direct tension test method for characterization of tensile...

Direct Tension Test Method for

Characterization of Tensile Behavior of Ultra

High Performance Concrete (UHPC)

Pizhong Qiao, Ph.D., PE, Professor

Zhidong Zhou, Ph.D. Candidate

Department of Civil and Environmental Engineering

Washington State University

[email protected]

08/10/2017

o Ultra High Performance Concrete (UHPC) is an innovative

cementitious composite that possesses a compressive strength

greater than 22,000 psi (ACI committee 239) and flexural strengths

greater than 1,500 psi at 28 days.

Performance: strength, durability, ductility and toughness, etc.

• Usually produced with cement, fine quartz sand, silica fume,

steel fibers, and high range water reducing admixture.

• Very low water-to-cementitious materials (w/cm) ratios (< 0.25,

per FHWA) were used to produce UHPC.

• Presetting pressure and/or heat treatment

Key: very dense microstructure & discontinuous pore structure

Smaller sections, reduced weight, more durable, longer service life,

low maintenance cost, seismic/impact resistance.

Background-What, How, Why

2

Normal concrete: flexural strength (ASTM C78) and splitting

tensile strength (ASTM C496).

Methods for Tensile Behavior of UHPC

Fiber reinforced concrete (FRC) and Engineered cementitious

composite (ECC): direct tensile strength

Direct tension method can more realistically predict the tensile

strength and ductile behavior

support, biaxial stress and geometry effects: ratio 1~3 (Victor Li, 1994)

3

4

Tensile Strength

ft (MOR) > ft (IDT) > ft (DTT)Small difference @ first cracking

Large difference @ post cracking

Notched beams or cylinders

Some fractures still do not occur at the notch

Local stress concentration caused by the notch, cannot

accurately predict the tensile strength

(Graybeal, 2006)

DTT for UHPC

5

Unnotched beams or cylinders

High requirements of grip system

Stress concentrations at ends, fractures occur beyond to the gauge

length of LVDTs

(Graybeal, 2006) (Graybeal & Baby, 2013)6

DTT for UHPC

(ASTM B557M, 2016)7

Dogbone shaped specimen - Examples

ASTM standard for other materials

ASTM B557M --- Aluminum- and Magnesium-Alloy

ASTM E8/E8M --- metallic materials

ASTM D638 --- plastic materials

Dogbone shaped specimen

Easily grip and avoid stress concentration

Constant section area and tensile stress at middle part

8

• No standard test protocol:

Specimen dimension

requirements and casting

molds

• Some rotation may form at

post cracking

DTT for UHPC

Modified from ASTM C190-85:

Small cross-section: 1” x 1” x 3”

(Wille et al., 2016)

9

No standard test protocolDTT for UHPC

Increased width and gauge length: 1” to 2”, 3” to 4”

1” x 2” x 4”

Still thin

(Kamal et al., 2008)

10


Increased width and gauge length:

1” x 1” x 8” and 1” x 2” x 8” but LVDT: 4” Thin-plate like

(Tran et al., 2016)

(Yoo et al., 2016)

11

DTT for UHPC

Increased thickness: 2” x 4” x 8”

12


Effects of Geometry: (Nguyen et al., 2014)

Direct Tension Test for Characterization of Tensile Behavior of

Ultra High Performance Concrete:

• Develop a practical standardized dogbone-shaped specimen

based on trial-and-error experiment and finite element models

• Investigate the tensile behaviors of UHPC using the proposed

DDT design.

• Provide idealized constitutive model to characterize the tensile

responses of UHPC.

Objectives

Development phase

Execution phase

Characterization Model

13

Specimen Design

14

Cross section area: 2” x 2”

ASTM C1609 S7.1.4, the width and the depth of fiber reinforced concrete beam

for flexural strength are required to above three times maximum fiber length

Neck-down type: Tapered vs. Concave curved portion

Abandon thin-plate

Abandon tapered design

• Tapered portion (incompetent Design)

Dimensions of Specimen for DTT Crack pattern

Direct Tension Test (DTT)

15

4.27"

1.73"

6"

1.73"

4.27"

18"2"

2"4"

Specimen Design

16

Middle portion of gauge length: 2”, 4”, 6”, 8”

The maximum stress is close among 4, 6, 8 inch, with 6 inch of min. smax.

2” (Stress concentration) 4”

6” 8”

Abandon too long

• 2” Middle portion (incompetent Design)

Dimensions of Specimen for DTT Crack pattern


17

2"4"

4.27"

3.73"

2"

3.73"

4.27"

18"2"

Stress-Strain Relationship-incompetent design

18

7 days

14 days

28 days

Specimen Design

19

End portion width: 3”, 4” Radius of Concave

3” radius

much uniform

stress

distribution

Choose 3” with larger radius

4” radius

•Direct Tension Test (DTT) (Final Design) The cross-section of 2” x 2” x 6” (6” of gauge length) and total length of 18” dog-bone shaped specimen

Dimensions of Specimen for DTT Mold


(To reduce/minimize the stress concentration at width change locations, the above mold is

smoothened so that the width is gradually and continuously changed from 3” to 2”)

DTT Setup

20

UHPC Mix Design

Mixture Type UnitAmoun

tType I/II Portland

Cementlb/yd3 1500

Silica Fume lb/yd3 260

Fine Sand lb/yd3 1574

Steel Fibers lb/yd3 236

HRWRA gal/yd3 11.5

Water lb/yd3 325

w/cm 0.18

Spread Testing In. 9.50

• Locally available sand and

cement

• Domestic steel fibers

• More importantly, the

expensive materials, such as

quartz powder and imported

fibers, are not used

• Within limited low w/cm ratio

and w/o steel aggregates

• Any procedures, such as

application of pre-setting

pressure and heating (which

needs costly equipment), are

not used.

21

Experimental Plan

22

Influencing Factor

Curing age, days

7 days (benchmark)

14 days

28 days

Fiber content

Vf 0%

Vf 1%

Vf 2% (benchmark)

Loading rate

0.01 in./min. (benchmark)

0.1 in./min.

1 in./min.

Direct Tension Test (DTT)-Curing Age

C3-28days

C3-7 days C3-14days

Cracking pattern of direct tension test with proposed design 23

Direct Tension Test (DTT)-Volume Fraction

Vf = 2%Cracking pattern of direct tension test with proposed design

24

Vf = 0% Vf = 1%

Direct Tension Test (DTT)-Loading Rate

1 in./min. Cracking pattern of direct tension test with proposed design 25

0.01 in./min. 0.1 in./min.

26

Idealized Constitutive Relationship

• Elastic

• Strain Hardening

• Strain Softening

27

Tensile parameters

• @first cracking strength (σcc)

• @post-cracking tensile strength (σpc)

• Strain @first cracking (εcc)

• Strain @post-cracking (εpc)

• Tangent modulus of elasticity (Ecc)

• Initial modulus of elasticity (Et) @ 40% of tensile strength

• Dissipated energy density prior to post cracking (Gp)

• Total dissipated energy density (Gt)

Stress-Strain Relationship-new design

28

Influence of curing age

29

ID

7days-1 218

224

0.89

0.94

1578

1941

0.98

1.027days-2 242 1.00 2144 1.04

7days-3 213 0.94 2101 1.04

14days-1 200

221

1.01

0.99

2330

2104

1.15

1.1314days-2 240 0.95 1864 1.14

14days-3 224 1.03 2117 1.10

28days-1 228

232

1.10

1.15

2382

2510

1.22

1.2828days-2 217 1.13 2450 1.27

28days-3 252 1.22 2697 1.34

Tensile parameters - Influence of curing age

30

ID

7days-1 4079

4213

4600

4386

1.474

1.868

18.04

18.497days-2 4132 4144 2.091 19.30

7days-3 4428 4413 2.038 18.13

14days-1 5058

4529

4681

4647

2.511

2.457

18.99

17.6914days-2 3946 4653 2.131 17.04

14days-3 4582 4606 2.729 17.05

28days-1 4823

4960

5324

5229

2.675

2.991

18.02

20.5028days-2 5204 5470 2.929 21.31

28days-3 4853 4894 3.370 22.19


31



32

Influence of fiber content

33

ID

Vf0%-1 123

121

0.42

0.39

141

154

0.43

0.41Vf0%-2 127 0.38 166 0.41

Vf0%-3 113 0.36 155 0.39

Vf1%-1 156

185

0.62

0.65

2462

2402

0.83

0.80Vf1%-2 187 0.69 2529 0.78

Vf1%-3 213 0.64 2216 0.78

Vf2%-1 218

224

0.89

0.94

1578

1941

0.98

1.02Vf2%-2 242 1.00 2144 1.04

Vf2%-3 213 0.94 2101 1.04

Tensile parameters - Influence of fiber content

34

ID

Vf0%-1 3403

3198

3535

3643

0.037

0.041

0.040

0.062Vf0%-2 3000 3655 0.044 0.059

Vf0%-3 3191 3740 0.041 0.086

Vf1%-1 4000

3576

4023

3954

1.769

1.753

10.98

11.01Vf1%-2 3707 3960 1.854 11.32

Vf1%-3 3021 3878 1.636 10.74

Vf2%-1 4079

4213

4600

4386

1.474

1.868

18.04

18.49Vf2%-2 4132 4144 2.091 19.30

Vf2%-3 4428 4413 2.038 18.13


35



36

Influence of loading rate

37

ID

0.01in./min.-1 218

224

0.89

0.94

1578

1941

0.98

1.020.01in./min.-2 242 1.00 2144 1.04

0.01in./min.-3 213 0.94 2101 1.04

0.1in./min.-1 220

211

0.88

0.94

2342

2062

1.00

1.050.1in./min.-2 213 0.87 2068 1.04

0.1in./min.-3 200 1.06 1777 1.10

1in./min.-1 226

221

1.09

1.04

2629

2271

1.18

1.121in./min.-2 214 0.98 1925 1.08

1in./min.-3 224 1.06 2258 1.09

Tensile parameters - Influence of loading rate

38

ID

0.01 in./min.-1 4079

4213

4600

4386

1.474

1.868

18.04

18.490.01 in./min.-2 4132 4144 2.091 19.30

0.01 in./min.-3 4428 4413 2.038 18.13

0.1 in./min.-1 3982

4459

4456

4447

2.135

1.944

18.42

18.890.1 in./min.-2 4099 4560 1.952 18.64

0.1 in./min.-3 5295 4385 1.764 19.62

1 in./min.-1 4816

4705

5211

4833

2.885

2.362

25.05

21.531 in./min.-2 4588 4775 1.900 20.06

1 in./min.-3 4711 4512 2.300 19.49



39

Constitutive Model

• Higher tensile strength, strain hardening & softening, and more

ductile behaviors than conventional fiber reinforced concrete

• Three phases: Linear elastic, strain hardening phase (multiple

cracking), strain softening phase (fiber pull-out).

•UHPC matrix and fibers resist

the tensile force cooperatively

•Numerous microcracks form,

interfacial bond starts working

•A visible macrocrack forms,

steel fibers start pulling out

𝜎 =

𝜀

𝜀𝑐𝑐𝜎𝑐𝑐 0 ≤ 𝜀 ≤ 𝜀𝑐𝑐

𝜎𝑐𝑐 +𝜀 − 𝜀𝑐𝑐𝜀𝑝𝑐 − 𝜀𝑐𝑐

(𝜎𝑝𝑐 − 𝜎𝑐𝑐) 𝜀𝑐𝑐 ≤ 𝜀 ≤ 𝜀𝑝𝑐

1 −𝜀 − 𝜀𝑐𝑐𝜀𝑝𝑐 − 𝜀𝑐𝑐

𝜎𝑝𝑐 𝜀 ≥ 𝜀𝑝𝑐

40(Hung et al., 2013)

Stress-Strain Relationship

41

Idealized constitutive relationship

42

Discussion

@first cracking strength (σcc)

@post-cracking tensile strength (σpc)

strain @first cracking (εcc) ~~~ 220 mε

strain @post-cracking (εpc) ~~~ 2200 mε

σcc = 90% σpc

43

Discussion

tangent modulus of elasticity (Ecc)

initial modulus of elasticity (Et), 40% of tensile strength

dissipated energy density prior to post cracking (Gp)

total dissipated energy density (Gt)

Ecc = 95.4% Et

Gp = 12.2% Gt

Conclusions

• A designated dogbone-shaped specimen was developed for

direct tension test based on trial-and-error experimental

characterization and finite element modeling, from which

drawbacks in specimen design from literature were minimized.

• Tensile behaviors of UHPC were investigated using the

proposed DDT specimen design: .

• An idealized constitutive model was used to characterize the

tensile responses of UHPC into three phases: linear elastic,

strain hardening, and strain softening.

• Four tensile parameters extracted from DTT were implemented

into the idealized constitutive model to reconstruct the three-

phase responses.

• Most of tensile parameters increased with the increasing of

affecting factors (fiber content, curing age, and loading rate).

Conclusions

• Curing age: 7 days of curing gains 85% of stiffness and 82% of

“mature” strength (early properties development).

• Fiber content: 2% fiber UHPC is about 1.32 (stiffness) and 2.41

(strength) times higher than no-fiber reinforced UHPC; but

significant energy dissipation increase (about 46 (up to post

cracking) and 298 times (total energy) higher) (contribution of fiber

to ductility and energy absorption).

• Loading rate: in the studied range of 0.1 to 1 in/min, the effect is

minimal.

45

σcc = 90% σpc Ecc = 95.4% Et Gp = 12.2% Gt

εcc = 220 uε εpc = 2200 uε

46

Acknowledgements

Washington State Department of Transportation

(WSDOT)

Center for Environmentally Sustainable

Transportation in Cold Climates (CESTiCC)

Thank you!

Questions?

47

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