Strain rate-dependent shear failure surfaces of ultra-high-performance fiber-reinforced concretes

Ngo, Tri Thuong,Kim, Dong Joo,Moon, Jae Heum,Kim, Sung Wook Elsevier 2018 Construction and Building Materials Vol.171 No.-

<P><B>Abstract</B></P> <P>Strain rate-dependent shear behavior of ultra-high-performance fiber-reinforced concretes (UHPFRCs) with confining pressure was investigated using a new shear test setup in an improved strain energy impact machine (I-SEFIM). Different confining pressures were applied to the specimens prior to shear testing at both static and high strain rates, and were maintained during testing. The shear strength of UHPFRCs was highly sensitive to the applied strain rate and confining pressure. The effect of confining pressure on the shear strength was more pronounced at a static rate rather than at high strain rates. Strain rate-dependent shear failure surfaces of UHPFRCs were proposed.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The shear strength of UHPFRCs increased as the strain rate increased beyond 111 s<SUP>–1</SUP>. </LI> <LI> The shear strength of UHPFRCs is dependent upon the confining pressure. </LI> <LI> An empirical equation predicting the confined shear strength was proposed. </LI> <LI> Shear failure surfaces at different strain rates were obtained. </LI> </UL> </P>

Shear resistance of ultra-high-performance fiber-reinforced concrete

Ngo, Tri Thuong,Park, Jun Kil,Pyo, Sukhoon,Kim, Dong Joo Elsevier BV 2017 Construction and Building Materials Vol.151 No.-

<P><B>Abstract</B></P> <P>The shear resistance of ultra-high-performance fiber-reinforced concrete (UHPFRC) was investigated by using a newly proposed shear testing system. UHPFRCs displayed strain-hardening responses, in both shear and tensile testing, accompanied with multiple microcracks. The shear resistance of UHPFRCs was clearly influenced by their tensile resistance in addition to shear span to depth ratio (<I>a/d</I>). The shear strengths of UHPFRCs generally exceed the direct tensile strengths about 1.6 times. A theoretical model predicting the shear strength of UHPFRCs was proposed based on the tensile strength and <I>a/d</I> ratio.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A newly shear testing system to investigate shear resistance of UHPFRC was proposed. </LI> <LI> The shear related hardening behavior, accompanied by multiple crack formation was obtained. </LI> <LI> The shear resistances of UHPFRCs significantly depended on both the fiber volume and <I>a/d</I>. </LI> <LI> The shear strengths of UHPFRCs containing 0.5 and 1.5vol.% smooth fibers were shown to exceed the direct tensile strengths about 1.6 times. </LI> <LI> A theoretical model predicting the shear strength of UHPFRC based on the direct tensile strength and <I>a/d</I> was proposed. </LI> </UL> </P>

Shear stress versus strain responses of ultra-high-performance fiber-reinforced concretes at high strain rates

NGO, Tri Thuong,KIM, Dong Joo Elsevier 2018 International journal of impact engineering Vol.111 No.-

<P><B>Abstract</B></P> <P>Shear stress versus strain response of ultra-high-performance fiber-reinforced concretes (UHPFRCs) at high strain rates up to 248 s<SUP>−1</SUP> was investigated by installing the shear test set-up in an improved strain energy frame impact machine (I-SEFIM). The tensile strain-hardening UHPFRCs also produced shear-related hardening response, even at high strain rates, accompanied with multiple cracks. The shear resistance was obviously sensitive to the applied strain rates even though the shear strain rate sensitivity was not as high as the tensile strain rate sensitivity: the dynamic increase factor (DIF)-1.5 for the shear strength of UHPFRCs with 1.5 vol.-% fibers was significantly lower than the DIF (3.2) for the tensile strength at the strain rate of 248 s<SUP>−1</SUP> owing to the different distribution of inertial force of mortar matrix surrounding fiber, resulting from difference of loading direction. A DIF predicting equation was finally proposed for the shear strength of UHPFRCs at high strain rates.</P> <P><B>Highlights</B></P> <P> <UL> <LI> An impact shear test method for UHPFRCs was proposed. Shear stress versus strain response of UHPFRCs at high strain rates could be successfully measured. </LI> <LI> The UHPFRCs produced shear related hardening response, accompanied by multiple crack formation, even at high strain rates. </LI> <LI> The shear strength and shear peak toughness of UHPFRCs were high rate sensitive, whereas their shear strain capacity was not. </LI> <LI> Equation for predicting strain rate dependent shear strength of UHPFRCs was proposed. </LI> </UL> </P>

Synergy in shear response of ultra-high-performance hybrid-fiber-reinforced concrete at high strain rates

Ngo, Tri Thuong,Kim, Dong Joo Elsevier 2018 COMPOSITE STRUCTURES -BARKING THEN OXFORD- Vol.195 No.-

<P><B>Abstract</B></P> <P>Synergistic effects on shear resistance of ultra-high-performance hybrid-fiber-reinforced concrete (UHP-HFRCs) at high strain rates was investigated using a new shear test setup. Two UHP-HFRCs: L05S10 (containing 0.5 vol-% long and 1.0 vol-% short fiber) or L10S05; and, two ultra-high-performance mono-fiber-reinforced concretes (UHP-MFRCs): L15S00 (containing 1.5 vol-% long fiber) or L00S15, were tested. The L05S10 generated high synergy in shear resistance whereas the L10S05 did little. Synergies were significant for shear strength, shear peak toughness, but not for shear strain capacity. Moreover, the L00S15 produced the highest rate-sensitivity although the shear response of both UHP-MFRCs and UHP-HFRCs was generally sensitive to the applied strain rates. The experimental shear strengths were well-matched with the theoretical calculations.</P>

Loading rate effect on crack velocity in ultra-high-performance fiber-reinforced concrete

Ngo, Tri Thuong,Park, Jun Kil,Kim, Dong Joo Elsevier 2019 Construction and Building Materials Vol.197 No.-

<P><B>Abstract</B></P> <P>Loading rate effect on crack propagation in ultra-high-performance fiber-reinforced concrete (UHPFRCs) was investigated using a pre-notched three-point bending specimen in an improved-strain energy frame impact machine (I-SEFIM) and image processing techniques. The crack velocity of up to 984 m/s and the crack initiation strain rate of up to 271 s<SUP>−1</SUP> were observed. Crack velocity in UHPFRCs increased as the applied strain rate increased. Fiber reinforcements significantly affected on the crack velocity in the UHPFRC at static rates, but slightly did at high strain rates. There is a strong correlation between the strain-rate sensitivity and the dynamic crack growth characteristics of UHPFRCs.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The crack initiation and velocity at high speed were successfully tested by using the modified I-SEFIM. </LI> <LI> The crack velocity increased as the applied strain rate increased. </LI> <LI> The fiber reinforcement significantly affected on the crack velocity in the UHPFRCs at static rates, but it slightly did at high strain rates. </LI> <LI> The strain-rate sensitivity of UHPFRC is strongly correlated with the dynamic crack growth mechanism. </LI> </UL> </P>

축방향 압축응력이 초고강도 섬유보강 콘크리트의 전단 저항성에 미치는 영향

노지트엉 ( Ngo Tri Thuong ),김동주 ( Kim Dong Joo ) 한국구조물진단유지관리공학회 2018 한국구조물진단유지관리공학회 학술발표대회 논문집 Vol.22 No.1

Effect of confining pressure on the shear strength of ultra-high-performance fiber-reinforced concrete (UHPFRCs) was investigated using a new shear test setup. Different confining pressures were applied and maintained to the specimens prior to shear testing. The shear strength of UHPFRCs was obviously sensitive to the confining pressure: the higher confining pressure produced higher shear strength. The confined shear strength could be expressed as a function of unconfined shear strength, confining pressure, and tensile strength.

Interfacial bond characteristics of steel fibers embedded in cementitious composites at high rates

Park, Jun Kil,Ngo, Tri Thuong,Kim, Dong Joo Elsevier 2019 Cement and concrete research Vol.123 No.-

<P><B>Abstract</B></P> <P>In this study, the source of the rate-sensitive steel fiber pullout response in cement-based matrices was investigated at pullout rates of 0.0167–500 mm/s. A new pullout impact machine utilizing elastic strain energy to generate high-rate pullout loads was used in high-rate fiber pullout tests. In ultra-high-performance concrete (UHPC), smooth (S) steel fibers generated a higher peak bond strength dynamic increase factor (DIF, 1.9) than hooked steel fibers (DIF, 1.25), whereas S fibers in high-strength mortar (96 MPa) did not exhibit noticeable rate sensitivity. Furthermore, the addition of a shrinkage-reducing agent to UHPC clearly decreased the rate sensitivity of S fibers owing to the reduced matrix shrinkage: the peak bond strength DIF decreased from 1.9 to 1.77. The theoretical analysis revealed that the fiber pullout rate sensitivity was correlated with the interfacial crack path in a manner that depended on the fiber geometry as well as the matrix composition.</P>

고속 충격에서 초고강도 섬유보강 콘크리트의 균열속도 조사

박준길(PARK, Jun Kil),노지트엉(NGO, Tri Thuong),김동주(KIM, Dong Joo) 대한토목학회 2018 대한토목학회 학술대회 Vol.2018 No.10

Evaluating Load-Carrying Capacity of Short Composite Beam Using Strain-Hardening HPFRC

Duy-Liem Nguyen,Vu-Tu Tran,Ngoc-Thanh Tran,Tri-Thuong Ngo,Manh-Tuan Nguyen 대한토목학회 2021 KSCE JOURNAL OF CIVIL ENGINEERING Vol.25 No.4

The main goal of this study was to develop the short composite beams using strain-hardening fiber-reinforced concrete (S_HPFRC) and conventional concrete (CC) together. Firstly, the sensitivity of the hybrid fiber system to the enhancement of mechanical properties of plain high performance concrete (P_mortar) was experimentally studied. The ranking of the mechanical properties in terms of sensitive coefficient was observed as follows: direct tensile > splitting tensile > compressive. Next, the responses of short composite beams with various thicknesses of S_HPFRC were investigated under a three-point bending test (3PBT). Six beams with no reinforcement (type A) and six beams with reinforcement were tested (type B) with their identical dimensions of 150 × 150 × 300 mm (depth × width × span length). The compressive strength of S_HPFRC and CC were about 80.65 MPa and 21.12 MPa, respectively. Most of the composite beams were observed to fail in shear mode. And, there was a favorable effect on enhancing the load-carrying capacity of a beam as S_HPFRC was placed at a critical tensile zone. Finally, based on the test data, the analytical equations were proposed for the purpose of predicting shear resistance of the S_HPFRC - CC beam.