Achieving High Strength, Ductility, and Durability in Flexural Members
Reinforced with Fiber-Reinforced Polymer Rebars by Using UHP-FRC
Shih-Ho (Simon) Chao, Ph.D., P.E.
Professor of Civil Engineering, University of Texas at Arlington
University of Texas at Arlington
(2nd largest campus of UT System)
2
Department of Civil Engineering
3
Dr. Surendra P. Shah
Presidential Distinguished Professor of Civil Engineering
Structures and Materials
Member of National Academy of Engineering
Director, Center for Advanced Construction Materials (CACM) at UT Arlington
Civil Engineering Laboratory Building
4
Ultra-high-performance fiber-
reinforced concrete (UHP-FRC)
▪ High compressive strength
▪ High compressive ductility
▪ High cracking resistance
▪ High shear strength
Fiber-reinforced
polymer (FRP) rebars
▪ High tensile strength
▪ Noncorrosive
Structural Members
▪ High durability (highly corrosion resistant)
▪ High flexural/shear strength
▪ High stiffness
▪ High ductility
▪ High resilience
5
0 0.8 1.6 2.4 3.2 4
Deflection (in.)
0
50
100
150
200
250L
oad
(kip
s)
0 20 40 60 80 100
Deflection (mm)
0
220
440
660
880
1100
Load
(kN
)
Load vs Deflection
RC #1 60S
RC #2 FRP
UHPFRC #4 FRP
UHPFRC #5 FRP
Load vs Deflection behavior
6
Fiber-Reinforced Polymer (FRP) reinforcing bars
✓ High-strength FRP bars can reduce
reinforcement congestion.
✓ Corrosion resistant – exposure to
deicing salts, seawater
✓ Lighter than steel (~ 75% lighter ).
Stress-strain relationship of various FRP bars,
Wu et al. 2012
7
RC #2 beam (Plain Concrete + BFRP bar)
Low rebar axial stiffness → large crack width (lower flexural stiffness) and reduced
shear resistance (due to reduced aggregate interlock, compression zone, and
dowel strength)
FRP rebars with conventional concrete
8
▪ The maximum usable compressive strain of plain concrete (at a post-peak stress
of approximately 80% of the peak stress), εcu, is 0.003 (ACI 318-19 and 2017
AASHTO LRFD).
▪ ACI 440 (2015) uses a conservative design for concrete members reinforced with FRP bars because both concrete and FRP bars are brittle materials.
Properties of Concrete Conventional Concrete UHP-FRC
Ultimate Compressive
Strength< 8,000 psi (55 MPa)
18,000 to 30,000 psi (124 to 207
MPa)
Early (24-hour)
compressive strength< 3000 psi (21 MPa)
10,000 – 12,000 psi (69 to 83
MPa)
Flexural Strength < 670 psi (4.6 MPa) 2,500 to 6,000 psi (17 to 41 MPa)
Shear strength < 180 psi (1.2 MPa) > 600 psi (4.1 MPa)
Direct Tension < 350 psi (2.5 MPa) up to 1,450 psi (10 MPa)
Rapid Chloride Penetration
Test2000-4000 Coulombs passed
Negligible (< 100 Coulombs
passed)
Ductility Negligible High ductility
Ultimate Compressive
Strain, εcu
0.003 0.015 to 0.03
Confining Negligible High confining capability
Comparison of typical conventional concrete and UHP-FRC
9
Traditional Design Concept for Reinforced Concrete
Transition Compression- ControlledTension - Controlled
t0.005 0.002
cu
t cu t
c
d
=
+Value of Ultimate Compressive
Strain in concrete (cu) = 0.003
11
Compressive Ductility of UHP-FRC
Aghdasi, P., Heid A. E., and Chao, S.-H. (2016). “Developing Ultra-High-Performance Fiber-
Reinforced Concrete for Large-Scale Structural Applications,” ACI Materials Journal, V. 113, No.
5, September-October 2016, pp. 559-570.
0.3%
Plain
concrete
0.0030.015
1
2
Full-field concrete longitudinal strain (εx) along moment region for UHP-
FRC#1 at an applied load of 317.7 kips (peak load)
Strains of UHP-FRC beam measured by high
resolution digital image correlation (DIC) technology
1
3 εcu of Plain Concrete, FRC, and UHP-FRC
400
350
300
250
200
150
100
50
0
Dep
thof
the
bea
m(m
m)
0.005 0.0025 0 -0.0025 -0.005 -0.0075
Strain (in./in.)
0.5 0.25 0 -0.25 -0.5 -0.75
Strain (%)
16
14
12
10
8
6
4
2
0
Dep
thof
the
bea
m(i
n.)
SFRC #2
Load: 25 kips
Load: 50 kips
Load: 100 kips
Load: 120.3 kips (peak)
Tension
Compression
400
350
300
250
200
150
100
50
0
Dep
thof
the
beam
(mm
)
0.015 0.01 0.005 0 -0.005
Strain (in./in.)
1.5 1 0.5 0 -0.5
Strain (%)
16
14
12
10
8
6
4
2
0
Dep
thof
the
beam
(in
.)
RC #1
Load: 10 kips
Load: 25 kips
Load: 50 kips
Load: 58 kips (design)
Load: 73 kips (peak)
Compression
Tension
400
350
300
250
200
150
100
50
0
Dep
thof
the
bea
m(m
m)
0.01 0.005 0 -0.005 -0.01 -0.015 -0.02
Strain (in./in.)
1 0.5 0 -0.5 -1 -1.5 -2
Strain (%)
16
14
12
10
8
6
4
2
0
Dep
thof
the
bea
m(i
n.)
UHP-FRC #1
Load: 50 kips
Load: 100 kips
Load: 200 kips
Load: 282.3 kips (design)
Load: 300 kips
Load: 317.6 kips (peak)
Tension
Compression
Plain concrete UHP-FRC
Ultra-High-Performance Fiber-Reinforced Concrete
(UHP-FRC): High Compressive Strength & Ductility
14
Note: The conventional RC column was designed based on ACI 318-14’s seismic provisions.
Full-scale Column Experimental Results (NSF Award No. 1041633)
15
1
6
εcu is approximately 0.015, five times of plain concrete’s εcu
New neutral
axisNeutral
axish d
As
s= 0.023 s=0.025 s=0.114
cu=0.003 cu=0.015 cu=0.015b
s=0.005
s=0.005
compressive strength and strain
Strain distribution for
increased compressive
strength
Strain distribution for
increased compressive
strain
Strain distribution for
combined increased
Singly reinforced
section
Effect of UHP-FRC’s High Compressive Strain and Strength on A
Flexural Member's Curvature Ductility
UHP-FRC (ductile element) + FRP bars (elastic element)
Steel bars (yielding element) + concrete (brittle element, crushed)
Opposite to conventional RC members
17
New Design Concept : Ductile-Concrete Strong-
Reinforcement concept (DCSR design concept)
▪ Using high reinforcement ratio of high-strength FRP bars can
achieve high structural efficiency (i.e., high flexural strength with a
relatively smaller cross-section).
▪ Keeping rebars elastic can minimize deterioration of bond strength,
limit the crack width (thereby maintaining the shear strength and
stiffness), and provide restoring force for reducing residual
deformation (i.e., self-centering capability).
▪ The high shear strength of UHP-FRC allows partial or total
elimination of shear reinforcement.
▪ FRP bars + UHP-FRC → a highly durable structural member.
18
Ductile-Concrete Strong-Reinforcement concept (Summary)
Strain profile and Design Sections
UHP-FRC #5 (8-#8 BFRP bars)
RC #1 (9-#5 Gr. 60 mild steel rebars) RC #2 (3-#7 BFRP bars)
The BFRP (baslt) bars had an ultimate tensile strength of approximately 125 ~ 150 ksi and an ultimate tensile strain of 0.017 to 0.025.
12
.0 i
n.
b=9.0 in.
14
.4 i
n.
c=7.45 in.
0.015cu =
0.009s =
0.014t =
14
.5 i
n.
9.0 in.
16
.0 i
n.
c=3.46 in.
0.003cu =
0.0095t =
13.25"
9.0"
1.5"
16.0"
60.0"20.0"
3.0" 20.0"
60.0"
57.0"
140.0"
57.0" 3.0"
6-#7
UHP-FRC #4 (6-#7 BFRP bars; no shear reinforcement)
20
Previous test results : Monotonic Loading
Geometry and reinforcement details
6.0"60.0"20.0"
3.0"20.0"
60.0"
57.0"
140.0"
57.0"3.0"
9-#5
12.0"
9.0"
1.5"
16.0"
20-#3 6.0"#3
7.0"60.0"20.0"
3.0" 20.0"
60.0"
57.0"
140.0"
57.0" 3.0"
7.0"
3-#7
14.5"
9.0"
1.5"
16.0"14-#3 #3
Beam RC #2 (plain concrete + BFRP longitudinal rebar)
Beam RC #1 (plain concrete +Grade 60 longitudinal rebar)
21
13.25"
9.0"
1.5"
16.0"
60.0"20.0"
3.0" 20.0"
60.0"
57.0"
140.0"
57.0" 3.0"
6-#7
Beam UHP-FRC #4 (BFRP longitudinal rebar)
Previous test results : Monotonic Loading
Geometry and reinforcement details
6.0"60.0"20.0"
3.0" 20.0"
60.0"
57.0"
140.0"
57.0" 3.0"
6.0"
8-#8
12.0"
9.0"
1.5"
16.0"18-#4
#4
Beam UHP-FRC #5 (BFRP longitudinal rebar)
22
0 0.8 1.6 2.4 3.2 4
Deflection (in.)
0
50
100
150
200
250L
oad
(kip
s)
0 20 40 60 80 100
Deflection (mm)
0
220
440
660
880
1100
Load
(kN
)
Load vs Deflection
RC #1 60S
RC #2 FRP
UHPFRC #4 FRP
UHPFRC #5 FRP
Load vs Deflection behavior
23
Cracking patterns at the end of test
RC #1 beam (Grade 60 steel) UHP-FRC #5 beam (UHP-FRC+ BFRP bar)
RC #2 beam (Plain Concrete + BFRP bar)
Specimen Effective
depth (d),
in. (mm)
Width of
compressio
n face (b),
in. (mm)
(%) Reinforceme
nt type
Fiber type Effective
span,
in. (mm)
UHP-FRC #1 4.311 (109) 6 (152) 15.5 MMFX Steel 49.5 (1257)
UHP-FRC #2 6.375 (162) 6 (152) 13.9 GFRP Steel 49.5 (1257)
UHP-FRC #3 5.35 (136) 8 (203) 14.8 BFRP UHMW-PE 34 (864)
UHP-FRC #4 5.35 (136) 8 (203) 14.8 BFRP Steel 34 (864)
Reinforcement type Diameter
in. (mm)
Tensile strength
ksi (MPa)
MMFX (Micro Composite Steel) Grade 100 (ASTM
1035)
1.125 (29) 100 (690)
GFRP (Glass Fiber-Reinforced Polymer) 0.75 (19) 90 (620)
BFRP (Basalt Fiber-Reinforced Polymer) 1.00 (25) 147 (1014)
Reinforcement Details
24
Design Summary
Ductile-Concrete Strong-Reinforcement concept – Cyclic Testing
25
Length (mm) Diameter (mm) Tensile Strength (ksi)
Micro Steel Fibers 13 0.12 313
UHMW Polyethylene Fibers 13 0.0015 375
Ductile-Concrete Strong-Reinforcement concept – Cyclic Testing : Fibers used
26
UHP-FRC beam #3 (UHMW PE fibers)
65"
8"
48" 15"
9"
38"
8"
8"
8"
8"
No. 4 BFRP stirrups at 6" o.c.
6"
6" UHP-FRC beam #4 (Steel fibers)
8 - #8 BFRP rebars
8 - #8 BFRP rebars
No. 4 BFRP stirrups
8"
1.75"
4"
6.25"
8"No. 8 BFRP rebar
UHP-FRC Beam #3 and #4
Detailed side view of the specimen showing specimens UHP-FRC #3 and UHP-FRC #4
Cross section of beam UHP-FRC #3 and UHP-FRC #4 (BFRP)
Loading Protocol
27
0 10 20 30 40 50
Cycle Number
-6
-4
-2
0
2
4
6D
rift
Rat
io (%
)
1/3 of 0.2% = 0.067%
0.2
0.25
0.35
0.5
0.75
1.0
1.4
1.75
2.20
2.75
3.5
4.375
5.5
28
-16 -12 -8 -4 0 4 8 12
Drift ratio (%)
-1600
-1200
-800
-400
0
400
800
1200
Mom
en
t(k
ips-
in.)
-16 -12 -8 -4 0 4 8 12
-150
-100
-50
0
50
100
Mom
ent (k
Nm
)-
-16 -12 -8 -4 0 4 8 12
Drift ratio (%)
-1600
-1200
-800
-400
0
400
800
1200
Mom
en
t(k
ips-
in.)
-16 -12 -8 -4 0 4 8 12
-150
-100
-50
0
50
100
Mom
en
t (k
Nm
)-
Moment vs Drift ratio for UHP-FRC #3 with UHMW PE fibers (BFRP) Moment vs Drift ratio for UHP-FRC #4 with steel fibers (BFRP)
UHP-FRC #3
Shear span-depth ratio = 4.25
8 - #8 BFRP bars
Total reinforcement ratio = 14.8 %
UHMW PE fiber
(Vf = 0.75%)
UHP-FRC #4
Shear span-depth ratio = 4.25
8 - #8 BFRP bars
Total reinforcement ratio = 14.8%
Micro steel fiber
(Vf = 3%)
29
BFRP 1% drift ratio (PE)
BFRP 3.5% drift ratio (PE)
BFRP 3.5% drift ratio (Steel)
BFRP 1% drift ratio (Steel)
5" 5" 2" 3" 3" 3"2"
8"
Effective span = 34"
Axis of Loading
3030
UHP-FRC #3
Shear span-depth ratio = 4.25
8 - #8 BFRP bars
Total reinforcement ratio = 14.8 %
UHMW PE fiber
(Vf = 0.75%)
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
Reinforcement strain
-1600
-1200
-800
-400
0
400
800
1200
Mom
ent (k
ips-
in.)
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
-150
-100
-50
0
50
100
Mom
ent
(kN
m)
Ru
ptu
re S
tra
in (
0.0
24
)
-
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
Reinforcement strain
-1600
-1200
-800
-400
0
400
800
1200
Mom
ent (k
ips-
in.)
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
-150
-100
-50
0
50
100
Mom
ent
(kN
m)
Ru
ptu
re S
train
(0.0
24)
-
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
Reinforcement strain
-1600
-1200
-800
-400
0
400
800
1200
Mom
ent (k
ips-
in.)
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
-150
-100
-50
0
50
100
Mom
ent
(kN
m)
Ru
ptu
re S
tra
in (
0.0
24
) -
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
Reinforcement strain
-1600
-1200
-800
-400
0
400
800
1200
Mom
en
t (k
ips-
in.)
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
-150
-100
-50
0
50
100
Mom
en
t (k
Nm
)
Ru
ptu
re S
train
(0.0
24)
-
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
Reinforcement strain
-1600
-1200
-800
-400
0
400
800
1200
Mom
en
t (k
ips-
in.)
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
-150
-100
-50
0
50
100
Mom
en
t (k
Nm
)
Ru
ptu
re S
train
(0.0
24) -
3" 3" 3"2"
8"
Effective span = 34"
Axis of Loading
3131
UHP-FRC #4
Shear span-depth ratio
= 4.25
8 - #8 BFRP bars
Total reinforcement
ratio = 14.8 %
Micro steel fiber
(Vf = 3%)-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
Reinforcement strain
-1600
-1200
-800
-400
0
400
800
1200
Mom
en
t (k
ips-
in.)
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
-150
-100
-50
0
50
100
Mom
en
t (k
Nm
)
Ru
ptu
re S
train
(0.0
24)
-
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
Reinforcement strain
-1600
-1200
-800
-400
0
400
800
1200
Mom
en
t (k
ips-
in.)
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
-150
-100
-50
0
50
100
Mom
en
t (k
Nm
)
Ru
ptu
re S
tra
in (
0.0
24
)
-
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
Reinforcement strain
-1600
-1200
-800
-400
0
400
800
1200M
om
en
t (k
ips-
in.)
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
-150
-100
-50
0
50
100
Mom
en
t (k
Nm
)
Ru
ptu
re S
train
(0.0
24) -
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
Reinforcement strain
-1600
-1200
-800
-400
0
400
800
1200
Mom
en
t (k
ips-
in.)
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
-150
-100
-50
0
50
100
Mom
en
t (k
Nm
)
Ru
ptu
re S
tra
in (
0.0
24
) -
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
Reinforcement strain
-1600
-1200
-800
-400
0
400
800
1200
Mom
en
t (k
ips-
in.)
-0.015 -0.0075 0 0.0075 0.015 0.0225 0.03
-150
-100
-50
0
50
100
Mom
en
t (k
Nm
)
Ru
ptu
re S
tra
in (
0.0
24) -
32
Ultra-high-performance fiber-
reinforced concrete (UHP-FRC)
▪ High compressive strength
▪ High compressive ductility
▪ High crack resistance
▪ High shear strength
Fiber-reinforced
polymer (FRP) rebars
▪ High tensile strength
▪ Noncorrosive
Structural Members
▪ High durability (highly corrosion resistant) → material properties of FRP and
UHP-FRC
▪ High flexural/shear strength → through DCSR design
▪ High stiffness/→ through DCSR design
▪ High ductility → through DCSR design (reminder: FRP is a brittle material)
▪ High resilience (small residual deformation) → through DCSR design