This study deals with the microstructure and tensile properties of 700 MPa-grade high-strength and seismic reinforced steel bars. The high-strength reinforced steel bars (600 D13, 600 D16 and 700 D13 specimens) are fabricated by a TempCore process, while the seismic reinforced steel bar (600S D16 specimen) is fabricated by air cooling after hot rolling. For specimens fabricated by the TempCore process, the 600 D13 and 600 D16 specimens have a microstructure of tempered martensite in the surface region and ferrite-pearlite in the center region, while the 700 D13 specimen has a microstructure of tempered martensite in the surface region and bainite in the center region. Therefore, their hardness is the highest in the surface region and shows a tendency to decrease from the surface region to the center region because tempered martensite has a higher hardness than ferrite-pearlite or bainite. However, the hardness of the 600S D16 specimen, which is composed of fully ferritepearlite, increases from the surface region to the center region because the pearlite volume fraction increases from the surface region to the center region. On the other hand, the tensile test results indicate that only the 700 D13 specimen with a higher carbon content exhibits continuous yielding behavior due to the formation of bainite in the center region. The 600S D16 specimen has the highest tensile-to-yield ratio because the presence of ferrite-pearlite and precipitates caused by vanadium addition largely enhances work hardening.

This present study deals with the microstructure and tensile properties of 600 MPa-grade high strength and seismic resistant reinforcing steels. The high strength reinforcing steel (SD 600) was fabricated by Tempcore processing, while the seismic resistant reinforcing steel (SD 600S) was air-cooled after hot-rolling treatment. The microstructure analysis results showed that the SD 600 steel specimen consisted of a tempered martensite and ferrite-pearlite structure after Tempcore processing, while the SD 600S steel specimen had a fully ferrite-pearlite structure. The room-temperature tensile test results indicate that, because of the enhanced solid solution and precipitation strengthening caused by relatively higher contents of C, Mn, Si and V in the SD 600S steel specimen, this specimen, with fully ferrite-pearlite structure, had yield and tensile strengths higher than those of the SD 600 specimen. On the other hand, the hardness of the SD 600 and SD 600S steel specimens changed in different ways according to location, dependent on the microstructure, ferrite grain size, and volume fraction.

In the moment frame subjected to earthquake loads, beam-column joint is structurally important for ductile behavior of a system. ACI Committee 352 proposed guidelines for designing beam-column joint details. The guidelines, however, need to be updated because of the lack of data regarding several factors that may improve the performance of joints. The purpose of this study is to investigate the seismic performance of reinforced concrete exterior joints with high-strength materials and unbonded tendons. Three specimens with different joint shear demand-to-strength ratios were constructed and tested, where headed bars were used to anchor the beam bars into the joint. All specimens showed satisfactory seismic behavior including moment strength of 1.3 times the nominal moment, ductile performance (ductility factor = at least 2.4), and sufficiently large dissipated energy.

최근 지진의 발생 빈도가 잦아지면서 고강도 내진 철근에 대한 관심이 급증하였다. 그러나 현재 콘크리트구조 학회기준 (2017)에서는 철근콘크리트 부재의 전단철근의 항복강도를 500MPa로 제한하고 있다. 이 논문에서는 이러한 설계기준항복강도 제한을 확장하기 위해 고강도 내진 철근을 사용한 철근콘크리트 보의 실험을 수행하였다. 실험에는 항복강도가 400MPa, 500MPa, 700MPa인 전단철근이 사용되었으며 전단철근의 배근간격을 변수로 하여 철근비에 따른 철근콘크리트 보의 전단거동을 비교하였다. 실험결과 철근량과 철근의 항복강도가 증가할수록 전단강도비가 감소하는 경향이 나타났다.

The purpose of this study is to establish a reasonable analytical method for the estimation of overall behavior characteristic from cracking to yielding of rebar and crushing of concrete and seismic performance of reinforced concrete shear wall with high-strength reinforcing bar. A total of 8 specimens of reinforced concrete walls which have constant aspect ratio and a variety of variables such as reinforcement ratio, reinforcement yielding strength, reinforcement details, concrete design strength, section shape and whether lateral restraint hoop were selected and the analysis was performed by using a non-linear finite element analysis program (RCAHEST) applying the proposed constitutive equation by the authors. The mean and coefficient of variation for maximum load from the experiment and analysis results was predicted 1.04 and 8%. The mean and coefficient of variation for displacement corresponding maximum load from the experiment and analysis results was predicted 1.17 and 19% respectively. The analytical results were predicted relatively well the fracture mode and the overall behavior until fracture for all specimens. These results are expected to be used as basic data for application of high-strength reinforcing bar to design codes in the future.