Effect of steel fibers on the flexural behavior of RC beams with very low reinforcement ratios

Yoo, Doo-Yeol,Moon, Do-Young Elsevier 2018 Construction & building materials Vol.188 No.-

<P><B>Abstract</B></P> <P>This study aims to investigate the implications of hooked-end steel fibers on the flexural performance of reinforced concrete (RC) beams with very low reinforcement ratios. For this, four different fiber volume fractions, v<SUB>f</SUB>, of 0.25%, 0.50%, 0.75%, and 1.00%, were incorporated into the concrete mixture and plain concrete without fibers was considered as a control specimen. Four reinforcement ratios of 0.178%, 0.267%, 0.317%, and 0.406%, which are 44%, 66%, 78%, and 100% of the minimum reinforcement ratio, ρ<SUB>min</SUB>, were also adopted to evaluate the steel fiber effect on the flexural behavior of RC beams with various very low reinforcement ratios. The test results indicated that the overall flexural performance of RC beams, in terms of flexural strength, deflection capacity, post-cracking flexural stiffness, and cracking behavior, was improved by increasing the reinforcement ratio up to ρ<SUB>min</SUB>. Higher initial cracking and yield loads, post-cracking stiffness, and better cracking performance of RC beams were also obtained by including steel fibers. However, the enhancement of ultimate load carrying capacity by steel fibers was relatively minor, and the ductility index and flexural strength margin, used to guarantee a ductile failure mode, deteriorated with the inclusion of steel fibers. The lower reinforcement ratios and higher fiber volume fractions clearly led to lower ductility indices. Therefore, it was concluded that longitudinal steel rebar could not be replaced with discontinuous steel fibers at moderate volume fractions, v<SUB>f</SUB> ≤ 1.0%, in terms of ultimate load carrying capacity, ductility, and flexural strength margin. Lastly, analytical results considering material models for steel fiber-reinforced concrete (SFRC), given by the RILEM recommendation, generally overestimated the flexural capacities of reinforced SFRC beams, and the inaccuracy increased with increasing fiber contents.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Flexural performance of RC beams with low reinforcement ratios is improved by adding steel fibers. </LI> <LI> Lower ductility index is obtained with lower reinforcement ratio and higher steel fiber content. </LI> <LI> Flexural strength margin and ductility of RC beams are deteriorated by adding steel fibers. </LI> <LI> Flexural strength decrease of RC beams with decreasing steel bar amount is not recovered by adding steel fibers up to 1%. </LI> <LI> Steel rebar cannot be replaced with discontinuous steel fibers at moderate amounts below 1%. </LI> </UL> </P>

Study on Mechanical and Thermal Properties of Fiber-Reinforced Epoxy/Hybrid-silica Composite

Kim, Dongho,Chung, Ildoo,Kim, Guni THE KOREAN FIBER SOCIETY 2013 FIBERS AND POLYMERS Vol.14 No.12

Recently, carbon fiber composites have been widely used as structural reinforcement materials of buildings, replacing reinforcing bars or concrete. And the increase in use of super fibers such as aramid and high strength PE, which is aimed at improving the reinforcement properties, has resulted in a demand for a resin system with excellent mechanical and thermal properties. In this research, a fiber-reinforced composite has been produced by using the super fibers such as carbon fiber or aramid fiber, reinforcement resin and the silica hybrid compound containing epoxy group. This study was carried out to confirm the effect of the silica hybrid on mechanical properties, heat resistance and adhesion strength of a fiber-reinforced epoxy composite, which was produced by blending silica or introducing silica hybrid through covalent bonds. And the silica hybrid containing epoxy group, which may be introduced to the structure of fiber-reinforced epoxy composite through covalent bonds caused by reaction with a hardener, has been used, so that the heat resistance and adhesion strength could be improved.

섬유보강 콘크리트의 역학적 특성에 대한 섬유 체적비와 길이의 영향

양근혁,오승진 한국건축시공학회 2008 한국건축시공학회지 Vol.8 No.1

Fifteen concrete specimens were mixed and tested to explore the significance and limitation of appling the polyvinyl alcohol (PVA) fiber and steel fiber with end hook to concrete. Main parameters investigated were volume fraction and length of the fibers. The measured mechanical properties of fiber reinforced concrete are analyzed according to the equivalent fiber amount index explaining the adding amount and length of fibers. Test results showed that compressive strength of fiber reinforced concrete was higher than that of concrete with no fiber by 10~20%. The normalized splitting tensile strength and flexural strength of PVA fiber reinforced concrete were similar to those of concrete with no fiber, whereas those of steel fiber reinforced concrete increased with the increase of the equivalent fiber amount index. In particular, much higher ductile behavior was observed in steel fiber reinforced concrete than in PVA reinforced concrete, indicating that the slope of descending branch of load-displacement relationship of steel fiber reinforced concrete decreased with the increase of the volume fraction and length of the fiber.

아리미드섬유와 PET섬유시트로 보강한 철근콘크리트 기둥의 구조성능평가

김동환,조민수,최진형,조우래,김길희 한국구조물진단유지관리공학회 2023 한국구조물진단유지관리공학회 논문집 Vol.27 No.1

This study evaluates the performance of reinforced concrete columns using hybrid fiber sheets for structural behavior. The purpose of thismethod is to improve the load-bearing capacity of the reinforced structure by impregnating a hybrid fiber sheet, which is woven by arranging aramidand glass fibers uniaxially and attached to an aged concrete structure requiring reinforcement with epoxy. In particular, not only the weight reductionof the material obtained by using a fiber lighter than the steel material, but also the low-strength, high-toughness fiber element among the fibers useddelays the brittle fracture of the high-strength, low-toughness fiber element. The low-strength, high-toughness fiber element among the fibers useddelays the brittle fracture of the high-strength, low-toughness fiber element, resulting in weight reduction compared to steel. The study conductedstructural tests on four specimens, with the hybrid reinforcement method and failure mode as main variables. Specimen size and loading conditionswere chosen to be comparable with previous studies. The structural performance of the specimen was evaluated using energy dissipation capacity andductility. Analysis shows that excellent results can be obtained with the hybrid fiber sheet reinforcement. 이 연구는 하이브리드 섬유시트를 이용하여 보강된 철근콘크리트 기둥의 구조성능평가에 관한 연구이다. 내진보강 공법은 보강이필요한 노후 콘크리트 구조물에 아라미드섬유와 PET섬유를 일축으로 배열하여 직조한 하이브리드 섬유시트를 에폭시로 함침하고, 이를 구조물에 부착시켜 보강 구조물의 내하력을 증진시키는데 그 목적이 있다. 특히, 강재보다 가벼운 섬유를 사용함으로써 얻어지는 재료의 경량화뿐만 아니라, 사용된 섬유 중 저강도 고인성의 섬유요소가 고강도 저인성 섬유요소의 취성적 파괴를 지연시켜 기존의 섬유보강 공법과 비교해 안전성 측면에서 우수하다. 연구는 구조실험과 그 결과에 대한 구조성능평가로 진행되었다. 총 4개의 실험체는 하이브리드 보강방법 및 파괴모드를 주요변수로 계획하였으며, 실험체 크기 및 가력조건 등은 기존연구에서 수행한 실험결과와 비교가 가능하도록 계획하였다. 실험체의 구조성능은 에너지소산능력, 연성평가등을 사용하여 평가하였다. 다음과 같은 분석을 통하여 하이브리드 섬유시트의 보강하였을 때 우수한 성능 결과를 보일 수 있다는 결론은 얻었다.

Use of steel fibers as transverse reinforcement in diagonally reinforced coupling beams with normal- and high-strength concrete

Jang, Seok-Joon,Jeong, Gwon-Young,Yun, Hyun-Do Elsevier 2018 Construction & building materials Vol.187 No.-

<P><B>Abstract</B></P> <P>This study investigates the effects of using steel fibers as transverse reinforcement on the seismic performance of diagonally reinforced coupling beams that are composed of normal- and high-strength concrete. Four types of coupling beams were fabricated and tested under quasi-static reversed cyclic loading. A normal-strength reinforced concrete (RC) coupling beam with the compressive strength of 40 MPa was designed with the full confinement of a beam section according to ACI 318-14. A second RC coupling beam specimen with the same reinforcement details and concrete with the compressive strength of 80 MPa was fabricated also to evaluate the effects of strength. In addition, steel fiber was used in an attempt to simplify the complex details of transverse reinforcements for diagonally RC coupling beams. To this end, two fiber volume fractions of 1.0% and 1.25% were used for 40 MPa and 80 MPa steel fiber-reinforced concrete (SFRC) coupling beams, respectively. Test results indicated that the shear strength of the coupling beams increased with an increase in compressive strength, whereas the energy dissipation capacity was similar for the normal- and high-strength concrete RC coupling beams. Furthermore, this study found that the inclusion of steel fiber prevented buckling of the diagonal steel rebar as well as provided additional transverse reinforcement. The overall performance, including strength, stiffness degradation, and the energy dissipation capacity, of the SFRC coupling beams was similar to that of the conventional concrete coupling beams. Comparisons of the conventional concrete and SFRC beam specimens indicate that the use of steel fiber allows for the simplification of transverse reinforcement construction details for diagonally reinforced coupling beams.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The inclusion of steel fibers improves flexural behavior and toughness of concrete. </LI> <LI> SFRC significantly contributed to prevent the buckling of the diagonal reinforcement. </LI> <LI> Using fibers as transverse reinforcement improves constructability of coupling beam. </LI> <LI> Shear strength is well predicted considering contribution of diagonal concrete strut. </LI> </UL> </P>

The effect of fiber reinforcement on behavior of Concrete-Filled Steel Tube Section (CFST) under transverse impact: Experimentally and numerically

Zeynep Yaman 국제구조공학회 2022 Structural Engineering and Mechanics, An Int'l Jou Vol.82 No.2

This study presents an experimental and numerically study about the effects of fiber reinforcement ratio on the behavior of concrete-filled steel tubes (CFST) under dynamic impact loading. In literature have examined the behavior of GFRP and FRP wrapped strengthened CFST elements impact loads. However, since the direction of potential impact force isn't too exact, there is always the probability of not being matched the impact force of the area where the reinforced. Therefore, instead of the fiber textile wrapping method which strengthens only a particular area of CFST element, we used fiber-added concrete-filled elements which allow strengthening the whole element. Thus, the effect of fiber-addition in concrete on the behavior of CFST elements under impact loads was examined. To do so, six simply supported CFST beams were constructed with none fiber, 2% fiber and 10% fiber reinforcement ratio on the concrete part of the CFST beam. CFST beams were examined under two different impact loads (75 kg and 225 kg). The impactors hit the beam from a 2000 mm free fall during the experimental study. Numerical models of the specimens were created using ABAQUS finite element software and validated with experimental data. The obtained results such as; mid-span displacement, acceleration, failure modes and energies from experimental and numerical studies were compared and discussed. Furthermore, the Von Misses stress distribution of the CFST beams with different ratio of fiber reinforcements were investigated numerically. To sum up, there is an optimum amount limit of the fiber reinforcement on CFST beams. Up to this limit, the fiber reinforcement increases the structural performances of the beam, beyond that limit the fiber reinforcement decreases the performances of the CFST beam under transverse impact loadings.

Strain rate effects on the compressive and tensile behavior of bundle-type polyamide fiber-reinforced cementitious composites

Kim, Hongseop,Kim, Gyuyong,Lee, Sangkyu,Son, Minjae,Choe, Gyeongcheol,Nam, Jeongsoo Elsevier 2019 Composites. Part B, Engineering Vol.160 No.-

<P><B>Abstract</B></P> <P>The compressive and tensile behavior of fiber-reinforced cementitious composites is significantly affected by the bonding and pull-out properties between matrix and reinforced fiber, as well as the fracture properties of the fibers. In addition, an increase in strain rate according to loading conditions influences the fracture behavior between the fiber and matrix. Steel fiber-reinforced cementitious composites with high flexural and tensile strength, toughness, and crack resistance are widely used in tunnels and plant structures. However, the high specific gravity and stiffness of steel fibers can cause rupture of concrete pump tubes, increase the rebound volume of shotcrete, and decrease durability by corrosion of fiber. Therefore, it is necessary to study the development and application of organic fiber which has similar mechanical properties to steel fiber and does not cause corrosion. In this study, polyamide fibers having the same aspect ratio as the hooked steel fibers, which are widely used as reinforcing fibers for concrete, have been developed. And strain rate effect on the compressive and tensile behaviors of bundle-type polyamide fiber-reinforced cementitious composite and hooked steel fiber-reinforced cementitious composite were evaluated. The results showed that the effect of strain rate over different fiber types influenced the tensile behavior more significantly than the compressive behavior. In polyamide fiber-reinforced cementitious composite (PAFRCC), a fracture behavior of fiber was observed regardless of a strain rate, and the tensile behavior of PAFRCC was influenced more by tensile strength of polyamide fiber itself than a bonding stress between fiber and matrix. In hooked steel fiber-reinforced cementitious composite (HSFRCC), a bonding stress between hooked steel fiber and matrix (frictional force at the interface between fiber and matrix, mechanical bond of the hooked part) influenced the tensile behavior significantly. Fracture properties that straightened pulled out the fiber from the matrix were observed at static tensile loading condition. However, non-straightened hooked steel fiber was observed along with the fracture of matrix due to an increase in mechanical bonding force of the hooked part and the bonding stress between the fiber and the matrix.</P>

섬유의 보강 형태에 따른 섬유-시멘트 혼합토의 일축압축강도특성

박성식,김영수,이종천 한국지반공학회 2007 한국지반공학회논문집 Vol.23 No.8

The behavior of fiber-reinforced cemented sands (FRCS) was studied to improve a brittle failure mode observed in cemented sands. Nak-dong River sand was mixed with ordinary Portland cement and a Polyvinyl alcohol (PVA) fiber. A PVA fiber is widely used in concrete and cement reinforcement. It has a good adhesive property to cement and a specific gravity of 1.3. A PVA fiber has a diameter of 0.1 mm that is thicker than general PVA fiber for reinforced cement. Clean Nak-dong River sand, cement and fiber at optimum water content were compacted in 5 layers giving 55 blows per layer. They were cured for 7 days. Cemented sands with a cement/sand ratio of 4% were fiber-reinforced at different locations and tested for unconfined compression tests. The effect of fiber reinforcement form and distribution on strength was investigated. A specimen with evenly distributed fiber showed two times more strength than not-evenly reinforced specimen. The strength of fiber-reinforced cemented sands increases as fiber reinforcement ratio increases. A fully reinforced specimen was 1.5 times stronger than a specimen reinforced at only middle part. FRCS behavior was controlled not only by a dosage of fiber but also by fiber distribution methods or fiber types.

Influence of steel fibers and fiber-reinforced polymers on the impact resistance of one-way concrete slabs

Yoo, Doo-Yeol,Yoon, Young-Soo SAGE Publications 2014 Journal of composite materials Vol.48 No.6

<P>In this study, to estimate the impact resistance of steel fiber-reinforced concrete slabs strengthened with fiber-reinforced polymer sheets, a series of 50 × 100 × 350 mm<SUP>3</SUP> sized slabs with 0.5%–1.5% (by volume) of steel fibers and two types of fiber-reinforced polymer sheets were impact-tested using drop-weight impact test machine. From the test results, the maximum impact load, dissipated energy and the number of drops before failure were all increased, whereas the maximum deflection and support rotation were reduced by strengthening the steel fiber-reinforced concrete slabs with externally bonded fiber-reinforced polymer sheets in the tensile zone. It was noticed that the impact resistance of the steel fiber-reinforced concrete slabs was substantially improved by externally strengthening the fiber-reinforced polymer sheets. In addition, the dynamic response of the steel fiber-reinforced concrete slabs strengthened with fiber-reinforced polymer sheets under a low-velocity impact load was analyzed using explicit code LS-DYNA with strain rate-dependent material models and debonding failure analysis. These numerical analyses were verified by comparing with the experimental results.</P>

PET 섬유 보강재를 사용한 섬유 보강 콘크리트의 성능 평가에 관한 연구

오리온,유용선,박찬기,박성기 사단법인 한국터널지하공간학회 2023 한국터널지하공간학회논문집 Vol.25 No.4

This study aimed to review the performance stability of PET (Polyethylene terephthalate) fiber reinforcing materials among the synthetic fiber types for which the application of performance reinforcing materials to fiber-reinforced concrete is being reviewed by examining short-term and long-term performance changes. To this end, the residual performance was analyzed after exposing the PET fiber to an acid/alkali environment, and the flexural strength and equivalent flexural strength of the PET fiber-reinforced concrete mixture by age were analyzed, and the surface of the PET fiber collected from the concrete specimen was examined using a scanning microscope (SEM). The changes in were analyzed. As a result of the acid/alkali environment exposure test of PET fiber, the strength retention rate was 83.4~96.4% in acidic environment and 42.4~97.9% in alkaline environment. It was confirmed that the strength retention rate of the fiber itself significantly decreased when exposed to high-temperature strong alkali conditions, and the strength retention rate increased in the finished yarn coated with epoxy. In the test results of the flexural strength and equivalent flexural strength of the PET fiber-reinforced concrete mixture, no reduction in flexural strength was found, and the equivalent flexural strength result also did not show any degradation in performance as a fiber reinforcement. Even in the SEM analysis results, no surface damage or cross-sectional change of the PET reinforcing fibers was observed. These results mean that no damage or cross-section reduction of PET reinforcing fibers occurs in cement concrete environments even when fiber-reinforced concrete is exposed to high temperatures in the early stage or depending on age, and the strength of PET fibers decreases in cement concrete environments. The impact is judged to be of no concern. As the flexural strength and equivalent flexural strength according to age were also stably expressed, it could be seen that performance degradation due to hydrolysis, which is a concern due to the use of PET fiber reinforcing materials, did not occur, and it was confirmed that stable residual strength retention characteristics were exhibited.