked bulb T beams allow adequate space between the beams for inspection
and maintenance. In addition, the top flange of
the beams is fabricated to act as a deck slab to
save time and effort and expedite the
construction of the bridge by eliminating the
need for a cast-in-place deck slab. However,
durability issues related to the corrosion of steel
reinforcement in decked bulb T beams is the
concern yet to be handled.
Through this investigation, the durability of the
decked bulb T beams is improved by replacing
the conventional steel reinforcement with CFCC
[3] or CFRP [4] reinforcement. CFCC and CFRP
are corrosion resistant and can significantly extend the lifespan of the bridge superstructure.
The use of CFRP in bridge construction has been
proven to be successful with the construction and monitoring of the ten-year old Bridge Street
Bridge in Southfield, Michigan, U.S.A. [5] and
four-year old Penobscot Narrows Cable Stayed
Bridge in Maine, U.S.A. [6].
The investigation presented in this paper
represents Phase #1 of a multi-task project
dedicated to establish comprehensive design
guidelines for Decked bulb T beam bridges
reinforced and prestressed with different kinds of FPR reinforcement for flexure and shear.
2. EXPERIMENTAL PROGRAM
To evaluate the performance of the developed
beams, three prestressed decked bulb T beams
were constructed, instrumented, and tested to
failure under vertical loads. The beams had a
span of 9,750 mm, top flange width of 457 mm, bottom flange width of 305 mm, and a total
depth of 356 mm. All three beams were
reinforced with an identical reinforcement scheme, shown in Fig.1, but with different
reinforcement materials. The first beam (steel
beam) served as a control beam and was pretensioned with four low relaxation
prestressing steel strands and reinforced with
non-prestressing steel reinforcement. The second
beam (CFCC beam) was pretensioned with four
prestressing CFCC strands and reinforced with
non-prestressing CFCC strands. The third beam
(CFRP beam) was pretensioned with four CFRP-
leadline tendons and reinforced with CFRP-DCI
tendons [7]. Details of reinforcement are provided in Table 1 and Table 2 for non-
prestressing and prestressing reinforcement,
respectively.
Table 1 Properties of non-prestressing
reinfocement Diam.
(mm)
Area
(mm2)
Yield
strength
(MPa)
Ultimate
strength
(MPa)
Tensile
modulus
(GPa)
Steel
16 200 413 620 200
CFCC
15.2 115 -- 2,590 159
CFRP 10 72 -- 2,344 157
Table 2 Properties of prestressing
reinforcement Diam.
(mm)
Area
(mm2)
Yield
strength
(MPa)
Ultimate
strength
(MPa)
Tensile
modulus
(GPa)
Steel
16 140 1585 1,861 201
CFCC
15.2 115 -- 2,590 159
CFRP 10 72 -- 2,744 163
Fig.1 Cross section of decked bulb T beams
In all beams, each strand/tendon was prestressed
with an initial prestressing force of 111 kN. The
prestressing force was applied using two bulkheads anchored to a heavily reinforced
concrete foundation with high strength bolts. In
addition, the CFCC pretensioning strands were
provided with a special mechanical anchorage
system at each end to facilitate pulling of the
strands without damaging their ends. This
anchorage system consisted of sleeve for CFCC,
wedges, joint coupler, mesh sheet, braided grip
and wedges for the steel strand. Fig.2 shows the
process of pretensioning the strands using hydraulic pump, while Fig.3 shows the newly
developed and tested anchorage system for the
CFCC strands that can be installed in field.
The transverse reinforcement for all beams was
made of steel stirrups with a diameter of 10 mm
and center-to-center spacing of 102 mm. Fig.4
shows the completed reinforcement cage in the
formwork while casting the concrete. The concrete mix was designed to achieve an
average 28-day compressive strength of 62 MPa.
However, the concrete in the CFRP beam
achieved an average 28-day compressive
strength of 50 MPa.
Fig.2 Applying pretensioning force to
longitudinal strands
Fig.3 Couplers for CFCC strands
Fig.4 Placing concrete in bulb T beam
The release of the prestressing forces took place
14 days after casting the concrete. At the time of prestress release, the concrete compressive
strength averaged 41 MPa. The release of
prestressing force in the steel beam was
performed by cutting the steel strands at the ends
of the beam using a gas/oxygen torch, while the
release of the prestressing force in the CFRP
beam was performed by saw cutting the CFRP
tendons using electric saw. On the other hand, the release of the prestressing forces in the
CFCC beam was performed by further pulling
the CFCC strands slightly beyond the prestressing force and untying the steel
anchorage couplers.
Both the steel and the CFRP beams were
designed to fail in tension by yielding of steel
strands or rupture of the CFRP tendons while the
CFCC beam was designed to fail in compression
by concrete crushing. The change in the failure
mode was necessary to evaluate both tension and
compression failure of the FRP-prestressed decked bulb T beams against the common
tension failure mode of steel-prestressed beams.
2.1 Instrumentation and Test Setup Strain gauges were attached to all prestressing
strands and non-prestressing reinforcement to
measure the strain in the strands at the mid-span
section during different stages of construction
and loading. In addition, strain gauges were mounted on the concrete surface at different
locations to measure the strain in the concrete.
Furthermore, a set of three strain gauges was attached to the concrete surface at the soffit of
the beam near the first initiated flexural crack to
predict the decompression load. Measurement of the decompression load was performed to
estimate the effective prestress in the
pretensioning tendons/strands. Calibrated load-
cells were mounted on the prestressing strands to
measure the initial prestressing forces. Linear
Motion transducers were attached to the beams
to measure the vertical deflection of each beam
at the mid-span under different load levels. All
sensors were calibrated and connected to a calibrated digital data acquisition system to
monitor the deformation of the beam specimens
during loading.
2.2 Loading Test As shown in Fig.5, the decked bulb T beams
were loaded under a four-point loading setup
over an effective span of 9,450 mm. The load
was applied through incremental cycles until
failure. Two steel reinforced neoprene bearing
pads were provided at the ends of each beam as
supports. The loading test was performed to
evaluate the flexural performance of each beam
under service limit state, post-cracking limit
state, and ultimate limit state. The performance
of the beams was examined by recording the
deflection at the mid-span, strain readings in
concrete and reinforcement, crack propagation, crack width, and crack pattern at different load
levels. The following sections present a
discussion for the results obtained from three beams.
Fig.5 Four-point Loading setup
2.3 Service Limit State For the purpose of this study, the service limit
state was defined by the state, at which the
concrete beam remained uncracked. The service
limit state ended with the initiation of the first flexural crack. The first crack was observed at a
load level between 42 and 44 kN in all beams.
All the beams exhibited nearly the same elastic
performance during the service limit state. In
addition, an estimate for the effective prestress force in each beam was made by recording the
concrete strain at the soffit of the beam. First,
the beam was loaded until the initiation of the
first crack. Second, the beam was unloaded and
stain gauges were attached to soffit of the beam
near the crack location. Third, the beam was
loaded again while the strain near the crack was recorded. During the second loading cycle, the
strain in the concrete near the first initiated
flexural crack increased nearly linearly with
increasing the load until reaching a certain load
level, at which the strain experienced no further
increase with increasing the load. This load level
was identified as the decompression load, which
defined the load level where the moment due to
dead plus applied load exceeded the moment due to prestressing force. The recoded strain value at
the decompression load represented the effect of
the prestress force and was used to backwards
calculate the effective prestressing force. The
decompression load for all the three beams
ranged between 26 and 31 kN. Based on the
elastic calculations, this level of decompression
load corresponded to an effective prestressing force of approximately 80 to 90 % of the
initially applied prestressing force.
2.4 Post-Cracking Limit State This state started with the initiation of the first
flexural crack and was marked by an apparent
change in the slope in the load-deflection curves.
Several flexural cracks developed in the beams
with increasing the load beyond the cracking
load. Consequently, the beams experienced
further reduction in their flexural stiffness with
each loading/unloading cycle. The load-
deflection curves for the CFCC and the CFRP Beams were nearly linear until failure while, the
load-deflection curve for steel beam showed a
ductile plateau near the failure. At load level of
approximately 169 kN, the steel beam exhibited
a steady increase in the deflection with a little or
no increase in the load carrying capacity.
Crack width and crack pattern were recorded
and plotted for each beam. Under a certain load level, all three beams experienced nearly similar
overall flexural crack pattern. However, the
crack width was slightly different. The CFRP beam exhibited the largest crack width followed
by the CFCC beam and then the steel beam. For
example, at load level of 80 kN, the maximum observed crack width was around 0.4, 0.3, and
0.25 mm in the CFRP, CFCC, and steel beams,
respectively. The wider crack width was
interpreted into an increase in the rotation and
deflection of the beam as the CFRP beam had
the largest mid-span deflection followed by the
CFCC beam and then the steel beam. However,
the situation changed when the bottom
reinforcement of the steel beam approached the yield. At the yield of the steel beam, the flexural
cracks progressively widened and the steel beam
exhibited rapid increase in the deflection. The CFCC beam and the CFRP beam, on the other
hand, showed a gradual increase in the crack
pattern and width since the initiation of the first
flexural crack until the failure of the beam.
2.5 Ultimate Limit State and Failure The steel beam failed at ultimate load of 191 kN
due to yielding of steel strands followed by
crushing of the concrete at the top flange (Fig.6).
The measured deflection at failure was 348 mm.
The CFCC beam failed at ultimate load of 205
kN with corresponding deflection of 329 mm.
The failure was characterized by crushing of the
concrete at the top flange near the mid-span
section (Fig.7). The CFRP beam failed at ultimate load level of 169 kN with a
corresponding mid-span deflection of 359 mm.
The failure was characterized by rupture of CFRP tendons followed by crushing of the
concrete at the top flange (Fig.8). The failure
patterns in all three beams matched their
designed and anticipated failure modes.
Fig.6 Reinfocement yield and concrete
crushing failure of the steel beam
Fig.7 Compression failure of the CFCC beam
Fig.8 Tensile failure of the CFRP beam
2.6 Strength and Energy The CFCC beam achieved the highest load
carrying capacity, followed by the steel beam,
and then the CFRP beam. The load carrying
capacity of the CFCC beam was 7 % higher than
the load carrying capacity of the steel beam,
while the load carrying capacity of the CFRP
beam was 12 % less than that of the steel beam.
On the other hand, the steel beam had the highest energy absorption capacity followed by
the CFCC beam and finally the CFRP beam. The
total energy absorbed by the steel, CFCC, and CFRP beams until failure (area under the load-
deflection curves) were approximately 49, 41,
and 37 kN.m, respectively. The energy
absorption capacities of the CFCC and CFRP
beams were approximately 17 and 25 % less
than the energy absorption capacity of the steel
beam, respectively. Table 3 shows the results of
tested beams.
Table 3 Results of experimental investigation
Crack
load
(kN)
Failure
Load
(kN)
Max.
deflection
(mm)
Energy
absorbed
(kN.m)
Failure
mode
Steel
42 191 348 49 Tension
CFCC
44 205 329 41 Comp.
CFRP 44 169 359 37 Tension
3. NUMERICAL SIMULATION
A numerical investigation was conducted to
examine the performance of the tested decked bulb T beams using commercially available
software ABAQUS. The concrete beam was
modeled using a three dimensional solid element
C3D8R. A continuum, plasticity-based, damage
model for concrete was used to model the
material behavior. The concrete damaged plasticity model uses concepts of isotropic
damaged elasticity in combination with isotropic
tensile and compressive plasticity to represent the inelastic behavior of concrete. It assumes
that the main two failure mechanisms are tensile
cracking and compressive crushing of the concrete. Consequently, the concrete was
defined by its uniaxial compressive and tensile
performance in addition to the elastic properties.
For the compressive behavior, the response was
assumed linear until the initial yield, which was
assumed to occur at stress equal to
approximately 60 % of the concrete ultimate
strength. After initial yield, the material started
the plastic response, which was typically
characterized by stress hardening followed by
strain softening beyond the ultimate stress. For
the tension side, the stress-strain response followed a linear elastic relationship until the
cracking stress was reached, which corresponded
to the onset of micro-cracking in the concrete material. Beyond the cracking stress, the
formation of micro-cracks was represented
macroscopically with a softening stress-strain
response, which included strain localization in
the concrete structure (Fig.9).
For the Steel and the CFCC beams, the modulus
of elasticity for the concrete was taken as 33.8
GPa, the direct tensile strength was taken as 4.8
MPa, and Poisson’s ratio was taken as 0.2. For the CFRP beam, the modulus of elasticity for the
concrete was taken as 30.5 GPa and the direct
tensile strength was taken as 4.3 MPa. These
values were calculated based on section 5.4.2 of
AASHTO LRFD [8] for the material properties
of the concrete.
The longitudinal reinforcement, with mechanical
properties as shown in Fig.10, and the shear reinforcement were modeled with a two-node
linear 3D truss element (T3D2). The truss
elements were embedded inside the host concrete brick elements. Each node of the truss
embedded elements had three degrees of
freedom ��� , �� , ��� and these degrees of
freedom were constrained to the interpolated values of the corresponding degrees of freedom
of the host element nodes.
The numerical cracking load was approximately
53 kN for all three beams, which was slightly
higher than the exhibited experimental cracking
load (44 kN). The difference between the
experimental and numerical cracking load was
attributed to various factors on both the
experimental and numerical sides. For instance,
it was difficult to precisely evaluate the tensile
capacity of the concrete in the beam and the exact loss in the prestressing force at the time of
testing. On the other hand, the element size in
the numerical analysis influenced the cracking load. Smaller element size tended to show lower
cracking load. However, the analysis became
intractable with such small element size.
Nevertheless, the response of the numerical
models closely matched that of the tested beams
after cracking until failure. For instance, in the
numerical model of the steel beam, when the
applied vertical load reached 169 kN, the strain
in the bottom reinforcement reached 5,080 µε
and the strain in the concrete at the top surface of the beam at the mid-span reached
approximately 1,100 µε. These values of strain
matched the measured strain values in the experimental investigation just before the yield
stage, where the strain in the concrete averaged
1,200 µε and the strain in the bottom
reinforcement averaged 4,700 µε. The ultimate
load predicted by the numerical model of the
steel beam was 198 kN, with a difference of 3 %
from the experimental ultimate load. At the
ultimate load, the FE analysis indicated
excessive yielding of the steel strands before the
crushing of the concrete in the top flange.
Fig.9 Stress-strain response of concrete
material as defined in the FEA
Fig.10 Stress-strain response of
reinforcement as specified in the FEA
The FE analysis of the CFCC beam showed that
the failure load of the beam was approximately 213 kN with a difference of 4 % from the
experimentally achieved ultimate load (205 kN).
In addition, the stress in the concrete at the top
flange reached 63 MPa in the FE model, which
was in close agreement with the ultimate
compressive strength of the concrete (64 MPa).
Furthermore, the strain in the bottom CFCC
strands reached 8,500 µε. This numerical CFCC
strain at failure matched the measured strain from the experimental investigation (8,900 µε).
It should be noted that when adding this strain to
the strain due to prestressing (4,220 µε), the total strain in the CFCC strands at failure would be
12,720 µε. This strain level is less than the
ultimate strain of the CFCC strands (16,000 µε)
and thus confirms the compression failure of the
CFCC beam. Likewise, the FE analysis of the
CFRP beam predicted a failure load of 165 kN
with a difference of 2 % from the experimentally
achieved ultimate load (169 kN). At failure, the
strain in the concrete at the top flange
approached 2,200 µε and the strain in the bottom CFRP tendons approached 10,130 µε. Adding
the CFRP strain to the strain due to prestressing
after losses (6,740 µε), the total strain would be
16,870 µε. This was the ultimate strain as
defined by the manufacturer and was provided
through the input file of the FE analysis. The
experimentally measured concrete strain at
failure was approximately 2,400 µε, while the
experimentally measured strain in the CFRP tendon before failure (excluding the strain due to
prestressing) was around 10,900 µε.
The experimental vs. numerical load-deflection
curves for all three beams are given in Fig.11.
The load-deflection curves obtained from the FE analysis of all three beams were in close
agreement with those obtained from the
experimental investigation. Therefore, it is
reasonable to extend the FE investigation to
model complete decked bulb T beam bridge
models with different geometries and different
loading configurations.
Fig.11 Load-deflection curves for tested
beams (experimental vs. numerical)
-70
-60
-50
-40
-30
-20
-10
0
10
-0.004 -0.003 -0.002 -0.001 0 0.001 0.002 0.003
Str
ess
(MP
a)
Strain (µε)
0
500
1,000
1,500
2,000
2,500
3,000
0 0.01 0.02 0.03 0.04 0.05
Str
ess
(MP
a)
Strain (µε)
Steel strands
CFCC strands
CFRP tendons
0
40
80
120
160
200
0 50 100 150 200 250 300 350
Lo
ad (
kN
)
Deflection (mm)
Steel-Exp.
CFCC-Exp.
CFRP-Exp.
Steel-FEA
CFCC-FEA
CFRP-FEA
4. CONCLUSIONS
Based on the results obtained from the
experimental investigation and the numerical
simulation, the following conclusions are drawn: 1. Under service limit state, the flexural
performance of the decked bulb T beams
prestressed with CFCC strands or CFRP tendons was comparable with the
performance of beams prestressed with
steel strands. No significant difference
was observed between tested beams.
2. Beyond service limit state, flexural
crack pattern and cracks spacing were
nearly similar in all tested beams. On the
other hand, the crack width in CFRP and
CFCC beams was slightly larger than
that in the steel beam. This suggests that flexural distress signs of the FRP-
prestressed decked bulb T beams are
similar to those of the steel-prestressed
beams but larger deflection is expected
in FRP-prestressed beams.
3. The flexural load carrying capacity and
the corresponding maximum deflection
of the CFCC beam were 107 % and 94
% of those of the steel beam, respectively. On the other hand, the
flexural load carrying capacity and
corresponding maximum deflection of the CFRP beam were 88 % and 103 %
of those of steel beam, respectively. In
addition, the total energy absorption capacity of the CFCC and the CFRP
beams were 84 % and 76 % of the total
energy absorption capacity of the steel
beam, respectively. Therefore, it is
reasonable to conclude that the overall
flexural performance of the CFCC and
the CFRP beams was comparable with
the flexural performance of the steel
beam with respect to the load carrying capacity and the deformation. However,
the FRP reinforced beams have the
advantage of corrosion resistance over the steel reinforced beams.
4. Numerical models for all the tested
beams accurately predicted the cracking
pattern, deflection, ultimate load, and
failure modes. Therefore, the numerical
approach can be adequately employed in
the design of the decked bulb T beams
reinforced and prestressed with FRP
materials.
ACKNOWLEDGMENT
This investigation was sponsored through a
consortium assembled of the National Science
Foundation, (Award No. CMMI-0969676), Michigan-DOT Center of Excellence, US-DOT
(Contract No. DTOS59-06-G-00030), Tokyo
Rope MFG. CO. LTD., Japan, and Diversified Composites, Inc. KY, U.S.A. The authors
gratefully acknowledge their supports.
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