탄소섬유보강근을 철근 대체재로 사용하기 위해서 단기 역학적 특성뿐 아니라 장기간 역학적특성에 대한 연구가 필히 수행 되어야 하고 현재도 진행 중이다. 이에 따라 본 연구에서는 CFRP bar의 지속하중에 대한 저항성을 평가하기 위해 ASTM 기준에 따라 약 1,000시간 동안 탄소섬유보강근 인장강도의 40%를 재하하는 크리프 시험을 진행 후 잔류 인장강도 확인을 위한 추가 인장시험을 진행하였다. 크리프 시험 결과, 탄소섬유보강근의 변형률은 지속하중 하에서 1,000시간 경과 후 하중재하 초기 변형률보다 약 4.9% 상 승하였고 크리프 파괴는 발생하지 않았다. 잔류 인장강도는 일반 인장강도의 95% 수준으로 측정되었고 잔류 탄성계수는 일반 탄성계 수의 85 % 수준이었다. 따라서 본 연구에서 진행한 인장강도의 40 %가 1,000시간 동안 재하되었을 때 탄소섬유보강근은 안전한 것으 로 확인되었다.

초고성능 콘크리트(UHPC)는 높은 압축강도를 위해 일반콘크리트에 비해 높은 시멘트 및 바인더 함량을 가지고 있다. UHPC 의 시멘트량을 줄이기 위한 연구가 지속적으로 수행되었으며, 그중 플라이애시와 고로슬래그는 각각 20%, 50% 수준까지는 강도 저하 없이 적용 가능하다는 연구가 확인되었다. 본 연구에서는 UHPC 배합에서 시멘트를 플라이애시와 고로슬래그로 치환하여 강도변화 및 유동성 변화를 분석하였다. 압축강도는 플라이애시 치환 실험체가 가장 낮으며, 고로슬래그는 치환 전과 유사한 강도를 보였다. 휨강도 는 고로슬래그, 플라이애시 치환 실험체 모두 감소하였다. 그러나 유동성은 플라이애시, 고로슬래그 실험체 모두 향상되면서 고성능감 수제 저감이 가능한 것을 확인하였다.

콘크리트의 변온저항성을 향상시키기 위해 상변화온도 4.5도와 44도의 상변환재료(Phase change material; PCM)를 캡슐화하여 혼입하였다. 그 후에 콘크리트가 기본적인 구조성능을 발휘할 수 있는 지 확인하기 위해 시멘트 부피의 10%, 20%, 30%에 해당하는 PCM을 혼입한 콘크리트의 온도 및 재 령에 따른 압축강도를 측정 및 비교하였다. PCM을 혼입하지 않은 콘크리트의 경우 PCM이 혼입된 다른 배합에 비해 온도에 따른 강도의 편차가 크지 않게 나타났으며 온도에 따른 강도의 편차는 PCM의 혼입률이 증가함에 따라 함께 증가하는 경향을 나타냈다. 4.5도의 PCM이 혼입된 시편은 0도 에서 25도로 온도가 증가 할 때 가장 높은 강도 감소률을 나타냈으며 45도 PCM은 25도에서 50도로 온도가 증가할 때 가장 높은 강도 하락률을 나타냈다. 0도에서는 대부분의 경우에서 4.5도 PCM 혼입 시편이 44도 PCM 혼입 시편보다 약간 높은 강도를 나타냈지만 25도에서는 44도 PCM 혼입 시편이 더 높은 강도를 나타냈으며 50도에서는 두 종류의 PCM 간의 유의미한 강도차이를 확인하기 어려웠 다. 시멘트 부피대비 10%정도의 PCM 혼입은 콘크리트의 압축강도 하락에 큰 영향을 주지 않으며 오 히려 높아지는 경향을 나타냈으며 그 원인은 잔골재 사용감소에 의한 상대적 시멘트-골재 비의 상승 에 의한 것으로 추정된다. 하지만 10%를 초과하는 혼입률은 시멘트-골재비 상승에 의한 강도 상승 효과보다 콘크리트 내부에서 하중에 저항하는 성능을 거의 낼 수 없는 PCM의 영향이 더욱 커지므로 강도가 감소하는 것으로 판단된다. PCM이 혼입된 콘크리트는 PCM의 상변화 온도에 따라 서로 다른 강도경향을 나타내는 것으로 확인되는데 4.5도 PCM 혼입 콘크리트의 경우 0도와 25도사이에서 상변 화 하므로 해당 구간에서 가장 큰 강도저하를 발생시킨 반면 44도 PCM 혼입 콘크리트의 경우 25도 와 50도사이에서 상변화하므로 해당 구간에서 가장 큰 강도저하가 발생하는 것으로 판단된다.

택코트란 아스팔트 포장 공사 시 기존 아스팔트층과 신설 아스팔트 층 사이에 부착성을 증가시키기 위하여 사용되는 재료이다. 교통 하중으로 인해 포장 경계면에서 수평전단응력 및 수직인장응력이 발생하게 되는데 택코트의 유실, 양생 부족 등의 문제로 접착 성능 이 부족하면 포장층의 분리, 밀림과 같은 도로 파괴가 형상이 나타날 수 있다. 현재 국내에서는 국토교통부 아스팔트 콘크리트 포장 시공 지침에 택코트 살포량에 대한 기준은 존재하지만 기존 및 신설 아스팔트 포장층 사이에 택코트의 접착강도에 대한 기준은 존재 하지 않는 실정이다, 이는 접착강도 특성이 분석되지 않은 택코트를 사용함에 따라 아스팔트 포장의 공용성 측면에서 문제를 초래할 수 있다. 따라서 본 연구에서는 PG등급이 다른 택코트 4종류에 대한 인장 및 전단접착강도를 확인하기 위하여 인장접착강도 시험, 전 단접착강도 시험을 진행하였다. 택코트의 양생정도에 따른 접착강도 특성을 확인해보기 위하여 택코트의 수분이 증발됨에 따라 중량 변화가 없는 상태를 양생 100%로 하여 양생 0%, 50%, 100%로 진행하였으며, 살포량은 국토교통부 아스팔트 콘크리트 시공 지침에 따 라 0.5ℓ/m2로 진행하였다. 사용된 택코트 종류에 관계 없이 양생 정도가 증가함에 따라 접착강도는 증가하는 추세를 보였으며, 인장 및 전단접착강도 시험에 관계없이 초기 양생(양생 0%~50%)보다 양생 50% 이후에서의 더 높게 강도가 발현된 것을 확인하였다. 또한 PG등급이 높은 택코트가 인장 및 전단접착강도에 관계없이 접착강도 성능이 우수한 것을 확인할 수 있었다. 추후 택코트의 종류를 추 가하여 PG등급이 분류가 이루어진 후에 접착강도 시험을 진행하여 결과를 확인할 예정이다.

Friction stir spot welding (FSSW) is a solid-state joining process and a rapidly growing dissimilar material welding technology for joining metallic alloys in the automotive industry. Welding tool shape and process conditions must be appropriately controlled to obtain high bonding characteristics. In this study, FSSW is performed on dissimilar materials AA5052-H32 aluminum alloy sheet and SPRC440 steel sheet, and the influence of the shape of joining tool and tool insertion depth during joining is investigated. A new intermetallic compound is produced at the aluminum and steel sheets joint. When the insertion depth of the tool is insufficient, the intermetallic compound between the two sheets did not form uniformly. As the insertion depth increased, the intermetallic compound layer become uniform and continuous. The joint specimen shows higher values of tensile shear load as the diameter and insertion depth of the tool increase. This shows that the uniform formation of the intermetallic compound strengthens the bonding force between the joining specimens and increases the tensile shear load.

This paper proposes the optimal molecular weight for a petroleum-based binder pitch to enhance the density and strength of the prepared graphite block. The effect of the molecular weight on the binder properties, which was quantified using solvent fractionation, was considered based on the evaluation of the coking and viscosity characteristics. The affinity of the pitch to coke influenced the carbonization yield of the block, and the proportion of closed pores was reduced via the use of a highaffinity binder pitch. In addition, the viscosity was found to influence the uniformity of the coke and pitch dispersions, and numerous open pores were formed in the graphite block under high-viscosity conditions. In terms of the molecular weight, a reduction in the content of the insoluble 1-methyl-2-pyrrolidone (NMP) fraction, which was the heaviest fraction present in the pitch, was found to reduce the affinity of the binder to coke while increasing its viscosity. Therefore, the density and strength of the prepared graphite block were reduced upon increasing the insoluble NMP content of the binder pitch. Consequently, it was necessary to control the content of this fraction within < 13.81 wt% to obtain high-density and high-strength graphite blocks.

In order to respond to environmental pollution, developed countries, including Korea, have begun to conduct research to utilize hydrogen energy. For mass transfer of hydrogen energy, storage as liquid hydrogen is advantageous, and in this case, the volume can be reduced to 1/800. As such, the transportation technology of liquefied hydrogen for ships is expected to be needed in the near future, but there is no commercialized method yet. This study is a study on the technology to test the performance of the components constituting the membrane type storage container in a cryogenic environment as a preparation for the above. It is a study to find a way to respond by analyzing in advance the problems that may occur during the shear test of adhesives. Through this study, the limitations of ISO4587 were analyzed, and in order to cope with this, the specimen was supplemented so that fracture occurred in the adhesive, not the adhesive gripper, by using stainless steel, a low-temperature steel, to reinforce the thickness. Based on this, shear evaluation was performed under conditions lowered to minus 243℃, and it was confirmed that the breaking strength was higher at cryogenic temperatures.

PURPOSES : The aim of this study is to evaluate the effects of air voids, binder content, and aggregate gradation on the indirect tensile strength (IDT) and cracking tolerance index (CTindex) of cored asphalt pavements. METHODS : Cored samples were obtained from roads in Incheon city, and several laboratory experiments were performed. First, the cored samples were first to cut into a size appropriate for the IDT test. Subsequently, the air voids of the samples were measured. The damaged sample from the IDT test was loose mixed at 150 ℃ before the binder content was determined, which was conducted via an asphalt extraction test. Finally, the clean aggregates obtained from asphalt extraction process were analyzed in the aggregate gradation test. RESULTS : The result shows that an increase in air voids from 4% to 8% decreases the IDT and cracking tolerance index (CTindex) by 30% and 28%, respectively. Incorporating a binder enhances the ductile behavior of the asphalt mixture, resulting in a higher CTindex. Finally, the contribution of the aggregate grade on the IDT and CTindex is negligible. CONCLUSIONS : The IDT and CTindex are primarily affected by the air voids and binder content. A higher percentage of air voids results in a lower IDT. In addition, a higher amount of binder increases the IDT and CTindex of the cored samples. Meanwhile, the aggregate grade does not affect the IDT.

Conversion to modern hydrogen energy is required, and research on liquefied hydrogen cargo containment systems is needed for large-capacity transport and storage. In this study, changes in the mechanical properties of the adhesive required for storage and transport in liquid hydrogen were confirmed. The lap shear test was performed by realizing cryogenic conditions in a small chamber using liquid nitrogen and liquid helium. There was an increase of 11.0% in the -180℃ condition compared to room temperature, and an increase of 1.8% in the -230℃ condition compared to the -180℃ condition was confirmed. In the case of shear strain, it is known that it decreases as the temperature goes down. As a result of the experiment, it was confirmed that the value at room temperature and the value at -180℃ reduced the shear strain by 5.0%, and -230˚ compared to the -180℃ condition. An increase of 1.5% was confirmed in the C condition. In the case of the specimen tested at -230℃, the deformation in the gripper part was larger than in other tests, and it is judged that the maximum shear strength and shear strain were affected. In addition, in this study, there is a limitation in the experiment at -230°C rather than 253°C, which is the boiling point of hydrogen

건축 및 토목 구조물의 대표 재료인 콘크리트의 핵심지표인 구조 성능을 개선하기 위해 많은 연구가 진행되고 있다. 아울러 구조물의 크기가 커질수록 강도가 높은 고기능 콘크리트의 필요성이 높아지고 있으며, 특히 내구성과 내후성이 우수한 콘크리트 재료의 개발이 필요한 실정이다. 따라서, 본 연구에서는 강섬유와 함께 파라-아라미드 원사를 이용하여 피복 및 꼬임, 원사 데니어 및 섬유 길이에 대한 차별화된 원사가공을 적용한 복합재료인 슈퍼섬유를 콘크리트에 혼입하여 구조적인 성능을 평가하고자 하였다. 본 연구를 통해 최적화된 슈퍼 섬유로 보강된 슈퍼 콘크리트는 도로, 교량, 상수도 및 하수도 등 기존 SOC 의 수리 및 보강에 적용될 유망한 성장 기술 분야가 될 것으로 예상된다.

Cured-in-place-pipe(CIPP) is the most adopted trenchless application for sewer rehabilitation to extend the life of the existing sewer without compromising both direct construction and indirect social costs especially applied in the congested urban area. This technology is globally and domestically known to be the most suitable for partial and full deteriorated pipe structure rehabilitation in a sewer system. The typical design of CIPP requires a significant thickness of lining to support loading causing sewage flow interruption and increasing material cost. This paper presents development of a high strength glass fiber composite lining material for the CIPP application and structural test results. The test results exhibit that the new glass fiber composite lining material has 12 times of flexural strength, 6.2 times of flexural modulus, and 0.5 Creep Retention Factor. These test results can reduce lining design thickness 35% at minimum. Even though taking into consideration extra materials such as outer and inner films for actual field applications, the structural capacity of the composite material significantly increases and it reduces 20 percent or more line thickness as compared to the conventional CIPP. We expect that the newly developed CIPP lining material lowers material costs and minimizes flow capacity reduction, and fully replaceable to the conventional CIPP lining materials.

This study deals with the effects of austempering time on the microstructure and mechanical properties of ultrahigh strength nanostructured bainitic steels with high carbon and silicon contents. The steels are composed of bainite, martensite and retained austenite by austempering and quenching. As the duration of austempering increases, the thickness of bainitic ferrite increases, but the thickness of retained austenite decreases. Some retained austenites with lower stability are more easily transformed to martensite during tensile testing, which has a detrimental effect on the elongation due to the brittleness of transformed martensite. With increasing austempering time, the hardness decreased and then remained stable because the transformation to nanostructured bainite compensates for the decrease in the volume fraction of martensite. Charpy impact test results indicated that increasing austempering time improved the impact toughness because the formation of brittle martensite was prevented by the decreased fraction and increased stability of retained austenite.

Commercial ultra-high-strength PAN-based carbon fibers (T1000G) were heat-treated at the temperature range of 2300– 2600 °C under a constant stretching of 600 cN. After continuous high-temperature graphitization treatment, microstructures, mechanical properties and thermal stability of the carbon fibers were investigated. The results show that the T1000G carbon fibers present the similar round shape with a smooth surface before and after graphitization, indicating the carbon fibers are fabricated by dry–wet spinning. In comparison, the commercial high-strength and high-modulus PAN-based carbon fibers (M40JB and M55JB) present elliptical shapes with ridges and grooves on the surface, indicating the carbon fibers are fabricated by wet spinning. After graphitization treatment from 2300 to 2600 °C under a constant stretching of 600 cN, the Young’s modulus of the T1000G carbon fibers increases from about 436 to 484 GPa, and their tensile strength decreases from about 5.26 to 4.45 GPa. The increase in Young’s modulus of the graphitized T1000G carbon fibers is attributed to the increase in the crystallite sizes and the preferred orientation of graphite crystallites along the fiber longitudinal direction under a constant stretching condition. In comparison with the M40JB and the M55JB carbon fibers, the graphitized T1000G carbon fibers are easier to be oxidized, which can be contributed to the formation of more micropores and defects during the graphitization process, thus leading to the decrease in the tensile strength.

In the current steel structures of high-rise buildings, high heat input welding techniques are used to improve productivity in the construction industry. Under the high heat input welding, however, the microstructures of the weld metal (WM) and heat-affected zone (HAZ) coarsen, resulting in the deterioration of impact toughness. This study focuses mainly on the effects of fine TiN precipitates dispersed in steel plates and B addition in welding materials on grain refinement of the HAZ microstructure under submerged arc welding (SAW) with a high heat input of 200 kJ/cm. The study reveals that, different from that in conventional steel, the γ grain coarsening is notably retarded in the coarse grain HAZ (CGHAZ) of a newly developed steel with TiN precipitates below 70 nm in size even under the high heat input welding, and the refinement of HAZ microstructure is confirmed to have improved impact toughness. Furthermore, energy dispersive spectroscopy (EDS) and secondary-ion mass spectrometry (SIMS) analyses demonstrate that B is was identified at the interface of TiN in CGHAZ. It is likely that B atoms in the WM are diffused to CGHAZ and are segregated at the outer part of undissolved TiN, which contributes partly to a further grain refinement, and consequently, improved mechanical properties are achieved.

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.

PURPOSES : The main purpose of this study is suggest of field bond strength evaluation method for more objective evaluation method through Evaluation of Bond Strength Properties with changing aspect ratio and temperature. METHODS : The evaluation is laboratory bond strength test. Using the core machine, the pull-off test method ; the bond strength test of interface layer the universal testing machine. RESULTS: As a result of the laboratory bond strength evaluation, it was verified that the bond strength by aspect ratio decreases linearly with increasing aspect ratio and the bond strength properties by temperature change existed at high and low temperature condition relative to odinary temperature condition. CONCLUSIONS: According to the results of laboratory bond strength evaluation, the field bond strength evaluation results suggest applying the proposed correction factor (0.8, 1.0, 1.4, 1.9) according to aspect ratio(0.5, 0.1, 1.5, 2.0), For more objective evaluation of the bond strength, it is analyzed that the evaluation value is within 6 ~ 32℃ and the result can be obtained within 5% of the coefficient of variation.

It is a well known that concrete is strong for compression and weak for tension. For reinforcing the weakness and improving the performance about concrete, various methods are used. Fiber reinforced concrete that is one of them has been investigated in this study. The function of fiber in concrete is to improve the stress strain relation and toughness, crack control. It’s applied from normal strength concrete to ultra-high performance concrete. But it is essential to disperse fiber uniformly and to prevent aggregation of fiber in concrete, in order that fiber reinforced concrete show the sufficient performance. The various properties of fiber affect the essential properties, for instance, length and diameter of fiber, source, etc. So, this study evaluated the ultra-high performance concrete with mixed in composite fiber.

The purpose of this study is to investigate the optimum conditions of dispersion and strength to maximize the mechanical properties of woody cellulose nano–crystal (CNC). As a dispersing method, ultrasonic dispersing machine and magnetic stirrer were used as the mechanical dispersion method. The mixing ratio of cellulose nano-crystals (CNCs) was 0.2% and the dispersion time was 10 minutes. Steam curing was carried out for 6, 24 and 48 hours. Based on the experimental results, we will propose source technology regarding CNC for construction materials.