• 한국안전학회지
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• 제28권2호
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• pp.55-59
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• 2013

The analytical model was constituted to evaluate the flexural strength of UHPFRC(ultra high performance fiber reinforced concrete) member. The analytical approach was attemped to study the effect of the joint and the result compared with the experimental study to verify the analytical model. The calculated value tends to underestimate about 23%~25% in comparison with the experimental result of the jointed test member because the bond stress between precast UHPFRC and cast-in-place UHPFRC surface is not considered in the analytical model. But in the case of the continuous test member, the analytical model provides reasonable results for the flexural strength of UHPFRC member.

• Advances in concrete construction
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• 제14권1호
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• pp.45-55
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• 2022

In this article, the flexural and shear capacity of ultra-high-performance fiber-reinforced concrete beams (UHPFRC) using two kinds of rebars, including GFRP and steel rebars, are experimentally investigated. For this purpose, six UHPFRC beams (250 × 300 × 1650 mm) with three reinforcement ratios (ρ) of 0.64, 1.05, and 1.45 were constructed using 2% steel fibers by volume. Half of the specimens were made of UHPFRC reinforced with GFRP rebars, while the other half were reinforced with conventional steel rebars. All specimens were tested to failure in four-point bending. Both the load-deformation at mid-span and the failure pattern were studied. The results showed that utilizing GFRP bars increases the flexural strength of UHPFRC beams in comparison to those made of steel bars, but at the same time, it reduces the post-cracking strain hardening. Furthermore, by increasing the percentage of longitudinal bars, both the post-cracking strain hardening and load-bearing capacity increase. Comparing the experiment results with some of the available equations and provisions cited in the valid design codes reveals that some of the equations to predict the flexural strength of UHPFRC beams reinforced with conventional steel and GFRP bars are reasonably conservative, while Khalil and Tayfur model is un-conservative. This issue makes it essential to modify the presented equations in this research for predicting the flexural strength of UHPFRC beams using GFRP bars.

• 한국구조물진단유지관리공학회 논문집
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• 제23권7호
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• pp.152-163
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• 2019

본 연구에서는 강섬유의 혼입, 매트릭스의 압축강도, 전단철근과 전단경간비가 UHPFRC 휨재에 미치는 영향에 대해 총 10개의 실험체에 대한 실험을 통해 검토하였다. 실험결과 2%의 부피비로 강섬유가 혼입된 경우 파괴 패턴을 전단파괴에서 휨파괴로 바꿀 정도로 높은 전단강도 증진효과를 보유하고 있는 것으로 나타났다. 또한 강섬유는 낮은 전단경간비에서 압축스트럿의 파괴를 지연시키는 효과를 가진 것으로 나타났다. 실험 결과 강섬유의 혼입과 전단경간비의 변화에 따라 균열각이 45도보다 낮은 것으로 나타났다. 실험 결과를 UHPC 설계권고안들과 비교해 본 결과 프랑스의 설계권고안은 보수적으로 평가하였고 한국의 설계권고안은 휨 강도에 대해 다소 과대평가하는 것으로 나타났다. 전단강도에 대해서는 두 설계권고안 모두 보수적으로 평가하는 것으로 나타났다.

• Structural Engineering and Mechanics
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• 제65권3호
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• pp.315-325
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• 2018

This paper predicts the flexural behaviour of reinforced concrete (RC) beams strengthened with a precast strip of ultra-high performance fiber-reinforced concrete (UHPFRC). In the first phase, ultimate load capacity of preloaded and strengthened RC beams by UHPFRC was predicted by using various analytical models available in the literature. RC beams were preloaded under static loading approximately to 70%, 80% and 90% of ultimate load of control beams. The models such as modified Kaar and sectional analysis predicted the ultimate load in close agreement to the corresponding experimental observations. In the second phase, the famous fatigue life models such as Papakonstantinou model and Ferrier model were employed to predict the number of cycles to failure and the corresponding deflection. The models were used to predict the life of the (i) strengthened RC beams after subjecting them to different pre-loadings (70%, 80% and 90% of ultimate load) under static loading and (ii) strengthened RC beams after subjecting them to different preloading cycles under fatigue loading. In both the cases precast UHPFRC strip of 10 mm thickness is attached on the tension face. It is found that both the models predicted the number of cycles to failure and the corresponding deflection very close to the experimental values. It can be concluded that the models are found to be robust and reliable for cement based strengthening systems also. Further, the Wang model which is based on Palmgren-Miner's rule is employed to predict the no. of cycles to failure and it is found that the predicted values are in very good agreement with the corresponding experimental observations.

• 한국방재학회 논문집
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• 제11권3호
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• pp.9-15
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• 2011

압축강도 100 MPa, 140 MPa 초고강도 섬유보강 콘크리트 직사각형보와 기하하적 조건 및 PS힘이 동일한 160 MPa RPC 직사각형보의 휨거동 비교를 통하여 압축강도가 낮은 초고강도 섬유보강 콘크리트 구조물의 거동 특성을 파악하였다. 직사각형보에 철근은 배근되지 않았으며 실험변수로는 강섬유, 콘크리트 압축강도, 프리스트레스 도입 방식, PS강재비 등이다. 모멘트-곡률관계와 하중-처짐관계를 통하여 압축응력이 작은 UHPC 직사각형보의 거동 특징을 분석하였다. 초고강도 섬유보강 콘크리트에 대한 간단한 형태의 응력-변형률 관계를 제안하였으며 이를 이용하여 UHPC 직사각형보에 대한 모멘트-곡률관계를 산정하였다. 실험에 의한 모멘트-곡률관계를 비교한 결과 상당히 일치함을 알 수 있었다.

• Advances in concrete construction
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• 제10권3호
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• pp.195-209
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• 2020

This study investigates the behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) with hybrid macro-micro steel and macro steel-polypropylene (PP) fibers. Compression, direct and indirect tension tests were carried out on cubic and cylindrical, dogbone and prismatic specimens, respectively. Three types of macro steel fibers, i.e., round crimped (RC), crimped (C), and hooked (H) were combined with micro steel (MS) and PP fibers in overall ratios of 2% by volume. Additionally, numerical analyses were performed to validate the test results. Parameters studied included, fracture energy, tensile strength, compressive strength, flexural strength, and residual strength. Tests showed that replacing PP fibers with MS significantly improves all parameters particularly flexural strength (17.38 MPa compared to 37.71 MPa). Additionally, the adopted numerical approach successfully captured the flexural load-deflection response of experimental beams. Lastly, the proposed regression model for the flexural load-deflection curve compared very well with experimental results, as evidenced by its coefficient of correlation (R²) of over 0.90.

• Computers and Concrete
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• 제23권1호
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• pp.11-23
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• 2019

Nowadays, the characterization of Ultra-High Performance Fiber-Reinforced Concrete (UHPFRC) tensile behavior still remains a challenge for researchers. For this purpose, a simplified closed-form non-linear hinge model based on the Third Point Bending Test (ThirdPBT) was developed by the authors. This model has been used as the basis of a simplified inverse analysis methodology to derive the tensile material properties from load-deflection response obtained from ThirdPBT experimental tests. In this paper, a non-linear finite element model (FEM) is presented with the objective of validate the closed-form non-linear hinge model. The state determination of the closed-form model is straightforward, which facilitates further inverse analysis methodologies to derive the tensile properties of UHPFRC. The accuracy of the closed-form non-linear hinge model is validated by a robust non-linear FEM analysis and a set of 15 Third-Point Bending tests with variable depths and a constant slenderness ratio of 4.5. The numerical validation shows excellent results in terms of load-deflection response, bending curvatures and average longitudinal strains when resorting to the discrete crack approach.

• Earthquakes and Structures
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• 제17권2호
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• pp.221-232
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• 2019

Material non-linearity of Reinforced Concrete (RC) framed structures is studied by modelling concrete using the Concrete Damage Plasticity (CDP) theory. The stress-strain data of concrete in compression is modelled using the Hsu model. The structures are analyzed using a finite element approach by modelling them in ABAQUS / CAE. Single bay single storey RC frames, designed according to Indian Standard (IS):456:2000 and IS:13920:2016 are considered for assessing their maximum load carrying capacity and failure behavior under the influence of gravity loads and lateral loads. It is found that the CDP model is effective in predicting the failure behaviors of RC frame structures. Under the influence of the lateral load, the structure designed according to IS:13920 had a higher load carrying capacity when compared with the structure designed according to IS:456. Ultra High Performance Fiber Reinforced Concrete (UHPFRC) strip is used for strengthening the columns and beam column joints of the RC frame individually against lateral loads. 10mm and 20mm thick strips are adopted for the numerical simulation of RC column and beam-column joint. Results obtained from the study indicated that UHPFRC with two different thickness strips acts as a very good strengthening material in increasing the load carrying capacity of columns and beam-column joint by more than 5%. UHPFRC also improved the performance of the RC frames against lateral loads with an increase of more than 3.5% with the two different strips adopted. 20 mm thick strip is found to be an ideal size to enhance the load carrying capacity of the columns and beam-column joints. Among the strengthening locations adopted in the study, column strengthening is found to be more efficient when compared with the beam column joint strengthening.

• 대한토목학회논문집
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• 제32권5A호
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• pp.317-325
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• 2012

본 연구에서는 초고성능 섬유보강 콘크리트(Ultra High Performance Fiber Reinforced Concrete, UHPFRC)의 구속 상태에서의 수축 거동을 평가하고자 국내 외에서 가장 보편적으로 사용되는 링-테스트(ring-test)를 이용하여 구속 수축 실험을 수행하였다. 특히, 다양한 구속도에서의 수축 거동을 평가하기 위하여 내부 강재 링의 두께와 내부 반경을 달리하여 실험을 수행하였으며, 자유 수축과 인장강도 실험을 수반하여 구속도 및 응력 이완, 수축 균열 가능성 등을 복합적으로 평가하였다. 실험 결과 내부 링의 두께가 증가할수록 내부 링의 평균 변형률과 잔류 인장응력은 감소하였으며, 반면에 구속도는 증가하는 경향을 보였다. 내부 링의 반경에 따라서는 변형률 및 잔류 인장응력, 구속도의 차이가 거의 없는 것으로 나타났다. 모든 시험체에서 잔류 인장응력이 인장강도에 비해 작은 것으로 나타났으며, 수축 균열은 발생하지 않았다. 지속적으로 작용하는 계면 구속 하중에 의해 탄성 수축 응력의 약 39~65%가 이완되는 것으로 나타났으며, 최대 이완 응력은 내부 링의 두께가 두꺼울수록 증가하는 것으로 나타났다. 마지막으로 본 연구에서는 비선형 회귀분석을 수행하여 재령에 따라 변하는 구속도를 예측하였으며, 실험 결과와 잘 일치하는 것으로 나타났다.

• 한국구조물진단유지관리공학회 논문집
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• 제26권5호
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• pp.57-65
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• 2022

본 연구에서는 smart ultra-high performance fiber reinforced concretes (S-UHPFRCs)의 자기감지 능력을 검증하기 위해 인장과 압축 영역에서 휨 하중에 따른 S-UHPFRCs의 전기역학적 거동을 조사하였다. 휨 하중 하에서 S-UHPFRCs의 전기저항률은 초기균열 이후 다수의 미세균열을 보이는 변형-경화 거동으로 인해 계속해서 변화된다. 압축 영역에서 S-UHPFRCs의 전기저항률은 등가 휨 응력이 증가함에 따라 976.57에서 514.05 kΩ-cm로 (47.00%) 감소하였으며, 인장 영역에서는 979.61에서 682.28 kΩ-cm로 (30.40%) 감소하였다. S-UHPFRCs의 응력 민감도 계수는 압축 영역과 인장 영역이 각각 1.709와 1.098 %/MPa이다. S-UHPFRCs의 처짐 감지 능력은 압축 영역 (30.06 %/mm)이 인장 영역 (19.72 %/mm)보다 높았다. 초기 처짐 감지 능력은 측정 영역과 관계없이 처짐 감지 능력의 약 50%로 초기 처짐에 대한 우수한 감지 능력을 가지는 것으로 확인되었다. 휨 하중 하에서 S-UHPFRCs의 자기감지 능력은 압축 영역에서 더 높았으나 S-UHPFRCs는 건설 현장에 적용할 자기 감지 재료로 충분하다.