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