Direct tensile behavior of ultra high performance fiber reinforced concrete subjected to impact loading

i ABSTRACT Ultra-high-performance fiber-reinforced concrete UHPFRC is much expected to enhance the resilience and sustainability of civil infrastructure under impacts or blasts owing to

Direct Tensile Behavior of Ultra-High-Performance Fiber-Reinforced Concrete Subjected to Impact Loading

Tran Ngoc Thanh

February 2016

Department of Civil and Environmental Engineering

The Graduate School Sejong University

Direct Tensile Behavior of Ultra-High-Performance Fiber-Reinforced Concrete Subjected to Impact Loading

Tran Ngoc Thanh

February 2016

Department of Civil and Environmental Engineering

The Graduate School Sejong University

Direct Tensile Behavior of Ultra-High-Performance Fiber-Reinforced Concrete Subjected to Impact Loading

Tran Ngoc Thanh

A dissertation submitted to the Faculty of the Sejong University in partial fulfillment of the requirements for the degree of Doctor of

Philosophy in Civil and Environmental Engineering

February 2016

Approved by Major Advisor Professor Dong Joo KIM

Direct Tensile Behavior of Ultra-High-Performance Fiber-Reinforced Concrete

Subjected to Impact Loading

DEDICATION

I would like to dedicate this dissertation to my parents, my brother and my sister who

have supported me all the way

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ABSTRACT

Ultra-high-performance fiber-reinforced concrete (UHPFRC) is much expected to enhance the resilience and sustainability of civil infrastructure under impacts or blasts owing to its outstanding tensile properties including high strength, ductility and energy absorption capacity at static rate; however, its tensile response at high strain rate is still not well understood In order to fill the knowledge gap, the objective of this research

is to develop better understanding of UHPFRC tensile response at high strain rates The research is divided into three parts, as follows:

In first part, the direct tensile stress versus strain response of UHPFRCs at high strain rates (5 to 24 /s) was investigated UHPFRCs exhibited tensile strain hardening behavior even at high strain rates and especially their tensile behavior was found to be very sensitive to the applied strain rates The tensile behavior of UHPFRCs at high strain rates was much influenced by the size of specimen and fiber type Unlike at static rate, UHPFRCs with smooth fibers produce higher tensile properties than those with twisted fibers at high strain rates

In second part, the fracture energies including peak toughness and softening fracture energy of UHPFRCs at high strain rates (5–92 /s) was investigated and the UHPFRCs with 11.5% of fibers volume contents exhibited very high entire fracture energy (2871 kJ/m2

) at high strain rates The peak toughness was highly sensitive to strain rate whereas the softening fracture energy was not In the investigation of UHPFRCs fracture energies, the un-notched specimens were more suitable than

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notched specimens The effects of fiber type and fiber volume on the UHPFRCs fracture energies were found to be totally different between at static rate and at high strain rates

Finally, a method for enhancing the tensile resistance of UHPFRCs at high strain rates (16 – 37 /s) was proposed by blending long and short steel fibers UHPFRCs blending small volume content of long and short steel fibers (total 1.5%) produced very high tensile resistance at high strain rates: post cracking strength up to 32.6 MPa, strain capacity up to 1.87%, peak toughness up to 412.6 kJ/m3 and softening fracture energy up to 31.3 kJ/m2 In particular, UHPFRCs with fibers blending, based on the synergistic performance, produced 86.4% higher strain capacity and 72.2% higher peak toughness than normal UHPFRCs with mono steel fibers The strain rate sensitivity model in tension of UHPFRCs based on the best fit of experimental test results was proposed and the tensile resistance of UHPFRCs was predicted even at high strain rates

Keywords: Direct tensile response, Fracture energy, Ultra-high-performance fiber-reinforced concrete, High strain rates, Fiber blending

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ABSTRACT i

TABLE OF CONTENTS iii

LIST OF FIGURES vii

LIST OF TABLES ix

CHAPTER 1 INTRODUCTION 1

1.1 Motivation 1

1.2 Literature review and background 2

1.2.1 Development of UHPFRC 2

1.2.2 Tensile response of UHPFRC at high strain rates 7

1.3 Goal and objectives 12

1.4 Organization of Dissertation 13

PUBLICATIONS FROM DISSERTATIONS 16

References 17

CHAPTER 2 INVESTIGATING TENSILE RESPONSE OF UHPFRC AT HIGH STRAIN RATE 22

2.1 Introduction 22

2.2 Experimental program 27

2.2.1 Materials and specimen preparation 28

2.2.2 Test setup and procedure 30

2.3 Test results 34

2.3.1 Static test results 34

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2.3.2 High strain rates test results 40

2.3.3 Strain rate sensitivities 43

2.4 Discussion 45

2.4.1 Effect of fiber type on the rate sensitive of UHPFRCs 45

2.4.2 Effect of specimen size on the rate sensitive of UHPFRCs 54

2.5 Conclusion 59

References 60

CHAPTER 3 INVESTIGATING FRACTURE ENERGY OF UHPFRC AT HIGH STRAIN RATE 66

3.1 Introduction 66

3.2 Fracture energy of UHPFRC at high strain rates 69

3.3 Experimental procedure 73

3.3.1 Materials and specimen preparation 74

3.3.2 Test setup and procedure 75

3.4 Test results 78

3.4.1 Static test results 79

3.4.2 High-strain-rate test results 85

3.5 Discussion 90

3.5.1 Strain-rate effect on UHPFRC fracture resistance 90

3.5.2 Effect of fiber type and fiber volume content on UHPFRC fracture resistan-ce 94

3.5.3 Effect of double edge notches on UHPFRC fracture resistance 97

3.6 Conclusions 101

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References 102

CHAPTER 4 ENHANCING TENSILE RESISTANCE OF UHPFRC AT HIGH STRAIN RATE 110

4.1 Introduction 110

4.2 Impact resistance of hybrid fiber-reinforced cementitious composites 114

4.3 Experimental procedure 116

4.3.1 Materials and specimen preparation 117

4.3.2 Test setup and procedure 119

4.4 Test results 120

4.4.1 Static test results 122

4.4.2 High strain rate test results 122

4.5 Discussion 127

4.5.1 Strain rate sensitivities 127

4.5.2 Comparative tensile resistance of UHP-HFRCs at high strain rates 133

4.5.3 Synergistic effects of blending long and short fibers 134

4.5.4 Predict the tensile resistance of UHPFRC at high strain rate 138

4.6 Conclusion 144

References 145

CHAPTER 5 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS 154

5.1 Summary and Conclusions 154

5.1.1 Conclusions related to the investigating direct tensile response of UHPFRC at high strain rate 155 5.1.2 Conclusions related to the investigating fracture energy of UHPFRC at high

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strain rate 156

5.1.3 Conclusions related to the enhancing tensile resistance of UHPFRC at high s train rate 157

5.2 Recommendations 158

국문초록 160

Acknowledgements 163

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Fig 1.1  Tensile response of UHPFRCs using deformed steel fiber [1] 4

Fig 1.2  Tensile response of UHPFRCs blending two fibers [13] 5

Fig 1.3  Behavior of UHPFRCs in comparison to other material 7

Fig 1.4  Indirect tensile test at high strain rates 8

Fig 1.5  Tensile response of UHPFRCs at low strain rates [3-5] 11

Fig 1.6  Tensile response of UHPFRCs at high strain rates [6] 12

Fig 1.7  Structure of the Dissertation 15

Fig 2.1 – Tensile response of UHPFRCs in comparison to normal concrete (NC), fiber reinforced concrete (FRC) 23

Fig 2.2 – Detail of experimental program 28

Fig 2.3 – The geometry of static tensile specimens 31

Fig 2.4 – High strain rate tensile test set up 32

Fig 2.5 – The geometry of high strain rate tensile specimens 32

Fig 2.6 – Tensile stress versus strain response of UHPFRCs at static rate 39

Fig 2.7 – Multiple cracking behavior within the gauge length of UHPFRCs at static rate 40

Fig 2.8 – Tensile stress versus strain response of UHPFRCs at high strain rates 42

Fig 2.10 – Strain rate effect on tensile parameters of UHPFRCs 45

Fig 2.11 – Effect of fiber types on tensile resistance of UHPFRCs 47

Fig 2.12 – Cracking behavior prior to crack localization of UHPFRCs at high rates 52 Fig 2.13 – Cracking opening displacement history 53

Fig 2.14 – Effect of fiber types on rate sensitivity of UHPFRCs 54

Fig 2.15 – Effect of specimen sizes on tensile resistance of UHPFRCs 55

Fig 2.16 – Acceleration and inertial force of large size and small size specimen 57

Fig 2.17 – Contribution of inertial force to the total resistance of specimen 58

Fig 2.18 – Effect of specimen sizes on rate sensitivity of UHPFRCs 58

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Fig 3.1 – Fracture energy of various cement based materials: NC, FRC, HPFRCCs,

UHPFRCs 69

Fig 3.2 – The geometry of specimens 77

Fig 3.3 – Typical tensile response and fracture parameters of UHPFRCs 79

Fig 3.4 – Tensile stress versus displacement curve of UHPFRCs at static rate 84

Fig 3.5 – Multiple cracking behavior of UHPFRCs within gauge length at static rate 85

Fig 3.6 – Tensile stress versus displacement curve of UHPFRCs at high strain rates 88 Fig 3.7 – Multiple cracking behavior of UHPFRCs within gauge length at high strain rates 89

Fig 3.8 – Fracture energy of UHPFRCs at high strain rates 90

Fig 3.9 – Strain rate effects on fracture parameters of UHPFRCs 91

Fig 3.10 – Effects of matrix strength on rate sensitivity of HPFRCCs and UHPFRCs 94

Fig 3.11 – Breakage of twisted fibers (in UT15B) at high strain rates 96

Fig 3.12 – Group effects on pullout resistance of single fiber 97

Fig 3.13 – Effects of double edge notches on the fracture parameters of UHPFRCs 99 Fig 4.1 – The images of fibers 118

Fig 4.2 – The geometry of specimens 120

Fig 4.3 – Typical tensile response and parameters of UHPFRCs 121

Fig 4.4 – Tensile stress versus displacement curve of UHPFRCs 125

Fig 4.5 – Multiple cracking behavior of UHPFRCs within gauge length 126

Fig 4.6 – Strain rate effects on fracture parameters of UHPFRCs 129

Fig 4.7 – Comparative tensile resistance of UHP-HFRCs 133

Fig 4.8 – Synergy response of tensile resistance of UHP-HFRCs 137

Fig 4.9 – DIF of post cracking strength of UHPFRCs 140

Fig 4.10 – Comparison of tensile resistance of UHPFRCs between the experiment and theory 141

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Table 2.1 – Test series of tensile specimens 27

Table 2.2 – Composition of matrix mixture by ratio and compressive strength 29

Table 2.3 – Properties of steel fibers 29

Table 2.4 – Test results: tensile parameters of UHPFRCs with twisted fibers 36

Table 2.5 – Test results: tensile parameters of UHPFRCs with long smooth fibers 37

Table 2.6 – Test results: tensile parameters of UHPFRCs with short smooth fibers 38

Table 3.1 – Test series of tensile specimens 74

Table 3.2 – Composition of matrix mixture by weight ratio and compressive strength 74

Table 3.3 – Properties of steel fibers 75

Table 3.4 – Fracture parameters of un-notched UHPFRCs 82

Table 3.5 – Fracture parameters of notched UHPFRCs 83

Table 3.6 – Characteristic length of UHPFRCs 84

Table 4.1 – Direct tensile properties of various concretes at high strain rates 113

Table 4.2 – Test series of tensile specimens 117

Table 4.3 – Composition of matrix mixture by weight ratio and compressive strength 118

Table 4.4 – Properties of steel fibers 118

Table 4.5 – Tensile parameters of UHP-MFRCs 130

Table 4.6 – Tensile parameters of UHP-HFRCs 131

However, the promise of UHPFRC for dynamic application is mostly based on their superior tensile response at static rate Their tensile response at high strain rates (impact or blast rates: 1 - 1000 /s) is still not well understood although a few research investigated tensile response of UHPFRCs, most of investigated strain rates were lower than seismic strain rates (lower than 1 /s) [3-5] According to the best knowledge of author, only Cadoni et al [6] investigated direct tensile response of notched cylindrical UHPFRC specimens at high strain rates (50, 100 and 200 /s) However, the notched specimen used in their experimental tests might not be appropriate for capturing the strain hardening behavior of UHPFRCs [7]

It is obviously that the tensile response of UHPFRCs at high strain rates can not be predicted using that at lower strain rates Moreover, the tensile response of UHPFRCs

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at high strain rates is crucial for most dynamic application because the impact or blast resistance of UHPFRCs is more closely related to their tensile properties than compressive properties at high strain rates [8] Thus, it is necessary to develop better understanding of their tensile response at high strain rates before utilizing UHPFRCs

in the practical structure subjected to impact or blast loadings

1.2 Literature review and background

A general literature review is presented in this chapter The further research background information is presented in each chapter

1.2.1 Development of UHPFRC

UHPFRC one of the greatest breakthrough in concrete technology in the 21stcentury, have been developed since 1960‘s [9] According to American Concrete Institute (ACI) Committee, UHPFRC can be defined as follow: ― Concrete, ultra-high performance concrete that has a minimum specified compressive strength of 150MPa with specified durability, tensile ductility and toughness requirement; fiber are generally included to achieve specified requirement.‖

Owing to its promissory performance, UHPFRCs have widely attracted much research in exploring material properties including durability and mechanical properties UHPFRCs have been demonstrated to have highly resistance to chemical, sulfate, chloride attack, abrasion, freeze thaw cycle, self healing capacity [10-12] In addition, they also exhibited high compressive resistance, tensile resistance, shear

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resistance, flexural resistance, penetration resistance, etc [13-14] Currently, various commercial UHPFRCs are available on the market: BSI/CERACEM with 2.5% steel fibers [15], CARDIFRC with 6% steel fibers [16], CEMTECmultiscale with 6% steel fibers [2], CRC with 12% steel fibers [17], RPC with 2.5% steel fibers [18] and DUCTAL with 6% steel fibers [30] Moreover, UHPFRCs have been primarily applied to new structures, e.g., bridge, slab, pre-stress structure and to existing structures as a repair material [19]

Among mechanical properties, the tensile properties of UHPFRCs have received a great amount of attention because their tensile resistance is much weaker than others

To achieve tensile strain hardening behavior and to improve tensile properties including tensile strength, ductility and energy absorption capacity, many methods have been proposed by (1) using deform steel fibers [1], (2) blending more than one fibers [13], and (3) increasing volume of fiber content [1] In particular, strain hardening UHPFRCs with very high tensile strength (over 10 MPa), high ductility (strain capacity over 0.5%) and high energy absorption capacity (fracture energy over

30 kJ/m2), could be obtained, with small amount of steel fibers (< 2.5% by volume) by using deformed steel fibers as shown in Fig 1.1 [1] or blending two fibers as shown in Fig 1.2 [13]

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a) Tensile stress and strain response of UHPFRCs

b) Multiple micro cracking behavior of UHPFRCs Fig 1.1  Tensile response of UHPFRCs using deformed steel fiber [1]

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waste or recycle material, e.g., palm oil fuel ash, rice husk ash, etc have been utilized

in UHPFRC to save cost [22-23] In addition, deformed fibers or fiber blending have been used to reduce fiber volume content and minimize the cost of UHPFRC [1,13] Besides, UHPFRC without heat or pressure curing have been developed to facilitate

on site application [24]

Overall, UHPFRCs have been developed to not only exhibit much higher compressive strength but also produce dominant tensile strength, ductility and energy absorption capacity than other concrete In particular, the compressive strength and ductility of UHPFRCs are relatively close to those of steel, as shown in Fig 1.3

1.2.2 Tensile response of UHPFRC at high strain rates

A considerable number of researchers have investigated tensile strength and fracture energy of UHPFRCs at high strain rates using indirect tensile test such as spalling or splitting test [25-28], as shown in Fig 1.4, although they could not obtain the direct tensile stress versus strain response of UHPFRCs at high strain rates Millon

et al [25], Noldgen et al [26] performed spalling test to evaluate the tensile strength and fracture energy of UHPFRCs at high strain rates from 100 to 160 /s They reported that the tensile strength of UHPFRCs was sensitive to the strain rate, whereas their fracture energy was not The DIFs of tensile strength was around 5, whereas the DIFs

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of fracture energy was around 1.1 In addition, UHPFRCs showed much higher tensile strength (3 times higher) and fracture energy (29 times higher) at high strain rate (120 /s) than normal concrete Rong et al [27] indicated that the tensile strength of UHPFRCs increased as the strain rate increased from 21 to 66 /s The DIFs of tensile strength was around 4 Moreover, the tensile strength at high strain rates increased as the fiber volume contents increased, specifically the tensile strength increased 2 to 2.5 times when the volume contents of fiber were between 3% and 4% By using splitting test, Bragov et al [28] reported that the tensile strength of UHPFRCs clearly increased

as the rate of loading became faster at the stress rates ranging 500×103 and 3000×103 MPa/s and the DIF of tensile strength was 2 at the highest stress rate 3000×103 MPa/s

a) Spalling test (Brara et al [29])

b) Splitting test (Bragov et al [28]) Fig 1.4  Indirect tensile test at high strain rates However, there are only a few researches could investigate the direct tensile responses of UHPFRCs at seismic rates [3-5], as shown in Fig 1.5 Fujikake et al [3]

in 2005 evaluated the effects of loading rates ranging 0.0001 mm/s to 50 mm/s on the tensile response of UHPFRCs In their experiment, double notches (12.5 mm depth and 3 mm wide) were cut at the center of specimen and four loading rates of 0.001, 0.2

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2 50 mm/s were tested The results showed that the first cracking strength and post cracking strength increased as the loading rates increased In addition, the relationship between the tensile stress and crack opening was proposed Habel et al [4] in 2008, performed the uniaxial tensile test on UHPFRCs specimen at different strain rates of 0.0000008 /s, 0.005 /s and 0.02 /s They indicated that UHPFRCs maintain strain hardening accumulated by multiple cracking at strain rate of 0.02 /s The post cracking strength increased by approximately 25% whereas the strain capacity reduced 33% as the strain rate increased from 0.0000008 /s to 0.02 /s In addition, the energy dissipation during strain softening increased whereas the energy dissipation during strain hardening reduced as the strain rate increased Wille et al [5] in 2012 investigated the direct tensile response of UHPFRCs at low strain rates ranging static (0.0001 /s) to seismic rate (0.1 /s) Three volume of fibers contents, 2%, 2.5%, and 3%, were examined According to their reports, UHPFRCs still maintained strain hardening tensile behavior accompanied with multiple micro-cracks at seismic rates The post cracking strength increased up to 20% and the energy absorption capacity increased up to 40% as the strain rates increased from 0.0001 /s to 0.1 /s Moreover, for a given strain rates the post cracking strength, the strain hardening modulus and the energy absorption capacity increased as the volume fiber content increased

10 a) Fujikake et al (2005)

b) Habel et al (2008)

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c) Wille et al (2012) Fig 1.5  Tensile response of UHPFRCs at low strain rates [3-5] Only Cadoni et al [6] investigated the tensile stress versus crack opening displacement responses of UHPFRCs at high strain rates (50, 100 and 200 /s), as shown in Fig 1.6 In their experiment, cylindrical specimens were notched with a depth of 35mm and a width of 4mm Moreover, two types of fibers, steel and PVA fibers, with fiber volume content of 2.5% were reinforced in ultra high performance concrete They reported that the investigated UHPFRCs showed softening behavior after first cracking, whereas it exhibited hardening behavior at static rate The tensile strength of UHPFRCs significantly increased as the strain rates increased from static

to 200 /s The steel fibers maintained the pullout mode at high strain rates whereas PVA fibers were broken

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a) UHPFRCs with PVA fibers b) UHPFRCs with steel fibers

Fig 1.6  Tensile response of UHPFRCs at high strain rates [6]

Based on above references, there is still not enough information about direct tensile response of UHPFRC at high strain rates In particular, the current research could not achieve the tensile strain hardening behavior of UHPFRC at high strain rates

1.3 Goal and objectives

The overall goal of this research is to develop better understanding about the tensile response of ultra-high-performance fiber-reinforced concrete at high strain rates Three major objectives have been conducted in order to achieve the goal, as follows:

Objective #1: Investigating direct tensile response of UHPFRCs at high strain

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rates The specific tasks are: (1) to investigate the direct tensile stress versus strain responses of UHPFRCs at high strain rates; (2) to find the most effective fiber for high tensile resistance of UHPFRCs especially at high strain rates; and, (3) to discover any effect of specimen size on the measured tensile responses of UHPFRCs at high strain rates

Objective #2: Investigating fracture energy of UHPFRCs at high strain rates The specific tasks are: (1) to discover and evaluate any rate effects on fracture energy of UHPFRCs; (2) to investigate the effects of fiber type and fiber volume content on the fracture energy of UHPFRCs at high strain rates; (3) to evaluate the effects of double edge notches on fracture energy of UHPFRCs at high strain rates

Objective #3: Enhancing the tensile resistance of UHPFRCs at high strain rates by blending small volume of long and short fibers The specific tasks are: (1) to investigate strain rate sensitivity of UHPFRCs with fiber blending; (2) to compare the tensile resistance of various UHPFRCs with fiber blending at high strain rates; (3) to evaluate synergistic response of UHPFRCs with blended fibers at high strain rates; and (4) to predict the tensile resistance of UHPFRCs at high strain rates

1.4 Organization of Dissertation

This dissertation includes five chapters covering three subjects: investigating tensile response of UHPFRC at high strain rate (Chapter II), investigating fracture energy of UHPFRC at high strain rate (Chapter III) and enhancing tensile resistance of UHPFRC at high strain rate (Chapter IV) The structure of the dissertation is shown in

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Fig 1.7 The details of each chapter are described below

Chapter I Introduction This chapter covers motivation, background, goal and objectives of this thesis

Chapter II Investigating tensile response of UHPFRC at high strain rate This chapter investigates tensile response of UHPFRC at high strain rates The effects of specimen size and fiber type on UHPFRC tensile response at high strain rate are evaluated

Chapter III Investigating fracture energy of UHPFRC at high strain rate This chapter investigates the fracture energy of UHPFRC at high strain rates The effects of specimen shape, fiber type and fiber volume on UHPFRC fracture energy at high strain rate are evaluated

Chapter IV Enhancing tensile resistance of UHPFRC at high strain rate This chapter proposes a method to enhance the tensile resistance of UHPFRC at high strain rate by blending long and short fibers The tensile resistance of UHPFRC even at high strain rate is predicted

Chapter VI Summary, conclusions, and recommendations This chapter provides summary, conclusions, and recommendations for future work

Enhancing the tensile resistance of UHPFRC at high strain rate

Summary, conclusions and recommendations

(Chapter V)

Fig 1.7  Structure of the Dissertation

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PUBLICATIONS FROM DISSERTATIONS

 N.T Tran, T.K Tran, D.J Kim, High rate response of

ultra-high-performance fiber-reinforced concretes under direct tension, Cement and

Concrete Research 69 (2015) 72-87

 N.T Tran, T.K Tran, J.K Jeon, J.K Park, D.J Kim, Fracture energy of

ultra-high-performance fiber-reinforced concrete at high strain rates, Cement

and Concrete Research (2015) (Accepted)

 N.T Tran, D.J Kim, Synergistic response of ultra-high-performance hybrid-

fiber-reinforced concrete at high strain rates, Cement and Concrete

Composites (Under reviewed)

[3] K Fujikake, T Senga, N Ueda, T Ohno, M Katagiri, Effects of Strain Rate on Tensile Behavior of Reactive Powder Concrete, Journal of Advanced Concrete Technology 4 (1) (2006) 79-84

[4] K Wille, S El-Tawil, A.E Naaman, Strain rate dependent tensile behavior of ultra-high performance fiber reinforced concrete, RILEM proceedings of HPFRCC 6, Ann Arbor, MI, USA (2012) 382-387

[5] K Habel, P Gauvreau, Response of ultra-high performance fiber reinforced concrete (UHPFRC) to impact and static loading, Cement and Concrete

[8] M Maalej, J Zhang, S.T Quek, S.C Lee, High-Velocity Impact Resistance of

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Hybrid-Fiber Engineered Cementitious Composites, International Association of Fracture Mechanics for Concrete and Concrete Structures, FraMCoS (2004) [9] A.E Naaman, K Wille, The path to Ultra-High Performance Fiber Reinforced Concrete (UHP-FRC): Five Decades of Progress In: Third international symposium on UHPC and nanotechnology for high performance construction materials, Kassel University Press, Kassel, Germany, 2012, pp 3-16

[10] B.A Graybeal, Material Property Characterization of Ultra-High Performance Concrete, Report No FHWA-HRT-06-103, Federal Highway Administration, Washington, DC, 2006

[11] C.G Pfeifer, B Moeser, C Giebson, J Stark, Durability of performance concrete, Tenth ACI International Conference on Recent Advances

ultra-high-in Concrete Technology and Sustaultra-high-inability Issues No SP-261-1, 2009

[12] B.A Graybeal, Ultra-High Performance Concrete, Report No

FHWA-HRT-11-038, Federal Highway Administration, Washington, DC, 2011

[13] S.H Park, D.J Kim, G.S Ryu, K.T Koh, Tensile behavior of Ultra High Performance Hybrid Fiber Reinforced Concrete, Cement and Concrete Composites 34 (2) (2012) 172-184

[14] D.J Kim, S.H Park, G.S Ryu, K.T Koh, Comparative flexural behavior of Hybrid Ultra High Performance Fiber Reinforced Concrete with different macro fibers, Construction and Building Materials 25 (11) (2011) 4144-4155

[15] J Jungwirth, Zum Tragverhalten von zugbeanspruchten Bauteilen aus Hochleistungs-Faserbeton, Ecole Polytechnique Federale de Lausanne, Ph.D

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thesis 2006

[16] S.D.P Benson, B.L Karihaloo, CARDIFRC – Development and mechanical properties, Part III: Uniaxial tensile response and other mechanical properties properties, Magazine of Concrete Research 57(8) (2005) 433 – 443

[17] N.H Bache, Principles of similitude in design of reinforced brittle matrix composites in H W Reinhardt & A E Naaman (eds) 1992, High Performance Fiber Rein-forced Cement Composites, Rilem Proceedings 15, Published by E

& FN Spon London (1992) 39 – 56

[18] P Richard and M Cheyrezy, Composition of reactive powder concretes, Cement and Concrete Research 25(7) (1995) 1501 – 1511

[19] N Behzad, M.R.S Raizal, S.J Mohd, V.L Yen, A review on ultra high performance ‗ductile‘ concrete (UHPdC) technology, International Journal of Civil and Structural Engineering 2(3) (2012) 1003-1018

[20] B.A Tayeh, B.H Abu Bakar, M.A Megat Johari, Y.L Voo, Mechanical and permeability properties of the interface between normal concrete substrate and Ultra-high Performance Fibre concrete overlay, Constr Build Mater 36 (2012) 538–48

[21] A.M.T Hassan, S.W Jones, G.H Mahmud, Experimental test methods to determine the uniaxial tensile and compressive behaviour of Ultra-high Performance Fibre Reinforced Concrete (UHPFRC), Constr Build Mater 37

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(2012) 874–82

[22] N.V Tuan, G Ye, K Breugel, A.L.A Fraaij, B.D Dai, The study of using rice husk ash to produce ultra-high performance concrete, Constr Build Mater 25 (2011) 2030–5

[23] S.L Yang, S.G Millard, M.N Soutsos, S.J Barnett, T.T Le, Influence of aggregate andcuring regime on the mechanical properties of ultra-high performance fibre reinforced concrete (UHPFRC) Constr Build Mater 23 (2009) 2291–8

[24] K Wille, A E Naaman, S El-Tawil, and G J Parra-Montesinos, Ultra-high performance concrete and fiber reinforced concrete: achieving strength and ductility without heat curing, Materials and Structures 45(3) (2012) 309–324 [25] O Millon, W Riedel, K Thoma, E Fehling, M Noldgen, Fiber-reinforced ultra-high performance concrete under tensile loads, International Conference

on the Mechanical and Physical Behaviour of Materials under Dynamic Loading (2009) 671–677

[26] M Noldgen, W Riedel, K Thoma, E Fehling, Properties of Ultra High Performance Concrete (UHPC) in tension at high strain rates, International Association of Fracture Mechanics for Concrete and Concrete Structures, FraMCoS (2013) 325

[27] Z Rong, W Sun, Experimental and numerical investigation on the dynamic tensile behavior of ultra-high performance cement based composites, Construction and Building Materials 31 (2012) 168-173

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[28] A.M Bragov, YuV Petrov, B.L Karihaloo, AYu Konstantinov, D.A Lamzin, Dynamic strengths and toughness of an ultra high performance fibre reinforced concrete, Engineering Fracture Mechanics 110 (2013) 477-488

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113

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2.1 Introduction

Since the September 11 attacks in 2001, to protect and to enhance the resistance of building and civil infrastructure under extreme loading conditions such as airplane impacts and blasts, numerous researches have been intensively carried out to prevent catastrophes [1-4] The September 11 attacks killed almost 3000 people, caused serious damage to the economy of Lower Manhattan and further generated a significant effect on global security system [5-6]

There have been various approaches in different levels for preventing those catastrophes One of approaches is to strengthen the national security systems [1] The other approach is to develop and apply structural systems with high impact and blast resistance under such extreme loads [2-4] However, the national security system might not eliminate all potential causes for the collapses or damages of building and civil infrastructure generated by manmade and especially by natural disaster In addition, the structural systems of existing buildings and infrastructure cannot be easily modified for improving their impact and blast resistance

Thus, in this study, it is proposed to improve the resistance of infrastructure under natural and manmade extreme events, e.g., airplane impacts, earthquake, blast, and

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typhoon, by simply attaching ultra-high-performance fiber-reinforced concretes (UHPFRCs) panels with high ductility and energy absorption capacity or by overlaying them with UHPFRCs Strain hardening UHPFRCs, with high tensile strength (over 10 MPa), high ductility (strain capacity over 0.5%) and high energy absorption capacity, could be recently obtained, with small amount of steel fibers (< 2.5% by volume), by combining dense ultra-high-performance concrete (UHPC) matrix containing very fine particles and tailored interfacial bond strength between fiber and matrix [7-8]; and, further enhanced by blending long deformed and short smooth steel fibers [9] In comparison with other cement based construction materials, strain hardening UHPFRCs showed much higher tensile resistance, in Fig 2.1

Fig 2.1 – Tensile response of UHPFRCs in comparison to normal concrete (NC),

fiber reinforced concrete (FRC)

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The superior direct tensile behavior of UHPFRCs is mostly based on their responses measured at static rate Owing to the static tensile behavior of UHPFRCs, it has been expected that UHPFRCs would produce higher tensile resistance even at higher strain rates Although several papers reported about the behavior of UHPFRCs under high strain rates, most of them reported about the flexural behavior of UHPFRCs [10-14] The flexural resistance of UHPFRCs under impact was found to

be much higher than that of steel fiber reinforced concrete (SFRC) and normal concrete [10] And, the flexural strength and toughness of UHPFRCs under impact significantly increased as the strain rate (or stress rate) increased The enhancement of flexural strength at high strain rates was reported to be much correlated to the matrix-fiber interface bond characteristics between fiber and matrix [11]

On the other hand, there are a few researches reporting the dynamic tensile strength and fracture energy of UHPFRCs by performing spalling test (indirect tensile test) [15-17] Millon et al 2009 [15] and Noldgen et al 2013 [16] reported that the spalling tensile strength of UHPFRCs was sensitive to the strain rate, whereas their fracture energy was not Rong et al 2012 [17] indicated that the spalling tensile strength of UHPFRCs increased as the strain rate increased from 21 to 66 /s

Very few studies have investigated the direct tensile stress versus strain responses

of UHPFRCs at seismic rates [12,18] Habel et al [12] and Wille et al [18] investigated the direct tensile response of UHPFRCs at the strain rates ranging static (0.0001 /s) to seismic rate (0.1 /s) They reported that as the strain rate increased from static to seismic rate, the post cracking tensile strength and the energy absorption

Recently, Tran and Kim 2012 [20-22] published several papers regarding the direct tensile behavior of high performance fiber reinforced cementitious composites (HPFRCCs), reinforced with deformed high strength steel fibers, at high strain rates (10-40 /s): they applied an innovative strain energy frame impact machine (SEFIM) to perform direct tensile tests, with a small test machine, for HPFRCCs requiring large size specimen [20] Tran and Kim [21] reported that the HPFRCCs with deformed steel fibers maintained their tensile strain hardening behavior even at higher strain rates and the interfacial bond strength is main source of the rate sensitivity of fiber reinforced cementitious composites They also found that the strength of matrix had more significant influence on the strain rate sensitivity on the tensile strength of