Transfer length of 2400 MPa seven-wire 15.2 mm steel strands in high-strength pretensioned prestressed concrete beam

Jin Kook Kim,Hong Jae Yim,Jun-Mo Yang 국제구조공학회 2016 Smart Structures and Systems, An International Jou Vol.17 No.4

In this study, the transfer length of 2400 MPa, seven-wire high-strength steel strands with a 15.2 mm diameter in pretensioned prestressed concrete (PSC) beams utilizing high strength concrete over 58 MPa at prestress release was evaluated experimentally. 32 specimens, which have the variables of concrete compressive strength, concrete cover depth, and the number of PS strands, were fabricated and corresponding transfer lengths were measured. The strands were released gradually by slowly reducing the pressure in the hydraulic stressing rams. The measured results of transfer length showed that the transfer length decreased as the concrete compressive strength and concrete cover depth increased. The number of strands had a very small effect, and the effect varied with both the concrete cover depth and concrete strength. The results were compared to current design codes and transfer lengths predicted by other researchers. The comparison results showed that the current transfer length prediction models in design codes may be conservatively used for 2400 MPa high-strength strands in high-strength concrete beams exceeding 58 MPa at prestress release.

초고성능 콘크리트 I형 거더의 전단강도에 프리스트레스가 미치는 영향

변남주,이정화,한상윤 한국복합신소재구조학회 2017 복합신소재구조학회 학술발표회 Vol.2017 No.04

Prestress is a reinforcement method to control crack due to moment on concrete girders with low tensile strength. In the existing literature, it is mentioned that prestress for ordinary concrete affects not only crack control but also shear strength enhancement. As the construction material improves, UHPC(Ultra-HIgh Performance Concrete) with excellent strength and ductility has been developed by combining ultra-high strength concrete and steel fiber. However, study on the effect of prestress on the shear strength of UHPC with different material properties from ordinary concrete is lacking. Therefore, in this paper, the effect of prestress on the shear behavior of UHPC I-girder is studied by finite element analysis. As a result of the analysis, it has been confirmed that the prestress increases crack strength and shear strength of UHPC.

Shear Cracking of Prestressed Girders with High Strength Concrete

Labib, Emad L.,Mo, Y.L.,Hsu, Thomas T.C. Korea Concrete Institute 2013 International Journal of Concrete Structures and M Vol.7 No.1

Prestressed concrete (PC) is the predominant material in highway bridge construction. The use of high-strength concrete has gained wide acceptance in the PC industry. The main target in the highway industry is to increase the durability and the life-span of bridges. Cracking of elements is one aspect which affects durability. Recently, nine 7.62 meter long PC I-beams made with different concrete strength were designed according to a simple, semi-empirical equation developed at the University of Houston (UH) (Laskar et al., ACI Journal 107(3): 330-339, 2010). The UH Method is a function of shear span-to-depth ratio (a/d), concrete strength $\sqrt{f^{\prime}_c}$, web area $b_wd$, and amount of transverse steel. Based on testing these girders, the shear cracking strength of girders with different concrete strength and different shear span-to-depth ratio was investigated and compared to the available approaches in current codes such as ACI 318-11 (2011) and AASHTO LRFD Specifications (2010).

Transfer lengths of pretensioned concrete members reinforced with 2400 MPa high-strength prestressing tendons

한선진,이득행,오재열,김강수,이성태 사단법인 한국계산역학회 2016 Computers and Concrete, An International Journal Vol.18 No.4

High-strength prestressing tendons with a tensile strength of 2400 MPa have been recently developed in the steel industry of South Korea, and is currently being mass produced for common commercial applications in structural engineering practices. Accordingly, since most of the existing transfer length estimation models were derived based on the test results of the specimens having the prestressing tendons with a tensile strength of 1860 MPa or less, modifications of the transfer length models are now required to consider the effect of the enhanced tensile strength of the prestressing tendons. In this study, six pretensioned concrete specimens reinforced with 2400 MPa tensile strength prestressing tendons were fabricated and tested to investigate their transfer lengths. In addition, a simplified design equation for the transfer length was developed based on the assumption of the linear strain profile curves of the prestressing tendon and surrounding concrete in transfer length zone. The accuracy of the proposed equation was verified in detail by comparing a total of 215 transfer length test results with analysis results. The simplified design equation provided very accurate results on the transfer lengths of all the test specimens, regardless of the tensile strength grades of prestressing tendons.

Shear Cracking of Prestressed Girders with High Strength Concrete

Emad L. Labib,Y. L. Mo,Thomas T. C. Hsu 한국콘크리트학회 2013 International Journal of Concrete Structures and M Vol.7 No.1

Prestressed concrete (PC) is the predominant material in highway bridge construction. The use of high-strength concrete has gained wide acceptance in the PC industry. The main target in the highway industry is to increase the durability and the life-span of bridges. Cracking of elements is one aspect which affects durability. Recently, nine 7.62 meter long PC I-beams made with different concrete strength were designed according to a simple, semi-empirical equation developed at the University of Houston (UH) (Laskar et al., ACI Journal 107(3): 330?339, 2010). The UH Method is a function of shear span-to-depth ratio (a/d), concrete strength f¹c, web area bwd, and amount of transverse steel. Based on testing these girders, the shear cracking strength of girders with different concrete strength and different shear span-to-depth ratio was investigated and compared to the available approaches in current codes such as ACI 318-11 (2011) and AASHTO LRFD Specifications (2010).

2,400 MPa급 고강도 강연선이 적용된 LNG 저장탱크의 프리스트레싱 효과

전세진,서해근,양준모,윤석구 대한토목학회 2016 대한토목학회논문집 Vol.36 No.6

High-strength strands have been increasingly applied to recent actual structures in Korea. Structural effect of the increased spacing of sheaths was investigated in this study when the usual 1,860 MPa strands of an LNG storage tank are replaced with 2,400 MPahigh-strength strands. First, finite element models of a cylindrical wall of an LNG tank were established and prestressing effect of the circumferential and vertical tendons was considered as equivalent loads. As a result of varying the tendon spacing and prestressing force with the total prestressing effect kept the same, the stress distribution required in design was obtained with the high-strength strands. Also, a full-scale specimen that corresponds to a part of an LNG tank wall was fabricated with 31 high-strength strands with 15.2 mm diameter inserted in each of two sheaths. It was observed that such a high level of prestressing force can be properly transferred to concrete. Moreover, an LNG tank with the world’s largest 270,000 kl capacity was modeled and the prestressing effect of high-strength strands was compared with that of normal strands. The watertightness specifications such as residual compressive stress and residual compression zone were also ensured in case of leakage accident. The results of this study can be effectively used when the 2,400 MPa high-strength strands are applied to actual LNG tanks. 최근 국내에서는 고강도 강연선이 실구조물에 적용되는 사례가 늘어나고 있다. 이 연구에서는 LNG 저장탱크에 통상적으로 적용되는 1,860 MPa급 강연선을 2,400 MPa급 고강도 강연선으로 대체할 때 쉬스 배치 간격 증가에 따른 구조적인 영향을 고찰해 보았다. 먼저, LNG 탱크의 원통형 벽체를모사하는 유한요소모델에 원환텐던 및 수직텐던의 프리스트레싱 효과를 등가하중으로 가하되 프리스트레싱 효과의 총합은 일정한 상태에서 텐던의 간격 및 긴장력을 조절한 결과, 고강도 강연선을 적용해도 설계 시 요구되는 소정의 응력 분포를 구현할 수 있는 것으로 나타났다. 또한, LNG 탱크벽체 일부분을 모사하는 실대형 실험체를 제작하고 15.2 mm 직경 고강도 강연선이 31개 삽입된 쉬스를 2개 배치하여 매우 높은 수준의 프리스트레싱 힘이 콘크리트에 적절히 전달되는지 관찰하였다. 한편, 세계 최대 용량인 270,000 kl급 LNG 탱크의 유한요소모델을 구축하고 고강도 강연선을 적용했을 때의 프리스트레싱 효과를 일반 강연선의 경우와 비교하였으며, LNG 누출 사고 시에도 여유압축응력 및 여유압축구간과 같은 수밀성 규정을 만족함을 확인하였다. 이 연구결과는 2,400 MPa급 고강도 강연선이 실제 LNG 탱크에 적용될 때 유용하게 활용될 것으로 기대된다.

The Effects of Cementitious Materials on the Mechanical and Durability Performance of High-Strength Concrete

이주하,윤영수 대한토목학회 2015 KSCE JOURNAL OF CIVIL ENGINEERING Vol.19 No.5

Various tests, focusing on the effects of the type and composition of cementitious materials (ordinary Portland cement, fly ash, slag, low-heat cement, and their combinations) on the mechanical properties and durability of high-strength concrete, were performed to provide experimental data for the application of high-strength concrete to prestressed bridges. Firstly, mix proportions were designed based on a number of trial mixes, taking into account their applicability to prestressed bridges. Mechanical properties, such as compressive strength, modulus of elasticity, splitting tensile strength and flexural strength, were determined. Durability related properties, such as temperature of hydration, resistance to chloride-ion penetration, resistance to freezing-thawing, autogenous and drying shrinkages and creep, were also determined. The effects of cementitious materials on the various properties of high-strength concrete have been demonstrated.

Bond Behavior of Pretensioned Strand Embedded in Ultra-High-Performance Fiber-Reinforced Concrete

Hyun-Oh Shin,Seung-Jung Lee,Doo-Yeol Yoo 한국콘크리트학회 2018 International Journal of Concrete Structures and M Vol.12 No.4

This study aimed to investigate the bond properties of prestressing strands embedded in ultra-high-performance fiberreinforced concrete (UHPFRC). Toward this end, two types of prestressing strands with diameters of 12.7 and 15.2 mm were considered, along with various concrete cover depths and initial prestressing force magnitudes. The average bond strength of the strands in UHPFRC was estimated by using pullout tests, and the transfer length was evaluated based on a 95% average maximum strain method. Test results indicated that the average bond strength of the pretensioned strand reduced as the diameter of the strand increased, and was between the bond strengths of round and deformed steel rebars. Higher bond strength was also obtained with a lower embedment length. Based on a comparison of p value, the bar diameter and embedment length most significantly influenced the bond strength of strands in UHPFRC, compared to a ratio of cover depth to diameter and initial prestressing force. Pretensioned strands in UHPFRC exhibited much higher bond strength and shorter transfer length compared with strands embedded in ordinary high-strength concrete. Lastly, ACI 318 and AASHTO LRFD codes significantly overestimated the transfer length of the strands embedded in UHPFRC.

Axial Compressive Performance of CFRP Confined Self-stressing High-strength Concrete Cylinders

Qi Cao,Xiaojun Liu,Rongxiong Gao 대한토목학회 2019 KSCE JOURNAL OF CIVIL ENGINEERING Vol.23 No.9

Due to the advantages of FRP (fiber reinforced polymer), such as light weight, high strength and corrosion resistance, FRP-confined concrete columns are gradually applied in civil engineering structures and have drawn wide attention from the engineering community. In order to avoid the stress hysteresis of CFRP (carbon fiber reinforced polymer) and make full use of FRP materials, expansive agent was added to concrete to make self-stressing high-strength concrete. In this study, the axial compression performance of 18 CFRP-confined self-stressing high-strength concrete cylinders was examined. The parameters include the CFRP layers and the level of prestress (with or without prestress). Experimental results show that 3.53 − 5.34 MPa prestress in concrete and 799.3 − 1,584.2 MPa prestress in CFRP are produced in the composite cylinder. The stress-strain curves obtained from the experiment shows that the intercept stress, inflection stress and peak stress of the self-stressing specimens are all higher than those of the non-prestressed specimens. With the application of prestress, the utilization ratio of CFRP increases as well. In the theoretical calculation of intercept and peak stress, by modifying the existing models, the experimental results are in good agreement with the calculated results.

프리스트레스 초고성능 콘크리트 I형 거더의 전단거동에 대한 해석 연구

변남주(Byun, NamJoo),이정화(Lee, JeongHwa),한상윤(Han, SangYoon),강영종(Kang, YoungJong) 한국방재학회 2017 한국방재학회논문집 Vol.17 No.3

본 연구에서는 초고성능 콘크리트 I형 거더의 전단거동에 프리스트레스가 미치는 영향을 고찰하고 기존 영향식의 적용 가능성을 평가하기 위해, 21개의 단순지지 해석모델에 대해 3점재하 형태의 하중조건으로 재료⋅기하 비선형 해석연구가 수행되었다. 해석모델의 변수는 복부판 두께를 10 mm, 20 mm 및 30 mm로 변화하였고, 프리스트레스를 긴장재 항복강도의 0%에서 60%까지 변화하였다. 해석결과는 프리스트레스가 증가할수록 강도가 증가하였으나 균열강도에 비해 전단강도의 증진 효과가 낮게 도출되었고, 초기균열 발생 위치가 복부판 상단으로 이동하는 현상이 나타났다. 또한, 초고성능 콘크리트의 강도 증진 효과는 기존 영향식에 비해 낮게 도출되었기 때문에, 기존 영향식의 적용은 한계점이 있어 새로운 기준개발이 요구된다. In this study, material⋅geometric nonlinear analysis study was performed under the load condition of three point load type for 21 simple support analytical models to investigate the effect of prestress on the shear behavior of Ultra-High Performance Concrete(UHPC) I-girder and to evaluate the applicability of the existing influence equation. The parameters of the analytical model varied from 10mm, 20mm, and 30mm in the thickness of web, and the prestress varied from 0% to 60% of the tendon yield strength. The analysis result showed that the strength incresed as the prestress increased, but the enhancement effect of the shear strength was lower than that of the crack strength, and the initial crack location moved to the top of the web. In addition, since the strength enhancement effect of UHPC is lower than that of existing influence formula, application of existing influence formula is limited and new standard development is required.